13.15 Hazardous Processes: Flooding

13.15 Hazardous Processes: Flooding G Benito, Spanish Council for Scientific Research (CSIC), Madrid, Spain r 2013 Elsevier Inc. All rights reserved.

13.15.1 13.15.2 13.15.3 13.15.3.1 13.15.3.2 13.15.3.3 13.15.4 13.15.4.1 13.15.4.2 13.15.4.3 13.15.5 References

Introduction Flood Causes and Their Magnitude Flood Hazards in Fluvial Environments Mountain Streams Alluvial Fans Alluvial Rivers Natural and Anthropogenic Drivers of Flood Hazard Variability Flood Response to Climate Variability Environmental Changes and Flood Hazards River Engineering Structures and Flood Hazards Concluding Remarks

243 244 245 246 248 250 254 254 255 256 257 257

Abstract This chapter demonstrates the value of fluvial geomorphology in flood hazard studies and identifies the links with hydrology and engineering to provide a holistic approach for flood hazard assessment. Applied flood geomorphology deals with the extension of flood records into the past from flood sediments, hydromorphological mapping of channel and floodplain landforms, and analysis and quantification of morphodynamic processes such as channel migration and sediment transport in response to individual or sequential flooding. General and specific approaches on the study of flood hazards are considered for three fluvial environments: mountain streams, alluvial fans, and alluvial rivers. Geomorphologic and stratigraphic signatures of floods are critical to understanding the linkages among climate change, environmental change, flood hydrology, and the geomorphic development of fluvial landscapes.

13.15.1

Introduction

A river flood is defined as a water level, above the average flow, that produces temporary inundation of land that is not usually submerged (Ward, 1978). Floods are natural phenomena, with hydrological, ecological, and geomorphic significance. Throughout history, humans have used and inhabited river valleys, which helped to provide reliable sources of water and rich alluvial soils. However, settlements next to rivers are often affected by high water levels, which pose a hazard to human life and property. Assessment of flood hazards is therefore critical for planning and is a basic tool to prevent and mitigate flood risk (UNISDR (United Nations International Strategy for Disaster Reduction Secretariat), 2004). Flood hazard assessment can be approached from different technical and scientific backgrounds (Matthai, 1990; Baker, 1994). From a technical perspective, flood hazards are commonly defined in statistical terms, based on analysis of the probabilistic properties of a sample of historical instrumented floods and expressed as annual exceedance probability

Benito, G., 2013. Hazardous processes: flooding. In: Shroder, J. (Editor in Chief), James, L.A., Harden, C.P., Clague, J.J. (Eds.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 13, Geomorphology of Human Disturbances, Climate Change, and Natural Hazards, pp. 243–261.

Treatise on Geomorphology, Volume 13

(PE ¼ m/(n þ 1), m is the rank in order of descending discharge and n is the total number of years). The exceedance probability may be expressed in terms of the average recurrence interval (TR in years; TR ¼ 1/PE). Geophysicists and hydrologists focus on water cycle components and mathematically simulate runoff associated with rainfall events (rainfall–runoff models) and its propagation along the drainage network. Earth and natural scientists are mostly interested in surface processes occurring during floods, namely erosion, sediment transport and deposition, and their effects on landforms and biota (Baker et al., 1988). Flood studies are becoming increasingly interdisciplinary, drawing on expertise, from geomorphology, sedimentology, hydrology, hydraulics, and statistics (Baker et al., 1988; House et al., 2002). Geomorphology provides a wide range of spatial and time scales for flood studies. Perhaps, more importantly, it enables the estimation of flood discharge and inundation extent from observations of sediments and landforms, based on the inference of processes and magnitudes required to produce them (Baker, 1994). This broad understanding of flood processes and landforms provides geomorphologists with a unique ability to formulate environmentally sustainable options for effective flood management. Three main research themes have emerged in the application of geomorphology to flood hazard studies: (1) quantitative paleoflood hydrology, with a focus on extending flood records (e.g., discharge, flood stages) back

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into the past based on evidence from sediments and landforms; (2) hydromorphological mapping of channel and floodplain landforms; and (3) quantification of morphodynamic processes and landform evolution (e.g., channel migration, sediment transport) in response to individual or sequential flood events. The characteristics and spatial distribution of the imprint of flooding on the landscape depend on (1) flow magnitude and frequency, which are mainly related to the causes of floods; (2) geomorphic and depositional environments, including valley and channel geometry; and (3) changes in water and sediment supply linked to hydroclimatic and anthropogenic changes. This chapter first describes major flood triggers and causes, which control the duration and spatial distribution of flooding, and then addresses extreme flows in selected geomorphic environments, including mountain streams, alluvial fans, and river floodplains. Flood hazards and associated risks are described in these environments, in qualitative and quantitative terms, based on evidence from geomorphic and sedimentary records and interpretation of dominant geomorphic processes. Finally, temporal flood hazard variability is analyzed in the context of climatic and human-induced land-use changes, and an appraisal of how geomorphic and stratigraphic records can contribute to understanding the long-term flood response to these drivers is presented.

13.15.2

Flood Causes and Their Magnitude

Flooding is caused by different geophysical mechanisms that control its geographic occurrence, magnitude, frequency, and timing (O’Connor et al., 2002). The most common causes of flooding are meteorological processes, namely intense rainfall, snowmelt, or combined rain and snowmelt. The specific combination of meteorological, topographic, and geomorphic factors substantially influences the resulting flood characteristics, such as its peak and hydrograph shape. Moreover, the climatic and geographic setting of a catchment determines the possible range of atmospheric conditions conducive to extreme precipitation and flooding (Figure 1; Hirschboeck et al., 2000). The largest rainfall-induced floods in terms of specific discharge (volume per unit drainage area) are produced by mesoscale convective systems (B100 km or more across in one direction) and by small convective cells (5–50 km in diameter) triggered by short-lived (few hours or less), high-intensity rainfall over localized areas (Figure 1; Hirschboeck, 1988). Convective cells and mesoscale convective complexes may lead to flash floods that can be highly destructive, particularly in high-gradient mountain catchments. For example, extreme convective rainfall of around 380 mm in 6 h produced a peak discharge of 1461 m3 s1 in Boxelder Creek (drainage area of 303 km2) and 883 m3 s1 in Rapid Creek (drainage area 135 km2), upstream of Rapid City, South Dakota, on 9–10 June 1972, claiming 238 lives (Schwarz et al., 1975). Peak flow associated with this flood is a high outlier of gauge records in the region, but sedimentological evidence shows that floods of similar magnitude have occurred five to seven times over the past 1000 years on both creeks (Harden et al., 2010a). Tropical cyclones are a type of mesoscale convective cloud cluster with local intensification that persists for days,

