Supplementary memorandum submitted by
the Environment Agency (FL 121a)
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
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.
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.
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.
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
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
PavingThe 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
2.2 The report puts forward a number of
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 vegetationpaving, 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
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
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
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 happensso
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 informationwe have
an estimated 1200 number of gauges in England and Wales recording
A network of river level and flow
recordersthese 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 forecastingmost
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 modelsthese predict
tide levels in estuaries and specific coastal lengths.
What is the process for forecasting flooding from
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
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
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 floodingRiver 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.
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
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
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.