Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management

GEOMOR-05220; No of Pages 17 Geomorphology xxx (2015) xxx–xxx

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Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management J.M. Hooke Department of Geography and Planning, School of Environmental Sciences, University of Liverpool, Roxby Building, Liverpool L69 7ZT, UK

a r t i c l e

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Article history: Received 27 May 2014 Received in revised form 7 April 2015 Accepted 12 May 2015 Available online xxxx Keywords: Flood event River channel Geomorphic effectiveness Magnitude–frequency Morphological change Sediment supply

a b s t r a c t In spite of major physical impacts from large floods, present river management rarely takes into account the possible dynamics and variation in magnitude–impact relations over time in flood risk mapping and assessment nor incorporates feedback effects of changes into modelling. Using examples from the literature and from field measurements over several decades in two contrasting environments, a semi-arid region and a humid–temperate region, temporal variations in channel response to flood events are evaluated. The evidence demonstrates how flood physical impacts can vary at a location over time. The factors influencing that variation on differing timescales are examined. The analysis indicates the importance of morphological changes and trajectory of adjustment in relation to thresholds, and that trends in force or resistance can take place over various timescales, altering those thresholds. Sediment supply can also change with altered connectivity upstream and changes in state of hillslope–channel coupling. It demonstrates that seasonal timing and sequence of events can affect response, particularly deposition through sediment supply. Duration can also have a significant effect and modify the magnitude relation. Lack of response or deposits in some events can mean that flood frequency using such evidence is underestimated. A framework for assessment of both past and possible future changes is provided which emphasises the uncertainty and the inconstancy of the magnitude–impact relation and highlights the dynamic factors and nature of variability that should be considered in sustainable management of river channels. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Floods are of major importance economically and socially because of their detrimental impacts and potential hazard to life. It is therefore essential to understand their occurrence and effects when designing flood and river management strategies and to recognise the range of possible impacts. Major concerns in flood management are to understand flood generation conditions and propagation through a catchment, to determine flood extent and depths for different size events, and to assess flood frequency. However, it is also important to understand and anticipate the possible physical impacts. Floods are the events in which major geomorphological changes naturally occur and in which geomorphological work is achieved by movement of sediment, so they are also fundamental to our understanding of fluvial landscape development. Many channels have been deliberately modified to constrain response but impacts may still occur. The issue of magnitude–frequency of flows, their relation to channel morphology, floodplain construction and sediment fluxes has long been discussed. The physical impacts can include channel widening, channel deepening, change in channel position, change in channel pattern and characteristics, erosion and movement of large amounts of sediment of various calibres, and deposition of sediment within channels, on floodplains and in sediment E-mail address: [email protected].

sinks. These changes can have feedback effects on subsequent flood events. Flood events also have major impacts, many of them beneficial, on ecology and maintenance of biodiversity and geodiversity and they deliver ecosystem services. Understanding flood impacts is therefore essential in conservation and ecological management. Major collections of geomorphological papers on floods have been published, many in the 1980s (e.g. Mayer and Nash, 1987; Baker et al., 1988; Beven and Carling, 1989) as well as many more associated with longer-term, palaeofloods. Several compilations of geomorphological effects of major floods, regionally and world-wide, were made (e.g. Baker and Costa, 1987; Kochel, 1988; Newson, 1989; Miller, 1990; Magilligan, 1992; Costa and O'Connor, 1995), and have remained as benchmarks against which more recent floods have been compared. A phase of high frequency of large magnitude floods in the last 15 years or more (e.g. Pattison and Lane, 2012) has stimulated renewed interest and urgency in quantifying flood occurrence and impact, including EU projects such as HYDRATE (Gaume et al., 2009). Numerous case studies of individual events have been published over the past few decades, many identifying individual factors that have influenced the impact of events. A few papers have provided syntheses and reviews of multiple factors influencing the magnitude impact relation, and developed conceptual models, notably Kochel (1988), Costa and O'Connor (1995) and Dean and Schmidt (2013). Kochel (1988) synthesised information from a large number of floods and attempted to identify which are the

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Please cite this article as: Hooke, J.M., Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management, Geomorphology (2015), http://dx.doi.org/10.1016/j.geomorph.2015.05.014

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most important variables in predicting channel response to large floods. He divided factors into drainage basin and channel factors. Basin factors include basin morphometry, climate and lithology, and channel factors are sediment load and channel characteristics (bedrock/alluvial and cross-section), all combining to affect the hydrograph flashiness, load, gradient, macroturbulent flow and erodibility of the channel. He also examined the effects of temporal ordering. The analysis of spatial controls on flood impacts has advanced considerably in recent years but evidence on the temporal interrelations is still limited, partly because of the need for long records and the infrequency of large floods. A few case studies have highlighted floods of similar peak magnitude that have had dissimilar effects (e.g. Kochel, 1988; Huckleberry, 1994; Magilligan et al., 1998). The long-term and cumulative geomorphological impacts of floods have received much more attention and various conceptual models have been proposed, examining the nature of the impacts, the persistence of impacts and the recovery of fluvial landscapes and features (see Section 1.2). The occurrence of floods and their physical impacts are often inferred from the evidence of morphological change and from deposits; sequences of evidence are used to infer frequency, especially in the longer-term. A common assumption in using such evidence is that the effects at any particular location are proportional to the magnitude of the flood as measured by the peak flow (discharge). However, that relationship may be complicated and factors other than peak magnitude may influence the impact and the record preserved. Some recent papers have identified that floods or their peak magnitude may be missing from the sedimentary record, though the issue of incompleteness of the geological record had been discussed much earlier (e.g. van Andel, 1981; Sadler, 1983; Tipper, 1983). Leclair (2011) recognised this in relation to dune features and Smith et al. (2010) identify it in the case of a large flood on the South Saskatchewan River where they show that the sedimentary record deposited is not commensurate with the size of flood. Magilligan et al. (1998) have previously raised this point in relation to analyses of deposition associated with Mississippi floods and Huckleberry (1994) in relation to floods on the Gila River, Arizona. Much flood management entails the calculation and modelling of flood levels and inundation extent for floods of various frequencies to produce assessments of risk and provide the basis for risk reduction strategies. At present flood risk mapping and modelling are almost entirely static (de Moel et al., 2009; Davies and McSaveney, 2011; Wong et al., 2015) and do not incorporate the effects of changes in morphology, either within-event process effects (Neuhold et al., 2009; Wong et al., 2015) or the feedback effects of morphological change in resetting the conditions for the next flood. Neuhold et al. (2009) tested the effects on flood levels and inundation of varying sediment inputs to a reach within events and found considerable effect on bed elevation levels. Wong et al. (2015) did test the effects of large changes in bed elevation associated with an extreme flood in northern England and found that it had little effect on flood extent but flow in the event was bounded by the valley walls. Erosional effects at peak flow were modelled and not depositional effects or major cross-sectional changes. Roughness effects were also tested as has also been done by Pappenberger et al. (2013) and others. Fewtrell et al. (2011) show one of the few cases of structural and geomorphic effects being tested, in this case on an urban area. One of the few models to incorporate feedback effects of morphological changes on a sequence of floods, and also the effects of changes in sediment and vegetation cover was that of Hooke et al. (2005), developed for ephemeral channels. The problems of uncertainty being generated by morphological changes as well as other factors are beginning to be considered in flood risk management and the problems of communicating and dealing with that uncertainty (e.g. Pappenberger et al., 2013). Ranges of uncertainty are being calculated from ensemble modelling (Pappenberger et al., 2013). Case studies of changes in flood conveyance produced by geomorphological processes have been applied in risk assessment and management strategies of individual reaches (e.g. Lane et al., 2007).

1.1. Purpose The purpose of this paper is to: 1. demonstrate that flood physical impacts can vary at a location over time; 2. examine the factors influencing that variation on differing timescales and how they operate; 3. identify if certain factors dominate and systematic effects are discernible; and 4. set these results in a conceptual framework and examine the implications for river management and for inferring flood occurrence. The analysis is based on a combination of evidence from the literature covering a range of river types and from field sites where flood impacts have been measured over decades, comprising some of the few field data on morphological impacts available over such timescales. The field evidence is from two contrasting environments, exemplifying a range of responses: ephemeral streams in a semi-arid area, episodic in flow regime and behaviour; and perennial streams of a humid temperate region, much more consistent in flows and behaviour. The results have implications for predicting the effects of floods and for inferring flood occurrence and frequency, especially from the sedimentary record. The aim is to highlight the need for river managers and those interpreting flood records to consider the complex relations of magnitude and impact, the range of river dynamics that may occur at a location, and that flood events may be missing from the physical record. The analysis is set within the current widely used conceptual frameworks, then comparative case studies of events of similar magnitude and case studies of single events and their impact are reviewed. The influences of individual factors producing temporal variation in response are analysed. Factors influencing spatial variations are also reviewed briefly. The possible dynamics and relation to force magnitude and resistance at any site are synthesised. The relevant timescales are from a few years to a few hundreds, i.e. those most relevant to management issues, not long-term environmental changes or records. 1.2. Conceptual frameworks Two major approaches tend to be taken to the assessment of geomorphological flood impacts; the magnitude–frequency approach based on work done as measured by sediment flux in an event (Wolman and Miller, 1960); and the flood effectiveness approach in which morphological change is used as the major measure of impact (Wolman and Gerson, 1978). The importance of geomorphic thresholds in the system influencing the response (Schumm, 1973, 1979) is well recognised in principle, though identifying and quantifying that threshold for any particular location or system are still somewhat challenging. The idea that there may be sudden and major changes in the channel characteristics was earlier recognised in the idea of river metamorphosis (e.g. Schumm and Lichty, 1963; Burkham, 1972) and cases of resetting the river have been documented (e.g. Brizga and Finlayson, 1990; Costa and O'Connor, 1995; Dean and Schmidt, 2013). Magilligan (1992) embedded the analysis of critical thresholds of flood power within the framework of magnitude–frequency relationships and effectiveness. Debate has long focused on the question of the relative contribution in the long-term of different size and frequency events, including the role of ‘catastrophic events’. This begs the question of what is catastrophic and the definition of different size events and impacts. Kochel (1988) uses the definition of large events as N50 yr RI (recurrence interval) and Conesa-Garcia (1995) classified events in ephemeral channels of SE Spain into four classes according to flow level and impact. Some authors consider that it is not absolute magnitude but relative size to mean annual flow or mean annual flood that is more important (Schumm, 1977; Kochel, 1988; Cenderelli and Wohl, 2003). Erskine and Peacock (2002), in examining a 1949 flood in New South Wales, Australia, that was 22 times the mean annual flood (MAF), classified floods 2–9 times MAF as large, floods N10 times MAF as catastrophic and flood events 40–50 times MAF, as occurred earlier in the Holocene period on Wollombi Brook, as cataclysmic. Several authors argue for alternative measures as having a closer

