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

Fig 1: The problem- downstream flooding caused by excessive flows

Fig 1: The problem- downstream flooding caused by excessive flows

Fig 2: The conventional soultion-relief sewer to pass flooding downstream
Fig 2: Individual plot storage of house run-off
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.


Table 1
Quality parameter
Event mean conc. (mg/I)
Unit load (kg/imp ha.yr)*
Suspended solids
BOD
COD
Ammoniacal nitrogen
Total nitrogen
Total phosphourus
Total lead
Total zinc
Hydrocarbons
Faecal coliforms

21-2582 (190)
7-22 (11)
20-365 (85)
0.2-4.6 (1.45)
0.4-20.0 (3.2)
0.02-4.3 (0.34)
0.01-3.1 (0.21)
0.01-3.68 (0.3)
0.09-2.8 (0.4)
400-50,000 (6430) (MPN/100ml)

347-2340 (487)
35-172 (59)
22-172 (358)
1.2-25.1 (1.76)
0.9-24.2 (9.0)
0.5-4.9 (1.8)
0.09-1.91 (0.83)
0.21-2.68 (1.15)
-
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 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.
2. Planning Policy Guideline 25 - Development and Flood Risk. (ISBN 0-11-753611-3) Stationery Office.
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.


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