Lane_et_al-2007-Earth_Surface_Processes_and_Landforms.pdf

Climate change and flood risk 429

Copyright 2006 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 32, 429446 (2007)DOI: 10.1002/esp

Earth Surface Processes and LandformsEarth Surf. Process. Landforms 32, 429446 (2007)Published online 5 September 2006 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/esp.1404

Interactions between sediment delivery, channelchange, climate change and flood risk in atemperate upland environmentS. N. Lane,1* V. Tayefi,2 S. C. Reid,3 D. Yu1 and R. J. Hardy11 Department of Geography, University of Durham, Science Laboratories, South Road, Durham, DH1 3LE, UK2 Scott Wilson Water, Unit 33, Mansfield i-centre, Oakham Business Park, Hamilton Way, Mansfield, Nottinghamshire, NG15 5BR, UK3 School of Geography, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK

AbstractThis paper uses numerical simulation of flood inundation based on a coupled one-dimensionaltwo-dimensional treatment to explore the impacts upon flood extent of both long-termclimate changes, predicted to the 2050s and 2080s, and short-term river channel changesin response to sediment delivery, for a temperate upland gravel-bed river. Results show that16 months of measured in-channel sedimentation in an upland gravel-bed river cause abouthalf of the increase in inundation extent that was simulated to arise from climate change.Consideration of the joint impacts of climate change and sedimentation emphasized the non-linear nature of system response, and the possibly severe and synergistic effects that comefrom combined direct effects of climate change and sediment delivery. Such effects are likelyto be exacerbated further as a result of the impacts of climate change upon coarse sedimentdelivery. In generic terms, these processes are commonly overlooked in flood risk mappingexercises and are likely to be important in any river system where there are high rates ofsediment delivery and long-term transfer of sediment to floodplain storage (i.e. alluviationinvolving active channel aggradation and migration). Similarly, attempts to reduce channelmigration through river bank stabilization are likely to exacerbate this process as withoutbank erosion, channel capacity cannot be maintained. Finally, many flood risk mappingstudies rely upon calibration based upon combining contemporary bed surveys with histor-ical flood outlines, and this will lead to underestimation of the magnitude and frequency offloodplain inundation in an aggrading system for a flood of a given magnitude. Copyright 2006 John Wiley & Sons, Ltd.

Keywords: flood risk mapping; flood inundation; sediment delivery; gravel-bed rivers

*Correspondence to: S. N. Lane,Department of Geography,University of Durham, ScienceLaboratories, South Road,Durham, DH1 3LE, UK.E-mail: [email protected]

Introduction

One of the key elements that can affect the hydrological response of a catchment is climate change. This can directlyaffect runoff generation through precipitation changes which, in turn, result in changes in flow characteristics. Inunda-tion extent is a function of flow discharge and so climate change may have a direct impact on flood inundation extent.There is evidence that climate change could be responsible for increases in the magnitude of peak flows (e.g.Middelkoop et al., 2001; Milley et al., 2002) and recent major flooding in the UK has raised the concern that climatechange is causing increases in flood frequency and magnitude (e.g. Hunt, 2002). Although the evidence that themagnitude and frequency of high flows is increasing is not clear (Robson, 2002), observations that: (a) climatevariability is seen to have a very strong impact on flood records; (b) rainfall has become more variable; and (c) rainfallintensity and the frequency of high intensity rainfall may have increased in some areas, including the UK (Osborn andHulme, 2002), suggest that future flood risk must take note of potential climate change.

However, the magnitude of a peak flow is not the only control on flood risk: a flood begins when main channelwater levels are sufficient to exceed local bank height. Thus, flood risk is driven by changes in river channel stage,which may be impacted upon by changes in both flow magnitude and river channel conveyance. In the literature, the

Received 12 July 2005;Revised 19 April 2006;Accepted 8 May 2006

430 S. N. Lane et al.


geomorphological impacts of river flows and flooding and the influence of different land management practices(e.g. land-use change, clear-cutting, urbanization) on downstream flooding have been hypothesized and researched butare still the subject of debate (see OConnell et al. (2005) for review). Much less attention has been given to theimpact of river channel configuration (i.e. within-channel morphology) on flood risk and inundation extent (Stover andMontgomery, 2001) and how sediment delivery impacts on conveyance. This is perhaps surprising given evidence thatthe sensitivity of alluviation to climate in temperate upland environments appears to have increased significantly overthe last 4000 years as a result of anthropogenic land-use change (Macklin and Lewin, 2003). In a study of theSkokomish river, Washington State, USA, Stover and Montgomery (2001) used historical data series to show thatwhilst peak discharge was either reducing slightly or not changing, there was evidence of significant stage increasesassociated with a given discharge. James (1999) described systematic shifts in stagedischarge relationships arisingfrom in-channel sedimentation from a Californian case-study. Korup et al. (2004) demonstrated the importance of highsediment delivery rates for floodplain inundation in the Poerua and Waitangitaona Rivers, New Zealand: exceptionallandslide events were required to cause floodplain inundation as otherwise flows were sufficient to erode into thevalley floors and remain in-bank. Finally, Pinter and Heine (2005) used equal discharge analysis (after Blench, 1969)to show that stages have systematically risen for equal discharge volumes in the Lower Missouri river. Floods thatwere completely in-bank during the early part of the twentieth century were now found to lead to flood inundation,and the most extreme floods now have a stage that is up to 37 m higher than at the start of the record. In two of thefive stations considered, they found that this was due to constriction in channel cross-sectional areas.

The above examples suggest that, in addition to understanding the impact of climate changes upon flood inundation,it is also important to consider the effects of changes in channel geometry, especially in systems undergoing long-termaggradation. The small number of publications that consider this issue, as well as the general lack of its considerationin flood risk mapping studies, is an interesting issue. This may be due to the traditional view of sediment delivery assome kind of local (in time and space) disturbance to an equilibrium channel morphology, where cross-sectional areasadjust to some characteristic bankfull flow. When sediment is delivered, it is thought to be just a matter of time beforechannel capacity re-establishes itself through transport of that sediment downstream through the river system. How-ever, this overlooks extensive geomorphological evidence of alluviation, potentially rapidly over short time periods(e.g. Macklin and Lewin, 2003), including aggradation. For example, Harvey (2002) reports that the long-termresponse of a major input of sediment to Bowderdale and Langdale Becks in the Howgill Fells was partial stabilizationof associated sedimentation zones and not downstream progradation.