Please cite this article as: Hooke, J.M., Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management, Geomorphology (2015), http://dx.doi.org/10.1016/j.geomorph.2015.05.014

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relationship to flood impact, such as peak stream power, total shear stress or unit (specific) stream power (Baker and Costa, 1987; Miller, 1990; Magilligan, 1992), or total discharge and total stream power (Huckleberry, 1994; Costa and O'Connor, 1995). Duration has also been shown to have a large effect (Huckleberry, 1994; Costa and O'Connor, 1995; Dean and Schmidt, 2013). Magilligan (1992) identified a threshold unit stream power of 300 W m−2 for major channel instability to occur in a flood. The differing sensitivity and robustness of channels to change from flood impacts spatially within systems and between systems of differing climatic and physiographic settings were identified by Wolman and Gerson (1978) and exemplified by many, now classic, studies (e.g. Graf, 1983a; Harvey, 1984; Miller, 1990; Wohl, 1992) but have also been demonstrated and to some extent quantified by many recent case studies (e.g. Fuller, 2008; Fryirs et al., 2009; Hauer and Habersack, 2009; Dean and Schmidt, 2013; Thompson and Croke, 2013). The importance of spatial variability of channel morphology at the local scale, as well as the regional controls and the widely considered role of valley confinement and boundary resistance (alluvial or bedrock channels) (Brierley and Fryirs, 2005), has emerged clearly. The concept of adjustment and equilibrium form adjusted to some particular flood frequency or level underlies much fluvial literature, with much debate on the controlling discharges. This is closely associated with ideas of persistence of features (Anderson and Calver, 1977; Calver and Anderson, 2004) and recovery times to equilibrium states. Many authors have shown the differing level of adjustment in different types of channel and numerous papers discuss patterns and rates of recovery (Knighton, 1998). These are influenced by sequencing and spacing in time (e.g. Beven, 1981), with some events building on one another to produce ramped or cumulative effects and continued transience (Brunsden and Thornes, 1979). The importance of inheritance is recognised in ideas of historical contingencies and memory effects, now widely incorporated in geomorphological conceptual landscape models (Phillips, 2009; Church, 2010; Huggett, 2011) but not in specific flood modelling used in management. Channels in disequilibrium have been identified and debated, particularly in relation to semi-arid and arid environments (e.g. Wolman and Gerson, 1978; Bull, 1997; Tooth and Nanson, 2000; Dean and Schmidt, 2013); these ideas are also closely related to notions of metamorphosis. In recent years there has been emphasis on feedbacks and the identification of trajectories of channel characteristics, some of it set within the context of non-linear and chaotic behaviour (Croke et al., 2013; Wohl, 2013). The importance of connectivity and the view that locations of reaches cannot be examined in isolation from upstream and downstream effects or the catchment and system dynamics is also increasingly recognised. Connectivity has a profound effect on sediment supply to a reach and can vary over time (Hooke, 2003; Kuo and Brierley, 2014). The connectivity concept is the subject of continuing research and development and is applied in the River Styles approach to river management (Brierley and Fryirs, 2005). In this analysis two major parameters are taken as measures of flood effects; amount of deposition and channel widening. These reflect ideas of work done and effectiveness respectively though are not entirely synonymous with those. They are also amongst the major forms of evidence occurring and preserved in the longer-term. One of the major problems in assessing and comparing flood impact, especially for large events is that until recently detailed morphological evidence from before and after the event was lacking. Very few locations have been monitored for a sufficiently long time to detect and quantify these effects. From their very nature evidence of flood impacts is often opportunistic. The availability of LIDAR and high precision satellite imagery, as well as aerial photography, is transforming this. However, evidence can only be used where there is independent verification of the flood magnitude such as from instrumental records or the direct measurement of flood marks for the event or documentary records from which flood peak stages and extents can be calculated (Gaume

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and Borga, 2008). There are still not enough data from individual sites to undertake multivariate analysis of temporal factors in relation to large floods. 2. Flood impact case studies 2.1. Comparative studies In a few cases the impacts of two floods of similar magnitude have been compared and the impacts are found to be very different. Huckleberry (1994) analysed two floods on the Gila River in Arizona, and the location of the classic study on channel metamorphosis by Burkham (1972). Floods of January and February 1993 resulted in the most dramatic channel widening on the middle Gila River since 1905. An earlier flood in October 1983 had a larger instantaneous discharge but resulted in little channel change. Comparison indicated that flow duration and volume can be more important than peak discharge in floodplain instability. Hooke (1996) also studied changes on the Gila up to 1992, including the effects of the 1983 event, building on Burkham's work. She concluded that thresholds for metamorphosis on this river had altered due to changes in morphology and possibly vegetation effects and that a simple single magnitude threshold over time is not applicable. Gomez et al. (1995, 1997) and Magilligan et al. (1998) measured small amounts of deposition on the Mississippi floodplain after the large 1993 flood. The cohesive soils and low power on the floodplains may account for general lack of sediment supply there. However, the 1993 event produced much less deposition than the comparable size 1983 event. They attribute this to either the summer timing of the event and/or the sequencing of the 1993 event, following a flood in a wet spring that transported available sediment. Summer floods do not have high loads because of the vegetation cover. Another comparative study was that of Newson (1980) on headwaters of the Wye and Severn on Plynlimon, central Wales. A flood in 1973 caused much mass failure on hillslopes but little effect in channels whereas a comparable size flood in 1977 caused considerable channel changes though little hillslope effect. In the first the hillslopes were primed for failure because it had been a long time since the last major rainfall and much weathering had occurred, producing material that became unstable. In the second event there was little unstable material on hillslopes but previous sediment had now been delivered to the channels. Newson (1989) brings together evidence from several major floods in the UK. He contrasts the example of two dam failure floods, Dolgarrog in North Wales with that of Lawn Lake in Colorado, in which the nature of the sediment supply caused differences in deposits and effects. The effects of two events of contrasting magnitude but similar impact were examined by Eaton and Lapointe (2001). They found that the morphologic response to a 7-year RI magnitude event and a 275-year flood was qualitatively similar and consider that morphologic stability can persist, even when transport rates are exceptionally high, if the stream channel has been conditioned by previous channel modification. Similarly, Kochel (1988) compared two major floods on the Pecos River, Texas, and found that the second had little effect because the first had enlarged and adjusted the channel. He suggested that recovery time is of the order of 500 yrs on the Pecos. Other examples are from the author's own work on ephemeral channels in SE Spain. Nine reaches of three ephemeral streams, Nogalte, Torrealvilla and Salada, in SE Spain have been monitored for flood effects since 1996 (Hooke, 2007). A relatively large flood, estimated at c. 7 year RI occurred in September 1997, after a wet period and several minor peaks (Bull et al., 1999). Hooke and Mant (2000) analysed the geomorphological impacts, though mainly comparing between sites, in relation to flood magnitude. They found a significant relationship in maximum erosion at a site to flood magnitude. In September 2012 another flood affected these channels. This was extremely large on the Nogalte channel but of comparable size to 1997 on parts of the Torrealvilla channel. Changes in cross-sections at one of the Torrealvilla

Please cite this article as: Hooke, J.M., Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management, Geomorphology (2015), http://dx.doi.org/10.1016/j.geomorph.2015.05.014

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sites over the whole period show the effects of both these events (Fig. 1). The 1997 event, with calculated peak discharge of 107 m3 s−1, produced a major change in the channel morphology, incising it by an average of 0.5 m from its pre-flood depth of 0.24 m. Flood stage was 2.10 m above a channel datum. Major deposition took place on the floodplain to a maximum depth of 0.64 m. The peak flow of the 2012 flood is calculated as 115 m3 s−1 and flood stage was 2.29 m

