Bridgwater Tidal Barrier Appraisal Geomorphological Baseline Report

Home , Bridgwater Bay, Port of Bridgwater, Steart Peninsula

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

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.

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

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. This has been supplemented with an additional literature review, including the following documents:  Environment Agency, March 2010. Parrett Estuary Flood Risk Management Strategy (PEFRMS).  Halcrow, March 2010. ‘Steart Coastal Management Project, Scoping Consultation Document, Appendix B Geomorphology Review’. Environment Agency.  Black & Veatch, August 2011. ‘River Parrett and Tone Channel Monitoring Project, Summary Report’. Environment Agency.  HR Wallingford, 2016. ‘Severn Estuary Long Term Morphology. An updated review of baseline changes – Report on analysis of LiDAR data’. The Crown Estate.  HR Wallingford, May 2016. ‘Somerset Levels and Moors Flood Action Plan Dredging Strategy (Draft)’. Somerset Rivers Authority. Where sufficient data exists, further geomorphological assessment utilising ‘top-down’ methods has been undertaken, including:  Historic trends analysis – analysis of LiDAR data to assess short-term changes in patterns of erosion and deposition, and analysis of historic change in the low water channel of the Parrett Estuary; and  Tidal regime analysis - updating the assessments used in the 2010 PEFRMS;  Observational data. From this information a series of conceptual models have been developed, which describe understanding in terms of hydrodynamic processes; the sediment system and estuary morphology. In addition to this Geomorphological Baseline, one-dimensional sediment transport modelling has been undertaken and reported on separately, to assess the impacts of the barrier. This modelling was used to appraise a range of environmental impacts and is documented in a technical report (CH2M, 2017a) and summarised in the Preliminary Environmental Report (Environment Agency,

Geomorphological Baseline Report 2017). Where model results help to inform the baseline and add to the understanding of the system this has been discussed. Report Structure This report is structured as follows: Chapter 1 Introduction - outline of the report aims and methodology. Chapter 2 Hydrodynamics - summary of current baseline conceptual understanding in terms of hydrodynamics which drive estuary morphology (tides, waves, freshwater flows). Chapter 3 Sediment System - summary of current baseline conceptual understanding in terms of sediment (sources, transport, and deposition). Chapter 4 Morphology - summary of current baseline conceptual understanding in terms of morphology (form and change), building on/synthesising the previous chapters. Chapter 5 Summary of geomorphology baseline for proposed barrier site For consistency, the following general geographical terminology has been used which reference to areas of the estuary: Outer Estuary – Bridgwater Bay (Hinkley Point to Burnham on Sea, covering Steart Flats) Middle Estuary – Parrett mouth up to Bridgwater Inner Estuary – upstream of Bridgwater

Geomorphological Baseline Report Chapter 2 Hydrodynamics

This chapter provides a summary of the hydrodynamics of the Parrett Estuary, comparing the influences of tidal and freshwater flows and wave action. A conceptual model has been developed to summarise this information at the end of the section. For place names/locations refer to Figure 1.1. Tidal flow CH2M 2015 (Table 4.2) reported that the mean spring and neap tidal ranges at Hinkley Point documented in Admiralty Tide Tables are 10.7 m and 4.8 m respectively, and this large tidal range produces very strong tidal currents. Numerical modelling carried out by Halcrow (2011) showed that peak flows occur around the time of mid flood and mid ebb tides. Peak flood speeds (1.3 to 1.8 m/s) were generally higher than ebb speeds (0.8 to 1.5 m/s), for a high spring tide (6.28 m at Hinkley). The Parrett Estuary is tidally influenced up to 34 km inland, from Stert Point to Oath Lock. The River Tone, the major tributary of the Parrett, is tidally influenced up to Newbridge Sluice, 35.6 km from the estuary mouth. Tidal flow brings with it more saline conditions. The average limit of saline intrusion on the Parrett is around 25 km upstream of Stert Point. Under mean neap and spring tidal conditions, the tidal limits (defined by tidally varying water levels) of the Parrett and Tone system are further upstream than the measured extents of saline intrusion (HR Wallingford, 2016). The flood tide on the Parrett Estuary is of significantly shorter duration than the ebb. HR Wallingford (2016) reported that in Bridgwater the duration of the flood tide is around 2.5 to 3 hours, while the ebb tide is around 9 to 10 hours. This flood dominance (shorter duration but higher flow speeds) has important consequences for the transport of sediment in the estuary. The higher velocities and greater turbulence on the flood tide mobilises relatively greater silt than on the ebb tide where velocities are lower.

Figure 2.1: Example tidal curves (Source: 2016 recorded tide data, West Quay Bridgwater) Freshwater flow The estuary is tidally dominated during periods of low fluvial flow, but is more strongly influenced by freshwater during periods of high fluvial flow (Defra, 2002). The degree of freshwater influence increases with distance upstream of the mouth.

