Forecasting Extreme Water Levels in Estuaries for Flood Warning

Forecasting Extreme Water Levels in Estuaries for Flood Warning.

Stage 1: Review of Current Practice and Recommendations for Stage 2

R&D Project Record W5/010/1

R Harpin, J M Wicks, R M J Scarrott, I D Cluckie*, J Qin*

Research Contractor:

Halcrow Water & University of Bristol* Publishing Organisation: Environment Agency Rio House Waterside Drive Aztec West Almondsbury Bristol BS32 4UD

Tel: 01454 624400 Fax: 01454 624409

All rights reserved. No part of this document may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior permission of the Environment Agency.

The views expressed in this document are not necessarily those of the Environment Agency. Its officers, servant or agents accept no liability whatsoever for any loss or damage arising from the interpretation or use of the information, or reliance upon views contained herein.

Dissemination status Internal: Released to Regions External: Restricted

Statement of use This report describes the first stage of a study aiming to improve the Agency’s flood warning systems in estuaries. It consists of a review of the current methods and data throughout all Regions and gives recommendations for improvement to them. It also outlines the approach to be adopted in the second stage of the project, which will focus on improved methodologies. The report will be of interest mainly to those flood defence staff specifically involved in providing flood warnings in estuaries.

Research contractor This document was produced under R&D Project W5-010 by:

Halcrow Water in association with Water Management Research Centre Burderop Park Dept. of Civil Engineering Swindon University of Bristol Wiltshire Lunsford House SN4 0QD Cantocks Close Bristol Tel: 01929 462314 BS8 1UP Fax: 01929 462180 Tel: 0117 9289769 Fax: 0117 9289770

Environment Agency Project Leader The Environment Agency’s Project Leader for R&D Project W5-010 was: Betty Ng (to September 1999) and Henry Brown (from October 1999) Environment Agency, Wales

Further copies of this report are available from: Environment Agency R&D Dissemination Centre, c/o WRc, Frankland Road, Swindon, Wilts SN5 8YF tel: 01793-865000 fax: 01793-514562 e-mail: [email protected] CONTENTS

Executive Summary 1

Key Words 2

1. Introduction 3 1.1 Objectives and Layout 3 1.2 Background 3

2. Overview 2.1 Estuaries in England and Wales 5 2.2 Information Gathering 5 2.3 Overview of Forecasting Methods 5 2.4 Overview of Data Availability and Use 13

3. Conclusions & Recommendations 26 3.1 Conclusions 26 3.2 Recommendations for Stage 2 29

4. References 30

Appendix 1 Anglian Region 31

1 Northern Area 31 1.1 Background 31 1.2 Current Forecasting Methods 31 1.3 Improvements to Water Level Forecasts 32

2 Central Area 39 2.1 Background 39 2.2 Current Forecasting Methods 39 2.3 Improvements to the Forecasting of Water Levels 41

3 Eastern Area 42 3.1 Background 42 3.2 Current Forecasting Methods 42 3.3 Improvements to Water Level Forecasts 45

Appendix 2 Midlands Region 47

2.1 Background 47 2.2 Current Forecasting Methods 47 2.3 Improvements to Water Level Forecasts 49

R&D Project Record W5/010/1 i Appendix 3 North East Region 54

3.1 Background 54 3.2 Method 54 3.3 Improvements to the Forecasting of Water Levels 58

Appendix 4 North West Region 65

4.1 Background 65 4.2 Forecasting Methods 65 4.3 Improvements to the Forecasting of Water Levels 68

Appendix 5 Southern Region 73

1 Kent Area 73 1.1 Background 73 1.2 Current Forecasting Methods 73 1.3 Improvements to Water Level Forecasts 75

2 Sussex Area 83 2.1 Background 83 2.2 Current Forecasting Methods 83 2.3 Improvements to Water Level Forecasts 85

3 Hampshire Area 87 3.1 Background 87 3.2 Current Forecasting Methods 87 3.3 Improvements to Water Level Forecasts 90

Appendix 6 South West Region 92

6.1 Background 92 6.2 Current Forecasting Methods 92 6.3 Improvement to Water Level Forecasts 98

Appendix 7 Thames Region 110

7.1 Background 110 7.2 Current Forecasting Method 110 7.3 Improvements to Water Level Forecasts 111

Appendix 8 Environment Agency – Wales 115

8.1 Background 115 8.2 Current Forecasting Methods 115 8.3 Improvements to Water Level Forecasts 117

R&D Project Record W5/010/1 ii List of Tables

Table A1 1.1 Forecasting Data Used Summary: Anglian Region Table A1 1.2 Forecasting and Warning Summary: Anglian Region Table A1 1.3 Factors Important in the Forecasting of Water Levels: Anglian Region Table A1 3.1 Level Forecasting at Monitoring Locations Table A2.1 Forecasting Data Used Summary: Midlands Region Table A2.2 Forecasting and Warning Summary: Midlands Region Table A2.3 Factors Important in the Forecasting of Water Levels: Midlands Region Table A3.1 Estuaries and Reference Sites Table A3.2 Humber Estuary Flood Warning Zones and Associated Forecast Stations Table A3.3 Humber Estuary Forecast Stations and Associated Reference Ports Table A3.4 Forecasting Data Used Summary: North East Region Table A3.5 Forecasting and Warning Summary: North East Region Table A3.6 Factors Important in the Forecasting of Water Levels: North East Region Table A4.1 Forecasting Data Used Summary: North West Region Table A4.2 Forecasting and Warning Summary: North West Region Table A4.3 Factors Important in the Forecasting of Water Levels: North West Region Table A5 1.1 Forecasting Data Used Summary: Southern Region Table A5 1.2 Forecasting and Warning Summary: Southern Region Table A5 1.3 Factors Important in the Forecasting of Water Levels: Southern Region Table A5 3.1 Coastal Zones and Hampshire Estuaries Table A5 3.2 Barometer Correction Table for Lymington Table A6.1 Estuary and Associated Coastal Flood Warning Reach Table A6.2 Water Level Forecast for the Coastal Reaches Table A6.3 Comments Received Regarding Data for Use in Water Level Forecasting Table A6.4 Comments Received Regarding Improvements to the Methods for Forecasting Water Levels Table A6.5 Forecasting Data Used Summary: South West Region Table A6.6 Forecasting and Warning Summary: South West Region Table A6.7 Factors Important in the Forecasting of Water Levels: South West Region Table A7.1 Real-Time Monitoring Gauges for the Thames Estuary Table A7.2 Forecasting Data Used Summary: Thames Region Table A7.3 Forecasting and Warning Summary: Thames Region Table A7.4 Factors Important in the Forecasting of Water Levels: Thames Region Table A8.1 Flood Risk Areas and Estuaries in the SE Wales Area Table A8.2 Forecasting Data Used Summary: Environment Agency Wales Table A8.3 Forecasting and Warning Summary: Environment Agency Wales Table A8.4 Factors Important in the Forecasting of Water Levels: Environment Agency Wales

R&D Project Record W5/010/1 iii EXECUTIVE SUMMARY

There is a need for rigorous but practical methods for the real time forecasting of extreme water levels in estuaries, which are suitable for incorporation into flood forecasting systems. As the first stage in developing appropriate methods, this report contains the findings of a detailed review of current practice for flood forecasting for estuaries in England and Wales.

Detailed descriptions of methods and data availability for all estuaries are presented, based on information gathered through interviews with Agency Flood Warning staff, questionnaires and previous reports. A database has been set up containing currently available data and it is intended to update this with further information, as more data on the accuracy of forecasts becomes available.

The current forecasting methods for predicting water levels in estuaries are classified into the following categories:

· Forecast levels derived from correlation with a reference port level; · Astronomic tide plus Storm Tide Forecasting Service (STFS) surge; · Tide table plus STFS surge residuals; · Astronomic tide time series and STFS surge residuals.

The accurate and reliable prediction of tide and surge residuals is key to the generation of extreme level forecasting and warning for estuaries. The report identifies the need to improve existing or develop new methods to overcome a range of deficiencies in current methods, including:

· Insufficient lead times; · Lack of real-time correction of forecast errors; · Available tidal, wind and wave information not being fully utilised; · Lack of reliable means of translating STFS surge related peak values and timings from the site where calculated to the site used for level forecasting.

Site specific recommendations are made for additional or improved real-time monitored or forecast data (astronomic tidal levels, surge residuals, wave data, wind speed, wind direction and fluvial flow) in order to improve forecast water levels. A caveat on the results of this report is that little objective information was readily available on the accuracy of current forecasting methods. As part of the next stage of the project a more detailed assessment of accuracy will be made at a limited set of representative locations. Following this review the research will focus on developing improved methodologies.

R&D Project Record W5/010/1 1 KEY WORDS

Flood warning; estuaries; forecast; review; current methods.

R&D Project Record W5/010/1 2 1. INTRODUCTION

1.1 Objectives and Layout

The overall objective of the study is to develop rigorous but practicable methods for the real-time forecasting of extreme water levels in estuaries, suitable for incorporation into existing Environment Agency flood warning systems (Environment Agency, 1998a). The specific report objectives are:

· to summarise the current methods used for the forecasting of estuary water levels in England and Wales; · to summarise the data available and that currently used for the forecasting of estuary water levels; · to review the current estuary water level forecasting methods and data availability; · to make recommendations for improvements in the estuary water level forecasting methods and data availability.

“Estuary” is defined in the Oxford English Dictionary as “the tidal mouth of a large river”. For the purposes of this research we propose that the definition includes all rivers for which the Agency has a possible flood warning role. We will consider the boundaries of a particular estuary to be from a point where the tidal effect starts to have a significant effect on water levels at the upstream end, to a point where the fluvial flow or the estuary shape has an insignificant effect on water levels at the downstream end.

This report contains an overview of flood forecasting methods and data availability in current use in Chapter 2, which have been distilled from detailed descriptions of current practice for all estuaries given by region in Chapters 3 to 14. Preliminary conclusions and recommendations are given in Chapter 15.

1.2 Background

The Environment Agency (Agency) has direct responsibility for flood defence and flood warning in England and Wales. This responsibility includes the provision and maintenance of suitable flood defences and the production of flood warnings for the estuaries of England and Wales.

The forecasting of water levels in estuaries is a complicated process due to the interaction of local tides with river flows, winds and waves. Currently there is a lack of an appropriate methodology (or methodologies) and inadequate data for the confident forecasting of extreme water levels in estuaries. This project was born out of the need to address these problems to ensure the Agency can comprehensively satisfy its flood warning duties.

The project is a three year National R&D study, scheduled for completion by 31 March 2001, that will examine the methods currently used for the forecasting of estuary water levels and develop improved techniques. This report contains the findings of the initial phase of the project aimed at reviewing the methods currently adopted and the data available to Agency flood warning staff. This report is based on

R&D Project Record W5/010/1 3 information gathered from Agency flood warning staff during meetings held between 12 July 1998 and 18 September 1998. To ensure the consistent use of data, the report also draws on information collected for the Environment Agency Tidal Flood Forecasting project (Environment Agency, 1998b).

R&D Project Record W5/010/1 4 2 OVERVIEW

2.1 Estuaries in England and Wales

The Environment Agency has seven regional offices and one in Cardiff representing the Environment Agency in Wales, which are responsible for forecasting water levels for flood warning on 72 principal estuaries.

The most important factor affecting extreme levels in estuaries is without exception the tidal effect. The astronomic tide is not so difficult to predict. Of particular importance is the surge component of the tide arising from adverse weather conditions (wind and pressure) over the seas. Of the 72 estuaries, 37 are subject to significant fluvial impact whilst wind speed and direction have been assessed as significant factors for 66 of them. Wave action is considered to be a significant factor in 39 estuaries. 14 are subjected to influence by small-scale pressure systems, 10 of these being within the South West Region, 2 in Wales and 2 in the Southern Region. 8 estuaries have significant double high tides, all of these being in the Southern Region. A summary of estuaries as well as risk factors is given in Table 2.1 and Figure 2.1.

2.2 Information Gathering

The information presented in this report has been obtained from a sequence of interviews carried out by the project team with Agency staff. Additionally a questionnaire has been sent out to all Agency regional and area offices responsible for flood warning in estuaries, and the information obtained has been used to populate a database for use by the project team. Information has been collected for all estuaries and is summarised region by region in subsequent chapters of this report.

The co-operation of all Agency staff who have contributed to the study is acknowledged and much appreciated. Agency staff have been pre-occupied by the Easter floods and, understandably, some information has taken time to provide. Very little information has been acquired on accuracy of the current forecasting methods, apart from subjective information in the responses to questionnaires based on opinions of flood warning staff. Comparisons of forecast and observed water levels in estuaries are said to be available in many cases, but are not easy to extract from archives. It was impossible to collect this information for all estuaries for this interim report, but will be collected for selected estuaries in the next stage of the study.

2.3 Overview of Forecasting Methods

2.3.1 Introduction

1. The current forecasting methods for predicting water levels in estuaries for England and Wales can be summarised by the following procedure: · Using predicted tide and surge information to estimate high water levels, which are either used as a final forecast level or as a primary forecast value; · Combining other factors, such as fluvial effects, with the primary forecast value to produce final forecast high water levels at the site of interest;

R&D Project Record W5/010/1 5 · Comparing the forecast level and taking into consideration other conditions, such as wind and wave effects with a warning threshold value to trigger a flood warning, · Undertaking real-time monitoring to assess the development of flooding when a warning is issued.

2. The key to successful flood warning is accurate forecasting and monitoring of:

· Tide height and time · Tidal surge · Wind speed and direction · Wave height and time · Fluvial – tide interaction

3. On the East Coast of England a reasonably reliable approach has been developed for storm tide forecasting, which covers most estuaries in this area. Improvements to STFS forecasts rely heavily on the real-time tracking of storm surges using data from ‘up tide’ gauges to monitor the progress of the observed surges down the east coast and comparison with predictions made by the STFS. This can be done with a reasonable forecast lead time, although there may well be scope for increased accuracy through improved modelling techniques.

4. Such real time measurements are not available on the South and West Coasts where the maximum time for the confirmation of a developing surge is barely one hour from the time of high water at most gauges. Consequently warnings have had to rely solely on predictions of surge from the Storm Tide Forecasting Service (STFS) and on severe weather forecasts. Performance has inevitably been unreliable and has failed to meet the Agency’s performance standards.

A summary of the above is given in Tables 2.1 and 2.2 and details of the current methods used in estuaries in England and Wales are given below:

2.3.2 Tide and Surge

Tide and Surge residuals are assessed as significant factors in all 72 estuaries. The accurate and reliable prediction of these is key to the generation of extreme level forecasts and warning for estuaries. The current method using tide and surge information depends on the availability of tidal gauges together with tidal forecasts and STFS surge information within or close to the specific estuary. Note that the comments on storm tides given below are based on discussions with Agency staff only and discussions with the STFS and POL have not yet been held. The review of external forecasts, notably surge predictions produced by the STFS, will be included in the next stage of the study.

A summary according to classification is given below:

1. Forecast levels derived from correlation with a reference port level This method uses the empirical level-to-level correlation to convert the forecast level at the reference site to a level at the site of interest. The empirical level-to-level correlation is derived from the analysis of historical data

R&D Project Record W5/010/1 6 including the analysis of the approximate time difference between the two sites.

The forecast level at each reference site is equal to the sum of the astronomic tide and the STFS surge residual forecast. The reference sites are normally the ‘A class’ tidal gauges for which the astronomic tidal time series and received STFS surge residual are available.

The accuracy of this method depends on the location of the reference ports and the historic data available, but in particular to the accuracy of the external forecast. Real-time monitoring values are only used to assess the development of the surge as compared to forecast values.

This method is mainly used in the North East region (6 estuaries). All of these use the same gauge (North Shields) as a reference port except for the Humber Estuary which uses 3 Class A gauges. Hydraulic models and a Regional Flood Forecasting System (RFFS) are also applied when forecasting levels in the Humber Estuary. Here, the astronomic tide level is produced by the tide generating software leased from the Proudman Oceanographic Laboratory (POL).

The significant deficiencies that exist are that:

· The spatial and temporal development of the tidal surge around an estuary will be unknown as only one site is used as a reference for each estuary; · There is no method for the real-time correction of forecast errors relating to the propagation of peak values and timing; · The empirical correlation is increasingly uncertain when estimating extreme events beyond the historic data.

2. Astronomic tide plus STFS surge residuals for coastal reaches This method directly applies the forecast levels at the specified coastal zone as the forecast level for the appropriate estuary. The forecast level at the coast zone is linked to specific tidal gauges that are generally the ‘A class’ gauges with an astronomic time series and STFS surge residual. The forecast level at a tidal gauge is given as:

Level = astronomic tide at tide gauge + STFS surge residual at the same or another gauge

The accuracy of this method is totally dependent on the relative location of the tidal gauges used and the relationship between the site within the estuary and the forecast site, but in particular to the accuracy of the external forecast. Real- time monitoring data is only used to assess the development of flooding.

This method is used in 37 estuaries, 17 of which are in the Southern Region, 13 in the Southwest Region, 5 in the Welsh Region and 2 in the Anglian Region.

R&D Project Record W5/010/1 7 The significant deficiencies that exist are that:

· The spatial and temporal development of the tidal surge around the estuary is unknown as only one site is used as a reference for each estuary; · Information about the tidal surge at the site of interest is not available. Many estuaries use the same tide gauge as a forecast site, especially in the Southern and South West Regions. In the former, only 2 sites with STFS surge residuals and 4 gauges with predicted astronomic tide information are used in the forecasting of levels for 17 estuaries. In the South West Region, 5 gauges with tide information are used for 13 estuaries. · No modification is made to the tidal surge predicted at the tidal gauge to obtain values at the site within the coastal zone, as well as from the coastal zone to the site of interest within the estuaries; · There is no method for the real-time correction of forecast errors relating to the propagation of peak values and timing.

3. Tide table plus STFS surge residuals for coastal zones The methodology for this category is the same as for Method 2, the difference being that the tidal information comes from a different source. Here the predicted tide comes from a tide table with some of the estuaries using ‘typical’ tide profiles to modify the water levels leading up to high levels. The tide tables are derived by POL, based upon a series of reference ports from the North East Region to Eastern Area in Anglian Region that include time difference and peak difference. Real-time monitoring data is only used to assess the development of flooding.

The accuracy and reliability of this method mainly depends on the relative location of the tide gauges used and the relationship of the site of interest within the estuary with the tide gauge, as well as the accuracy of the tide table and especially the prediction of the surge residual.

This method is used in 14 estuaries, 12 of which are in the Anglian Region and 2 are in the South West Region (these being Camel and Stour/Avon). 4 of the estuaries within this category use a ‘typical’ tide profile technique. They are Witham, Haven, Welland and Nene. This method is thought to work well during extreme events in the Anglian Region, Eastern Area. This is probably due to three factors. First the large area rather than specific estuaries that is at risk during extreme events. Secondly, the number of gauges used is relatively much higher than for others Regions so that each estuary is matched with one or more tide monitoring sites. Thirdly, this Region is located within a relatively closed environment (the North Sea) where the local conditions are more stable than for other Regions, such as South West and Southern where secondary pressures are more influential.

Significant deficiencies exist in that:

· There is no method for the real-time correction of forecast errors relating to the propagation of peak values and timing, except for the Great Ouse;

R&D Project Record W5/010/1 8 · The propagation of tidal surges from downstream to upstream is not identified and tide tables need further improvement in the Central Area of the Anglian Region; · Two methods are used to provide level forecasts in the Great Ouse which could give conflicting results when issuing warnings.

4. Astronomic tide time series and STFS surge residuals This method is used to forecast water levels directly at sites of interest where,

Level = predicted astronomic tide at + STFS forecast surge residual the site within the estuary at the same or a different site

The astronomic tide comes from the time series produced by POL. The STFS forecast residual, produced by a digital model, is received for the site of interest or the reference site.

