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Article for 'Civil
Engineering Surveyor'
September 2003
Stephen Winpenny BSc CEng FICE FConsE, Fenland Hydrotech
PPG25 and Flood Risk Assessment
PPG25 is here to stay, and is starting to have a major impact on the development industry. Flooding is now explicitly considered as a part of the planning process, and flooding is a material planning consideration. The Environment Agency is responsible for advising the planning authorities on flooding. The planners do not have to take the advice they are given, but usually they do.
The principles are simple, and obviously sensible:
Sounds easy. The devil, as always, is in the detail.
PPG25 says that most new development should not take place in areas with a risk of flooding from rivers of more than once in 100 years, or from coastal flooding of more than once in 200 years. There are exceptions for essential infrastructure, brownfield sites and areas where nowhere else is available, but for most of the developments most of us are involved with, these guidelines will apply.
This means that a risk assessment has to be carried out to consider the probability of a flood occurring which would affect the development site.
The starting point is the indicative flood plain maps published by the Environment
Agency, which are available on the internet. If your site is anywhere near
the indicative flood plain, the EA will oppose any planning application without
a flood risk assessment and the application will probably be turned down.
The flood plain maps are only indicative. They vary in reliability and are
little help in identifying areas at risk from flash floods. The risk of flooding
from sewers does not fall within PPG25 and is not included in the flood risk
maps.
It is not up to the EA or the planners to show that your site will not flood, it is up to you. This may be easy. One of our projects was for a site near the river, but 5 metres above the highest ever recorded water level. The planning application was refused until we prepared a flood risk assessment which showed there was minimal risk to the site.
Usually, the problem is defining the level the water will reach in a 1 in 100 or 1 in 200 year flood event. Computer-based digital hydraulic models have been built for some of the major rivers, and extreme floods are generally recorded, but for most of the country, no-one knows the predicted flood level with any degree of accuracy.
The techniques for predicting inland flood levels have developed enormously
in recent years, and specialist consultancies like ours have an array of tools
available. This involves predicting the critical storm duration and intensity
for the site under study, and simulating the response of the watercourse to
the storm event.
It is possible to predict the rise and fall of water levels in the river as
the storm develops, and to allow for storage of the flood peak in the floodplain
as the river bursts its banks. This is carried out for a range of flood event
return periods. The response of the site drainage system to various storm
events is also simulated.
This kind of mathematical modelling can be expensive, particularly if large
areas have to be modelled, but may be the only way to demonstrate that the
site can be developed at all.
The modelling considers theoretical flood events of 100 or 200 year return
periods, and must also make an allowance for increased flows due to global
warming. This can create enormous flows in streams which are little more than
ditches in normal conditions, and can show areas with no recorded history
of flooding disappearing under water. Development within this predicted flood
boundary will normally be opposed
Avoiding building in flood risk areas is only part of the requirements of
PPG25. The second part is to avoid increasing flood risk elsewhere.
Developments in the flood plain can reduce flood storage, or obstruct flow along the flood plain. The water displaced from the lost storage must go somewhere, making someone else's flooding worse. There is no point in arguing that only a small volume has been lost, as dozens of separate developments in a catchment, each with a small loss of flood storage, will add up to a significant impact. Any loss of flood plain will be opposed.
PPG25 does not only apply to the flood plain. Any development anywhere in the catchment will have an impact on the watercourses downstream.
Very little of the rainwater falling on the rural landscape ends up in the rivers. Most is retained by the vegetation, and evaporates directly or through evapo-transpiration, or soaks into the ground to be released over months or years as base flow in the streams. The rainwater which does reach the rivers does so slowly, taking hours or days.
Developing a site, with hard paved areas and roofs drained by a piped network, prevents this natural loss of rainwater and increases both the rate of runoff, from hours to minutes, and the total volume of runoff, possibly by four or five times. Changing land use from rural to urban increases flood risk.
PPG25 suggests the use of sustainable drainage systems (SUDS) which mimic the natural processes of recycling rainwater to the air and the ground. The remaining water is stored close to the source and released slowly to reduce the peak flow rates. These systems have been implemented in Scotland, but are largely ignored in England.
Most property developers offer the drainage systems to the sewerage undertaker
for adoption. SUDS systems involve swales, filter drains and ponds, none of
which will be adopted by the sewerage undertakers.
They will adopt pipes. Developers are putting in oversized pipes as tank sewers
with outlet flow controls to reduce the outflow to the pre-development rates.
This is controlling the rate of outflow from each development, but doing nothing
to control the increased volume of water entering the river systems as the
catchment is developed.
The tank sewers are emptying over 12-24 hours, which is less than the response rate of a river basin. Some watercourses are benefiting from a reduction in flow rates, but the overall impact of these flow balancing devices on the whole catchment is less certain.
These tank sewers are big. Storage volumes and be hundreds of cubic metres and pipes can easily be 1.8m dia. Large sums of money are being poured into holes in the ground, yet increasing urbanisation is increasing the volume of water reaching our rivers.
The Construction (Design and Management) Regulations (CDM) require us to design out hazards at source wherever possible. We are building large underground chambers which do not achieve self cleansing velocities and will silt up in time, requiring maintenance in a hazardous confined space. These hazardous working conditions would not arise with ponds and shallow open channels, which would also help to reduce the volume of water reaching the rivers.
Flooding has blighted the lives of thousands. As engineers in society, should we be asking ourselves if the present policies are the most effective means of reducing the threat?
The objectives of PPG25 are excellent. No-one can argue that we should be building houses that may flood, or cause their neighbours to flood. The implementation is a cause for concern.
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