Geomorphological Baseline Report The main sources of freshwater flow into the tidal Parrett Estuary come from the upstream catchment, above Oath Lock on the River Parrett and Newbridge Sluice on the River Tone. In addition, there are a number of freshwater inputs from land drainage which are managed through control structures. The largest inputs downstream of Bridgwater are the River Brue at Highbridge, Huntspill River, King’s Sedgemoor Drain at Dunball, and Cannington Brook. These are all located downstream of the preferred barrier site, the closest being King’s Sedgemoor Drain 2 km downstream. Durleigh Brook (3 km upstream) and Petherton Stream discharge into the Parrett upstream of Chilton Trinity. Freshwater flows through these sluices are controlled by the Environment Agency (except for Cannington Brook which is controlled by a flapped outfall) and they discharge into the Parrett Estuary when water levels in the estuary are low enough. Typically winter and spring are the wettest seasons whilst summer and autumn are the driest. Data for monthly freshwater flows at Combwich over a 10 year period from 2004-2014 were presented in CH2M, 2015 (Figure 4.2). Fluvial flows ranged from a maximum of 75 m3/s in the winter (recorded in February 2014) to less than 10 m3/s in autumn (minimum flow of 3 m3/s) recorded in October 2005. The Parrett is a well-mixed estuary (in terms of mixing of saline and freshwater flow) due to the high tidal range, which generates complex currents and results in thorough mixing within the water column. Salinity measurements reported in CH2M 2015 from the 1970s showed that overall mean salinity decreased from 26 ppt at Stert Point to 5 ppt at Bridgwater (Thorn and Maskell, 1979). During summer when the freshwater input at the tidal limit is small, the salinity of the estuary increases. During winter the occurrence of freshwater floods causes estuary salinity to reduce (Thorn and Maskell, 1979). Position of the turbidity maximum (TM) Sediment is deposited in an estuary during periods of slack water and is mobilised during periods of high flow. As shown in figure 2.1, the flood tide is relatively rapid in the River Parrett estuary and tends to mobilise sediment and carry it upstream. Conversely the ebb tide is slower and so less deposited silt is mobilised and carried downstream. Because of the tidal asymmetry there tends to be a landward movement of tidally driven sediment. This process is referred to as ‘tidal pumping’ (see Figure 2.2). However high fluvial flows in the winter carry sediment downstream and it is this balance between tidal pumping and fluvial movement of sediment which drives the sediment regime. Up and down estuary flows converge at some point, with the downstream flows having a significant fluvial component. Sediment concentrations tend to increase at this convergence, causing a turbidity maximum (TM) and increase sedimentation (due to the low velocities and high sediment concentrations). Due to the complex interactions of flow, salinity and sediment, this mixing zone has important consequences for the mobility and transport of suspended particles, so it is a key control on sediment processes in the estuary The deposition of large quantities of fine sediment on the estuary bed and on to adjacent intertidal areas generally occurs during slack water periods when flow speeds decrease. Sedimentation is directly related to the concentration of suspended sediments, which is highest near to the TM.

Geomorphological Baseline Report

Downstream Upstream

Figure 2.2: Conceptualisation of tidal sediment pumping mechanism (Source: HR Wallingford, 2016) The geographical location of the TM is dependent on river discharge and tidal conditions at any given time. It can vary on a daily basis but a significant pattern has been observed seasonally in the Parrett (typically summer vs winter), see Figures 2.3 and 2.4. Concentrations of suspended sediment alter constantly with entrainment and settling of sediment. In general the TM during spring tides is pushed further up the estuary than at neap tides, due to increased forcing from the higher mean water level at the mouth of the estuary. The recent dredging strategy for the Parrett Estuary produced by HR Wallingford (2016) compiled literature from different sources to describe the location of the TM under various states of tide (spring and neap) and river flows (high and low). It commented on research from Burt (1980), which suggested that the average location of the TM in the Parrett was broadly represented by the limit of saline intrusion, some 25 km upstream of Steart (Figure 2.3).

Figure 2.3: Average salinity along the tidal section of the River Parrett. (Sources: Burt, 1980/HR Wallingford 2016) During neap tides and in particular during corresponding periods of strong fluvial flow, typically winter and spring, the TM is likely to be pushed further downstream to Bridgwater (HR Wallingford,

Geomorphological Baseline Report 2016). This is supported by visual evidence of extensive deposition at Dunball following winter floods in 2014, indicating the TM was located nearby (personal communication, Port of Bridgwater). During the lowest fluvial flow conditions, typically summer and autumn, the TM can move further upstream than the average location. At such times the limit of saline intrusion (and therefore the position of the TM) has been recorded up to 28 km (during spring tides) and 20 km (during neap tides) upstream from Stert Point (HR Wallingford, 2016). This corresponds with evidence from the Parrett Monitoring Project (Black & Veatch, 2011) which estimated that during spring tides along the River Parrett, the TM is located upstream of Bridgwater between the M5 motorway and Burrowbridge (see Figure 2.4). Tidal bore The River Parrett is a relatively sheltered estuary with limited influence from wind and waves. Its funnel shape, combined with high tidal range, means that the estuary develops a small tidal wave, or bore. The bore travels twice daily up the estuary through Bridgwater, and usually occurs around 1-2 hours before high water. A key feature of tidal bores is intense turbulence and turbulent mixing that can be generated and extend for considerable distances. Typical speeds of the bore measured along the Parrett Estuary are around 10 km/hr, which can result in some erosion of sediment beneath the bore front and along the channel banks. The height is dependent on meteorological conditions at the time of high water; the largest bore waves occurring with spring tides. The Parrett bore is small in comparison with the Severn bore and is expected to have limited geomorphological impact. Conceptual Model – Hydrodynamic Processes The information documented in this chapter has been used to develop a conceptual model (Figure 2.4) to describe estuary hydrodynamics under different seasonal conditions. It has been subdivided into winter/spring and summer/autumn flows. The purpose of the conceptual model is to summarise and inform the understanding of complex tidal processes within the Parrett Estuary system.