The method is used in estuaries where the tidal-fluvial interaction is important in relation to flooding in middle and/or upper areas along the estuary. The accuracy depends on the relative location of the site for which the STFS surge is calculated, the accuracy of the fluvial flow forecast and the methods used to handle the fluvial-tidal interaction.

There are 14 estuaries that use this method. 3 of these are in the South West Region and 3 are in the North West Region where they provide flood warnings through the Operation Neptune System which triggers the forecast procedures and places the Agency flooding warning staff on stand by. 1 is in the Midlands Region, where the Severn estuary receives surge forecasts for Avonmouth from the Bristol Channel and Severn Estuary models and 1 is in the Anglian Region. 2 of them are in Wales and 2 of them use the results as boundary conditions for hydrodynamic models (e.g. The Humber and Thames).

Significant deficiencies exist in this method in that:

· There is no method to propagate tidal surges from one site to another as well as from downstream to upstream. · There is a lack of an appropriate method to forecast the interaction of tidal and river flows which is discussed in Section 2.2.3. · In the case of the hydrodynamic models the impact of local wind stress, not allowed for in the model, can produce significant errors.

2.3.3 Fluvial-Tidal Interaction

The impact of fluvial flows is assessed as a significant factor in 37 estuaries. However, only 18 estuaries incorporate it in the forecast procedure. The methods of incorporation can be divided into two categories:

1. Empirical method · The level-to-level correlation including ‘bands’ for different flow values is used in the North East Region (Coquet and The Humber).

R&D Project Record W5/010/1 9 · The empirical correlation between tidal levels and fluvial flows is used in the Southern Region (7 estuaries), Wales (Cleddau and Tywi), on the River Lune, Trent Region, and in the Central Area of the Anglian Region (3 estuaries). The correlation is derived from the experience of flood warning staff and/or analysis of historic events. The level is forecast from look-up-tables or is produced automatically (e.g. the ‘Hiwater’ system in the Trent Region).

Significant deficiencies exist in this methodology in that:

· The empirical method lacks ability to estimate extreme events beyond the historic data, particularly at sites where extreme flooding results from the combination of fluvial and tidal effects. An example of this is the Cleddau estuary where the impact of fluvial flows on water levels varies and, typically, flooding occurs with a high tide and moderate fluvial flows or a moderate tide and high fluvial flows. A full understanding of the tidal-fluvial interaction is therefore needed. · In most cases, the river flow is forecast from level-to-level correlations (only a few use rainfall runoff models, e.g. the Cleddau and Trent). This may result in a significant deficiency in warning lead time.

2. Hydrodynamic models · A hydrodynamic model within the ‘Regional Flood Forecasting System (RFFS)’, is used for the Humber Estuary (where two methods are used). · A 1-D hydrodynamic model is used to estimate the water levels at sites along the Thames. · Correlations between tidal levels and fluvial flow are derived from hydraulic modelling studies for use in forecasting. This method has been adopted for the Rivers Severn and Ribble. 1-D and 2-D models exist for the River Severn but are not routinely used.

A significant deficiency exists in the procedures adopted for the Severn and Ribble and the currently available research models could be converted for routine use.

2.3.4 Wind Speed and Direction

Of the 69 estuaries where local winds are a significant factor, 46 incorporate wind data into forecast or warning procedures.

There are three estuaries that incorporate wind data in level forecasts. 2 use empirical correlations to simply add levels (the Frome and Piddle) whilst for the Thames, the wind factor is considered in the North Sea Model (a cut-down version of the POL-CS3 model).

The remaining 43 estuaries incorporate wind data into warning procedures in which the wind data together with other information is compared with warning thresholds to trigger flooding warnings. These thresholds include an analysis of the wind factor based on historic data.

2.3.5 Wave Height and Time

R&D Project Record W5/010/1 10 Of the 40 estuaries where local waves are a significant factor, only 5 use data in the warning procedure to trigger flood warnings. They are the Tees (North East Region) and the Adur, Arun, Ouse and Cuckmere (Southern Region). This deficiency results from the lack of onshore wave data and the lack of a method to convert offshore waves to onshore waves.

2.3.6 Other Meteorological Conditions

Several estuaries have been identified where other conditions present a significant risk.

· 14 estuaries are influenced by small-scale pressure systems. This relates to conditions that may invalidate the surge forecasts. It is considered in 10 estuaries in the South West Region via the Lennon criteria. At Lymington the low-pressure data is used to modify the forecast levels based on experience. · 8 estuaries in the Southern Region have significant double high tides which are quantified using a typical tide curve.

Further research is needed in regard to these aspects.

2.3.7 Warning

The Agency’s target for passing colour phase warnings to the public is 2 hours before flooding occurs (minimum standards). For operational response and disaster planning this time-scale is too short.

Liaison with Police and Local Authorities has indicated that possible rather than probable risk should be considered and 6-12 hour lead times have been proposed (in addition to improved real-time liaison).

7 Environment Agency Regions are currently able to generate a warning about 4-6 hours before high tide. Only in Environment Agency Wales is this not provided at present. In Anglian Region a preliminary assessment is given at 12 hours before high tide.

The proposed national warning lead time/targets are:

· Initial colour phase warning estimate at T-12 hours; · Warning to emergency/operational services at T-6 hours; · Warning to the public at T-4 hours (currently 2 hours minimum) These targets are achievable in estuaries where tidal surges play a dominant role in flooding. However in estuaries where the fluvial flow is dominant, this target should be varied depending on the catchment characteristics such as its size and lag time. In such estuaries, the initial colour phase warning estimate may be set up between 12 and 6 hours ahead.

The proposed level of accuracy is that not more than 1 in 5 warnings should be inaccurate to the extent that flooding did not occur when it was forecast (Tidal Flood Warning In The Bristol Channel, Summary Report, April 1994, Bristol Channel Tidal Monitoring and Forecast Group).

R&D Project Record W5/010/1 11 R&D Project Record W5/010/1 12 2.3.8 Deficiencies and Comments on Improvement

1. Deficiencies of the current forecasting methods

· The current methods lack real-time correction of the forecast errors in relation to peak values and the timing of the forecast levels. · Most of the current methods use look-up tables to forecast levels and issue flooding warning. This will result in insufficient lead times for warnings in some cases. · Tidal information from forecasts and monitoring is not utilised fully in those areas where forecast levels for coastal reaches are provided, especially in the Southern and South West Region. · Wind and Wave information are not utilised fully in most areas, especially in the Central Area of Anglian Region, Southern Region and North West Region. · The lack of extrapolation of the forecast values in areas where the first and third types of method are used and also when the empirical method is used to handle the tidal-fluvial interaction. This deficiency is significant in the North East, South West and Southern Regions. · The current methods lack reliable means of translating STFS surge related peak values and timing from the site where it is calculated to the site used for level forecasting. This reduces the accuracy of the current forecast method, especially in the Northern Area of the Anglian Region, the South West Region, North West Region, Southern Region and Thames. · The current methods lack reliable and accurate ways of estimating peak values and timing of tidal propagation from downstream to upstream sites in the South West Region.

2. Improvements

The following points summarise improvements that could be made to the methods for forecasting water levels based on the summary of deficiencies above.

R&D Project Record W5/010/1 13 · Development of a method to translate wave height and period measured offshore to onshore locations, particularly in Wales. · Development of forecast methods for the River Kent and Solway Firth. · Derivation of astronomic tidal time series for estuaries where tidal tables are used. · Improvement of hydraulic modelling to give a thorough understanding of areas at risk and to enhance the reliability of forecast levels during extreme events (particularly for the Humber, Severn and Trent) upon which, further developments of the hydrodynamic models can be based.

2.3. 9 Accuracy of Forecasts

As described above no data has yet been made available on accuracy of the current forecasts, except for subjective information in response to the questionnaires. A summary of this information and initial conclusions on where methods are currently insufficiently accurate is presented below:

The feedback from the regions has been limited at the time of completion of the interim report. Southern, North West and South West have indicated that the preliminary information provided was in general accurate. As the project develops it is intended to update the information contained in the database.

It is proposed that information on accuracy of current forecasts is principally collected from the three regions from which pilot study estuaries are to be selected.

2.4 Overview of Data Availability and Use

All of the data accessed in the interim report has been derived from EA sources. Indirectly this has included input from the STFS and POL. A considerable body of information has also been extracted from the draft Tidal Flood Forecasting Project (EA, 1986b). In order to be accurate and reliable forecasts require real-time monitoring of the principal factors contributing to tidal flood risk in all estuaries, including;

· astronomic tidal levels; · surge residuals; · wave height, period etc.; · wind speed and direction; · river flow.

2.4.1 Tidal Data

The forecasting of astronomic tides comes from Agency Forecasting Centres where Admiralty tables are used, or from POL tide generating software outputs.

The real-time monitoring of tidal data is available for all estuaries (see Table 2.3 and Figure 2.4). An extensive network of tide level gauges comprising the national ‘A class’ network and local gauges already exists. Of the 114 tide gauges available only 40 are used. 15 of these are ‘A class’ gauges. A summary of use and availability of tide gauges is given in Table 2.1. and Figures 2.2 and 2.3.

R&D Project Record W5/010/1 14 2.4.2 Surge Predictions

Of the 198 available sites that receive STFS surge predictions from three different model outputs (CS3, ECNS, and BSEM) 38 are used in the forecasting of tidal levels for England and Wales. 18 are used in forecasting levels for 72 principal estuaries. There are accuracy/reliability deficiencies in some estuaries (e.g. Humber, Wash, Solent, and Morecambe Bay) and along south and west coasts, where surge forecasts have been invalidated on a number of occasions by the influence of secondary low pressure.

A summary of the use and availability of STFS surge forecast sites is given in Table 2.1. and Figure 2.2.

2.4.3 Wind Data

68 STFS wind forecast sites and 60 real-time monitoring sites, supplemented by meteorological sites, can provide wind speed and direction data for inclusion in the forecasting of water levels. Of the 72 estuaries, 46 incorporate wind factors in the flooding procedure and 3 use wind data in the forecast procedure. There are 7 estuaries that have no wind data. They are the Great Ouse (Kings Lynn), Welland, Kent, Solway Firth, Cleddau, Wye and Tywi. The available real-time monitoring sites for wind speed and direction, as well as the sites receiving STFS forecast wind data are listed in Table 2.1 and Figures 2.2 and 2.5.

2.4.4 Wave Data

Real-time monitored wave data is available only for the Humber. Forecast wave data is available for four estuaries. They are the Tees, Humber, Thames and Dee. Only the forecast wave data is used in the forecast procedure. A summary of wave data is given Table 2.1 and Figures 2.1 and 2.6.

2.4.5 Fluvial Data

Real-time monitored fluvial data is available for 6 estuaries. They are the Great Ouse (Brampton), Crouch/Roach, Trent, Thames, Ribble and Lune. Forecast fluvial data is available for 19 estuaries, these being the Crouch/Roach, Severn, Trent, Wear, Ouse (at Skelton), Ribble, Lune, Kent, Torridge, Camel, Truro/Falmouth (Fal estuary), Fowey, Tamar, Dart, Teign, Exe, Thames, Cleddau and Tywi. The Severn and Thames use rainfall runoff models to predict the fluvial flow. The others use level-to-level correlations to predict the fluvial level. There are 13 estuaries where the fluvial flow is assessed as having an important impact on water level forecast but for which there is no available data. Available fluvial data is summarised in Table 2.1 and Figure 2.4.

2.4.6 Recommendations for the collection of future data

The following points summarise comments regarding data for use in improved forecasting of water levels.

R&D Project Record W5/010/1 15 · More level gauges are needed at the following sites: North East: Wear, Tyne, Warkworth, Amble, Hawkhill and Blyth Midlands: Trent (improved gauge reliability)

· Tidal information. Anglian: more tidal gauges needed in Central, Essex and the Channel Areas and need to derive astronomic tide-series data for South Ferriby, Boston, Fosdyke Bridge and West Lighthouse instead of using tide tables Wales: additional tidal gauge needed in Haverfordwest. Midlands: access to all ‘Class A’ gauges for the Severn Estuary. South West: Improved astronomic tide predictions for Bournemouth and Padstow.

· STFS forecast surge residuals. North East: needed at more sites along the coastline and at a finer grid size for the Humber. Anglian Northern: require more sites to give improved resolution and an improvement in the reliability of the timing of forecast peak surge. Eastern: needed at more sites along the Essex and Suffolk coastline to give better resolution. Southern Kent: needed for the Rye estuary Hampshire: needed at more sites Wales: Carmarthen estuary Northwest: needed to improve timing and accuracy at Liverpool

· More wind speed and direction data North East: Tees and Humber (local wind) Anglian: The Wash Southern: Northern: along the coastline Sussex: at more sites (for local winds) Hampshire: additional local wind forecast sites to help assess the impact of local weather patterns on water levels and more real-time wind gauges needed. South West: real-time wind needed for Poole and Christchurch. Wales: access required to real-time wind data. Midlands: additional gauged wind data needed in the Bristol Channel.

· Wave data Anglian: more data needed in The Wash. Wales: eliminate wave effects on the tide gauge at Carmarthen.

R&D Project Record W5/010/1 16 · Real-time monitoring systems Southern Hampshire: development of a real-time monitoring system to assess if conditions are improving or deteriorating. Midlands: improved system for the real-time monitoring of levels, flow, wind and air pressure data. Wales: install real-time barometric pressure recorders to enable the monitoring and tracing of low-pressure systems. Obtain access to real-time level and wind data to allow the monitoring of conditions. South West: Real-time atmospheric pressure data for Christchurch to be made available.

R&D Project Record W5/010/1 17 Table 2.1 Summary of Important Factors, Data Available and Data Used

Estuary By Region NE A Th S SW M W NW TOTAL 8 15 1 17 16 2 8 5 72 Significiant Tide level (No. used) 8 (8) 15 (15) 1 (1) 17 (17) 16 (16) 2 (2) 8 (7) 5 (3) 72 (69) influnces on water Surge (No. used) 8 (8) 15 (15) 1 (1) 17 (17) 16 (16) 2 (2) 8 (7) 5 (3) 72 (69) level forecast Fluvial (No. used) 5 (2) 6 (5) 1 (1) 3 (0) 14 (4) 2 (2) 3 (2) 2 (2) 37 (18) Wind (No. used) 8 (8) 15 (4) 1 (1) 17 (13) 16 (16) 2 (2) 5 (5) 5 (2) 69 (46) Wave (No. used) 7 (1) 9 (0) 1 (0) 12 (4) 6 (0) 1 (0) 2 (0) 2 (0) 40 (5) Other Met. Conditions 0 0 0 8 (8) 10 (10) 0 2 (0) 0 20 (18) No. of real-time Tide level (No. used) 12 (11) 20 (20) 18(18) 20 (18) 22 (21) 15 (15)* 16 (13) 9 (9) 132(125) monitoring sites Fluvial level (No. used) 1 (1) 2 (2) 1 (1) 0 (0) 0 (0) 1 (1) 0 (0) 2 (2) 7 (7) available Wind (No. used) 7 (1) 13 (2) 1 (1) 9 (1) 11 (1) 1 (1)* 11 (0) 7 (0) 60 (7) Wave (No. Used) 0 (0) 1 (1) 0 (0) 0 (0) 1 (0) 0 (0) 0 (0) 0 (0) 2 (1) No. of forecasting Tide surge (No. used) 22 (3) 30 (7) 9 (9)* 30 (2) 46 (5) 6 (3) 40 (3) 15 (3) 198 (35) site available Fluvial level (No. used) 1 (1) 3 (3) 0 (0) 0 (0) 8 (4) 1 (1) 2 (2) 0 (0) 12 (11) On shore wind (No. used) 4 (4) 3 (3) 1 (1) 3(3) 4 (4) 0 (0) 1 (1) 3 (3) 19 (19) Offshore wind (No. used) 9 (4) 10 (3) 1 (1) 12 (3) 16 (4) 0 (0) 16 (1) 4 (3) 68 (19) Onshore wave (No. used) 3 (2) 0 (0) 0 (0) 1 (1) 0 (0) 0 (0) 0 (0) 0 (0) 4 (3) Offshore wave (No. used)* 9 (4) 10 (0) 0 (0) 12 (2) 16 (4) 0 (0) 16 (0) 4 (0) 67 (12) Notes: 1. * Data come from TIDAL FLOOD FORECASTING PROJECT REPORT October 1998 Environment Agency Final Report 2. The method of offshore wave is unknown at the time of writing. 3. ( No. used) in section of No. of real-time monitoring sites available—How many sites are used in assessing the development of flooding.

R&D Project Record W5/010/1 18 Table 2.2 Summary of Methods Forecast Methods Estuaries by Region NE A Th S SW M W NW TOTAL Tide and Surge Correlation with the forecast levels in 6 0 0 0 0 1 0 0 7 reference port Tide table/typical tide profile + STFS 0 12 0 0 2 0 0 0 14 surge for coast zone Astronomic tide + STFS surge for coast 0 2 0 17 11 0 5 0 35 zone Astronomic tide + STFS surge for specific 2 1 1 0 3 1 2 3 13 estuary No method 0 0 0 0 0 0 1 2 3 Fluvial-tide Empirical correlation 1 2 0 0 4 1 1 1 10 interaction Hydrodynamic model 1 0 1 0 0 1 0 1 4 No used 3 2 0 3 10 0 2 0 20 No Impact 3 11 0 14 2 0 4 3 37 Wind Used in forecast procedure 0 0 1 0 2 0 0 0 3 Used in warning procedure 5 2 0 13 14 2 5 2 43 No used 2 13 0 4 0 0 0 3 22 No impact 1 0 0 0 0 0 2 0 3 Wave Used in forecast procedure 0 0 0 0 0 0 0 0 0 Used in warning procedure 1 0 0 4 0 0 0 0 5 No used 6 9 1 8 5 1 2 2 34 No impact 1 6 0 5 11 1 5 3 32 Real-time Forecast and warning 0 1 0 0 0 1 0 0 monitoring level used in: Assessing development of flooding 8 14 1 17 16 0 7 3 69 Warning lead time > 6 Hr. (% of total) 8 (100) 15(100) 1(100) 17(100) 16(100) 2(100) 7(88) 2(40) 68

R&D Project Record W5/010/1 19 Table2.3 Tide level monitoring sites for Figure 2.4

1. North Shields (A) 39. Charlton 77. Padstow 2. Tees Dock 40. Tower Pier 78. Wadebridge 3. Whitby (A) 41. Westminster 79. Bude 4. Scarborugh 42. Chelsea 80. Bideford 5. Bridlington 43. Hammersmith 81. Appledore 6. Withernsea 44. Richmond 82. Yelland 7. Immingham (A) 45. Dartford 83. Ilfracombe (A) 8. Paull 46. Thames Barrier 84. Bridgewater 9. Hull 47. Queensborough 85. Hinckley Pt (A) 10. Hessle 48. Sheerness (A) 86. Avonmouth (A) 11. Blacktoft 49. Dover (A) 87. Avonmouth 12. Selby 50. Rye 88. Sharpness 13. South Ferriby 51. Pevensey 89. Chepstow 14. Keadby 52. Newhaven (A) 90. Newport (A) 15. Gainsborough 53. Shoreham 91. Mumbles (A) 16. Boygrift 54. Littlehampton 92. Neath 17. Boston 55. Medway 93. Pontardulais 18. Fosdyke Bridge 56. Holy Island 94. Carmarthen 19. Lawyers Sluice 57. Bembridge 95. Milford Haven (A) 20. West Lighthouse 58. Portsmouth (A) 96. Barmouth/Fishguard 21. Kings Lynn 59. Cowes (A) 22. Hunstanton 60. Yarmouth 97. Fishguard (A) 23. Wells 61. Woolston 98. Porthmadog 24. Cromer (A) 62. Southampton (A) 99. Holyhead (A) 25. Mundesley 63. Lymington 100. Llandudno (A) 26. Broadland 64. Christchurch 101. Conwy 27. Great Yarmouth 65. Bournemouth (A) 102. Rye 28. Lowestoft (A) 66. Poole 103. Connah’s Quay 29. Southwold 67. Wareham 104. Liverpool (A) 30. Aldeburgh 68. Weymouth (A) 105. Warrington 31. Felixstowe (A) 69. Portland 106. Mersey 32. Holland 70. West Bay 107. Rhuddlan 33. Wivenhoe 71. Exmouth 108. Preston 34. Southend 72. Totnes 109. Blackpool 35. Coryton 73. Davenport (A) 110. Fleetwood 36. Tilbury 74. Truro 111. Lancaster 37. Erith 75. Newlyn (A) 112. Heysham (A) 38. Silvertown 76. Hayle Gate 113. Workington (A)

R&D Project Record W5/010/1 20 R&D Project Record W5/010/1 21 R&D Project Record W5/010/1 22 R&D Project Record W5/010/1 23 R&D Project Record W5/010/1 24 R&D Project Record W5/010/1 25 R&D Project Record W5/010/1 26 3 CONCLUSIONS AND RECOMMENDATIONS

3.1 Conclusions

This report concludes the first stage of the project, which was to collect information and review current estuarial forecasting methods in use by the Agency. The specific report objectives were:

1 to summarise the current methods used for the forecasting of estuary water levels in England and Wales; 2 to summarise the data available and that currently used for the forecasting of estuary water levels; 3 to review the current estuary water level forecasting methods and data availability; 4 to make recommendations for improvements in the estuary water level forecasting methods and data availability.