delivering heavy rainfall, in some cases more than 500 mm (e.g., Brahmaputra River; Goswami, 1998). If they stall or move slowly, they can cause severe flooding and destruction, particularly in small watersheds in mountainous regions (Hirschboeck et al., 2000). The plains north of Bihar in India, for example, have experienced extensive and frequent loss of life and property from heavy monsoon rains (Kale, 1997, 1998). In extratropical regions, macroscale and synoptic-scale atmospheric processes (e.g., blocking circulation types) can affect large geographic regions, producing floods that last days to weeks, with moderate specific discharges (e.g., flooding in the upper Mississippi River basin; Knox, 2006). In cold-winter regions, large floods can be generated from snow or ice melt, particularly in combination with rainfall (e.g., 1976 flood in Indus River valley, Pakistan) or exacerbated by ice jams (e.g., 1967 flood on Lena River, Russia). In mountainous regions, snowmelt or rainfall-generated floods are strongly related to elevation (Wohl and Cenderelli, 1998). For instance, in the Rocky Mountains, floods on streams above 2300 m are caused dominantly by snowmelt, whereas, at lower elevations, floods are also generated by intense rainfall (Jarrett, 1989). Several types of meteorological events and different storm types result in mixed flood distributions, each with its own probability distribution (Hirschboeck, 1987). Climatic variability can lead to changes in the magnitude and frequency of snowmelt or rainfall flood events, with implications for understanding future impacts of anthropogenic climate change on flood hazards (Milly et al., 2008). Floods may also result from failures of natural or man-made dams. Glaciers, moraines, volcanic lava flows, and landslides can form natural dams (O’Connor et al., 2002). The volumes and magnitudes of floods from dam failures are controlled primarily by the size of the outlet channel and the volume of impounded water (O’Connor and Beebee, 2009). As a general rule, floods from dam failures have a much greater specific discharge than those originating from rainfall or snowmelt (Figure 1) and are often unprecedented in the history of the affected valley (Costa, 1988). Some of the largest historic floods on Earth have been generated by the rapid release of water stored behind natural ice dams, including the 1986 flood with a peak of 105 000 m3 s1 resulting from the ice-dam failure of Hubbard Glacier across Russell Fiord, Alaska (Figure 1; O’Connor and Costa, 2004). Very large floods can also result from melting of snow and ice during eruptions of ice-covered volcanoes, by heavy rains that may accompany volcanic eruptions, and by transformation of lahars to stream flow (Hoblitt et al., 1987). The largest jo¨kulhlaup (literally ‘glacier-burst’) in recent history was caused by a subglacial volcanic eruption at Katla, Iceland, in 1918; it had a peak discharge of 300 000 m3 s1 (To´masson, 1996). Recent jo¨kulhlaups from subglacial lake Grı´msvo¨tn, located in an ice-covered caldera in Iceland, have occurred at intervals of 1–10 years, with peak discharges of 600–40 000 m3 s1, durations of 2 days to 4 weeks, and total volumes of 0.5–4.0 km3 (Bjo¨rnsson, 1998). A primary effect of recent glacier retreat is the formation and disappearance of glacier- and moraine-dammed lakes. Many large proglacial lakes in the Himalayas, Andes, Alps, central Caucasus, and the Cordillera of western North America have disappeared, although new ones are forming. Moraine

Hazardous Processes: Flooding

245

Duration Week Day

Year Month

Century Decade

10 Ka 1Ka

1000 Ka 100 Ka

10 000 Ka 108 Global

108 Global climatic phenomena: Interannual (ENSO) Amazon, 1953 Multidecadal (PDO) Extra-tropical Centennial (Little ice age) cyclons Millennial: (Ice-ages) Mississippi River, Elbe/Vltava Rivers 1973, 1996 Czech R., 2002 Columbia River, 1894 Bramaputra, Tropical Bangladesh, 1890 cyclons Russell Fiord, 1986 Regional snowmelt Mesoscale Gardon River, France, 2002 Columbia River, Holocene convective Seasonally persistent circulation patterns

Areal scale of influence, in square kilometers

107

106

105

104

complexes Llobregat River, Spain, 1617 Guadalentin River, Spain, 1973 Aniakchak, AK, Late Holocene Thunderstorms Katla, Iceland, 1918 Convection Indus River, Pakistan, 1841 Biescas flood, Spain, 1996

103

102

101

107 Hemisphere

106 Continent

105 Sub-continent

104 Basin 103 Catchment

Floods from geologic mechanisms

Small 102 catchment

Floods from meteorologic mechanisms Space-time domain for meteorological floods

101 Stream 100 Slope/gully

100 0

10

1

10

10

2

3

10

10

4

5

10

6

10

10

7

8

10

9

10

10

10

11

10

Time scale of controlling processes, in hours Figure 1 Space–time domain for selected meteorological processes producing floods and selected floods caused by failure of natural dams (ice, moraine, landslide, structural, basin sill, volcanic caldera) and constructed dams. Reproduced from O’Connor, J.E., Grant, G.E., Costa, J.E., 2002. The geology and geography of floods. In: House, P.K., Webb, R.H., Baker, V.R., Levish, D.R. (Eds.), Ancient Floods, Modern Hazards. Principles and Applications of Paleoflood Hydrology. American Geophysical Union Water Science and Application Series 5, pp. 359–385.

dams have failed due to melting of ice cores (Richardson and Reynolds, 2000) and erosion and seepage (Clague and Evans, 2000; O’Connor et al., 2001). Glacier-dammed lakes drain catastrophically through ice-marginal drainage, mechanical failure of the ice dam, enlargement of a tunnel at the base of the glacier, or by a combination of the latter two processes (Walder and Costa, 1996). Climate change has been identified as a primary driver for recent, more frequent failures of glacier and moraine dams (Rosenzweig et al., 2007). In the Northern Patagonia Ice Field, glacier-dammed Lake Cachet 2 (230 million m3) released six large outburst floods between 2008 and 2011, after more than 45 years without one (Dussaillant et al., 2010). Failures of man-made dams have produced catastrophic floods and are a major concern for dam engineers. There have been more than 2000 failures of constructed dams since the twelfth century (Jansen, 1980), although detailed information is limited to about 100 of these events (Wahl, 1998). Data are sparse for dams higher than 20 m, but substantial data exist for failures of dams 6–15 m high (Wahl, 1998). The most common causes of failure are spillway failure (34%), foundation defects (30%), and piping and seepage (28%) (ICOLD (International Commission on Large Dams), 1973). Tailings dams are particularly prone to failure (Rico et al., 2008a, b). They are built to store waste tailings from mining activities. Currently, thousands of tailings dams

worldwide contain billions of tons of contaminated water and mineral waste. Sensitivity studies have shown that reservoir volume (V) and dam height (H) are critical factors in dam failure hydrographs (Hagen, 1982; Petrascheck and Sydler, 1984). The simplest proposed relations are functions of these two parameters (H  V), considering that most of the water volume stored behind the dam is released (Costa, 1988). Other approaches are based on physically based models (Walder and O’Connor, 1997) that estimate the dimensionless peak outflow as a function of a dimensionless parameter that reflects the drop in reservoir level, volume of released water, and the mean vertical erosion rate of the breach.

13.15.3

Flood Hazards in Fluvial Environments

Fluvial geomorphologists have long recognized the importance of different flood magnitudes on the development of channel and valley morphology and on sediment transport flux along the fluvial system (Baker et al., 1988; House et al., 2002). Detailed analyses of floodplain geomorphology and stratigraphy provide data on extraordinary floods that represent the most extreme hazards for a given hydrological system. The following sections describe general and specific approaches for the study of flood hazards in three characteristic

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Hazardous Processes: Flooding

fluvial environments, namely mountain (bedrock-confined) rivers, alluvial fans, and alluvial floodplains.

13.15.3.1

Mountain Streams

Mountain streams have a gradient greater than or equal to 0.002 m m1 along most of their length (Jarrett, 1992), although some reaches may have much lower gradients (Wohl, 2000). The diversity of hydroclimatic conditions in mountain regions is reflected in the wide variety of stream morphologies. In terms of flood processes, however, most mountain rivers share a number of characteristics (Wohl, 2000): (1) high average channel gradient; (2) high channel-boundary resistance (bedrock or coarse alluvium) with high boundary roughness (coarse clasts); (3) potentially highly turbulent flows; (4) strong differences in seasonal discharge, including flash flood discharges due to snow and ice melt or seasonally intense rainfall; and (5) high spatial variability in channel morphology due to an external control of geology. Convective storms typically affect small catchments within mountain regions due to strong thermal gradients of air masses with altitude. The resulting intense rainfall typically generates Hortonian overland flow, even on vegetated and soil-covered hillslopes (Pearce and McKerchar, 1979; GarciaRuiz et al., 1996). Runoff may be particularly high if a highintensity storm is preceded by rain that saturates soils in the catchment (Costa, 1987). Under such circumstances, high hydrological connectivity of the slopes, gullies, and streams can generate a fast runoff response and subsequent flash floods (Bracken and Croke, 2007). The largest discharges are produced by an optimal combination of basin morphometry and storm intensity (Costa, 1987). Data from the U.S. (Costa, 1987) and Europe (Gaume et al., 2009) show that most of these floods are generated by rainfall intensities of over 100 mm h1, with a recorded maximum of 738 mm in 24 h (North Fork Hubbard Creek, near Albany, TX). Flash floods are highly hazardous due to short lag times between the storm and the peak discharge, and their common occurrence during nighttime hours. Intense storms runoff activates geomorphic processes, including gully erosion, sheetflow erosion, and mass movements (creep, rock falls, and landslides), increasing sediment delivery to the channel network (Corominas and Alonso, 1990). Of particular relevance in flood studies is the relation between flood deposits and landforms, on the one hand, and the flow regime (Froude number) and behavior (Newtonian fluid or Bingham plastic), on the other. These relations dictate the selection of appropriate governing hydraulic equations that enable interpretation of the deposits (Costa and Jarrett, 1981). Geomorphic approaches for estimating flood discharge of ungauged mountain streams rely on drainage basin morphology and on erosional and depositional evidence (Table 1). At the drainage-basin scale, discharge estimates can be obtained from simple exponential relations (QT ¼aAb) that link drainage area (A) and annual peak discharge (QT) for a particular return period (T), with an exponent b ranging from 0.5 (arid basins) to 1.0 (humid basins) (Jarvis, 1936; Thomas and Benson, 1970). Similar empirical equations can be constructed using drainage density and morphometric