Geomorphological Baseline Report Figure 2.4: Conceptual model of seasonal hydrodynamic processes in the Parrett Estuary (for locations refer to Study Area Figure 1.1)

Geomorphological Baseline Report Chapter 3 Sediment System

This chapter provides a summary of the sediment system of the Parrett Estuary, in terms of sediment sources, transport and deposition. Sediment Sources Channel bed material  In the outer estuary, bed material is predominantly fine sand (Thorn and Maskell, 1979).  Average grain size decreases upstream, and coarser material nearer the mouth is likely to be of marine origin (Black and Veatch, 2011).  Upriver of Dunball, bed material becomes progressively more silty (Thorn and Maskell, 1979).  In the upper estuary and into the Parrett and Tone, bed sediments are typically composed of very fine sand and very coarse silt (Black and Veatch, 2011).  Sediments in the River Tone have a redder colour than those in the River Parrett and this difference is due to different fluvial sources (Black & Veatch, 2011).  Thorn and Maskell (1979) commented on a clear demarcation between the fine, cohesive sediments found in suspension in the water column and on the estuary margins, and the coarser, non-cohesive sand on the bed of the main channel (general comment related to middle estuary). Channel banks  Typically the banks along the Parrett Estuary are composed of very fine sand to very coarse silt, which is compositionally homogenous (i.e. of relatively uniform grain size) (Black & Veatch, 2010).  Recent testing undertaken during trials for the Somerset Internal Drainage Board (Ambios, 2017) has indicated that the banks in the upper estuary contain cohesive material with a high shear strength.  Median (D50) grain size for sampled locations along the River Parrett banks indicated values in the range of 22 to 46 µm, with a general decrease in median grain size with increasing distance upstream (Black & Veatch, 2010).  River banks and intertidal slopes are of variable erodibility along the cross-section, with lower banks more easily eroded than the upper banks (leading to mass failures), and the increased resistance of upper banks attributed to the presence of vegetation (Black & Veatch, 2010). Suspended sediment concentrations  Suspended sediment concentrations in the Parrett Estuary, as well as the wider Severn Estuary and Bristol Channel are very high. Mantz and Wakeling (1981) measured concentrations in Bridgwater bay for spring tides of 0 to 2,000 mg/l and for neap tides of 0 to 500 mg/l.  HR Wallingford (1989) estimated suspended sediment concentrations in the fluid mud layer in the outer estuary to be as high as 200,000 mg/l. Turner & Burt (1985) measured suspended solid concentrations within fluid muds in the River Parrett in the region of 50,000-180,000 mg/l.

Geomorphological Baseline Report  At Stert Point, Thorn & Burt (1983) reported values of 300-77,000 mg/l. Near the present day Steart Marshes, Thorn & Maskell (1979) observed peak concentrations of up to ~80,000ppm at time of highest water slack period, but lower concentrations of approximately 10,000-20,000 ppm during other periods.  Derbyshire and West (1993) showed that there was a very strong relationship between suspended solids concentrations and tidal level, with higher tides being associated with high suspended sediment levels. This was especially the case downstream of Combwich. Figure 3.1 shows that the relationship between tidal level and suspended sediment concentration is strongly non-linear and indicates the effect of higher tide levels on suspended sediment moving up the estuary (this supports the idea of the TM moving further upstream during higher tides).  Measurements of suspended sediment indicate an order of magnitude of difference between concentrations during the flood tide (order 1000s mg/l) and the ebb tide (order 10s to 100s mg/l) (HR Wallingford 2016).

Figure 3.1: Relationship between suspended sediment concentration and tidal (high water) level at Bridgwater. (Reproduced from HR Wallingford, 2016). Fluid mud Fluid mud has been identified within the Parrett Estuary. Fluid mud is a near-bed, high density, cohesive sediment suspension layer which forms during the slack water periods of the tidal cycle and is easily re-suspended on the next phase of the tide. It greatly increases the concentration of mud in suspension in the water column at the main run of the tide. Evidence from Van Rijn (2016) suggests that fluid mud can accumulate between 2-3 m thick on the bed of the channel within the wider Severn Estuary, but that this is normally re-entrained by accelerating tidal currents. It has been observed that during lower energy conditions during neap tides, fluid mud is not completely re-entrained, and instead remains in partial suspension within the

Geomorphological Baseline Report water column. In this way dense mobile sediment suspensions can persist from neap to intermediate portions of the neap-spring cycle, but not evenly dispersed through the water column. Fluid mud is likely to form at slack water where the concentration of mud in suspension in the lower water column exceeds about 2,500 ppm (Maskell, 1984 reported in Dronkers, 1988), which is common throughout the Parrett Estuary based on measured concentrations. As well as tidal flows, wave action and disturbance by drag heads of dredgers or ship propellers with small keel clearance may fluidize a mud bed. Figure 3.2 is taken from Dronkers (1988) and shows that mud concentrations on the incoming flood tide at the seaward boundary (Bridgwater Bay) within the Severn Estuary peak at about 2,000 mg/l (ppm) on a spring tide. Their modelled computations in the Parrett Estuary itself show that the suspended mud concentrations are amplified by a factor of about 10, and rise up to 15,000 mg/l (ppm) immediately upstream of Bridgwater, (see Figure 3.2). This gives rise to concentrations sufficient for fluid mud to develop.

