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Understanding River Restoration Fluvial Processes Within River Restoration Design

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Understanding River Restoration Fluvial Processes Within River Restoration Design Understanding River Restoration fluvial processes within river restoration design Dr Jenny Mant Dr Philip J. Soar Overview • accounting for sediment in river restoration • channel-forming discharge • what to think about for restoration design • class exercise • FORM, PROCESSES AND DYNAMICS (change over time) Context Fluvial geomorphology is still an emerging science Interest in ‘applied’ fluvial geomorphology grew rapidly during the 1980s Key NRA/EA R&D reports in during the 1990s setting out a geomorphological approach to river management Consideration of river processes, sediment transfer and physical habitats recognised as vital for ‘sustainable’ river management and restoration In the UK there are few consultant geomorphologists (but this is changing!) Training courses are not widely available and there are no standard approaches to river restoration Accounting for Sediment in river restoration Starting point Guidebook of Applied Fluvial Geomorphology Technical Report FD1914 (Thorne, Sear and Newson 2003) Starting point Applied Fluvial Geomorphology for River Engineering and Management (Thorne, Hey and Newson 1997) Starting point Fluvial forms and process A new perspective ( Knighton, D 1998) Sustainable restoration design Why should we ‘account for sediment’? What do we mean by ‘continuity’ (or connectivity)? Sediment Continuity… as a design ‘principle’ as a design ‘process’ in the design ‘procedure’ Define Fluvial Geomorphology... Fluvial geomorphology is: Fluvial is defined as: found in, or produced by a river or rivers Morphology is defined as: the scientific study of form and structure Geo relates to the surface of the earth Fluvial geomorphology is the study of sediment sources, fluxes and storages within the river channel over short, medium and longer timescales and of the resultant floodplain morphology (Sear and Newson, 1993) Why ‘account’ for sediment in restoration “Consider the amount and type of sediment supplied to a stream channel. Why? Sediment is part of the balance (i.e. between energy and material load) that determines channel stability.” “Lack of sediment relative to stream power, shear stress or amount of energy in the flow (discharge = m3/s) usually results in erosion of sediment from the channel boundary of an alluvial channel.” (indexes of transport capacity) “Conversely, an oversupply of sediment relative to the transport capacity usually results in deposition of sediment in that reach” Federal Interagency Stream Restoration Working Group 1998. Stream Corridor Restoration: Principles, Processes and Practices Why ‘account’ for sediment in design For sustainable channel restoration design... “a reconstruction that modifies the size of a cross section and sinuosity should be analysed to ensure that upstream sediment loads can be transported through the reconstructed reach with minimal deposition or erosion.” Federal Interagency Stream Restoration Working Group (FISRWG), 1998. Stream Corridor Restoration: Principles, Processes and Practices River restoration objectives What is the ‘primary’ objective of river restoration? Support a diverse biodiversity! Improve fisheries Improve conservation value of the river landscape Restore meanders! Further objectives might include flood protection and recreation Geomorphology and sediment transport are often given a lower priority or not considered at all! ‘Form without function’ Without accounting for sediment transport and river processes there is a risk of designing ‘form without function’ By imposing an unsustainable condition, the designed channel might not be able to support the targeted habitats over the long term. The river can reject the imposed changes very quickly, especially if there is a high sediment load and sufficient energy, resulting in complex responses. River management NOT working with nature ‘Detroit riprap’ Geomorphology offers a better solution! Discharge Velocity distribution in channel velocity profile bed roughness Channel Bed material Bed shear geometry characteristics stress availability selective transport of sediment competence & capacity erosion & Bedload deposition transport Sediment supply after Ashworth and Ferguson (1986) The ‘sediment system’ Schumm, 1977 Characterising the sediment system (relate to previous slide) Hawkcombe Stream, Somerset Channel-slope coupling a) Gorge Fully coupled: delivery of slope material to channel at all flows b) Confined channel / floodplain Partially coupled: delivery of slope material to channel. Spatially discrete and sporadic activity of sources. Some limited storage. c) Unconfined channel / floodplain Uncoupled /weakly coupled: delivery of slope