The methods used for forecasting estuary levels have been investigated and are presented region by region in Chapters 3 to 14. A database has been set up and contains information on all estuaries for which returns were made to the questionnaire circulated to EA flood warning staff.

The data available has been summarised in a set of tables in Chapters 3 to 14. Requirements on additional data requirements on the seaward side has been presented in the Tidal Forecasting Project Report. Figures extracted from the Tidal Forecasting report have been reproduced in Chapter 2.

The current estuary water level forecasting methods and data availability have been reviewed and summarised in Chapter 2.

The major caveat on the results of this report is that little information is readily available on accuracy of forecasts, which makes an objective evaluation of the methods currently in use difficult. The results and conclusions presented here are based largely on information given by flood warning staff rather than an independent assessment. To overcome this problem, it is intended to focus future activities on a limited number of regions and engage the EA forecast groups directly in order to try to determine the quantitative accuracy of the current procedures.

It is quite apparent that the majority of the regions do not pool experience, data or procedures at the present time. The review of the Tidal Flood Forecasting System has been long overdue and has brought attention to the lack of corporate distribution inherent in data and procedures throughout the Agency.

The majority of procedures currently adopted are relatively simple and reflect the historical problems of inadequate telemetry and investment in forecasting system development. The need to introduce a more central distribution and collection of data on tidal levels is likely to have the greatest impact on improving coastal forecasting services than any other effort in recent years.

Many hydrodynamic models have been constructed at a variety of locations around the coast. Most are unemployed and previous investment under-utilised. A lack of focus

R&D Project Record W5/010/1 27 at a central level is generally associated with such a situation. Little overview is apparent in many regions and areas have done their best in difficult circumstances.

The following points summarise perceived improvements that could be made to the methods for forecasting water levels:

· Development of real-time forecasting methods including real-time error correction and greater utilisation of monitored data; · Development of an automatic forecast system based on existing methods to increase warning lead times. In locations influenced by fluvial flow, a possible improvement is the coupling of existing Flood Forecasting systems with hydrodynamic models (or empirical models) to form an automatic system. · Development of a method to translate the surge residual from one point to another site, including peak values and timing, in order to improve accuracy; · Development of a method to estimate the peak value and timing of tidal propagation from downstream to upstream to improve the accuracy of forecasts at upstream sites; · Development of a method, or use of new data to evaluate the tidal-fluvial interaction and further enhance fluvial factors in the forecast or warning procedure in estuaries where empirical correlations are used. · Development of methods to evaluate the impact of wind, wave and local pressure anomalies on water level, and further utilise and enhance these factors in the generation of forecast and warning procedures. · Development of a method to translate wave height and period measured offshore to onshore locations, particularly in Wales. · Development of forecast methods for the River Kent and Solway Firth. · Derivation of astronomic tidal time series for estuaries where tidal tables are used. · Improvement of hydraulic modelling to give a thorough understanding of areas at risk and to enhance the reliability of forecast levels during extreme events (particularly for the Humber, Severn and Trent) upon which, further developments of the hydrodynamic models can be based.

No attempt has been made to rank these points due to lack of information on accuracy and relative level of importance of the contributing factors. This will come form a more focussed look at estuaries in the regions selected for further study.

It is our view that considerable potential exists for improving the current capability of the Agency in relation to estuarial flood forecasting. Some of our preliminary recommendations are provided below.

The following points summarise comments made regarding data for use in improved forecasting of water levels.

· More level gauges are needed at the following sites: North East: Wear, Tyne, Warkworth, Amble, Hawkhill and Blyth Midlands: Trent (improved gauge reliability)

R&D Project Record W5/010/1 28 · Tidal information. Anglian: more tidal gauges needed in Central, Essex and the Channel Areas and need to derive astronomic tide-series data for South Ferriby, Boston, Fosdyke Bridge and West Lighthouse instead of using tide tables Wales: additional tidal gauge needed in Haverfordwest. Midlands: access to all ‘Class A’ gauges for the Severn Estuary. South West: Improved astronomic tide predictions for Bournemouth and Padstow.

· Wave data Anglian: more data needed in The Wash. Wales: eliminate wave effects on the tide gauge at Carmarthen.

· Real-time monitoring systems Southern Hampshire: development of a real-time monitoring system to assess if conditions are improving or deteriorating.

R&D Project Record W5/010/1 29 Midlands: improved system for the real-time monitoring of levels, flow, wind and air pressure data. Wales: install real-time barometric pressure recorders to enable the monitoring and tracing of low-pressure systems. Obtain access to real-time level and wind data to allow the monitoring of conditions. South West: Real-time atmospheric pressure data for Christchurch to be made available.

3.2 Recommendations for Stage 2

Evaluation of the suitability of methods in current use is difficult on a national basis since many estuaries have their own unique needs. It is proposed that the research will focus on two or three regions of the Agency and that a more detailed evaluation of a selection of estuaries, which represent the forecasting challenges met nationally, will be carried out. The most important task at present is to gather objective information on accuracy of forecasts. It is also important to gain a more thorough understanding of the relative contribution of the various causal mechanisms for the variety of estuaries encountered.

It is proposed that this will be attempted in the first instance by concentrating future efforts on the North West, South West and Thames Regions. Work on the Thames will include a review of surge forecasting techniques along the East Coast with recommendations for improvements. With the direct support of key agency personnel objective information will be collected and studied. As part of this task visits to the STFS and POL will be arranged in order to gain directly from the experience of current key agency service providers.

The project will also choose, as a consequence of the more detailed data obtained, a set of specific estuaries which will be used to develop where possible generic techniques which can be applied in other locations. The revision of some of the simple procedures which are widely used within the Agency to incorporate an element of real- time feedback to reduce forecast errors is an obvious initial step. More accurate prediction of storm tides is another area where significant improvements would be envisaged. The review of external forecasts during Stage 2 will therefore be very important. Alongside this work a thorough literature search on methods for forecasting extreme levels in estuaries for flood warning purposes is proposed.

R&D Project Record W5/010/1 30 4. REFERENCES

Environment Agency (1998a), Letter of Invitation, dated 26 February 1998

Environment Agency (1998b), Tidal Flood Forecasting Project Report, Draft Report Version 4, April 1998

R&D Project Record W5/010/1 31 APPENDIX 1

ANGLIAN REGION

1 Northern Area

1.1 Background

The Area office is responsible for the forecasting of levels for South Humberside, the East Lincolnshire coast and the Wash.

The principal estuaries in the Area are:

· The Humber · The Wash, including: Haven to Grand Sluice at Boston River Welland to Spalding River Nene to Dog-in-Doublet Sluice

The information contained in the following sections is based upon that gathered and requested during the meeting of 1 September 1998.

1.2 Current Forecasting Methods

1.2.1 Overview

The highest estuary water level is forecast for each tidal cycle. The forecast level is given by:

Level = predicted tide + forecast STFS surge residual

Water levels are forecast for 3 hours either side of the predicted high tide and warnings are issued based on the highest forecast level.

If forecast levels trigger the issue of a flood warning then the following data are monitored in real-time to assess the situation:

· Tide levels at North Shields and Whitby (which provide an early indication of what will happen on the Northern Area coastline) · Wind conditions from telemetry sites, STFS and MRSC Humber · Wave condition from Boygrift, MRSC Humber and coastal patrols.

Warnings are issued at least six hours before high water level and are updated as necessary based on the monitoring of conditions.

R&D Project Record W5/010/1 32 1.2.2 Specific Estuaries

South Humberside Estuary water levels are forecast for two sites: Immingham and South Ferriby.

The level at Immingham is the sum of the forecast surge residual at Immingham from STFS and the predicted tide level based on the astronomic time series. The highest water level three hours either side of the predicted high tide is used for the issue of flood warnings.

The forecast level at South Ferriby is the sum of the forecast surge residual at Immingham and the predicted high tide from tide tables. The same three hours window as at Immingham is used to forecast the high water level. A ‘typical’ tide curve for the predicted high tide level is used to define the change in tide level during this period.

The predicted water levels together with forecast winds are used in a look-up table to ascertain the degree of any flood warning.

The Wash (Witham, Haven, Welland, Nene) Estuary water levels are forecast for three sites: the Wash and Witham Haven (Boston); the tidal Welland (Fosdyke Bridge) and the tidal Nene (West Lighthouse).

The level at each site is given by:

Level = predicted tide + STFS surge residual for King’s Lynn

The predicted tide is taken from tide tables for each site. The highest water level three hours either side of the predicted high tide is used for the issue of flood warnings. For each site the change in tide level about the high tide is given by a ‘typical’ profile based on historic data.

Flood warnings are based on forecast water levels alone for the River Welland (Fosdyke Bridge). For Witham Haven (Boston) and the River Nene (West Lighthouse) flood warnings are issued based on a look-up table containing water level with wind speed and direction forecasts.

1.3 Improvements to Water Level Forecasts

1.3.1 Data Needs

The following comments were made regarding data for use in forecasting water levels for Northern Area estuaries:

· STFS surge residual forecasts to be made available at more sites to give improved resolution · Further forecast wind data for the Wash (Environment Agency, 1998) · Further forecast wave data for the Wash (Environment Agency, 1998) · Astronomic tide-series data to be derived for South Ferriby, Boston, Fosdyke Bridge and West Lighthouse so that ‘typical’ tide curves are no longer required

R&D Project Record W5/010/1 33 · Improved reliability of the timing of forecast peak surge.

1.3.2 Forecasting Method

The following comments were made regarding where improvements to the water level forecasting methods could be made:

· The derivation of a method for translating forecast surge residuals from one site to another, e.g. Immingham to South Ferriby; · The inclusion of the effect of wave action on water levels in The Wash and South Humberside estuaries.

R&D Project Record W5/010/1 34 Table A1 1.1 Forecasting Data Used Summary: Anglian Region

Forecast Data Used1 Real-time Monitoring Sites2 Real-time Estuary Tide Surge wind wave fluvial level wind wave fluvial monitoring3 Immingham Boygrift STFS East South Ferriby Hull Barrier Immingham Immingham MRSC Humber 4 Humber Humber North Shields MRSC Humber Coastal Patrol Whitby

STFS Kings Great Ouse Kings Lynn Kings Lynn Great Freebridge Kings Lynn Lynn 4 Freebridge Freebridge Ouse @ ? (Brampton) STFS Boygrift Great Ouse West Lynn Kings Lynn Freebridge 4 (Kings Lynn) Kings Lynn Immingham Witham STFS Kings Boston Sluice Kings Lynn Lawyers Sluice 4 Lynn Haven Fosdyke Bridge W. Lighthouse

Kings Lynn Immingham Fosdyke Kings Lynn Lawyers Sluice 4 Welland Bridge Fosdyke Bridge W. Lighthouse

Kings Lynn Immingham West STFS Kings Kings Lynn Lawyers Sluice 4 Nene Lighthouse Lynn Fosdyke Bridge W. Lighthouse

Great Lowestoft Lowestoft Felixstowe 4 The Broads Yarmouth Gt. Yarmouth

Lowestoft Lowestoft Lowestoft Felixstowe 4 Blyth Aldeburgh Felixstowe Southwold Lowestoft Southwold Alde Aldeburgh Felixstowe Aldeburgh Felixstowe 4 Wivenhoe Orford (in 1999)

R&D Project Record W5/010/1 35 Table A1 1.2 Forecasting and Warning Summary: Anglian Region

1 2 Forecast Data Used Real-time Monitoring Sites Real-time Estuary tide Surge wind wave fluvial level wind wave fluvial monitoring3

Felixstowe Felixstowe Felixstowe Felixstowe 4 Deben Woodbridge

Orwell/ Felixstowe Felixstowe Felixstowe Felixstowe 4 Stour Ipswich

Felixstowe Wivenhoe Felixstowe Holland Felixstowe 4 Colne Holland Colne Wivenhoe

Felixstowe Felixstowe Felixstowe Felixstowe 4 Blackwater Holland Holland Colne @ Lexden Crouch/ Felixstowe mailto: Felixstowe Colne @ Felixstowe Felixstowe 4 Roach Holland Colne Holland Lexden @Lex den Southend Thames Tilbury Southend Tilbury Thames barrier 4 Thames Barrier

Notes: 1Forecast Data Used – forecast data used in the flood warning procedures. Refer to the ‘Forecasting and Warning Summary’ table for how the data are used in the flood warning procedures. 2Real-time Monitoring Sites – details the real-time monitoring sites that are available near to the estuary and/or those that are used in the real-time monitoring of events 3Real-time Monitoring – details whether real-time monitoring of events is performed during the flood warning procedures. * denotes that the site is not currently used in the real-time monitoring of conditions

R&D Project Record W5/010/1 36 Table A1 1.2 Forecasting and Warning Summary: Anglian Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Humber level = predicted astronomic tide + forecast surge residual

where, 4 4 8 8 8 4 4 8 8

Warnings issued for the maximum forecast level 3 hours either side of the astronomic high tide Great Ouse (Brampton) level = predicted astronomic tide + forecast surge residual 4 4 8 8 8 4 4 8 4 where,

levels are forecast for each hour up to high tide Great Ouse level = predicted astronomic high tide + forecast (Kings Lynn) surge residual

where, 4 4 8 8 8 4 8 8 8

A ‘typical’ tide profile is used to estimate the water level leading up to high tide. Witham Haven level = predicted high tide from the tide table + forecast surge residual

where, 4 4 8 8 8 4 8 8 8 Warnings issued for the maximum forecast level 3 hours either side of the tide table high tide

A ‘typical’ tide profile is used to estimate water levels either side of the high tide.

R&D Project Record W5/010/1 37 Table A1 1.2 (Cont) Forecasting and Warning Summary: Anglian Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Welland As Witham Haven 4 4 8 8 8 4 8 8 8 Nene As Witham Haven 4 4 8 8 8 4 8 8 8 The Broads HW = predicted high tide from the tide table + forecast surge residual where, 4 4 8 8 8 4 8 8 8 Warnings issued for the maximum forecast level 2 hours either side of the tide table high tide Blyth As The Broads 4 4 8 8 8 4 8 8 8

Alde As The Broads 4 4 8 8 8 4 8 8 8 Deben As The Broads 4 4 8 8 8 4 8 8 8

Orwell/Stour As The Broads 4 4 8 8 8 4 8 8 8 Colne As The Broads 4 4 8 8 8 4 8 8 4

Blackwater As The Broads 4 4 8 8 8 4 8 8 8 Crouch/Roach As The Broads 4 4 8 8 8 4 8 8 8

Thames As The Broads 4 4 8 8 8 4 8 8 8 Notes: 1Method for forecasting estuary water level – a general explanation of the method by which water levels are forecast for the estuary. The method may apply to a coastal reach containing the estuary. 2Data used for forecasting – details the data that are used in the actual forecasting of the water level for flood warning. Only those data that are actually used in deriving the water level are used: for example, wind data may be used to determine the degree of flood warning required but not the actual water level itself. 3Data used for the issue of flood warnings – details the additional data types that are used to determine the degree of any flood warning required but are not used in the actual forecasting of water level.

R&D Project Record W5/010/1 38 Table A1 1.3 Factors Important in the Forecasting of Water Levels: Anglian Region

Factors1 Estuary tide surge wind wave fluvial 2-D Effects Other Humber 4 4 4 4 Great Ouse (Brampton) 4 4 4 Great Ouse 4 4 4 (Kings Lynn) Witham Haven 4 4 4 4 Welland 4 4 4 4 Nene 4 4 4 4 The Broads 4 4 4 4 4 Blyth 4 4 4 4 Alde 4 4 4 4 Deben 4 4 4 4 Orwell/Stour 4 4 4 4 4 Colne 4 4 4 4 Blackwater 4 4 4 Crouch/Roach 4 4 4 Thames 4 4 4

Total 15/15 15/15 15/15 9/15 3/15 0/15 0/15

Notes: 1Factors – physical factors that in the opinion of the Agency Flood Warning Staff are important in relation to the forecasting of water levels

R&D Project Record W5/010/1 39 APPENDIX 1

ANGLIAN REGION

2 Central Area

2.1 Background

The Area office is responsible for the forecasting of levels for the Great Ouse. The Great Ouse is the only principal estuary in the Area.

The information contained in the following sections is based upon that gathered and requested during the meeting of 22 July 1998.

2.2 Current Forecasting Methods

2.2.1 Overview

The Anglian Region Central Area currently has two methods for forecasting the water level for the Great Ouse. In both cases the water level is forecast for King’s Lynn. One method is undertaken at the Brampton Control Room and the other at the King’s Lynn office: for the purpose of this study they are referred to as the Brampton and King’s Lynn methods respectively.

Flood warnings are issued if the predicted water level exceeds a predetermined threshold value. The two methods adopted can on occasions, produce conflicting results. Under such circumstances the King’s Lynn method takes precedence.

2.2.2 Brampton Method

Water levels are forecast for Freebridge for each hour up to high tide where,

Level = astronomic tide at Freebridge + STFS forecast surge residual for King’s Lynn

Flood warnings are issued if the predicted level exceeds a given threshold. The high water level may not occur at the time of the predicted astronomic high tide.

The forecasting procedure is automated and the Flood Warning Staff routinely monitor levels during each tide cycle.

During routine monitoring the actual level at Freebridge is plotted with the forecast level. At –3, -2.5 and –2 hours before the predicted high water the actual versus predicted residual is calculated, where:

Residual = actual level - predicted level

R&D Project Record W5/010/1 40 A set of guidelines based on past experience is used to re-forecast the high water level based upon these residual valves. The final forecast is made 2 hours before the predicted high water, as at least 2 hours are needed to mobilise staff and close the necessary flood protection gates if required. In addition to the revised forecasts based on the residuals, flood warnings may also be upgraded (from yellow to amber only) depending on the forecast weather conditions. The conditions that give rise to revisions of the flood warnings are:

i) South West or Westerly winds where: · STFS prediction of wind force 5 or above and to persist in the period 3 hours before high water up to high water time · Astronomic tide >4 m · North Shields gauged high water level is >3.3 m.

ii) Northerly winds (N, NNE and NNW) where · STFS prediction of wind force 7 or above and to persist in the period 3 hours before high water up to high water time · Residuals in excess of 1.0 m at 2.5 hours before high water (where residual = actual – predicted level) · High astronomic tide (>4 m AOD).

Wind speed and direction data is monitored in real-time for the Kings Lynn and Boygrift gauges to assess if conditions are improving or deteriorating. The data is used together with gauged flows in the Great Ouse when considering the issue of red warnings on the tidal river stretch (Denver to Cut Bridge). The warning is issued based on experience.