Bridgwater

Figure 3.2: Computed longitudinal variation of suspended mud concentration in the Parrett Estuary, UK (annotated from Dronkers, 1988). Red dash line indicates 2500ppm (indicative concentration for fluid mud development). Sediment Transport Outer Estuary (Bridgwater Bay)  On the north-facing coast (from Hinkley Point to the Parrett Estuary), sediment transport is generally from west to east, towards the mouth of the Parrett (Defra, 2002).  On the west-facing coast (north of the Parrett Estuary), sediment transport is generally from north to south towards the mouth of the estuary (Defra, 2002).  Gore Sand may act as a transport pathway for sand between the intertidal and offshore zones, aided by ebb flows along the Parrett Estuary channel (Defra, 2002).  The exchange of suspended sediment between the Parrett and the wider Severn Estuary during a mean spring tide (estimated by Thorn and Burt (1983) from a continuous tidal survey measurement) was 60,000 tonnes.

Geomorphological Baseline Report  Thorn and Burt (1983) calculated that the total weight of sediment in dynamic equilibrium (i.e. moving upstream and downstream within the Parrett estuary, but generally staying within the system as a whole) is approximately 200,000 tonnes on a mean spring tide. Middle and Inner Estuary The following general observations have been made:  Sediment transport within the Parrett Estuary is strongly influenced by tidal flow (spring versus neap) and freshwater flow (summer versus winter).  The tidal flood dominance of the Parrett Estuary has led to the dominance of marine sediment within the Parrett Estuary (Tinkler, 1979; Thorn and Maskell, 1979).  The strong tidal flood speeds carry large quantities of sediment into and up the estuary, whilst the lower speeds on the ebb only carry a more limited amount seaward, leading to a net inward transport of sediment. This is known as flood dominance.  In the winter, marine sediment entering the estuary is much reduced due to freshwater flushing effects, whilst during the summer; marine sediment becomes the dominant source within the estuary (Black and Veatch, 2011).  A recent sediment monitoring programme along the River Parrett (upstream of Bridgwater) concluded that large volumes of marine sediment are transported into the system and upstream of Bridgwater on spring tides (>4.8 m AOD at Burnham) but not under normal tidal conditions (Black & Veatch, 2010). Tidally deposited material was observed to be removed on the ebb tide or during minor fluvial events.  Tinkler (1979) believed the annual cycle of deposition within the estuary represents a dynamic equilibrium, characterised by a build-up of silt in the summer when river flows are low, and the scouring out of sediment during winter floods.  The flushing effect of freshwater flows is consistent with observations made in other UK estuaries. For example, in the Humber, the highest suspended solid concentrations are found in the vicinity of the TM in the upper reaches of the estuary.  At about high water the TM is located well up the estuary and concentrations of suspended sediment reduce because of settling. At low water the TM is located further down the estuary, and reduced concentrations occur due to settling at slack water. Material is then re- mobilised during flood and ebb tides, and the cycling of sediment between the channel bed and banks and the water column continues (HR Wallingford, 2016).  Spring tides enhance the deposition of sediment on intertidal areas between Steart and Dunball due to the long slack water period of high water, together with the higher concentration (HR Wallingford, 2016).  Under large freshwater discharges the TM moves downstream, and within the River Parrett freshwater flows can enhance the ebb tide sufficiently to induce net erosion of tidal sediments (HR Wallingford, 2016).  During baseline conditions relatively small quantities of sediment are being transported into the inner estuary from upstream (recorded as 30 tonnes per day during the winter and <100 tonnes per day summer). During fluvial flood events large volumes of sediment can be transported into the Parrett and Tone from upstream (recorded as 1,278 tonnes per day in the winter and 1,186 tonnes per day in the summer) (Black and Veatch, 2011).

Geomorphological Baseline Report  During winter, upstream of the Parrett and Tone confluence, sediment transport can be dominated by river flows. It has been observed that high spring tides can reverse flows and create periods of slack water (impounding freshwater flows upstream) which encourage sediment deposition further up the estuary (Black and Veatch, 2011).  The mean input of fluvial sediment derived from rivers is estimated at around 5 tonnes per tidal cycle although this will vary considerably. The sediment input from fluvial sources is generally considered to be very low compared to the marine supply in regard to the overall sediment balance (Thorn and Burt, 1983). Role of extreme events in transporting sediment The CH2M 2015 review highlighted the need to better understand the role of ‘extreme’ events in transporting sediment within the Parrett Estuary. This includes both storm surge events and large fluvial flood events. There is little information available regarding the impact of storm surges on sediment transport within the Parrett Estuary. However, it is general accepted that the effects will be similar to a spring tide, in that there is likely to be a net import of sediment further upstream along the estuary. In addition, there is likely to be greater vertical mixing within the water column, so that coarse marine sediment will be deposited much further upstream than occurs under a normal spring tidal event. While this may be significant at the time of the event, these events occur infrequently and are not expected to have a big overall impact on the overall sediment dynamics of the estuary. For fluvial flood events, there is substantial evidence which shows that, especially during winter, freshwater floods can cause deposited sediments to be flushed out of the estuary. During summer when the freshwater input at the tidal limit is very small (typically less than 3 m3/s), there is a net accretion of bed sediments brought into the estuary by the tide from the Severn Estuary. During winter, the more regular occurrence of combined freshwater floods and long duration ebb tidal flow causes accumulated bed sediments to be flushed out of the estuary. Thorn and Maskell, 1979 (reported in CH2M, 2015) estimated that a discharge of approximately 125 m3/s could result in sediment flushing. However, this may be superseded by more recent modelling which indicates maximum fluvial discharges are around 65 m3/s within the Parrett. Higher fluvial discharge causes the TM to move downstream, and within the River Parrett freshwater flows have been known to enhance the ebb tide sufficiently to induce net erosion of tidal sediments (HR Wallingford, 2016). Flood events also increase turbidity within the estuary. A monitoring survey undertaken on the Parrett Estuary upstream of Bridgwater recorded a storm event on the 4th December 2008. It was shown to increase flow velocities, and increase suspended sediment concentrations up from 30 mg/l measured at low (baseflow) conditions to 711 mg/l during the event. This massively raised turbidity throughout the system as a result. Another issue in relation to large fluvial flows is that it can lead to stratification occurring within the water column i.e. much reduced mixing of freshwater and tidal/saline flows. In the report by HR Wallingford (2016) it is suggested that at flows over 80 m3/s this is likely to occur in the Parrett Estuary, particularly when a large fluvial flow coincides with neap tidal conditions. Sediment Deposition Outer Estuary Bridgwater Bay is a sink for sediment, and a thick layer of mud (the Bridgwater Bay mudbelt) has accreted here (Defra, 2002). An exchange of mud occurs between the intertidal and offshore