material to channel at high flows or where tributaries join. Storage on floodplain. Sediment transfer Transfer Reach Flow Bank conditions Site constraints Width Depth Slope Sediment transfer (Soar and Thorne, 2001) Reference Transfer Downstream Reach Reach Reach (Supply) (Demand) Stream type Local variability Bank conditions Width Channel stability Channel-forming discharge Sediment load Depth Sediment and gradation Slope load Sedimentation Braided channel? Time 0 Channel Change Channel Unstable Channel Design Stable Channel Design Erosion Impact of channelisation on sediment transporting capacity discharge sediment discharge QwS+ ~ Qs+D50 bed slope median bed material size A . Planform .B .A .B Long profile Impact of channelisation on aggradation and degradation Brookes, 1988 The channel is trying to ‘recover’ a new equilibrium through a combination of degradation and aggradation: negative feedback Know your river! Where in the fluvial system? ‘Type’ of river (meandering? mobile bed? hydrology? etc) Channel typing and characterisation: Reference reach(es) Natural morphologies to use as analogues? Supported sediment forms and features Supply reach Sediment transfer and connectivity Flow regime ( i.e. stream power) Bed and bank materials/vegetation/typology Project reach Bed and bank materials/vegetation Valley gradient and site constraints Where in the fluvial system? Stream Power Stream Power: f(QS) Q = discharge S = slope FISRWG, 1998 Q = width x mean depth x mean v Independent and dependent variables controlling channel form Sediment transport classification When discussing the sediment load it is vital to be clear about which component of the load is being dealt with. Predicting Sediment Transport Rate In the UK measured sediment transport data are not available other than for research sites There are numerous equations to calculate the sediment (bed material) load in a stream. Accuracy can range between +/- 50% of actual transport rates unless calibrated against measured loads. The key to selecting an appropriate equation and improve accuracy is to find an equation developed for conditions that match those under which it is to be applied. Sediment Transport Rate Bed material size Basis Sample Applications Comments Formula Bagnold (1980) sand, gravel Stream Mimmshall Brook, Performed well in power R. Sence, R. Idle, tests against field Shelf Brook data using reach- average values. Both under and over predicts. Bathurst, Graf gravel, cobble Discharge Shelf Brook, (Newson Performed well for and Cao and Bathurst 1991) steep, headwater (1987) streams (S > 0.1). Over-predicts and can produce negative loads. Ackers-White silt, sand Shear stress R. Sence, Usk, Performed well in tests (1973) updated gravel Colne, Stour, based on flumes and by HR Ecclesborne (HR rivers. Much better when Wallingford Wallingford 1992) calibrated against data (1990) from site in question. Over-predicts. Newson (1986) silt, sand Catchment Shelf Brook, Sence, Provides estimate of gravel area Tawe, Idle Dunsop, annual sediment yield to Whitendale river. (Newson and Bathurst 1991) ‘Typing’ the bed and banks Bed type: implications for slope, bed material composition, roughness and stable channel dimensions Bathurst, 1997 Bank material and vegetation: implications for roughness and stable channel dimensions River profile Riffle-pool sequence characteristic of both straight and meandering channels with heterogeneous bed material in the size range 2- 256mm (mostly gravel-cobble bed streams). Slope range generally 0.001 to 0.02. Important sediment storage and sorting for channel stability and high ecological value. Step-pool sequence slopes are steep > 0.03 – 0.1, typically formed from accumulation of boulders and cobbles in confined valleys. Riffle-pool spacing measured in rivers Diagram from Leopold and Wolman (1957). Riffle spacing in low sinuous channels has a spacing (on average) or 5-7 times the channel width. Pool-riffle characteristics Regular spacing between successive units of 5-7 times the width (thus, the spacing is scale-related!) However, Keller and Melhorn (1978) showed that the spacing can range between 1.5 and 23.3 times the channel width with a mean of 5.9. Riffles generally absent from sand bed rivers and cobble- boulder bed streams, where they are replaced by step- pool units. Riffles have a coarser bed material than pools suggesting that a sorting mechanism is present. How is this maintained? Riffles River Cole, nr Birmingham River Gaunless, Co Durham WANDERING POOL- RIFFLE PLANE BED STEP-POOL CASCADE Transport Capacity 40 Know your river Stream Reconnaissance (Thorne, 1998) Fluvial Audit (Defra FD1914) And … RIVERS MOVE! Mississippi River, Mississippi ? and… RIVERS MOVE! Gilwiskaw Brook, Leicestershire Stability and Equilibrium ‘Geomorphological’ stability: A river that maintains the same average cross
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