JBA Consulting Report Template 2015
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River Rother Structures Assessment Report Final Report July 2021 www.jbaconsulting.com Environment Agency – South East ENV0002754C-JBA-XX-60-RP-EN-0001-A0-C02-River_Rother_Structures_Assessment i JBA Project Manager Rebecca Thrower The Library St Philip’s Courtyard Church Hill COLESHILL Warwickshire B46 3AD Revision History Revision Ref/Date Amendments Issued to P01 / 10th May 2021 Draft Report Environment Agency P02 / 4th June 2021 Final Draft Report, updated following Environment Agency review comments C01 / 25th June 2021 Final Report, minor updates following Environment Agency review comments C02 / 8th July 2021 Final Report, minor update to Table 2-1 Environment Agency Contract This report describes work commissioned by the Environment Agency as part of the SSD Invest to Save Package of projects (ref: ENV0002754C) through Lot 1 of the Collaborative Delivery Framework. The Environment Agency’s managers for the contract were Maria Fröberg and Tristan Herbert and the project Senior User was Adam Hammerton. Francess Haine and Rebecca Thrower of JBA Consulting carried out this work. Prepared by .................................. Fran Haine BSc Assistant Analyst .................................................... Rebecca Thrower BSc MSc MCIWEM C.WEM Chartered Senior Geomorphologist Reviewed by .................................. Matthew Hemsworth BSc MSc FRGS MCIWEM C.WEM Principal Geomorphologist Purpose This document has been prepared as a Final Report for Environment Agency. 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. JBA Consulting has no liability regarding the use of this report except to Environment Agency. Copyright © Jeremy Benn Associates Limited 2021. ENV0002754C-JBA-XX-60-RP-EN-0001-A0-C02-River_Rother_Structures_Assessment i Carbon Footprint A printed copy of the main text in this document will result in a carbon footprint of 478g if 100% post-consumer recycled paper is used and 609g if primary-source paper is used. These figures assume the report is printed in black and white on A4 paper and in duplex. JBA is aiming to reduce its per capita carbon emissions. ENV0002754C-JBA-XX-60-RP-EN-0001-A0-C02-River_Rother_Structures_Assessment ii Executive Summary Study overview The River Rother is located in the South Downs National Park and the River Lod is a tributary. There are a number of identified structures through the catchment that are creating obstacles for fish passage and impacting the WFD status. This project aims to carry out a geomorphology assessment to identify the impact of the interventions proposed by the Arun and Rother Adaptation Study (ARAS) on each structure on geomorphological characteristics. A more detailed survey of Stedham Mill has been completed and presented in the Appendix. Arun and Rother Adaption Study The Arun and Rother Adaptation Study (ARAS) considered various options for 12 structures along the River Rother and River Lod, primarily to improve environmental conditions under the Water Framework Directive. However, this study represented an initial feasibility stage and further steps were recommended to progress the preferred options for each structure. Geomorphology Assessment To understand the anticipated response of the river to the interventions recommended by ARAS, and to establish if they remain the most appropriate from a geomorphology viewpoint, a targeted geomorphology assessment of the River Rother and River Lod has been undertaken. The following table summarises the key findings of the study. Full removal will have many benefits to geomorphic processes, morphology and ecological functioning. However, given the degree to which the rivers Rother and Lod have been extensively modified, it is important to note that full removal of any structure will not restore the rivers to pre-modification conditions. Mitigation of potential detrimental impacts, such as reduced channel to floodplain connectivity, temporary bank instability and release of high volumes of sediment, should be considered during the design process and detailed assessment at each structure. The suggestions made in the table below are unconstrained and relate only to river geomorphology; a full feasibility assessment taking account of different receptors, constraints and the views of stakeholders, would be required as a next step. Structure Study Outcome Durford Mill ARAS recommendation appraisal: Full removal of the weir would (River Rother) reinstate uninterrupted sediment transport processes and restore natural functioning of the river, with the exception of natural interactions between the channel and its floodplain which are expected to reduce following removal. Provided the impacts