This paper aims to understand how future climate change might interact with river channel change and so impactflood risk, for an upland gravel-bed riverfloodplain system. Equal discharge analysis (e.g. Blench, 1969; Pinter andHeine, 2005), where stage changes through time associated with a given discharge are used to detect conveyancechanges, is not adopted here. Such an approach is difficult except in systems with a very long history of measurementas: (a) extreme events do not happen very often; (b) many of our instrumental records are short and/or incomplete; and(c) discharge is often inferred from a stage record, especially in smaller tributaries that are not well-suited to moreadvanced discharge measurement methods, making such an analysis circular. Instead, we use a hydrodynamic model-ling approach. We measure changes in channel geometry over a short timescale (16 months), in a reasonably wellunderstood reach of aggrading gravel-bed river. These are then applied to a baseline hydrograph, as well as hydrographsscaled for climate change, based upon the predicted monthly percentage precipitation changes simulated by HADRM3for periods 2050s and 2080s (Hulme et al., 2002).

Methodology

Study siteThe test site in this study is based upon a 6 km upper reach of the River Wharfe, UK (Figure 1). This comprises atypical upland gravel-bed river with a range of 10 to 15 m in width. It is generally single thread, with individualmeander bend series linked by relatively straight reaches. The floodplain is relatively wide (Figure 1), has a gentleslope (c. 00040) but is divided by a mixture of dry-stone walls, hedges and fences. Observations suggest that the dry-stone walls are largely impervious to flow except where there are gates, and exert an important impact on floodrouting. Coulthard et al. (2005) produce Holocene timescale estimates of the aggradation/erosion characteristics ofthis reach (which corresponds to their Reach 2), in comparison with other reaches of the Yorkshire Dales system. Aswith the upper reaches of the Swale and the Nidd, this reach is modelled to have undergone rapid erosion followed bygradual aggradation, commonly over hundreds of years. The timescale for the focus of this paper (present to 100 yearsinto the future) is shorter than that in Coulthard et al. (2005) and the shorter term fluctuations in aggradation and



Figure 1. The catchment and reach under study: the Upper Wharfe, Yorkshire Dales National Park, North Yorkshire. Sedimentsensors were installed in the Oughtershaw and Greenfield subcatchments, and at Beckermonds, Deepdale, Hubberholme, Buckdenand Starbotton.



erosion associated with our timescale will determine whether or not the reach is aggradational. Thus, we includesurveyed results to assess current sedimentation characteristics.

Flood inundation modellingThis research is underpinned by a key development in the modelling of floodplain inundation with complex topolo-gical structure (Lane, 2005; Yu and Lane, 2006a,b; Tayefi et al., in press). A range of studies have demonstrated thatfloodplain topographic features have a controlling role in flow distribution on floodplains (see Zanobetti et al., 1968;Cunge, 1975; Hardy et al., 1999; Marks and Bates, 2000; Bates and De Roo, 2000; Horritt and Bates, 2001a,b;Nicholas and Mitchell, 2003). This has resulted in the development of two-dimensional (2D) floodplain flow modelsin which the depth-averaged form of the NavierStokes equations is simplified through ignoring the associated inertialterms (e.g. Bates and De Roo, 2000; Horritt and Bates, 2001a,b; Bradbrook et al., 2004). Commonly, the channelflows are represented using one-dimensional (1D) models. Strictly speaking, the simplification of the depth-averagedequations restricts the validity of the modelling process to situations where the flood inundation depths are low. Thesecoupled 1D2D models have allowed a much more ready incorporation of topographic data into the flood modellingprocess. However, they have still represented subgrid-scale topographic features using friction rules (e.g. the Manningfriction law), often outside the range of conditions for which the rules were developed and overlooking the arbitrarynature of the original friction laws as they were developed (e.g. see Manning, 1891; Lane, 2005). Yu and Lane (2006a)show that these models have very low sensitivity to friction parameters, amplifying the need to use very high valuesof roughness in order to control model response. Thus, Yu and Lane (2006b) develop a subgrid-scale treatment(FLOODMAP) in which topography is represented through blockage and porosity terms rather than roughnessparameterization, and show that this results in a significant improvement in model performance for an urban area,including better time-step control and the recovery of sensitivity to roughness perturbation. Tayefi et al. (in press)show for the case study presented here that this model development is equally necessary for rural floodplains withcomplex topological structures. They compared three options for modelling floodplain inundation of upland floodplainswith complex topographic and topological structure: (1) lateral extension of cross-sections across the floodplain; (2)representation of the floodplain as a series of connected storage cells; and (3) a coupled one-dimensional model of themain channel and two-dimensional diffusion wave model of the floodplain. Model assessment was based upon anevent that occurred on 4 February 2004. This showed that a significantly better level of agreement was obtained withthe coupled 1D2D model than with either the extended section or storage cell approaches, and so the 1D2D modelis used in this study.

Tayefi et al. (in press) detail the 1D2D modelling approach and only a summary is provided here. The mainchannel flow is modelled using HEC-RAS, a hydraulic model developed by the Hydrologic Engineering Centre (HEC)of the US Army Corps of Engineers. This model allows description of the river channel as a series of discrete cross-sections perpendicular to the flow direction. The finite difference procedure for solving the flow equations then definesthe topography of the system domain according to the cross-sections. Water surface elevation and other hydrauliccharacteristics of flow are calculated at each cross-section and all hydraulic structures defined in the domain (e.g.lateral weirs).