characteristics (basin shape, area, stream length, and relief) (Mosley and McKerchar, 1993). Regional envelope curves can also provide a first-order upper limit of flood magnitude, duration, and volume that a basin can generate based on its size (Wolman and Costa, 1984). These enveloped curves can be compared and constrained with paleoflood data (Enzel et al., 1993). Erosion and depositional evidence left by floodwaters may be used as stage indicators to estimate flood discharges (Kochel and Baker, 1982; Baker, 2008). Types of physical evidence include high-water marks and paleostage indicators. High-water marks typically only persist for weeks in humid climates to several years in arid climates, and include mud, silt, seed lines, and flotsam (fine organic debris, grass, or woody debris) that closely mark the peak flood stage (Williams and Costa, 1988). In contrast, paleostage indicators provide longer lasting evidence of peak flow stages; they include fine-textured flood sediment (slack-water flood deposits; Figure 2(a); Kochel and Baker, 1982), gravel and boulder bars (Grimm et al., 1995), erosion features (Webb and Jarrett, 2002), and botanical evidence such as scars and damage to riparian trees (tilted stems, eccentric ring growth, adventitious roots; Sigafoos, 1964; Yanosky and Jarrett, 2002; Ballesteros et al., 2010). Paleodischarges can be derived using paleocompetence or paleostage methods. Paleocompetence methods rely on selective sorting of particles being transported by flood flows, whose hydraulic forces succeed in transporting small particles but fail to transport large particles (Carling, 1983; Parker, 1991). Reviews of flow competence methods, including their application and uncertainties, are provided by Maizels (1983), Williams (1983), Komar (1989, 1996), Komar and Carling (1991), Wilcock (1992), O’Connor (1993), and Carling et al. (2002). Site-specific empirical relationships (e.g., Costa, 1983) can be defined from computed hydraulic parameters and the largest particles moved by floods (e.g., mean diameter of the largest 10 boulders). Flow competence estimates based on the largest sampled mobile grains have been criticized as being subject to large errors (Wilcock, 2001), related in part to the effect of insufficient sample size. Paleostage-based discharge estimates can be made using hydraulic calculations, assuming stable channel geometry (Figure 2(b)). Discharge estimates follow from the assumption that the position of paleostage evidence relates closely to the maximum stage attained by an identified flood (Jarrett and England, 2002). Water surface elevations associated with the paleostage evidence are converted into discharge values (Figure 2(b); O’Connor and Webb, 1988). A number of formulae and models are available to estimate flood discharge from known water surface elevations, ranging from simple hydraulic equations to more sophisticated, multi-dimensional hydraulic modeling (O’Connor and Webb, 1988; Webb and Jarrett, 2002). The models assume a fixed bed, and required data include slope, roughness (usually Manning’s n), crosssectional geometry, and, for the step-backwater method (Figure 2(b)), an upstream or downstream boundary condition depending on the flow type. Cross-section geometry is the most uncertain data for past flood discharge estimation; for this reason, it is desirable to make estimates in bedrock canyons where bed changes are minimal.

Table 1 Geomorphic techniques used to estimate discharges of past floods based on geological and botanical flood indicators Technique

Hydrological estimation

Geologic indicator

River environment

Method of estimation

References

Largest particles, boulders

Alluvial–colluvial and bedrock channels

High-water marks, paleostage indictors, gravel bars, drift wood, sand-silt flood deposits Erosion scars on trees, damage on vegetation

Bedrock channels, consolidated colluvium

Empirical regression of particle entrainment, transport, and deposition Slope area, Manning’s equation

Costa (1983), O’Connor (1993), Carling et al. (2002) Jarrett (1984)

Upland bedrock and alluvial valleys

Hydraulic methods, slope area

Sigafoos (1964), Yanosky and Jarrett (2002)

Bedrock channels, consolidated colluviumalluvium Bedrock or mixed channels

Slope area, Step-backwater modeling, 2D flow hydraulic models Hydraulic modeling

O’Connor and Webb (1988), Webb and Jarrett (2002)

Paleochannel dimension

Alluvial channel

Linear regression, observation from modern alluvial channels Empirical equations. ratio flow depth-bankfull depth to sediment size Hydraulic methods, geomorphology

Dury (1973), Williams (1988)

1

Mountain river channels (Channel slope 40.002 m m ) Paleocompetence Largest flood: peak discharge, shear stress, velocity, stream power Paleostage-based flow Peak discharge

Dendro-geomorphologicalbased flow

Peak flow

Bedrock channels (medium to low channel slope) Paleostage-based flow Peak discharge (minimum estimates) Paleohydrologic bounds

Peak flows (minimummaximum estimates)

Alluvial channels (low channel slope) Regime-based paleoflow Mean annual flow, bankfull estimates discharge

Slackwater flood deposits, drift wood, erosion marks Slackwater flood deposits, soil disturbance/evolution

Mean annual flow, annual maximum flow

Overbank sedimentology, stratigraphy

Alluvial channels

Hydro-geomorphic

Peak flow

Floodplain and channel geomorphology

Alluvial or mixed channels

Drainage basin characteristics

Peak flow, annual flow, Probable Maximum Flood

Basin area, network density

Bedrock alluvial systems

Empirical equations, envelope curves

Knox (1987), Knox and Daniels (2002) Garry et al. (2002), DIRENPACA (Direction Re´gional de l’Environment-PACA) (2007), Lastra et al. (2008) Gregory and Gardiner (1975), Baker (1976)

Hazardous Processes: Flooding

Floodplain stratigraphy

Levish (2002)

247

248

Hazardous Processes: Flooding

92 BM-8

91 BM-9

89

Water surface elevation Q = 510 m3s–1

BM-1 BM-7 MB-3

88

Water surface Q = 460 m3s–1

87

BM-2

Channel bottom

86

XS-10

85 84 0

100

50

A.D. 1925−2006 (n = 10) A.D. 1715−1925 (n = 7) A.D. 1440−1715 (n = 5) < A.D. 1440 (n = 7)

Flow

BM-5

BM-4

XS-9

Elevation (m)

90

150

200

250

300

350

Distance downstream (m) (a)

(b)

800 1

Palaeoflood record

Qh1

Qh3 Qh2

Discharge below threshold level

Systematic record

Discharge, m3s–1

Discharge, m3s–1

500

200

Return period (yr) 100 200 10 20

1000

400

Palaeoflood data Modelled annual peak discharge Modelled annual peak discharges and palaeoflood data

300 Modelled annual peak discharges only

200 100

0

0 1500

(c)

5

600

600

400

2

1600

1700

1800

1900

2000

Years AD

50 (d)

20

10

5

1 0.5

0.1

Annual exceedence probability (%)

