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Severn Estuary Long Term Morphology Synthesis of data and modelling studies

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Page 1: Synthesis of data and modelling studies - Crown Estate · PDF fileDDR5432-RT002-R03-00 July 2016 Severn Estuary Long Term Morphology Synthesis of data and modelling studies

Severn Estuary Long Term Morphology

Synthesis of data and modelling studies

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© Crown copyright 2016 ISBN: 978-1-906410-72-8 Published by The Crown Estate The basis of this report was work undertaken by HR Wallingford, on behalf of The Crown Estate. Disclaimer The opinions expressed in this report are entirely those of the authors and do not necessarily reflect the view of The Crown Estate. The Crown Estate is not liable for the accuracy of the information provided or responsible for any use of the content. Dissemination Statement This publication (excluding the logos) may be re-used free of charge in any format or medium. It may only be used accurately and not in a misleading context. The material must be acknowledged as The Crown Estate copyright and use of it must give the title of the source publication. Where third party copyright material has been identified, further use of that material requires permission from the copyright holders concerned. Suggested Citation HR Wallingford 2016, DDR5432-RT002-R03-00, Severn Estuary Long Term Morphology, Synthesis of data and modelling studies, The Crown Estate, 60 pages, ISBN: 978-1-906410-72-8. This report is available on The Crown Estate website at www.thecrownestate.co.uk

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July 2016DDR5432-RT002-R03-00

Severn Estuary Long Term MorphologySynthesis of data and modelling studies

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Summary Mindful of the great potential for tidal power from the Severn Estuary, The Crown Estate (TCE) continues to support work that improves general knowledge of the physical processes of the Severn estuary.

The objective of the study reported here is to provide an update on our understanding of the baseline evolution of the intertidal morphology of the Severn Estuary, through compilation and analysis of the latest information and modelling work. This is report is supported by ‘An updated review of baseline changes - Report on analysis of LiDAR data’, HR Wallingford 2016.

Through the Holocene period the sediment eroded from intertidal areas within the Severn Estuary was re-deposited at the margins of the estuary, preserving the nature of the shoreline and allowing intertidal areas to transgress landwards. The imposition of fixed margins since Roman times has limited the ability of the intertidal margins to transgress, with the result that these margins are slowly reducing in size: a process often referred to as ‘coastal squeeze’. Given the valuable resource of the Severn Estuary intertidal zone to both birds and fish, this process is of considerable ecological importance.

LiDAR evidence was analysed for the period 2003-2014 for the coastline between Hinkley Point and the 2nd Severn Crossing (on the English coast) and for the period 2006-2014 for the coastline between Goldcliffe and the 2nd Severn Crossing (on the Welsh coast). Over the period 2003-2007 the English coastline of the Outer Severn as a whole experienced an average rise in bed level of 24-30 mm/year throughout the intertidal profile. Over the period 2007-2014 the changes were more mixed with observed erosion of 6 mm/year on the upper profile and accretion of 5 mm/year on the mid-low profile. Over the period 2006-2014 the Welsh coastline between Goldcliff and the Second Severn Crossing experienced accretion on the upper and mid-low profiles (23 mm/year and 11 mm/year respectively) and erosion on the mid-upper profile (2 mm/year). The data taken as a whole indicates that there is more erosion/less accretion for the higher part of the intertidal profile compared with the lower part of the intertidal profile, but the data also shows that expanses of saltmarsh on the Welsh upper profile experienced accretion throughout the period studied. The highest rates of erosion were experienced on the mid-lower profile between Brean Down and Sand Point (Area 5) on the English side.

Due to limitations in the LiDAR data coverage, changes in total intertidal area at Low Water along the coastline covered by the data are not available. Taking areas of the English and Welsh coastline for which LiDAR data exists as a whole, changes at 0m ODN (close to mean water) and above were derived. These showed that over the period 2006/7-2014 the total intertidal area above 0m ODN, and above +3m ODN, increased by 4.4 ha/year and 1.2 ha/year, respectively. These changes at 0m ODN represent an average seaward advance of approximately +0.5 m/year of the English and Welsh coastline over the ~90 km stretch for which LiDAR data exists over the 7-8 year period from 2006/7 to 2014.

This study has analysed recent LiDAR measurements of the intertidal evolution of the Outer Severn Estuary and compared the results of this analysis against evidence from 19th and 20th century surveys, geomorphological features, archaeology, geomorphological theory and other contemporary measurements of intertidal change. The short-term LiDAR evidence indicates overall accretion and stability; while the long-term evidence indicates a clear signal of continued erosion since before Roman times. These two, apparently contradictory, trends have been reconciled by studying the trends in wave action in the Severn,

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and UK trends in wind climate generally, prior to and throughout the LiDAR period. The recent period of LiDAR measurement coincides with a period of relatively low wind and wave activity. It is not known if this will continue into the future or whether the previous long-term trend of coastal squeeze may reassert itself.

Evidence both from previous monitoring and the more recent LiDAR monitoring points to the rate of erosion being highest for mudflat high in the tidal frame. Both these studies show, however, that the rate of vertical change of the saltmarsh surface (which is even higher in the tidal frame) is much more accretionary than mudflat – a result which can be linked to the attenuation of waves by the enhanced friction associated with the vegetation.

This report also presents the application of morphological modelling to scenarios in the estuary, and compares the results with field observations. The morphological model is able to predict reasonably well the effects of coastal development, as well as the ongoing effects of natural variation in waves, currents and water levels. Modelling highlights the importance of wave action on the estuary margins, but also shows how coastal development can lead to changes in tidal currents which can cause significant relocation of the shoreline. Lastly the model provides a resource for diagnosis and verification of observed changes which can be key to understanding intertidal evolution and development of long-term shoreline strategy.

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Contents

Summary

1. Introduction _________________________________________________________ 1 1.1. Background ....................................................................................................................................... 1 1.2. Report Structure ................................................................................................................................ 2

2. Summary of LiDAR analysis ____________________________________________ 2 2.1. Measured changes in bed level ........................................................................................................ 2 2.2. Measured recession/ advance of intertidal contours ......................................................................... 6

3. Evidence of longer-term change _________________________________________ 7 3.1. Evidence from profile surveys from the 20th century ........................................................................ 7 3.2. Evidence from historical Ordnance Survey Maps ........................................................................... 10

4. Geomorphological and archaeological evidence of long-term change ___________ 11 4.1. Evidence for the marine transgression of the Severn Estuary ........................................................ 11 4.2. Geomorphological considerations ................................................................................................... 12

5. Application of morphodynamic modelling _________________________________ 13 5.1. Introduction ...................................................................................................................................... 13 5.2. Application of the model to the Clevedon shoreline ........................................................................ 14

5.2.1. Observed changes on the Clevedon shoreline 2003-2014 ............................................... 14 5.2.2. Predicted changes on the Clevedon shoreline 2003-2014 ................................................ 15

5.3. Application of the model to the Welsh Grounds shoreline .............................................................. 18 5.3.1. Observed changes on the Welsh Grounds shoreline 2006-2014 ...................................... 18 5.3.2. Predicted changes on the Welsh Grounds shoreline 2006-2014 ...................................... 20

5.4. Application of the model to evolution of the coast at Portbury ........................................................ 21 5.5. Application of the model to the effects of potential barrage construction ....................................... 23 5.6. Summary of modelling outcome ...................................................................................................... 24

6. Discussion _________________________________________________________ 25 6.1. Reconciling the observed long-term and short-term intertidal evolution ......................................... 25 6.2. Other influences on intertidal evolution ........................................................................................... 29

7. Conclusions ________________________________________________________ 29

8. References ________________________________________________________ 31

Appendices ____________________________________________________________ 36

A. Long-term profile measurements

B. HR-MUDPROF model description

Figures Figure 2.1: Areas of intertidal used for LiDAR analysis .............................................................................. 3 Figure 2.2: Observed annual rates of bed level change for 2003-2007 from LiDAR measurements ......... 4 Figure 2.3: Observed annual rates of bed level change for 2007-2014 from LiDAR measurements ......... 5