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above datum. Deposition was 0.16 m on the floodplain but by then the channel had further incised (Fig. 1). The relations of maximum amount of erosion and deposition measured by DGPS survey of crosssections in relation to flood stage for all flows 1997–2012 for this site (Fig. 2) indicate a wide scatter in the relationship as well as the difference between the 1997 and 2012 events. Other smaller floods in between the two large floods had not deposited on the floodplain because, since the morphological change of 1997, only the large flows reach the floodplain. Bankfull channel capacity was increased by the 1997 event but floodplain capacity was decreased by the deposition. In a contrasting environment, river channel changes on the meandering River Dane in NW England have been monitored annually since 1980. This is a very active perennial river with up to several overbank flows per year. Analysis of cumulative bank erosion in relation to flood magnitude and number of peak flows over a threshold value indicates a very strong relationship of bank erosion to peak winter discharge (Hooke, 2012). A scaling relationship for annual mean amount of erosion in each year was produced from total amount of erosion proportioned between annual winter peak floods for the period of 1984–96 (Fig. 3a) then tested against actual amounts in the period 1997–2001 and 2001–2007. This relationship predicted length of eroded bank and maximum erosion to within 76–87% of actual amounts for the two time periods, indicating a high level of explanation by winter flood magnitude alone. Fig. 3b shows the peak flows over threshold in the whole period. In the period 2007–8 more intensive monitoring took place and the effects of individual events can be isolated. Several comparable flows of moderate magnitude occurred but these had differing effects (Table 1).

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The literature contains many examples of individual flood impacts including studies of large floods having large impacts, even catastrophic

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Fig. 1. Cross-section surveys at Torrealvilla Oliva site, SE Spain. Profiles before and after September 1997 event (thin lines) and before and after September 2012 event (thick lines) plotted. Other annual surveys omitted for clarity. Flood level is that of September 2012 event. Cross-sections are approximately 20 m apart, successively downstream.

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Fig. 2. Maximum amounts of a) erosion and b) deposition, on cross-sections in relation to flow stage for individual events at Torrealvilla Oliva site, SE Spain, for the period 1997– 2012. Dashed lines are envelope curves to distributions.

Please cite this article as: Hooke, J.M., Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management, Geomorphology (2015), http://dx.doi.org/10.1016/j.geomorph.2015.05.014

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a)

well-established vegetation. Similarly, Heritage et al. (1999) calculated the potential effects of a large flood on the Sabie River, South Africa, but when such an event occurred it did not have the expected effects. Heritage et al. attribute this to the dense vegetation preventing erosion of the cohesive sediment, but possibly also the high sediment load and the short duration. In some cases large magnitude floods have little morphological impact but transport high sediment flux. This is often the case for confined bedrock reaches (e.g. Cenderelli and Wohl, 2003); where morphological effects are constrained, these reaches act as major sediment conduits in floods. Other cases of little morphological effect but major sediment flux include the Narmada River, India (Kale, 2007), where sections were adjusted to large floods so altered little but conveyed large sediment loads. A major flood on the Saskatchewan River, Canada, left relatively little deposition in spite of complete reworking of the braid plain (Smith et al., 2010). Smith et al. attribute this to the channel morphology producing relatively low overbank shear stresses. The impact of large floods can also be seen from the monitored channel of the Nogalte in SE Spain. This is an ephemerally flowing stream with a channel that is mainly very wide and braided and composed of very loose schist gravel. The September 2012 flood in SE Spain was the highest flood for N 40 years on the Nogalte channel, exceeding the 1973 flood which resulted in 83 casualties (because a market was being held in the river channel) (Mairota et al., 1998). Three reaches, in the upper, middle and, lower parts of the course, have been monitored since 1996 and the effects of the 2012 flood were also mapped along the 25 km length of main channel. Crest stage recorder readings for the period 1997–2012 at the downstream site, NogMon, show that the 2012 flow was much higher than any other in that period (Fig. 4a). The peak discharge at the downstream end was measured at the catchment authority continuous gauge as 2400 m3 s−1 and discharges have been calculated for each of the sites and at other cross-sections all down the course. The main effects of the flow were to transport massive sediment loads and deposit material of depths up to a 1 m on the broad braid bars (Fig. 4b, c). Some bank erosion occurred and some chute cuts on bends but general widening and change in channel form did not take place. The relief of the braided channel was increased. Fig. 4d shows the relationship of depth of deposition at the generally aggrading site downstream in relation to all flows measured in the period 1996–2012, of which there were very few. The extent to which the relationship is non-linear cannot be seen because of the lack of flows of intermediate magnitude in the rather dry period of monitoring. The 2012 event did not have massive effect on type of morphology and overall dimensions but did transport very large amounts of sediment. It reduced flood capacity by an average of 25% in this reach. An example of a high flow event that had little physical impact either in morphological change or sediment flux and deposits was on another of the monitored sites in SE Spain (Hooke, 2007), the lowermost Salada site (Sal3), in a marl catchment. It experiences relatively frequent flows for an ephemeral stream (Fig. 5a). Since 1999 this channel has been subject to a virtually persistent, very low flow due to the construction of a water-purifying plant upstream and a small effluent flow routed into the channel. This has resulted in prolific vegetation growth and

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Fig. 3. a) Calibration relation of amount of bank erosion on the River Dane, NW England, to winter peak flow. The total amount of erosion in the period 1984–1996 was distributed in proportion to the annual winter peak flow each year then predicted tested against actual values for periods 1996–2001 and 2001–2007. b) Peak flows over threshold 51.038 m3 s−1 on the River Dane at Rudheath gauging station (SJ667718) (from Environment Agency record) for period of channel observations, 1980–2012.

impacts (Knighton, 1998). Recently, Croke and associates have published a series of papers about the massive floods in Queensland, Australia showing spatially variable but very large effects in some locations (Croke et al., 2013; Grove et al., 2013; Thompson and Croke, 2013; Thompson et al., 2013). Amongst other examples, Dean and Schmidt (2013) demonstrate that the impact of a flood in 2008 on the Rio Grande was related to the state of channel recovery. A major storm in Israel in 2006 produced major changes on hillslopes and in the channel (Grodek et al., 2012). Milan (2012) studied the July 2007 flood effects in Thinhope Burn, a small stream in N England and concludes that rare large floods are the geomorphologically effective flows in that catchment. In a few cases floods with low stream power have been found to have large effect e.g. the Plum Creek flood in 1965 (Costa and O'Connor, 1995). In other cases, large magnitude floods have had little effect (e.g. Costa, 1974; Costa and O'Connor, 1995; Magilligan et al., 1998). Gardner (1977) reckons a 500 yr RI event had little effect because the channel was well adjusted to that size event. In Moss and Kochel's (1978) case a 200–400 yr RI event had little effect because wide flow on the floodplain meant low stream power and erosion was limited by lack of coarse abrasive load, low velocity, cohesive banks, and

Table 1 Details of four consecutive flood events and their depositional impact on the River Dane, NW England (SJ820650). Rank is the order of flows in the peaks over threshold record from the Environment Agency gauge at Rudheath (SJ667718) for the period 1980 to 2012. Date of flow Peak Q Rank Date of observation Bar deposition Floodplain deposition Max depth on floodplain

4 July 2007 3

−1

72.2 m s 14 11/7/2007 Some gravel Little sand 1 cm

7 December 2007 3

−1

86.3 m s 2 27/1/2008 Deep deposits Much sand 20 cm

6 September 2008 3

−1

76.1 m s 9 18/9/2008 Few fresh deposits Very little sediment b1 cm

26 October 2008 73.1 m3 s−1 12 7/11/2008 Cobbles Some sand 4 cm

Please cite this article as: Hooke, J.M., Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management, Geomorphology (2015), http://dx.doi.org/10.1016/j.geomorph.2015.05.014

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Fig. 4. a) Peak flow stage as measured at crest stage recorder at Nogalte downstream site (NogMon), SE Spain. (Flow is plotted at date of recorder reading but likely date of flow events has been identified from rainfall records.) b) Photograph of NogMon reach after the September 2012 flood, looking downstream. c) Channel cross-section profiles (X2) before and after the September 2012 flood. d) Relation of maximum deposition to peak stage recorded at NogMon site in the period 1997–2012 on two cross-sections, X1 and X2. Dashed line is envelope curve to distribution.

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deposition of cohesive mud within the channel bed. On 16 October 2003 an unusually high flow occurred as recorded in flood marks at 3.5 m above the channel bed in a 3 m wide channel. Staff at the water plant reported high runoff rates and high flows of short duration. The rainfall recorded at a gauge downstream (Alcantarilla) was only 16 mm that day, though possibly 35 mm occurred locally in the catchment, but the regional meteorological office reports intensities of 80–122 mm h− 1 (INM pers. comm., internal report). Peak discharge at cross-sections in the site was calculated as 357.9 m3 s− 1. Recently available gauge records from CHS (Confederación Hidrográfica del Segura) give peak flow for the gauge, 6 km downstream, as 336 m3 s−1. The hydrograph shows that the flow rose from zero to this peak in 2 h and had a duration of 3.5 h above 100 m3 s−1 and a total significant flow duration of 9–10 h. Measurements of the effects of the 3.5 m stage flow on repeat crosssections surveys showed no morphological change, little sediment deposition and little effect on vegetation in the centre of the channel, though some mortality of bushes at the upper edges (Fig. 5b). Calculations of the hydraulics show that the forces were high, with shear stress possibly up to 525 N m−2 and unit stream power up to 762 W m− 2 (Sandercock and Hooke, 2010). The possible reasons for the lack of effect include the dense vegetation cover developed, the cohesive mud layer produced by the effluent flows over the previous four years, and the assumed short duration of the flood. The CHS flow gauge record confirms the short duration generated by the high intensity, but low total rainfall.

