The magnitude and frequency characteristics of fluvial transport in a devon drainage basin and some geomorphological implications

CATENA

Vol. 9, 9-23

Braunschweig 1982

THE MAGNITUDE AND FREQUENCY CHARACTERISTICS OF FLUVIAL TRANSPORT IN A DEVON DRAINAGE BASIN AND SOME GEOMORPHOLOGICAL IMPLICATIONS

B.W. Webb & D.E. Walling, Exeter

SUMMARY Magnitude and frequency analysis of fluvial transport based on long and detailed record has been lacking for British rivers. Continuous monitoring of suspended sediment and solute concentrations for the River Creedy, Devon since October, 1972 has provided, an opportunity to establish the m~dium-term magnitude and frequency characteristics of river transport in a sizeable (262 km') drainge basin. Results reveal the importance of solute removal, substantial variations in the relative magnitude of annual dissolved and suspended sediment yields, clear differences in the temporal distribution of sediment and solute removal, and contrasts in the magnitude and recurrence of extreme values of dissolved and suspended solids transport. Consideration of discharge levels most effective for transporting river loads in relation to a natural channel cross-section has stressed the importance of events of moderate magnitude and frequency in this drainage system. ZUSAMMENFASSUNG Bisher ist keine Untersuchung der Transporte in britischen FlieBgew[issem nach Gr6Be und H/iufigkeit vorgelegt worden, die auf langen Zeitreihen mit hoher Aufl6sung beruht. Die durchgehende Messung von Schweb und gel6sten Stoffen seit 1972 im River Creedy, Devon, erm6glicht es, Gr6Be und H/iufigkeit des FluBtralasports f'tir einen mittleren Zeitraum und ein ziemlich groBes FluBeinzugsgebiet (262 km-L) zu bestimmen. Die Ergebnisse offenbaren die Bedeutung des Austrags der gel6sten Stoffe, deutliche Varianzen in der relativen Gr6Be der j/ihrlichen L6sungs- und Schwebfracht, deutliche Unterschiede in der zeitlichen Verteilunges Austrags der gel6sten Stoffe und des Schwebs sowie Gegens/itze in er Gr6Be und der Eintrittsh/iufigkeit der Extremwerte des Transports tier gel6sten Stoffe einerseits und des Schwebs andererseits. Eine Untersuchung der Abfliisse, bezogen auf den natiJrlichen AbfluBqueschnitt, die am st/irksten beitragen zum Transport im FlieBgerinne, hebt die Bedeutung von Ereignissen mit mittlerer Gr6Be und H/iufigkeit in diesem Einzugsgebiet hervor. 1. INTRODUCTION In the twenty years since magnitude and frequency concepts were first introduced into fluvial geomorphology by WOLMAN & MILLER (1960), there have been surprisingly few attempts to investigate the magnitude and frequency characteristics of sediment and solute transport by British rivers. This lack of attention reflects a number of factors which include first, the general absence of detailed sediment and solute records in Britain, secondly, the

10

WEBB & WALLING

short-term nature of many research investigations into sediment and solute dynamics which provide very limited scope for magnitude and frequency analysis, and thirdly, a changing emphasis since the classical work of the 1960s. In this latter context, recent studies have focussed on the effectiveness ofgeomorphological events in terms of landscape modification (BAKER 1977, WOLMAN & GERSON 1978, ANDERSON & CALVER 1980) rather than their efficacy in removing material from the drainge basin system (LEOPOLD et al. 1964). Furthermore, such studies that have been undertaken to date (e.g. SMITH & NEWSON 1974, FINLAYSON 1977) have largely related to small headwater streams, so that the magnitude and frequency characteristics of sediment and solute transport in many larger British rivers are virtually unknown. Continuous monitoring of suspended sediment and solute concentrations at the outlet of a sizeable Devon catchment since October, 1972 has furnished what is probably the longest detailed record of river transport available in Britain, and has provided an opportunity to define unequivocally the medium-term magnitude and frequency properties of suspended sediment and dissolved solids removal and to explore the geomorphological implications of these characteristics.