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Figure 3.1: Location of intertidal profile information from previous studies around the Severn Estuary ........................................................................................................................................................ 8 Figure 4.1: Schematic view of the spatial distribution of processes within a tide-dominated estuary ...... 12 Figure 5.1: Locations of profiles used for modelling ................................................................................. 14 Figure 5.2: Profiles extracted from LiDAR data sets between 2003 and 2014 for Clevedon ................... 15 Figure 5.3: Predicted intertidal evolution at Clevedon 2003-2014 ............................................................ 16 Figure 5.4: Comparison of average predicted and observed rates of intertidal change at Clevedon over the periods 2003-2007 and 2007-2014 ............................................................................................. 17 Figure 5.5: Profiles extracted from LiDAR data sets between 2000 and 2014 for Welsh Grounds ......... 19 Figure 5.6: Predicted evolution at Welsh Grounds 2006-2014 ................................................................. 21 Figure 5.7: The coastline before and after the construction of Royal Portbury Dock in 1972-1997. Top: coastline in 1972. Bottom: Coastline in 2004. .................................................................................. 22 Figure 5.8: Predicted and observed evolution of the intertidal profile at Portbury over the period 2006-2014 ................................................................................................................................................. 23 Figure 5.9: Predicted effect of a flood and ebb tide generating tidal barrage between Cardiff and Weston on the intertidal profile at Woodhill Bay ....................................................................................... 24 Figure 6.1: Hindcast mean annual and 95%-ile significant (Hs) wave heights mid-profile on the Clevedon shore, 1972-2014 ...................................................................................................................... 26 Figure 6.2: Hindcast mean annual and 95%-ile significant (Hs) wave heights mid-profile on the Welsh Grounds shore, 1972-2014 ............................................................................................................ 27 Figure 6.3: Winter (December to March) North Atlantic Oscillation index (differences between normalised monthly SLP at Gibraltar and Iceland, averaged over December–March expressed as anomalies from the 1961–1990 mean) for the period 1823 to 2014 ......................................................... 28

Tables Table 2.1: Measured changes in bed levels of the Welsh and English coastlines for 2003-2007 and 2007-2014 in mm/year ................................................................................................................................ 5 Table 2.2: Intertidal area changes .............................................................................................................. 6 Table 3.1: Average long-term erosion rates for profiles in the Severn Estuary .......................................... 9

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1. Introduction 1.1. Background

The Crown Estate (TCE) marine estate includes almost the entire UK seabed out to 12 nautical miles and around half the UK’s foreshore, tidal beds and estuaries. This includes the majority of the Severn Estuary.

Owing to the Estuary’s potential to act as a source of renewable power, TCE continues to take an active interest in the physical processes of the estuary, and how these may interact with tidal power generation. Before reaching conclusions on the effects of power generation on the estuary system, it is first necessary to develop an understanding of estuary change under baseline (i.e. no power generation) conditions.

To our knowledge, the most recent and comprehensive review of baseline morphological change on the Estuary, was conducted between 2008 and 2010 as part of the DECC studies of tidal power. The DECC project used data and studies available up to 2007.

The objective of this study is to update our understanding of the intertidal morphology of the Severn Estuary, through compilation and analysis of information that have become available since the DECC studies. The study entails four key activities:

1. a review and comparison of additional sources of data on intertidal morphology, including recent LiDAR data;

2. a review of historical information of changes in intertidal morphology, including 19th and 20th Century surveys, archaeological and geomorphological findings;

3. use of an intertidal profile model, to enable better understanding of the drivers of intertidal evolution and of the predictability of intertidal morphology; and,

4. a review of the scale of change in intertidal area within the estuary.

These activities provide a better definition of the current intertidal evolution in the Severn Estuary and of its long term development, enabling further insights into the drivers and mechanisms of change.

An accompanying report (HR Wallingford 2016) presents the work undertaken for the first of these activities - the acquisition and analysis of LiDAR data to examine changes that have occurred over the intertidal areas of the Severn Estuary since the publication of the results of the DECC studies in April 2010 (HR Wallingford, 2010).

This report presents the work undertaken for the remaining three tasks, including a synthesis of the information that has come from data analysis and modelling.

The focus of this report is the Outer Severn Estuary (i.e. the estuary southwest of the Second Severn Crossing), as this is the part of the estuary of greatest interest for tidal power, and is best described by the existing data.

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1.2. Report Structure This report comprises a further five Sections. Section 2 summarises the LiDAR analysis presented in an accompanying report (HR Wallingford, 2016). Sections 3 and 4 present evidence concerning longer term change – including survey data, historical maps, geomorphology and archaeology. Section 5 describes how greater insight into intertidal evolution can be deduced using morphological modelling. Section 6 discusses and reconciles the various strands of evidence and the conclusions of the studies are presented in Section 7.

2. Summary of LiDAR analysis This section presents a brief summary of the results of the LiDAR analysis presented in the accompanying report (HR Wallingford, 2016). The analysis extends the LiDAR data analysis undertaken for the DECC study (HR Wallingford, 2009).

2.1. Measured changes in bed level The accompanying review (HR Wallingford, 2016) examined LiDAR data from the English side of the estuary from 2007, 2009, 2012 and 2014 and from Lafarge Tarmac Marine Dredging Ltd (for the Welsh side between Goldcliff and the Second Severn Crossing) from 2011, 2012, 2013 and 2014. The data was processed to give information on average vertical change and the area change and recession/advance of the mudflat over this period in six different areas (See Figure 2.1). No additional recent LiDAR data was available for the Welsh coastline between Penarth and Goldcliff since the 2006 data already analysed in the DECC studies (HR Wallingford, 2009).

The observed changes in each of these six areas for the lower-mid (-3m to 0m ODN), upper-mid (0m to +3m ODN) and upper (above +3m ODN) intertidal profiles area are summarised in Figure 2.2, Figure 2.3 and Table 2.1. Figure 2.2 shows the observed changes for 2003-2007 and Figure 2.3 shows the observed changes for 2007-2014.

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Figure 2.1: Areas of intertidal used for LiDAR analysis

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Figure 2.2: Observed annual rates of bed level change for 2003-2007 from LiDAR measurements

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Figure 2.3: Observed annual rates of bed level change for 2007-2014 from LiDAR measurements *Changes shown for Area 1 are for period 2006-2014

Table 2.1: Measured changes in bed levels of the Welsh and English coastlines for 2003-2007 and 2007-2014 in mm/year

Profile range

(m ODN)

2003-2007 2007-2014

Wales (Area 1) England (Areas 2-6) Wales (Area 1)* England (Areas 2-6)

+3 to +7 - 30 23 -6

0 to +3 - 24 -2 0

-3 to 0 - 26 11 5

*Changes shown are for period 2006-2014

Over the measurement period the English coastline of the Outer Severn as a whole experienced an average rise in bed level of 24-30 mm/year in 2003-2007 throughout the intertidal profile. Over the period 2007-2014 the changes were more mixed with observed erosion of 6 mm/year on the upper profile and accretion of 5 mm/year on the mid-low profile. The Welsh coastline between Goldcliff and the Second Severn Crossing experienced accretion on the upper and mid-low profiles (23 mm/year and 11 mm/year respectively) and erosion on the mid-upper profile (2 mm/year). The analysis (HR Wallingford 2016) shows that both erosion and accretion can and does occur, varying from year to year, and often there are significant differences between areas and even between adjacent locations within the same area. The data taken as a whole indicates that there is more erosion/less accretion for the higher part of the intertidal profile compared with

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the lower part of the intertidal profile. This is accompanied by expanses of saltmarsh on the Welsh upper profile that experienced accretion throughout the period studied. The highest rates of erosion were experienced on the mid-lower profile between Brean Down and Sand Point on the English side.

2.2. Measured recession/ advance of intertidal contours The accompanying review of LiDAR data (HR Wallingford, 2016) provides up-to-date information in the changes of intertidal area, but cannot give a complete picture for the estuary. Firstly, in any one year, we do not necessarily have complete survey coverage for the estuary. Secondly, LiDAR surveys only provide information above the water line at the time of survey, and not the full intertidal. Most of the surveys occur at times when the water level is above MLW (Mean Low Water), at around mean low water neaps (MLWN). Data is available from most of the surveys for elevations down to MLWN but some of the surveys are incomplete along the water line.

The accompanying review therefore identifies the changes above a level similar to MLWN where possible and at Ordinance Datum (which is close to, but slightly below mean water level). However, because of the incomplete nature of the data close to MLWN level, presenting the total changes to intertidal area at MLWN would be misleading. Therefore, only the changes in intertidal area at 0m ODN and +3m ODN are presented here in detail. Table 2.2 below presents the changes in the total intertidal area above these contours, for the areas surveyed. See HR Wallingford, 2016 for more information.

