Reach-scale sediment and barform dynamics of the wandering gravel-bed River Nent, Cumbria, UK *George Heritage Division of Geography, School of Environment and Life Sciences, University of Salford, Manchester, M5 4WT. UK (*corresponding author. g.l.heritage@salford.ac.uk) David Milan & Andrew Large (Department of Geography, University of Newcastle upon Tyne, NE1 7RU, UK Ian Fuller Division of Geography & Environmental Management, University of Northumbria at Newcastle, NE1 8ST The use of DEMs to investigate large scale channel change is particularly relevant to systems displaying widespread lateral and downstream instability such as braided and wandering channels. This paper presents the first results of a study initiated on the River Nent, Cumbria that aimed to determine morphological change and process operation through the repeat survey of two unstable multi-thread reaches. Detailed tachometric techniques generated the coordinate data for the construction of DEMs based on rigorous geostatistical criteria settings to define the interpolated surface (Chappell et al. in prep.). The surveys of both unstable reaches record the effects of a major flood followed by an extended series of lower flow events over a 9 month period on channel and bar morphology. Channel change patterns are presented based on comparison of successive survey DEM surfaces in order to highlight loci of instability and generate a volumetric estimate of sediment storage change between surveys. The results of the three surveys are presented in the context of local sediment supply changes and mechanisms of sediment transfer and barform change. Abstract The study site is located on the River Nent, in the North Pennines, England. It drains an upland area (basin altitude 341 m AOD) of 29.4km 2 before flowing into the River South Tyne (Fig. 1). The geology of the basin consists of sedimentary rocks, principally sandstone shale, and limestone that has undergone extensive base metal mineralisation to produce deposits of galena (PbS) and Sphalerite (ZnS) along with other economically exploitable minerals. Mining of these ores reached a peak between 1850 and 1900, declining to zero in 1950. The study reach is located on the River Nent at Blagill, Cumbria (NT 743469). The river may be described as wandering in nature, while the study reach is approximately 0.8km and consists of two unstable multi-thread channel zones, 0.2-0.3km in length, separated by a stable single-thread reach of approximately 150m (Fig. 2). Upstream instability zone This is the smaller instability zone of the two (6796 m 2 ) consisting of 3 primary bar units (Fig. 2). The zone is composite in nature and displays extensive evidence of sediment erosion and chute channel development. Bars 1 and 3, towards the left bank of the instability zone are low relief features extending only 0.1-0.3m above the summer low flow water surface and grade into riffle heads at their downstream ends. Bar 2 is larger and displays several surfaces ranging in elevation from 0.1m to 0.5m above the low flow water surface on a vegetated surface close to the right bank of the instability zone. Downstream instability zone The second instability zone is larger (12545 m 2 ) and more complex, comprising 4 principal bar units (Fig. 2): an extension of the point bar complex at the entrance to the reach (Bar 1) and a large mid-channel bar sequence (Bar 2) bounded by bar complexes on either side close to the left and right banks of the unstable zone (Bars 3 and 4). Bar 1 consists of the downstream extension of the right bank point bar, heavily dissected by distributary channels as the flow crosses the channel from the left to the right bank. This process has formed small low relief active gravel bars alternating with steep riffle fronts. Bar 2 is the largest feature in the downstream instability zone and is characterised by several higher bar surfaces, towards the upstream end of the feature, that are well vegetated. Transport activity is evident on lower surfaces that represent the eroded remnants of larger bars and these surfaces remain unvegetated. Bar dissection is also evident, particularly on the distal surface of the feature where several chute channels can be identified. Bar 3, towards the left bank of the unstable zone is an older feature, whileBar 4 is a small lower elevation feature that has evolved significantly over the two years of survey. 3 2 1 4 1 2 3 0m 100m 200m Downstream Instability Zone Upstream Instability Zone 1. Study reach characteristics Morphological survey and hydrological response Channel change was monitored across the whole study site on three occasions between the summer of 1998 and the summer of 1999 (July 1998, October 1998 and June 1999). July 1998 forms the baseline against which subsequent change was evaluated, the October 1998 survey was conducted following a significant rainfall event within the Nent catchment which resulted in flood conditions within the study reach. The effects of a series of lower flow events was captured by the June 1999 survey (Fig. 3). The accuracy of surfaces created using kriging was evaluated by conducting a geostatistical analysis of the spatial elevation data, which compared the interpolated surface height at the measured points over the reach with the surveyed data (Chapell et al. in prep). 