Choosing a hard or soft option

The flooding events of recent times continue to increase both in terms of frequency

and severity. They have heightened public awareness on the issues of urban drainage

with recent broadsheet newspaper articles(1) considering the impact on insurance

companies and their likely response on providing cover to properties located

within flood risk regions – the autumn/winter floods of 2000 caused the flooding

of over 10,000 properties.

Conventional v SUDS

What can be regarded as a conventional solution to the urban drainage problem

of flooding? The growth of urbanisation took place with the migration of people

from rural regions to urban/suburban regions. This migration resulted in the

need to build more houses and provide a better infrastructure (more roads) all

contributing to the increase in impermeability and the resultant increase in

storm response times or peakedness of the storm hydrograph. Engineers tasked

with resolving the urban drainage problem tended to build bigger relief sewers

to run parallel with the existing overloaded sewer (Figures 1 and 2) involving

heavy civil engineering construction and its associated disruption.

During periods of heavy rainfall the relief sewer would transfer excess flows

from one point of the catchment to a downstream location, which would hopefully

have sufficient capacity to accept the increased volume. This curative approach

attempted to handle the quantity of rainfall resulting from a standard design

return period storm (RPS). Analysis suggests that what was once a 1 in 10 RPS

may well soon be considered a 1 in 2 RPS.

An assessment of future changes in extreme precipitation over Britain indicate

an increase in magnitude under enhanced greenhouse conditions(³). Hence, using

the conventional approach alone to solving flooding problems cannot be considered

sustainable.

Hard Solutions

In one of it’s simplest forms, source control starts with disconnecting downspouts.

In many circumstances this is not practical, however, this does not preclude

the use of other source control techniques. A recent example of using source

control at the uppermost point in a drainage system was the use of vortex flow

controls on the roof of Lakeside shopping centre in Thurrock. More than 70 vortex

flow controls were installed in the flat roof rainwater outlets at the top of

the down pipes. The solution retains excess flows during heavy rainfall and

allows discharge to the underground system at a controlled rate to prevent flooding.

Another example of using a hard engineered solution in a sustainable way is

demonstrated by the application of geoplastic matrix used at the Grantham Garden

Centre in Cambridge. Here the run-off is collected in an underground detention

basin constructed from a geoplastic matrix, with a high structural integrity

capable of withstanding vehicular loads and a 95% void ratio. From the detention

basin the run-off is returned to a pH control facility before it is reused for

irrigation and watering of the plants and vegetation at the centre. The system

provided the benefits of a localised storage facility without the need and costs

of heavy civil engineering construction.

A third example is the 1.1Ha housing development in Hertfordshire, known as

Churchfield Nurseries, which used the concept of individual plot storage to

satisfy the run off restrictions of the site while adopting a sustainable design.

The house run-off remains private whereas the highway run off has been adopted.

Figure 3 shows pictorially how the site’s stormwater drainage is configured.

Flow quantity continues to be the area on which we are focussing much of our

attention. However, increasing recognition is being given to the water quality

aspect of storm run-off. For example, it is recognised that high-way run-off

is a major contributor to pollution. Table 1 highlights the variability of storm

water quality and shows typical values and ranges of pollutant discharges from

stormwater systems in the UK (Ellis, 1986)(⁴). The table shows that

a sizeable portion of pollutant load derives from suspended solids. Other pollutants

are also attached to the suspended solids fraction. In the US this is already

recognised in Federal statute with the implementation of the Phase 2 Stormwater

Regulations.

Under the permit system of the phase 2 regulations all new developments greater

than 1 acre require control of polluted stormwater. Most states have incorporated

this federal requirement as an 80% net annual total suspended solids (TSS) removal,

although some also specifically target phosphorus and occasionally nitrogen.

The regulations largely target non point source pollution from urban areas where

land is often scarce and expensive. In Europe, the introduction of the European

Union (EU) Water Framework Directive, which requires all waters within member

states to achieve good status by 2015, also adds weight to the drive for a water

quality focus in the UK.

A wider acceptance of SUDS and the benefits of source control techniques would

prevail if certain hard engineered solutions were also considered as a sustainable

method of solving both urban and rural drainage problems. There is no one-size

fits all and both soft and hard solutions should be considered as a means to

meeting the sustainable objectives. There already exists a solid track record

for many hard solutions incorporating a sustainable approach. It is suggested

that sediment control and removal remains an area which can be readily addressed

using innovative technologies to improve water quality as a result of stormwater

run-off

References

1. Sunday Times. 28 October 2001, Money Section.