FLOODING IN WORCESTER

Background

  1.1  Worcester has a long history of widespread flooding. One of the first areas flooded each time (and therefore most frequently) is at Hylton Road. The area was therefore selected as part of trials of the use of temporary barriers. The site was also useful to test how the equipment performed in particular circumstances, ie a long stretch of construction. This pilot was the first of its type in the Midlands for trialling this form of flood alleviation. The barriers were always to be deployed on a "best endeavours" basis, and were to be seen as a possible replacement for sandbags. The barriers also enabled us to identify issues such as groundwater seepage or sewer flooding.

  1.2  When deploying the equipment it is necessary to close the road. We have occasionally been criticised in the past for deploying when the levels don't subsequently justify it because of the unnecessary disruption to the traffic. This has been an ongoing issue of debate with both Worcester City and County Councils.

  1.3  The temporary defence protects a combination of light industrial units, commercial units, and residential property and affords a benefit to maintaining highway access into the city. There are approximately 20 properties protected.

SUMMER FLOODS

June 2007

  1.4  A decision was taken not to deploy the temporary defences in the June event because the river was predicted to peak at a lower level than that required to trigger deployment of the barriers. It was marginal, however, and we sent a team to Worcester to sandbag any low points in the bank and to monitor the situation as a contingency.

  1.5  There was then an increase in the rate of rise of river levels at Worcester, caused by an influx of water from the River Teme. The rate of rise on the River Teme in this area was unprecedented, with levels being higher than they had been for 47 years. This caused water in the River Severn, at its confluence with the Teme just downstream of Worcester, to back up very quickly.

  1.6  As a consequence of not deploying the barriers, the highway was flooded and became impassable. No property was flooded on this occasion.

  1.7  We would not normally deploy the barriers just in order to maintain highway access, as to do so would require the road to be closed in any event, so it would not have any benefit.

July 2007

  1.8  A decision to deploy the barriers to Hylton Road in July was taken in good time and the necessary workforce, plant and materials were deployed based on what is normally an easy journey on good roads. However, widespread disruption caused by the severe weather led to the closure of the M5 and grid locking of other routes. We got the barriers through but, despite having a police escort, we couldn't get the workforce there in time to erect them in a safe manner as, by then, they would have had to be working in water.

  1.9  In Worcester, 120 properties flooded from main river and 85 properties flooded from surface water sources. Approximately 20 of those properties were in Hylton Road and would have been protected had the barriers been installed.

THE WAY FORWARD

  1.10  Regional Flood Defence Committees are developing a scheme using Local Levy funding (subject to RFDC approval) which will provide the same level of protection at Hylton Road as is provided by the barriers. This would cost £650k. We are also exploring the possibility of a contribution from the City Council which could mean a scheme that gives a higher protection may be possible. A scheme could be in place within the next 12-18 months.

2.  The number of gardens paved over in London over the last five years

  2.1  In September 2005, the Greater London Assembly's Environment Committee published a report entitled "Crazy Paving—The environmental importance of London's Front Gardens". The report draws attention to the detrimental effects on wildlife and visual amenity, and the potential increase in flood risk caused by the "urban creep" caused by concreting over London's front gardens.

  2.2  The report puts forward a number of facts:

—  Front gardens cover between 3% and 5% of London's land area and for the purposes of the report, a conservative estimate of 3% is adopted. This equates to 47.8 km2 (18.5 square miles).

—  Research commissioned by the Committee and carried out on its behalf by the Greater London Authority's Data Management and Analysis Group suggests that at least two thirds of London's front gardens are already at least partially covered by surfacing other than vegetation—paving, bricks, concrete and gravel being the most likely.

—  London's front gardens have given way on a huge scale to parking bays, covering an area of 32 km2 (12 square miles). This area is compared with the size of the boroughs of Islington, Hammersmith and Fulham combined; 22 times the size of Hyde Park; 16 times the size of the new Olympic Park in east London; and 5,200 football pitches.

  2.3  Further, the Committee makes the following recommendations:

—  The Mayor, in partnership with relevant stakeholders, should initiate an awareness-raising campaign to inform Londoners about the detrimental environmental impact of paving over front gardens, and to raise the profile of environmental sustainable alternatives to concrete and paving slabs. We invite the Mayor to tell us in his response to this report what action he plans to take towards this end.

—  The Greater London Authority should make available its data for use by the London Wildlife Trust or other suitable organisation to conduct a more detailed analysis of the extent and patterns of hard surfacing in front gardens, in order to inform future policy and initiatives.