The floodplain was discretized to an 8 m resolution, following recommendations for complex floodplains in Yu andLane (2006a,b). Fluxes between cells were set to zero where cells fell adjacent to impervious boundaries. Flowexchanges between the main river and floodplain were calculated based on water levels in the adjacent cells and someform of boundary condition, in this case a weir equation. We assume that the weir segment and water level over theweir are horizontal, allowing weir flow to be computed using the standard equation for a broad-crested weir:

Q = CLH3/2 (1)

where Q is discharge, C is the weir flow coefficient, L is the length of spillway crest and H is the upstream energyhead above the spillway crest (i.e. the evacuating weir segment). It should be emphasized that this type of boundarycondition will exert a crucial control on the magnitude of the flux from river to floodplain. Whilst there has been muchresearch into the parameterization of fluxes from river to floodplain (e.g. Knight and Shiono, 1996), much less of thisresearch has been incorporated into models that couple 1D river and 2D floodplain representations and this remains anarea of significant uncertainty.

Flows from floodplain to river are based upon evaluating the water surface elevation difference between 2Ddiffusion wave model predictions on the floodplain and the 1D model predictions in the river using the 2D diffusionwave approximation. The models are loosely coupled, i.e. the 1D model was used to generate the boundary conditionsnecessary for the 2D model, and it was assumed that return flow to the channel would have a minimal impact upon



downstream flooding. The latter is justified by the relatively small floodplain length being modelled as compared withthe duration of the event.

Geometric dataBoth the 1D and 2D treatments require geometric data, although they use these data in different ways. The channeltopography was described by cross-sections surveyed using a combination of total station and real-time, kinematic,Global Positioning System (GPS) surveys. Both of these provided data precise to better than 002 m. A total of 58sections were surveyed over the 6 km reach, with the greatest survey density within sinuous reaches (at approximatelyone-quarter of the meander bend wavelength) and lower survey densities on straighter reaches (maximum spacing250 m). These surveys were collected on two occasions: December 2002 and March 2004. The GPS system was usedto guarantee cross-section relocation to exactly the same end points in each survey. We then used both the 2002 and2004 data with a February 2004 flood event (see below) in order to assess the impacts of geometrical change uponflood inundation. We also scaled the 2004 event from a 1-in-05-year event to a 1-in-2-year event (also see below) inorder to assess the interactions between a larger event and the same two geometrical configurations.

The floodplain topography was described using remotely sensed Light Detection and Range (LiDAR) data. Thesedata were filtered for trees and hedgerows (assumed to be pervious to flow at the scale of model application, butcompare with Cobby et al., 2003) using automated image processing techniques. Field tests suggested that theprocessed data were precise to better than 020 m. These data were supported by Ordnance Survey Landline data,held in the same co-ordinate system as that used for both the cross-section and LiDAR data surveys. These dataneeded additional field-based classification into whether or not the boundaries were pervious, and over what spatialextent. Parameterization of Equation 1 required additional field survey of individual gates and their width and eleva-tion characteristics, and this was also undertaken using the GPS survey.

Boundary conditionsTwo flood events were modelled in this research: one with an estimated 1-in-05-year return period and one with anestimated 1-in-2-year return period. The 1-in-05-year return period flood was based upon stage data measured duringan event that occurred on 4 February 2004. The stage record was upstream of the study site in a relatively stablebedrock cross-section. Tayefi et al. (in press) explain how these data were used with a Flood Estimation Handbook(Institute of Hydrology, 1999) rainfallrunoff model to provide the discharge data required at the upstream end of thedomain. The 1-in-2-year event was based upon an analysis of measured maximum stages at the upstream end of thedomain to identify a representative 1-in-2-year stage. This was then used to scale the measured 1-in-05-year floodevent linearly. At the upstream end of this reach, both of these flows were well in-bank. Here, the section wasrelatively simple and there were minimal backwater effects, meaning that the stagedischarge relationship is likely tobe linear in the zone to which the scaling was applied. The downstream boundary conditions were set as normal depthusing the local channel bed slope, but the reach was extended beyond the zone of interest such that this boundarycondition had no impact on model predictions. Sensitivity tests, in which both n and channel bed slope were varied,showed that this was the case.

Detailed sensitivity analysis was used to identify the most sensitive model parameters in relation to main channelwater levels. This showed that Mannings n was by far the most sensitive model parameter (Tayefi et al., in press) andthis was the focus of model calibration. It was found that n = 0055 gave the optimum prediction of water levels basedupon 14 locations of estimated maximum water surface elevation, reconstructed from wrack lines measured immedi-ately after the flood event. The 1D model was then used to determine the stage boundary condition for application tothe lateral weirs (using Equation 1) alongside the main river to derive the fluxes required for the 2D model. Flux couldbe bidirectional (i.e. river to floodplain and/or floodplain to river), determined by the difference between water levelsin the river and on the floodplain, and dependent upon the channel adjacent bank height.

System drivers: sediment deliveryAs we wish to situate these results within broader geomorphological processes operating in the catchment, we alsosought to collect geomorphologically relevant data over a longer time period. First, in order to assess the medium-termpatterns of channel change in the river system, we: (a) undertook repeat survey of the cross-sections that we model inthis study; and (b) synthesized together, as far as possible, the cross-section data collected in previous studies of theriver dating back to the 1980s (Reid et al., in press, a). The repeat section survey was limited to the upper 50 per centof the reach, extending from Hubberholme to Heber, until December 2002 but extended to the full, modelled reach,