Table 2.2: Intertidal area changes

Area

Elevation

Range

(m ODN)

Period of

change

Change in total intertidal

area above contour

(ha/year)

1 Above +3m 2006 to 2014 -0.14

Above 0m 2006 to 2014 +1.52

2 Above +3m 2007 to 2014 -0.24

Above 0 2007 to 2014 -0.31

3 Above +3m 2007 to 2014 -0.01

Above 0m 2007 to 2014 -0.05

4 Above +3m 2007 to 2014 +0.47

Above 0m 2007 to 2014 +1.07

5 Above +3m 2007 to 2014 +0.39

Above 0m 2007 to 2014 +0.42

6 Above +3m 2007 to 2014 +0.70

Above 0m 2007 to 2014 +1.76

Taking Areas 1 to 6 as a whole and over the period 2006/7-2014 the total area above 0m ODN, and above +3m ODN, increased by 4.4 ha/year and 1.2 ha/year, respectively. These changes represent an average seaward advance by the contour of approximately +0.5 m/year (at 0m ODN) of the coastline over the ~90 km stretch for which LiDAR data exists over the 7-8 year period from 2006/7 to 2014. Equivalent figures for the change in total area above MLWN are dominated by changes and data gaps in Areas 1 and 6. Here the

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data, such as it is, suggests rates of seaward advance by the contour an order of magnitude larger than that at 0m ODN, with fairly minimal rates of advance/retreat in the other areas.

3. Evidence of longer-term change 3.1. Evidence from profile surveys from the 20th century Based on locations originally identified by Kirby (1988,1989) the DECC study of the evolution of intertidal morphology in the Severn (HR Wallingford, 2009a) identified a number of previous studies surveying intertidal profile elevations in various locations around the Severn Estuary (Figure 3.1). This data includes the following intertidal profile surveys:

Hydrographic surveys between Penarth and Rumney taken by HR Wallingford in 1987;

Intertidal profiles along sewer pipe locations for the Rumney Valley Sewer (1920) and Ystradyfodwg and Pontypridd sewer (1889) on the Welsh coast;

Theodolite surveys carried out at various locations in the Severn Estuary by Somerset River Authority (SRA) in 1954.

The Penarth-Rumney, Sewer and SRA surveys provide a basis for long-term estimates of intertidal change when compared with contemporary LiDAR.

km

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Figure 3.1: Location of intertidal profile information from previous studies around the Severn Estuary Source: based on data presented in Kirby, 1989, figure reproduced from HR Wallingford, 2009a

Appendix A shows a number of profiles surveyed by Hydraulics Research Station (1987) between Penarth and Rumney (see Figure 3.1 for locations), compared with the same profiles extracted from the LiDAR data. The Hydraulics Research Station (HRS) profiles were measured from a small boat using an echo sounder relative to three step gauges used to determine the vertical datum. In general the figures suggest long-term erosion, with the LiDAR profiles being up to 1.5 m lower than, and on average 0.3-0.9 m lower (2000 LiDAR), or up to 0.7 m lower (2006 LiDAR), than the historical profiles. The comparison of the historic data with the 2006 LiDAR results in reduced overall rates of erosion (0.016 m/year on average) than the corresponding comparison with the 2000 LiDAR (0.047 m/year on average). The annual volume changes are presented in Table 3.1 and are based on differences between the historical and 2006/2007 LiDAR data sets.

Figure A.7 and Figure A.8 compare intertidal profiles shown on sewer construction diagrams surveyed around 100 years ago (from Kirby, 1989) with LiDAR from 2000 and 2006. Both sewers were on the Welsh coast between Cardiff and Newport. The comparisons show significant erosion has occurred over the last century, with the LiDAR profiles 2 to 6 m lower than the historic profiles. Further evidence for this erosion is from piles, inserted to keep the sewer pipe in place, which now stand well above the sediment surface (Kirby, 1989). It is worth pointing out that these historical profiles covered the entire foreshore to Low Water.

A comparison between Somerset River Authority (SRA) theodolite surveys carried out in 1954 and LiDAR data for 8 sites between Clevedon and Bridgwater Bay is shown in Figure A.9 to Figure A.16. Profiles 1 to 4

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are backed by embankments or seawalls, 5 (Sand Bay) and 10-12 (Steart Flats) are backed by dunes. The seaward extent of each profile is the extent of the original SRA survey, and was generally above mean water level in 1954. Profiles 10 and 11 consisted largely of Spartina grass whilst profile 12 was mostly sand and pebbles with only a short stretch of mud on the lower profile. Profile 10 appears to have eroded, in line with observed trends elsewhere in the estuary. Profile 11 however appears to have accreted. Profile 12 appears to have experienced only a relatively small change in bed level. The remaining profiles (1-5) mainly comprised muddy sediments in the 1954 surveys, although the high intertidal zone varied and included shingle and Spartina grass as well as mud. Of these, profiles 1 to 4 showed long-term erosion between 1954 and 2003/2007 while profile 5 showed accretion, mainly on the upper flat.

A summary of the long term erosion rates, calculated between the historical data sets and LiDAR data are shown in Table 3.1 below. These rates are averaged over the length of the profile and simply assume a constant rate of change between the two snapshot surveys.

Table 3.1: Average long-term erosion rates for profiles in the Severn Estuary

Survey reference Location Period Average erosion

rate (mm/year)*

SRA survey 1 Clevedon 1954-2007 21

SRA survey 2 Clevedon 1954-2007 35

SRA survey 3 Kingston Seymour 1954-2007 23

SRA survey 4 Kingston Seymour 1954-2007 31

SRA survey 5 Sand Bay 1954-2007 -1

SRA survey 101 Steart Flats 1954-2007 9

SRA survey 111 Steart Flats 1954-2007 -21

SRA survey 121 Steart Flats 1954-2007 -5

HRS 1 Penarth 1987-2006 8

HRS 2 Penarth 1987-2006 1

HRS 3 Penarth 1987-2006 1

HRS 4 Rumney 1987-2006 21

HRS 5 Rumney 1987-2006 25

HRS 6 Rumney 1987-2006 36

Rumney Valley Sewer Wentlooge 1920-2006 44 †

Y & P Sewer Rumney 1889-2006 35

Source: HR Wallingford (2009) * positive rates imply erosion and negative rates imply accretion. † Similar to the value of 0.043 m/year reported by Kirby and Kirby (2008)

The long-term data available is based on a limited number of data sets which do not cover the whole of the intertidal area and which can show quite varied rates of erosion, even from the same area. The data sets from the Welsh side of the estuary are more consistent and show long term erosion of the order of a few centimetres per year while those from the English side are more varied and tend to show lower rates of change. The highest calculated erosion rates were observed at Kingston Seymour on the English side and around Rumney on the Welsh side. Erosion rates appear to increase from Penarth to Rumney, possibly influenced by increasing wave exposure.

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The long term changes in bed level from the sewer pipe profiles and the HRS profiles show erosion in the region of 1-44 mm/year (mean of 21 mm/year of erosion). Similarly the mudflats on the English side show changes of between 21 mm/year accretion to 35 mm/year erosion (mean of 12 mm/year of erosion). As noted in the original study for DECC (HR Wallingford, 2010), some of these profiles are likely to have been measured because they were showing erosion (at a rate which gave rise to concern) while neighbouring accreting or quasi-stationery profiles may not have presented a problem and therefore not required measurement. There is a possibility, therefore, that these longer term measurements are not representative of the estuary as a whole.

In addition to these longer term comparisons Kirby and Kirby (2008) measured annual changes in elevation at two upper intertidal profiles at Steart Flats (see Figure 3.1) over a period of 17 years between 1990 and 2006. These authors calculated an erosion rate of 16 mm/year for the mudflats in this area, based upon analysis of the yearly surveys.

3.2. Evidence from historical Ordnance Survey Maps Pye and Blott (2010) undertook an analysis of Ordnance Survey (OS) maps to evaluate the historical changes in mean low water (MLW) and mean high water (MHW) to inform an analysis of extreme events for a proposed power station at Hinkley Point. Pye and Blott used the “first edition” OS maps of the Hinkley Point area published surveyed between 1883 and 1887 at scales of 1:2500 and 1:105600 to identify the MHW and MLW lines displayed by the maps. These maps were compared to corresponding maps surveyed between 1955-1957, the 1970s and to LiDAR flown in 2007-2008. Using this information an assessment was made of the progradation1 /recession of the foreshore between Kilve and Brean (very roughly corresponding to Area 1 of Section 2).

The MHW analysis indicated that:

At the site of the existing A and B power stations, construction (between the 1950s and 1970s) had led to seaward movement of MHW by of the order of 100m;

Between Steart and Brean, MHW has mainly moved seaward by between 100 m and 300 m, this movement having occurred prior to the 1970s and in many cases prior to the 1950s. Some minor recession was observed following the 1970s;

Otherwise movement of MHW was generally mixed and of the order of 0.1 m/year (seaward or landward) since the 1880s.

The movement of MLW was very different however, the chart analysis showed that in general over the 1880s to 2008 period there had been (South of Steart) net shoreward movement of MLW of a few hundreds of metres or in the order of 1 m/year, and (north of Steart) net shoreward movement of many hundreds of metres or in the order of 10 m/year.

