Hydromorphological Appraisal of the Use of Large Woody Debris in the Restoration of the River Lathkill, Derbyshire
John M. E. Cowx & Ian B. Drew
Adapted from Gordon et al., 2004 Focus on Hydromorphology – but recognising the link with ecology
Raven et al., (2002) - the importance of the physical (hydromorphological) dimension as a supporting element in the ecological restoration of rivers is profound. Chessman et al., (2006) indicate that rehabilitation of geomorphic condition can be critical for the improving the biodiversity. Clarke et al., (2002) argues that both morphological and ecological components of a river system are inherently linked and that ecological goals can only be achieved through geomorphic improvements.
Successful river restoration can only be achieved by a multidisciplinary approach, fully understanding ecological, hydrological and geomorphological process and form Lathkill Dale 1 of 5 valleys comprising the Derbyshire Dales National Nature Reserve managed by Natural England Historical channel modification by human intervention 18th and 19th century lead mining was associated with channelisation and the excavation of drainage soughs. Diagrammatic profile through the edge of the Derbyshire plateau, showing a sough cut to drain the limestone for deeper mining access (Ford and Rieuwerts, 2007). Note: contrary to this diagram the sough at Lathkill Dale was driven in below the level of the river.
The drainage provided by the sough combined with the permeable limestone geology has causes surface flow to dry up in summer months.
Environment Agency daily discharge data for the River Lathkill (1997-2009) (data provided courtesy of Professor John Gunn).
The River Lathkill indicating the perennial and intermittent springs. The reach of the river in which this study was conducted is highlighted in red (adapted from Wood et al., 2005). In the Victorian era the river channel was straightened, clay lined and controlled by weirs in order to establish good conditions for trout fishing.
Natural England & Wildlife Trust Restoration began in 2003 by removing the grasses and reeds which occupied the river and by digging a narrower channel through the otherwise flat topography of the river bed.
In 2006 the topography of the river bed was still relatively even and bed material uniformly distributed. 5 LWD structures of various design, stabilised and anchored with timber stakes and wire, were installed in 2008 There are five anchored wooden structures in this reach all of which are comprised of locally sourced sycamore and elm.
The Lathkill site characteristics offered a unique opportunity to study & sample the river bed around LWD in detail The Project The aim of the research was to undertake a post project analysis in order to evaluate the success of LWD in creating a hydromorphologically diverse river channel with a view to identifying its potential impact on the ecology of the stream.
To identify the hydromorpholoical characteristics of the river, a programme of detailed field mapping was undertaken. Spring 2010: Flow direction & velocity was measured at up to 14 points across 41 cross-sections in the 30m reach. Readings were all taken at a height above the bed equivalent to 0.6 of the water depth. Spring 2010: River bed elevation was determined along the 14 cross sections to produce a topographic map of the river bed Summer 2010: bed material around the woody debris was mapped and sampled for analysis. Initially visual characterisation of the bed material size was used to identify bed units. These were mapped and sampled. From each bed unit a sample was taken up to a trowels depth (approximately 15cm). There was no division into surface and subsurface samples. Trowel sampling was chosen rather than an infield grid pebble count grid technique because it was thought a sub-surface sampling of the river bed would be more representative rather than just a surface sample (Rice and Haschenburger, 2004). The samples were subsequently analysed in the lab using standard techniques: dry sieving of the whole sample, followed by axis measurements and sediment shape and angularity analysis for the coarsest fraction. Flow Patterns - 17th of June 2010
Mapping confirms that the LWD structures are having an influence on flow patterns and shows precisely how direction and magnitude are modified. At the entrance to the reach the channel plan form and flow is relatively uniform in direction but with the line of fastest flow towards the left hand bank. Subsequently the first LWD structure seems to have a minor influence in deflecting flow, while further downstream the effects are more marked demonstrating how the course of the thalweg can be manipulated. This will consequently influence the plan form of the channel which has been made less uniform in the LWD reach with bank erosion has occurring where water flow velocity is increased and deflected towards the channel side. Flow has also forced its way round stream side structures resulting in additional areas of bank erosion – only the first side structure built well into the bank has avoided this. Flow is generally slowed down behind the LWD where in some case areas of still water or reversed flow have been created. Bed Topography Summer 2010