3. Influences on temporal variation in magnitude–impact relations Fig. 5. a) Peak flow stage as measured at crest stage recorder at Salada downstream site (Sal3), SE Spain. (Flow is plotted at the date of recorder reading but likely dates of flow events have been identified from rainfall records.) b) Photograph of the effects of the 16 October 2003 event at the Sal3 site.

From these case studies, particularly the comparative ones, various factors emerge, acting on various timescales that influence the response to floods. The nature of these influences on temporal variations in response will now be considered in more detail.

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3.1. Morphological and hydrological states The morphologic state of a channel when a major flood occurs affects the response of the channel, though this is mediated through the ability of the channel to respond morphologically, mainly governed by its resistance. Schumm (1973, 1979) emphasises the importance of the closeness to an inherent threshold influencing response. Richards (1999) also stressed the importance of pre-existing morphology influencing impact. Once the morphology of the channel has been altered by a flood event it may gradually or fairly rapidly adjust back

a) 12

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to pre-flood morphology. This is mainly governed by the intervening events and flows and the supply of sediment (Wolman and Gerson, 1978). However, in some cases the new channel morphology is maintained or even enhanced by positive feedback and the channel form continues on a differing trajectory from previously. As noted in the case studies, a change in morphology caused by a previous flood is a reason given in several cases of little impact of a later flood because the channel is already adjusted to that higher discharge and the recovery time is extremely long. Eaton and Lapointe (2001) point out that the morphological modification can be natural (e.g., Gardner, 1977) or anthropogenic. In other cases, particularly of wide alluvial channels, low overbank shear stresses account for lack of flood signal commensurate with magnitude (e.g. Smith et al., 2010) because of the channel shape. It can be seen from Fig. 1 that the altered morphology at the Torrealvilla Oliva site in SE Spain, with a much narrower and deeper channel, produced by the 1997 flood then altered subsequent responses and the 2012 flood effects. Once the channel incised there was a positive feedback of further flows in increasing incision because the smaller flows were more confined (Fig. 6a). The consistency between crosssections on this generally incising reach can also be seen. This changed trajectory compares with the nearby Prado tributary site where the 2012 peak discharge was at least double the 1997 flow. The 1997 flow did not alter the morphology markedly; the 2012 event produced some channel erosion but large amounts of deposition on the upper parts of the floodplain (Fig. 6b, c). Change in responsiveness and setting into a new trajectory can also take place due to alteration of channel resistance, not just morphology. For example, at another of the Spanish study sites, Torrealvilla Pintor, the channel sediment coarsened so that the threshold for sediment movement became much higher and the site has become relatively insensitive. It changed little morphologically in the 2012 flood in spite of 4 m stage peak flow, though some large sediment was moved. The change and deposition are influenced by the hydraulics, controlled by the morphology and a resistant boundary, and by recession conditions, though scour and fill may take place within the event. Another example of the influence of state of the system is that of meandering rivers and occurrence of cut-offs. Hooke (2004) suggested that a series of cut-offs on the River Bollin, NW England, in winter 2000–2001 was mainly due to the sinuosity of the channel being in critical state and that cut-offs were inevitable. They were not due solely to the occurrence of high flows that winter. On the neighbouring river Dane, with comparable high flows but lower sinuosity, such major morphological changes did not take place (Hooke, 2006). It was argued that this behaviour can be interpreted through the theory of selforganisation (Hooke, 2004) and that occurrence of multiple cut-offs can be related to the closeness of the system to an inherent threshold of critical sinuosity. In other cases it has been shown that the short-term hydrological state of the catchment in terms of wetness can influence stability on hillslopes and likelihood of gullying and debris flows. These in turn influence hillslope–channel coupling and sediment supply (e.g. Newson, 1980, 1989).

435 3.2. Sequences of events

434 433 432 0

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Fig. 6. a) Phase space plot of trajectories of cross-sectional area for successive flows for five profiles at Torrealvilla Oliva site, SE Spain for the period of measurement, January 1997 to January 2014. X2 to XCR are sequential downstream. b) X10 and c) XCR cross-sectional profiles for Prado Aqueduct site in Torrealvilla catchment, SE Spain. Profiles before and after September 1997 event and before and after September 2012 event plotted. Other annual surveys omitted for clarity. Horizontal line is the 2012 flood level.

Wolman and Gerson (1978) and Brunsden and Thornes (1979) explain that sequence of events, time between events, and relative size of events between large floods can significantly alter the response. The role of inter-arrival time in governing effectiveness of events was shown by Beven (1981). Carling and Beven (1989) consider that sequence of events, together with sediment availability, may be as important as magnitude in determining flood impact and that effectiveness of sequences is associated with complexity. Although it is well known in principle, it is often not acknowledged or applied in interpreting changes and deposits. The persistence of events partly

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depends on whether comparable or effective events have occurred since and on the magnitude and nature of the original changes. Anderson and Calver studied the persistence of features created by the very high magnitude Lynmouth flood in 1952 in uplands of SW England after 25 years (Anderson and Calver, 1977) and again after 50 years (Calver and Anderson, 2004). They showed that the persistence of features varies, with differential persistence of different kinds of features. Some features such as hillslope landslides were obliterated after 50 years. Planform changes were found to have the greatest persistence but some features such as riparian slips have been renewed and reactivated in subsequent, smaller flood events. Calver and Anderson (2004) divided the persistence into static and dynamic. Harvey (2007) examined the persistence of features and processes created by a large flood in the Howgill Fells of northern England (Harvey, 1986) and found differential recovery in two adjacent valleys, related to differential coupling of hillslopes and sediment supply and closeness to the geomorphic threshold for instability. Dean and Schmidt (2013) demonstrate that on the Rio Grande in the US, although the peak magnitude of the 2008 flood was not unprecedented, it followed 17 low flow years. Record flood stages occurred because the narrow pre-flood channel restricted the ability of the channel to convey the flood flow. The greatest reset occurred where the channel was very sinuous, near tributaries that had contributed large volumes of sediment, and at sites where rapid declines in specific stream power occurred. However, they also consider that the rate of recovery from channel widening or resetting events is faster now than in the past. Fryirs et al. (2009) identified what they called a ‘response gradient’ to the impacts of European settlement, with differing rates of recovery spatially and temporally as the channels adjusted to these impacts. Hooke's (1996) work on the Gila, extending the timescale of Burkham's (1972) analysis demonstrated the differing effect of sequence of flows and also of interaction with periods of drought. She showed that feedback effects of morphology can induce ramped responses and that simple size measurements, such as of width, cannot be taken as directly indicative of flood magnitude or even hydrological phase. Thus the morphology is in disequilibrium with current flood regime. A succession of high flows can keep an enlarged channel in that state instead of the adjustment back, as Gupta and Fox (1974) showed with multiple high floods in 2 years. Frequent high floods can maintain the bigger channel thus moving it to a new state and altered thresholds. Equilibrium theory would expect adjustment back by lesser flows if the new regime was not maintained. Such changes raise the question of a how long a phase and how many events are needed for a channel to take on a new adjusted morphology. Several other of the comparative examples above showed that a second high flow had little impact because of a preceding high flow, even if some years previously (Newson, 1980; Kochel, 1988). Beven (1981) used Hooke's data (1979) on bank erosion on the River Exe in SW England to test the effect s of differing order of events. He uses data on flows and erosion to model different sequences and also a variable or dynamic threshold and finds large differences in the outcomes in terms of total amount of erosion (a range from 3 to 6 m in cumulative erosion over 20 years). The interval and timing of high flow events can also have an effect on whether slump blocks stabilise at the base of eroding banks, as Hooke (Hooke, 1979; Hooke and Yorke, 2010) has observed on active meandering rivers in NW and SW England. If the interval is long and particularly through the summer, then the blocks may be stabilised, becoming resistant to removal in the next flow and reducing or even stopping erosion of the bank (Parker et al., 2011). Much evidence is available from suspended sediment flux measurements and some from bedload that the effect of a short-term, rapid succession of high flows is sediment exhaustion. Walling and Webb (1987) illustrated this for the River Dart and state that it applied to all the rivers in the Exe catchment, which includes the location of the