Pye and Blott (2010) also reviewed other sources of historical and map and survey data (Kidson, 1960, 1963; Ravensrodd Consultants, 1996; Long et al, 2002) which indicated that that the shoreline of the Steart Peninsula has experienced net erosion (shoreward movement) between (around) 1800 and 1928. In 1928 the SRA built an artificial gravel bank along the shore near Steart village and planted Spartina on the upper foreshore. Ranwell (1964) and Carr (1965) report that expansion of the Spartina occurred between the 1930s and mid 1960s after which erosion of the seaward marsh set in and has continued to the present day (Long et al, 2002; Kirby and Kirby, 2008). 1 seaward growth of a beach, delta, fan, etc., by progressive deposition of sediment by rivers or shoreline processes.

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4. Geomorphological and archaeological evidence of long-term change

As well as the evidence of long-term change throughout the 20th century presented in Section 3 there is also longer term evidence from archaeological and geomorphological studies which provides information about millennial-scale intertidal changes in the Severn Estuary.

4.1. Evidence for the marine transgression of the Severn Estuary Around 2500 years ago mean sea level in the Severn Estuary was roughly 2.5 m below what it is today (Shennan,1983; Allen, 1990). Since that time sea level has risen, and continues to rise. The rise in sea level has historically resulted in the immersion of previously exposed land and the movement of the coastline up the estuary, i.e. landwards. This process is called marine transgression which is typically accompanied by geomorphological features resulting from increase in wave exposure, transport of intertidal sediment onto the HW margins and head of the estuary. Allen (1990) describes four different strands of evidence that all point to this ongoing process:

The erosion of bedrock cliffs at the estuary margin.

The present-day wide intertidal exposure of mid post-glacial peats formed on early tidal marshes.

These marshes represent wetlands which were high in the tidal frame, which were at their time of formation infrequently flooded by the sea. The peats are resistant to erosion and typically form broad ledges around mid-intertidal profile levels.

The discovery of prehistoric (e.g. peat beds and submerged forests) and Bronze/Iron age and Romano-British cultural debris (including within the intertidal zone representing previous wetland settlement, which is now eroding.

In particular Allen (1990) pointed to the evidence from researchers identifying Roman sea defences which visibly lie inland from those of today (Allen and Fulford, 1986, 1988; Allen, 1987, 1989).

The erosive relationship between modern sand-flat deposits and the older post-glacial silts and clays. Along the margins of the coast the sand flats peter out against cliffs of older silt-clay sequences which lie stratigraphically underneath the post-glacial peats. The cliffs are in retreat as indicated by cantilever-type failures forming collections of debris at the base of the cliffs.

Further evidence of coastal erosion is provided by the progressive loss of salt marsh. For instance Otto (1998) cites the loss of salt marsh from the Welsh Severn coast at the Wentlooge Levels (loss of 49%) and Caldicott Levels (loss of 69%) over the 19th and 20th centuries. Royal Haskoning (2004) analysed changes in Saltmarsh on the English side of the Severn and concluded that there has been a loss of 21% of the saltmarsh surface between 1946-8 and 2000.

These strands of evidence together form a compelling story of the creation over thousands of years of wetland areas formed by the deposition of alluvial sediment on the margins of the estuary and long term erosion (say over the last 2000 years) of intertidal areas which previously formed these wetland areas (albeit at a lower sea level). Further evidence of marine transgression and the erosion of mudflats is provided by geomorphological analysis and measurements of wind and current processes (Section 4.2).

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4.2. Geomorphological considerations Dalrymple et al (2011) identified the characteristic physical processes within tidally dominated estuaries and summarised in the form of a figure which is reproduced in Figure 4.1. Tidally dominated estuaries tend to be funnel shaped and the evolution of tidal range is a function of the relative balance of the convergence of this “funnel” (which tends to increase tidal range) and friction (which tends to reduce tidal range) (Friedrichs et al, 1998; Savenije, 2005). Strongly convergent estuaries like the Severn, where tidal range increases in a landwards direction, are known as hypersynchronous. In tidally dominated estuaries friction induces shorter flood tides and longer ebb tides resulting in higher flood tide currents and net landward transport of sediment (Friedrichs and Aubrey, 1994; Friedrichs et al, 1998). In general, the higher the tidal range, the greater the asymmetry (Friedrichs and Aubrey, 1988) and so hypersynchronous estuaries tend to have strong landward residual transport over much of the length of the estuary (Wells, 1995).

In general in estuaries the relative importance of tides reduces with proximity to the head of the estuary while the importance of fluvial input increases (in Figure 4.1). Dalrymple et al (2011) characterises this change in tidal-dominance with distance as tidal-dominance becoming tidally dominated with river influence, then river dominated with tidal influence and then final river dominated. For hypersyncronous estuaries, this reduction in tidal dominance may occur over a relatively short distance near the head while estuaries with a smaller tidal range and more dominant river discharge (e.g. the Thames Estuary) may experience a more gradual change from tidal to fluvial dominance. At some point the net seaward sediment transport induced by river discharges dominates the landward transport induced by tides and there is a zone of convergence in sediment transport (see Figure 4.1). In the case of the Thames Estuary this occurs roughly at the mid-point of the estuary at a location known as the “Mud Reaches” (Abbot, 1960; Inglis and Allen, 1957) while in the Severn this occurs near the head of the estuary (although a second zone of convergence or turbidity maximum also occurs in the vicinity of Bridgewater Bay) (Manning et al, 2010).

Figure 4.1: Schematic view of the spatial distribution of processes within a tide-dominated estuary Source: Dalrymple (2011)

As noted by Dalrymple et al (2011) the margins of tide-dominated estuaries experience a weaker effect of tides and instead are wave dominated in the outer estuary where fetches are larger and coastlines are more exposed. This means that there will be ongoing coastal erosion as the estuary transgresses upstream due to

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the rising sea level and the greater exposure to wave action. This key role of wave action on the estuary margin is the driving mechanism for coastal erosion that accompanies marine transgression.

The idea of wave action having a key role on the morphology of the estuary margin is supported by measurements of erosion and accretion on intertidal areas within the Severn Estuary by Allen and Duffy (1998a). Allen and Duffy measured the change in mudflat and saltmarsh level at six locations (Goldcliff, Portishead, Mathern, Cake Pill, Woolaston and Purton) on the upper 40% or so of the intertidal zone over a 2 year period. The measurements showed that mudflat erosion in the Severn Estuary is greatest at stations high in the tidal frame where wind fetches are larger and decreases with distance landward as the significance of waves reduces.

Although the evidence collated within this report presents the argument for wave action being the key driver for intertidal erosion and marine transgression, it should be noted that the role of wave action in dictating the rate of accretion of the surfaces (rather than the cliffs) of saltmarshes in the Severn is much less and the Dalrymple (2011) conceptual model (see Figure 4.1) does not make this distinction. Allen and Duffy (1998a) found that on the surface of saltmarsh the effect of waves is an order of magnitude less than that on the mudflat and the rate of accretion is determined by the supply of sediment and depth of water (accommodation space) above the saltmarsh (which increases with tidal range). The reduction in the effect of waves accords with prevailing theory regarding the attenuation of wave action by saltmarsh (e.g. Dalrymple, 1984; Kobayashi, 1993; Möller et al, 1996, 1999, 2014) while the link between saltmarsh accretion and accommodation space in the Severn Estuary is supported by measurements of historical saltmarsh accretion along the estuary increases with distance landwards along the estuary (Allen and Rae, 1988; Allen, 1991).

5. Application of morphodynamic modelling 5.1. Introduction Numerical modelling can be used both to explore the significance of key processes in the Severn Estuary and in a predictive capability. We discuss here the application of a profile cohesive sediment transport model, HR-MUDPROF, originally based on work by Roberts et al (2000) and Roberts and Whitehouse (2001) and which has been further developed to investigate the effects of Severn Barrage and Lagoon options on the intertidal morphology of the Severn Estuary (Rossington et al, 2009; HR Wallingford, 2009b, 2014). A brief description of the model is provided in Appendix B.

In this Section we illustrate the insights on the sedimentary processes within the Severn Estuary gained from the application of the model to four different situations:

Natural changes that have been observed at Clevedon on the English coast over the period 2003-2014.

Natural changes that have been observed at Welsh Grounds on the Welsh coast over the period 2006-2014.

Historical development of the shoreline at Portbury following the construction of the Royal Docks in 1972-1977.

Proposed barrage construction.

The locations of the profiles used in this modelling are shown in Figure 5.1.