2.2.3 King’s Lynn Method

Water levels are forecast for West Lynn (West Lynn is downstream of Freebridge). The King’s Lynn method is not written up as a procedure, however, the following description is based upon comments received from the practitioners. The high water level is predicted for the predicted astronomic high tide time, where:

Level = astronomic high tide for West Lynn + STFS forecast surge residual for King’s Lynn

The forecast level is then used with a Type Curve during real-time monitoring of conditions. The Type Curve is an historic tidal curve that typifies a certain level. The Curve that best fits the predicted high water level is used. Conditions are monitored and the residuals calculated. If the residuals exceed a certain value at 3 and 2.5 hours before high tide then a warning may be upgraded from yellow to amber.

The results of the calculation are compared with those of the Brampton method and take priority on the upgrading of warnings and during any conflict in results.

R&D Project Record W5/010/1 41 2.3 Improvements to the Forecasting of Water Levels

2.3.1 Data Needs

The following comments were made regarding data needs for the improved forecasting of estuary water levels: · Additional wind data for the Wash – both forecast and gauged.

2.3.2 Forecasting Methods

The following comments were made regarding where improvements could be made to the methods for forecasting water levels:

· Improved accuracy of the timing of the STFS surge residuals (the amplitude of errors in the levels is thought to be small); · Review wind effects and their impact on forecast accuracy; · Improved fluvial forecasts.

R&D Project Record W5/010/1 42 APPENDIX 1

ANGLIAN REGION

3 Eastern Area

3.1 Background

The Area office is responsible for the forecasting of levels from Hunstanton to Tilbury (Thames Estuary).

The principal estuaries in the Area are:

· The Broads - (including the Rivers Yare, Waveney, Ant, Thurne, Wensum and Bure). · River Blyth · River Alde · River Deben · River Orwell/Stour · River Colne · River Blackwater · River Crouch/Roach · River Thames.

The information contained in the following sections are based upon that gathered during the meeting of 14 September 1998.

3.2 Current Forecasting Methods

3.2.1 Overview

In the Anglian Region Eastern Area water level forecasts are made for a reach of coastline and not for specific estuaries. The forecast water level for each reach of coastline is equal to the value for a specific monitoring location. The monitoring locations are:

· Wells · Cromer · Great Yarmouth · Lowestoft · Aldeburgh · Felixstowe · Holland · Wivenhoe (for Colne barrier operation) · Southend · Tilbury.

R&D Project Record W5/010/1 43 The forecast level at each site is given by:

Level = predicted tide + forecast STFS surge residual

The highest forecast surge residual two hours either side of the predicted high tide is used in the forecast of high water level. Details of the method by which the levels are forecast at each site are given in Table A1 3.1.

If the forecast high water level at a monitoring location exceeds a pre-determined threshold then a flood warning is issued. The thresholds have ‘evolved’ and are based on historical events and past experience.

If forecast levels exceed a threshold then tide level and wind data are monitored in real-time to assess if conditions are improving or deteriorating.

3.2.2 Specific Estuaries

The Broads Water levels are not forecast for specific estuaries within The Broads. Current water level forecasts associated with The Broads are for Lowestoft and Great Yarmouth. Refer to Table A1 3.1 for the method by which water levels are forecast at these sites.

Forecast wind and wave data are received for Lowestoft but are not directly incorporated into the forecasting of water levels.

Water levels at Lowestoft and Great Yarmouth can be monitored in real-time.

River Blyth Water levels are not directly forecast for the River Blyth. The closest monitoring locations for which water levels are forecast are Lowestoft and Aldeburgh. Refer to Table A1 3.1 for the method by which water levels are forecast at these sites.

Forecast wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels. Water levels at Lowestoft and Southwold can be monitored in real-time.

River Alde Water levels are not directly forecast for the River Alde. The closest monitoring location for which water levels are forecast is at Aldeburgh. Refer to Table A1 3.1 for the method by which water levels are forecast at this site.

Forecast wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Lowestoft, Southwold, Aldeburgh, Wivenhoe and Orford (from 1999) can be monitored in real-time.

R&D Project Record W5/010/1 44 River Deben Water levels are not directly forecast for the River Deben. The closest monitoring location for which water levels are forecast is Felixstowe. Refer to Table A1 3.1 for the method by which water levels are forecast at this site.

Forecast wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Felixstowe and Woodbridge can be monitored in real-time.

The River Deben can exhibit unpredictable surge behaviour, particularly the occurrence of a positive surge at the upstream boundary but none at the downstream end.

River Orwell/Stour Water levels are not directly forecast for the River Orwell/Stour. The closest monitoring location for which water levels are forecast is Felixstowe. Refer to Table A1 3.1 for the method by which water levels are forecast at this site.

Forecast wind and wave data from STFS are not directly incorporated into forecasting of water levels.

Water levels at Felixstowe and Ipswich can be monitored in real-time. Fluvial tidal interactions are important in relation to flooding, particularly at Ipswich.

River Colne Water level forecasts are made at Wivenhoe and Holland, and are used to decide if the Colne Barrier is to be closed. Refer to Table A1 3.1 for the method by which water levels are forecast at Wivenhoe and Holland.

A matrix of forecast high water levels at Holland and Wivenhoe together with predicted fluvial flows for the Colne at Lexden is used in the decision to close the barrier.

The method by which the flows at Lexden are predicted was unknown at the time of writing.

Forecast wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Felixstowe, Holland and Wivenhoe can be monitored in real-time.

River Blackwater Water levels are not directly forecast for the River Blackwater. The closest monitoring locations for which water level forecasts are made are Felixstowe and Holland. Refer to Table A1 3.1 for the method by which water levels are forecast at these sites.

R&D Project Record W5/010/1 45 Forecast wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels. Water levels at Felixstowe and Holland can be monitored in real-time.

Rivers Crouch and Roach Water levels are not directly forecast for the Rivers Crouch and Roach. The closest monitoring locations for which water levels are forecast are Felixstowe and Holland. Refer to Table A1 3.1 for the method by which water levels are forecast at these sites.

Forecast wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Felixstowe and Holland can be monitored in real-time.

River Thames Anglian Region, Eastern Area is responsible for the closure of barriers around Canvey Island and that at Tilbury Docks.

The decision to close the barriers is based on forecast water levels at Southend.

3.3 Improvements to Water Level Forecasts

3.3.1 Data Needs

The following comments were made regarding data needs for the forecasting of water levels in Eastern Area estuaries:

· Additional wind forecasts along the coastline in order to assess the effect of wave action and prolonging of high tides; · More tide gauges in Central Essex and the Channel (the latter would help recognise surges propagating from the South West); · Additional surge forecasts for the Essex and Suffolk coastline to give better resolution.

3.3.2 Forecasting Methods

The following comments were made regarding where improvements could be made to the existing forecasting methods:

· The current methods are perceived to work well during extreme events (red warnings) but less so at lower levels (yellow and amber warnings); · During extreme events, i.e. when water levels exceed red warning thresholds, large areas are at risk rather than specific estuaries. (This is why forecasts for each estuary have not been developed.)

R&D Project Record W5/010/1 46 Table A1 3.1 Level Forecasting at Monitoring Locations

Data Tide Method Tide Surge level = predicted tide + highest Wells Wells Wells forecast surge 2 hours either side Cromer Cromer Cromer as above Great Yarmouth Great Yarmouth Lowestoft as above Lowestoft Lowestoft Lowestoft as above Aldeburgh Aldeburgh Felixstowe as above Felixstowe Felixstowe Felixstowe as above Holland Holland Felixstowe as above Wivenhoe Wivenhoe Colne as above Southend Southend Southend as above Tilbury Tilbury Southend as above

R&D Project Record W5/010/1 47 APPENDIX 2

2 MIDLANDS REGION

2.1 Background

The Midlands Region is responsible for the forecasting of water levels in the Severn and Trent estuaries.

These two rivers are the only estuaries in the Region. The River Trent is tidal to Newark and the Severn to Gloucester (although during extreme events it reaches Tewkesbury).

The information contained in the following sections is based on that gathered and requested during the meeting of 6 August 1998.

2.2 Current Forecasting Methods

2.2.1 Overview

Water levels for the Trent and Severn estuaries are forecast at the Nottingham and Tewkesbury offices respectively. The current water level forecasting methods for the River Trent and Severn predict the level during each tidal cycle.

2.2.2 Specific Estuaries

Severn Estuary Estuary water levels are forecast for upstream and downstream reaches. The downstream reach relates to the Severn Beach Flood Warning Zone, and the upstream reach to Gloucester.

The forecast water level at the downstream section of the estuary is given by:

Level = astronomic high tide at Avonmouth + STFS forecast surge residual at Avonmouth

The Tewkesbury office receives 36-hour surge forecasts for Avonmouth from the STFS Bristol Channel and Severn Estuary models.

The water level is forecast for the high tide only.

Forecast wind speed and direction data are available from:

· Birmingham Weather Department; · Bristol Weather Centre (currently lapsed); · Bristol Port Signal Station gauge at Avonmouth; · Severn Bridge Control Centre; · Oldbury and Berkeley Nuclear Power Station Control Rooms.

R&D Project Record W5/010/1 48 Flood warnings for the downstream reach are issued based on the forecast water level and wind speed and direction.

The upstream water level is not forecast per se. A look-up table relating the predicted Avonmouth level at high tide with the predicted fluvial flow at Gloucester is used to ascertain the degree of any warning required. The predicted fluvial flow at Gloucester is calculated from the forecast peak flows for the Rivers Severn, Teme and Avon from a rainfall–runoff model. The look-up table was produced from the results of a hydraulic modelling study that analysed water levels during a variety of tide level and fluvial flow combinations.

Real-time monitoring is not performed although level data is available at Avonmouth, Sharpness, Epney, Minsterworth and Gloucester and wind data at Avonmouth is also available.

The current forecasting method is satisfactory but the look-up table for the upstream reach of the Severn can be unreliable.

Trent Estuary Water levels in the Trent estuary are forecast for Keadby at the mouth through to Collingham, where water levels are virtually unaffected by tides. Water levels are forecast automatically by the bespoke programme ‘Hiwater’. Underlying the program are a series of relationships between fluvial flows and tidal levels for sites along the estuary. The relationships are empirical and based on the analysis of historic event data.

The forecasting procedure is summarised below.

i) Observed fluvial levels are recorded for the Trent at Colwick. The travel time from Colwick to Gainsborough is approximately 24 hours.

ii) The observed water level is corrected to Ordnance Datum (level = level + 16 m).

iii) A table and/or graph is used to convert the observed level to a flow at Colwick and Muskham.

iv) The same table is used to calculate the corresponding low water levels for all sites from Collingham to Keadby.

v) The tide table for Keadby is used to obtain the predicted high tide.

vi) The high water level at Keadby is then calculated using a correction for predicted surge and wind speed and direction where,

High water level = predicted high tide from tide table + Level correction

Where Level correction is the maximum of (STFS forecast surge residual for Immingham + 0.25 m) and (a correction factor from a wind speed–direction– Immingham water level matrix).

R&D Project Record W5/010/1 49 vii)The high water level is further adjusted for high fluvial flows as follows:

Low water levels at Gainsborough +3.0 m +4.6 m +5.3 m +5.8 m Addition to calculated Keadby tide 0.00 m 0.18 m 0.30 m 0.38 m

viii) The final forecast level at Keadby is thus given by:

High water level = predicted high tide from tide table + Level correction + High fluvial flow correction

ix) A correlation matrix of flow at Muskham (from (iii)) and Keadby high water level (from (viii)) is used to predict estuary water levels between Gainsborough and Collingham.

Conditions are monitored in real-time using level recorders at Colwick (fluvial), Immingham, North Shields, Whitby, Keadby, Gainsborough, Torksey and Blacktoft.

2.3 Improvements to Water Level Forecasts

2.3.1 Data Needs

The following comments were made regarding data needs for the improved forecasting of estuary water levels.

Severn · Improved astronomic tide predictions for Avonmouth. · Wind and pressure forecasts to be received from the Bristol Weather Centre (cost permitting). · Additional gauged wind data in the Bristol Channel. · Improved systems for the real-time monitoring of levels, flow, wind and air pressure data. · Access to tidal levels at all Class A gauges.

Trent · Additional level gauges. · Improved gauge reliability.

2.3.2 Forecasting Methods

The following comments were made regarding where improvements to the current forecasting methods could be made.

Severn and Trent · The development of hydrodynamic models for use in level prediction.

R&D Project Record W5/010/1 50 Severn · The development of a hydrodynamic model from Worcester to the boundary of the Severn Estuary Model to help predict flooding from fluvial-tidal interaction. 1-D and 2-D models of the estuary do exist but are not routinely used. · Improved accuracy of surge predictions from the Bristol Channel and Severn Estuary Models.

Trent · No comments made.

R&D Project Record W5/010/1 51

Table A2.1 Forecasting Data Used Summary: Midlands Region

Forecast Data Used1 Real-time Monitoring Sites2 Real-time Estuary tide surge wind wave fluvial level wind wave fluvial monitoring3 Severn, Avonmouth Teme, Avon Sharpness forecasts Avonmouth Avonmouth Epney Avonmouth 4 Severn from a Minsterworth rainfall runoff Gloucester model

Immingham Keadby North Shields Trent @ Trent @ Keadby Immingham Met Office Whitby 4 Trent Colwick Colwick Gainsborough Torksey Blacktoft

R&D Project Record W5/010/1 53 Table A2.2 Forecasting and Warning Summary: Midlands Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Severn d/s: level = predicted astronomic tide + forecast surge residual

u/s: level = value based on d/s level and the forecast 4 4 8 8 4 4 4 8 4 fluvial flows for the Severn, Avon and Teme produced by a rainfall runoff model.

Trent d/s: level = predicted astronomic tide + forecast surge residual + correction for fluvial flow 4 4 8 8 4 4 4 8 4 u/s: level = correlation between fluvial flow and the predicted d/s level

Notes: 1Method for forecasting estuary water level – a general explanation of the method by which water levels are forecast for the estuary. The method may apply to a coastal reach containing the estuary. 2Data used for forecasting – details the data that are used in the actual forecasting of the water level for flood warning. Only those data that are actually used in deriving the water level are used: for example, wind data may be used to determine the degree of flood warning required but not the actual water level itself. 3Data used for the issue of flood warnings – details the additional data types that are used to determine the degree of any flood warning required but are not used in the actual forecasting of water level.

Abbreviations: d/s – downstream u/s – upstream

R&D Project Record W5/010/1 54 Table A2.3 Factors Important in the Forecasting of Water Levels: Midlands Region

Factors1 Estuary tide surge wind wave fluvial 2-D Effects Other Severn 4 4 42 42 43 Trent 4 4 4

Total 2/2 2/2 2/2 1/2 1/2 0/2 0/2

Notes: 1Factors – physical factors that in the opinion of the Agency Flood Warning Staff are important in relation to the forecasting of water levels 2Below Arlington Bend only. 3Above Arlington Bend only.

R&D Project Record W5/010/1 55 APPENDIX 3

NORTH EAST REGION

3.1 Background

The North East Region is responsible for forecasting water levels for flood warning along the coastline from the Tweed to the Humber.

The principal estuaries in the Region are:

· River Tweed · River Aln · River Coquet · River Blyth · River Tyne · River Wear · River Tees · River Ouse/Humber estuary.

The information contained in the following sections is that requested and gathered following the meeting of 16 July 1998.

3.2 Method

3.2.1 Overview

The current forecasting methods for predicting water levels in estuaries for the Northumbria Area are based on level to level correlation matrices. The matrices relate the forecast level at a reference site to a corresponding level at the site of interest. The reference sites are:

· North Shields · Whitby · Immingham

The reference site associated with each estuary is given in Table 3.1. These sites are all Class A gauges and receive STFS surge residual forecasts.

The forecast level at each reference site is produced for each tide where,

Level = astronomic high tide + STFS surge residual at time of high tide

No correction for errors in timing of the forecast surge is made.

Level–to–level correlation matrices are then used to correct the forecast level at the reference site to a level at the site of interest. The correlation matrices are based on the analysis of historic data and include the approximate time difference between the two

R&D Project Record W5/010/1 56 sites. For locations influenced by fluvial flow, the level-to-level correlation matrix includes ‘bands’ for different flow valves.

The forecast water levels are then compared to flood warning thresholds. If the forecast level exceeds a threshold then the corresponding level is issued. The thresholds incorporate wind and/or wave effects where important and are based on historic flood data and known defence levels.

The Northumbria Area has a well-developed telemetry system and water levels are monitored in real-time. Wind data is monitored in real-time for the Humber Estuary.

3.2.2 Specific Estuaries

River Tweed The estuary water level is forecast based on the predicted level at North Shields. A level-to-level correlation is used to convert the predicted level at North Shields to one for the Tweed. A ‘critical condition’ table contains the water level thresholds that trigger the flood warnings.

Forecast wind and wave data for East Farne received from STFS are used in the flood warning procedure but not in the actual forecasting of levels. Water levels at North Shields are monitored in real-time. There is no real time monitoring of wind and wave data.

River Aln The estuary water level is forecast based on the predicted level at North Shields. A level-to-level correlation is used to convert the predicted level at North Shields to one for the Aln.

River Coquet The estuary water level is forecast based on the predicted level at North Shields. A level-to-level correlation is used to convert the predicted level at North Shields to one for the Coquet. A ‘critical condition’ table contains the water level thresholds that trigger the flood warnings.

River Blyth The estuary water level is forecast based on the predicted level at North Shields. A level-to-level correlation is used to convert the predicted level at North Shields to one for the Blyth. A ‘critical condition’ table contains the water level thresholds that trigger the flood warnings.

R&D Project Record W5/010/1 57 Forecast wind and wave data for East Farne received from STFS are used in the flood warning procedure but not in the actual forecasting of levels. Water levels at North Shields are monitored in real-time. There is no real time monitoring of wind and wave data.

River Tyne The forecast estuary water level is equal to that for North Shields. The forecast water level for North Shields is given by:

Level = astronomic high tide at North Shields + STFS forecast surge residual for North Shields.

The water level is predicted for the time of the high tide only.

Forecast wind and wave data are received from STFS for East Farne. The data are not used for the forecasting of water levels, as wave and wind action are less significant in relation to flooding compared to the astronomic tide and surge effect. Fluvial flows have little impact with respect to flooding.

Flood warnings are issued when forecast water levels exceed predetermined thresholds.

Water levels at North Shields are monitored in real-time to assess if conditions are improving or deteriorating. Wind data at Tynemouth are gauged in real-time but are not used in the flood warning procedures.

River Wear An estuary water level is not forecast per se. Flood warnings are issued based on a look-up table of the forecast water level for North Shields and the gauged stage for the Wear at Chester-le-Street. The forecast water level at North Shields is that for the high tide only.

Thresholds are set for different combinations of forecast tide level and gauged river stage and conditions exceeding these limits trigger flood warnings.

Forecast wind and wave data are received from STFS for East Farne. The data are not used in the flood warnings as wind and wave action have only a small impact in relation to potential flooding episodes.

River Tees The estuary water level is forecast for Tees Dock. The level is derived based on a correlation with the forecast level at North Shields. The forecast level at North Shields is given by: Level = astronomic high tide at North Shields + STFS forecast surge residual for North Shields.

R&D Project Record W5/010/1 58 The water level is forecast only for the time of the predicted high tide. The level-to- level correlation between North Shields and Tees Dock includes an estimate of the travel time.

Flood warnings are issued based on thresholds given in critical condition tables. The critical condition tables combine forecast water levels and forecast wave height and direction to give associated warnings. The forecast wave data is received from STFS for NE Middlesborough. The offshore data is translated to an onshore wave forecast but the method of translation is unknown at the time of writing.

Water levels are monitored at the North Shields and Aberdeen gauges in real-time to assess if conditions are improving or deteriorating. Real-time wind data is available from the Tynemouth and Loftus gauges but are not currently monitored for the purposes of flood warning.

