Choosing a hard or soft option
Chris Williams of Hydro International looks at why hard engineered solutions should be considered as sustainable methods of solving both urban and rural drainage problems
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
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.
Event mean conc. (mg/I)
Unit load (kg/imp ha.yr)*
400-50,000 (6430) (MPN/100ml)
0.9-3.8 (2.1) (x106 counts/ha)
|*imp ha = impervious area measured in hectares
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
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
1. Sunday Times. 28 October 2001, Money Section.
2. Planning Policy Guideline 25 – Development and Flood Risk. (ISBN 0-11-753611-3)
3. New Civil Engineer – Assessing Future Changes in Extreme Precipitation over
Britain using Regional Climate Model Integrations. P D Jones & P A Reid,
(submitted to) International Journal of Climatology.
4. Ellis JB (1986) Pollution aspects of Urban Run Off in Urban Pollution Runoff,
NATO SI SeriesG: Ecological Sciences -Vol 10 Springer – Verlag, 1-3.