These applications illustrate the ability of the model to reproduce behaviour on both the Welsh (in conditions of overall accretion) and English (in conditions of overall erosion) coastlines, to reproduce the effects of

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localised coastal development and the wider scale effects of barrage construction. But they also illustrate how the model can be used to identify the key drivers for change and to verify the observations of change.

Figure 5.1: Locations of profiles used for modelling

5.2. Application of the model to the Clevedon shoreline

5.2.1. Observed changes on the Clevedon shoreline 2003-2014

The Clevedon profile we consider here is shown in Figure 5.1. The LiDAR measurements of the bed elevation at this profile over the period 2003-2014 are shown in Figure 5.2. The figure shows that over this period there has been little observed change to the lower intertidal since 2003. The LiDAR data indicates erosion of the upper intertidal between 2003 and 2007 and a small and temporary amount of accretion on the lower intertidal in 2012.

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Figure 5.2: Profiles extracted from LiDAR data sets between 2003 and 2014 for Clevedon

5.2.2. Predicted changes on the Clevedon shoreline 2003-2014

There are two steps to validation of the mud profile model. The first is to test whether the model, starting with the measured profile, is able to preserve the profile shape thereby indicating that the model has reproduced the spatial distribution of waves, currents and suspended sediment concentration correctly along the profile. The second step is to model the evolution of the profile over a long period (e.g. decade) and compare it with the observed changes in the profile.

The model was used to reproduce the observed evolution of the Clevedon profile (Profile 12 in the LiDAR review accompanying this report (HR Wallingford, 2016)). Firstly the model was initialised over a spin-up period of 1 year starting from the 2003 LiDAR profile 2. The initial profile and the predicted profile at the end of the spin-up year are shown in Figure 5.3. The model preserves the profile, demonstrating that the model is capturing the distribution of waves, currents and sediment well. The model prediction of evolution over the period 2003-2014 is also shown in Figure 5.3 and the prediction is compared to the observed changes for the epochs 2003-2007 and 2007-2014 in Figure 5.4. For the purposes of summarising the results we separate the profile into the area of mudflat below high water neaps and an area, which we will loosely term “saltmarsh” which is above high water neaps. In this saltmarsh zone saltmarsh may occur but in this case it is very patchy.

2 ‘Spin-up’ is the period allowed for a model to reach a state of equilibrium.

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The figures show that the model predicts the right order of change during 2003-2007 for both mudflat and saltmarsh and predicts the (overall) reduction in erosion over the period 2007-2014. Overall though, the model significantly underpredicts the magnitude of saltmarsh erosion. This is in part because the model does not explicitly include saltmarsh processes and has no mechanism for representing eroding marshes. In addition, the saltmarsh at this particular location is very fragmented and appears to be breaking up into smaller clumps (see Photograph 5.1) which may be picked up by LiDAR as more general erosion of the upper mudflat surface, possibly leading to an exaggerated erosion rate when examining the LiDAR data (fewer or smaller saltmarsh clumps lead to a lower average height, but the mudflat surface between the clumps may not actually have eroded).

Figure 5.3: Predicted intertidal evolution at Clevedon 2003-2014

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Figure 5.4: Comparison of average predicted and observed rates of intertidal change at Clevedon over the periods 2003-2007 and 2007-2014

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Photograph 5.1: Eroding saltmarsh at Gullhouse Point, Clevedon Source: © Copyright Jim Mitchell. Licensed for reuse under the Creative Commons Licence.

5.3. Application of the model to the Welsh Grounds shoreline

5.3.1. Observed changes on the Welsh Grounds shoreline 2006-2014

The LiDAR measurements of the bed elevation at this profile (Welsh Grounds Profile 6 from the LiDAR review (HR Wallingford, 2016) over the period 2006-2014 are shown in Figure 5.5. The figure shows that over the period 2006-2014 the seabed has been relatively stable between chainages 800 m and 1200 m. Between chainage 100 m and 300 m there has been up to 0.8 m of accretion; between chainage 300 m and 800 m there has been erosion of up to 0.1 m with some additional but temporary modification of the low water channel at the lower end of the profile.

When corrected for datum error between the 2006 and 2014 LiDAR surveys (100 mm in the region of Caldicot - see Table 5.1 of HR Wallingford 2016) the measured mudflat change of -8 mm/year (erosion) over this period becomes 4-5 mm/year (accretion). The corresponding observed evolution for the saltmarsh part of the profile (100-300 m) is in the region of 80 mm/year. This value of accretion is an order of magnitude higher than long term measurements of salt marsh evolution (e.g. Allen and Rae, 1988; Allen, 1991)

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although the same behaviour is observed for all of the profiles at Welsh Grounds (see HR Wallingford 2016). The accretion is not seen at Goldcliff however, suggesting that there is a local but unknown cause to the rapid saltmarsh accretion observed over this period. A photograph of the saltmarsh taken in 2009 is shown in Photograph 5.2.

Figure 5.5: Profiles extracted from LiDAR data sets between 2000 and 2014 for Welsh Grounds

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Photograph 5.2: Saltmarsh at Welsh Grounds in 2009 Source: © Copyright Roger Cornfoot. Licensed for reuse under the Creative Commons Licence.

5.3.2. Predicted changes on the Welsh Grounds shoreline 2006-2014

The model was initialised over a spin-up period of 6 years starting from the 2000 LiDAR profile. The initial profile and the predicted profile at the end of the spin-up year are shown in Figure 5.6. The model preserves the profile well confirming that the model is capturing the distribution of waves, currents and sediment well. The model prediction of evolution over the period 2006-2014 is also shown in Figure 5.6.

The model prediction for mudflat evolution the period 2006-2014 is of accretion of 5 mm/year (over the chainage 300m to 1300m) which matches the observed evolution over this period when corrected for datum error (see Section 5.3.1). Notably, the model predicts saltmarsh accretion of 6 mm/year which is much lower than the LiDAR measurement of around 80 mm/year. The model value is more typical of measured rates from on-the-ground surveys (e.g. Allen and Rae, 1988; Allen, 1991). This result, together with the discussion about LiDAR and saltmarsh (HR Wallingford, 2016), suggests that the disparity between LiDAR and model is caused by the LiDAR signal being reflected from the saltmarsh canopy at different times over the growing season, giving the illusion of large changes in bed elevation where in fact more modest accretion has occurred.

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Figure 5.6: Predicted evolution at Welsh Grounds 2006-2014

5.4. Application of the model to evolution of the coast at Portbury In this application, the mud profile model is used to simulate evolution of the shore profile following construction of the Docks at Portbury between 1972 and 1977 (Figure 5.7). Following the construction of the east Pier, the currents along the shoreline to the west were greatly reduced, leading to accretion and advance of the neighbouring shoreline by a few hundred metres.

The model was used to simulate the evolution of the shoreline profile over the period 1972-2014. The predicted evolution is shown in Figure 5.8 together with Admiralty Chart and LiDAR data for this period. The figure shows that the model on the whole predicted the evolution well but slightly over-predicted the extent of the advance of the shoreline and the rate of early evolution. Notably, the model correctly predicted the deepening of the profile on the lower shoreline.

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Figure 5.7: The coastline before and after the construction of Royal Portbury Dock in 1972-1997. Top: coastline in 1972. Bottom: Coastline in 2004 Source: © Crown Copyright and/or database rights. Reproduced by permission of the Controller of Her Majesty’s

Stationery Office and the UK Hydrographic Office (www.ukho.gov.uk).

Approx. location of model profile

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Figure 5.8: Predicted and observed evolution of the intertidal profile at Portbury over the period 2006-2014

The principal driver for the observed change in this case is the reduction in tidal currents immediately offshore of the coastline, coupled with the large sediment supply that exists in this part of the Severn Estuary. The effect of waves influences the final shape of the equilibrium profile as the shoreline adjusts, but does not affect the scale of change.

5.5. Application of the model to the effects of potential barrage construction

In HR Wallingford (2009b) and HR Wallingford (2014) the mud profile model was applied to investigate the effects of potential tidal barrage options on intertidal morphology, using the model to study the effects of both ebb-tide-only generation and flood and ebb tide generation. This case differs from the previous three, in that ‘like for like’ data is not available for comparison against the modelled situation.

An example of wave-erosion arising from the effect of a flood and ebb tide generating barrage (between Cardiff and Weston) on the shoreline at Woodhill Bay is given in Figure 5.9. As shown in the figure the model predicted significant recession of the mudflat.

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Figure 5.9: Predicted effect of a flood and ebb tide generating tidal barrage between Cardiff and Weston on the intertidal profile at Woodhill Bay Source: HR Wallingford (2014)

This application illustrates the key role of wave action in controlling the evolution of intertidal areas in the Severn Estuary and how the existing profiles are a function of a delicate (albeit very dynamic) balance between these waves together with tidal currents and sediment supply.