The Humber Estuary water levels are forecast for 11 flood warning zones that cover the length of the estuary. The zones are given in Table 3.2.

The forecast estuary water levels for each zone are equal to those for a specific Forecast Station. The Forecast Station associated with each warning zone is given in Table 3.2. The level at each Forecast Station is derived from the astronomic tide and forecast surge residual at a Reference Port. The Reference Port associated with each Forecast Station is given in Table 3.3.

The forecasting procedures are initiated if the sum of the astronomic high tide and the forecast surge residual for the reference port exceeds a minimum threshold.

The water level at the Reference Port is forecast based on astronomic tide levels and STFS forecast surge residual. The water level can be forecast in one of two ways, using level–to–level correlation plots; or, the Regional Flood Forecasting System (RFFS).

a) Level-to-Level Correlation Plots Water levels within the estuary are forecast based on level-to-level correlation plots relating the level at the Reference Port with that at the Forecast Station. The forecast level at the Reference Port is equal to the astronomic high tide and the STFS forecast surge residual for the site. The level-to-level correlation is derived from historical data.

Critical condition tables relating the forecast water level and the predicted wind speed and direction are used to ascertain the degree of any flood warning issued. The forecast wind speed and direction data is that received from STFS for East Humber.

b) Regional Flood Forecasting System (RFFS) Estuary water levels can be predicted as part of the hydrodynamic module of the fluvial forecasting system. The upstream boundary is formed by forecast fluvial levels based on gauged river levels and weather data. The downstream boundary is given by the predicted

R&D Project Record W5/010/1 59 astronomic tide, which is produced by the tide generating software leased from the Proudman Oceanographic Laboratory (POL). These data are used together with the 36 hourly STFS surge residual forecasts to predict total water levels.

The level at each Forecast Station is produced automatically from the digitised level- to-level correlation graphs.

Flood warnings are issued based on the highest water level forecast during the tidal cycle. The high water level may not be coincident with the astronomic high tide time.

The critical condition tables are included in the database and can be searched automatically to ascertain the degree of warning required. The critical condition tables include the effect of wind speed and direction.

For both procedures, following the initiation of the forecasting, conditions are monitored in real-time. Water levels at Aberdeen, North Shields, Whitby, Immingham, Bridlington, Paull Hessle and Blacktoft are monitored in real-time. Wind speed and direction data are monitored in real-time for the Bridlington and Hull Bank gauges. Real-time wind data is also available at Cowden, Donna Nook and Boygrift but the data are not used in the flood warning procedures.

3.3 Improvements to the Forecasting of Water Levels

3.3.1 Data Needs

The following comments were made regarding data for use in the improved forecasting of estuary levels:

· More local inshore wind and wave data to be made available (for the Tees and Humber in particular); · Level gauges in areas at risk (for the Wear and Tyne in particular); · Level gauges to be installed at Warkworth, Amble and Hawkhill; · A level gauge to be installed in the Blyth estuary; · STFS forecast surge residuals to be made available at more points along the coastline, and at a finer grid size for the Humber.

3.3.2 Forecasting Methods

The following comments were made regarding where improvements could be made to the current forecasting methods: · Verification of existing procedures (for the Wear, Tyne, Blyth, Coquet, Aln, Tweed and Humber); · Improved hydraulic modelling of the Humber estuary to give a more detailed understanding of the areas at risk and allow more specific warnings to be issued; · Extrapolation of existing methods to give greater accuracy during extreme events (particularly for the Humber).

R&D Project Record W5/010/1 60 Table 3.1 Estuaries and Reference Sites

Estuary Reference Site Tweed North Shields Aln North Shields Coquet North Shields Blyth North Shields Tyne North Shields Wear North Shields Tees North Shields Ouse/Humber North Shields, Immingham, Whitby

R&D Project Record W5/010/1 61 Table 3.2 Humber Estuary Flood Warning Zones and Associated Forecast Stations

Flood Warning Zone Forecast Station EA No Name 3 Skeffling to Kilnsea Spurn Point 4 Sunk Island Hull 5 Paull Holme to Sattend Chemical Works Hull 6 Hull Hull 7 North Ferriby to Faxfleet Brough 8 Faxfleet to Saltmarshe Blacktoft 9 N/A N/A 10 Saltmarshe to Old Boothferry Bridge Goole 11 - Goole 12 - Barnby Barrage 13 - Selby Toll Bridge

Table 3.3 Humber Estuary Forecast Stations and Associated Reference Ports

Forecast Station Reference Port Spurn Point Whitby Hull Immingham Brough Immingham Blacktoft Immingham Goole Immingham Barnby Barrage Immingham Selby Toll Bridge Immingham

R&D Project Record W5/010/1 62 Table 3.4 Forecasting Data Used Summary: North East Region

Forecast Data Used1 Real-time Monitoring Sites2 Real-time Estuary Tide surge wind wave fluvial level wind wave fluvial monitoring3 STFS North Shields North Shields North Shields 4 Tweed East Farne

STFS North Shields North Shields North Shields 4 Aln East Farne

STFS North Shields North Shields North Shields 4 Coquet East Farne

STFS North Shields North Shields North Shields 4 Blyth East Farne

STFS North Shields North Shields North Shields Tynemouth* 4 Tyne East Farne

Wear @ STFS North Shields North Shields Chester-le- North Shields Tynemouth* 4 Wear East Farne Street

STFS NE STFS NE North Shields Tynemouth* Tees Dock North Shields 4 Tees Middlesboro’ Middlesboro’ Aberdeen Loftus*

Aberdeen North Shields Bridlington STFS NE STFS NE Whitby North Shields Hull Bank Whitby Flamborough, Flamborough, Ouse @ Immingham Ouse @ Whitby Cowden* 4 Humber Immingham STFS East STFS East Skelton Bridlington Skelton Immingham Donna Nook* Humber Humber Paull Boygrift* Hessle Blacktoft

R&D Project Record W5/010/1 63 Table 3.5 Forecasting and Warning Summary: North East Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Tweed level = level derived from a correlation with a reference port level

where, 4 4 8 8 8 4 4 8 8 reference port level = predicted astronomic high tide + forecast surge residual

Aln As River Tweed 4 4 8 8 8 4 4 8 8 Coquet As River Tweed 4 4 8 8 8 4 4 8 8 Blyth As River Tweed 4 4 8 8 8 4 4 8 8 Tyne level = predicted astronomic tide + forecast surge residual 4 4 8 8 8 4 8 8 8

Wear Water level is not forecast per se, but warning are issued based on the level at North Shields and gauged NA NA NA NA NA 4 8 8 4 fluvial levels

Tees level = level derived from a correlation with a reference port level

where, 4 4 8 8 8 4 8 4 8 reference port level = predicted astronomic high tide + forecast surge residual

R&D Project Record W5/010/1 64 Table 3.5 (cont) Forecasting and Warning Summary: North East Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Humber level = level derived from a correlation with a reference port level

where, 4 4 8 8 4 4 4 8 8 Level is forecast automatically using the RFFS system

R&D Project Record W5/010/1 65 Table 3.6 Factors Important in the Forecasting of Water Levels: North East Region

Factors1 Estuary tide surge wind wave fluvial 2-D Effects Other Tweed 4 4 4 4 Aln 4 4 4 4 4 Coquet 4 4 4 4 4 Blyth 4 4 4 4 Tyne 4 4 Wear 4 4 4 Tees 4 4 4 4 Humber 4 4 4 4 4

Total 8/8 8/8 6/8 6/8 4/8 0/8 0/8

Notes: 1Factors – physical factors that in the opinion of the Agency Flood Warning Staff are important in relation to the forecasting of water levels

R&D Project Record W5/010/1 66 APPENDIX 4

4 NORTH WEST REGION

4.1 Background

The North West Region is responsible for forecasting levels for the North West Coast from the River Mersey to the Solway Firth (Cumbrian side only).

The principal estuaries in the region are:

· River Mersey · River Ribble · River Lune · River Kent · Solway Firth

The information contained in the following section is that gathered and requested during the meeting of 12 July 1998.

4.2 Forecasting Methods

4.2.1 Overview

The North West Region performs flood warning duties through Operation Neptune. Operation Neptune triggers the flood forecasting procedures and places the Agency Flood Warning staff on stand-by.

Operation Neptune is initiated based on the forecast high water level at Gladstone Dock, Liverpool, where:

Level = astronomic high tide at Gladstone Dock + STFS forecast surge residual for Liverpool at high tide

Once initiated, Operation Neptune involves the real-time monitoring of conditions and comparison to the forecast levels for four sites on the North West Coast. The sites are:

· Liverpool (Gladstone Dock) · Heysham · Blackpool · Workington.

The real-time monitoring enables Flood Warning staff to assess if conditions are improving or deteriorating.

Water levels at the following gauges are monitored in real-time:

· Liverpool

R&D Project Record W5/010/1 67 · Blackpool · Heysham · Workington · Fleetwood · Lancaster · Preston · Mersey (3 sites) · Warrington

Wind speed and direction at the following gauges are monitored in real-time:

· Heysham · Liverpool · Fleetwood

4.2.2 Specific Estuaries

River Mersey Water levels are forecast for the Mersey at Liverpool where:

Level = astronomic tide at Liverpool + STFS forecast surge residual for Liverpool

The water level is calculated for each hour during the rising limbs of the tidal cycle.

The major flood warning zone on the Mersey estuary is Warrington. Flood warnings are issued based on the forecast level at Liverpool. No transformation of the figure is made to give an estimated level at Warrington although a level-to-level correlation based on historic data does exist but is used only infrequently.

Fluvial flows are not included in the forecasting of the estuary level. During a high fluvial flow event, the majority of the flow is diverted into the Manchester ship canal and consequently bypasses the estuary.

Forecast wind data is received from STFS for Heysham, Fleetwood and Liverpool but is not directly incorporated into the forecasting of water level.

During an event, water levels and wind speed and direction data are monitored in real time at the sites given in Section 4.2.1.

River Ribble Flooding on the River Ribble is predominantly fluvial although there are some tidal effects at high tide. The degree of the tidal effect on flooding varies from location to location, and the forecasting method has been developed to account for this.

The current flood warning procedure is based on the results of a hydraulic modelling study and water levels are not forecast per se. The modelling study analysed a combination of fluvial flows and tides to estimate water levels. The results were then used to produce a ‘flow-tide’ graph that details those areas along the Ribble that are at risk from flooding.

R&D Project Record W5/010/1 68 The flood warning procedure is summarised as follows:

i) Water levels are monitored for the Ribble at Samlesbury. When the level reaches 4.25 m, Flood Warning Staff are placed on stand-by.

ii) Forecast peak flow at Samlesbury is produced from a rainfall-runoff model. This is converted to a flow at Walton-le-Dale by shifting the time base by +1 hr.

iii) The forecast tide level is calculated where,

Level = astronomic tide at Liverpool + STFS forecast surge residual for Liverpool

iv) For the tide height at peak flow and the flow at high tide, the ‘flow-tide’ graph is used to assess which areas may flood.

v) Conditions are monitored in real-time to assess if they are improving or deteriorating.

The ‘flow-tide’ graph is given in Figure 4.1. Level and wind data are monitored in real-time as detailed in Section 4.2.1.

Water levels are affected by wind speed and direction, but the forecast data are not used in the flood warning procedure.

River Lune Water levels are forecast for the Lune at Lancaster Quay. The water level is forecast based on both the predicted high tide and forecast fluvial flows. A correlation graph of tide level and fluvial flow is used to forecast the water level at Lancaster Quay.

The predicted high tide level is forecast for Fleetwood where,

Level = astronomic high tide at Fleetwood + STFS forecast surge residual for Heysham

The fluvial flow is forecast for the Lune at Caton and is equal to the gauged flow for the Lune at Killington five hours previously; i.e. the forecast flow in 5 hours time equals the current gauge flow.

The forecast tide level at Fleetwood is projected onto the forecast fluvial flow at Caton on a correlation graph to give the predicted level at Lancaster Quay. The correlation graph is based on the analysis of historic data. The data show that the high water level at Lancaster Quay occurs approximately 15 minutes after that at Fleetwood.

Conditions are monitored in real-time for the level and wind gauges listed in Section 4.2.1 to assess if conditions are improving or deteriorating.

Wind action affects levels at Fleetwood but forecast wind data are not used in level prediction.

R&D Project Record W5/010/1 69 River Kent There is currently no method for the forecasting of water levels in the Kent estuary. The nearest Class A gauge and STFS forecast are at Heysham.

Solway Firth There is currently no method for the forecasting of water levels in the Solway Firth. The nearest Class A tide gauge and STFS forecast are at Workington.

4.3 Improvements to the Forecasting of Water Levels

4.3.1 Data Needs

The following comments were made regarding data for use in improved forecasting of water levels in the NW Region estuaries.

· Improved timing of the STFS surge predictions (the surge tends to peak approximately 3 hours before high tide) · Improved accuracy of STFS surge levels for Liverpool as forecast levels are typically 0.2 to 0.3 m higher than observed. Heysham and Workington forecasts are more accurate although the latter is more variable.

4.3.2 Improvements to Forecasting Methods

The following comments were made regarding improvements that would be made to the forecasting of estuary water levels:

· The development of forecasting methods for the River Kent and the Solway Firth. The methods should include the analysis of water levels and wind data. · The inclusion of more variables in the level forecasting method for the River Mersey, e.g. wind, wave and fluvial flows. · The development of a method to translate point forecast surge predictions to other locations.

R&D Project Record W5/010/1 70 Table A4.1 Forecasting Data Used Summary: North West Region

Forecast Data Used1 Real-time Monitoring Sites2 Real-time Estuary tide surge wind wave fluvial level wind wave fluvial monitoring3 Liverpool Blackpool STFS Heysham Heysham, Workington STFS Liverpool Liverpool Fleetwood 4 Mersey Liverpool, Lancaster STFS Preston Blackpool Mersey (x3) Warrington

Liverpool Blackpool STFS Heysham Heysham, Workington STFS Ribble @ Ribble @ Liverpool Liverpool Fleetwood 4 Ribble Liverpool, Salmesbury Salmesbury Lancaster STFS Preston Blackpool Mersey (x3) Warrington

Liverpool Blackpool STFS Heysham Heysham, Workington STFS Lune @ Lune @ Fleetwood Heysham Fleetwood 4 Lune Liverpool, Caton Caton Lancaster STFS Preston Blackpool Mersey (x3) Warrington Liverpool Blackpool Heysham Workington Kent NA NA NA NA NA Fleetwood 4 Lancaster Preston Mersey (x3) Warrington

R&D Project Record W5/010/1 71 Table A4.1 (cont) Forecasting Data Used Summary: North West Region

1 2 Forecast Data Used Real-time Monitoring Sites Real-time Estuary Tide Surge wind wave fluvial level wind wave fluvial monitoring3 Liverpool Blackpool Heysham Workington Solway Firth NA NA NA NA NA Fleetwood 4 Lancaster Preston Mersey (x3) Warrington

R&D Project Record W5/010/1 72 Table A4.3 Forecasting and Warning Summary: North West Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Mersey level = predicted astronomic high tide + forecast surge residual 4 4 8 8 8 4 8 8 8

Ribble Water levels are not forecast per se. A hydraulic model has been used to produce a graph detailing the combinations of fluvial flow and tide that cause 4 4 8 8 4 4 4 8 4 flooding.

Lune level = correlation high water and fluvial flow

where,

high water = astronomic high tide + forecast surge 4 4 8 8 4 4 4 8 8 residual at time of high tide

fluvial flow = forecast fluvial flow at Caton

Kent No method 8 8 8 8 8 8 8 8 8 Solway Firth No method 8 8 8 8 8 8 8 8 8

R&D Project Record W5/010/1 73 Table A4.3 Factors Important in the Forecasting of Water Levels: North West Region

Factors1 Estuary tide surge wind wave fluvial 2-D Effects Other Mersey 4 4 4 Ribble 4 4 4 4 Lune 4 4 4 4 Kent 4 4 4 4 4 Solway Firth 4 4 4 4 4

Total 2/2 2/2 2/2 1/2 1/2 0/2 0/2

Notes: 1Factors – physical factors that in the opinion of the Agency Flood Warning Staff are important in relation to the forecasting of water levels

R&D Project Record W5/010/1 74 APPENDIX 5

SOUTHERN REGION

1 Kent Area

1.1 Background

The Environment Agency Kent Area office is responsible for flood warnings from the River Darent to the Eastern Rother.

The principal estuaries in the Area are:

· River Darent · River Medway · River Rother (Eastern) · ‘The Swale’ · Great Stour.

The information contained in the following sections are based upon that gathered during the meeting of 21 September 1998.

1.2 Current Forecasting Methods

1.2.1 Overview

The highest estuary water level is forecast for each tide cycle. The forecast level is given by:

Level = predicted astronomic hightide + STFS forecast surge residual

No correction to the timing of the maximum surge is formally made in the forecasting procedure.

Water levels are forecast for coastal reaches some of which incorporate an estuary, or estuaries. For the purpose of flood warning, the coastal reaches are termed zones.

Flood warnings are issued based on the highest forecast water level during the tidal cycle. A table correlating forecast water level and forecast wind speed and direction is used to assess the severity of any event and the degree of warning to be issued, e.g. yellow, amber or red.

R&D Project Record W5/010/1 75 1.2.2 Specific Estuaries

River Darent Estuary water levels are forecast as for coastal zone 1 (Dartford Creek to Higham Marshes). The level is the sum of the forecast surge residual at Sheerness and the predicted astronomic tide level at Sheerness.

A barrier protects the upstream sections of the river from flooding as a result of high tides. The decision to close the barrier is based on the forecast levels at Sheerness and is related to the closure of the Thames Barrier (see Section 7.2.2).

Forecast wind data are used in the forecasting of water levels and are used in the decision to issue flood warnings.

Real-time monitoring of events is performed by Area Staff and in the Regional Office in Worthing. Level data is available at Sheerness and Dartford gauges and wind speed and direction data at the Dartford Barrier (Environment Agency, 1998 – Tidal Flood Forecasting).

River Medway Estuary water levels are forecast as for coastal zone 2 (Higham Marshes to Seasalter). The level is the sum of the forecast surge and the predicated astronomic tide for Sheerness.

The tidal influence extends as far upstream as East Farleigh during the highest tides. This upstream end of the estuary is covered by fluvial flood warning zone 8A6. Flooding can occur in this area due to fluvial-tidal interaction.

The interaction is assessed based on experience and is not written down as a formal procedure. For example, if there is a large gauged fluvial flow and the tide is in, then a warning may be issued as the flow cannot escape to sea.

Forecast wind data are used in the forecasting of water levels and are used in the decision to issue flood warnings.

Real-time monitoring is performed by Area Staff and the Regional Office in Worthing during an event. Level data is available at Sheerness and wind data at the Dartford Barrier.

The Swale Estuary water levels are forecast as for coastal zone 2, 4 and 9A1. The level for coastal zone 2 is the sum of the forecast surge and the astronomic tide for Sheerness.

Flood warning zone 9A1 is the upstream reach of the estuary. Water levels are forecast based on rainfall, SMD, gauged flows and the predicted high water level. The forecasting procedure for fluvial-tidal interaction is an informal one based on experience and is not written down explicitly.

The forecast level for coastal zone 4 is the same as that for zone 2.

R&D Project Record W5/010/1 76 Forecast wind data are used in the forecasting of water levels and are used in the decision to issue flood warnings.

Real-time monitoring is performed by Area Staff and the Regional Office in Worthing during an event. Level data is available for Sheerness and Queenborough gauges.

Great Stour Estuary water levels are forecast as for coastal zone 6. The level for coastal zone 6 is the sum of the forecast surge and predicted astronomic tide for Sheerness.

Forecast wind data are not used in the forecasting of water levels, but are used in the decision to issue flood warnings.

Real-time monitoring is performed during an event. Level data is available for Sheerness.

The tidal influence extends as far inland as Fordwich and is the dominant factor with respect to flood risk in this area.