Figure 2 Paleoflood case study of the Buffels River (NW South Africa). (a) General view of the upper section of the study reach, and the location of three profiles: BM-7, BM-8, and BM-9. Person (circled) for scale approximately 1.7 m. (b) Longitudinal profile of the stream channel bed and water surface profiles obtained from HEC-RAS modeling (Hydrologic Engineering Center, 2010) for the highest paleoflood deposits (510 m3 s1) and for a reference discharge of 460 m3 s1. Stratigraphic profiles are represented as vertical bars, with colors sketching sections of flood sediments of different ages. (c) Paleoflood and modeled discharges (rainfall–runoff modeling from seven rain gauges over the period 1965–2006). The horizontal shaded areas represent the discharge threshold values (Qh) used in the flood frequency analysis. (d) Two-component extreme value distributions fitted to annual series of modeled peak discharges and paleoflood information (censored data). Reproduced from Benito, G., Botero, B.A., Thorndycraft, V.R., et al., 2011b. Rainfall-runoff modelling and palaeoflood hydrology applied to reconstruct centennial scale records of flooding and aquifer recharge in ungauged ephemeral rivers. Hydrology and Earth System Sciences 15, 1185–1196.

Paleostage-based discharge estimates from slackwater flood deposits are the basis of quantitative paleoflood hydrology (Kochel and Baker, 1982). This method has been applied in many parts of the world to improve flood risk assessments (latest reviews by Baker, 2008 and Benito and O’Connor, 2012). A flood frequency analysis can be conducted by combining paleoflood discharges with annual peak flows recorded at gauge stations (Figure 2(c); Stedinger and Baker, 1987). This analysis provides better estimates of flood quantiles related to high return intervals, given that data on extreme floods are included (Benito and Thorndycraft, 2005). For instance, minimum discharge estimates associated with elevations of high flood deposits along the lower Buffels River in South Africa indicate that paleofloods were up to five times (510 m3 s1) larger than the largest recorded and rainfall–runoff-modeled peak discharges (Figure 2; Benito et al., 2011a, b). In this case, flood frequency analysis using the combined modeled and paleoflood discharges, and

based on maximum likelihood estimators, was successfully fitted to a two-component extreme value distribution (Figure 2(d)). The fitted distribution shows an average return period of c. 100 years for the largest paleoflood discharges, whereas the largest flood from the systematic record since 1965 (106 m3 s1) has an average return interval of 10 years. These paleoflood records provide physical evidence of the largest floods, thus improving design discharge estimates for hazard studies and even for assessing sustainability of water resources in dryland environments where floods are an important source of water to alluvial aquifers (Benito et al., 2011a, b).

13.15.3.2

Alluvial Fans

Alluvial fans occur where confined mountain streams open up into valleys or onto plains (Figure 3). They are widespread in

Hazardous Processes: Flooding

249

Zone G: Active trunk channel Zones C−D Active fanhead

Zone H: Active alluvial fan surface Zone F: Active subplanar fan surface

Zone B: Non-active fan surface Potentially active if fanhead blocked

Zone E: Distributary channels

Zone A: Old fan surface

C D A

B

H F

E

G F

(a)

b

B +

F + E+ G+

Relative probability

Geo-indicators

Geological mapping and modelling

H +

Time since deposition, years (b)

B GF E FH Inundation at geomorphic surface

Depth, m

Depth, m

From geologic mapping

Chronosequence

Identical hazard on fan surface

Error

Flow, m3s–1

B GF E FH Inundation at given location

Recurrence (years)

d Flood hazard

Uncertainty

2

From hydraulic calculations

100 Recurrence (years)

Flood hazards composite method G

F E Flow, m3s–1 Based on paleostage indicators

G Depth, m

100

Area = 1

Depth, m

2

c From regime theory

From default assumption Probability density

Flow, m3s–1

Conventional hydrology

Steps a approach Frequency analysis from hydrologic methods

E F H B

Annual probability Based on assigning an ocurrence frequency to mapped surface

Figure 3 Flood hazards on alluvial fans. (a) Basic components of an alluvial fan showing the zones that require various forms of flood risk assessment. (b) Two approaches on flood hazard delineation. Flood hazard regulation based on identification of areas that have a one percent chance of being flooded in any given year (e.g., FEMA). In a conventional hydrological approach, a discharge–recurrence relationship is estimated for the apex of the fan; dashed lines indicate error bands – a. It is assumed that floods may inundate any location of the fan – b. A relationship between depth, velocity, and discharge is then determined – c. The predicted degrees of flood hazard on the fan surfaces are similar when considering the high uncertainties (gray area) of this process – d. A second approach follows a combination of geomorphological mapping and modeling (see Pelletier et al., 2005) that considers the time since the last flood on each fan surface (chronosequence based on soil development, sedimentary indicators) – a. A relative timing and probability of flooding is provided at each geomorphic unit – b. Hydraulic modeling can be validated against geological mapping – c, to determine flood hazards based on assigning an occurrence frequency to each mapped surface – d. See Table 2 for an explanation on sedimentary facies, geomorphic processes, potential vulnerability, and human perception of flood risk in each geomorphic unit. Adapted from NRC (National Research Council), 1996. Alluvial Fan Flooding. National Academy Press, Washington, DC, 172 pp.

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Hazardous Processes: Flooding

dryland environments, but also occur in subtropical, temperate, and even arctic and subarctic zones (Oguchi et al., 2001; Harvey et al., 2005). They typically are gravelly or bouldery at their apex, grading to finer sediments toward the distal fan areas (Bull, 1977). Sediments in alluvial fans may be deposited by both water flows and debris flows (commonly 20–50% of the sediment volume). Velocities of flows that deposit sediments on fans range from 3 to 10 m s1 (see Wells and Harvey, 1987; Blair and McPherson, 1994). Most geomorphic research on alluvial fans has focused on relations of fan morphology and sediments to the factors that control their development, including tectonic and climatic controls and base-level changes (Harvey et al., 2005). Flooding on alluvial fans is characterized by high flow velocities, multiple flow paths, and highly active processes of erosion, sediment transport, and deposition (Figure 3(a); Table 2). Flow processes include debris flow (from viscous to dilute flows), in-channel flow, and unconfined overland flow (Costa, 1984; Field and Pearthree, 1997; Harvey, 1997). Sediment load is an important factor in assessing the flood hazard because rapid sedimentation or bank erosion favors channel avulsion and migration to a completely new location radiating from the apex fan (Volker et al., 2007). In catchments with a high sediment yield and steep gradients, most sediment is transported and deposited by debris flows, with up to 85–90% of the fan comprising successive and overlapping debris flows (e.g., White Mountains, California and Nevada; Beaty, 1974). Studies dealing with flood hazard assessment on fans are scarce, perhaps due to the inherent complexity in applying probability-based flood hazard assessments to alluvial fan environments (Figure 3(b); Baker et al., 1990; FEMA (Federal Emergency Management Agency), 2000). Difficulties arise from the uncertainty of flow paths, high flow velocities, and heavy loads of debris and floating wood (French, 1987; NRC (National Research Council), 1996). Conventional hydraulic modeling has failed to provide an accurate representation of historical flood size and recurrence on alluvial fan surfaces (Figure 3), supporting efforts to use geologic information to improve flood hazard delineation (Baker et al., 1990; Pearthree et al., 1992; House, 2005). Research based on soil characteristics, sedimentary facies and landforms, and plant communities is now considered a feasible option for assessing flood hazard on alluvial fans (House, 2005; Robins et al., 2009). A three-step approach for flood hazard delineation on fans has been proposed by FEMA (Federal Emergency Management Agency) (2000; Figure 3(b)): (1) identification and characterization of the alluvial fan landform; (2) characterization of the alluvial fan environment and location of areas of active erosion and deposition; and (3) definition and characterization of areas affected by the 1% exceedance probability flood (‘100-year flood’). Several authors have found a significant disparity between flood hazard maps based on the FEMA (Federal Emergency Management Agency) (2000) regulatory model (FAN model) and those based on geologic mapping of geomorphic surfaces (House, 2005; Robins et al., 2009). Geologic evidence (landforms and soil ages) relates flood hazards to the timing of flow processes on different fan surfaces (mainly tied to elevations). These processes are

