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Bridgwater Tidal Barrier Appraisal Geomorphological Baseline Report

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Bridgwater Tidal Barrier Appraisal Geomorphological Baseline Report REPORT Bridgwater Tidal Barrier Appraisal Geomorphological Baseline Report Prepared for Environment Agency 23 August 2017 CH2M HILL Ash House Falcon Road Exeter EX2 7LB Geomorphological Baseline Report Contents Section Page Chapter 1 Introduction ........................................................................................................................... 1 Background ...................................................................................................................................... 1 Purpose of this document ................................................................................................................ 1 Methodology .................................................................................................................................... 3 Report Structure .............................................................................................................................. 4 Chapter 2 Hydrodynamics ....................................................................................................................... 5 Tidal flow .......................................................................................................................................... 5 Freshwater flow ............................................................................................................................... 5 Position of the turbidity maximum (TM) ......................................................................................... 6 Tidal bore ......................................................................................................................................... 8 Conceptual Model – Hydrodynamic Processes................................................................................ 8 Chapter 3 Sediment System .................................................................................................................. 10 Sediment Sources .......................................................................................................................... 10 Channel bed material ....................................................................................................... 10 Channel banks ................................................................................................................... 10 Suspended sediment concentrations ............................................................................... 10 Fluid mud .......................................................................................................................... 11 Sediment Transport ....................................................................................................................... 12 Outer Estuary (Bridgwater Bay) ........................................................................................ 12 Middle and Inner Estuary.................................................................................................. 13 Role of extreme events in transporting sediment ............................................................ 14 Sediment Deposition...................................................................................................................... 14 Outer Estuary .................................................................................................................... 14 Middle and Inner Estuary.................................................................................................. 15 Conceptual Model – Sediment System .......................................................................................... 16 Chapter 4 Morphology .......................................................................................................................... 19 Morphology.................................................................................................................................... 19 Historic evolution ........................................................................................................................... 20 Conceptual Model – Morphology .................................................................................................. 21 Chapter 5 Summary of geomorphology baseline for proposed barrier site ......................................... 23 Tidal prism and turbidity maximum ............................................................................................... 23 Morphology.................................................................................................................................... 23 Considerations for barrier impacts ................................................................................................ 24 References ............................................................................................................................................ 25 i Geomorphological Baseline Report Chapter 1 Introduction Background The Bridgwater Tidal Barrier Project aims to put in place a tidal barrier which will reduce the risk from flooding to Bridgwater and further upstream along the River Parrett. The construction and operation of a tidal barrier has the potential to impact estuary hydrodynamics, which could lead to impacts on estuarine processes including sediment transport and resulting geomorphology. A robust baseline understanding of estuarine processes and geomorphology is required in order to inform the assessment of the potential impacts from the construction and operation of a tidal barrier within the Parrett Estuary. This report summarises the understanding of baseline estuary morphology and the underpinning dynamics. The main aim is to present a robust understanding to enable assessment of potential impacts on geomorphology and associated features/receptors during the project decision making process. The impact assessment was undertaken during the option appraisal process and is documented in the ‘Bridgwater Tidal Barrier - Preliminary Environmental Information’ report (Environment Agency, 2017). Purpose of this document The ‘Parrett Barrier: Geomorphology Assessment and Peer Review’ report (CH2M, Feb 2015) highlighted a number of gaps in the current understanding of estuary dynamics and geomorphology, and recommended areas for further investigation, which included: Determining qualitatively and quantitatively the role of marine versus fluvial sedimentation within the estuary; Determining the role of extreme events in governing estuary morphology; Determining the position of the turbidity maximum1; Determining the influence of suspended sediment versus bedload sediments; Assess the influence of fluid mud2 on estuary sediment transport and morphological response; Determining historic rates of siltation along the estuary; Improving conceptual understanding of the estuary regime (tidal prism3); Improving understanding of previous estuary response to the presence of man-made structures. 1 The body of water in which the saline and freshwaters meet is known as the mixing zone or Turbidity Maximum (TM). 2 Fluid mud is a near-bed, high density, cohesive sediment suspension layer that forms during slack water. Its presence greatly increases the concentration of mud in suspension. 3 The morphology and size of an estuary is a direct reflection of the tidal volume it has to accommodate, known as the tidal prism. 1 Geomorphological Baseline Report STERT POINT Figure 1.1: Study area A number of location specific questions were also highlighted in the 2015 review, including questions relating to the stability and variability of channel morphology at the tidal barrier site. This report includes information about the wider estuary as well as the preferred barrier location (‘Site 5’ at Chilton Trinity), covering aspects such as estuary width/cross-section, meander position, stability of the low water channel, erosion and deposition. Details for all the potential barrier locations were 2 Geomorphological Baseline Report documented and used during the options appraisal process, which resulted in the selection of Chilton Trinity as the preferred location (CH2M, 2016). The purpose of this report is to supplement the information provided in the 2015 ‘Parrett Barrier: Geomorphology Assessment and Peer Review’, focusing on the key gaps. The aim is to develop a fuller conceptual understanding of the Parrett Estuary at an appropriate spatial and temporal extent to better understand the processes in the estuary, including morphology, sediment dynamics and hydrodynamics. The information contained within this report formed the baseline for assessment of potential impacts of the barrier (location/design/operation) and has been used in the Option Appraisal, Preliminary Environmental Information Report and Water Framework Directive Compliance Assessments for the Bridgwater Tidal Barrier project. Methodology The geomorphological baseline methodology has been based on existing best practice guidance for understanding estuary morphology using synthesis and conceptual model approaches, as recommended by the Defra/Environment Agency Estuary Guide http://www.estuary-guide.net/. It is desk based and draws together available existing information from published literature, previous studies and reports to make evidence-based statements of how the estuary works, informing the potential impacts of the tidal barrier scheme. The main supporting reference is the ‘Parrett Barrier: Geomorphology Assessment and Peer Review’ (CH2M, 2015), which provided an initial conceptual understanding of the Parrett Estuary.
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