extending from Hubberholme to Starbotton, for the period December 2002 to March 2004. Second, we installed aseries of sediment sensors (see Figure 1), designed to record the onset and duration of coarse sediment movementduring transport-effective events, in order to understand the spatial patterns of sediment delivery and deposition (Reidet al., in press, a). The sensors comprise pressure plates, 0150 m 0130 m in area, installed flush with the bed andconnected to loggers buried within the bed. The detection of sediment movement rests upon grains rolling or saltatingalong the bed (i.e. there is no detection of particles in suspension) and if they impact upon the pressure plate, a pulseis recorded by the logger. The plates are calibrated to be sensitive to any grain coarser than 0020 m in diameter. Themain appeal of using these is that we were able to install a relatively large number of them (ten), distributed across thecatchment, in order to explore possible coarse sediment sources. There are three main disadvantages to using them.First, each sensor occupies only a small proportion of the potentially active channel width and we must assume thatthe plate location is representative of transport activity across the entire channel width. This is less likely to be thecase at lower transport rates when there is a greater probability that a narrower part of the bed is experiencing activesediment transport. Second, for saltating particles, as saltation step lengths (not particle step lengths as conventionallyexplored when particle transport distances are determined at event timescales) increase, so there is a greater probabil-ity of overpassing of the plate. This probability should be a negative function of grain size and a positive function ofdischarge. These two issues mean that care must be shown in using these sensors to infer absolute coarse sedimenttransport rates. Rather, they are likely to give an index of relative transport variability as well as very useful informa-tion on the onset and termination of transport (Reid et al., in press, a). The cross-section and sediment sensor data areused to contextualize the channel change findings for the December 2002 to March 2004 period.

System drivers: climate changeBy accepting the conclusion that flooding in the future is likely to be affected by climate change, through changes inrainfall event characteristics, future climate conditions must be modelled. Here, we use a simple method to translatespecified changes in climatic inputs into changes in hydrological responses. This involved (i) the application of ahydrological model using current (i.e. observed) climate data (see Tayefi et al. (in press); (ii) the definition of climatescenarios; (iii) perturbation of the original (i.e. observed) input climate data accordingly; and (iv) running the hydro-logical model under future climatic condition. At present, the prime source of information regarding potential climatechanges are Global Climate Models (GCMs) whose results can be interpreted at the scale of individual grid boxes(typically 300 km). In turn, these can be used in combination with regional topographic and climatic characteristics todrive regional climate models like HADCM3.

GCMs attempt to represent the climate and physical process of the atmosphere, ocean, cryosphere and land surfaceusing a three-dimensional grid across the globe (Prudhomme et al., 2002). Although the.y are often run at finetemporal resolutions (e.g. 1530 minute time-steps), the most reliable output variables are at the daily scale (for large-scale circulation indices) and the monthly scale (for weather variables such as temperatures and precipitation)(Prudhomme et al., 2002). GCMs simulate current and future climate elements, according to different global emissionscenarios. The emission scenarios are based upon varying assumptions about future population and economic growth,technological advancement and social attitudes towards energy use (Reynard et al., 2001). None of these scenariosincludes targeted global or national strategies but they assume different development paths for the world. The mostcommonly applied emission scenarios are the SRES scenarios developed by the Intergovernmental Panel for ClimateChange (IPCC) and labelled Low Emissions, Medium-Low-Emissions, Medium-High-Emissions and High-Emissions(IPCC, 2001).

The period 19611990 has been chosen internationally as the baseline for predicting future climate changes (IPCC,2001; Hulme et al., 2002). Future climate change results are mostly presented for three different time periods: the2020s, the 2050s and the 2080s. The 2020s, 2050s and 2080s present the periods 20112040, 20412070 and 20712100, respectively. HADCM3 has been run for all four of the SRES scenarios and for three future time slices. The datasets available from the UK Climate Impacts Programme 2002 (Hulme et al., 2002) include 14 climate variables at aspatial resolution of 50 km in the form of monthly averages for the 2020s, 2050s and 2080s time slices. Due to thehigh degree of variability exhibited in the UK climate, average results for a 30-year period are generally consideredmore reliable than results for a single year (Reynard et al., 2001). In this paper, in terms of climate change, we focusonly on precipitation changes and then only on 2050 and 2080 simulations. UKCIP02 scenarios have been producedfor all four of the SRES greenhouse gas emission scenarios. However, only A2, the medium-high emissions scenario,is considered here as it is the only one to have been fully simulated by HADRM3.

As discussed, the HADRM3 precipitation predictions are only reliable at the monthly timescale. Given the temporalresolution required for hydrological modelling they must be further downscaled. The monthly percentage precipitationchanges simulated by HADRM3 for periods 2050s and 2080s under emission A2, for a 50-km grid within which the



study site, are shown in Table I. As Table I shows, the UKCIP02 scenarios indicate an increase in the seasonality ofprecipitation for North Yorkshire in the future with a progressive increase in winter rainfall and decrease in summerrainfall (Hulme et al., 2002). In order to generate flows scaled for climate change effects, we scaled the 15-minuteprecipitation data used to generate the 4 February 2004 peak discharge using a proportional approach. From Table I,the proportional upscaling was 85 and 150 per cent for scenarios in the 2050s and 2080s, respectively. Due to itssimplicity, this type of change has been frequently applied in literature (e.g. Arnell, 2003; Prudhomme et al., 2003).The scaled precipitation data were applied to the Flood Estimation Handbook (Institute of Hydrology, 1999) rainfallrunoff model assuming no change in the associated parameter values. The rainfallrunoff method includes parameterssuch as standard percentage runoff (SPR) and base flow, which depend on parameters that may be climate-dependent(e.g. due to antecedent condition effects). We are currently exploring continuous hydrological simulation for thecatchment to address this issue. However, the estimated increases in the 2004 peak flow for the 2050s and 2080sscenarios of 82 per cent and 147 per cent matched the findings of Dugdale (2003) who applied a modified version ofTOPMODEL (Lane et al., 2004) to a subcatchment of the study area and found similar percentage increases under aproportional downscaling of climate parameters.