—  The Mayor's revised London Plan should include consideration of the strategic importance of London's gardens as a crucial environmental resources, wildlife habitat, amenity resource and flood protection system. It should set objectives for the promotion and protection of the large area of green space that is made up by front gardens, and should encourage and enable London boroughs to do the same in their own development plans.

—  The Association of London Government host a seminar at its next Liveable London conference to share knowledge and experience of using planning, transport and other policies to manage the numbers of new pavement crossovers, enforce the law relating to illegal crossovers, take account of the likely impact on front gardens when introducing parking restrictions, and more generally promote the environmental significance of front gardens.

—  The Government amend the Town and Country Planning (General Permitted Development) Order 1995, to enable local authorities to require planning applications to be submitted for all proposals to install pavement crossovers.

  2.4  The Environment Agency shares the Greater London Authority's concerns on this issue.

3.  Understanding how the Environment Agency can forecast flooding from rivers and the sea

What do the Environment Agency and Met Office do to forecast river and sea flooding?

  3.1  The Environment Agency and Met Office work closely together to forecast flooding from the rivers and the sea. Both have unique capabilities and skills which come into play at different stages during a developing flood incident.

  3.2  The Met Office forecast changes in the weather and the Environment Agency forecasts changes in river flows and levels and local tide levels. The Met Office employs meteorologists and the Environment Agency hydrologists.

  3.3  The Met Office are constantly monitoring the weather conditions, updating their forecasts and sharing their information with organisations including the Environment Agency.

  3.4  The Environment Agency monitors rainfall amounts and the locations where it falls, monitors river and tide levels and uses this information to forecast flooding. We do this 24 hours a day, 365 days of the year.

Why don't we just issue flood warnings when we know of heavy rain?

  3.5  In a typical year we are likely to receive over 500 Severe Weather Warnings from the Met Office. We need to understand the impact of a weather warning before we issue flood warnings or mobilise our workforce to operate flood defences.

  3.6  Not all heavy rainfall results in flooding and, although heavy rainfall can cause serious flooding from rivers, there are many factors that need to be considered before the true extent of flooding can be predicted if indeed flooding is predicted at all.

  3.7  It is consideration of these factors that allows the Environment Agency to translate general warnings of heavy rain from the Met Office into targeted warnings to individual homes and businesses.

  3.8  By reacting prematurely we can waste a lot of time and effort which in turn could affect our ability to respond when there is a real need. The public can easily become immune to receiving many warnings when nothing happens—so called false warnings, and as a consequence do not take action.

What do we need to know to forecast flooding?

  3.9  Before we can forecast flooding it is necessary to answer as far as possible the following questions:

—  What is the current state of the river and catchment?

—  How much will rain will fall and with what intensity? and

—  Where will the rain land?

  3.10  Although these questions might appear simple, answering them is a complex and expensive business. In conjunction with the Met Office we have developed many tools that we use to try answer these questions. The information and tools we use are:

—  Weather forecasts (from the Met Office)—providing information on amounts of expected rainfall on a regional basis

—  Weather radar information (shared EA and MO)—provides information on the rate of rainfall and where it is falling

—  Outputs from the Storm Tide Forecasting system (from the Met Office)

  3.11  We have also developed our own tools to predict the impact of rainfall on river levels and high tides on the coast which include:.

—  Rain gauge information—we have an estimated 1200 number of gauges in England and Wales recording rainfall amounts.

—  A network of river level and flow recorders—these are devices that measure the level and in some cases the flow of water in a river at a given location. All principal rivers in England and Wales have a number of these recorders which total in number 2800.

—  River models for forecasting—most principal rivers are modelled in some form or other and we can use the models to predict whether or not flooding will occur using rainfall and river level information.

—  Local tide models—these predict tide levels in estuaries and specific coastal lengths.

What is the process for forecasting flooding from rivers?

  3.12  In order to understand the process of forecasting floods it is necessary to appreciate what information is available as we lead into a flood incident which is represented in the following sections:

A few days before flooding

  3.13  We need to know the state of the catchment conditions. This is the state of the land and the soil before it rains. If rainfall is preceded by a dry period, the land will be dry normally allowing more absorption which reduces the amount that runs into the river. This is not the case after a prolonged period of drought when heavy rain on parched soils can run straight off into rivers. A similar phenomenon occurs in winter with frozen soils. Floodplains, large areas of low lying land next to rivers, are empty after dry spells which means they all available to store their maximum capacity and likewise most reservoirs will be low which means they can also store some of the flow. The Environment Agency monitors these conditions and takes them into account when making flood predictions. If it rains for a few days then flows on rivers can increase significantly once floodplains are full of water and there is no storage left.

