By Partner, Dimitris Linardatos.
A few years ago, Price & Myers were appointed to undertake the Flood Risk Assessment and the Drainage Feasibility Study for Dunning’s Mill, a new residential development in East Grinstead.
Last Saturday, I took the opportunity to revisit Dunning’s Mill to see how the site looks now that it is fully occupied, and the landscaping is well established. The development includes 12 houses with an associated access road which is located to the east of an unnamed stream. See image below for the site layout.
We have allocated a very large space between the access road and the stream for flood water storage during extreme storm events. We produced a hydraulic model of the stream, and the results were used to set the proposed site levels, creating an amenity space for the site users that will occasionally be utilised to store and convey water from the stream. The ground levels at the location of the proposed buildings have been raised to ensure that flood water will not reach them during extreme storm events. The impact of climate change on flood water levels was also taken into consideration in our design.
The photos below show the site’s open space before (left) and after (right) construction.
During our first site visit, we discovered a surface water drainage pipe that was left to flow freely into the field. Southern Water’s records show that this pipe should have been extended to connect to the stream. We used this defect effectively by creating a water feature that blends well with the site’s landscaping and provides biodiversity benefits.
Photos below show the uncontrolled surface water flow pre-development (left), and the new water feature (right).
Last but not least, below is a photo of the permeable paving system that forms the access road. The purpose of the permeable paving is to collect, store and treat surface water from the new roofs and impermeable areas before releasing it into the stream.
Water brings threats. The stream was responsible for flooding the Dunning’s Mill pub and its surrounding area in the past. Tree branches and debris blocked the culvert that runs under the Dunning’s Mill pub forcing flood water to overtop the banks of the stream and flood the local area. The historical flood incident was the main reason for the lengthy planning consultation period for the proposed development. Our hydraulic model was simulated for various scenarios including for blockages in the culverts and the records from the flood incident were used to evaluate the accuracy of our results.
Planning requirements for the provision of maintenance and management plans for new developments also helps in flood management. Frequent inspection and maintenance of the watercourse will prevent the accumulation of debris in the culverts and reduce the risk of flooding.
This development would have been denser than it is if the stream was not there, providing limited amenity spaces and little to no biodiversity benefits. As long as the threats that water brings can be evaluated and managed effectively, water will also bring opportunities for a sustainable development.