2. THE STUDY BASIN The study basin of the River Creedy in mid-Devon drains an area of 262 km 2 above the South West Water Authority flow gauging station at Cowley. It exhibits moderate relief and is underlain predominantly by Carboniferous strata although a variety of Permian lithologies are preserved in the Credition trough (Fig. 1). Hydrometeorological conditions reflect topographic differences across the basin, and mean annual precipitation and runoff respectively vary from more than 1000 mm and 700 mm on the highest ground to less than 800 mm and 300 mm in the vicinity of the catchment outlet. Agricultural land use, in particular permanent pasture, dominates the study basin, and the Creedy system is unaffected by major domestic or industrial pollution. Continuous records of discharge at Cowley are collected by the South West Water Authority, and continuous monitoring of river loads at this site has been achived by the installation of sensors mounted directly in the river channel (c.f. WALLING 1978). Suspended sediment (total filterable solids) concentrations are measured using a photoelectric turbidity meter. This equipment has proved well-suited to the predominantly fine-grained particulate load of the Creedy system, and provides a continuous record of turbidity which is readily converted to values of average suspended sediment concentration in the measuring cross-section using a calibration relationship derived from field sampling (WALLING 1977). Storm-period suspended sediment concentrations rarely exceed 2000 mgl -~ in the study basin. Furthermore, sediment transport is greatly dominated by movement of material in suspension and no attempt has been made to measure accurately the small amount of bedload transport which takes place in storm events. Dissolved solids concentrations are continuously monitored using measurements of specific electrical conductance. Conductivity values have been adjusted for water temperature fluctuations and converted to values,of total dissolved solids concentration by application of a conversion equation derived frem the laboratory analysis of more than 100 water samples collected at the gauging site. Continuous records of suspended sediment and dissolved solids concentrations ir~the River Creedy are currently available for the period October 1st, 1972 to June 30th, 1980, and can be adequately defined by the abstraction of hourly instantaneous values. Combination of

MAGNITUDE AND FREQUENCY OF FLUVIAL TRANSPORT

11

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MEAN ANNUAL .... PRECIPITATION (mm)

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Fig. 1: Characteristicsof the study catchment.

hourly concentration values with hourly discharge data provides a very detailed record of river loads over a period of 7.75 years which forms the basis for the present magnitude and frequency analysis. i"

3. 3.1.

MAGNITUDE AND FREQUENCY CHARACTERISTICS OF RIVER TRANSPORT ANNUAL AND TOTAL LOADS

Total and annual suspended and dissolved solids loads carried by the River Creedy during the study period are listed in Table 1. This indicates that the total transport of dissolved material (153,325 tonnes) exceeds the total removal of suspended material (82,832 tonnes)

12

WEBB & WALLING

Tab. 1: ANNUAL AND TOTAL YIELDS OF SUSPENDED SEDIMENT AND DISSOLVED SOLIDS FROM THE RIVER CREEDY AT COWLEY DURING THE STUDY PERIOD Water Year*

Dissolved Solids Load (tonnes) 72/73 14284.7 73/74 22805.5 74/75 19848.7 75/76 7411.7 76/77 30779.3 77/78 21041.3 78/79 18325.3 Total** 153325.1 * Water year runs from let October to 30th September ** Refers to period of 1.10.1972 to 30.6.1980

Suspended Sediment Load (tonnes) 7482.2 20618.7 10546.5 1941.3 16234.4 10213.6 4717.4 82832.4

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Annual Discharge (m a. 1 0 a) Fig. 2: Relationships between annual runoffand annual yields of suspended sediment and dissolved solids for individualwater years. Inset shows total loads for the study period.

MAGNITUDE AND FREQUENCY OF FLUVIAL TRANSPORT

13

by a factor of 1.85. This ratio serves to emphasise the importance of solute removal, which is a continuous process in contrast to the more sporadic nature of significant suspended sediment transport. A plot of annual loads against annual discharge for individual (vater years (Fig. 2) demonstrates that solute yields always exceeded suspended sediment removal on an annual basis during the study period. However, relationships between annual loads and annual discharge fitted by least-squares regression (Fig. 2) reveal a tendency for the differential between annual dissolved and suspended sediment transport to decline with increasing annual runoff. Figure 2 demonstrates that solute removal is very closely and directly related to the annual volume of runoff. However, the slope of 0.85 for the fitted relationship indicates that the increase in solute load during wetter years is less than proportional to the increase in discharge because of enhanced dilution of solute concentrations under such conditions. In contrast, the fitted relationship for suspended sediment load has a slope of 1.4, demonstrating that the increase in transport during wetter years is more than proportional to the increase in runoff.