River Rother (Eastern) Estuary water levels are forecast for coastal zone 10, which is dedicated to the estuary itself. The predicted water level for the Rother estuary is the sum of the astronomic tide at Rye and the average of the forecast surge residuals at Newhaven and Dover.

Forecast wind data are used in the forecasting of water levels and are used in the decision to issue flood warnings.

Real-time monitoring is performed by Area Staff and the Regional Office in Worthing during an event. Level data is available at Dover, Newhaven and Rye tide gauges.

The Brede and Tillingham tributaries and the upstream section of the Rother are protected from flooding at high tide by sluice gates.

1.3 Improvements to Water Level Forecasts

1.3.1 Data Needs

The following comments were made regarding data for use in forecasting estuary water levels within the Kent Area:

· Obtain additional STFS surge residual forecasts for Rye; · Obtain additional STFS surge residual forecasts for the North Kent coast to aid in the gate closure decision making process; · Obtain additional forecast wind speed and direction data along the coastline for use in flood warning procedures; · Improve data transfer within the Southern Region and with other Regions, e.g. to be able to view Thames Barrier forecasts.

R&D Project Record W5/010/1 77 1.3.2 Forecasting Method

The following comments were made regarding where improvements to the method(s) for forecasting estuary water levels could be made.

· Improved accuracy of STFS surge residual forecasts. · An assessment of the confidence STFS has in the surge residuals to accompany each forecast. · The development of a system to allow the accessing and display of actual and forecast tide levels around England and Wales. This would allow flood warning staff to assess if conditions are improving or deteriorating. · The development of a method or method(s) for assessing the impact of fluvial flows on estuary water levels. · The development of a method, or methods, for forecasting the change in water levels between the upstream and downstream section of an estuary.

R&D Project Record W5/010/1 78 Table A5 1.1 Forecasting Data Used Summary: Southern Region

Forecast Data Used1 Real-time Monitoring Sites2 Real-time Estuary tide Surge wind wave fluvial level wind wave fluvial monitoring3 Sheerness Sheerness Sheerness Dartford 8 Darent Dartford

Medway Sheerness Sheerness Sheerness Dartford 4 8 Sheerness Sheerness Sheerness 4 8 The Swale Queenborough Great Stour Sheerness Sheerness Sheerness 4 8

Dover Eastern Rother Rye Sheerness Newhaven 8 Rye

Newhaven STFS Shoreham Adur Shoreham Portsmouth STFS S.Selsey Newhaven 8 S.Selsey Littlehampton Medmerry

Newhaven STFS Shoreham Arun Shoreham Portsmouth STFS S.Selsey Newhaven 8 S.Selsey Littlehampton Medmerry Newhaven STFS Shoreham Newhaven Portsmouth Newhaven Newhaven 8 Ouse S.Selsey Littlehampton Pevensey Newhaven STFS Shoreham Newhaven Portsmouth Newhaven Newhaven 8 Cuckmere S.Selsey Littlehampton Pevensey

Portsmouth St Catherines Pt Southampton Portsmouth STFS S.Selsey 8 Meon Southampton Solent MRSC

Portsmouth St Catherines Pt Southampton Portsmouth STFS S.Selsey 8 Hamble Southampton Solent MRSC Itchen Portsmouth Portsmouth St Catherines Pt Southampton STFS S.Selsey 8 Southampton Solent MRSC

R&D Project Record W5/010/1 79 Table A5 1.1 Forecasting and Warning Summary: Southern Region

1 2 Forecast Data Used Real-time Monitoring Sites Real-time Estuary tide surge wind wave fluvial level wind wave fluvial monitoring3

Test Portsmouth Woolston St Catherines Pt Southampton Portsmouth STFS S.Selsey 8 (Southampton Eling Solent MRSC Water) Southampton

STFS Portsmouth St Catherines Pt Southampton Portsmouth S. Portsmouth, Lymington 8 Lymington Solent MRSC Met. Office Southampton

Portsmouth St Catherines Pt Southampton Portsmouth STFS S.Selsey 8 Medina (IoW) Southampton Solent MRSC

Portsmouth St Catherines Pt Southampton Portsmouth STFS S.Selsey Southampton 8 Yar (IoW) Solent MRSC Yarmouth Portsmouth Bembridge St Catherines Pt Southampton Portsmouth STFS S.Selsey Southampton 8 Solent MRSC (IoW) Bembridge

Notes: 1Forecast Data Used – forecast data used in the flood warning procedures. Refer to the ‘Forecasting and Warning Summary’ table for how the data are used in the flood warning procedures. 2Real-time Monitoring Sites – details the real-time monitoring sites that are available near to the estuary and/or those that are used in the real-time monitoring of events 3Real-time Monitoring – details whether real-time monitoring of events is performed during the flood warning procedures. 4Fluvial flows are monitored in real-time for the fluvial flood warning zones at the upstream end of the estuary * denotes that the site is not currently used in the real-time monitoring of conditions

R&D Project Record W5/010/1 80 Table A5 1.2 Forecasting and Warning Summary: Southern Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial level = predicted astronomic tide + forecast surge residual

where, Darent 4 4 8 8 8 4 4 8 8 Warnings issued for the maximum forecast level during the tide cycle

As Darent Medway 4 4 8 8 8 4 4 8 8 As Darent The Swale 4 4 8 8 8 4 4 8 8 As Darent Great Stour 4 4 8 8 8 4 4 8 8 level = predicted astronomic tide + the average of the two nearest surge residual forecasts Eastern Rother 4 4 8 8 8 4 4 8 8 Warnings issued for the maximum forecast level during the tide cycle

level = predicted astronomic tide + forecast surge residual Adur 4 4 8 8 8 4 8 4 4 Warnings are issued based on the maximum water level 0.5 hours either side of the predicted high tide Arun As Adur 4 4 8 8 8 4 8 4 4

Ouse As Adur 4 4 8 8 8 4 8 4 4

R&D Project Record W5/010/1 81 Table A5 1.2 (Cont) Forecasting and Warning Summary: Southern Region

Data used for the issue of flood Data used for forecasting2 warnings3 Estuary Method for forecasting estuary water level1 fluvia tide surge wind wave fluvial level wind wave l Cuckmere As Cuckmere 4 4 8 8 8 4 8 4 4 level = predicted astronomic high tide + maximum forecast surge residual 2 hours either side of the predicted high tide. Meon 4 4 8 8 8 4 4 8 8 The predicted astronomic high tide is the first of the double peak experienced in the Solent.

As Meon Hamble 4 4 8 8 8 4 4 8 8 As Meon Itchen 4 4 8 8 8 4 4 8 8 Test (Southampton As Meon 4 4 8 8 8 4 4 8 8 Water) level = predicted astronomic high tide + maximum forecast surge residual 2 hours either side of the high tide. Lymington 4 4 8 8 8 4 4 8 8 The level is adjusted for exaggerated surge residuals caused by very low pressure systems (<991mb)

As Meon Medina (IoW) 4 4 8 8 8 4 4 8 8 As Meon Yar (IoW) 4 4 8 8 8 4 4 8 8 As Meon Bembridge (IoW) 4 4 8 8 8 4 4 8 8

R&D Project Record W5/010/1 82 Notes: 1Method for forecasting estuary water level – a general explanation of the method by which water levels are forecast for the estuary. The method may apply to a coastal reach containing the estuary. 2Data used for forecasting – details the data that are used in the actual forecasting of the water level for flood warning. Only those data that are actually used in deriving the water level are used: for example, wind data may be used to determine the degree of flood warning required but not the actual water level itself. 3Data used for the issue of flood warnings – details the additional data types that are used to determine the degree of any flood warning required but are not used in the actual forecasting of water level. 4Fluvial forecasts are considered with the tide forecast for the fluvial flood warning zones where there is a tidal influence. The impact on water level through the fluvial-tidal interaction is made based on the experience of the flood warning staff.

R&D Project Record W5/010/1 83 Table A5 1.3 Factors Important in the Forecasting of Water Levels: Southern Region

Factors1 Estuary tide surge wind wave fluvial 2-D Effects Other Darent 4 4 4 Medway 4 4 4 The Swale 4 4 4 Great Stour 4 4 4 4 Eastern Rother 4 4 4 4 Arun 4 4 4 4 Adur 4 4 4 4 Ouse 4 4 4 4 Cuckmere 4 4 4 4 Meon 4 4 4 4 double high tide Hamble 4 4 4 double high tide Itchen 4 4 4 4 4 double high tide Test (Southampton 4 4 4 4 double high tide Water) Lymington 4 4 4 4 4 low pressure, double high tide Medina (IoW) 4 4 4 4 double high tide Yar (IoW) 4 4 4 4 double high tide Bembridge (IoW) 4 4 4 4 double high tide

Total ?/? ?/? ?/? ?/? ?/? ?/? ?/?

Notes: 1Factors – physical factors that in the opinion of the Agency Flood Warning Staff are important in relation to the forecasting of water levels

R&D Project Record W5/010/1 84 APPENDIX 5

SOUTHERN REGION

2 SUSSEX AREA

2.1 Background

The Area office is responsible for the forecasting of levels for the East and West Sussex coast (Hastings to Chichester).

The principal estuaries in the Area are:

· River Arun · River Ader · River Ouse · River Cuckmere.

The Area also includes Chichester harbour.

The information contained in the following sections is based upon that gathered during the meeting of 8 September 1998.

2.2 Current Forecasting Methods

2.2.1 Overview

In the Sussex Area, water levels are forecast for coastal zones and not for specific estuaries. The coastal zones are given in Table A5 2.1 together with the estuaries found in each zone. For both coastal zones the forecast water level is given by:

Level = predicted astronomic hightide + STFS forecast surge residual

The sites used for the astronomic tide and the STFS forecast surge residual are given in Table A5 2.2. The predicted water level for the coastal zone applies to each estuary in the Agency Area.

Flood warnings are issued based on a look-up table of predicted water level and forecast wave height data from STFS. The highest water level ±0.5 hours of the predicted astronomic high tide is used in the decision to issue any warning.

R&D Project Record W5/010/1 85 2.2.2 Specific Estuaries

River Arun The water level for the Arun estuary is equal to that forecast for the Chichester Harbour to Shoreham coastal zone. Refer to Table A5 2.2 for the method by which levels are forecast for this reach. The forecast levels apply on the Arun up to Littlehampton; however, the tidal influence extends inland to Pulborough and passes through fluvial flood warning zones ‘Littlehampton to Arundel ( 3C4)’ and ‘Arundel to Pulborough (3C3)’. The impact on water levels due to fluvial-tidal interaction in these zones is based on experience. The ‘fluvial only’ level is forecast and compared to the to forecast tidal level and flood warnings issued based upon the experience of the Flood Warning Staff.

Forecast wind and wave data are received from STFS for South Selsey.

Real-time level data is available for Newhaven, Shoreham, Littlehampton and Medmerry gauges. Real-time wind data is available for the Newhaven gauge. Real- time monitoring is performed by Area Staff.

River Adur The water level for the Adur estuary is equal to that forecast for the Emsworth to Shoreham coastal zone. Refer to Table A5 2.2 for the method by which levels are forecast for this zone.

The forecast levels apply on the Adur up to Shoreham. However, the tidal influence extends inland to Henfield and passes through fluvial flood warning zones ‘Shoreham to Upper Beeding (3F4)’ and ‘Upper Beeding to Henfield (3F3)’. The impact on water levels due to fluvial-tidal interaction in these zones is based on experience. The ‘fluvial only’ level is forecast and compared to the to forecast tidal level and flood warnings issued based upon the experience of the Flood Warning Staff.

Forecast wind and wave data are received from STFS for South Selsey.

Real-time level data is available for Newhaven, Shoreham, Littlehampton and Medmerry gauges. Real-time wind data is available for the Newhaven gauge. Real- time monitoring is performed by Area Staff.

River Ouse The water level for the Ouse estuary is equal to that forecast for the Shoreham to Hastings coastal reach. Refer to Table A5 2.2 for the method by which levels are forecast for this reach.

The forecast levels apply on the Ouse up to Newhaven, however, the tidal influence extends inland to Isfield and passes through fluvial flood warning zones ‘Newhaven to Lewes (4A6)’, ‘Lewes to Barcombe (4A5)’ and ‘Barcombe to Isfield (4A4)’. The impact on water levels due to fluvial-tidal interaction in these zones is based on experience. The ‘fluvial only’ level is forecast and compared to the forecast tidal level and flood warnings are issued based upon the experience of the Flood Warning Staff.

Forecast wind and wave data are received from STFS for South Hastings.

R&D Project Record W5/010/1 86 Real-time level data is available for Newhaven, Shoreham, Littlehampton and Pevensey gauges. Real-time wind data is available for the Newhaven gauge and real- time monitoring is performed by Area Staff.

River Cuckmere The water level for the Cuckmere estuary is equal to that forecast for the Shoreham to Hastings coastal zone. Refer to Table A5 2.2 for the method by which levels are forecast for this zone.

The forecast levels apply on the Ouse up to Exceat Bridge, however, the tidal influence extends inland to Hellingly and passes through fluvial flood warning zones ‘Exceat Bridge to Alfriston (4B3)’ and ‘Shermans Bridge to Hellingly (4B2)’. The impact on water levels due to fluvial-tidal interaction in these zones is based on experience. The ‘fluvial only’ level is forecast and compared to the to forecast tidal level and flood warnings issued based upon the experience of the Flood Warning Staff.

Forecast wind and wave data are received from STFS for South Hastings.

Real-time level data is available for Newhaven, Shoreham, Littlehampton and Pevensey gauges. Real-time wind data is available for the Newhaven gauge. Real-time monitoring is performed by Area Staff.

2.3 Improvements to Water Level Forecasts

2.3.1 Data Needs

The following comments were made regarding data for use in forecasting estuary water levels within the Sussex Area:

· Obtain additional wave forecast data; · Obtain additional local wind forecast data;

2.3.2 Forecasting Method

The following comments were made regarding where improvements to the method(s) for forecasting estuary water levels could be made.

· Improved accuracy of STFS surge residual forecasts, especially under localised low pressure conditions · To include more thoroughly the effect of wind and wave action on forecast water levels · To include the effect of wave frequency for flood warnings (as wave frequency affects defence stability)

R&D Project Record W5/010/1 87 Table A5 2.1 Coastal Zones and Estuaries

Coastal Zone Estuaries in Zone Chichester Harbour to River Adur Shoreham Harbour (No 12) River Arun

Shoreham to Hastings River Cuckmere Harbour (No 11) River Ouse

Table A5 2.2 Coastal Zones and Forecast Water Level

Predicted Astronomic Coastal Zone STFS site Tide Site Chichester Harbour to Shoreham Portsmouth Shoreham (No 12)

Shoreham to Hastings Newhaven Portsmouth Harbour (No 11)

R&D Project Record W5/010/1 88 APPENDIX 5

SOUTHERN REGION

3 HAMPSHIRE AREA

3.1 Background

The Area office is responsible for the forecasting of levels along the Hampshire coastline (Milford on Sea to Chichester Harbour) and the Isle of Wight.

The main estuaries in the Area are:

· River Itchen · River Lymington · River Test (Southampton water) · River Hamble · River Meon · River Medina (Isle of Wight) · River Yar (Isle of Wight) · River Bembridge (Isle of Wight).

The Area also includes Portsmouth, Langstone harbours and the west side of Chichester Harbour.

The information contained in the following sections is based upon that gathered during the meeting of 8 September 1998.

3.2 Current Forecasting Methods

3.2.1 Overview

Water levels are not forecast for specific estuaries but for coastal zones – the exception being Southampton Water. The estuaries within each coastal zone are given in Table A5 3.1.

The same water level is forecast for each zone, and hence the estuaries, where:

Level = astronomic high tide for Southampton + STFS forecast surge residual for Portsmouth

The Solent experiences a double ‘high’ tide. The two ‘high’ tides are approximately two hours apart. The time from low to high tide is the same as for typical tides (6 hours) but the fall from the second high to low tide is compressed into 4 hours. The double high is most pronounced during Spring tides. The first high tide is always listed in tide tables as being greater than the second, however, in practice this is not always the case.

R&D Project Record W5/010/1 89 The highest surge residual 2 hours either side of the predicted astronomic high tide is used to calculate the high water level. Flood warnings are issued based on a look-up table correlating the predicted high water level with the forecast wind speed and direction. The predicted water level is also compared against return period levels to assess the severity of any event.

There is no formal procedure for assessing the impact of fluvial-tidal interaction on water levels. It is perceived that for the estuaries in the Area, the impact of fluvial-tidal interaction on the risk of flooding is low, however, it has not been investigated thoroughly.

3.2.2 Specific Estuaries

River Lymington Water levels for the River Lymington are forecast as described in Section 3.2.1. However, the level may be adjusted if the forecast pressure is less than or equal to 991mb. Such an atmospheric low exaggerates the surge and the forecast level needs to be adjusted. The adjustment is the lower of either:

1.5 x STFS Portsmouth surge or The value from a Barometer Correction Table (see Table A5 3.2)

Forecast wind speed and direction data from the Met. Office (in the form of gale warnings) and from STFS for Portsmouth are used to assess the degree of any warning required but are not used in the forecasting of the water level itself.

Real-time level data is available at the Portsmouth, Lymington and Southampton gauges. Real-time wind data is available at St Catherine’s Point and Solent MRSC. Real-time monitoring is performed by Area Staff.

The River Lymington is prone to tide lock but fluvial and tidal predictions are not combined. The tide lock is exaggerated by a tidal gate on the Lymington.

Test (Southampton Water) The water level for the River Test is forecast as described in Section 3.2.1.

Forecast wind data from the Met. Office (in the form of gale warnings) and from STFS for South Selsey are used in a look-up table to assess the degree of any warning required. The data are not used directly to calculate water levels.

Real-time level data is available for the Portsmouth, Woolston, Eling and Southampton gauges. Real-time wind data is available at St Catherine’s Point and Solent MRSC gauges. However, currently no real-time monitoring is performed.

The impact of fluvial-tidal interaction on water levels is not included in the forecasting procedures.

R&D Project Record W5/010/1 90 Southampton Water is well defended but if the defences were breached then the inundated area would be extensive.

Itchen The water level for the Itchen is forecast as described in Section 3.2.1.

Real-time level data is available for the Southampton and Woolston gauges. Real-time wind data is available for St Catherine’s Point and Solent MRSC gauges. Real-time monitoring is performed by Area Staff.

The estuary is affected by fluvial-tidal interactions that can flood designated areas. The impact of fluvial-tidal interactions on water levels is not included in the forecasting procedures.

Hamble The water level for the Hamble is forecast as described in Section 3.2.1.

Real-time level data is available for the Southampton and Woolston gauges. Real-time wind data is available for the St Catherine’s Point and Solent MRSC gauges. Real- time monitoring is performed by Area Staff.

The impact of fluvial-tidal interactions on water levels is not included in the forecasting procedures.

Meon The water level for the Meon is forecast as described in Section 3.2.1.

Real-time level data is available for the Southampton, Hayling Island and Woolston gauges. Real-time wind data is available for the St Catherine’s Point and Solent MRSC gauges. Real-time monitoring is performed by Area Staff.

The estuary is affected by fluvial-tidal interactions that can flood designated areas. The impact of fluvial-tidal interaction on water levels is not included in the forecasting procedures.

Medina The water level for the Medina is forecast as described in Section 3.2.1.

R&D Project Record W5/010/1 91 Forecast wind data from the Met. Office (in the form of gale warnings) and from STFS for South Selsey are used in a look-up table to assess the degree of any warning required. The data are not used directly to calculate water levels.

Real-time level data is available for the Portsmouth, Southampton and Bembridge gauges. Real-time wind data is available for the St Catherine’s Point and Solent MRSC. Real-time monitoring is currently performed by Area Staff.

The Medina is prone to tide lock and flooding can occur at Cowes. The impact of fluvial-tidal interactions on water levels is not incorporated into the current forecasting procedures.