0 20 40 60 80 100 120 1 45 89 133 177 221 265 309 353 397 441 485 529 573 617 661 705 749 793 837 881 925 969 1013 1057 Hours Rainfall (mm) Changes to the upstream instability zone (Figures 4a and 4b) are the result of the large flood of October 1998 and the minor flows which followed. Between July 1998 and October 1998 Bar 3 experienced significant deposition upstream and towards the left bank, reducing the capacity of the entrance to the left bank distributary. The surface of Bar 2 experienced minor scour over its distal surface and associated deposition at the bar head, partially infilling the entrance to the right bank distributary. Bar 1 also displayed minor overall deposition, partially infilling the entrance to the left bank distributary. There was a net loss of sediment across the zone which resulted in an increase in riffle area and a better defined chute network (which is consistent with a volumetric loss of 114 m 3 of sediment from this sub-reach). Volume changes between successive surveys were calculated to establish the temporal losses and gains of sediment within each of the sub-reaches. No statements can be made of flux rates as inputs and outputs to and from the wandering reach were not measured over the 13 month monitoring period. It is clear that, between July 1998 and October 1998, the upstream instability zone experienced sediment transfer resulting in minor losses whilst the downstream instability zone displayed a corresponding but higher magnitude volumetric gain. The effects of a series of lower magnitude flows between October 1998 and June 1999 resulted in a small increase in sedimentary deposits in the upstream instability zone and a significant loss from the downstream instability zone. Over the entire study period, minimal overall change in deposited volume in the upstream instability zone was observed, whereas the downstream instability zone experienced some sediment loss. 2. Volumetric and morphological changes Table 1. Estimated volumetric sediment changes to the instability zones on the River Nent and Blagill between July 1998 and June 1999. Instability zone Upstream July 1998 – October 1998 October 1998 – June 1999 Deposition 1284 1400 Upstream (area=6796m 2 ) erosion 1398 1250 Deposition 2073 1007 Volume transfer (m 3 ) Downstream (area=12545m 2 ) erosion 1623 1950 Upstream -114 150 Net volume change (m 3 ) Downstream 450 -943 Upstream 17 22 Aerial change (mm) Downstream 36 75 Phase 1: Transverse chute activated splitting bar into 2. Phase 2: Deposition at the head of the central distributary on the margins of the new main channel. Blocking of the entrances to the two side distributaries. Chute channel activated across bar top creating new riffle front . Phase 3: Large central bar formed through the cutoff of the central distributary. Phase 1: Left bank distributary blocked flow concentrated through central chute , . Phase 2: Central distributaries blocked, flow concentrated through right bank chute. Phase 3: Bar extension and distributary rationalisation. Large scale bar extension and distributary rationalisation appears to be occurring with the infilling of riffle fronts at the downstream end of linear bar complexes. Examples of this phenomenon exist between Bar 1 and Bar 4 in the downstream instability zone and between Bar 3 and Bar 1 in the upstream instability zone (Fig. 6 and Fig. 7). At high flows discharge is concentrated along the main distributary within both instability zones. Flow, is however, sufficient across the bar surfaces to transport sediment across the feature and into the low flow riffle fronts. This material is retained in these topographic lows away from the main distributary channel effectively blocking these low flow channels and linking the upstream and downstream sub-bar surfaces. Within the upstream instability zone, sediment has accumulated in the riffle front between the two sub-bar surfaces of Bar 3 and, on a larger scale, between Bar 2 and Bar 1. This has created a larger bar complex towards the right bank (Fig. 6). Upstream instability zone Within the upstream instability zone sediment has accumulated, causing the flow to expand at the entrance to the two subordinate distributary channels between Bar 3 and the left bank and Bar 2 and the right bank. The primary flow is concentrated along the central steeper principal distributary channel. This channelling has activated a steep riffle front across Bar 3 which now accommodates the main flow reducing the discharge in the distributary between Bar 1 and Bar 2. In turn this has allowed these 2 features to amalgamate as a single bar complex following deposition at the head of the distributary channel (Fig. 6). Downstream instability zone The blocking of minor distributary and chute channel entrances also appears to be a major cause of reach- scale channel and barform change. The downstream instability zone displays a sequence of deposition beginning with the blocking of the left bank distributary channel as a result of flow divergence at high discharge (Fig. 7). This results in the preferential channelling of most of the high energy flow across the instability zone within a steep distributary channel towards the right bank. In turn this creates a lower energy zone in the left bank distributary that accumulates sediment transported from upstream further concentrating the flow in the right distributary. Two further zones of flow divergence further along this distributary result in similar modes of sediment accumulation in the subordinate distributary. The process of sediment deposition at these locations is aided by the antecedent bar geometry which acts as a high flow obstruction to sediment movement. The rationalisation of the majority of the flow into a single principal distributary as the discharge reduces has had the effect of concentrating the flow energy between Bar 2 and Bar 4 resulting in downcutting within this distributary, increasing the channel gradient locally, and reinforcing its position as the new principal distributary. 