Use of Space-for-Time Substitution in River Restoration: examples from SE England

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1 Use of Space-for-Time Substitution in River Restoration: examples from SE England Drs Andrew Brookes and Niamh Burke (Jacobs) Lizzie Rhymes and Graham Scholey (Environment Agency, SE)

2 What is Space-for-Time Substitution? An approach assuming causes of channel change can be inferred from the association between spatial variation in process and in form Has been used extensively to document man-induced changes of river channel form and sediment yield For example in UK understanding of channel response to impoundment has been based principally on this approach Sediment injected by tributaries and resulting from upstream degradation and bank erosion will be redistributed downstream resulting in bench development and channel narrowing

3 Urbanised Channels These methods have also been used for establishing relationships for un-urbanised areas so that they can be compared to actual field-measured values under urbanised conditions. The proportion between urbanised values and those estimated for pre-urban natural conditions are then calculated to indicate the magnitude of change, commonly referred to as the enlargement This enlargement may occur in response to increased peak flows from paved surfaces

4 What is the relevance to river restoration? Natural channel dimensions can sometimes be recreated from historic data/ information held in County Records Offices Typically however river condition is assessed relative to a natural reference reach but this can be difficult in heavily modified catchments Could try to use dimensions from a neighbouring catchment but needs to be identical geology etc Often there has to be a mix of approaches including an element of professional judgement The use of space-for-time substitution based on regional relationships is therefore explored in this study

5 Importance of low flow width (Lower Colne)

6 Regional relationships Leopold and Maddock (1953) (USGS) used the concept of downstream hydraulic geometry to show how simple fluvial parameters along a reach (width, depth etc) vary in a downstream direction with discharge (or catchment area as a surrogate for discharge) Some studies have extended this to entire river networks Where observed channel dimensions depart from those predicted then this can be indicative of a modified channel For river restoration purposes relationships between the few remaining natural or recovered reaches with modified reaches can be used to determine deviations from expected conditions and provide new channel dimensions

7 Relationships developed for some N. American Rivers

8 Example: River Ray, Oxfordshire

9 Spoil versus field furrows

10 Method Estimating average width, depth and cross-sectional area for the few suitable reference reaches found during survey (these were often old channel remnants found in the floodplain Estimating average width, depth and cross-sectional area for the pre-modification channels. For example, by assuming that an adjusted width (created by fine silt deposition) is similar to what would have existed previously (this relies heavily on professional judgement) Need to recognise that recovery of natural width may be obscured by slipping of channel banks, particularly on main stem Ray Main stem Ray and tributaries dealt with separately Catchment area used as a surrogate for discharge. Broad estimates based on following the contours on a 1:25,000 map

11 Summary of Ray Fluvial Audit 83km walkover survey Relatively low slope (including through the historical poorly drained Otmoor Fen) through clay soils Extensively modified (regrading and deepening) as part of arterial drainage schemes (especially 1950 s) Ubiquitous removal of channel bed (including substrate) and destruction of pools and riffles Subsequent bank slippage causing unusual channel profile Some sediment accumulation at the toe of the slopes Absence of reference reaches Some types of restoration recommended: channel narrowing; local bed raising; restoration of meanders Space-for-time included as part of a Fluvial Audit in 2012

12 Extent of Fluvial Audit

13 Conservation Status Conservation Status 2% 12% 86% Low Med High Uniformity created by extensive channel works (reduction of flow heterogeneity and absence of morphological feaures) Corresponds with the WFD findings that the Ray is failing to reach Good Ecological Status

14 Collection of natural channel dimensions from residual 2% high status Collection of measurements from relict channels in floodplains Supplemented by expert judgement Approach

15 Channel width (m) Log plot channel low-flow width vs drainage area Ray-Mainstem-Template Tributaries-Template Ray-Mainstem-Artificial Ray Tributaries-Artificial Power (Tributaries-Template) Power (Ray-Mainstem-Template) Power (Ray-Mainstem-Artificial) Power (Ray Tributaries-Artificial) Ray-Template R 2 = Tribs-Template R 2 = Ray- Artificial R 2 = Tribs- Artificial R 2 = Catchment area (km2)

16 Findings Compared with previous studies there is a good power relationship for the main stem Ray and tributaries using relict reference/ template reaches Poor relationships for the modified channel sections but a linear relationship would not be expected (cf natural channels) Modified channels can be between 3 and 32 times greater in crosssectional area than for a natural channel anticipated at that location reflecting different capital and maintenance works at different times Channels have been slow to adjust to a more natural width through sedimentation Aided by bank slips in places (although this is not an adjustment process caused by fluvial processes and slips may still be in progress)

17 Complex Response

18 Adjustment Processes on Ray

19 Limitations of Method Not where geological controls are different Use of power laws tend to have a seductive quality and may disguise local variability Works relatively well for width dimension but not for other parameters such as channel shape (which is more a measure of channel complexity) Other parameters which may be suitable: average depth, bankfull width and width:depth ratio (needs further work)

20 Conclusions It is perhaps easy to lose sight of what might have been the pre-modification channel dimensions when there is a lack of reference reaches Space-for-time has been found to be a useful tool in the river restorers toolkit (ideally combined with other approaches to allow sensitivity testing etc) Can be a useful tool, particularly where the geology is uniform throughout a catchment Accuracy of a single method needs to be caveated and it is recommended that complementary tools are used in the same study (together with sensitivity testing) Thank you for listening!

21 14 TH ANNUAL NETWORK CONFERENCE Scaling up our Aspirations for River Restoration and Management The RRC would like to thank the sponsors of the RRC Annual Conference 2013 who support discounted places