The levels in the Medina are very sensitive to wind direction with elevated levels experienced during Northerly winds.

Yar The water level for the Yar is forecast as described in Section 3.2.1. The Lymington correction is not made to the level to compensate for very low atmospheric pressures.

Real-time level data is available for the Portsmouth, Yarmouth, Southampton and Cowes gauges. Real-time wind data is available for the St Catherine’s Point and Solent MRSC gauges. Real time monitoring is currently performed by Area Staff.

Bembridge The water level for Bembridge (Eastern Yar) is forecast as described in Section 3.2.1.

Real-time level data is available for the Portsmouth, Southampton and Bembridge gauges. Real time wind data is available for the St Catherine’s Point and Solent MRSC gauges. Real time monitoring is currently performed by Area Staff.

3.3 Improvements to Water Level Forecasts

3.3.1 Data Needs

The following comments were made regarding the data needs for improved level forecasts:

· Additional local wind forecast sites to help assess local weather patterns which can affect water levels; · The installation of more real time wind gauges; · More detailed surge forecast resolution;

R&D Project Record W5/010/1 92 · The development of a real-time monitoring system to assess if conditions are improving or deteriorating.

R&D Project Record W5/010/1 93 3.3.2 Forecasting Methods

The following comments were made regarding improvements to the current methods for forecasting water levels:

· Improved forecasting of the maximum surge residual and its timing; · Improved correlation between wind data and wave height (due to the effects of over-topping); · The development of methods for assessing the impact of fluvial-tidal interactions on water levels.

Table A5 3.1 Coastal Zones and Hampshire Estuaries

Estuary Corresponding Coastal Zones Lymington Milford on Sea to Calshot (13A) Test Southampton Water (13B) Itchen Southampton Water (13B) Hamble Southampton Water (13B) Meon Hamble to Chichester Harbour (13D) Medina East Cowes to Sandown (C14B) Yar Needles to East Cowes (C14A) Bembridge East Cowes to Sandown (C14B)

Table A5 3.2 Barometer Correction Table for Lymington

Pressure [mb] Correction [m] 965 +0.8 969 +0.75 974 +0.70 978 +0.65 982 +0.60 986 +055 991 +0.50

R&D Project Record W5/010/1 94 APPENDIX 6

SOUTH WEST REGION

6.1 Background

The South West Region is responsible for the forecasting of levels in the south west from Christchurch to Avonmouth.

The principal estuaries in the Region are:

· River Avon (Avonmouth) · River Parrett · River Taw · River Torridge · River Camel · River Fal (Truro and Falmouth) · River Tamar · River Dart · River Exe · River Frome and Piddle (Poole Harbour) · River Avon and Stour (Christchurch) · River Fowey · Kingsbridge · River Teign.

The information contained in the following sections is based upon that gathered and requested during the meeting of 5 August 1998.

6.2 Current Forecasting Methods

6.2.1 Overview

The current forecasting methods in the South West Region predict water levels for reaches of coastline rather than for specific estuaries. The coastal reach associated with each estuary is given in Table 6.1. Exceptions to this rule are the River Parrett (Bridgewater), Rivers Frome and Piddle (Poole Harbour), and the River Stour (Christchurch Harbour) where specific forecasts are produced.

In all cases the water levels are forecast for each tidal cycle. The forecasts are based on the STFS predicted surge residual together with the astronomic tide level for specific sites. A summary of the forecast calculation for each location is given in Table 6.2.

The South West Region is subject to extreme surges caused by secondary depressions. The predicted surges are modified for such conditions based on the Lennon Criteria. The Lennon Criteria and guidelines apply to all estuaries in the Region except those in the Bristol Channel (Avon, Bristol Channel and Parrett).

R&D Project Record W5/010/1 95 Flood warnings are issued based on a matrix of predicted water levels with wind speed and direction forecasts.

If a flood warning is or may be issued then conditions are monitored in real-time to assess if they are improving or deteriorating.

6.2.2 Specific Estuaries

Avon (Bristol) The estuary water level is forecast as for the Bristol Channel Coast. Refer to Table 6.2 for the method by which water levels are forecast for this site.

Forecast offshore wind and wave data are received from STFS but are not directly incorporated in the prediction of water levels.

The forecast water levels apply only to the downstream reach of the estuary at Avonmouth. There is no method for the forecasting of water levels upstream of this area.

Water levels at Avonmouth, Hinkley Point and Bridgwater can be monitored in real- time. The Avonmouth gauge has a history of unreliability.

The downstream reach of the estuary at Avonmouth is affected by wave action. In the intermediate and upstream sections of the estuary, the risk of flooding due to fluvial- tidal interaction has not been quantified.

Bristol Channel Water levels are forecast for the Bristol Channel “coastline” between Porlock and Avonmouth. Refer to Table 6.2 for the method by which water levels are forecast for this reach.

Forecast offshore wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Avonmouth, Hinkley Point and Bridgwater can be monitored in real- time.

The accuracy of the forecast water levels is good but decrease between Porlock and Avonmouth. The overall accuracy of the forecasts, during extreme events decreases, as the surge behaviour becomes less predictable.

River Parrett (Bridgwater) Water levels are forecast for the downstream section of the estuary and also for the upstream reach at Bridgwater.

The level at the downstream reach is equal to that forecast for the Somerset/Bristol Channel Coast. Refer to Table 6.2 for the method by which levels are forecast for this reach.

At Bridgwater, water levels are forecast as follows:

R&D Project Record W5/010/1 96 Level = level at downstream reach + 300mm

Forecast offshore wind and wave data are received from STFS but are not directly incorporated into the forecasting of levels.

Water levels at Avonmouth, Hinkley Point and Bridgwater are monitored in real-time.

The downstream reach of the estuary is affected by wave action. The River Parrett is prone to tide lock and there is a strong tidal-fluvial interaction affecting water levels at Bridgwater.

River Taw No specific estuary water level forecasts are made. Instead, flood warnings are issued based on forecast water levels for the North Devon coast. Refer to Table 6.2 for the method by which water levels are forecast for this reach. The fluvial and tide level predictions are not combined to give a total estuary water level.

Forecast offshore wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Ilfracombe, Yelland and Bideford can be monitored in real-time. Wind speed and direction data at Dawlish can be monitored in real-time.

The estuary does experience 2-D effects that influence water levels.

River Torridge Water levels are forecast for the upstream and downstream reaches of the estuary.

The downstream water level is equal to that forecast for the North Devon coast. Refer to Table 6.2 for the method by which water levels are forecast for this reach.

Forecast offshore wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Ilfracombe, Yelland and Bideford can be monitored in real-time. Wind speed and direction data at Dawlish can be monitored in real-time.

River Camel Water levels are forecast for the upstream and downstream reaches of the estuary. The downstream water level is equal to that forecast for the North Cornwall coast. Refer to Table 6.2 for the method by which water levels are forecast for this reach of coast.

Water levels for the upstream reach of the estuary are forecast based on gauged river level data using an empirical model. The model is based on the experience of the Flood Warning staff and is not a mathematical or physical model.

Forecast offshore wind and wave data are received from STFS but are not directly incorporated into the forecasting of water levels.

R&D Project Record W5/010/1 97 Water levels at Ilfracombe, Milford Haven, Padstow, Hayle Gate, Wadebridge and Bude can be monitored in real-time. Wind speed and direction at St Mawgan can be monitored in real-time.

Observed tide levels can often be higher than predicted values.

Truro/Falmouth (Fal estuary) Water levels are forecast for the upstream and downstream reaches of the estuary.

The downstream water level is equal to that forecast for the South Cornwall coast. Refer to Table 6.2 for the method by which water levels are forecast for this reach of coast.

Water levels for the upstream reach of the estuary are as for those forecast for the South Cornwall coast but with empirical relationships for fluvial flows based on the experience of the Flood Warning staff.

Forecast wind speed and direction data are received from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Newlyn, Truro and Devonport can be monitored in real-time. Wind speed and direction at Culdrose can be monitored in real-time.

There are floodgates within the estuary to protect areas from flooding during high tide levels. There are detention reservoirs above the estuary, which are used to store fluvial flows and prevent high water levels from fluvial-tidal interactions when the estuary floodgates are closed.

River Fowey No specific forecasts of water levels are made for the estuary. Instead water levels as forecast for the South Cornwall coast are used. Refer to Table 6.2 for the method by which water levels are forecast for this reach of coast.

In the upstream section of the estuary, water levels are forecast based on gauged river levels and the experience of the Flood Warning staff.

Forecast wind speed and direction data are received from STFS but are not directly incorporated into the forecasting of water levels.

Water level data at Newlyn, Truro and Devonport can be monitored in real-time. Real- time wind data at Culdrose, Lizard Lighthouse and Pendennis Point is available.

River Tamar Water levels are forecast for the upstream and downstream reaches of the estuary.

The downstream water levels are equal to those forecast for the South Cornwall coast. Refer to Table 6.2 for the method by which water levels are forecast for this reach of coastline.

R&D Project Record W5/010/1 98 The upstream water levels are forecast based on gauged river levels and the experience of the Flood Warning staff.

Forecast wind and wave data are received for SE Eddystone from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Devonport, Truro and Newlyn can be monitored in real-time. Real- time wind speed and direction data at Plymouth, Pendennis Point, Culdrose and Lizard Lighthouse is available.

Kingsbridge Estuary water levels are not forecast specifically for this site. Instead water levels are forecast as for the South Devon coast. Refer to Table 6.2 for the method by which water levels are forecast for this reach of coastline.

Forecast wind speed and direction data are received for SE Eddystone from STFS but are not directly incorporated into the forecast levels.

Water levels at Devonport can be monitored in real-time. Wind speed and direction data at Plymouth and Brixham is available in real-time.

River Dart Water levels are forecast for the upstream and downstream reaches of the estuary.

Downstream water levels are equal to those forecast for the South Devon coast. Refer to Table 6.2 for the method by which water levels are forecast for this reach of coastline.

The upstream water levels are forecast based on gauged river levels and the experience of the Flood Warning staff.

Forecast wind speed and direction data are available for Plymouth and Portland from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Devonport and Totnes can be monitored in real-time. Wind speed and direction data for Brixham and Plymouth is available in real-time.

River Teign Water levels are forecast for the upstream and downstream reaches of the estuary.

Downstream water levels are equal to those forecast for the South Devon coast. Refer to Table 6.2 for the method by which water levels are forecast for this reach of coastline.

The upstream water levels are forecast based on gauged river levels and an empirical model.

Forecast wind speed and direction data are available for Plymouth and Portland from STFS but are not directly incorporated into the forecasting of water levels.

R&D Project Record W5/010/1 99 Water levels at Devonport and Exmouth can be monitored in real-time. Wind speed and direction data for Brixham and Plymouth is available in real-time.

River Exe Water levels are forecast for the upstream and downstream reaches of the estuary.

Downstream water levels are equal to those forecast for the South Devon coast. Refer to Table 6.2 for the method by which water levels are forecast for this reach of coastline.

Water levels at the upstream boundary are forecast based on the STFS predicted surge residual for Devonport and the observed levels at Exmouth on the preceding tide.

Forecast wind speed and direction data are available for Plymouth and Portland but are not directly incorporated into the forecasting of water levels.

Water levels at Devonport can be monitored in real-time. Wind speed and direction data at Brixham, Lyme Bay and the Isle of Portland can be monitored in real-time.

River Frome (Wareham) Water levels for the River Frome estuary are forecast for Wareham.

The forecast water level is given by:

Level = astronomic tide at Wareham + STFS forecast surge residual for Bournemouth

Forecast wind speed and direction data are received from STFS for Bournemouth. The predicted water levels are modified based on the forecast and actual wind conditions as follows:

i) For 12 hours or more of strong (Force 6) S to E winds, a significant build up of surge can occur in the harbour. For such, the forecast surge is multiplied by two in the period before the astronomic tide peak.

ii) For SWW to SSE winds of strength > Force 6 wave action can raise water levels significantly (a 1:10 year storm is a 0.6 m wave height on a 1.61 m water level). For such conditions, 0.5 m is added to the basic predicted water level to allow for wave effects.

Water levels can be monitored in real-time at Wareham, Bournemouth and Portland. Water levels cannot be monitored at the Poole Harbour gauge, as there is no telemetry system at present.

River Piddle (Poole Harbour) Water levels are forecast for Poole Harbour. The forecast water level is given by:

Level = astronomic tide at Poole + STFS forecast surge residual for Bournemouth

A floodgate protects part of Poole from flooding during high tide levels. The decision to close the gate is based on the forecast water level.

R&D Project Record W5/010/1 100 River Stour and Avon (Christchurch Harbour) Water levels for the River Stour and Avon are forecast for Christchurch Harbour.

The forecast water level for the harbour is given by:

Level = predicted tide at Bournemouth + STFS forecast surge residual at Bournemouth

The area experiences two double high tides during every 24 hours due to its close proximity to the Solent.

The tide is monitored at least 2 hours ahead of the high tide using curve matching. Two curves are used each of which describes the typical change in water levels as the tide nears high tide. One curve describes a gradual smooth increase in level through to high tide and the other more rapid increase in level as the high tide is approached. The predicted level is revised if necessary depending on which tide curve the actual levels are following.

Forecast wind speed and direction date are received from STFS but are not directly incorporated into the forecasting of water levels.

Water levels at Bournemouth and Christchurch are monitored in real-time and are used to revise the predicted levels if necessary.

Christchurch Harbour is very susceptible to rapid drops in atmospheric pressure resulting in unexpected surges.

6.3 Improvement to Water Level Forecasts

6.3.1 Data Needs

The following points summarise the data needs for the improved forecasting of water levels. Detailed comments for each estuary are given in Table 6.3

· The installation of additional tide gauges along the south coast of the Region · Real-time wind speed and direction data for Poole and Christchurch to be made available · Real-time atmospheric pressure data for Christchurch to be made available · Improved astronomic tide predictions for Bournemouth and Padstow · The collection and analysis of more event data to assess forecast performance.

6.3.2 Forecasting Method

The following points summarise improvements that could be made to the method(s) for forecasting water levels. Detailed comments for each estuary are given in Table 6.4:

· Review the performance of the existing methods

R&D Project Record W5/010/1 101 · Develop methods for forecasting water levels in the intermediate area of an estuary (i.e. between the mouth and the tidal limit) · Develop a method for translating point STFS surge residual forecasts to another location · Improve the forecast accuracy of STFS surge residuals along the south coast of the Region. STFS and POL are already aware of the need for this improvement · Improve the forecast accuracy of STFS surge residuals within the Bristol Channel during extreme events when behaviour can be ‘unpredictable’ · Develop a method for assessing increases in water levels due to fluvial-tidal interaction · Develop a method for estimating the time it takes the high tide levels to propagate up an estuary.

Table A6 6.1 Estuary and Associated Coastal Flood Warning Reach

Estuary Coastal Reach Bristol Channel (Porlock to Somerset/Bristol Channel Coast Avonmouth) Avon Bristol Channel Coast Parrett Somerset/Bristol Channel Coast Taw North Devon Coast Torridge North Devon Coast Camel North Cornwall Coast Fal (Truro/Falmouth) South Cornwall Coast Fowey South Cornwall Coast Tamar South Cornwall Coast Kingsbridge South Devon Coast Dart South Devon Coast Teign South Devon Coast Exe South Devon Coast Frome South Devon Coast Frome + Piddle (Poole Harbour) South Devon Coast River Avon and Stour South Devon Coast (Christchurch)

R&D Project Record W5/010/1 102 Table A6 1.2 Water Level Forecast for the Coastal Reaches

Coastal Reach Forecast Calculation Somerset/Bristol Channel Coast Hinkley Point surge + Hinkley Point Astronomic Tide

Bristol Channel Coast Avonmouth surge + Avonmouth Astronomic Tide

North Devon Coast Ilfracombe surge + Ilfracombe Astronomic Tide

North Cornwall Coast Ilfracombe surge + Padstow Astronomic Tide

Plymouth surge + Devonport Astronomic Tide (the South Cornwall Coast forecast also examines Newyln surge + Newyln astronomic tide) South Devon Coast Plymouth surge + Devonport Astronomic Tide

R&D Project Record W5/010/1 103 Table A6 1.3 Comments Received Regarding Data for Use in Water Level Forecasting

Estuary Comment Bristol Channel Improved reliability of Avonmouth tide gauge required. Avon Improved reliability of Avonmouth tide gauge required.

Additional level gauges required upstream of Avonmouth Parrett None. Taw New tide level gauges have been installed but more are needed in the fluvial-tidal interaction zone. Torridge None. Camel Improved accuracy of the Padstow tide tables required. More information required during events. Fal Additional tide level gauges are required at the downstream reach of (Truro/Falmouth) the estuary.

Event data needs to be collected for the intermediate area of the estuary. Fowey Tide level gauges are required at Fowey and Lostwithiel. Tamar Event data needs to be collected at intermediate area. Kingsbridge Tide gauges are required in the estuary.

There is a general lack of data. Dart Tide gauges are required at Dartmouth and Totnes. Teign Tide gauges are required at Teignmouth and Newton Abbot. Exe General poor data availability.

Tide gauges required at Exmouth and Topsham or further upstream. Frome + Piddle Wind speed and direction real-time data required. (Poole Harbour) A link to Poole Harbour Commission level, wind speed and direction gauge data is required.

Wind speed and direction forecasts are required. Stour/Avon Improved tide tables required for Bournemouth (this is currently (Christchurch) under review at the time of writing).

Real-time wind speed, wind direction and atmospheric pressure data is required.

R&D Project Record W5/010/1 104 Table A6 1.4 Comments Received Regarding Improvements to the Methods for Forecasting Water Levels

Estuary Comment Bristol Channel Improved forecast surge residuals are required during extreme events when the behaviour is less predictable. Avon The accuracy of the forecasting method needs to be improved in the intermediate and upstream section of the estuary, e.g. for tidal- fluvial interaction. Parrett Improved forecast accuracy is required during extreme surges. Improved forecast level accuracy is required at the upstream boundary. Taw Improved forecast accuracy is required in the upstream and intermediate areas of the estuary. Torridge (As Taw). Camel Improved accuracy of forecast surge residuals for Padstow is required.The current method tends to under predict levels and the reasons for this need to be assessed. Fal (Truro/Falmouth) Improved forecast accuracy is required in the intermediate area of the estuary. Fowey Improved forecast accuracy is required in the intermediate reach of the estuary. Tamar The accuracy of the current forecasting method needs to be assessed based on data collected from past events. Kingsbridge Improved forecast accuracy is required. Dart None. Teign None. Exe Improved forecast accuracy is required in the intermediate reach of the estuary. There is, however, a lack of event data. Frome + Piddle Improved reliability of the forecast surge residual for Portland and (Poole Harbour) Bournemouth is required. POL and STFS are aware of this. Greater understanding of the complex surge behaviour in the harbour. Improved forecasting of ‘static’ high tides caused by South to East winds. Greater understanding of the effect of flows from the Frome and Piddle on estuary water levels. Stour/Avon Improved reliability of the forecast surge residual for (Christchurch) Bournemouth. Improvements are required. POL is aware that improvements are required.

