The old Cogan Hall farm development site is adjacent to the Cosmeston Country Park, which is a Site of Special Scientific Interest (SSSI), and includes a medieval village of national archaeological value. The park has a high recreational use by many who come to walk, visit the medieval village, and take part in water sports. The development site falls towards the Cogan Hall Brook, which runs between it and the park.

The Cogan Hall land owner intended to sell the site on the open market to a series of house builders. His drainage solution did not place restrictions on the form of development. It provided for a large-volume, underground, surface-water attenuation tank to handle discharge for the design rainfall event to the brook.

This lack of development restrictions limited the source control options and the opportunities to consider grey water reuse. Further, the tenders for the land were less than expected because of the high cost of the tank, so the land owner withdrew the site from sale. PB was appointed to provide an economic solution to the surface water drainage strategy for the site.

Sustainable Urban Drainage Systems

Sustainable urban drainage systems (SUDS) as known in the UK, or water sensitive urban design (WSUD) as known elsewhere, seek to achieve a balance among three factors:

• Quality: Ensure discharge is of a standard appropriate to the receiving water body.
• Quantity: Ensure the quantity and rate of surface water runoff can be maintained by existing hydraulic regime, including during peak flow and low flow.
• Amenity: Ensure an amenity benefit is provided at either the site or adjacent lands that may be affected by the discharge.

SUDS is the process of thinking about a drainage system in a way that considers the wider issues other than the immediate need to transfer runoff. There is nothing new about its implementation. SUDS is simply the formalization of the processes and technologies that have been readily available for some considerable time, yet the best efforts in this direction can be stifled by traditional and commercial approaches of public and other adopting and regulatory authorities and end users. The ability to create a win-win situation is the truly sustainable solution to the issues of sustainable surface water management.

Existing Site Hydrology

The UK’s Environment Agency (EA) gave the pre-development green), an amount that we field runoff as 5 liters/second/hectare (l/s/ha), or 0.05 gallons per second per acre(gal/s/acre considered to be low for this steeply sloping site with the recorded geology. Our infiltration tests showed the underlying limestone substrata to have no potential for infiltration; because the site was overlain by heavy clay that had filled the karst voids. With this information, we recalculated the green field run off at 13 l/s/ha (1.4 gal/s/ acre), resulting in a reduction in attenuation storage volume of 1,400 m3 (1,860 cubic yards) over the preceding design.

Traditional Solutions

We considered many forms and arrangements of multiple underground tanks, as this was the only form of attenuation feature the water company was prepared to accept for adoption as a public sewer. Although more cost effective due to the reduction in required volume, and improving the cash flow on the development, these still did not return the required land value and did not meet the three SUDS criteria.

The client had control over land to the east side of Cogan Hall Brook, so next we considered on- and off-line ponds for attenuation. These ponds were close to the site and provided the required volume at a lower cost than buried tanks. After negotiation this option was discounted, however, because the ponds were not eligible for adoption by a public maintaining body as they were an open water feature and did not meet standard criteria. Any drainage system connecting to these ponds would not have an adopted outfall and, therefore, not be eligible for adoption itself.

Opportunity for a SUDS Solution

One of the reasons Cosmeston County Park is an SSSI is the high presence of damsel flies, predatory insects similar to the dragon fly that deposit their eggs on the stems and leaves of water plants. The park is maintained by the Council. The council park warden advised that the flies lacked sufficient breeding areas. We saw this as an opportunity and suggested that a separate area for the fly would be of benefit to the park. The landowner had a smaller area of land adjacent to the country park some 600 m (2,000 feet) to the south of the development site. The area was overgrown and surrounded by trees and dense vegetation.

It was agreed that a pond constructed in this area could add value to the park by providing both a fly breeding habitat and attenuation and treatment of the runoff from the site. As this pond would form part of the country park, it could be adopted as a public asset and maintained by the public body.

Three methods of conveying the flow from the site to the pond were considered:

• A Swale. A swale would provide attenuation and water quality treatment of the runoff while on route to the pond. Topography dictated that an overland swale would have to pass through an area of known archaeological interest, however, and upon consultation with the controlling body for Welsh archaeology we learned that the swale would disturb this area. The swale also provided a physical disruption to agricultural land use. In addition, it would not be eligible for public adoption as it was not recognised in the water company’s design guide. The swale was discounted.
• The Brook. The use of Cogan Hall Brook was considered, with the flows being discharged directly into it. Flows in excess of the allowed discharge would be diverted out of the brook and into the pond by a flow control structure downstream. Again, adoption proved to be a difficulty and there were concerns over the effect of the increased flows in the transfer lengths of brook.
• Piped Connection. The adopted solution was to pipe the discharge to the pond. The piped connection was eligible for adoption by the water company as it complied with its design guides. As the Council had adopted the pond, the whole surface water system was adoptable and maintained by a public body.

The pond was designed to have a puddle clay liner with varying water depths and areas of reed bed planting. The variation in depths provided maintenance access with various habitats with water being retained at all times.

The solution addressed the three SUDS criteria:

• Quality of discharge was maintained by the processes of sedimentation, filtration, absorption of nutrients, microbial decomposition and vegetative uptake.
• Quantity and rate of the flow was maintained and with the lower cost of providing the storage volume, the pond was designed to have no detriment to the watercourse for the 1:1000 year event. The construction cost of the pond was some £5 million less that the traditional system proposed originally.
• Amenity value was provided by the reed bed, an additional undisturbed open water body that enhanced the value of the adjacent country park.

Conclusions

The Cogan Hall project is but one illustration of how SUDS systems can provide economic solutions to development needs. SUDS tools are not new; it is the thought process behind them and their application that is now to be considered on a site-by-site basis. Greater flexibility by adopting, regulatory and maintaining bodies would further increase the application of SUDS techniques.