3. Three-phase model of morphological response 4. Discussion and conclusions The morphologic change recorded across the instability zones of the River Nent at Blagill between July 1998 and June 1999 offer additional support for the mechanisms of barform development proposed in the literature. Channel adjustment occurred with only relatively small changes to the volume of sediment within each instability zone (Table 1). Although the Nent instability zones are very sensitive to change, it does not appear that major changes in sediment supply regime are required to induce distributary rationalisation. This study has highlighted the effect of localised deposition on reach-scale activity with the blocking of small secondary distributaries at elevated flows redirecting the flow energy along steeper distributary channels. Several other factors also appear to have influenced the deposition of sediment and subsequent distributary alteration including: • Deposition at zones of high flow expansion at the head of bar units where the steeper of the two distributary channels directs the principal flow path allowing deposition at the entrance to the subordinate distributary away from the main flow. • Deposition occurs over riffle fronts at elevated flow as gravels are moved over bar surfaces, and extend across riffles at the tail of the unit that are no longer acting as a locus for flow. Relative to the bar tops, riffles effectively form topographic lows which are filled at their upstream end as sediment is deposited in the resultant high flow reduced energy zones. • At high flows, existing bar units act as barriers to the main flow direction. Deposition associated with these units can have the effect of blocking the distributary channel associated with one side of the bar unit. • Bar coalescence is the result of localised blocking of the distributary and chute channel entrances. As a consequence, the bars may split and the former channels be reoccupied if flood flows are sufficient to remove the deposited material and activate the channel again. As such, channel switching between old chutes and distributaries is more likely to form entirely new channels than headcutting. • Morphologic response to the changing flow regime at the instability zone scale is complex, with the downstream sub-reach displaying opposite trends in volumetric sediment change to the upstream sub-reach. This reflects the interrelationships between the flow regime, mobilisation of local sediment stores and upstream sediment inputs. • Significant morphologic alteration is possible as a result of sediment redistribution within the study reach, rather than inputs from upstream of the instability zones. The ability of relatively small volumes of accumulated sediment at strategic locations within each instability zone can generate significant sub- reach scale morphologic change. July 1998 October 1998 June 1999 Days Fig.1 Fig. 2 Fig. 3 July98 - Oct98 NENT UPSTREAM VOLUME CHANGE The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. 5100 5150 5200 5250 5300 5350 0m 40m 80m 4650 4750 4850 Oct98 - June99 NENT UPSTREAM VOLUME CHANGE The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. 5100 5150 5200 5250 5300 5350 0m 40m 80m 4650 4750 4850 -3 -2 -1 0 1 2 3 Upstream instability zone Fig. 4 (a) (b) Fig.5 - - - Downstream instability zone Changes to the downstream instability zone are shown in Figs. 5a and 5b. Between July 1998 and October 1998, the most significant activity involved sedimentation at the entrance to many of the chute channels in the reach. This was allied to the reorganisation of Bar 2 and Bar 4. The period October 1998 to June 1999 is characterised by significant overall sediment loss as a result of upstream deposition on the left bank blocking the entrance to the left bank distributary and diverting flow across the front of Bar 2. This flow concentration led to distributary erosion and rationalisation. Bar 1 extended due to deposition across the old riffle fronts dissecting the downstream end of the feature. Volumetric sediment storage between July 1998 and October 1998 was reduced by bar and chute erosion between October 1998 and June 1999 resulting in a minor net volumetric change to this reach over the entire study period (Table 1). The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. 5000 5050 5100 5150 4 9 0 0 4 9 5 0 5 0 0 0 5 0 5 0 5 1 0 0 5 1 5 0 5 2 0 0 5 2 5 0 5 3 0 0 July98 - Oct 98 NENT DOWNSTREAM VOLUME CHANGE The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. 5000 5050 5100 5150 4 9 0 0 4 9 5 0 5 0 0 0 5 0 5 0 5 1 0 0 5 1 5 0 5 2 0 0 5 2 5 0 5 3 0 0 Oct98 - June99 0m 50m 100m 0m 50m 100m 3 2 1 0 1 2 3 (b) (a) Fig. 6 Fig. 7 Title: UniCrest #1 [DH] (Converted)-5 Creator: FreeHand 8.0 Prev iew: This EPS pi cture was not saved wit h a prev iew i ncluded in it. Comment: This EP S pi cture will print to a PostScript printer, but not to other ty pes of pri nters. UNIVERSITY of NORTHUMBRIA at NEWCASTLE