Less obvious, however is the role of tidal-varying water levels on this delicate balance. With a barrage in place the tidal range is decreased and any wave action is focused over a smaller part of the profile. This causes an increase in the wave-induced shear stress experienced at a given position on the profile. The wave focusing effect is predicted to be more significant than the reduced current-induced shear stress arising from the smaller tidal range and as a result recession of the mud flat is predicted to occur.

5.6. Summary of modelling outcome These model applications highlight the applicability of morphological modelling as a tool for predicting the effects of coastal development as well as the ongoing effects of natural variation in waves, currents and water level. These applications highlight the importance of wave action on the estuary margins but also show how coastal development can lead to changes in tidal currents which can cause significant relocation of the shoreline. Lastly the model is a resource for diagnosis and verification of observed changes which can be key to understanding intertidal evolution and development of long-term shoreline strategy.

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6. Discussion 6.1. Reconciling the observed long-term and short-term intertidal

evolution On the whole the LiDAR analysis indicates that the last decade or so has seen accretion or relative stability on the intertidal areas of the Severn Estuary. Longer term measurements from the 20th century (Section 3.1), from surveys since the late 19th century (Section 3.2), from archaeology (Section 4.1) and geomorphology (Section 4.2) indicate that the intertidal trend within the Severn Estuary has been one of erosion and recession since Roman times. This pattern has continued through to (at least) the turn of the recent millennium.

This longer term behaviour – of coastal erosion – is to be expected as it is the inevitable consequence of sea level rise during the Holocene and driven by wave action at the margins of the estuary. The fact that there is an apparent contradiction between these two observed types of behaviour: erosion over the long-term and accretion/stability over recent decadal time scales was identified but not explained during the DECC (HR Wallingford, 2009) studies. The additional evidence presented in this report, drawn from a wide variety of sources, suggests ways that this apparent contradiction to be reconciled.

We argue that the recent period (2003-2014) of the LiDAR monitoring is an unrepresentative period of lower wave activity and storminess compared to the long-term average. This is based on two pieces of evidence: hindcasts of wave activity on the Welsh and English shores and analysis of the North Atlantic Oscillation Index.

Figure 6.1 and Figure 6.2 show the variation in mean annual significant (Hs) wave height and the 95%-ile annual significant wave height over the period 1973-2014 at points roughly mid-profile on the Clevedon and Welsh Grounds on the English and Welsh shorelines. The figures show a reduction in both mean annual and 95%-ile annual wave heights from about 1995 onwards with a much reduced mean and 95%-ile over the period 2003-2014 compared to that of the preceding period 1973-2003.

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Figure 6.1: Hindcast mean annual and 95%-ile significant (Hs) wave heights mid-profile on the Clevedon shore, 1972-2014

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Figure 6.2: Hindcast mean annual and 95%-ile significant (Hs) wave heights mid-profile on the Welsh Grounds shore, 1972-2014

This reduction in hindcast wave height is driven by variation in wind strength measured at locations in the Severn Estuary area. This reduction in wind strength is a more general feature of UK wind strengths since the 1990’s (Atkinson et al, 2006; Boccard, 2009) and this trend, in both mean speeds and gusts, has been shown to continue until at least 2010 (Earl et al, 2013; Watson et al, 2013).

The North Atlantic Oscillation (NAO) is an irregular cycle in regional atmospheric circulation defined as the pressure difference (which determines wind strength) between the Icelandic low and the Azores high. The balance between the relative strength of the dominant pressure systems determines the pathways and the intensity of depressions as they track across the North Atlantic into Western Europe. The index of the NAO is a measure, amongst other things, of the degree of winter windiness of the British climate (Woolf and Coll, 2006; Kirby and Kirby, 2008).

When the NAO index is positive, moderately strong and more persistent westerly winds are characteristic, whereas when in negative phase, lighter and more northerly and easterly winds are more usual with less persistent and weak westerly winds (Hurrell et al. 2003; Jenkins et al., 2007; Earl et al, 2013). There are numerous methods to calculate the NAO index. All of these involve records from Reykjavik in the north but comparisons are made with the Azores, Gibraltar or Lisbon in the south. In this report we used a long term series from Gibraltar as this index has the advantage of the longest record, helping place the UK wind variability for the period of interest into context (Earl et al, 2013).

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Figure 6.3 shows estimates of the winter averaged (i.e. averaged over the period December to March) NAO index estimated by Jones et al. (1997) and updated up to the winter of 2014/2015 by Osborn (2004, 2006, 2011 and http://www.cru.uea.ac.uk/~timo/datapages/naoi.htm).

Figure 6.3: Winter (December to March) North Atlantic Oscillation index (differences between normalised monthly SLP at Gibraltar and Iceland, averaged over December–March expressed as anomalies from the 1961–1990 mean) for the period 1823 to 2014 Source: Jones et al. (1997), Osborn (2004,2006,2011, http://www.cru.uea.ac.uk/~timo/datapages/naoi.htm). Figure reproduced from http://www.cru.uea.ac.uk/~timo/datapages/naoi.htm

Key: bars: individual winter values; thick curve: 10-year low-pass filter

Figure 6.3 shows that the NAO index is significantly reduced for periods in the 1850s, around 1940, the mid 1950s, the 1960s and around the period 2004-2011. This latest period accords with the reduced wave activity indicated in Figure 6.1 and Figure 6.2. The rise in the index for 2013/14 corresponds well with persistent storm activity which was experienced in December 2013/January 2014 in the Severn Estuary. The evidence of changes in the NAO and in wind speed thus confirms the results of the wave hindcasts, i.e. that the period of LiDAR measurement coincides with a period of generally lower wave activity. Kirby and Kirby (2008) also found a similar link between the NAO and annual changes in intertidal level on two transects at Steart (see Figure 5.1), noting a marked reduction in erosion in 1996 when the NAO index was strongly negative.

This analysis of wind and wave action, in combination with intertidal measurements and Severn geomorphology, which confirm the predominant importance of wave action on the intertidal areas in the Severn, strongly suggests that the period of apparent accretion or near stability measured using LiDAR over the period 2003-2014 is not representative of the longer-term signal. This long-term signal is clearly erosive, a conclusion supported by archaeological evidence (pre-Roman, Roman, medieval and post-medieval), a variety of eroding geomorphological features, 19th century and 20th century survey measurements and the expected behaviour accompanying marine transgression. The long-term average intertidal evolution, based on Table 3.1, is in the region of 1-2 cm/year of erosion. It is unclear whether the recent period of apparent accretion or near stability over the period 2003-2014 or the long term trend is more indicative of the likely changes in intertidal morphology going into the future.

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6.2. Other influences on intertidal evolution Whilst wave activity and storminess are identified as the most important driving processes to the evolution of intertidal areas in the Severn, there are other processes which are also important. Over yearly time scales, variations in tidal range arise as a result of the 8.85-year cycle of lunar perigee, which influence tides as a quasi-4.4 year cycle, and the 18.6-year lunar nodal cycle (Pye and Blott, 2010; Haigh et al, 2011). Besides driving the variation in tidal currents which flow over intertidal areas inducing an additional shear stress to that of waves, this variation in tidal range manifests as:

changes in intertidal suspended sediment supply (Kirby and Parker, 1983; Dyer, 1995);

influences on wave activity (through change in water depth) (Dyer, 1995);

wave-induced shear stress being distributed over a greater (or lesser) expanse of intertidal profile (Dyer, 1995, HR Wallingford, 2009b; 2014);

the movement of offshore sand banks (Otto, 1998); and,

variations in water level above saltmarsh (Allen and Rae, 1988, Allen and Duffy, 1998b).

The influence of wind-induced waves at any particular location is therefore complicated by many factors and may not correlate in a simple and clear way against intertidal change (Allen and Duffy, 1998a).

7. Conclusions Introduction

The intertidal habitat in the Severn Estuary alone represents represent 3% of the UK intertidal area3,4 and representing 4% of the total area of saltmarsh in the UK (Dargie, 2000). These intertidal areas are nationally (Widgeon, Gadwall, Shoveller and Pochard) and internationally important (Dunlin, Bewick’s Swans, European White-fronted Geese) for a number of bird species and the estuary may support as much as 10% of the British wintering population. The estuary is considered to be one of the most diverse fish assemblages in the UK (Potts and Swaby, 1991) and one of the more important nursery areas in Britain (Bird, 2008), the intertidal areas playing a key role in this 3.

