        |
|
 |
 |
 |
 |
When the Rains Drain
Mainly to the Plain…
Stephen Winpenny BSc CEng FICE FConsE, Fenland Hydrotech
What are rivers for? Fishing, canoeing, swimming, picnics on the banks, wildlife habitats… Yet the most important use of our rivers is what Nature originally created them for - as a vital network of drainage channels that carry rainwater away to the sea.
Recently, as urban development has expanded, engineers have devised more efficient drainage systems to remove rainwater from them and pipe it into rivers. Yet it is only comparatively recently that we have begun to look seriously at our rivers' capacity to handle the increasing volumes and rates of water that are being channelled into them.
Flooding is already a serious problem, and could get worse as more and more land is turned over to urban development. DEFRA stated recently that ' …nearly two million properties are located in flood plains in England and Wales… 25 people have died as a result of floods and many more suffered trauma and devastating damage to homes and valuables.' Apart from damage to property, flooding causes many detrimental effects including soil erosion, destruction of wildlife habitats, traffic delays, and damage to bridge foundations. Financial losses due to floods commonly amount to millions of pounds every year.
Urbanisation encourages flooding
When it rains, some of the water is retained in the soil, some is absorbed by vegetation, some evaporates, and the remainder - called runoff - reaches stream channels and rivers. Flooding occurs when a river cannot remove or store the volume and rate of runoff coming into it. Periodic floods occur naturally on many rivers, and areas where flooding is endemic are known as flood plains.
In a rural landscape, much rainfall soaks into the ground or evaporates. Perhaps only 15-30 per cent of the water ends up in rivers, and this may be spread over a period of days. However, urbanised areas produce a much greater problem. Piped drainage systems remove rainfall from roofs, roads and hard-standings in minutes rather than hours or days. Losses by evaporation or soakage are minimal, and some 85 per cent of the rainfall in an urban area will quickly arrive in rivers and watercourses.
Locating an urban development in a flood plain makes things even worse. Not only is more water being collected and piped to rivers, but the transfer of rural land to development may reduce the area available for natural flood water storage. Flood defence works can reduce the problem locally, but may have the effect of creating greater flooding elsewhere.
What can be done to minimise flooding, especially in urban areas?
As no one has devised ways of controlling what falls out of the clouds, we are left with three areas to work on:
(1) Development siting and planning;
(2) Sustainable drainage systems (SDS), to minimise volumes and rates of run-off arriving in rivers;
(3) River engineering, to maximise the efficient removal of run-off to the sea.
Development siting
Last year the government published PPG25 (Planning Policy Guidance) for planning authorities. This states that the susceptibility of land to flooding must be a material planning consideration. It requires assessments for proposed developments to consider flood risk and whether they would increase flood risk elsewhere. Some planning applications have already been rejected because such assessments have not been carried out. PPG25 calls for a Sequential Test in which developments can be categorised into one of three 'flood zones' - little or no risk, low to medium risk, and high risk. Local planners are urged to give priority to sites within the lower risk zones.
Detailed flood risk assessments are complex, and include factors such as the effects of climate change, degree of liability to flooding, and likely flood depths and rates of flow. Although flood risk maps are available on the Internet, they are only indicative. They show overall areas that may be subject to flooding, but are little use for specific sites, and may not take local topography into account. Any developer needing to prepare a detailed assessment would be well advised to seek professional help from consultants with specialised knowledge and experience. For example, further information about PPG25, and a useful summary of the guidelines, is available from specialist consulting engineers Fenland Hydrotech via the Internet, www.fenhydro.co.uk/PPG25
In some areas the local Environment Agency (EA) has produced policy and guidance documents for proposed developments in specific areas liable to flooding. One example is the Nene Valley area, following serious flooding around Northampton in 1998. Its principle conclusions are that additional flood volumes cannot be accepted in Northampton; completed developments must discharge at lower flow rates and lower total flow volumes; and any development in the catchment should contribute to the reduction of flood risk for storms up to the '1 in 200 year' event.
