Department for Environment, Food and Rural Affairs Ripon Land Management Project June 2007 DRAFT REPORT JBA Consulting Department for Environment, Food South Barn and Rural Affairs Broughton Hall Flood Defence Division SKIPTON Quantock House North Yorkshire Paul Street BD23 3AE Taunton UK TA1 3NX t: +44 (0)1756 799 919 t: +44 (0)1823 348403 f: +44 (0)1756 799 449 [email protected] www.defra.gov.uk Department for Environment, Food and Rural Affairs Ripon Land Management Project Draft Final Report REVISION HISTORY Revision Ref./ Amendments Issued to Date Issued Preliminary Draft Final Report L.Aucott, D.Murphy, W.Brooks March 2007 (1 electronic copy each) 2nd Draft Final Report Revised to include additional L.Aucott, D.Murphy, W.Brooks April 2007 material and comments from (1 electronic copy each) L.Aucott & D.Murphy received on 4 April 07 3rd Draft Final Report Revised to include additional L.Aucott, D.Murphy, W.Brooks June 2007 comments from L.Aucott and (1 electronic copy each) D.Murphy received on 5 June 07 CONTRACT This report describes work commissioned by the Department for Environment, Food and Rural Affairs (Defra) under the Consultancy Project ref: SLD2332 of 1 March 2007. Defra’s representative for the contract was Linda Aucott (Defra, Flood Management Division, Taunton). Steve Rose, Zdenka Rosolova and Rob Lamb of JBA Consulting carried out the work. Prepared by: ................................................... Steve Rose, BSc, MSc, FRGS, CGeog ======mêáåÅáé~ä=^å~äóëí= = = ============ÁÁÁÁÁÁÁÁÁÁÁÁÁÁKK=Zdenka Rosolova, MEng, MSc = = = = = = ^ëëáëí~åí=^å~äóëí= Reviewed by: ................................................... Rob Lamb, MA, PhD ======qÉÅÜåáÅ~ä=aáêÉÅíçê= Approved by: ....................................…............ Gary Deakin ======qÉÅÜåáÅ~ä=aáêÉÅíçê= PURPOSE This document has been prepared solely as a Final Report for Defra. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared. ACKNOWLEDGMENTS We would like to acknowledge a number of staff from Defra, Environment Agency, British Geological Survey, Cranfield University, Newcastle University, Natural England, Meteorological Office, Forest Research and the Ripon Multi-Objective Project for the supply of datasets and other information to this project. JBA Consulting ïïïKàÄ~ÅçåëìäíáåÖKÅçKìâ= N:\2007\Projects\2007s2273 - Defra - Land Management Study\Reports\Project\Draft report\Ripon draft final report17 (20 June 07).doc: 20/06/2007 i Department for Environment, Food and Rural Affairs Ripon Land Management Project Draft Final Report EXECUTIVE SUMMARY To date, research has not been able to quantify how the likely effects of land management change at the field or farm scale will affect the dynamics of flood generation at the catchment scale. This short project aims to augment the current state of knowledge by using sensitivity testing as a means of indicating the potential impact that changes in runoff characteristics from farms and sub-catchment areas may have on the generation of floods at the catchment scale. The case study catchment chosen for this work was the 120km2 catchment draining through Ripon in North Yorkshire, which includes the rivers Skell and Laver, and Kex Beck. The approach used existing modelling techniques. Ten individual sub-catchment rainfall-runoff models, in the form of Probability Distributed Moisture (PDM) models, were developed, each representing an area with similar physical characteristics, such as topography, rainfall, soils and land use. All the individual PDM sub- catchment models were then linked together via an ISIS flood routing model in order to simulate flows at the outlet to the study catchment at Alma Weir on the River Skell in Ripon. The PDM and ISIS routing models were initially calibrated to the current ‘baseline’ conditions in the catchment, which is assumed to be degraded to some extent given the general agricultural intensification and moorland improvements that have taken place over the last 50 years or so. Seven scenarios were developed to represent plausible changes to the land management or land use in all or parts of the Ripon catchment. The scenarios chosen were principally aimed at practices that would lead to the degradation of the soils within the catchment (e.g. compaction or crusting of vulnerable soils, conversion of improved grassland to maize production) or a change to the management of the moorland in the headwaters (e.g. maintenance of moorland grip drainage or moorland grip blocking). In particular, the impact of the proposed land management changes were represented in the PDM models by alterations to specific PDM model parameters affecting the rapid runoff component, the condition of soil moisture store and the hydrograph timing. The impact of moorland grip drainage