The contours shown depth BELOW bank height. They identify areas of deeper channel which indicates scouring is taking place, particularly beside channel side LWD structures and in front of the mid channel structures and generally reflecting the new line of fastest flow (as indicated previously). In future the scour could result in undercutting of the structures which would result in greater uncertainty in regards to the nature of longer term channel changes. Bed elevation is generally higher downstream of the structures as a consequence of deposition associated with the lower flow velocities. The patterns of bed topography seem to increase in asymmetry when associated with LWD set at an angle to the flow rather than perpendicular to it – the middle structure showing the most symmetrical pattern. Bed Material Size Distribution
The map suggests that the bed material size distribution reflects the increased complexity of flow once the LWD reach is entered. Before the first structure the pattern of bed material distribution is more uniform. Once the reach is entered finer sediments are generally located downstream of LWD structures which also correspond with shallower areas of channel and areas of low/no flow. Bed units over 4 mm in composition share general patterns being generally located in the areas of fastest velocity seen in figure 4 associated with deeper sections of the river. Roundness index numbers were relatively low probably due to the discontinuous flow. Sample 25 contained high levels of clay in a scour zone, this could represent the Victorian channel lining efforts. The tufa deposits are calcareous precipitates which can cement materials river bed materials; their position could indicate incision of the channel into a older bed. Unfortunately plans to undertake further study of flow around the LWS at different river stages during winter 2011 were not possible due to a delay in flow becoming re-established. Success in meeting objectives of hydromorphological diversity
Creating pool habitats - Enhanced scour has created deeper areas directly in front of and beside the LWD structures. Promote riffle and bar formation through induced sediment deposition - Sediment deposition is evident in areas of the riverbed with lower flow and located directly downstream of LWD structures. These shallow areas were identified as having potential for riffle and bar formation. Diversion of flow – has resulted in some evidence of plan form variation from the straight cut section. In summary it is clear that geomorphic thresholds for sediment transportation, erosion and deposition vary throughout the channel as a result of the diverse hydraulic conditions promoted by the LWD. Implications for Habitats The material of the riverbed has been highlighted as being fundamental to the aquatic habitat of invertebrates and fish (Robert 2003). Bed type Range of Relative Density of Diversity of Fish use of particle size frequency of benthic benthic bed (mm) bed macro- macro- sediments movement invertebrates invertebrates Boulder – ≥64 Rare High High Cover, Cobble spawning, feeding Gravel – 2-64 Rare to Moderate Moderate Spawning, pebble – 64-256 periodic Feeding cobble Sand 0.063-2 Continual High Low Off-channel fine deposit used for feeding Fine <0.063 Continual or High Low Feeding. material rare Gurnell (2006) determined that LWD were sites that serve as food for grazing organisms with high organic matter retention, nursery habitat for fish and perches for birds and other animals. References Chessman, B. C., Fryirs, K. A., and Brierley, G. J. (2006) Linking geomorphic character, behaviour and condition to fluvial biodiversity: Implications for river management. Aquatic Conservation: Marine and Freshwater Ecosystems 16, 267–288. Clarke SJ, Bruce-Burgess L, Wharton G. (2003). Linking form and function: towards an eco- hydromorphic approach to sustainable river restoration. Aquatic Conservation. Marine and Freshwater Ecosystems 13, 439–450. Ford, T.D., and Rieuwerts J.H., (2007). Lead mining in the Peak District. The Peak District Mines Historical Society Gordon, N. D. McMahon, T. A., Finlayson, B.L., Gippel, C. J. and Nathan, R. J. 2004, Stream hydrology: an introduction for ecologists, 2nd edn, John Wiley and Sons Ltd, West Sussex. Gurnell, A., K. Tockner, P. Edwards, and G. Petts. (2005). Effects of deposited wood on biocomplexity of river corridors. Frontiers in Ecology and the Environment 7, 377–382. Raven, P. J., Holmes, N.T., Charrier, P., Dawson, F. H., Naura, M. and Boon, P. J. (2002). Towards a harmonized approach for hydromorphological assessment of rivers in Europe: A qualitative comparison of three survey methods. Aquatic Conservation: Marine and Freshwater Ecosystems 12, 405–424. Rice, S. P., and Haschenburger, J.K. (2004). A hybrid method for characterization of course subsurface fluvial sediments. Surface Processes and Landforms 29, 373–389 Robert A. (2003) River processes. Arnold. London