Results

Short-term channel morphological changes and sediment delivery.In order to contextualize the subsequent geometrical changes and flood inundation response, this section summarizesthe short-term changes in channel morphology and sediment delivery that occurred during and immediately before thestudy period. The cross-section surveys suggested that, over the period December 2001 to March 2004, the upper partof the study reach was aggradational, with a reach- and width-averaged mean rise in bed levels of 022 0021 m, andthis fits with much longer timescale simulations of this reach which also suggest aggradation (Coulthard et al., 2005).Over 80 per cent of the net in-channel aggradation measured during the period December 2003 to March 2004occurred in the February 2004 flow event modelled here. This change was not distributed evenly throughout the studyperiod (Figure 2). The bed-level changes were aggradational except for the period December 2002 to March 2003,which was relatively dry for a winter period and, unlike all of the periods shown, experienced no bankfull floodevents. The study reach was surveyed in full from December 2002 onwards and, during this period, the reach- andwidth-averaged mean rise in bed levels was 012 0014 m. This corresponds to the aggradation that occurredbetween the two geometrical boundary conditions applied to the flood inundation model.

Figure 3 shows the spatial distribution of the measured mean bed level changes in terms of erosion and deposition.This shows substantial spatial variability associated with these mean changes. It is tempting to infer the passage ofwaves of deposition and erosion from these data, but our observations suggest that this is problematic, as thesepatterns hide substantial transfer of material into store for periods greater than the measurement period, as well as the

Table I. Percentage changes in precipitation relative to the 19611990 baseline, as simulated by HADRM3 for the 2050s and2080s using emission A2 (Hulme et al., 2002)

Month Percentage change in precipitation

2050s 2080s

January 111 195February 85 150March 45 80April 13 23May 86 152Jun 157 277July 207 364August 206 362September 138 244October 37 64November 55 96December 107 188



release of material that has been in store for considerable time. However, the observations do suggest that there issubstantial spatial variability superimposed upon the net aggradational trend, and that this will have implications forwhich reaches deliver water to the floodplain and which parts of the floodplain are inundated. Situating this reach in alonger time frame of analysis is more difficult as the cross-section measurements have been undertaken for a numberof purposes (including flood risk assessment and geomorphological analysis) and by a number of different surveyors.The cross-sections were rarely co-located and cross-sections were not always surveyed into a reliable fixed datum.However, Figure 4 shows a schematic of river channel response dating back to the 1980s for the upper 50 per cent ofthe reach. Between 1982 and 1990 a gravel trap was installed towards the upstream end of the reach and extensive

Figure 2. Changes in reach- and width-averaged mean bed level from December 2001 to March 2004. Vertical error bars showestimated mean bed level uncertainty.

Figure 3. Spatial distribution of measured erosion and deposition for the five study periods. Each epoch is centred on ahorizontal grid line with the measured change corresponding to erosion when above the grid line and deposition when below: 25is the no change line for the December 2001 to March 2002 epoch; 50 is the no change line for the March 2002 to December2002 epoch; etc. The vertical grid line is the downstream end of the gravel trap.



shoals of gravel were removed and the channel realigned. In both periods shown in Figure 4, the gravel trap reach wasdepositional and, through time, this deposition appears to have extended upstream: much of the river downstream ofthe sudden reduction in valley gradient at Hubberholme (Figure 4a) was depositional. Downstream of the gravel trapreach, there was evidence of erosion (Figure 4a) that appeared to extend downstream during the 1990s (Figure 4b),although our cross-section surveys suggested that deposition had begun to occur in parts of this zone from 2000onwards. This emphasizes the difficulty of generating clear conclusions about secular erosion and deposition trendswithout higher frequency data (in space and time) before 2000. From the Buckden Bridge bend, patterns of erosionand deposition appear to be relatively stable over the time period of available data: the bend itself is generallydepositional; there is then a straight reach that is generally erosional; and finally there is a weakly sinuous anddepositional reach until a sudden realigment of the river at the downstream end of the upper reach, where there is asmall amount of erosion evident.

Our general interpretation of these results is that the short-term (2000 to present) response of the study reach isexpressed in terms of aggradation. In general, from the restricted data that are available between 1980 and 2000, thisshort-term pattern mirrors longer term change. Indeed, the rationale for gravel trap introduction during the 1980s wasto manage gravel accumulation which was blamed for a high frequency of flood events in the system: sedimentaccumulation was estimated to be between 043 m and 14 m in depth in the gravel trap reach between 1966 and 1981(Stewart, 1984), and this was linked to a frequency of flood inundation of between 20 and 40 times per year

Figure 4. Schematic maps of change inferred from available cross-sectional data for the period 1982 to 1990 and 1990 to 2000.Tributary streams are shown, some of which connect with the river, some of which do not. Lighter shading is depositional, darkershading is erosional.



(Yorkshire Water Authority, 1983; Hey and Winterbottom, 1990). After installation, the gravel trap appears to haveacted as a sediment transfer discontinuity for a short period of time. However, it filled extremely rapidly (within twoyears of becoming operational) and it was deemed too expensive to empty it again. In July 2002 it was removed andit is possible to detect the effects of this in the March 2002 to December 2002 survey (Figure 3, erosion upstream ofgravel trap associated with gravel removal) and establishment of a clear zone of gravel accumulation downstreamfrom the trap in the subsequent survey periods.

The second major source of evidence that can be used to understand the geomorphological setting of the study sitecomes from the sediment sensors. Based upon data given in Reid et al. (in press, a), Figure 5 shows the relative intensityindex for sensed sediment based upon Hubberholme, Buckden Bridge and Starbotton (see Figure 1b for locations). Thisshows a dramatic reduction in the amount of sediment that is sensed between Hubberholme and Starbotton (approach-ing three orders of magnitude) and this is likely to be a realistic trend, even given the sensor uncertainties describedabove. However, with distance downstream, the maximum intensity during the largest event recorded (4 February2004) increases. This helps us to explain why the study tends to be depositional. There is significant deliveryof sediment from the catchment upstream of Hubberholme, and this appears to be associated with a very large numberof transporting events, not least because the main river and tributaries are much steeper and often bedrock, allowingmuch more ready transfer of coarse sediment (Reid et al., in press, a). Only the largest events are capable oftransporting material within and through the Hubberholme to Starbotton reach. Little sediment was observed to movebeyond Starbotton during the study period (Reid et al., in press, a) which can be attributed to an alluvial fan thatextends across the valley at this point. The net effect of high sediment delivery to Hubberholme and low sedimenttransfer beyond Starbotton is the substantial bed aggradation recorded in our cross-section surveys, and it does seemto be the case that the alluvial fan at Starbotton is contributing to aggradation upstream (Coulthard et al., 2002).