Tab. 2: RANKINGOF WATERYEARS IN THE STUDYPERIOD WITH RESPECTTO RUNOFF, INSTANTANEOUS PEAK FLOW AND SUSPENDED SEDIMENT YIELD OF THE RIVER CREEDY AT COWLEY Annual Rtlnoff

Annual Suspended Sediment Load

iiiii iiiii

Annual Instantaneous Peak Flow

iiiii.

iiii! !iiii iiiii 75/76

75/76

75/76

* Influenced by snowmelt runoff

The relationship between annual sediment load and annual discharge exhibits considerable scatter, and this is largely responsible for the appreciable variation in the dissolved to suspended load ratio from year to year (Fig. 2). Ranking of annual suspended sediment loads in relation to annual runoffvolume and annual flow maxima (Table 2) shows that suspended sediment transport is not solely a function of either total or peak discharge in a given year. the pattern of runoff may also be very significant and there is a good rank correspondence between annual load and number of major flood events as indexed by exceedence of

14

WEBB & WALLING

Tab. 3: RANKED SUSPENDED SEDIMENT LOADS AND NUMBER OF BANKFULL EXCEEDENCES AT COWLEYFOR WATERYEARS IN THE STUDY PERIOD Ranked Annual Suspended Sediment Load 73/74 76/77 74/75 77/78 72/73 78/79 75/76 * Influenced bysnowmelt runoff

Number of Events Exceeding Bankful Stage 6 7 3 4* 1 1 0

bankfull stage (Table 3). However, other factors may be equally important and include the sequence of wet and dry years and the origins of runoff. For example, water year 1977/78 experienced a relatively high total runoff, peak flow and number of bankfull exceedences. However, it was not associated with a particularly large suspended sediment load, mainly because three of the major discharge events in this year were dominated by snowmelt which carried relatively low sediment concentrations. Similarly, the ordering of individual events will clearly influence the effectiveness of particular flood periods (e.g. BEVEN 1981). In the past, considerable attention has been given to spatial variations in sediment and solute loads and their ratio (e.g. LANGBEIN & DAWDY 1964, JUDSON & RITTER 1964, VAN DENBURGH & FETH 1965, MEYBECK 1976) but preliminary analysis of the Creedy records suggests that marked temporal variations in the relative magnitude of the load components also occur and are worthy of further attention.

3.2.

LOAD DURATION DATA

Another perspective on the transport of suspended sediment and dissolved solids during the study period is provided by the construction of duration curves based on hourly load values (Fig. 3). These highlight the much greater variability of suspended .sediment loads which fluctuated over approximately six orders of magnitude during the study period, whereas dissolved loads ranged over only three orders of magnitude and exhibited less variation than discharge levels. The lower variability of the dissolved solids loads reflects the offsetting of low discharges by high solute concentrations, and vice-versa at high flows. The point at which the two load duration curves intersect indicates that suspended sediment loads were in excess of dissolved loads for only 2.5% of the period. This represents a total of 71 days or apl~roximately 9 days per year. . The timing of suspended sediment and dissolved solids transport in the Riv.er Creedy may be examined in more detail by reference to load duration information expx'essed in cumulative form. Figure 4Aillustrates, in a conventional way, the percentage of time.taken to transport different proportions of the total suspended and dissolved load of the study period, whereas Figure 4B has been contructed to indicate the cumulative pecentages of the tw 9 load components associated with the cumulative percentage of total streamflow. The results expressed in these graphs confirm the findings of previous studies in Britain (e.g. DOUGLAS 1964, WALLING 1971, BROOKES 1974, SMITH & NEWSON 1974, FINLAYSON 1977),