of this can be mitigated, full removal is the most beneficial option from a geomorphology viewpoint. Terwick Mill ARAS recommendation appraisal: Do Minimum will have minimal (River Rother) impact on geomorphological processes and the current condition will continue. Alternative: Full removal would offer the most benefits to geomorphic process, form and function in the river. A naturalised bypass channel through the floodplain on the wide, left bank floodplain could be considered a beneficial alternative. Iping Weir ARAS recommendation appraisal: The long-term solution of full removal (River Rother) would be the most beneficial for sediment continuity and river restoration, although important constraints would need to be considered during the design. The ARAS short-term solution of installing a fish pass would have minimal benefit to the river’s geomorphic condition. Stedham Mill ARAS recommendation appraisal: The installation of a fish pass ENV0002754C-JBA-XX-60-RP-EN-0001-A0-C02-River_Rother_Structures_Assessment iii (River Rother) through the ford would have minimal impact on existing geomorphological processes. Alternative: Full removal would offer the most benefits to geomorphic process, form and function in the river. However, as with many of the weirs in this catchment, a high volume of sediment has accumulated upstream and the potential for this to cause hydromorphological harm needs to be better understood before the weir can be removed. Woolbeding ARAS recommendation appraisal: Neither ARAS preferred options offer (River Rother) improvements to geomorphic processes in this reach and the current condition will continue. Alternative: Full removal would offer the most benefits to geomorphic process, form and function in the river. A naturalised bypass channel through the floodplain on the wide, right bank floodplain could be considered a beneficial alternative. North Mill ARAS recommendation appraisal: A bypass channel would be beneficial (River Rother) from a geomorphology viewpoint; however, it is recommended that a more naturalised channel be created to mimic natural habitat and restore uninterrupted sediment transport. Moorland Farm ARAS recommendation appraisal: Do Minimum will have minimal (River Rother) impact on geomorphological processes and the current condition will continue. An alternative suggestion made by the ARAS report of bypassing the structure would be highly beneficial to geomorphic processes in this reach, particularly if the former river meander is reinstated and the weir at the top of this channel is removed. Kilsham Farm ARAS recommendation appraisal: Installing rock ramps would have (River Rother) minimal benefit to wider geomorphic processes. The alternative solution suggested by ARAS of full removal would be the most beneficial for sediment continuity and river restoration. Fittleworth Mil ARAS recommendation appraisal: Installation of a fish pass offers (River Rother) minimal improvements to geomorphic processes in this reach and the current condition will continue. Alternative: Full removal would offer the most benefits to geomorphic process, form and function in the river. A naturalised bypass channel through the floodplain on the wide, right bank floodplain could be considered a beneficial alternative. Hardham (River ARAS recommendation appraisal: Installation of a fish pass offers Rother) minimal improvements to geomorphic processes in this reach and the current condition will continue. The alternative suggestion made by ARAS for full removal would offer the most benefits to geomorphic process, form and function in the river. Halfway Bridge ARAS recommendation appraisal: Do minimum would have minimal (River Lod – change on the current system and siltation upstream will continue, with tributary of severe bank instability likely to continue downstream River Rother) Further investigation is required. Lurgashall ARAS recommendation appraisal: Installation of a fish pass offers (River Lod – minimal improvements to geomorphic processes in this reach and the tributary of current condition will continue. River Rother) Alternative: Full removal would offer the most benefits to geomorphic process, form and function in the river. ENV0002754C-JBA-XX-60-RP-EN-0001-A0-C02-River_Rother_Structures_Assessment iv Contents 1 Introduction 1 1.1 Purpose and context 1 1.2 Objective 1 1.3 Study Area 1 1.4 Methodology 2 1.4.1 Data review, collation and gap analysis 2 1.4.2 Baseline Geomorphology Assessment 2 1.4.3 Option review 3 1.4.4 Additional detailed assessment at Stedham Mill 3 2 Existing Data Review and Gap Analysis 4 2.1 Overview 4 2.2 Heritage value 4 2.3 Catchment overview 5 2.4 Sediment Dynamics 5 2.5 Arun and Rother Adaptation Study 6 2.6 Summary of Gap Analysis