Impact of channel geometry changesGiven the above evidence of bed aggradation, this section assesses the effects of measured aggradation upon theinundation associated with a 1-in-05-year event that occurred on 4 February 2004 and the upscaling of the 1-in-05-year event to a 1-in-2-year event. The effects of the geometrical changes were a reduction in the magnitude of thebankfull discharge, not because of a reduction in flow magnitude, but because of a rise in bed level. For those sectionsthat were bankfull in both the December 2002 and March 2004 cases, the average bankfull discharge fell from 306 to287 m3 s1 (i.e. a 61 per cent decrease), associated with an average rise of 012 m in bed levels along the reach,although there was substantial spatial variation in both the magnitude of bankfull discharge and the bankfull dischargereduction. Figure 6 shows the inundated area through time for the 2002 and 2004 geometrical scenarios, using the1-in-05-year and 1-in-2-year floods. It is clear that the return period of the two floods has a major impact upon themaximum inundated area. However, the 16 months of measured sedimentation resulted in increases in inundationarea of 57 per cent and 71 per cent for the 1-in-05-year and 1-in-2-year floods, respectively. Visual comparison ofthe inundated areas obtained for the time at which the peak flow passed the downstream end of the reach, using the

Figure 5. Downstream changes in sediment sensor results for the period March 2003 to March 2004. The relative intensity indexis the number of clasts per second scaled by the channel width (see Reid et al., in press, a). The maximum recorded intensity is thelargest number of clasts recorded per second at each sensor.



two geometry scenarios and the two flood return periods (Figure 7), shows that these increases are predominantlyassociated with the upstream part of the study site. The average out-of-bank flow duration increased from 302 minutesto 330 minutes (i.e. 95 per cent) and the downstream distribution of durations for the 1-in-05-year event and the twogeometries is shown in Figure 8. It is possible to associate the changes in bed level shown in Figure 3 to the durationchanges shown in Figure 8. The relevant change series is the sum of the top three curves in Figure 3. Figure 3 showssubstantial aggradation, that begins about 300 m downstream and peaks at about 600 m downstream. In Figure 8, fromabout 300 m downstream, there is a rapid rise in inundation duration to a peak at about 400 m downstream. Thedifference between the location of the peak increase in inundation duration and the peak aggradation is interesting andimplies that the onset of upstream aggradation causes flow to go out-of-bank further upstream. The increased flux tothe floodplain reduces the flux in the river such that whilst the in-channel aggradation continues to increase in magnitudedownstream of 400 m, this is countered by there being smaller flows in the channel. This type of process occurs overa larger scale between 300 m downstream and 1600 m downstream, where the increase in duration of inundation withthe March 2004 geometry to 800 m downstream is countered by a decrease in duration of inundation after 800 mdownstream: the increased flux to the floodplain in the upstream reach is countered by reduced flows further down-stream. The key point emerging from this discussion is that the effects of in-channel aggradation can lead to quitecomplex changes in flood inundation as a result of a changing partitioning of flow between channel and floodplain.

Under both geometric scenarios and flood events, changes in the downstream inundation extent were partly limitedby high levels of valley confinement. Thus, whether or not geometrical changes impact upon inundated area dependsupon both the magnitude of the flood event and the geometrical change in relation to the degree of valley confinement.However, as the geometrical changes lead to a reduction in bankfull discharges, they will also increase the frequencyof onset of flooding, so reducing the return period associated with a given inundation area. Similarly, flows with returnperiods close to bankfull will be most sensitive to geometrical changes although, for the two geometries simulatedhere, the peak flow magnitudes were somewhat larger than the mean bankfull discharge.

Impact of predicted climate changesFigure 9 shows the time-series of inundated area for the present event, the 2050s and the 2080s, for the 1-in-05-yearevent. The peak inundation extent is estimated to increase by 122 per cent for the 2050s and 216 per cent for the2080s. The inundation maps (Figure 10) show that the patterns of inundation remain similar and that the prime sensitivityof the system is to changes in the volume and duration of out-of-bank flow, which results in the increase in inundated area.

Combined geometrical and climate changeFinally, we combined the 1-in-05-year event with the 2002 to 2004, 2050 and 2080 simulations. The effects ofthe December 2002 to March 2004 sedimentation in isolation were a 57 per cent increase in inundated area by 2004.The 2050s climate scenario resulted in an increase of 122 per cent and the 2080s in an increase of 147 per cent.

Figure 6. Inundation through time for the December 2002 and March 2004 bed geometries and two different flood magnitudes.



When the 2002 to 2004 sedimentation was combined with the 2050s climate scenario, the increase was 382 per centand with the 2080s, the increase was 521 per cent. This shows non-linear interaction between the two drivers of theflood system, not surprising as the lateral inundation extent will be controlled by the number of cross-sectionsexperiencing out-of-bank flow and the magnitude and duration of those out-of-bank flows, i.e. a threshold process.

Discussion

The main finding that emerges from this research is that relatively short duration channel configuration changes, in thiscase over a 16-month period, can lead to substantial changes in inundated areas for 1-in-05 and 1-in-2-year floods,

Figure 7. Predictions of areas inundated in the 1 in 05 and 1 in 2 year events simulated in this study, with 2002 and 2004geometries respectively, both at the time of peak flow at the downstream boundary.



Figure 8. Duration for which out-of-bank flow was exceeded as a function of distance measured upstream from the downstreamend of the reach.