Geomorphological Baseline Report sections of the Bridgwater Bay mudbelt, with erosion of the intertidal areas putting mud into suspension, which is then transported offshore to be deposited on the seaward face of the mudbelt (Defra, 2002). The River Parrett acts as a long term source of fine-grained sediment to Bridgwater Bay and the wider Bristol Channel (Posford Duvivier and ABP Research, 2000). Recent analysis of LiDAR data, undertaken by HR Wallingford for the Crown Estate (HR Wallingford, 2016) shows that between 2003 and 2014 there was a trend for slight accretion observed within Bridgwater Bay. The Futurecoast study reported that the Parrett Estuary was almost ‘full to capacity’ with sediment, and is therefore only likely to be a weak sink for mud in the wider context of the Severn Estuary and Bridgwater Bay (Defra, 2002). Middle and Inner Estuary The following general observations have been made:  Marine derived coarse sediment tends to settle out at slack water in the bed of the low flow channel (Black and Veatch, 2011).  Fine, cohesive sediment (carried in suspension) tends to be deposited on the banks in intertidal areas rather than on the bed (Black and Veatch, 2011).  The banks build up until they become unstable and then ‘slump’ or collapse into the channel. Because of this mechanism, recent dredging works (2014-15) focussed on removing sediment from the intertidal banks rather than the bed.  Spring tides enhance the deposition of sediment on intertidal areas between Steart and Dunball due to the long slack water period of high water, together with the higher concentration (HR Wallingford, 2016).  Siltation rates within the intertidal zone can be locally very high if the natural regime is disrupted, such as by dredging. For example, the berth at Combwich was dredged in September 1972 and in the following two years up to 5m of siltation occurred there (Thorn and Burt, 1983).  At the confluence between the Parrett and Tone the river bed gradient is very flat, which means that high water levels in the Parrett can cause water levels to back up in the Tone, reducing flow and resulting in sedimentation within the channel.  The geomorphology of the Parrett Estuary is dynamic and can be responsive to changes in natural forcings such as tidal variation, fresh water flow or resulting man-made activities such as dredging and increased freshwater discharges4.  High siltation rates have been observed following dredging operations in the tidal reaches.  Sedimentation rates in the Parrett and Tone are rapid - a year after dredging along the River Tone in the late 1960s, approximately 12 tonnes of silt per metre length of channel had been re-deposited (Tinkler, 1979).  The Environment Agency manages a number of outfalls from surface channel land drains throughout the Parrett Estuary. Generally these outfalls are de-silted every year so they can operate correctly. In 2014 there was significantly more siltation at the outfalls than in previous years, which may be a response to the dredging carried out in 2014 (CH2M, 2015).

4 Following the winter floods of 2013/2014 the pumps from the King’s Sedgemoor Drain at Dunball changed the rate and pattern of sedimentation within the estuary, resulting in rapid deposition of sediment along the intertidal area on the opposite banks to the pumps. Personal communication: Harbour Master, Port of Bridgwater.

Geomorphological Baseline Report  HR Wallingford (2016) described the behaviour of sediment and water within the Parrett system under different conditions: o Neap tide, low freshwater flow - Sedimentation is most likely to occur between Bridgwater and Burrowbridge. Sedimentation rates are relatively low due to relatively low suspended sediment concentrations. o Neap tide, high freshwater flow - Sedimentation is unlikely to occur anywhere in the system. Fluvial flow prevents deposition and augments the ebb tide flow. Relatively low suspended sediment concentrations during neap tides. o Spring tide, low freshwater flow - Sedimentation is possible across the whole Parrett system, through a combination of the influence of marine waters and tide locking. Sedimentation rates are expected to be very high along the Bridgwater to Burrowbridge section and moderate to high (dependent upon the level of tidal influence) along the Tone and Parrett rivers upstream of the confluence. o Spring tide, high freshwater flow - The sedimentation pattern is not clear under these conditions, as it will depend upon the specific conditions that occur. However, upstream of the Parrett and Tone confluence, sedimentation is possible due to tide locking. Closer to the tidal limits erosion could occur due to the increased dominance of freshwater flow. Downstream of the confluence, flood tide sedimentation could be balanced by ebb tide erosion.  The sediment transport modelling undertaken to assess the impact of the proposed barrier provides further evidence regarding the impact of brief changes in the system (by temporarily closing the barrier). The modelling supports the view that the overall behaviour of sediment is driven by the longer term processes rather than single events, and any temporary disturbance in the sediment system tends to return to the longer term regime over time. Conceptual Model – Sediment System A conceptual model has been developed to summarise the information in this chapter. The first model (Figure 3.3) describes the sediment sources of the Parrett Estuary and the second model summarises information regarding sediment dynamics (transport and deposition), see Figure 3.4.