intimately linked to climate and landscape variables that function over a temporal scale of hundreds to thousands years (House, 2005). These surface stability maps predict neither when nor how frequently floods will occur, but instead illustrate where water is most likely to flow, given a precipitation event at or upslope of a given location (Figure 3; Robins et al., 2009). In contrast, FEMA models designate flood hazards based on probabilities of storm magnitude recurrence on a human time scale, with an emphasis on 100-year floodplains. In order to bridge this gap, Pelletier et al. (2005) used four complementary methods to assess flood hazards at the Tortolita alluvial fans in Arizona: two-dimensional, raster-based hydraulic modeling; satellite-image change detection; fieldbased mapping of recent flood inundation; and surficial geology mapping and dating of geomorphic surfaces based on flood criteria (e.g., last time since inundation, paleostage evidence). This flood hazard assessment had the advantage of providing information on flow depth and velocity (Pelletier et al., 2005), as well as discriminating fan active channels, preferential flow paths, and dominant processes (Robins et al., 2009).

13.15.3.3

Alluvial Rivers

An alluvial river comprises two basic geomorphic elements: the stream channel and the floodplain surface (Figure 4). A corpus of studies has examined these two components based on hydraulic geometry concepts, rooted in the classic studies by Schumm (1968) and Dury (1973). Channel-forming discharge is commonly related to bankfull discharge, representing the maximum flow conveyance capacity of the channel prior inundation of the floodplain (Leopold et al., 1964). Above bankfull, increasing storage on the floodplain and associated delays due to frictional resistance may lead to flood peak attenuation and, in some rivers, to a break on the slope of the flood frequency distribution at bankfull discharge (Figure 4(b); Archer, 1989). Field determination of bankfull discharge and its hydrological characteristics is less than straightforward due to the commonly poorly defined transition between the main channel and the flat floodplain (Leopold et al., 1964; Navratil et al., 2006). Moreover, application of bankfull discharge to flood hazard studies is limited to frequent, smaller floods (e.g., Wharton, 1992). A floodplain is the relatively flat land adjacent to a stream or river that experiences occasional or periodic flooding under a modern hydrologic regime (Wolman and Leopold, 1957; Nanson and Croke, 1992; Knighton, 1998; Bridge, 2003). In reality, most active floodplains are complex, products of environmental change and hydroclimatic variability over periods of hundreds to thousands of years (Brakenridge, 1988; Nanson and Croke, 1992). The topography of a river valley and its floodplain geomorphology adjust to flood hydraulics but also influence the style of flooding over short time scales (Hudson et al., 2006). Consideration of discreet depositional processes in relation to floodplain geomorphology results in the definition of several categories of floodplains (Nanson and Croke, 1992) that are relevant on flood hazard assessment (Benito and Hudson, 2010). In high-energy, noncohesive floodplains with small, mixed bedrock-alluvial valleys (Nanson and Croke’s Class A),

Table 2 Description of flood hazard zones on an alluvial fan in relation to landforms and processes Flood zone/relative magnitude

Geomorphic unit

Sedimentary facies

Dominant geomorphic processes

Vulnerable land-use/threat to human life during inundation of flood zone

Human perception of flood risk (relative to life span)

A: Nonactive flood surface

Very old fan surface

Old debris flows, fluvial gravel bars and bedrock. Sheet-flow deposits from adjacent slopes

Highly dissected fan remnants. Runoff from side slopes. It is not being undermined.

Very low risk

B: Nonactive flood surface

Abandoned fan surface (entrenched but stands at an elevation below that of A)

Sheet-flow deposits and remnants of distributary channels.

C–D: Active during large floods

Active fanhead

Highly threatening to people when large flood occurs

High risk: (41 flood/life span). Evidence of historical floods

E: Moderate flood

Distributary channels

Bouldery lobes and levees from debris flow. Water flow deposits (gravel and bouldery bars). Gravel and sand bars. Subdued bar and swale forms

Dangerous to people

High risk (active several times in life span)

F: Moderate flood

Active subplanar fan surface

Moderate to weakly dissected fan remnants. Desert pavement and varnish, well developed soils and bedrock, possible rounded landforms and well vegetated. Debris flows, transitional and fluvial deposits. High energy environment with erosion and deposition Distributary channels. Evidence of major scour, fill, migration, or avulsion during recent large floods Broad and flat. Sheet-flow deposits

Highly to moderate dissected remnants. No evidence of flooding for several thousand years Unlikely to be flooded unless channel becomes blocked by debris. Riparian vegetation

Moderately dangerous to people. Natural wild area

Moderate to high risk (active several times in life span)

G: All floods

Active trunk channel (active trench)

Highly unstable zone. Highly dangerous to people

High risk (flooded during each flow event)

H: Moderate and large floods

Potentially active fan surface

Surface subject to flooding. Moderate to low danger to people (except on proximal fan)

Low risk (1–2 floods per life span)

Overbank flood deposits. Subdued bar and swale landforms

Vertical incision and aggradation; banks steepsided; immature vegetation. Trench incision observed or not on oldest maps and aerial photos Subject to overbank flooding, channel shifting, or invasion from a distributary channel

Notes: These zones may differ for specific fluvial systems, hydroclimatological conditions, and fan geometries. Flood zones are depicted in Figure 3.

Hazardous Processes: Flooding

Sheet-flow deposits. Intermixed with sandy and silty fluvial deposits with windblown sand on lower fan Single-trunk channel entrenched into old deposits. Gravel bars and channel facies. Overbank deposits and chute channels. Unconsolidated sediments

Very low to low risk. Not flooded for several hundred to thousand years

251

252

Hazardous Processes: Flooding

Flood zone B Risk of death or serious harm Floodway Prevent construction zone A of buildings Risk of death Permit only the construction of channel infrastructure Flood zone C Risk to people when d > 1m Place restrictions on rural buildings CMZ

Flood zone D Low risk to people Restrictions on urban developments Prevent construction of sensitive facilities

PMF

D B C A

25 T (years)

20

2 5 10 20 100 1000

15

C

B

Abkf

10

T ~ 10−100 T ~ 2−10 T~2 QA QB

5 0

Cross-section

Discharge (m3s−1) Rating curve from hydraulic modelling

1

(b)

m

Qbkf

00

Station (× 10

150 0

Probability

QD

0.

1−104),

QC

01 0. 05 0. 1 0. 2 0. 5

100

50

Area

0.

0

D

Area flooded (%)

Elevation (× 1−10), m

(a)

Excedence probability P=1/T Flood hazard linked to geomorphological mapping

Figure 4 (a) Sketch of a typical floodplain showing hazard zonation in both plan and cross-sectional views with hazardous potential to people. (b) Characteristic cross-section, general shape of rating curve and area inundated. The area of flood inundation relates directly to cross-sectional morphology and can be applied to assessing flood risk if a curve of exposure and property value is available. See Table 3 for an explanation on sedimentary facies, geomorphic processes, potential vulnerability, and human perception of flood risk in each geomorphic unit. Modified from Benito, G., Hudson, P., 2010. Flood hazards: The context of fluvial geomorphology In: Alca´ntara-Ayala, I., Goudie, A. (Eds.), Geomorphological Hazards and Disaster Prevention. Cambridge University Press, Cambridge, UK, pp. 111–128.