erosion measurements above (Hooke, 1979). This implies lesser deposition from flows of comparable magnitude in the succession. Rovira and Batalla (2006) obtained similar results for a catchment in NE Spain. According to Magilligan et al. (1998), the lack of effects of the 1993 flood on the Mississippi could have been due to a wet spring, prior to the major event, having already removed loose sediment and therefore affecting sediment available for deposition. From studies of suspended sediment concentration in four contrasting catchments in Puerto Rico, Gellis (2013) considers that the effect of the sequence of flows depends on the sediment supply status of the catchment and channel reach. The effects of multiple large events in succession can also go the other way in that a major event can destabilise a hillslope and/or channel and this is then susceptible to further erosion and sediment movement in subsequent events, thus increasing sediment supply and deposition downstream. If the flood removes vegetation from banks or hillslopes then erosion and sediment supply may increase. This effect is illustrated by Gintz et al. (1996) in a study from Bavaria on bedload transport, where sediment fluxes in moderate events after a large event were much higher than in comparable events before the large flood. One of the few models that incorporates the feedback effects of events, and actually includes effects of sediment calibre and vegetation state (Brookes et al., 2000) is that of Hooke et al. (2005). This model was produced to simulate the effects of flow events on the ephemeral channels in SE Spain. The sites already discussed were established to provide data to validate the model. The effects of different sequencing of various magnitude flows over a 30 year period were simulated. They did produce different outcomes in terms of amounts of erosion and deposition though the effects were not large on some of the channels. 3.3. Trends A gradually changing relation of deposition may arise with regard to the position of sampling relative to the location of a channel or the locus of activity. This may be due to channel migration in a meandering river; it may be due to progradation of a delta in a lake or due to gradual build-up of an alluvial fan or successive movement of the apex (e.g. Bull, 1997). Any of these could mean that the amount and calibre of deposition change over time for the same size event at a specific location. In floodplains the amount of deposition tends to decrease with distance from the channel (e.g. Rossl et al., 2004). In fans and deltas sediment will generally build forward over time so equivalent size of flow and size of sediment will be deposited more distally, though much depends on the configuration of distributary channels (Erskine and Borgert, 2013). Likewise, in cut-offs and ox-bow lakes the channel will be increasingly censored by infill of the ends (Hooke, 1995). In a similar way a gradual aggrading or incising channel will have a changing relation to the floodplain and therefore likely occurrence and depth of deposition. This has been recognised in floodplain construction models (Nanson and Croke, 1992) and is similar to the slackwater deposit concept where censoring is recognised (House et al., 2002) but it is often not applied when sections of floodplain deposits are analysed. Nanson (1986) demonstrated that certain channels may reach a stage where cumulative deposits are suddenly stripped from the floodplain. Vegetation has been shown to have a significant effect on both channel morphology (e.g. Hession et al., 2003) and channel processes (e.g. Abernethy, and Rutherfurd, 1998; Darby, 1999; Hopkinson and Wynn, 2009). Therefore, any change or trend in vegetation over time can have an effect on response to flood events. Such trends include gradual growth of vegetation after a resetting, at the site itself or upstream e.g. due to a previous flood or other land management, fire or other natural event. These changes all exert an influence on the amount of deposition and on responsiveness by altering channel flood capacity, stream power and distribution of forces and sediment supply. For example, growth of vegetation at a site can decrease erodibility of banks and floodplain and increase roughness, thus increasing resistance

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and decreasing velocity of flow. Two examples of trends associated with vegetation change and the possible effects on channel response are illustrated from the work in SE Spain and in NW England. In the ephemeral channels of SE Spain vegetation grows in many parts of the channel (Sandercock et al., 2007). Herb and some shrub vegetation tend to be removed by even low and moderate flows (Sandercock and Hooke, 2010) but phreatophyte vegetation such as Tamarisk and Retama is highly persistent. In the Nogalte, channel vegetation grew over the period 1998–2012 (Fig. 7a, b); however, the September 2012 flood of N50 year RI was so high that it destroyed or buried most of the vegetation (Fig. 7c). This is the only event that has been found on any of the measured channels that actually destroyed such vegetation on a large scale. In ephemeral channels increased vegetation growth within the channel in low flow periods could increase channel roughness and reduce flow forces. It needs a very large event to reset the vegetation component of the channel and the threshold force increases as vegetation grows. It is expected that the Nogalte channel will now be more susceptible since the 2012 flood effects to subsequent flows, though some of the remaining Retama stumps are quickly resprouting.

Fig. 7. Photographs of NogMon site, SE Spain, in a) 1998, b) 2010 and c) 2012 showing vegetation cover, mainly Retama bushes. Vegetation grew between 1998 and 2010 but was destroyed in the September 2012 flood. The position of photograph Fig. 4b is indicated.

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On the River Dane a detailed GIS mapping and analysis of riparian vegetation cover from aerial photographs has shown that the coverage of woody vegetation increased by up to 40 times in the period from 1984 to 2007 (Hooke and Chen, 2015) (Fig. 8). It is possible that this growth is having a stabilising effect on the floodplains, and to some extent channel banks, and that it is having a moderating effect in channel response (Hooke, 2008). The extent and number of gravel bars have decreased markedly since 1984 (Hooke and Yorke, 2011), including during phases of high and frequent floods (Fig. 3b) and this may be related to the vegetation increases reducing bank erodibility and sediment supply. 3.4. Sediment supply and connectivity The depositional response is closely related to sediment supply to the location, in combination with morphology to give the conditions for deposition. Abundant sediment supply also tends to produce channel lateral instability. The marked non-linearity of the relationship of sediment transport to flow magnitude is well established so, in general, for a higher magnitude flood proportionally much more sediment is expected to be transported. However, this depends on availability. Obviously, major environmental factors, climate, land use and human activities and modification of the channel upstream of a site can alter sediment availability downstream. The development of the concept of connectivity in geomorphology has emphasised the importance of location of a reach in relation to upstream conditions and spatial sequences and their effects on sediment delivery (Hooke, 2003; Brierley and Fryirs, 2005) Applications of the connectivity concept have shown that it cannot be assumed that even fine sediment eroded in the upper parts of catchment is transported all the way through the system (Fryirs and Brierley, 2001). Much is deposited in stores on hillslopes and in the channel system. Much sediment is also derived from the channel so the immediate upstream conditions are very important in response of a reach (Hooke, 2003). The position of the site relative to the spatial sequence of sources is important and supply could change over time if a new source is activated or one stabilised. It can also alter if barriers are emplaced or breached. Thus assessing the context of the site is essential. An example of the effects of a breach upstream is given by Rathburn et al. (2013) in which the recovery and propagation of effects downstream are described. Connectivity itself can be dynamic as Kuo and Brierley (2014) demonstrated from the active environment of Taiwan, where landslides are frequent. Impacts of landslides on connectivity were shown to be spatially differentiated. Kuo and Brierley produced a conceptual model of differing connectivity in relation to magnitude–frequency of flood events since occurrence of the landslides themselves is also influenced by such events. Likewise, the magnitude–frequency of other events can influence sediment supply as Davies and McSaveney (2011) demonstrated in relation to volcanic eruptions and earthquakes, as well as landslides, in New Zealand. Newson (1980), Harvey (1986) and Milan (2012) showed how, in floods in the British uplands, the effects were very much influenced by the sediment supply from the hillslopes on which landslides, debris flows and gullying took place. Coupling, or connectivity, of hillslopes and channels is an important influence on sediment availability in the channel system (Harvey, 2002), exemplified by a large event in the uplands of NW England that had a very large effect on connectivity and greatly increased sediment supply to the channel (Harvey, 1986). Schwendel and Fuller (2011) identify that, despite similar geographical and geological characteristics of the catchments upstream of studied mountain stream reaches of the eastern Ruahine Range in New Zealand's North Island, the morphological response to flood events ranges from almost none to frequent migration of the channel in a wide active channel zone. They consider that the dynamics are mainly driven by connectivity of sediment supply from hillslopes and gullies

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Fig. 8. Riparian woody vegetation cover on the River Dane, NW England, mapped in ArcGIS from aerial photographs. a) April 1984 and b) April 2007. The large increase in area of riparian woody vegetation is evident.

over the last decade and from in-channel storage. Similarly, Dean and Schmidt (2013) demonstrate how the input from tributaries influences the channel response and the importance of high bedload transport in producing lateral instability. Variations in connectivity on hillslopes with different rainfall events have been shown by Marchamalo et al. (2014). Examples of cases in which deposition was low because of little sediment supply may be related to short-term variations due to flood sequence, often producing sediment exhaustion (see above), or to seasonality (see below) for example, the 1993 Mississippi flood (Magilligan et al., 1998). Morphology is altered by sediment supply and delivery in individual events or over the longer-term and this in turn can alter flood conveyance and cause problems (e.g. Lane et al., 2007). Neuhold et al. (2009) and Wong et al. (2015) are some of the few to have modelled the effect of erosion on inundation levels within events. Sediment input was found to affect bed elevations but in those cases had little effect on flood levels. Scour and fill can take place within an event so the net effect may be very little morphologically but sediment flux may be large. In some cases, particularly confined sites, the hydraulics of flow are such that similar deposits and morphology may be created by a range of different magnitude flood events, mainly related to the recession limb of the hydrograph. Lack of differentiation of deposits has been noted in ephemeral channels (e.g. Laronne et al., 1994). Of course, deposits created by one event may be obliterated in the next. For example, at one of the Spanish study sites (Serrata) (since destroyed by gravel extraction) a sequence of flows and sediment changes was recorded (Fig. 9a–d) (Hooke, 2007). Major scouring took place in the September 1997 event, producing a coarser, sparse sediment bed (Fig. 9a before, Fig. 9b after). A fine layer was deposited in the next flow (Fig. 9c) but was subsequently removed by the following flow (Fig. 9d). The maximum size of sediment measured in quadrats at each of these dates has a non-linear relationship to flow stage (Fig. 9e). This emphasises that not all events are recorded in the sedimentary record, at least not in active channels and also, that such events may be missed completely without detailed monitoring. The source of the fine material in October 2000 is not known. In summary, deposition and channel change at a site can be affected by sediment supply from upstream which can alter on various timescales. These range from individual events associated with hillslope destabilisation, to seasonal changes associated with vegetation, to short-term modifications such as by woody debris dams, to longerterm emplacement or breaching of natural barriers, e.g. rock steps, or artificial dams and embankments, and to long-term changes in land use and supply from the wider catchment. Complex response means that spatial and temporal responses are connected. Thus, at a single site, sediment supply can alter over time through transmission of responses spatially and consequent morphological changes upstream (Schumm, 1979; Bull, 1997).