Developers must identify, implement and cover the costs of any necessary measures, including undertaking flood risk assessments and, possibly, detailed surveys and modelling of river reaches and other watercourses. Developers will be expected to contribute to the cost of using and enhancing hydrological models of the catchment. A summary of the Nene Valley guidance document is also available on the Internet, www.fenhydro.co.uk/northampton.
Sustainable Drainage Systems
Flooding occurs when rivers cannot remove or store the volume and rate of runoff coming into them. A method of control attracting increasing attention nowadays is Sustainable Drainage Systems, or SUDS. The main aim of SUDS is to control run-off as close to the source as possible, using natural processes of attenuation to decrease and delay the volume and rate of water arriving in rivers. SUDS does this by storage systems, and by encouraging as much water as possible to drain into the ground and/or evaporate before reaching the rivers.
There are a number of techniques, some old, some relatively new. One of the oldest is the soakaway - a gravelled area in which water collects and gradually soaks away into the ground. Storage in lakes and reservoirs provides some evaporation, and controls the rate of discharge into rivers. In some areas such as the Fens, networks of ditches are also used to slow rates of water discharging to rivers.
We are seeing increasing use of what Americans call 'swales' - shallow depressions, often found in places such as golf courses. Swales normally resemble dried-up ponds but, at times of flooding, they fill with run-off water to delay discharge into a river. Another recent technique is porous pavements, used in large paved areas such as car parks and estates. The surface is of concrete or tarmac, but using a different mix to provide permeability so that water can drain through into the ground. On areas where natural drainage is difficult, such as heavy clay soils, perforated pipes may be located just under the porous surface to channel some of the water away.
SUDS must be designed, built and maintained in the context of the development control system, and developers must arrange for their systems to be adopted by a statutory body - usually the local drainage authority. Unfortunately there is currently little agreement between the various statutory authorities. Adopting authorities often oppose open ponds, there is little agreement on the use of porous pavements, and many householders are reluctant to accept the cost of long-term maintenance of structures away from their own properties. SUDS' potential benefits of reductions in flooding cannot become fully available until these matters have been resolved.
Computerised hydraulic modelling is available for most existing and proposed drainage systems. Modern software packages such as Hydroworks allow engineers to drain electronic storm water through electronic sewers to predict how it will behave before anything is built.
River engineering and management
Very few of our rivers - particularly those in urban environments - are what might be described as 'natural'. Alignments, levels and channel profiles have been artificially altered; many rivers are managed and their levels controlled as far as possible. Look at the river in the photograph; weir, bridge and banks are all artificial, and the level in the river is managed to benefit the amenities of the nearby town.
Basic methods of flood control include reforestation and the construction of levees, dams, reservoirs and floodways - artificial channels that divert floodwater. If surplus water can be stored across the catchment area in manageable quantities, minimal inconvenience results. However, there is a limit to what can effectively be done to increase a river's flood water handling capacities. Gradients are difficult to change over any appreciable distance, and dredging out a channel to increase its depth will result in siltation during normal flows. Measures to increase capacity on one stretch will move floodwater downstream faster, making matters worse elsewhere.
Hydraulic modelling systems are now helping engineers to understand why rivers behave as they do, and the effect of possible modifications. There are two methods of modelling; by building a physical model and introducing water into it at measured volumes and rates, or by computer modelling. Computer models are built on one of the powerful systems now used by hydraulic engineers, incorporating factors such as river levels, widths, depths, soil types, and cross-sections of the river at various points. The modellers can then introduce varying volumes and rates of run-off, so that the behaviour of the river in various conditions can be studied and assessments made of its capacity to handle volumes and rates of run-off.
Flooding is not a simple problem with one simple answer. Its primary cause, rainfall, is uncontrollable. We can only minimise the effects, and it is vital that developers, planners, engineers and statutory authorities take a holistic approach, looking at each case equally in terms of development siting, drainage techniques and river management.
        |