MAGNITUDE AND FREQUENCY OF FLUVIAL TRANSPORT

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Percentage of Time Discharge or Load Equalled or Exceeded Fig. 3: Load and runoffduration curves for the period 1st October, 1972 to 30th June, 1980. which have shown suspended sediment transport to be mainly accomplished during infrequent stortn events of short duration, whereas solute removal is a more continuous process. In the RK,er Creedy catchment, 50% of the total suspended sediment load is transported in only 0.8% of the time, or approximately three days per year, whereas a period of 44 days per year, or 12% of the time, is required to remove 50% of the total dissolved load (Fig. 4A). Equivalent duration figures for 90% of the suspended and dissolved loads are 22 days per yearJor 6% of the time, and 204 days per year or 56% of the time respectively. The ineffectiveness of suspended sediment transport during lower flows is clearly apparent from Figure 4A, which indicates that events recurring 50% of the time and more frequently contribute

16

WEBB & WALLING

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MAGNITUDE AND FREQUENCY OF FLUVIAL TRANSPORT

17

only 0.06°/0 of the total suspended load carried in the River Creedy. In contrast, events with the same recurrence account for approximately 14% of the dissolved load transport. The relationship between cumulative load and cumulative discharge (Fig. 4B) for the study period also reveals that 50% of the suspended sediment was moved by only 9% of the total discharge, whereas 56% of the total flow volume was needed to remove half of the dissolved solids load. Figure 4B again emphasises that the relationship between load and discharge is closer for dissolved solids than for suspended sediment. A

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Fig. 5: Extremes of transport evidenced by maximum daily yields of individual water years (A), and by the variability and recurrence interval of maximum two-day loads and peak runoff(B). ~ refers to average percentage from seven years of record.

18 3.3.

WEBB & WALLING EXTREMES OF TRANSPORT

The relative importance of extreme events to the removal of suspended sediment, dissolved solids and runoff from the Creedy basin may be investigated by plotting maximum values of daily load and runoffvolume, derived from continuous records, as a percentage of total yields for individual water years (Fig. 5A). Major storms clearly have the greatest influence on the transport of suspended solids, since peak daily loads account on average for 13.4% of the annual suspended sediment yield. In contrast, peak daily yields remove on average only 3.5% and 2.5% of the annual runoff and annual dissolved solids load respectively (Fig. 5A).

Tab. 4: RECURRENCE INTERVALS OF MAXIMUM TWO-DAY LOADS DERIVED FROM ANNUAL SERIES Return Period (years) 9 4.5 3 2.25 1.8 1.5 1.29 1.125

Suspended Sediment Load (tonnes) 3508.2 2758.3 2173.7 1967.9 1928.6 1544.3 750.9 438.3

Dissolved Solids Load (tonnes) 1240.9 990.9 933.1 891.1 839.7 569.6 483.1 286.8

The frequency of extreme events and the relative variability of peak values of sediment load and discharge must also be considered (e.g. DICKINSON & WALL 1978). Return periods for peak suspended sediment and dissolved solids yields are presented in Table 4 and have been calculated using the standard formula for recurrence interval proposed by LEOPOLD et al. (1964), and the annual series of maximum two-day loads. Yields for a twoday period have been employed because large events in the study catchment typically have a duration of between 24 and 48 hours. Table 4 indicates that events with a return period of 2.33 years, equivalent to the recurrence interval of the mean annual flood, transport twice as much suspended sediment as dissolved material. Plotting of the ratio of annual peak yields to the smallest annual maximum recorded during the study period (Fig. 5B) clearly indicates that events of increasing recurrence interval become more extreme in terms of suspended sediment transport than for the removal of dissolved solids. Annual maximum values of suspended sediment yield are also more variable than the annual series ofdischarge peaks (Fig. 5B), and in this respect the study catchment is comparable to several Canadian rivers studied by DICKINSON & WALL (1978). The latter study demontrated that variability ofsediment load and flow extremes differed considerably between catchments, and similar comparisons between the River Creedy and other local and national rivers would seem a worthwhile topic for future investigation.