3.5. Seasonality The effects of seasonality are arguably particularly neglected with regard to flood magnitude relations, though they are well known for other fluvial phenomena and processes. In one of the few studies relating to this factor, Old et al. (2014) have demonstrated that seasonal growth of submerged macrophytes can have a very large effect on flood conveyance. They tested this by measuring flows and velocities before and after a weed cut in summer. They found that weed cutting increased flood conveyance by 84–141%. This is a management strategy commonly applied in chalk streams in the UK but also demonstrates the potential difference between winter and summer conditions of weed growth. Additional seasonal effects of vegetation on resistance to morphological change and on sediment supply are not well documented, though seasonality of suspended sediment is well known. On the River Dane, the evidence from the few summer floods that have occurred is that they deposit very little sediment. As seen in Table 1, summer floods of comparable magnitude to winter floods that deposited thick layers of sediment on bars and floodplain produced only very thin veneers of fine sediment on the vegetation, illustrated in photographs after the events (Fig. 10) (though the thin veneers are barely visible). The little evidence available suggests that coarse material in the channel bed may still be mobilised. From the field observations and measurements of channel mobility and budgets (Hooke and Yorke, 2010) it is suggested that the reason for the lack of fine sediment load and deposits in the summer floods is that most of the sediment in this river reach comes from bank erosion and in summer the banks are dry and much more vegetated than in winter. Thus there is little erosion and little supply of sediment. This is corroborated by Hooke (1979) in a 2.5 year record of bank erosion on similar types of stream in SW England in which erosion after every high flow event was recorded. This showed little effect of flow until banks were wetted in autumn; multivariate analysis indicated that soil moisture state was as important as peak flow in amount of erosion in an event with a distinct threshold at several sites, an influence also found by Wolman (1959). (These are the data also used by Beven (1981) in his modelling of effects of sequencing of flows.) Marked seasonality in river suspended sediment load has been demonstrated for the same catchments (Walling and Webb, 1987; Harlow et al., 2006). As indicated above, the lack of deposits from the 1993 Mississippi floods is explained by Magilligan et al. (1998) as partly due to seasonality, as well as the sequence of flows. In contrast, on the Paria River in Utah/Arizona Graf et al. (1991) showed that sediment loads are much higher in summer and this was related to storm type and possibly frozen ground in winter. The effects of seasonality are mainly due to drying of banks making them less erodible, thus reducing response and also sediment load,

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and to vegetation cover increasing resistance and decreasing velocities. Velocities on floodplains will also be lower in summer meaning that deposition of any load being carried will occur more readily and thus sediment may not be carried as far through the system. This seasonality effect may be greater in floodplain than in upland streams. The seasonality effect could also be related to duration and hydrograph characteristics, discussed below, if winter and summer floods tend to differ, summer floods often tending to be flashier. The destabilisation effect on hillslopes is also likely to be much less in summer/dry season than wet season. Overall, the evidence indicates that summer (dry season) floods may produce much less deposition than equivalent winter (wet season) floods. Thus there is a danger that large floods may be missed by examining deposits and their occurrence could be underestimated in the long-term. This may especially be the case for flash floods because this is the common kind of flooding in summer; these can be some of the most difficult events to manage and on which greater knowledge is needed.

3.6. Duration and hydrograph characteristics The relationship between flood magnitude and flood duration as a control on channel reset was suggested by Costa and O'Connor (1995). Likewise, Dean and Schmidt (2013) argue that geomorphic reset does not begin to occur until the flood exceeds a reach-averaged erosional threshold. Thus, a large, short duration flood pulse may cause rapid rates of geomorphic change. However, a longer duration flood of smaller magnitude may result in greater total channel reset because the length of time above the erosion threshold is greater than that of the other flood. They illustrate this with the evidence from the Rio Grande floods where it was the 7th highest flood ever measured which had the greatest morphological effect but it was the longest duration flood. Huckleberry (1994) considered that differences in flood duration accounted for the difference in morphological impact of the 1983 and 1993 floods on the Gila River in Arizona. It may be correlated with seasonal effect if floods of differing seasons tend to have different hydrograph characteristics. Huckleberry suggested that there was a difference in floods coming from Pacific and Gulf storm systems. Duration of floods may change over time as a result of climatic variability. For example, on the Paria River, Arizona, Graf et al. (1991) show that sediment load duration is closely related to flow duration and this had changed with climate over decadal timescales, producing profound effects on sediment flux and floodplain aggradation. Of course, catchment modifications such as urbanisation and dam construction will modify duration of events. For the River Dane the hypothesis that the seasonal difference detected in sediment transport and deposition was due to differing duration of summer and winter hydrographs was tested. It was hypothesised that summer storms are mainly thunderstormgenerated and of shorter duration than cyclonic-generated floods in winter. However, the distributions of flow duration above bankfull in relation to peak flow for summer and winter floods emerge as almost identical (Fig. 11). It would therefore seem that it is the seasonal effects of vegetation and dry banks that are significant in producing much less channel change and deposition rather than flood duration on that river and this corresponds with other results of bank erosion research (Hooke, 1979; Julian and Torres, 2006). On the Sabie River, South Africa, Heritage et al. (1999) consider that short duration may have been a contributory factor to the lack of flood effect on the cohesive and highly vegetated channel and floodplain. Fig. 9. Sediment quadrats at Torrealvilla Serrata site, SE Spain, in: a) September 1997, b) November 1999, c) October 2000, and d) January 2001; e) maximum particle size (b axis) in sediment quadrats in relation to peak flow stage measured at a crest stage recorder on the site at the same dates.

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Fig. 10. Photographs of sediment deposits on the River Dane for four events (as in Table 1) showing seasonal differences: a) 4 July 2007 (summer), b) 7 December 2007 (winter), c) 6 September 2008 (summer), and d) 7 November 2008 (winter). Only a thin veneer of fine sediment is present in a) and c).

Similarly, this is the reason suggested above for the lack of effect in the 16 October 2003 Salada flood in Spain, also on a cohesive and highly vegetated channel. Although in both cases forces calculated to have occurred should have been above the threshold for erosion, this did not occur. It seems that a longer duration high peak flow is needed to produce erosion in such cases or resistance has been underestimated. Within event effects on erosion have been modelled by Wong et al. (2015) using different sediment equations, for a large flood in the English Lake District. On the Nogalte in SE Spain, the net effect of the September 2012 flood was massive deposition but it is hypothesised that the extremely rapid rise of the hydrograph (of the order of 2000 m3 s− 1 at the downstream end in b1 h), meant that sediment was mobilised very quickly, producing very high loads early on and that there was little time or capacity for erosion. The effects of duration can be incorporated in flood assessment by using variables such as total flood power, or total discharge as alternative measures of magnitude. However, these require a continuous record or some reconstruction of hydrographs as Costa and O'Connor (1995) did but cannot easily be reconstructed from the sediment record (Huckleberry, 1994). Thus the occurrence of flow duration events that do not produce significant deposits may be underestimated from sedimentary evidence. 4. Spatial factors Wolman and Gerson (1978) and Brunsden and Thornes (1979) emphasised the importance of channel setting and sensitivity in geomorphic impact. That flood effects are spatially variable has been demonstrated in many case studies (e.g. Graf, 1983b; Harvey, 1984; 2500

Duration (mins)

2000 summer winter Linear(summer) R = 0.53 Linear(winter) R = 0.70

1500 1000 500

Kochel, 1988; Erskine, 1996; Cenderelli and Wohl, 2003) including recent studies (e.g. Hauer and Habersack, 2009; Croke et al., 2013; Dean and Schmidt, 2013). For example, Fuller (2008) assessed the geomorphic impacts of a 100-year flood in the Kiwitea Stream (254 km2), a tributary within the Manawatu River catchment (New Zealand). High stream powers generated in confined channels at bends produced catastrophic channel transformation. Where flood flows dissipated overbank, stream powers and the extent of channel transformation were reduced. A key factor emerging from these studies is the influence of confinement. Where deep narrow channels are in bedrock then this may produce little erosion and channel change and high stream powers combined with lack of accommodation space will restrict deposition. In other cases, if the confined sections are somewhat erodible, then the high stream powers can produce very large changes. For example, Thompson and Croke (2013) examined the effects of a 2000 year RI event in Queensland and found dramatic differences in geomorphic responses between the two adjacent reaches of contrasting valley configuration. The confined reach experienced large-scale erosion and reorganisation of the channel morphology and was net erosional, whilst morphological changes were much less in the unconfined reach and it was net depositional. Croke et al. (2013) found differences between expansion and contraction reaches in the extent of floodplain–channel linkage and impacts in each zone. Though the threshold of 300 W m−2 has been identified as a threshold force for change to occur (Miller, 1990; Magilligan, 1992) Miller found this a poor predictor and local factors made a difference in response. He identifies the effects of climate, boundary materials, flood frequency, valley confinement, and availability of coarse bedload as significant. Kochel (1988) identifies topography, bedload and channel gradient as primary factors in channel response and Dean and Schmidt (2013) summarise major factors as including valley confinement (Wohl, 1992), sediment supply, grain size (Newson, 1980), and the flow resistance provided by floodplain vegetation. In their own study on the Rio Grande Dean and Schmidt (2013) find that major widening takes place in zones of high sinuosity, bedload supply, and large divergences in sediment transport capacity. Complete spatial analysis and comparison of reaches for an event are now much more feasible given the availability of remote sensing and rapid data acquisition, allowing differences and association to be measured and thresholds quantified. 5. Synthesis and discussion

0 0

20

40 60 80 Peak discharge (m3 s-1)

100

Fig. 11. Duration of summer and winter peak flows of various magnitudes on River Dane, NW England, in the period 1980–2009. Linear regression and correlation of each plotted, showing lack of significant difference in duration with season.