Geomorphological Baseline Report

Figure 3.3: Conceptual model of sediment sources in the Parrett Estuary (for locations refer to Study Area Figure 1.1)

Geomorphological Baseline Report

Figure 3.4: Conceptual model of sediment dynamic processes in the Parrett under Spring and Neap tides (Modified from HR Wallingford, 2016) (for locations refer to Study Area Figure 1.1)

Geomorphological Baseline Report Chapter 4 Morphology

This chapter provides a summary of understanding of the overall geomorphology of the Parrett Estuary, in terms of its overall historic evolution, development and movement of the low water channel, and wider morphology. Morphology The Parrett Estuary is a filled river valley (Ria) type estuary. The estuary mouth is constrained along its western side by Stert Point, but beyond this opens into the extensive tidal mudflats and sand banks in Bridgwater Bay. These mudflats represent the ebb tidal delta of the Parrett Estuary (Defra, 2002). Over its entire length (31 km) from Stert Point to the tidal limits at Newbridge Sluice (on the River Tone) and Oath Lock (on the Parrett) it is characterised by steep-sided muddy banks and a relatively flat bottom. The channel width is about 250 m at Combwich, reducing to about 50 m at Bridgwater, and to about 20 m at the tidal limits in the fluvial sections of the rivers. At the site of the proposed barrier at Chilton Trinity the channel is approximately 118 m wide between flood banks, which are measured to be approximately 60 m wide from the mean high water spring level. The reach is generally straight, but is a short distance downstream from a sharp meander bend. Generally the low water channel is confined to a single channel within the centre of the tidal channel. Intertidal areas are exposed on both banks. Pethick (2002) considered that the form of the outer channel is related to the balance between:  ebb tidal flows in the Severn estuary/Bristol Channel, which are directed east to west on the lower foreshore within Bridgwater Bay;  momentum of the flows exiting the Parrett Estuary; and  wave dominated flows and transport on the upper foreshore of Bridgwater Bay which are directed from west to east. The morphology and size of the Parrett Estuary channel reflects the tidal volume it accommodates, as well as modifications to the estuary over time. The large meanders (typically 4 km wave length) and high channel width (typically 200 m to 500 m) in the middle estuary downstream of Bridgwater are influenced more strongly by tidal rather than fluvial flow. Pethick (2002) carried out preliminary regime modelling of the estuary. This work was developed further by Black and Veatch (2006). The work concluded that:  The estuary is mostly stable, although some parts of the estuary are experiencing erosion or deposition due to the varying position of the low water channel.  The present middle and inner estuary are believed to be close to an equilibrium regime state, but without modifications and resulting constraints it would be expected to be wider/larger to be exactly ‘in regime’.  The outer Parrett estuary is wider than regime, and is therefore likely to tend towards sedimentation, particularly from tidally derived sediments. This may also explain the expansion of saltmarsh in this area. The tidal prism volume of the Parrett is approximately 36 x 106 m3 during a mean high water spring tide (MHWS). Of this 1.1 x 106 m3 is upstream of the proposed barrier structure (this represents approximately 3% of the overall tidal prism volume of the Parrett Estuary). The total baseline tidal prism volume is likely to increase in the future due to:

Geomorphological Baseline Report  Sea level rise5, which is predicted to increase the tidal prism volume of the Parrett Estuary by 9% (to 39.5 x 106 m3).  Managed realignment schemes, such as those proposed in the Parrett Estuary Flood Risk Management Strategy (PEFRMS, Environment Agency 2010), which are estimated to increase the Parrett Estuary tidal prism volume somewhere between 0.5% and 4% depending on the location developed. In both cases the regime in the estuary will change over time depending on the available sediment supply in response to the increasing tidal prism. This will tend to offset part of the increase in tidal prism. Historic evolution There has been a history of anthropogenic interventions in the Parrett Estuary including reclamations (large areas of former marshes have been reclaimed over the centuries), dredging operations, construction of port facilities and bridge crossings. In the past, the mouth of the estuary at Bridgwater Bay has moved westwards or anticlockwise (see Figure 1.1), which is believed to have been due to an overall decrease in the estuary tidal prism due to land reclamation (Pethick, 2002). The estuary channel has also changed in terms of the position of the high and low water marks. From the 1700s onwards, the mouth of the Parrett drifted towards Burnham-on-Sea due to the eastward drift of sand and shingle along the upper shore (Royal Haskoning, 2009). Additionally, near Stockland (which lies to the west of the current Parrett Estuary), two apparent former channels have been replaced by a single channel, following infilling of the western channel and the accretion of saltmarsh on the eastern side of Steart Peninsula. An island that formed at the mouth of the Parrett Estuary (Fenning Island) has also changed in form and become attached to the peninsula through the growth of saltmarsh, and the main flow channel west of Burnham has widened and narrowed over time. The high tidal range means that the estuary dries extensively at low water, which exposes a meandering low water channel (thalweg) at the base of the channel. This low water channel extends for 11.3 km downstream beyond Stert Point, curving westwards before it ends at the Bridgwater Bar (CH2M, 2015) The course of the Parrett Estuary, especially its low water channel, has varied over time, and significant changes have occurred in the outer estuary since the 1800s (CH2M, 2015). As part of the Steart Coastal Management Project, Halcrow (2011) carried out analysis of LiDAR data (2007, 2008 and 2009) for the outer channel in the vicinity of Burnham. This showed that the western bank of the low water channel showed significant variability, with lateral movement of 20 to 50 m over the three year period. By contrast, the landward (eastern) side of the channel remained stable. The results indicate an interaction of the estuary channel with open coast littoral processes in the outer reaches of the Parrett Estuary. Within the middle estuary, the meandering low water channel is narrow and muddy, and is constrained in its ability to adjust laterally by flood defences that have been constructed along its entire length (Defra, 2002). The low water channel is actively mobile in places, migrating in response to the erosion and deposition of basal mud, silt and sand. Monitoring being undertaken for the portion of the Parrett adjacent to the Stert Marshes habitat creation scheme (CH2M, 2017b) documents channel behaviour in this region of the estuary. This work showed extensive lateral variations in channel position below the level of mean low water neap tides. At higher levels, around the edges of saltmarshes (mean high water neap) the channel is less variable.