MAGNITUDE AND FREQUENCY OF FLUVIAL TRANSPORT 4.

19

GEOMORPHOLOGICAL IMPLICATIONS

The magnitude and frequency characteristics of suspended sediment and dissolved solids transport outlined above clearly highlight differences in the temporal distribution of mechanical and solutional removal in the study catchment which are of geomorphological interest. However, interpretation of river loads in a geomorphological context requires caution because the existence of non-denudational components (e.g. MEADE 1969, JANDA 1971, FIN LAYSON 1977) and the effects of sediment conveyance processes (e.g. ROEHL 1962, ROBINSON 1977) may lead respectively to over- and underestimation of denudation rates. Furthermore, recent studies have suggested that the effectiveness of events in achieving landform modification depends not only on work done in transporting material but also on the sensitivity and lability ofthe landscape to change (TRUDGILL 1976, BRUNSDEN & THORNES 1979, BRUNSDEN 1980). The landscape changes associated with major hydrological events may be influenced by antecedent conditions (NEWSON 1980), interflood timing (BEVEN 1981) and local factors of catchment area, valley width and sediment supply (ANDERSON & CALVER 1980). Similarly, the degree of landform alteration may reflect the transgression ofthresholds (HARVEY 1977, SCHUMM 1979), complex reactions to single inputs (SCHUMM 1977), and the ability ofrecovery processes to repair the,effects of geomorphological events (ANDERSON & CALVER 1977, WOLMAN & GERS.ON 1978). A result of this recent work has been a questioning of the classic findings of WOLMAN & MILLER (1960) which stressed the importance of events of moderate magnitude and frequency in fluvial transport and landform adjustment. Increasing interest is being directed towards the effectiveness of catastrophic events (e.g. STEWART & LaMARCHE 1967, STARKEL 1976, FROEHLICH 1975, USGS 1979, DURY 1980), the adjustment of fluvial features to more than one dominant discharge level (PICKUP & WARNER 1976, HARVEY et al. 1979), and the modification of the Wolman-Miller principle for environments with highly wlriable flow regimes and very resistant channel materials (BAKER 1977). In contrast, some recent investigations have reported only minor changes following catastrophic events in drainage systems (e.g. DURY 1973, COSTA 1974, GUPTA & FOX 1974) or have demontrated moderate events to be mainly responsible for both significant bank erosion and substantial sediment transport (HOOKE 1980, GRIFFITHS 1979, ANDREWS 1980). Detailed records available for the River Creedy allow the relative roles ofevents ofvarying frequency in river transport to be defined, and Figure 6A indicates the total load carried during the study period in each of 23 equal discharge classes spanning the total flow range. In tile case ofsuspended sediment, three flow classes between 12m3s-l and 30m3s -l each have loads in excess of 10000 tonnes and represent the most effective discharge range for the transport of suspended solids, whereas for dissolved solids removal the most effective range is represented by a single class of flows less than 6m3s -l. These discharge ranges have been plotted in relation to stage levels in a naural channel cross-section in the vicinity of the River Creedy gauging station at Cowley (Fig. 6B). This diagram indicates that flows at half-bankfull and below, including mean and median discharge levels, are most effective in solutional removal. In contrast, a range of flows close to bankfull level (Fig. 6B) are most effective for mechanical removal, and it is suggested that the channel cross-section may also be adjusted to discharges around bankfull level. In this respect, the present study supports the findings of ANDREWS (1980) who demonstrated that discharges most effective in transporting sediment were also those responsible lbr channel lbrmation in the Yampa Basin of Wyoming and Colorado. It appears that more extreme flows recorded in the study basin are less effec-

20

WEBB & WALLING

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Fig. 6: Effectivenessof differentflowclasses for transporting suspended sediment and dissolved solids in the river Creedy (A), and their relationship to stage levels in a natural channel cross-section in the vicinityof the Cowleygauging station (B).

tive in removing suspended sediment. The maximum discharge recorded during the study period involved inundation of the flood plain by approximately 2 m, and was the third largest peak recorded in the 16 year flow record at Cowley. This event may not represent a truly catastrophic occurrence, but it seem likely that the River Creedy, because ofits climatic setting and physiographic characteristics, does not have the potential for catastrophic response (BAKER 1977). In consequence, at least some of its fluvial features may be adjusted to events of moderate magnitude and frequency.