Through the Holocene period the sediment eroded from intertidal areas within the Severn Estuary was re-deposited at the margins of the estuary, preserving the nature of the shoreline and allowing intertidal areas to transgress landwards. The imposition of fixed margins since Roman times has limited the ability of the intertidal margins to transgress, with the result that these margins are slowly reducing in size: a process often referred to as ‘coastal squeeze’. Given the valuable resource of the Severn Estuary intertidal zone to both birds and fish, this process is of considerable ecological importance. This report has confirmed that, over the last decade or so, the rate of loss of intertidal has slowed or even reversed due to changes in weather patterns. It is unknown whether this recent trend may extend into the future.

This study has analysed recent LiDAR measurements of the intertidal evolution of the Outer Severn Estuary and compared the results of this analysis against evidence from 19th and 20th century surveys, geomorphological features, archaeology, geomorphological theory and other contemporary measurements of intertidal change.

3 Severn Estuary Partnership http://www.severnestuary.net/sep/estuary.html 4 JNCC (http://jncc.defra.gov.uk/PDF/UKBAP_PriorityHabitatDesc-Rev2011.pdf)

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Summary of the recent LiDAR evidence

LiDAR evidence was analysed for the period 2003-2014 for the coastline between Hinkley Point and the 2nd Severn Crossing (on the English coast) and for the period 2006-2014 for the coastline between Goldcliffe and the 2nd Severn Crossing (on the Welsh coast). Over the period 2003-2007 the English coastline of the Outer Severn as a whole experienced an average rise in bed level of 24-30 mm/year throughout the intertidal profile. Over the period 2007-2014 the changes were more mixed with observed erosion of 6 mm/year on the upper profile and accretion of 5 mm/year on the mid-low profile. Over the period 2006-2014 the Welsh coastline between Goldcliff and the Second Severn Crossing experienced accretion on the upper and mid-low profiles (23 mm/year and 11 mm/year respectively) and erosion on the mid-upper profile (2 mm/year). The data taken as a whole indicates that there is more erosion/less accretion for the higher part of the intertidal profile compared with the lower part of the intertidal profile, but the data also shows that expanses of saltmarsh on the Welsh upper profile experienced accretion throughout the period studied. The highest rates of erosion were experienced on the mid-lower profile between Brean Down and Sand Point (Area 5) on the English side.

Due to limitations in the LiDAR data coverage, changes in total intertidal area at Low Water along the coastline covered by the data are not available. Taking areas of the English and Welsh coastline for which LiDAR data exists as a whole, changes at 0m ODN (close to mean water) and above were derived. These showed that over the period 2006/7-2014 the total intertidal area above 0m ODN, and above +3m ODN, increased by 4.4 ha/year and 1.2 ha/year, respectively. These changes at 0m ODN represent an average seaward advance of approximately +0.5 m/year of the English and Welsh coastline over the ~90 km stretch for which LiDAR data exists over the 7-8 year period from 2006/7 to 2014.

Summary of the longer-term evidence

Various pieces of evidence indicate that there has been a long term trend of erosion of intertidal areas since at least Roman Times. This evidence includes erosion of cliffs at the estuary margin; the exposure of post-glacial peats; the discovery of Bronze/Iron Age and Romano-British cultural debris; the progressive loss of saltmarsh; the mapped recession of the low water line since 1850 and intertidal profile surveys in a number of locations over the last 130 years. This evidence outlines the creation over thousands of years of wetland areas formed by the deposition of alluvial sediment on the margins of the estuary and long term erosion (say over the last 2000 years) of intertidal areas which previously formed these wetland areas (albeit at a lower sea level).

The evidence of long term changes in bed level from intertidal profile surveys over the last century indicate overall rates of erosion which vary (on average) between 12-16 mm/year on the English side and 21 mm/year on the Welsh side. Analysis of changes in low water from historical OS maps of the coast between Kilve and Brean (roughly corresponding to Area 1 of the LiDAR analysis) shows that since the end of the 19th century there had been net recession of the LW line of 1 m/year south of Steart and recession of the order of 10 m/year north of Steart. These long term signals confirm much more erosive historical trend than the one observed in the LiDAR data over the last decade or so.

Synthesis of evidence from short-term and long-term observations, archaeology and geomorphology

The short-term LiDAR evidence indicates overall accretion and stability while the long-term evidence indicates a clear signal of continued erosion since before Roman times. These two, apparently contradictory, trends have been reconciled by studying the trends in wave action in the Severn, and UK trends in wind climate generally, prior to and throughout the LiDAR period. The recent period of LiDAR measurement

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coincides with a period of relatively low wind and wave activity which may or may not continue into the future.

Different observed rates of change for different elevations in the tidal profile

Evidence both from older monitoring by Allen and Duffy (1988a) and the more recent LiDAR monitoring points to the rate of erosion being highest for mudflat high in the tidal frame. Both these studies show, however, that the rate of vertical change of the saltmarsh surface (which is even higher in the tidal frame) is much more accretionary than mudflat – a result which can be linked to the attenuation of waves by the enhanced friction associated with the vegetation.

The role of morphological modelling

This report has presented a number applications highlighting the applicability of morphological modelling as a tool for predicting the effects of coastal development as well as the ongoing effects of natural variation in waves, currents and water level. These applications illustrate the importance of different drivers for shoreline change and highlight the use of well-chosen modelling to improve understanding of intertidal evolution and to enable the development of long-term shoreline strategy.

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Appendices

A. Long-term profile measurements A.1. Comparison between 1987 HR Wallingford profiles and

LiDAR between Penarth and Rumney Notes: Comparison between profile surveys taken by Hydraulics Research Station in 1987 and

2000/2006 LiDAR data.

Profiles numbered 1- 6 moving from Penarth to Rumney.

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Figure A.1: Profile 1 - Hydraulics Research Station 1987 survey and 2000/2006 LiDAR data Source: HR Wallingford (2009a)

Figure A.2: Profile 2 - Hydraulics Research Station 1987 survey and 2000/2006 LiDAR data Source: HR Wallingford (2009a)

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Figure A.3: Profile 3 - Hydraulics Research Station 1987 survey and 2000/2006 LiDAR data Source: HR Wallingford (2009a)

Figure A.4: Profile 4 - Hydraulics Research Station 1987 survey and 2000/2006 LiDAR data Source: HR Wallingford (2009a)

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Figure A.5: Profile 5 - Hydraulics Research Station 1987 survey and 2000/2006 LiDAR data Source: HR Wallingford (2009a)

Figure A.6: Profile 6 - Hydraulics Research Station 1987 survey and 2000/2006 LiDAR data Source: HR Wallingford (2009a)

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A.2. Comparison between Sewer construction drawings and LiDAR data

Figure A.7: Comparison between Y&P Sewer construction drawing from 1889 (from Kirby, 1989) and LiDAR data

Source: HR Wallingford (2009a)

Source: HR Wallingford (2009a)

Figure A.8: Comparison between Rumney Valley Sewer construction drawing from 1920 (from Kirby, 1989) and LiDAR data

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A.3. Comparison between Somerset River Authority profiles and LiDAR data

Notes: Comparison between profiles from the Somerset Rivers Authority theodolite surveys in 1954

and 2003/2007 LiDAR data.

Profiles are numbered from north to south, with 1 to 4 between Clevedon and Kingston Seymour, 5 in

Sand Bay and 10 to 12 in Bridgwater Bay (Steart Flats).

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Figure A.9: Profile 1 - Somerset Rivers Authority 1954 survey and 2003/2007 LiDAR data Source: HR Wallingford (2009a)

Figure A.10: Profile 2 - Somerset Rivers Authority 1954 survey and 2003/2007 LiDAR data Source: HR Wallingford (2009a)

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Figure A.11: Profile 3 - Somerset Rivers Authority 1954 survey and 2003/2007 LiDAR data Source: HR Wallingford (2009a)

Figure A.12: Profile 4 - Somerset Rivers Authority 1954 survey and 2003/2007 LiDAR data Source: HR Wallingford (2009a)

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Figure A.13: Profile 5 - Somerset Rivers Authority 1954 survey and 2003/2007 LiDAR data Source: HR Wallingford (2009a)

Figure A.14: Profile 10 - Somerset Rivers Authority 1954 survey and 2003/2007 LiDAR data Source: HR Wallingford (2009a)

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Figure A.15: Profile 11 - Somerset Rivers Authority 1954 survey and 2003/2007 LiDAR data Source: HR Wallingford (2009a)

Figure A.16: Profile 12 - Somerset Rivers Authority 1954 survey and 2003/2007 LiDAR data Source: HR Wallingford (2009a)

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Severn Estuary Long Term Morphology Synthesis of data and modelling studies

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B. HR-MUDPROF model description B.1. Model concept HR-MUDPROF is based on the mathematical model described by Roberts et al (2000) exploring the effects of tidal currents and waves on the shape of intertidal mud flats. The Roberts et al model predicted the changes to the profile of a mudflat in response to different tidal ranges, sediment concentrations and wave conditions. This methodology has been used in similar models by Pritchard et al (2002), Wood and Widdows (2002), Pritchard and Hogg (2003) and Le Hir et al. (2007).