which are typical of upland headwaters, flood stages may be reached rapidly during flashy events (short lag times), with large energy expenditures along the floodway (unit stream power o4300 W m2). Flood processes are dominated by simple overbank deposition with vertical and lateral accretion during small-to-medium floods or by catastrophic floodplain erosion during large floods (Nanson, 1986). Hazardous floods may produce severe physical damage, for example, by undermining bridges and roads and washing away human settlements (Benito and Hudson, 2010). Sediment load plays a critical role in flow processes and construction of medium-energy, noncohesive floodplains characteristic of unconfined middle reaches of rivers (o¼ 10–300 W m2; Nanson and Croke’s Class B). Channel

instability and overbank vertical accretion are common on floodplains of braided rivers, even during regular flow events. In systems characterized by high ratios of water to sediment load, floodplain construction is dominated by lateral pointbar accretion and vertically accreted, overbank, fine-grained flood deposits (Brakenridge, 1988). Floodplain morphology is characterized by two different surfaces: A low floodplain inundated on average every B2–10 years and a higher surface flooded on average once every 10–100 years (Figure 4; Table 3; Ballais et al., 2005; Benito and Hudson, 2010). Low-energy cohesive floodplains (Nanson and Croke’s Class C), typically located in distal, wide alluvial valleys and deltas, undergo long-duration flooding (weeks to months) with low unit stream power (o¼ o10 W m2). Here, the

Table 3 Description of flood hazard zones in relation to geomorphic units on an alluvial floodplain Flood zone/flood magnitude

Geomorphic unit

Sedimentary facies

Dominant geomorphic processes

Vulnerable land-use/threat to human life during inundation of flood zone

Human perception of flood risk (relative to life span)

A: Minor flood TBo2

Channel

Channel lag, point bar, side bar, longitudinal bar, chute, low-stage slackwater facies (e.g., clay drapes)

Seasonal sediment flushing, coarse sediment mobility, incision, erosion-deposition zone

In channel infrastructure and economic activities (fishing, shipping)

High risk (flooded every year)

B: Moderate flood TB2–10

Low floodplain

Lateral accretion, natural levees (sandy ripples and dunes adjacent to channel fining to silt laminations on natural levee flanks); high flow channel (floodway); crevasse splays, oxbow infilling, slackwater sedimentation in low depressions and backswamps

Channel migration and channel bar development, bank erosion, coarse floodplain aggradation

C: Large flood TB10–100

High floodplain

Slackwater flood deposits on low Holocene terraces. Significant backswamp sedimentation (fine sediments in slackwater ponds and sloughs)

Irregular topography Dominant vertical accretion, infilling of paleomeanders, activation of recently abandoned channels; channel avulsion; floodplain reworking

D: Extreme flood TB4100

Low terrace/highest floodplain

Slackwater flood deposits on older Quaternary terraces and high bedrock ledges; coarse overbank ‘stringers’ on distal floodplains

Colluvial, tributary, and alluvial fan accumulation; floodplain ‘stripping’ (e.g., Nanson ‘cyclic disequilibrium model’)

Highly dangerous to humans Riparian vegetation, gravel mining; agriculture and floodplain irrigation (in lesser developed nations)

Low risk (1–2 floods per life span)

Very low risk (0–1 floods per life span)

Note: These zones may differ for specific fluvial systems, hydroclimatological conditions, and floodplain geometry. Flood zones are depicted in Figure 4. Source: Modified from Benito, G., Hudson, P., 2010. Flood hazards: The context of fluvial geomorphology. In: Alca´ntara-Ayala, I., Goudie, A. (Eds.), Geomorphological Hazards and Disaster Prevention. Cambridge University Press, Cambridge, UK, pp.111–128.

Hazardous Processes: Flooding

Dangerous to human life Agriculture and floodplain irrigation, recreation areas, transportation infrastructure; rural and urban habitation (in lesser developed nations) Only dangerous to humans when water depth 41 m Agriculture and irrigation; flood management infrastructure; urban areas with high population densities (developed nations). Not dangerous to human life

Medium risk (4–6 floods per life span)

253

254

Hazardous Processes: Flooding

sedimentology and topography of the floodplain exert an important control on flooding, with a channel constrained by natural levees perched above the lower lying floodplain (Hudson and Colditz, 2003). Flooding of natural levees occurs less frequently than floodplain bottoms due to high lateral flow connectivity. Frequent inundation of floodplains poses a serious hazard to human infrastructure and activities, including agriculture (Benito and Hudson, 2010). An important product stemming from these concepts is flood hazard maps (Figure 4; Table 3). These maps may depict flood-prone inundation zones and their hydraulic characteristics, including depth, velocity, frequency, and areal extent of flooding for specific flood recurrence intervals (Figure 4; Wolman, 1971). They are based on identification of low-flow and high-flow channels, lower and upper floodplains (Garry et al., 2002; Benito and Hudson, 2010), and channel migration zones derived from historical morphodynamic studies (Rapp and Abbe, 2003). In applied studies, a flood hazard map is accompanied by flood risk maps that show the potential damage to people, property, and the environment. The fundamental purposes of flood hazard maps are to support flood risk management, land-use planning, emergency response planning, and to increase public awareness of flood hazards and risks. Flood hazard mapping is commonly based on geomorphic interpretation of aerial photographs or satellite imagery, in combination with field study of flow indicators including sediments and landforms (Dunne, 1988), soils, drainage (Smith and Boardman, 1989), and distinctive vegetation related to frequent standing water (Foxcroft et al., 2008). Geomorphic criteria for flood hazard delineation (Wolman, 1971; Dunne, 1988) have increased in sophistication in parallel with advances in remote sensing and geospatial methodologies (Charlton et al., 2003; Jain et al., 2005). Rapid advances are being made in the acquisition and processing of geospatial data using global positioning systems (GPS), digital photogrammetry, and high-resolution ground and airborne remote-sensing techniques airborne laser scanning (ALS), synthetic aperture radar (SAR), light detection and ranging or laser imaging detection and ranging (LiDAR) French, 2003). Alluvial stratigraphies can be used to extend flood histories beyond the period of instrumental records. They are based on the characteristics of the alluvial deposits and apply sedimentological evidence to derive flood-related high-water stages (Knox and Daniels, 2002). Most of these paleoflood studies have been carried out on meandering rivers, where vertical floodplain accretion facies are dominated by finegrained sediments (sand, silt, and clay) deposited by relatively low-energy tractive and suspension processes during floods. The sediments are best preserved in depressions away from the stream channel and in adjacent low-lying areas that carry flood flows, for example, behind levees and abandoned meanders. The depositional unit recording a former large flood typically has a high sand fraction, provided that sand is available in the drainage system (Knox and Daniels, 2002). Intense postflood bioturbation on the floodplain surface may complicate the identification of discrete flood deposits (Knox, 1987). A more complex task is estimating paleoflood magnitudes from floodplain stratigraphy. Knox and Daniels (2002) derived peak flood discharges from estimates of competent

flow depth necessary to transport coarse cobbles and boulders occurring in floodplain alluvium. They indicate that coarse clasts suitable for estimating flow depths decrease rapidly in the downstream direction, and consequently, this method becomes difficult to apply in large watersheds.

13.15.4

Natural and Anthropogenic Drivers of Flood Hazard Variability

Flood hydrology is controlled by a variety of allogenic (climate) and authigenic (channel shape and size) factors. Those factors representing the characteristics of the basin (area, shape, slope, aspect, elevation) can be considered unvarying on the human time scale. Other driving variables may vary in time and space on time scales from years to decades, in particular, climate and land-use changes induced by human activities. In fact, a fundamental assumption in conventional hydrology, stationarity of flood records – that is floods are randomly generated and follow a probability distribution with stable moments – can be questioned due to the effects of climate change and land use on flood hydrology (Milly et al., 2008). The assumption of stationarity can be tested through geomorphology, specifically through analysis of long-term flood records, their magnitudefrequency patterns, and their links to local and regional natural and anthropogenic forcing mechanisms.

