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SuDS: Understanding the Cost Benefits
A recent study commissioned by DEFRA provided an evidence base for drainage design and operation costs. A WSP-led consortium compared New Build Sewers Under Different Conditions and Standards and Increasing the Uptake of Sustainable Drainage Systems in New Developments.
The aim was to provide costs associated with the construction and operation of drainage systems for three typical residential developments based on the three standards; the Building Regulations, Sewers for Adoption and the new SuDS Standards.
Three pilot sites were assessed, based on real locations, comprising 8 dwellings (small site), 32 dwellings (medium site) and 210 dwellings (large site).
Key Survey Findings
- SuDS provide greater cost benefits on larger sites, less for small sites.
- SuDS have a very great advantage on permeable catchments where rainfall runoff can be infiltrated compared to the drainage designs based on the other Standards.
- On “impermeable” pilot sites, where some degree of infiltration was available for very small rainfall events, permeable pavements were advantageous over the other Standards.
- Permeable pavements avoid large land take issues related to the use of vegetative systems.
- SuDS have little cost advantage for medium and large sites where ground conditions require lining (high groundwater, contaminated soils, soil strength), and small sites are likely to be significantly more expensive. This is primarily due to the additional SuDS components that are needed to meet design criteria on both interception and treatment.
- Infiltration trenches and rainwater garden features are the least cost effective for meeting Interception criterion in addressing runoff from roofs if permeable pavements are lined.
- Sites that have slightly larger rainfall storage requirements will tend to favour SuDS schemes – more so in the South West and North East of the country, less so in the South East
- SuDS work well for flat sites, but on steep sites they tend to constrain site layout and may cost more than drainage schemes designed using the other Standards.
- The approach to SuDS design is a major factor and has a large impact on the cost and affordability of the SuDS scheme.
WSP estimated annual average maintenance costs of 0.5% of capital costs of drainage construction. This indicates a whole life cost (WLC) approach to SuDS would focus on capital costs.
This figure is an average and will be more for situations when a liner is needed (to protect the groundwater or due to soil contamination), or when the soil is totally impermeable such as with blue London clay.
WLC is a troublesome calculation as there are 3 different sets of stakeholders – each with their own cost interest;
- the capital cost is incurred by the developer
- SuDS in the public realm will be owned by the Local Authority
- SuDS in private property will be owned by these individuals.
There is generally little difference in maintenance costs between traditional drainage and the use of SuDS, though this is dependent on the type of SuDS used. The maintenance frequency for de-silting structures is a key determinant.
Although permeable pavements are becoming common-place for infiltration close to properties, not all Local Authorities have been prepared to adopt them or use them in certain road categories.
Case Study – Elvetham Park, Hampshire
Elvetham Heath storage pond – Image courtesy of Susdrain
This estate was completed in 2008 across 63ha offering 1,868 housing units and ancillary services including a school, village centre, retail outlets and sports pitches.
Elvetham Heath storage pond – Image courtesy of Susdrain
The primary drainage strategy was to use soakaway systems on higher ground (30% of the land area), therefore deeper groundwater storage capacity. For the remainder, Swales and linear ponds were used on the flattest parts of the site, while detention basins were scattered throughout the site to provide some attenuation. Stormwater pipework was used to connect the majority of residential and commercial units to a retention pond upstream from a nature reserve.
The SuDS management train works by runoff discharging into the swales, detention basins and then into a retention pond which forms part of a public park. It functions to a 50 year return period Greenfield rate of 7 litres per second per hectare, with no flooding within a 30 year return period.
Since the estate was completed, analysis by Susdrain has shown a 10% increase in property value for those properties in the vicinity of the SuDS components.
This response demonstrates the benefit to both residents but also to developers in terms of the marketing message to selling properties closest to these features.
From a commercial perspective, this helps to add either a premium to first sale prices and to prospective buyers on the capital growth benefit. This sale premium can also help mitigate the initial and maintenance costs of the SuDS.
In this case, the developer also benefited from Thames Water adopting a third of the SuDS scheme. They also had to pay Hart District Council £500,000 for the maintenance of vegetative aspects of the scheme.
Fitting the right SuDS to the Site
SuDS have a strong cost benefit across larger developments in areas where infiltration is most suitable, but there are other low-cost options in smaller schemes. Understanding the site, its drainage profile and capacity and the appropriate SuDS scheme design are therefore critical as part of the master planning response to Local Authority requirements.
GeoSmart’s highly experienced consultants can develop a bespoke combination of contaminated land risk assessment, flood risk assessment and a drainage strategy for the Site. This will inform the best land management strategy to discharge planning conditions on contaminated land, flooding and drainage.
We are also able to offer more detailed run off calculations and offer preliminary SuDS designs based on the site’s unique characteristics.
For more information, call us on 01743 298 100 or email email@example.com