Figure 9. Inundation through time for the February 2004 event as baseline and with scaling to the 2050s and 2080s.

in an upland environment associated with coarse sediment transfer. For the 1-in-05-year event, the increase ininundated area was almost one-half of the increase in inundated area estimated as resulting from simulated climatechange to the 2050s. Key to this increase was coarse sediment aggradation associated with a depositional reach ofriver (Figure 2), on which was superimposed quite significant spatial variability in the deposition process (Figure 3).The non-linear nature of the onset of floodplain inundation (i.e. associated with the onset of overbank flow) meant thatwhen these geometrical changes were combined with forecast climate changes, the increases in inundated area wereamplified over the increases associated with each change in isolation. In other words, in this case study, in-channelsedimentation increases the sensitivity of flood inundation to future climate changes. The comparison of the 1-in-05and 1-in-2-year flood events emphasizes that when this kind of analysis is couched in terms of event return periods,this appears to be a much more important impact upon the inundated area (Figure 6) than either short-termgeomorphological response or climate change. However, return periods can be misleading, not simply because of thereliability in their determination, but also because they are commonly based upon the peak magnitude of flow, whichmay be only weakly related to the magnitude and duration of out-of-bank flow. The kinds of geomorphologicalchanges described here do not impact upon discharge return periods, but they do impact upon the return periods offloodplain inundation, such that there could be a major increase in the magnitude and frequency of inundation eventsin any river, or river reach, that is aggradational, with no change in the magnitude and frequency of imposed flows.Some distinction is required here between changes in inundated area and changes in inundation depth. Once a flood islarge enough to occupy the full valley width, bed aggradation will only impact upon the inundation depth. For larger



floods, for which all sections are at bankfull, aggradation will result primarily in increases in the duration of out-of-bank flow, which will be reflected in increases in inundation depth and duration. For the size of floods shown inFigure 7 (i.e. 1-in-05-year, 1-in-2-year), aggradation effects will be manifest as changes in both return periods ofout-of-bank flow and increases in inundated area.

There are interesting parallels here with the common distinction between climate variability and climate change.Robson (2002) notes that some current hydrological changes can be attributed to hydrological variability and thatperiods longer than 40 years are needed to distinguish between climate variability and climate change. Under climatevariability, a climate may differ from one period to the next but under climate change, a long-term alteration in theclimate is occurring. However, what may appear to be variability in the short term may be diagnostic of change in thelonger term. This is the sense in which the measured aggradation since 2000 (Figure 2) combined with the probabletendency towards aggradation within this system that can be traced back to the 1980s (Figure 4) may be: (1) a short-term response to temporarily enhanced sediment delivery associated with a period of more intense storm activity,particularly given empirical (Reid et al., in press, a) and modelling evidence (Reid et al., in press, b) of the sensitivityof sediment delivery to extreme rainfall events that do not require high levels of antecedent catchment saturation;(2) part of a long-term trend towards higher sediment delivery rates due to a long-term increase in storm activitylinked to climate change; and/or (3) associated with a reach of river that is aggradational as a result of larger-scalegeomorphological controls.

Coulthard et al. (2002) and Reid et al. (in press, a) describe the large-scale geomorphological setting that leads tothis reach being aggradational, but it is quite possible that variability and change in sediment delivery rates are also acontributing factor. We know that UK upland rivers can go through distinct cycles of alluviation in response to bothshort- and long-term climatic fluctuations (e.g. Macklin et al., 1992) and that, at present, upland landscapes are moresensitive to climatic fluctuations as a result of historical land-use change (Macklin and Lewin, 2003). For instance,Coulthard and Macklin (2001) used modelling to show a severely non-linear interaction between climate change anddeforestation in a similar upland river catchment. S. C. Reid et al. (unpublished work) applied the same climatechange scenarios used in this paper to a continuous simulation, coupled hydrologysediment delivery model (Reidet al., in press, b). For a subcatchment of this study reach (Buckden Beck, Figure 1) they estimated that, by the 2030s,the volume of sediment delivered annually would increase by between 7 and 40 per cent according to the nature ofthe changes in precipitation (essentially, the extent to which the precipitation increases is distributed evenly across allrainfall events as opposed to being associated with a smaller number of additional extreme events). By the 2080s, theincrease was estimated to be between 28 per cent and 68 per cent. It appears that the indirect impacts of the climate

Figure 10. Predicted inundation patterns for a 1 in 05 year event for the 2050s (a) and the 2080s (b), both at the time of peakflow at the downstream boundary.



change signal upon sediment delivery will act so as to reinforce the importance of short-term aggradation upon themagnitude and frequency of floodplain inundation, and that short-term variability in aggradation rates may be super-imposed on top of a longer-term, aggradational trend. This emphasizes that the changing flood risk identified here willbe strongly impacted upon by climate change, not only because of the direct impacts of climate upon the magnitudeand frequency of high flow events, but also because of the indirect effects of changing climate upon coarse sedimentdelivery which in turn will impact upon coarse sediment delivery and hence aggradation rates. There has been muchdebate over the extent to which land management impacts on catchment hydrology might be contributing to changingflood magnitude and frequency (see review in OConnell et al., 2005). There has been much less acknowledgementof the extent to which land management practices have sensitized river basins to climate change (Coulthard et al.,2002; Macklin and Lewin, 2003) and in so doing are indirectly exacerbating flood magnitude and frequency. Thiscase study emphasizes the need to broaden the current focus of debate on the impacts of climate change upon riverflows to include a much stronger engagement with catchment sediment delivery processes and how these interact withclimate change.

The observations from this case study point to two important management implications. The first is in terms ofpractical aspects of flood risk mapping. Common practice, at least in the UK, involves specially commissionedgeometrical surveys, application to one-dimensional hydraulic models, and model calibration based upon adjustmentof model parameters, notably channel friction (commonly Mannings n), to predict the water level recorded in histori-cal events. If there has been bed aggradation since that historical event, then a lower value of n will be required toreproduce that event than if the historical flood had been combined with historical geometrical survey. Thus, whenmodelling future events, models calibrated in this way will underestimate water levels, and hence inundated area,inundation depth and flood risk, as a result of having a calibrated value of n that is too low. The converse would occurin systems that are degradational.