It has been demonstrated that various factors influence the effectiveness and impacts of a flood event of particular magnitude over time. In management of flood risks and of channels and floodplains it is important to take into account state of a channel, changed state over various timescales and likely variability of response. There is a need to recognise

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Table 2 Factors influencing temporal variation of geomorphic impacts to flood magnitude. Morphological and hydrological states

Sequence of events Trends Sediment supply and connectivity

Seasonality Hydrograph characteristics

Closeness to intrinsic threshold, e.g. sinuosity for cut-off. Flood capacity and hydraulics; restricted deposition because of accommodation space, boundary resistance, and changing hydraulic relations with flow increase Soil moisture for hillslope instability Exhaustion effect on sediment supply; destabilisation; previous major impact; timescale of readjustment Channel position; vegetation growth; progradation of deposits; aggrading or degrading trajectory Hillslope destabilisation and coupling Short term barriers e.g. woody debris dams; barrier breached or inserted upstream Morphological change upstream Dry banks; vegetated banks, bars and floodplain; seasonally different flood events Duration primarily; possibly rate of rise, rate of recession Total volume

Confined alluvial sections High R, High F >Threshold Change

a)

old

esh

Thr

Erodible alluvial channels Low R, High F >Threshold Change

Force

Confined bedrock gorges High R, High F >T >Threshold No change

Wide alluvial floodplains Low R, Low F
Channels with cohesive materials dense vegetation Med R, Med F
Resistance

b) Narrowing erosion

old

esh

Thr

Greater effect

Sediment supply

Widening deposition

Duration

Force

this varying relation of impact to magnitude at any particular location and the possibility that evidence is missing in assessing frequency of any particular magnitude. This also applies in assessing the contribution of different floods to geomorphological change. The factors are summarised in Table 2. The condition of a particular site at any point in time can be considered in relation to the force and resistance status of the site and the relation to the threshold for significant flood impact. Fig. 12a indicates the different types of zone and channels, setting the channel reach in its spatial context. These could be more formally related to specific river classifications and typologies being used in any particular management application; for example, use of River Styles (Brierley and Fryirs, 2005) or Rosgen's (1994) scheme. The impact of individual events can be set in the context of coefficient of variability, which is widely used for hydrological analysis of the possible and likely ranges of flow in flood management (e.g. Maheshwari et al., 1995) and can inform on the flood magnitude to which a channel is adjusted and the range of flood impacts to be considered. Fig. 12b demonstrates the changes which might occur, some of which alter resistance and others altering force, moving a site away from or across the threshold. Thus, the relation to the threshold for morphological change from a flood in any particular channel reach is dynamic. For example, growth of vegetation may increase resistance and therefore increase the threshold force needed to cause impact. Individual sites can be plotted on this space according to their characteristics and the possible effects and the likelihood of variability arising from the different factors considered. The kind of changes indicated by the arrows in Fig. 12b could be anywhere along the diagram depending on the site characteristics. This analysis leads to explicit consideration of the possibilities of changed relations and probability of crossing a threshold. Peak discharge still emerges as an overall indicator but peak shear stress or peak unit stream power is an often better measure of magnitude because they take into account channel morphology and energy gradient. The pre-existing state, its relation to thresholds and its trajectory of adjustment are very important. There is a need to assess to what scale of flow the channel is adjusted and whether it is still adjusting or likely to adjust back and the possible timescales. Depending on that state then sequences of high flow will have different effects; time of last major flood impact should be ascertained, if possible. Overall trends in behaviour affecting either the forces in the channel, through morphological changes by aggradation or degradation, narrowing or widening, or trends in resistance, such as from vegetation, or trends in sediment supply, will all have effects, usually on decadal timescales. Wider catchment and local reach changes can affect sediment supply and spatial sequence downstream, having a crucial effect on response. Sediment supply can be affected by duration of flow, particularly in influencing time for erosion. Timing of event in relation to season can

13

Morphological changes

Seasonality bank moisture, vegetation

Trend eg. vegetation growth

Resistance Fig. 12. a) Exemplification of types of channel in relation to general force–resistance relation and thresholds for flood impact. Change will take place when force exceeds resistance, i.e. the channel reach falls above the threshold relation (F = force, R = resistance). Confinement increases stream power but boundary materials affect resistance. b) Possible types of changes in factors influencing force and resistance over time in relation to flood impact threshold. These variations may occur anywhere along the threshold line and several factors may have influence in specific reach.

be very significant, particularly in climates where summer produces drying out of banks and floodplains and growth of vegetation, so reducing erodibility and sediment supply, as well as increasing roughness and decreasing flood capacity. Several of these factors are often interrelated. This type of analysis emphasises the possible dynamics of the channel and changes in the magnitude–impact relation over time. Fig. 13 summarises these effects and some of the feedback loops acting over various timescales. The analysis has indicated several circumstances and factors that may cause a large flood event to leave little record in terms of stratigraphy or morphology where at other times it has major impact. This is particularly the case for short duration floods, for summer floods in certain climates, for later floods in a succession of peak flows and where resistance has increased over longer timescales than events and seasons. Lack of sediment supply may mean that a large magnitude flood is not recorded in the flood deposits or that the deposit does not reflect the magnitude of the flood. Occurrence of floods may also be underestimated where peak magnitude was relatively high but duration was low. Duration is difficult to derive from sediment records; as Huckleberry (1994, p. 1086) states “This is an important notion because only peak discharge can be reconstructed from the stratigraphic

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Timescale

External Change

EVENT FACTORS Sequence Duration Peak flow Total force

Catchment land use Channel modification

Autogenic Change

Morphology

Vegetation Channel location Trajectory

(cross section, pattern, gradient)

Geological events Upstream changes

Sediment supply Seasonality

Adjustment Resetting Flow regime

Erosion Deposition

FLOOD EFFECTS Channel and floodplain morphology Flood capacity Resistance

Sediment availability

Fig. 13. Summary of factors and feedback effects influencing temporal variation in flood impact. Short-term event characteristics can affect flood impacts through channel morphology and sediment supply. Response will also depend on upstream changes, particularly influencing sediment supply to the reach. These may be superimposed on longer-term autogenic changes and trends and trajectories of adjustment to previous floods. Trajectories of change will also be influenced by external changes and direct channel modifications causing changes in flow regime, sediment supply and channel morphology. The flood impacts have feedback effects on channel morphology, sediment supply within the reach and on trajectories of adjustment and recovery.

record in paleohydrological studies. Consequently, interpretations of prehistoric channel changes based on the paleoflood record are limited by our inability to reconstruct flood volume”. Therefore use of such evidence to estimate flood frequency–magnitude relations may omit events that could still have devastating effects in terms of inundation and human impact. Conversely, in some cases peak magnitude is overestimated because of large effects due to long duration and high total flow, but, importantly, impact is still not underestimated. All these changes are superimposed on or taking place at the same time as climatic fluctuations, with marked phases at decadal timescales and now the trends of climate change due to global warming. These considerations discussed here have important implications for flood risk assessment and need to be incorporated into river management. The present situation in relation to flood risk mapping, modelling and assessment is that much of the focus is on calculating and mapping the inundation extent of floods of specific magnitude or return period, usually the Q100. This is as required by the Floods Directive (FD) (2007/60/EC) in Europe. De Moel et al. (2009) outline the procedures used in standard flood mapping. A review of flood mapping in Europe demonstrated that some maps of flood depth, a few of velocity and some limited flood risk maps are produced as well as inundation extent (de Moel et al., 2009). Typically the flood mapping is static with the effects of embankment breaching tending to be the only changes modelled. The Floods Directive requires some revision of mapping but most of the changes actually incorporated are structural. Some flood mapping does not even incorporate the effects of defences. Extents of historical floods have been mapped in some countries (de Moel et al., 2009). As Wong et al. (2015) stated, recent research into flood modelling has concentrated on simulation of inundation flow without considering influence of channel morphology. They test the effects of major channel bed erosion in the extreme 2009 Cockermouth flood in the English Lake District, using the LISFLOOD model, and find that changes in bed elevation have little effect on flood inundation extent. However, this was a flood that filled the whole valley floor and was bounded by the valley walls. They did not test the effects of major changes in cross-sectional form or the effects of deposition. They conclude that