5 Based on sea level rise allowances used in the PEFRMS (Environment Agency, 2010).

Geomorphological Baseline Report The present day intertidal areas are governed by the position of the low water channel and the modifications constraining the channel margins. Where the channel lies close to the present defences, the intertidal mudflats are narrow and steep. In other parts where the channel lies some distance from the estuary margins, the intertidal mudflats are more extensive. The areas of intertidal marshes lie at an elevation above mudflats and are typically more limited in their extent due to the presence of flood defences (CH2M, 2015). There is a significantly larger area of intertidal habitat downstream of the preferred barrier location (Chilton Trinity) than upstream. Table 4.1 shows the area of intertidal habitats calculated from GIS analysis, extracting the area between high and low water levels from OS Mastermap. The majority of intertidal habitats are in the middle-outer estuary, comprising mudflats between Steart and Combwich, saltmarsh nearer to Steart (including the developing area from habitat creation) and larger sand and mudflats into Bridgwater Bay. Intertidal areas in the majority of the middle and into the upper estuary, including through Bridgwater, are narrow and steep, with vegetation in the upper banks. Location of intertidal habitat Upstream of Downstream of barrier site (ha) barrier site (ha) Left bank 10 2603 Right bank 19 2313 In channel 0 368

Table 4.1: Intertidal habitat area (hectares) within the Parrett Estuary, total area upstream to the tidal limit and downstream to Bridgwater Bay from the preferred barrier location at Chilton Trinity. Further assessment of LiDAR data has been undertaken, comparing data from 2007 and 2014 (but no data for intervening years). This was undertaken to address some of the gaps identified in previous work and informed the options appraisal / short-listing for tidal barrier site selection. The analysis provided information on the current (2007-2014) stability of the low water channel; the stability of the existing channel cross-section, and considered how planform may influence the operation of the barrier. Conceptual Model – Morphology A conceptual model has been developed to summarise this information and is shown in Figure 4.1 below.

Geomorphological Baseline Report Figure 4.1: Conceptual model of morphological features of the Parrett Estuary (red text relates to regime analysis; green text highlights morphological processes and features)

Geomorphological Baseline Report Chapter 5 Summary of geomorphology baseline for proposed barrier site

This section summarises the baseline conditions from the wider review of the estuary geomorphology which were used to inform the decision on the preferred barrier site (Site 5, Chilton Trinity) and for assessment of the potential impacts of the barrier operation. The option appraisal process and the assessment of environmental effects have been documented in the scheme Options Appraisal and Preliminary Environmental Information reports. Tidal prism and turbidity maximum The tidal volume of the estuary upstream of Chilton Trinity has been estimated to be 1.1 x 106 m3, which is approximately 3% of the overall tidal prism of the Parrett Estuary. The prism relationship appears to be close to the 'ideal', or slightly edging towards erosional in this area of the estuary. Chilton Trinity is located downstream of the influence of the TM, which in summer is usually located upstream of Bridgwater between the M5 motorway and Burrowbridge (refer to Figure 3.4). The TM has not been directly recorded from field data in the winter, but from the understanding of the tidal and fluvial flow interactions it is expected to be located further downstream due to the stronger influence of fluvial flows (see Figure 2.4). Despite being outside of the TM, this area of the estuary is known to have a very high suspended sediment concentrations (>10,000 ppm). Fluid mud is known to form at slack water where the concentration of mud in suspension in the bottom of the water column exceeds about 2,500 ppm. There is therefore a high likelihood that fluid mud will occur in this area of the Parrett Estuary and may have some significance for ongoing maintenance of the barrier. Morphology The channel width (MHWS) at Chilton Trinity is approximately 60 m, and is confined by wider flood embankments along both banks (120 m width between the flood banks). The flood embankments are located close to the edge of the channel along the left bank in particular, and are potentially vulnerable to erosion and coastal squeeze. From reviewing historic maps and LiDAR information from 2007 and 2014, the overall capacity of the channel has reduced in recent years at the barrier site (net change) due to deposition along both intertidal banks. This is particularly notable along the right hand bank. There is also evidence of erosion of the intertidal banks (along the left hand side), which appears to be associated with discharge from a tributary that outfalls at Pims Clyce. The tidal prism analysis discussed above suggests that the River Parrett can be classified as being between “within regime” and slightly erosional; however, examination of LiDAR time series data indicates an accretional trend. Although these results appear to be contradictory it is more likely that LiDAR data reflects variability over a shorter temporal and spatial scale whereas the tidal prism analysis describes the overall long term trend of the estuary. Therefore the results of both analyses are valid. Short term fluctuations around a longer term regime are to be expected. The barrier site is located along a straight section of the estuary channel, downstream of a sharp meander bend which extends around Chilton Trinity Sewage Treatment Works. From review of the low water channel location, the highest velocities on the incoming flood tide are likely to be fairly centrally located within the channel cross-section. The low water channel has been relatively stable in this area of the estuary, being well confined in a central single channel. There are intertidal areas along both banks.