13.15.4.1

Flood Response to Climate Variability

Recent advances in climate change science have not been accompanied by equivalent attention to impacts of climate change on hydrological extremes. Current knowledge on how floods might respond to climate change is highly uncertain (Trenberth et al., 2007). Long series of stream gauge data do not show a clear climate-related trend in flooding during the twentieth century (Kundzewicz et al., 2007). Additionally, the downscaling necessary to establish hydrological impacts cannot be adequately achieved using existing instrumental hydrological records at contemporary timescales. It may be necessary to build extended flood records using paleoflood reconstructions as a way to improve understanding of flood response over century-scale climate shifts (Knox, 1993; Ely, 1997). Climate affects flooding across a wide spectrum of spatial and temporal scales (Redmond et al., 2002). Analysis of long instrumental and paleoflood records has revealed that subtle variations in the atmospheric circulation affect primarily the largest floods (Knox, 2000). Knox (1983) examined an instrumental record extending back to 1860 for the Mississippi River and identified four periods of different prevailing global atmospheric circulation patterns, that is, zonal and meridional circulation modes. He computed separate estimates of flood probability for each period and showed that the occurrence of large rare floods (50-year flood and higher) is more sensitive to the atmospheric circulation pattern than the occurrence of smaller, more frequent floods (2-year floods). Temporal analysis of annual floods and peaks-overthreshold series have shown cyclic or clustered flood occurrences within instrumental (Robson et al., 1998), historical

Hazardous Processes: Flooding

(Machado et al., 2011), and paleoflood records (Ely et al., 1993; Thorndycraft and Benito, 2006). Flood clusters are ascribed to changing atmospheric circulation patterns modulated by climate variability at decadal and secular time scales (Enzel, 1992). A Holocene sedimentary record in Spain indicated a high flood frequency and magnitude at 10 750–10 240, 9550–9130, 4820–4440, 2865–2350, 960–790, and 520–290 cal BP (Figure 5 shows the last 4000 years; Benito et al., 2008). A period of increased flooding by Iberian Atlantic rivers between AD 1000 and 1200 is also apparent in written documentary records (Benito et al., 2003b), and has been associated with unusually wet winters during the Medieval Climate Anomaly (Benito et al., 2003a). The largest historical floods since AD 1500 (Benito et al., 2008) occurred when the North Atlantic Oscillation during the winter months was in its negative phase, as reconstructed by Luterbacher et al. (2002). Pall et al. (2011) argue that global warming is contributing to increased flood risk in the UK, although no gauge-based evidence has been found for a climate-related trend in the magnitude or the frequency of floods during the past several decades (Rosenzweig et al., 2007). Paleoflood records provide evidence of rare, large floods that are commonly larger than those recorded by stream gauges (Enzel et al., 1993). In September 2002, an extreme flood on the Gardon River in France claimed 22 lives and caused damage worth millions of euros. The flood was produced by 680 mm of rain in a 20-h period

255

and was considered by both the media and the water authorities to be ‘the largest flood on record.’ The flood’s peak discharge was estimated to be 6000 m3 s1; the next largest recorded historical flood, in 1890, had an estimated discharge of 4500 m3 s1. Sheffer et al. (2008) identified a rock shelter in the Gardon River gorge that contained stratigraphic evidence of at least five floods larger than the September 2002 event over the past 500 years. Documentary evidences suggest that five floods, in AD 1403, 1604, 1741, 1768, and 1846, were probably the most catastrophic ones (Lescure, 2004). Flood records in the Mediterranean over the past 500 years reflect periods of intense climatic variability lasting 30–40 years coinciding with frequent floods; the periods AD 1580–1620 and 1840–70 are notable (Barriendos and Martı´n Vide, 1998).

13.15.4.2

Environmental Changes and Flood Hazards

Natural and human-induced environmental changes may produce significant hydrological adjustments. Natural environmental changes occur on a variety of temporal (decades to millennia) and spatial (regional and global) scales. Humaninduced environmental change may occur over time scales as short as years, with impacts at catchment and regional spatial scales. Both types of environmental change can indirectly impact flood hazards by altering the hydrological

Probability per year

Floodplain deposits (n = 23) 0.06

Slackwater flood deposits (n = 40)

0.04

0.02

0 4000

3000

2000

1000

0

Calibrated years BP

Floodplain deposit clusters (n >2) Overlapping SWD and floodplain dating clusters SWD dating clusters (n >2) Figure 5 Summed probability plots of radiocarbon-dated samples collected from slackwater flood deposits (red line) and floodplain deposits (blue area) from 13 rivers in Spain. Modified from Benito, G., Thorndycraft, V.R., Rico, M., Sa´nchez-Moya, Y., Sopen˜a, A., 2008. Palaeoflood and floodplain records from Spain: evidence for long-term climate variability and environmental changes. Geomorphology 101, 68–77. The bottom bar shows overlapping periods of slackwater and floodplain deposition (diagonal line pattern dipping left on green background), exclusively slackwater flood date clusters (red colour), exclusively floodplain deposition (diagonal line pattern dipping right on blue background), and periods with minor or lack of fluvial activity (white rectangles). Radiocarbon dates from aggraded floodplain sediments are clustered at 2710–2320, 2000–1830, and 910–500 cal BP. The first cluster period is in phase with the timing of slackwater deposition, whereas the third (910–500 cal BP) occurs in between two periods of increased flood frequency and likely reflects the first post-Roman evidence of environmental change related to generalized land-use changes at the catchment scale.

256

Hazardous Processes: Flooding

characteristics of watersheds (e.g., runoff volumes and peak discharge) and sediment yields. Land-use changes may intensify river flooding by adversely affecting soil structure and reducing infiltration rates and water storage capacity (Smith and Ward, 1998). Land clearing may lead to rapid hydrological responses and larger flood peaks, both of which affect flood severity and the rate and intensity of geomorphic processes. The effects on hydrology are more significant during frequent low-magnitude storms and diminish as prolonged heavy rainfall creates conditions of surface saturation (Haigh et al., 1990). The impacts are also more significant in small headwater and tributary watersheds than in large watersheds (Knox, 1977). An increase in the frequency of overbank flows of alluvial rivers is generally accompanied by an increase in floodplain sedimentation rates and by major changes in channel geometry (e.g., Knox, 2006). As bank heights are increased by overbank sedimentation, channels may convey more frequent, deep, high-energy flows that exacerbate bank erosion and lateral channel migration. Accelerated aggradation episodes on floodplains during the late Holocene are commonly interpreted as being caused by human disturbance, although it is difficult to disentangle the roles played by climate and human activity (Foulds and Macklin, 2006). Large-scale agriculture and other forms of intensified land use may have led to greater sediment availability, but frequent and large effective water flows driven by climate may be required to enhance sediment transport and subsequent alluviation (Foster et al., 2000). In fact, desertification, floods, fires, droughts, and loss of biodiversity may be enhanced by a combination of climate and human factors (Benito et al., 2010). Regionally synchronous alluvial stratigraphies are widely cited as an indicator that climatic forcing is a major driver of flooding at specific times (Macklin and Lewin, 1993; Sancho et al., 2008), whereas asynchronous fluvial activity across a number of catchments is an argument for anthropogenic forcing (Burrin and Scaife, 1988). The relative roles that human and climate effects play in flooding can possibly be determined by considering the insensitivity of large floods to land changes. Flood chronologies from alluvial floodplain sequences, commonly built by frequent floods that are highly sensitive to land-use changes, can be compared with those derived from slackwater flood deposits at bedrock reaches, mainly recording extreme floods responding to climate variability (Benito et al., 2008; Harden et al., 2010b). Comparison of flood histories of bedrock and alluvial rivers in Spain, based on a probability analysis of 80 radiocarbon ages, demonstrates temporal coincidence of floodplain aggradation and slackwater deposition at 2710– 2320 cal BP, showing a major influence of enhanced flooding related to climate (Figure 5; Benito et al., 2008). However, a historical period of floodplain aggradation at 910–500 cal BP occurred between two phases of slackwater deposition (960–790 and 520–290 cal BP), suggesting a major role of deforestation and expansion of agriculture. In the southwestern U.S., Harden et al. (2010b) have documented similar out-of-phase flood chronologies from arroyo alluvial environments and slackwater deposition at bedrock reaches. Slackwater flood records in this region provide evidence for periods of increased precipitation and lower temperatures,

whereas arroyo alluvial stratigraphy reflects episodes of reduced annual precipitation.