The model allows prediction of the changes to intertidal profile form in response to changes in the forcing effects of tides, tidal currents, waves, and suspended sediment concentrations and includes the use of spring-neap tidal cycles, longshore tidal currents (Rossington et al., 2009) and the stirring effect of waves using the formulation of Soulsby (2006).

B.2. Model equations The intertidal profile is described by bed elevations at fixed cross shore intervals or nodes. Cross-shore currents are driven by changes in the water level at the offshore boundary. Changes in water level imposed at the offshore boundary force water onto/off of the intertidal flat and generate currents determined by the volume of water passing each node.

Cross-shore tidal currents are represented with simplified hydrodynamics, taking only conservation of mass into account (equation 1) and ignoring conservation of momentum (Roberts et al, 2000).

0

x

uh

t

h

1

where h is water depth, u is the depth average velocity and x is the cross shore distance.

Sediment transport in the model is described by an advection equation with source and sink terms for erosion and deposition (Equation 2). The model represents the cross-shore and long-shore transport of sediment but assumes that the longshore transport is homogeneous.

The seaward boundary is assigned a sediment concentration which is proportional to the tidal range (see Section B.6.3).

de QQ

x

uch

t

ch

2

where c is the depth averaged suspended sediment concentration and Qe is the erosion flux and Qd is the deposition flux.

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Severn Estuary Long Term Morphology Synthesis of data and modelling studies

DDR5432-RT002-R03-00

Figure B.1: Schematic of the intertidal profile model

Erosion occurs when the predicted shear stress from currents and waves exceeds the erosion shear stress. The erosion flux is calculated using the Partheniades (1965) formulation, (Equation 3). Deposition is calculated as suggested by Winterwerp (2007) assuming that that there is no threshold of deposition (Equation 4).

1

e

bee mQ

(positive bed flux) 3

sd cwQ (negative bed flux) 4

where me is an erosion rate constant, e is the critical bed shear stress for erosion and ws is the settling

velocity. b is the bed shear stress calculated from combined cross shore and longshore currents ( c ) and

waves ( w ).

wcb 5

gDc uu lnC 6

2

2

1pww uf 7

-6

-4

-2

0

2

4

6

8

10

12

0 200 400 600 800 1000 1200

Cross shore distance (x) (m)

Bed

ele

vati

on

(zb

) (m

)

Sea wall

zs(t)

zs(t+1)δzs

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Severn Estuary Long Term Morphology Synthesis of data and modelling studies

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where is the water density, u is the cross shore current velocity, ulng is the longshore current velocity, CD is the drag coefficient (z0 is the roughness length) (Equation 8), Up is the peak wave orbital velocity (equation 9) (Soulsby, 2006) and fw is the wave friction factor (equation 10).

2

0 1)/log(

4.0

zhCD

8

1.22/12/1

65.3exp

42

g

h

Th

gHU

z

s

p

9

N

ww BRf

10

where Hs is the significant wave height, g is acceleration due to gravity (9.81 m/s), h is water depth, Tz is the zero crossing period of the wave (Tp=1.28Tz), Rw is the wave Reynolds number (UpA/ ν, A=UwTp/2π, ν is kinematic viscosity, 1.36x10-6) and B and N are coefficients for smooth turbulent flow (B=0.0521 and N=0.187) (Soulsby, 1997). Smooth turbulent friction factors are used because combined waves and currents make laminar friction inappropriate (Soulsby and Clarke, 2004).

Long shore currents are taken from flow model results for a spring-neap cycle. These are adjusted within HR-MUDPROF to account for changes in bathymetry, assuming the water surface slope is unaffected by changes in the morphology of the mudflat. No wave driven currents are included in the model.

B.3. Boundary conditions and constraints The suspended sediment concentration at the seaward boundary (Cbnd) in the intertidal profile model varies over a spring neap cycle so that it is larger on spring tides and smaller on neaps. It is assumed that suspended sediment concentrations in the Severn Estuary are saturated. This is a reasonable assumption because of the continual pick up and collapse of the vertical sediment profile (e.g. Kirby and Parker, 1975). Winterwerp (2001) showed that the saturated depth-averaged concentration in a suspension scales as the cube of the current speed. It is therefore assumed that the boundary concentration, Cbnd, scales as the cube of the ratio between the current tidal range and the spring tidal range (Equation 11):

3

_

espringrang

rangecc inbndbnd

11

Sea-level rise is included in the model by including the cumulative amount of sea-level rise to the water level so that water levels progressively move up the slope. As the sea-level rises new nodes can become wet.

The model can include the role of resistant material underlying modern sediment layers in controlling erosion. A profile with different erosion properties (higher shear stress for erosion, different sediment density) can be specified below the modern sediment surface. If this layer is exposed, erosion is limited by the greater resistance of the sediment. Alternatively, if the constraint is rocky, erosion of this layer can be prevented entirely. For the profiles modelled here, a constraint was nominally set 10 metres below the present-day bed level, and had no effect on the modelled morphological changes.

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Severn Estuary Long Term Morphology Synthesis of data and modelling studies

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B.4. Input data Water levels for the offshore boundary are taken at half hour intervals throughout a neap-spring-neap tidal cycle from a TELEMAC-2D flow model (HR Wallingford, 2014). Long shore currents at each node across the profile were taken from the flow model. This ensures that water levels and longshore currents are in phase and include realistic tidal asymmetry.

Wave conditions were input into the model as time series of significant wave heights and periods. Time series of waves have the merit that they allow the representation of seasonal influences, such as winter storms, to shape the profile causing variations around equilibrium in response to changes in forcing.

The boundary suspended sediment concentration must be estimated for each site, as must sediment properties such as the shear stress for erosion, settling velocity, erosion rate coefficient and sediment density. Where previous studies have been carried out these parameters may have been measured; however at many locations little information will be available and these parameters will have to be estimated and fitted within an appropriate range. For this study, boundary concentrations were estimated using data from the DECC study (HR Wallingford, 2009b).

B.5. Key assumptions The along-shore currents are locally homogeneous in a long shore direction.

The along-shore water free surface slope is assumed unaffected by changes in morphology, but the along-shore currents, cross-shore currents, and shear stresses are updated in the model.

Boundary suspended sediment concentrations to the model are assumed to be saturated and to be reasonably described by the scaling law put forward by Winterwerp (2001). This scaling law was combined with empirical relationships derived from the data between suspended sediment concentration and tide range developed in HR Wallingford (2009b).

B.6. Model application

B.6.1. Hydrodynamics

Changes in the intertidal profile in the model are driven by changes to cross-shore currents, long-shore currents, waves, and concentrations. Cross-shore currents are calculated within the model from changes in water level at each time-step, as described in Section 2. Water levels and longshore currents were taken from flow model results (HR Wallingford 2014) for each profile.

B.6.2. Waves

Wave height and period were taken from the SWAN wave modelling (HR Wallingford, 2009c) at a position corresponding to approximately 8m water depth (with scheme) at MHWS. A one-year time-series of wave conditions was generated for each location, and waves were then further limited by water depth internally within the intertidal profile model, according to time-varying water levels.

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Severn Estuary Long Term Morphology Synthesis of data and modelling studies

DDR5432-RT002-R03-00

B.6.3. Boundary suspended sediment concentration

Boundary suspended sediment concentrations were defined for each profile, based upon the findings from analysis of the Kirby/IOS survey data presented in (HR Wallingford, 2009a). For each profile location, five survey points closest to the profile were selected and the relationship of concentrations with tidal range analysed. Curves of the form:

3

max

eSpringrang

Rangeconcconc

12

were fitted to the data for surface and mid depth concentration profiles (based upon the scaling developed by Winterwerp, 2001). These curves were then used to assign the boundary concentration for each profile, both before and after the barrage by using the change in tidal range to determine the change in boundary concentration.

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© HR Wallingford

FS 516431EMS 558310OHS 595357

HR Wallingford is an independent engineering and environmental hydraulics organisation. We deliver practical solutions to the complex water-related challenges faced by our international clients. A dynamic research programme underpins all that we do and keeps us at the leading edge. Our unique mix of know-how, assets and facilities includes state of the art physical modelling laboratories, a full range of numerical modelling tools and, above all, enthusiastic people with world-renowned skills and expertise.

HR Wallingford, Howbery Park, Wallingford, Oxfordshire OX10 8BA, United Kingdomtel +44 (0)1491 835381 fax +44 (0)1491 832233 email [email protected]

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