Geomorphological Baseline Report Considerations for barrier impacts Closing the barrier will result in a temporary reduction in estuary tidal prism of approximately 3% and is likely to result in a similar reduction in tidal flow. Whilst the barrier is closed the normal location of the TM upstream of Bridgwater will be moved downstream of the barrier. The effects of the barrier will therefore be directly dependant on its operational frequency in the short, medium and long term epochs, and uncertainty increasing with time. In general, the barrier location selected at Chilton Trinity was preferred over other locations, from the perspective of potential impacts on geomorphological processes. This was because of the location along a fairly straight section of channel and overall stability of the low water channel. However, there is evidence of deposition causing a slight reduction in channel capacity in recent years, and this will require monitoring during the operational period of the barrier. A detailed assessment of the interaction of the barrier with geomorphological processes is reported in the Options Appraisal and Preliminary Environmental Information reports.

Geomorphological Baseline Report References

Ambios, 2017. Dredging Trials Monitoring Programme November-December 2016. Report for Somerset Drainage Boards Consortium. Report ref AmbSDBC02 (draft).

Black and Veatch, Dec 2006. Parrett Tidal Flood Defence: Sluice/Embankment technical review. Report produced by Black and Veatch Ltd for the Environment Agency South West Region.

Black & Veatch, August 2010. River Parrett and Tone Channel Monitoring Project, Summary Report. Environment Agency.

Black and Veatch, 2011. River Parrett and Tone Channel Monitoring Project, Summary Report, Final Report, Report produced by Black and Veatch Ltd for the Environment Agency, 41pp, February 2011

Burt,1980. Pilot Study of the River Parrett: Interim Discussion Paper.

CH2M, 2015. Parrett Barrier: Geomorphology Assessment and Peer Review.

CH2M, 2016. Bridgwater Tidal Barrier Appraisal. Options Appraisal Report. Version 3: Short List. September 2016.

CH2M, 2017a. Bridgwater Tidal Barrier. Barrier Morphological Impacts Modelling. Technical Memorandum. March 2017.

CH2M, 2017b. Far Field Effect Report – Note 3. Prepared for Environment Agency June 2017. Report prepared for the Environment Agency by CH2M Halcrow, Version 1.1, August 2017.

Dronkers, J & Leussen, W.V., 1988. Physical Processes in Estuaries, Springer-Verlag. 512pp + Figure. Defra, 2002. Managed Realignment Review. Joint report produced by Defra and the EA.

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Environment Agency, 2010. Steart Coastal Management Project, Scoping Consultation Document, Appendix B, Geomorphology Review. Report produced by Halcrow group Ltd for the Environment Agency. 110 pp + Annex.

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HR Wallingford, 1989. Fluid mud in estuaries – Field measurements 1989. HR Report EX2076. Report produced by HR Wallingford for the Energy Technology Support Unit of the Department of Energy’s Renewable Energy Research Programme. January, 2009, 7pp + Figs.

HR Wallingford, May 2016. ‘Somerset Levels and Moors Flood Action Plan Dredging Strategy (Draft)’. Somerset Rivers Authority.

HR Wallingford, 2016. ‘Severn Estuary Long Term Morphology. An updated review of baseline changes – Report on analysis of LiDAR data’. The Crown Estate.

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Mantz, P.A. and Wakeling, H.L., 1981. Aspects of sediment movement near to Bridgwater bar, Bristol Channel. Proc. Instn Civ. Engrs, Volume 73, Issue 1, 1 – 23p.

Pethick, J. S., 2002. Managed realignment within Bridgwater Bay and the River Parrett. Preliminary geomorphological assessment. Report to Babtie Group by Professor John Pethick. 22pp August 2002.

Royal Haskoning, 2009. Steart Compensation Scheme – Environmental Scoping Report. Report for the Bristol Port Company (4 February 2009), Final Report 9R4093, 90pp. Thorn and Burt, 1979.

Thorn, M.F.C. and Burt, T.N., 1983. Sediments and metal pollutants in a turbid tidal estuary. Can. J. Fish. Aquat. Sci, 40 (suppl.), 207-215pp.

Thorn, M.F.C. and Maskell, J.M. 1979. River Parrett Tidal Barrier: hydraulic investigations, 1979. Paper presented at the summer meeting of the Institution at Taunton on 29th of June, 1979.

Tinkler, B.A., 1979. The Parrett Barrage Investigation: Its effects and problems. Paper presented at the summer meeting of the Institution at Taunton on 29th of June, 1979. Van Rijn, L.C. July 2016. Fluid mud formation. www.leovanrijn-sediment.com.