13.15.4.3

River Engineering Structures and Flood Hazards

Flood control structures have at least one of the following objectives (Hudson et al., 2008): (1) reduce the area of inundation on floodplains; (2) reduce flood stage and peak discharge; and (3) reduce flood duration. These structures, however, can adversely affect the fluvial morphodynamics of the channel and floodplain (Benito and Hudson, 2010). The river system adjusts to the new conditions imposed by the flood control structures, producing, in some cases, an unintended geomorphic response (Smith and Winkley, 1996; Hesselink et al., 2003; Hudson et al., 2008). The timescale of the adjustments depends on channel sensitivity (alluvial vs. bedrock), geology, and climate. Rivers in equilibrium in humid environments may require several decades and several floods to produce significant adjustments, whereas rivers in semiarid areas may adjust much more rapidly, in some cases, during a single flood. Flood control structures on floodplains include dikes (levees), drainage canals, and floodways. Dike construction alters floodplain hydrology and sedimentation, and also inhibits some of the geomorphic functions of floodplains, including floodwater storage capacity (Hudson et al., 2008). For example, historical dike construction along the Rhine River has resulted in a reduction in floodplain extent from 1400 to 950 km2, with a consequent decrease in water storage on floodplains during floods (Ebel and Engel, 1994). A common consequence of a reduction in floodplain water storage is more severe downstream flooding. Other fluvial engineering modifications are straightening (artificial cutoffs), bank protection (revetments and rip-rap), groynes (wing dikes), and dredging. Channel straightening affects flood hydrology by reducing flood stage and increasing flood conveyance (Benito and Hudson, 2010). The secondary effects of river straightening include channel incision and bank erosion, which may cause changes in geomorphic and hydraulic processes and alter downstream flood hazards. Straightening of a 5-km reach of the River Main channel in the UK increased the channel slope and hydraulic radius, causing a subsequent increase in flow velocity that necessitated a downstream extension of bank protection works, including sheet piles and rock armoring (Wilcock and Essery, 1991). Flood mitigation structures (canals, drop structures, check dams) in mountain catchments with high sediment yields may also increase hazards. Difficult access to mountain streams may result in poor maintenance of flood mitigation structures, increasing the risk of deterioration or failure. For example, in 1996, a summer flood in a small catchment (25 km2) located in the Central Pyrenees (Spain) destroyed 31 of a series of 36 check dams, built 50 years earlier (Figures 6(a) and 6(b)). Breaching of the dams triggered debris flows that reached the apex of an alluvial fan at the mouth of the valley, blocking a drainage canal that crossed the fan surface (Figure 6(a); Benito et al., 1998). As a result, flood waters were diverted down the fan and into a campsite, causing the loss of 87 lives (Garcia-Ruiz et al., 1996). A long civil trial ended in 2005 with payments of h11.2

Hazardous Processes: Flooding

13.15.5

Stream channel Camp site Road

Concrete canal (a)

(b)

Figure 6 Alluvial fan of the Ara´s stream (Spanish Pyrenees) and flood deposition left by the 1996 flood. This flash flood caused the loss of 87 lives in a campsite area located in the trees in the right side of the alluvial fan (Garcia-Ruiz et al., 1996; Benito et al., 1998). (a) Concrete canal connects the upper fan segment to the Gallego River (foreground). Note that the canal apex was buried by the flood debris, and diverted most of the flood waters into the camp site zone. (b) View of six retention dams, about 5–7 m in height, which collapsed during the 1996 flood. The filled dams increased the sediment supply and favored the development of debris flows.

million by water authorities to the affected families. The Supreme Court considered expert reports based on geomorphic and botanical indicators that advised against siting the camp location on the fan. In developed countries, traditional flood management based on structural solutions is now being replaced by nonstructural ones in which understanding of geomorphic and ecological processes play a relevant role (Silva et al., 2001). Potential adverse effects of river rehabilitation and restoration are complex and remain difficult to predict, but on-going fluvial geomorphic research is addressing the problem (Gregory et al., 2008). Restoration of floodplain areas to increase water storage is an affordable nonstructural practice in flood management, although impacts on geomorphic processes need to be carefully assessed (Hudson et al., 2008). An example of this nonstructural solution is the ‘Room for the River’ program along the lower Rhine River in the Netherlands in response to large floods in 1993 and 1995 (Middelkoop and Van Haselen, 1999). This program brings together new ideas of spatial physical planning based on geomorphology, hydrology, ecology, and climate change forecasts (Fokkens, 2007).

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Concluding Remarks

Flooding is the most common natural hazard on Earth (Smith and Ward, 1998). As the human population increases, occupation and alteration of riverine areas are dramatically increasing flood damage. Flood damage reduction plans require a detailed delineation and characterization of alluvial zones prone to flooding. Credible and accurate characterization of flood frequency and magnitudes and causative mechanisms are needed to reduce flood risk. Moreover, it is important to assess how flood attributes can be affected by human activities and climate change. There is a general appreciation that flood risk is a complex problem whose assessment, management, and mitigation require a multidisciplinary approach. This chapter has reviewed methods of flood hazard assessment, based on landforms and stratigraphic signatures, as well as the short-term and long-term response of flood hazards to natural and human-induced changes. Geomorphology can contribute in three ways: (1) by documenting and assessing flood evidence, including timing, along fluvial corridors; (2) by relating identified flood evidence to flow magnitude, typically peak discharge; and (3) by understanding and quantifying flood processes to anticipate potential geomorphic changes to stream channels and floodplains. Application of fluvial geomorphology to flood hazard studies provides robust methods to extend flow magnitude and frequency back beyond instrumental records. Selection of appropriate methods depends on the characteristics of the fluvial environments (e.g., mountain streams, alluvial fans and alluvial rivers). Quantitative paleoflood estimates are most reliable for rivers and streams with a stable geometry, such as in bedrock-confined canyons. On alluvial fans and unconfined rivers, effective flood hazard assessments can be obtained from a combination of paleoflood hydraulic estimates and morphosedimentary mapping. Long-term flood records can be linked to meteorological causes and climatic conditions to better understand flood response to century-scale climate and environmental changes. Large floods are more sensitive to climate variability than smaller, more frequent ones. Most paleoflood studies have recorded periods of clustered floods, reflecting changes in prevailing atmospheric circulation patterns associated with climate. Flood frequency may also be altered by humaninduced environmental changes at the watershed scale (e.g., land-use changes) or by modification of channel morphology. Land-use changes at the watershed scale may be reflected in the hydrology of small flood events and appear to be more significant in small catchments than larger ones. Construction of flood mitigation structures may trigger geomorphic changes in river channels and floodplains, with significant implications for flood risk. Because humans are extensively modifying river channels and floodplains, geomorphology is making critical contributions to understanding how river engineering and human activities affect the natural conditions of rivers.

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Biographical Sketch Gerardo Benito, PhD in Geosciences (1989) from the University of Zaragoza (Spain), was associate research fellow at the University of Arizona (1989–92), and since 1992, is a member of the research staff at the Spanish National Research Council (CSIC); he currently holds a Research Prof. position. His research interests are in Quaternary Geology, with an emphasis on paleohydrology and geomorphology, and particularly on the study of past and recent floods. Regarding the latter, his works have focused on flood hazards in relation to climate and environmental changes, with field sites in Europe, North and South America, and Africa. Dr. Benito is currently President of the Global Continental Palaeohydrology group of INQUA, Leader of Focus Area ‘Hydrological Change and Climate’ in INQUA, Spanish Delegate in PAGES (Past Global Changes), and Member of the IGBPSpain and INQUA-Spain committees. His research activity is combined with active participation as Lead Author of the IPCC Special Report on ‘Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation’ (SREX), and of the IPCC 5th Assessment Report, WG II Impacts, Adaptation and Vulnerability.