Study of Interaction Between Broad Oak Reservoir and Richborough Connection Project South East Water & National Grid
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Study of Interaction between Broad Oak Reservoir and Richborough Connection Project South East Water & National Grid Geomorphology Fluvial Audit B14000AG/BORStudy/1016 | 0 July 16 Geomorphology Fluvial Audit South East Water & National Grid Geomorphology Fluvial Audit Study of Interaction between Broad Oak Reservoir and Richborough Connection Project Project no: B14000AG Document title: Geomorphology Fluvial Audit Document No.: B14000AG/BORStudy/1016 Revision: 0 Date: July 16 Client name: South East Water & National Grid Client no: Project manager: Alaistair Smith Author: Sera Roberts and Katy Kemble Jacobs U.K. Limited 1180 Eskdale Road Winnersh, Wokingham Reading RG41 5TU United Kingdom T +44 (0)118 946 7000 F +44 (0)118 946 7001 www.jacobs.com © Copyright 2016 Jacobs U.K. Limited. The concepts and information contained in this document are the property of Jacobs. Use or copying of this document in whole or in part without the written permission of Jacobs constitutes an infringement of copyright. Limitation: This report has been prepared on behalf of, and for the exclusive use of Jacobs’ Client, and is subject to, and issued in accordance with, the provisions of the contract between Jacobs and the Client. Jacobs accepts no liability or responsibility whatsoever for, or in respect of, any use of, or reliance upon, this report by any third party. Document history and status Revision Date Description By Review Approved 0 2nd Dec Draft for client comment Sera Roberts Andrew Brookes Ros Vincent 2015 Katy Kemble 1 4th July 2016 Final for Stage 1b report rev 1 Sera Roberts Andrew Brookes Ros Vincent Katy Kemble Distribution of copies Revision Issue Date issued Issued to Comments approved As appendix to report 0 A J Smith 15-12-15 Report issued under control of main report B14000AG-BORStudy-901 As appendix to report 1 A J Smith 04-07-15 Report issued under control of main report B14000AG-BORStudy-901 B14000AG/BORStudy/1016 1 Geomorphology Fluvial Audit Contents Executive Summary ............................................................................................................................................. 34 1. Introduction .............................................................................................................................................. 45 1.1 Background ............................................................................................................................................... 45 1.2 Report Aim and Objectives ........................................................................................................................ 45 2. Fluvial Geomorphology .......................................................................................................................... 67 2.1 What is Fluvial Geomorphology? .............................................................................................................. 67 3. Methodology ............................................................................................................................................ 78 3.1 Geomorphological Reconnaissance Survey and Spot Checks ................................................................. 78 3.2 Data Collection .......................................................................................................................................... 78 3.3 Analysis and Reporting ............................................................................................................................. 78 4. Desk Study ............................................................................................................................................. 910 4.1 Catchment Overview ............................................................................................................................... 910 4.2 Historical Channel Change ...................................................................................................................... 910 4.3 Geology and Soils ................................................................................................................................. 1011 4.4 Hydrology .............................................................................................................................................. 1011 5. Contemporary River Characteristics ................................................................................................. 1112 5.1 Geomorphological Characteristics ........................................................................................................ 1112 5.1.1 Channel Planform .................................................................................................................................. 1112 5.1.2 Floodplain Connectivity ......................................................................................................................... 1213 5.1.3 Flow Variation ........................................................................................................................................ 1314 5.1.4 Bed Substrate ........................................................................................................................................ 1415 5.1.5 Sediment Sources ................................................................................................................................. 1516 5.1.6 Erosion .................................................................................................................................................. 1617 5.1.7 Deposition .............................................................................................................................................. 1718 5.1.8 Land Use and Riparian Zones ............................................................................................................... 1920 5.1.9 Channel Modifications and Structures ................................................................................................... 1920 5.2 Analysis ................................................................................................................................................. 2122 5.2.1 Channel Planform Changes .................................................................................................................. 2122 5.2.2 Channel Substrate and Bank Erosion ................................................................................................... 2122 5.2.3 Potential Sediment Sources .................................................................................................................. 2223 6. Summary and Recommendations ..................................................................................................... 2324 6.1 Summary of the Sarre Penn Characterisation ...................................................................................... 2324 6.2 Recommendations for Diversion Design ............................................................................................... 2324 References ....................................................................................................................................................... 2627 Appendix A. Point Sediment Sources Appendix B. Structure Types B14000AG/BORStudy/1016 2 Geomorphology Fluvial Audit Executive Summary South East Water’s Water Resources Management Plan (2014) identified a new reservoir at Broad Oak as a ‘preferred plan water resource option’ for development within the 25 year plan period. A concept stage study proposed a top water level of 36.0mOD. The reservoir would be filled principally from an abstraction from the River Stour at Plucks Gutter (in an adjacent catchment) and pumped to the reservoir through a pipeline. The proposed scheme includes a water treatment works located downstream of the dam from where potable water would be pumped into the supply. Approximately 2km of the Sarre Penn runs through the footprint of the proposed reservoir. Construction of the reservoir would lead to the loss of the river channel and associated riparian habitats. The Sarre Penn is classified under the Water Framework Directive and a preliminary assessment suggests that without mitigation the reservoir development could result in a deterioration of the status or potential of the waterbody. At this stage the option discussed with the Environment Agency to address this WFD impact, is to create a river diversion channel to maintain the continuity of the stream and its habitat function. This will involve a diversion channel perched on the southern edge of the reservoir within SEW’s land holding. A geomorphological reconnaissance survey of the river has been undertaken, particularly focusing on the upstream lengths, to establish the character of the river and the parameters for an acceptable design of a river diversion. This assessment aims to establish the baseline conditions based on a walkover survey and also analysis of data/information. Where possible these findings are supported by literature and professional judgement. The Sarre Penn is not an active alluvial gravel bed river. Whilst secondary currents are present in the channel, they are insufficiently powerful to erode the bed and banks. It has been assessed that the Sarre Penn has historically been heavily modified, leading to a modified channel that is slowly attempting to adjust within the bank limits to the channel changes. The clay banks provide a definitive barrier to lateral migration, with the course of the river not having adjusted in