  3.14  Another complicating factor is one of lying snow. Any snow within a catchment is additional stored water that may run off with any predicted rainfall if the temperature conditions are right. This can significantly increase the flood risk in the catchment. 300mm depth of lying snow is equivalent to about 25mm of rain.

  3.15  The amount of rainfall, peak intensities and where it lands are never the same from one flood to the next making it more difficult to predict the impact of the rainfall directly. Even small differences in the predicted pattern of rainfall in a catchment can radically change where the rainfall is channelled and the subsequent timings and size of flood peaks.

  3.16  In order for us to predict flooding we need information on rainfall amounts, intensity and where it will land. The short term weather forecasts, up to 48 hrs ahead, give rainfall amounts in more specific locations. We can now begin to use this information in our river models to predict flooding. However we are using a forecast to predict a forecast and the accuracy and our confidence in flooding at this stage is still uncertain.

  3.17  If the Met Office predict very heavy rainfall they send us a Severe Weather Warning. This tells us where the heavy rainfall is predicted at a regional level. This is useful for us to know as we can expect heavy rainfall but not all heavy rainfall results in flooding so we still need to get more accurate information before we can predict flooding with reasonable certainty. As mentioned earlier in a typical year we are likely to receive over 500 Severe Weather Warnings from the Met Office generally resulting in less than 10 significant flooding incidents.

12 to 24 hours before flooding

  3.18  We can supplement the rainfall information with information from our network of radars. These radar images can give us information on how much rainfall is actually falling, its intensity and just as important where it is falling. It can also be used to as a tool to forecast, about 6 hours ahead, and where the rain is heading. Information from radar and other Met Office information are combined in our models to help improve the accuracy. Even at this stage we may still not be in a position to predict flooding for certain although we may have issued flood watches and began to mobilise our workforce.

  3.19  There is a network of rain gauges across the British Isles that are used to measure rainfall amounts in a specific location. They send rainfall amounts to our models as frequently as every 15 minutes. These measured amounts, together with the radar information, begin to give us certainty in the forecasts of our models. We are now beginning to give, with reasonable confidence, flood warnings to the public at specific locations and deploying our workforce to operate our defences. We will alert local authorities and other partners such as the emergency services that flooding is likely and that they need to start implementing their response plans.

  3.20  From 24 hours in, the forecast from the Met Office was accurate, but this is not always the case and much further work will be required if greater accuracy is to be achieved. However, the ability to forecast accurately heavy rainfall sufficiently far in advance and at an appropriately local scale will almost certainly always be difficult to achieve.

6 to 12 hours before flooding—River Levels

  3.21  In dry spells river levels will be low to start with and so can accommodate more rainfall and flow before flooding occurs. If there has been a wet spell before the heavy rainfall then rivers may be running higher than normal and react more quickly to smaller amounts of rain.

  3.22  These conditions also vary depending on the time of year. For example it is normal in the summer for larger rivers to be running at lower levels than in the winter and that the ground will be drier. Also in the summer there is more plant growth and tree cover which affects how fast rainfall runs off the land into rivers and the rate at which water is soaked into the land.

  3.23  Once the rain has landed and run into the river we can detect river level rises using our network of river level recorders. This gives us the best information possible to help forecast whether flooding will occur. We will normally track a flood wave(s) down the river. Once we know what the peak is upstream we can more accurately predict the peak downstream. At this stage we can also predict if there is likely to be overtopping of defences or extreme flooding. If this is the case severe flood warnings will be issued to the public and command centres will be operating. The timing of flood waves is very significant where major tributaries join rivers. If flood waves meet at river confluences then levels can be much higher and this is difficult to predict.

During the flood

  3.24  During flooding we would continue to monitor the rivers and would be looking ahead up to 48 hours to try and predict if any further rainfall is likely to occur. Further rainfall will prolong flooding and could generate an extreme flood such as those in 1998, 2000 and 2007.

  3.25  If no further rainfall is predicted then we would be forecasting when river levels are likely to begin to fall and the process of clean up and recovery can begin.

3.26  Summary

—  We work closely with the Met Office to get the best available information to help us predict flooding but we cannot always get sufficiently precise location information sufficiently early.

—  The forecast of heavy rainfall does not necessarily imply that there will be flooding from rivers.

—  We do not know for certain how much rain is coming into the rivers until it has landed on the ground and we have measured it.

—  We are using all available tools and technology to help predict flooding but even this has its limits.

—  We continue to develop our network of river level gauges and rainfall gauges

—  We are still developing our capability to confidently predict river flooding to provide accurate flood warnings and operate our defences in a timely and efficient manner.

—  We currently use the output from models for local forecasters to add their experience and professional judgement to produce forecasts that decisions can be taken on to issue warnings and close flood gates, etc.

Environment Agency

November 2007