5.

CONCLUSIONS

Magnitude and frequency characteristics of suspended and dissolved solids transport have been defined for a Devon river system from detailed records assembled over a period of nearly eight years. Analysis of the River Creedy data has confirmed the distinction noted in

MAGNITUDE AND FREQUENCY OF FLUVIAL TRANSPORT

21

other studies between the character of sediment and solute removal in fluvial systems, bu.t has also suggested that more attention could be profitably paid to variations in the relative magnitude of annual sediment and solute loads, to the relationship between magnitude and recurrence interval for transport extremes and to comparisons of magnitude and frequency characteristics in catchments of different character. Investigation of discharge levels most effective for the transport of river loads has revealed that solute removal is mainly accomplished by flows at less then half'-bankfull stage. A range of discharges close to bankfull level carry more suspended sediment than the discharge extremes, and events of moderate magnitude and frequency appear to strongly influence this fluvial system. In this context, the River Creedy may be viewed as similar to other drainage basins in humid-temperate landscapes of subdued relief and with a good cover of soil and vegetation (e.g. BAKER 1977) and to other catchments of moderate or large area (e.g. FISK 1977, ANDREWS 1980). In other river systems characterized by smaller size, semi-arid climate, or thin soils and sparse vegetation, the potential for, and the effects of, extreme events may be much more significant for river transport and for the formation of fluvial features (e.g. BAKER 1977). Equally, contrasts in fluvial adjustment within a climatic zone, for example between British rivers, would be worthy of further investigation.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support for work on suspended sediment and solute dynamics provided by a Natural Environment Research Council research grant, and the assistance of the South West Water Authority in supplying river flow data.

BIBLIOGRAPHY

ANDERSON, M.G. & CALVER, A. (1977): On the persistence of landscape features formed by a large flood. Transactions of the Institute of British Geographers New Series 2, 243-254. ANDERSON, M.G. & CALVER, A. (1980): Channel plan changes following large floods. In: CULLINGFORD et al. (eds). Timescales in Geomorphology. Wiley-Interscience, Chichester, 43-52, 360 pp. ANDREWS, E.D. (1980): Effectiveand bankfull discharges of streams in the Yampa River Basin, Colorado and Wyoming. Journal of Hydrology 46, 311-330. BAKER, V.R. (1977): Stream-channel response to floods, with examples from central Texas. Geological Society of America Bulletin 88, 1057-1071. BEVEN, K. (1981): The effects of ordering on the geomorphic effectiveness of hydrologic events. In: Erosion and Sediment Transport in Pacific Rim Steeplands. I.A.H.S. Publication No. 132, Christchurch, 510-526. BROOKES, R.E. (1974): Suspended sediment and solute transport for rivers entering the Severn Estuary. Unpublished Ph.D. Thesis, University of Bristol. BRUNSDEN, D. (1980): Applicable models of long term landform evolution. Zeitschrift fiir Geomorphologie Supplement Band 36, 16-20. BRUNSDEN, D. & THORNES, J.B. (1979): Landscape sensitivity and change. Tranactions of the Institute of British Geographers New Series 4, 463-484. COSTA, J.E. (1974): Response and recovery of a piedmont watershed from tropical storm Agnes, June 1972. Water Resources Research 10, 106-112. DICKINSON, W.T. & WALL, G.J. (1978): Temporal and spatial patterns in erosion and fluvial processes. In: DAVIDSON-ARNOTT, R. & NICKLING, W. (eds). Research in Fluvial Geomorphology. Geo-Abstracts, Norwich, 133-148, 214 pp. DOUGLAS, I. (1964): Intensity and periodicity in denudation processes with special reference to the removal of material in solution by rivers. Zeischrift Rir Geomorphologie 8, 453-473.

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Anschrift der Autoren: B.W. Webb, D.E. Walling, Department of Geography, University of Exeter Amory Building, Rennes Drive, Exeter, EX4 4RJ. U.K.