The second management issue is in relation to river management more generally, especially in upland environmentsand where large-scale geomorphological controls have a major impact upon the sediment transfer process. In thisstudy, despite the delivery of large volumes of sediment being linked to major flood flows, the sudden reduction inmain valley slope at Hubberholme (Figure 1) coupled to the valley constriction at Starbotton (Figure 1) associatedwith a major alluvial fan, creates a zone of natural sediment deposition. The extent to which aggradation is a moregeneric characteristic of upland temperate environments, and hence the representativeness of this case study, needsfurther consideration. However, this case study does raise a more generic question in relation to the appropriateness ofcommon assumptions made in river engineering with regard to river management. Our field observations show that,for this case study, the natural river channel response to coarse sediment delivery is: (a) localized sedimentation inpoint bars, commonly attached to either meander bends or the inside edge of weakly sinuous reaches; coupled to (b)erosion of mixed fine and coarse material from channel banks on the channel margin opposite the zone of deposition.We can hypothesize that this transfer of coarse sediment to storage, through channel migration, should be an integralcontribution to the observed rapid downstream fining, as well as a mechanism that leads to long-term valley fill.Increasing sediment throughput through river realignment will simply transfer the sediment aggradation downstream,eventually introducing coarser material into river reaches where it would not normally be found.

Where river bank stabilization has been advocated to protect farmland, properties and footpaths, the erosionrequired to maintain channel capacity will be prevented. There is still a general view in some river managementcommunities that bank erosion (as opposed to artificially enhanced bank erosion, such as due to stock poaching) is aproblem that has to be prevented, rather than a natural geomorphological response to high rates of sediment delivery.Consultant reports from the late 1990s for the reach of river considered in this paper advocated focusing managementon stretches where eroding banks should be stabilized (e.g. RKL Arup, 1999a,b). This assumes that a channel can beengineered to be robust, to transfer sediment through it and so to absorb changes in sediment supply with only minoradjustment in its form. However, channel morphology and stability in fact reflect the net sediment budget (e.g. Hooke,2003), which in turn reflects: (i) the net erosion and deposition observed; and (ii) the supply and connectivity ofsediment from the hillslopes and upstream reaches. Where river management restricts (a) lateral movement of the riverand (b) transfer of sediment into floodplain storage, the channel bed becomes responsive (e.g. Schumm, 1979; Harvey,2001; Hooke, 2003), and one response is aggradation. This will reduce channel conveyance and lead to the kinds ofimpacts described above in relation to flood risk. Thus, in addition to naturally high rates of sediment delivery, thiskind of river management exacerbates flood risk. The Holocene record (e.g. Macklin and Lewin, 2003) tells us thatthese systems can experience large increases in sediment delivery over relatively short time periods and it is thishistorical record that emphasizes the need to incorporate changes in conveyance due to sediment aggradation moreexplicitly into flood risk assessment. Whilst this research is based upon an upland case study, the observations willapply to flood risk issues in any upland or piedmont system, where there is coarse sediment delivery and accumula-tion, but also where active river management precludes the possibility of coarse sediment transfer into floodplain



storage. Research by Pinter and Heine (2005) shows that river engineering on a river with a sand and silt bed may alsolead to systematic increases in stage for a given flow, and this may make these kinds of issues of relevance in otherriver systems.

Conclusions

This paper has shown the importance of considering the effects of coarse sediment delivery and the associated bedaggradation in the case study of a temperate upland environment. Whilst based on a case study, it has some veryimportant implications for climate change studies, flood risk assessment and river management more generally. In thisstudy, for a flood with a 1-in-05-year event, short-term bed aggradation (over a 16-month period) was shown to resultin about half the inundated area increase that results from applying climate changes estimated for the 2050s to thesame event. Consideration of the joint impacts of climate change and bed aggradation emphasized the non-linearnature of system response, and the possibly severe and synergistic effects that come from combined direct effects ofclimate change and sediment delivery. Such effects are likely to be exacerbated further as a result of the impacts ofclimate change upon coarse sediment delivery. When couched in conventional flood frequency terms, these changesdo not result in a change in the return period of extreme flow events, but rather a reduction in the return period of theflow events that lead to floodplain inundation. Thus, analyses that focus upon the changing magnitude and frequencyof flow events will overlook a key influence upon the return period and magnitude of flood inundation events. This isparticularly important as future climate changes will impact upon sediment delivery rates (S. C. Reid et al., unpub-lished work) as well as upon flows. Currently, the effects of bed aggradation are commonly overlooked in flood riskmapping exercises that require calibration based upon combining contemporary bed surveys with historical floodoutlines, and this will lead to underestimation of the magnitude and frequency of floodplain inundation in an aggradingsystem. Finally, the research emphasizes the importance of considering the natural process by which a river adjuststhrough lateral migration to high rates of coarse sediment delivery, as river management activities that seek to slowthis process may exacerbate the effects of coarse sediment delivery upon river conveyance and hence upon flood risk.Ultimately, and despite repeated calls (e.g. Sear et al., 1995), we still do not properly address geomorphologicalprocesses in flood risk management studies and we desperately need tools that can predict the medium-term (annual todecadal) response of river beds and river banks to sediment delivery in order to assess flood risk impacts. This shouldinclude consideration of those types of system where aggradation issues are relevant. Whilst the prime candidates mayseem to be those with gravel, or coarser, bed material, aggradation-related shifts in stagedischarge curves have alsobeen observed in some systems with sand/silt boundaries (e.g. Pinter and Heine, 2005).

AcknowledgementsThis research was funded by a PhD scholarship awarded to V.T. by the Iranian Science, Research and Technology Ministry and byNERC Connect Grant NER/D/S/2000/01269 awarded to S.N.L., Professor A. McDonald (A.McD) and Professor M. J. Kirkby, andby Environment Agency R&D award E1-108 awarded to A.McD and S.N.L. Downscaled climate data were provided by the UKClimate Impacts Programme funded by the UKs Department of the Environment, Farming and Rural Affairs. The Design andImaging Unit, Durham, assisted with figure production. Tom Coulthard and an anonymous reviewer provided very valuable reviewsof this paper.

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