R&D Project Record W5/010/1 105

Table A6 6.5 Forecasting and Warning Summary: South West Region

Forecast Data Used1 Real-time Monitoring Sites2 Real-time Estuary tide Surge wind wave fluvial level wind wave fluvial monitoring3 Avonmouth STFS N. Avonmouth Avonmouth Hinkley 4 Avon Cornwall Bridgewater

Avonmouth Bristol STFS N. Avonmouth Avonmouth Hinkley 4 Cornwall Channel Bridgewater

Avonmouth STFS N. Hinkley Hinkley Hinkley 4 Parrett Cornwall Bridgewater

Ilfracombe STFS N. Ilfracombe Ilfracombe Yelland Gawlish* 4 Taw Cornwall Bideford

Torridge @ Ilfracombe STFS N. Ilfracombe Ilfracombe Weare Yelland Gawlish* 4 Torridge Cornwall Gifford Bideford

Ilfracombe Camel @ Milford Haven STFS N. Denby, Padstow Ilfracombe Ilfracombe St. Mawgan* 4 Camel Cornwall Camel @ Hayle Gate Wadebridge Wadebridge Bude

Truro/ Fal @ Newlyn Devonport Plymouth STFS Falmouth (Fal 4 4 Tregony, Truro Culdrose* 4 (Newlyn) (Newlyn) Eddystone estuary) Truro Barrier Devonport

Newlyn Culdrose* Devonport Plymouth STFS Fowey @ Truro Lizard L.House* 4 Fowey 4 4 Eddystone Restormel (Newlyn) (Newlyn) Devonport Pendennis Pt*

Plymouth* Newlyn Devonport Plymouth STFS Tamar @ Pendennis Pt.* Truro 4 Tamar 4 4 Eddystone Gunnislake Culdrose* (Newlyn) (Newlyn) Devonport Lizard L.House*

R&D Project Record W5/010/1 107 Table A6 6.5 (Cont) Forecasting and Warning Summary: South West Region

Forecast Data Used1 Real-time Monitoring Sites2 Real-time Estuary tide surge wind wave fluvial Level wind wave fluvial monitoring3

STFS Plymouth* Devonport Plymouth Devonport 4 Kingsbridge Eddystone Brixham*

STFS Dart @ Plymouth* Devonport Plymouth Eddystone, Austins Devonport 4 Dart Portland* STFS Portland Bridge

STFS Teign @ Plymouth* Devonport Plymouth Eddystone, Devonport 4 Teign Preston Brixham* STFS Portland

STFS Exe @ Brixham* Exe Devonport Plymouth Eddystone, Whitford Devonport Lyme Bay* 4 STFS Portland Bridge Portland*

Wareham STFS Wareham Bournemouth Bournemouth 4 Frome Bournemouth Portland

Wareham STFS Poole Bournemouth Bournemouth 4 Piddle Bournemouth Portland

STFS Bournemouth Bournemouth Bournemouth 4 Avon/Stour Bournemouth Christchurch H.

Notes: 1Forecast Data Used – forecast data used in the flood warning procedures. Refer to the ‘Forecasting and Warning Summary’ table for how the data are used in the flood warning procedures. 2Real-time Monitoring Sites – details the real-time monitoring sites that are available near to the estuary and/or those that are used in the real-time monitoring of events 3Real-time Monitoring – details whether real-time monitoring of events is performed during the flood warning procedures. 4Forecasts for Newlyn are also used to assess the severity of any event * denotes that the site is not currently used in the real-time monitoring of conditions.

R&D Project Record W5/010/1 108 Table A6 6.6 Forecasting and Warning Summary: South West Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Avon level = predicted astronomic high tide + forecast surge residual 4 4 8 8 8 4 4 8 8

Bristol Channel As River Avon 4 4 8 8 8 4 4 8 8 Parrett d/s: level = predicted astronomic high tide + forecast surge residual 4 4 8 8 8 4 4 8 8 u/s: level = d/s level + 300mm

Taw As River Avon 4 4 8 8 8 4 4 8 8 Torridge d/s: level = predicted astronomic high tide + forecast surge residual

u/s: level = d/s level + a correction to allow for fluvial flows. 4 4 8 8 4 4 4 8 8

The correction is empirical and based on the experience of the Flood Warning Staff.

Camel d/s: level = predicted astronomic high tide + forecast surge residual

u/s: level = d/s level + a correction to allow for fluvial flows. 4 4 8 8 4 4 4 8 8

The correction is empirical and based on the experience of the Flood Warning Staff.

R&D Project Record W5/010/1 109 Table A6 6.6 (cont) Forecasting and Warning Summary: South West Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Truro/Falmouth (Fal d/s: level = predicted astronomic high tide + forecast estuary) surge residual

u/s: level = d/s level + a correction to allow for fluvial flows. 4 4 8 8 4 4 4 8 8

The correction is empirical and based on the experience of the Flood Warning Staff.

Fowey level = predicted astronomic high tide + forecast surge residual 4 4 8 8 8 4 4 8 8

Tamar d/s: level = predicted astronomic high tide + forecast surge residual

u/s: level = d/s level + a correction to allow for fluvial flows. 4 4 8 8 4 4 4 8 8

The correction is empirical and based on the experience of the Flood Warning Staff.

Kingsbridge level = predicted astronomic high tide + forecast surge residual 4 4 8 8 8 4 4 8 8

R&D Project Record W5/010/1 110 Table A6 6.6 (cont) Forecasting and Warning Summary: South West Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Dart d/s: level = predicted astronomic high tide + forecast surge residual

u/s: level = d/s level + a correction to allow for fluvial flows. 4 4 8 8 4 4 4 8 8

The correction is empirical and based on the experience of the Flood Warning Staff.

Teign d/s: level = predicted astronomic high tide + forecast surge residual

u/s: level = d/s level + a correction to allow for fluvial flows. 4 4 8 8 4 4 4 8 8

The correction is empirical and based on the experience of the Flood Warning Staff.

Exe d/s: level = predicted astronomic high tide + forecast surge residual

u/s: level = d/s level + a correction to allow for fluvial flows. 4 4 8 8 4 4 4 8 8

The correction is empirical and based on the experience of the Flood Warning Staff.

R&D Project Record W5/010/1 111 Table A6 6.6 (cont) Forecasting and Warning Summary: South West Region

Estuary Method for forecasting estuary water level1 Data used for the issue of flood Data used for forecasting2 warnings3 tide surge wind wave fluvial level wind wave fluvial Frome level = predicted astronomic high tide + forecast surge residual

For certain wind conditions, the surge forecast is 4 4 4 8 8 4 8 8 8 increased (the increase is based on historical events and past experience)

Piddle level = predicted astronomic high tide + forecast surge residual

For certain wind conditions, the surge forecast is 4 4 4 8 8 4 4 8 8 increased (the increase is based on historical events and past experience)

Avon/Stour level = predicted astronomic high tide + forecast surge residual

A tide curve describing two typical tide shapes is 4 4 8 8 8 4 4 8 8 used 2 hours prior to high tide to predict how the level is likely to change.

Abbreviations: d/s – downstream u/s – upstream

R&D Project Record W5/010/1 112 Table A6 6.7 Factors Important in the Forecasting of Water Levels: South West Region

Factors1 Estuary tide surge wind wave fluvial 2-D Effects Other Avon 4 4 4 Bristol Channel 4 4 4 4 Parrett 4 4 4 Taw 4 4 4 4 Torridge 4 4 4 runoff rates Camel 4 4 4 runoff rates Truro/Falmouth (Fal 4 4 4 runoff rates, estuary) Lennon criteria Fowey 4 4 4 4 runoff rates, Lennon criteria Tamar 4 4 4 Lennon criteria Kingsbridge 4 4 runoff rate, Lennon criteria Dart 4 4 4 runoff rate, Lennon criteria Teign 4 4 4 runoff rate, Lennon criteria Exe 4 4 4 Lennon criteria Frome 4 4 4 4 4 4 Lennon criteria Piddle 4 4 4 4 4 4 Lennon criteria Avon/Stour 4 4 4 4 4 4 Lennon criteria

Total 16/16 16/16 4/16 4/16 14/16 5/16 12/16

Notes: 1Factors – physical factors that in the opinion of the Agency Flood Warning Staff are important in relation to the forecasting of water levels

R&D Project Record W5/010/1 113 APPENDIX 7

7 THAMES REGION

7.1 Background

The Thames Region is responsible for the forecasting of water levels for the Thames estuary and any subsequent closure of the Thames Barrier.

The River Thames is the principal estuary in the Region, however the following rivers are prone to flooding from fluvial-tidal interaction:

· River Crane (prone to tide lock); · River Darent (protected by a barrier); · River Roding (protected by a barrier).

The information contained in the following sections are based upon that gathered during the meeting of 23 July 1998.

7.2 Current Forecasting Method

7.2.1 Overview

The forecast water level for the Thames estuary is given by predicted values at Southend. The water levels at Southend are derived from the sum of the astronomic time series for the site together with the forecast surge residual.

Total water level = astronomic tide at Southend + STFS forecast surge residual for Southend

The forecast surge residual for Southend is provided from the North Sea Model which is a cut-down version of the POL CS3 model. The model is run twice daily to produce 36 hourly surge residual forecasts.

The actual level at Southend is continually compared to the forecast values to ascertain if conditions are improving or deteriorating.

Water levels can be monitored in real-time at 8 Class A and 10 local Thames gauges. Wind speed and direction data can be monitored in real-time for North Shields, Immingham, Lowestoft, Southend, Sheerness and Newhaven. Fluvial flows can be monitored in real-time at Teddington Weir. Details of the real-time gauges are given in Table 7.1.

7.2.2 Thames Barrier Closure

The decision to close the Thames Barrier is based upon a closure matrix relating forecast levels at Southend with the observed flow over Teddington Weir. For a given fluvial flow there is a corresponding level at Southend which triggers Barrier closure.

R&D Project Record W5/010/1 114 The closure of the Thames Barrier also initiates the closure of the River Roding and Darent Barriers.

7.2.3 Water Levels Upstream of the Thames Barrier

The forecast water levels at Southend and the flow over Teddington Weir may not be sufficient to cause Barrier closure although localised flooding may still occur along the Thames if levels are high enough.

Water levels are predicted along the Thames from Teddington to Southend using a 1- D hydrodynamic model. The forecast water level output from the Thames River Model is used to issue local flood warnings but not Barrier closure.

7.3 Improvements to Water Level Forecasts

7.3.1 Data Needs

There are no additional data needs required for forecasting levels on the Thames estuary at present.

7.3.2 Forecasting Methods

The following comments were made regarding where improvements to the current forecasting method could be made:

· Enhancement of the River Thames Model to incorporate wave effects; · Improved forecast timing of peak surge valves; · Improved modelling of tidal surges propagating from the South and South West; · Assess what impact local changes in weather conditions have on estuary levels.

R&D Project Record W5/010/1 115 Table 7.1 Real-Time Monitoring Gauges for the Thames Estuary

Type Gauge Name Tide Level Sheerness North Shields Immingham Lowestoft Wick Dover Newhaven Southend Coryton Tilbury Erith Silvertown Tower Pier Westminister Chelsea Hammersmith Richmond River Level Thames @ Teddington Wind Speed and Direction North Shields Immington Lowestoft Sheerness Newhaven

R&D Project Record W5/010/1 116 Table 7.2 Forecasting Data Used Summary: Thames Region

Forecast Data Used1 Real-time Monitoring Sites2 Real-time Estuary Tide surge wind wave fluvial level wind wave fluvial monitoring3 Wick North Shields Immingham Lowestoft Southend Dover North Shields Newhaven North Shields Immingham STFS Immingham Lowestoft STFS Southend Thames @ Coryton Thames @ Southend Southend Lowestoft 4 Thames Southend COSMOS Teddington Tilbury Teddington COSMOS Southend Dover Erith Newhaven Newhaven Silvertown Tower Pier Westminster Chelsea Hammersmith Richmond

R&D Project Record W5/010/1 117 Table 7.3 Forecasting and Warning Summary: Thames Region

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Thames Water level at Southend is predicted using astronomic tide data and the forecast surge residuals from the North Sea model (which is based on the CS3 model) 4 4 4 8 4 4 8 8 4 Water levels between Southend and Teddington Weir forecast using a 1-D hydraulic model.

Table 7.4 Factors Important in the Forecasting of Water Levels: Thames Region

Factors1 Estuary tide surge wind wave fluvial 2-D Effects Other Thames 4 4 4 4 4

Total 1/1 1/1 1/1 1/1 1/1 0/1 0/1

Notes: 1Factors – physical factors that in the opinion of the Agency Flood Warning Staff are important in relation to the forecasting of water levels

R&D Project Record W5/010/1 118 APPENDIX 8

8 ENVIRONMENT AGENCY - WALES

8.1 Background

The Welsh Region is responsible for the forecasting of levels along the Welsh coastline from Chepstow to the River Dee.

The Region is divided into 3 areas:

· SE Area - covering Chepstow to Aberthaw · SW Area - covering Aberthaw to Borth · Northern Area - covering Borth to the River Dee.

The principal estuaries in each area are:

· SE Area: Bristol Channel River Usk River Taff River Ely River Wye

· SW Area: Cleddau Tywi

· Northern Area: River Dee.

The information contained in the following section is based upon that gathered during the meeting of 24 July 1998 and subsequent data received from the SW and Northern Area offices.

8.2 Current Forecasting Methods

8.2.1 South East Area

The current forecasting method predicts the water level for the reach from Chepstow to Aberthaw. Water level forecasts are not made for individual estuaries.

The forecasts are based upon the output from the STFS Bristol Channel and Severn Estuary models. The forecast water level for each tidal cycle is given by:

Level = astronomic high tide at Newport + STFS forecast surge residual for Newport

The Chepstow to Aberthaw reach comprises 8 Flood Risk Areas (see Table 8.1), each of which may receive a flood warning.

R&D Project Record W5/010/1 119 A look-up table, for each Flood Risk Area, containing a trigger threshold based on the forecast level at Newport is used to ascertain the degree of any warning required (e.g. yellow, amber or red). The thresholds are based on historic flood events.

If the wind is forecast to be from a SW direction and above Force 8, a warning may be upgraded from no warning to yellow or from yellow to amber. Upgrading from amber to red is only done following consultation with other Agency staff.

There is no formal method for the inclusion of high fluvial flows on water levels, but they are considered during the flood warning decision-making process.

Forecast wind speed and direction are received from the Met. Office.

No real-time monitoring of conditions is performed although level data at Newport, Chepstow and Neath is available.

8.2.2 South West Area

The South West Area predicts water levels for two estuaries: the Cleddau and Tywi.

Cleddau The water level is forecast for the downstream and upstream sections of the estuary.

The downstream level is given by:

Level = astronomic tide at Milford Haven + STFS forecast surge residual for Milford Haven

The water level is forecast for high tide only.

The upstream forecast water level (at Haverfordwest) is equal to the downstream level with a correction for time of travel (approximately 15 to 20 minutes) and the effects of river flows. The river flow is forecast using a rainfall-runoff model and the impact on estuary water level is based on experience. The impact of fluvial flows on water levels varies. Typically flooding occurs with a high tide and moderate fluvial flows or a moderate tide and high fluvial flows.

Forecast offshore wave and wind speed and direction forecasts are received for Carmarthen Bay but are not directly used in the forecasting of water levels, however, they are used when deciding what warning is to be issued.

No real-time monitoring is performed but level data is available at Milford Haven and Fishguard. Real-time wind speed and direction data is available at Milford Haven and St Gowan.

Tywi The water level is forecast for the upstream and downstream sections of the estuary.

The downstream water level is given by: Level = astronomic tide at Carmarthen + STFS forecast surge residual for

R&D Project Record W5/010/1 120 Milford Haven

The level is forecast for high tide only.

The upstream forecast water level (at Carmarthen) is equal to the downstream level with a correction for the effect of river flow. The river flow is forecast based on a correlation with observed flows further upstream based on experience. The tide travel time from the downstream to upstream reach of the estuary is approximately 15 to 20 minutes. Fluvial flows are important in relation to flooding.

No real-time monitoring is performed but level data at Carmarthen and Milford Haven is available. Real-time wind speed and direction data at St Gowan and Pendine is available.

8.2.3 Northern Area

The Northern Area does not forecast water levels for the Dee estuary but uses those forecasts produced by the North West Region Neptune system.

8.3 Improvements to Water Level Forecasts

8.3.1 Data Needs

The following points summarise the data needs for the improved forecasting of water levels.

· Obtain access to real-time level and wind data to allow the monitoring of conditions; · Install real-time barometric pressure recorders to enable the monitoring and tracking of low pressure systems;

· Install an additional tide gauge at Haverfordwest; · Obtain surge residual forecasts for Carmarthen; · Eliminate wave effects on the tide gauge at Carmarthen.

8.3.2 Forecasting Methods

The following points summarise the comments received regarding where improvements to the forecasting methods could be made:

· Review the current performance of the existing methods; · Improved STFS surge residual forecasts for the Bristol Channel, particularly during extreme conditions (includes both level and timing); · The development of a method for the offshore to onshore translation of surge forecasts;

R&D Project Record W5/010/1 121 · Automate the existing methods employed in the Southwest Area to improve flood warning lead-times; · The development of a method to assess the effect of wave action on water levels.

Table 8.1 Flood Risk Areas and Estuaries in SE Wales Area

Flood Risk Area Estuary within Area Lower, Wye, Tintern and Chepstow Wye Caldicot Levels Bristol Channel Lower Usk, Caerleon and Newport Usk Wentloog Levels Cardiff Taff, Ely Penarth Barry Aberthaw

R&D Project Record W5/010/1 122 Table 8.2 Forecasting Data Used Summary: Environment Agency - Wales

Forecast Data Used1 Real-time Monitoring Sites2 Real-time Estuary tide Surge wind wave fluvial level wind wave fluvial monitoring3

Bristol Newport Newport Newport Met Office Chepstow 8 Channel Neath

Newport Usk Newport Newport Met Office Chepstow 8 Neath

Newport Taff Newport Newport Met Office Chepstow 8 Neath

Newport Ely Newport Newport Met Office Chepstow 8 Neath

Newport Wye Newport Newport Chepstow 8 Neath

Cleddau @ Milford Carmarthen Milford Haven Canaston 8 Cleddau Haven Milford Haven Bridge Tywi @ Carmarthen Camarthen Milford Haven 8 Tywi Nantgaredig Milford Haven

Dee Op. Neptune Op. Neptune Op. Neptune Op. Neptune 8

R&D Project Record W5/010/1 123 Table 8.3 Forecasting and Warning Summary: Environment Agency - Wales

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Bristol Channel level = predicted astronomic high tide + forecast surge residual 4 4 8 8 8 4 4 8 8 Water level is forecast for the time of high tide only.

Usk As Bristol Channel 4 4 8 8 8 4 4 8 8 Taff As Bristol Channel 4 4 8 8 8 4 4 8 8

Ely As Bristol Channel 4 4 8 8 8 4 4 8 8 Wye As Bristol Channel 4 4 8 8 4 4 4 8 8 Cleddau d/s: level = predicted astronomic high tide + forecast surge residual

u/s: level = d/s level + a correction to allow for fluvial flows. 4 4 8 8 4 4 8 8 8

The correction is empirical and based on the experience of the Flood Warning Staff.

R&D Project Record W5/010/1 124 Table 8.3 (Cont) Forecasting and Warning Summary: Environment Agency - Wales

Data used for the issue of flood Data used for forecasting2 Estuary Method for forecasting estuary water level1 warnings3 tide surge wind wave fluvial level wind wave fluvial Tywi d/s: level = predicted astronomic high tide + forecast surge residual

u/s: level = d/s level + a correction to allow for fluvial flows. 4 4 8 8 4 4 8 8 8

The correction is empirical and based on the experience of the Flood Warning Staff.

Dee Use data from North West Region Neptune System NA NA NA NA NA 4 8 8 8

Abbreviations: d/s – downstream u/s – upstream

R&D Project Record W5/010/1 125 Table 8.4 Factors Important in the Forecasting of Water Levels: Environment Agency - Wales

Factors1 Estuary tide surge wind wave fluvial 2-D Effects Other Bristol Channel 4 4 4 4 air pressure Usk 4 4 4 Taff 4 4 4 Ely 4 4 4 Wye 4 4 4 Cleddau 4 4 4 4 air pressure Tywi 4 4 4 Dee no info no info no info no info no info no info no info

Total2 7/7 7/7 5/7 2/7 2/7 0/7 2/7

Notes: 1Factors – physical factors that in the opinion of the Agency Flood Warning Staff are important in relation to the forecasting of water levels 2The River Dee has not been included in the totals as no information was available at the time of writing.

R&D Project Record W5/010/1 126