blocking in controlling the generation and rate of runoff was also investigated. Each of the scenarios was run through the PDM models for both observed winter/summer events, and for design events with 10 year, 50 year and 100 year return periods that were generated from a separate stochastic rainfall model based on raingauge datasets from the catchment. Initial investigations of land cover and soil type indicated that the Ripon catchment was likely to be susceptible to land management impacts. Results indicated that the worst case plausible degradation scenario (combining soil structural degradation across the whole catchment and additional moorland grip maintenance) led to increased peak flows in Ripon compared to the baseline case of between 20% for smaller scale floods and 10% for more extreme floods. A less extensive scenario assuming soil degradation over 30% of the catchment led to increased peak flows of 10% for smaller scale floods and 3% for more extreme events. In contrast, the best case plausible improvement scenario (moorland grip blocking) led to a reduction of flood peak magnitudes in Ripon by up to about 8% when compared to the baseline case. The timing of the flood peak in Ripon was altered by up to ±1.5 hours as a result of the scenarios, though changes to the timing of the hydrographs generated in the moorland areas were attenuated by the time they had reached Ripon. The study may help to shed light on why land management effects can be difficult to detect in historic datasets. It should be remembered that there are certain other features or practices in the catchment that might have an effect to increase or reduce runoff that have not been directly tested as part of this project. This is because the modelling approach taken in this project was specifically developed to address the shortfall in knowledge of the effect on floods of land management induced soil degradation. Other interventions, for example, changing floodplain roughness, installing online farm storage ponds and the effects of in-field underdrainage on certain soil types could also have produced similar effects on flood peaks depending on their extent and location. The work undertaken in this project complements more detailed modelling being done elsewhere. It suggests a generic framework for scenario testing, using the Ripon catchment as a prototype to help in interpreting the relevance of the scenarios and results. However, there could be alternative results produced for other locations. It is hoped that the information gained from this project will help inform the development of policy in using land management change as a potential flood mitigation or attenuation measure. JBA Consulting ïïïKàÄ~ÅçåëìäíáåÖKÅçKìâ= N:\2007\Projects\2007s2273 - Defra - Land Management Study\Reports\Project\Draft report\Ripon draft final report17 (20 June 07).doc: 20/06/2007 ii Department for Environment, Food and Rural Affairs Ripon Land Management Project Draft Final Report This page is intentionally left blank. JBA Consulting ïïïKàÄ~ÅçåëìäíáåÖKÅçKìâ= N:\2007\Projects\2007s2273 - Defra - Land Management Study\Reports\Project\Draft report\Ripon draft final report17 (20 June 07).doc: 20/06/2007 iii Department for Environment, Food and Rural Affairs Ripon Land Management Project Draft Final Report CONTENTS Page REVISION HISTORY i CONTRACT i PURPOSE i ACKNOWLEDGEMENTS i EXECUTIVE SUMMARY ii CONTENTS iv 1 INTRODUCTION ---------------------------------------------------------------------------------- 1 1.1 Introduction ................................................................................................................................ 1 1.2 Study Area.................................................................................................................................. 1 1.3 Project Brief................................................................................................................................ 2 1.4 Project Methodology .................................................................................................................. 3 2 STUDY AREA-------------------------------------------------------------------------------------- 5 2.1 Background ................................................................................................................................ 5 2.2 Topography ................................................................................................................................ 5 2.3 Geology ...................................................................................................................................... 6 2.4 Soils...........................................................................................................................................