cumulative effects of morphodynamic changes over a series of events need to be modelled. Examples produced here, such as that of the 2012 flood effects in the Nogalte channel (Fig. 4), demonstrate the large changes in flood conveyance that can take place due to morphological changes. In that reach a large amount of deposition took place that decreased the channel flood capacity for the 2012 flood level by 27%. It decreased the flood capacity for just below floodplain level, between the embankments, by 32% from the 2011 state, and it decreased the capacity for flows that in 2011 just came to below the upper bar, reaching the level of the track that is widely used along the channel, by 24%. Thus the risks of inundation to the floodplain and to the whole active channel are significantly increased. The 2012 flood is calculated to be of at least 50 years recurrence interval (RI) but the embankment level is of the order of 20 years and the road level is c. 1 in 5 years RI (and much less in other sections). A limited amount of other research has demonstrated the effects of channel changes on flood risk or inundation, particularly through the effects of sediment delivery to reaches, for example the work of Lane et al. (2007) on a reach in the English uplands. Davies and McSaveney (2011) have discussed the effects of ‘geological events’, such as landslides, earthquakes and volcanic eruptions, on channel morphology and on flood incidence, directly and indirectly via sediment supply, in the extremely active environment of New Zealand. They advocate that, in such an environment, these effects need to be incorporated in flood risk analysis and management. They consider that their present omission leads to underestimation of flood risk. Work is in progress to develop the ideas discussed here into a more usable set of procedures and protocol that can feed into decisionmaking on river management in relation to flood risk, flood impacts and channel dynamics. However, at this stage, some guidance can be suggested on checks to be made: • Measure present morphology. Calculate hydraulics and resistance. Calculate effect of variation in resistance and flood capacity. Assess likely seasonal range in reach (likely to be high for silt-day material, herbaceous vegetation).

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• Find evidence for last flood that altered the channel significantly (enough to cause concern or management problems). Can be used as a reference flood. • Find the characteristics of the most recent large flood (may or may not be the same as last having major effect). Use to assess threshold for change for that flood. Calculate possible range in channel resistance and capacity with seasonal factors for such a flood. • Compile evidence of longer-term (multi-decadal) change and trajectory of channel reach. Identify adjustments to external changes and autogenic changes and trends. • Assess upstream status–sediment supply and connectivity and channel stability. • Calculate possible change in response.

Even if detailed evidence is not collected this can act as a check list of factors that may have altered or be different and therefore have altered flood risk. Part of the procedure would be to compare present flood impact risk with a reference flood in the past, preferably one that had large impact (to be avoided or allowed for in the future) and therefore crossed the threshold of major response. Likely impacts of another flood of comparable magnitude in the near future could be assessed by comparing peak flow, duration, stream power, position in flow sequence, season, and trajectory since that time, together with external catchment changes, upstream reach alterations and inherent trends, such as vegetation growth or state of morphology. Alternatively, the effects of changing some of the parameters in the commonly used design flood, often Q100, could be incorporated in modelling. Of course, Q100 frequency may itself be underestimated due to various changes. Uncertainty range may be assessed by ensemble modelling. The evidence provided here emphasises the need for awareness of, and allowance for, the variability in flood impact. It also demonstrates the importance of morphological feedback in models of flood risk. It provides a framework for consideration of channel management actions after a flood and indicates that it may not be appropriate to try to reinstate the previous morphology (as happened at Cockermouth in England). The dynamics of channels are beginning to be incorporated in some of the procedures used in managing channels and are the focus or incorporated into the results and recommendations in several of the papers in this Special Issue. These include procedures associated with WFD and FD in Europe and ideas for coping with channel instability and mobility world-wide. One of the major messages emerging from this review is the danger of underestimation of flood frequency from sedimentary evidence. Geomorphologists and sedimentologists, and those using flood event information derived from sedimentary evidence, need to be aware of the likelihood of underestimating flood frequency or the magnitude of on event due to lack of deposits associated with summer floods, short duration events, and later floods in close sequence, as well as the longer-tem effects of catchment and channel modifications on the depositional record. For example, on the River Dane, in a 50 year peaks over threshold record of discharge, four out of the highest 20 peaks in the series were summer floods and are therefore likely not to have left deposits which would be detected in a sedimentary record. Further research is underway there to test the correspondence of the instrumental and depositional records. Flood benefits should be recognised in river management as well as detrimental effects. These include: flood peak attenuation and reduction of hazard downstream; addition of sediment and soil to floodplains; contributions to biodiversity on floodplains; addition of moisture and groundwater recharge; and aesthetic and geodiversity value of naturally adjusting river reaches. In a move towards more sustainable flood management of rivers, use of strategies that work with nature and use natural processes are being widely advocated now and much river restoration is taking place. The variability of flood impacts and neglect

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of certain factors and changes identified here may mean that flood defence designs under conventional methods are inadequate and do not provide sustainable strategies for the future, allowing for these adjustments and variability. 6. Conclusions Examples from the literature and from field studies in a semi-arid region and a humid–temperate region over several decades have been used to assess the influence of various factors on channel response to flood events and how that may change over time. These have demonstrated the need to allow for variability of flood impact, especially associated with seasonality, duration and sequence of events, and prior state and upstream status of the channel reach. The evidence provided raises awareness of the possibility that not all impacts are commensurate with flood magnitude and have a constant relation, and that the sedimentary and morphological records may be incomplete and therefore flood frequency underestimated using such records. The variability of response of similar magnitude floods in the same location also needs to be allowed for in interpretation of past events. The appraisal and summary of factors and the possible dynamics provided here emphasise the importance of incorporating geomorphological considerations into flood risk assessment and decisions on channel management. They can act as a check list both for the interpretation of past changes and deposits and as a predictor of liability to future changes. It emphasises the need to understand the channel dynamics. The analysis points to the variations that can occur and the uncertainties that surround the prediction of flood impact, and provides a framework for assessment of the factors having influence on physical impacts of floods. In modelling, where the starting point is usually calculation of the hydraulics and flood inundation limits, variation associated with change in morphology, resistance and duration should be calculated to provide the risk and uncertainty bounds. Sustainable management of river channels should allow for dynamics of flood impact by incorporating variation in factors and effects of feedback into flood modelling. References Abernethy, B., Rutherfurd, I.D., 1998. Where along a river's length will vegetation most effectively stabilise stream banks? Geomorphology 23, 55–75. Anderson, M.G., Calver, A., 1977. Persistence of landscape features formed by a large flood. Trans. Inst. Br. Geogr. 2, 243–254. Baker, V.R., Costa, J.E., 1987. Flood power. In: Mayer, L., Nash, D. (Eds.), Catastrophic Flooding. Allen and Unwin, Boston, pp. 1–21. Baker, V.R., Kochel, R.C., Patton, P.C., 1988. Flood Geomorphology. Wiley, New York (503 pp.). Beven, K., 1981. The effect of ordering on the geomorphic effectiveness of hydrologic events. IASH Publ. 132, 510–526. Beven, K., Carling, P. (Eds.), 1989. Floods: Hydrological, Sedimentological and Geomorphological Implications. Wiley, Chichester (290 pp). Brierley, G., Fryirs, K., 2005. Geomorphology and River Management: Application of the River Styles Framework. Blackwell, Oxford (398 pp.). Brizga, S.O., Finlayson, B.L., 1990. Channel avulsion and river metamorphosis — the case of the Thomson River, Victoria, Australia. Earth Surf. Process. Landf. 15, 391–404. http:// dx.doi.org/10.1002/esp.3290150503. Brookes, C.J., Hooke, J.M., Mant, J.M., 2000. Modelling vegetation interactions with channel flow in river valleys of the Mediterranean region. Catena 40, 93–118. Brunsden, D., Thornes, J.B., 1979. Landscape sensitivity and change. Trans. Inst. Br. Geogr. 4, 463–484. Bull, W.B., 1997. Discontinuous ephemeral streams. Geomorphology 19, 227–276. Bull, L.J., Kirkby, M.J., Shannon, J., Hooke, J.M., 1999. The impact of rainstorms on floods in ephemeral channels in southeast Spain. Catena 38, 191–209. Burkham, D.E., 1972. Channel changes of the Gila River in Safford Valley, Arizona, 1846–1970. Professional Paper 655-G. U.S. Geological Survey, Washington, D.C. (24 pp.). Calver, A., Anderson, M.G., 2004. Conceptual framework for the persistence of flood-initiated geomorphological features. Trans. Inst. Br. Geogr. 29, 129–137. Carling, P., Beven, K., 1989. The hydrology, sedimentology and geomorphological implications of floods: an overview. In: Beven, K., Carling, P. (Eds.), Floods: Hydrological. Sedimentological and Geomorphological Implications. Wiley, Chichester, pp. 1–9. Cenderelli, D.A., Wohl, E.E., 2003. Flow hydraulics and geomorphic effects of glacial-lake outburst floods in the Mount Everest region, Nepal. Earth Surf. Process. Landf. 28, 385–407. Church, M., 2010. The trajectory of geomorphology. Prog. Phys. Geogr. 34, 265–286. Conesa-Garcia, C., 1995. Torrential flow frequency and morphological adjustments of ephemeral channels in south-east Spain. In: Hickin, E.J. (Ed.), River Geomorphology. Wiley, Chichester, pp. 171–192.

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