Surface water is instrumental to every planning application.
Pluvial or surface water is now a key component in any planning application, whether the site is located within a flood zone or not.
The surface water solution needs to be addressed at the outset and it should be the surface water scheme that drives the layout rather than the layout driving the surface water solution.
If your site is going to increase the impermeable area then there will be an increase in the rate at which the surface water will travel across and leave the site (guttering, downpipes, underground pipework etc) potentially increasing flood risk off site.
Case Study 1
We were appointed to undertake a surface water assessment for a client wanting to build a 2nd house in their garden. The pre-existing conditions (a garden) enabled the rainwater to freely infiltrate across the whole surface of the lawn and the flower borders. Post development the impermeable area increased by 150m2 - which included the footprint of the building, parking and the access drive. We undertook percolation testing in the garden to calculate the infiltration rates and designed a surface water soakaway which was compliant with the Local Authority and Building Control.
Soakaways or other infiltration methods are the preferred method of disposing of surface water and should be considered and discounted first.
Soakaway sizing is undertaken using our Micro Drainage Hydraulic Modelling Software which is the industry recognised software, for undertaking soakaway calculations.
Case Study 2
We were appointed to undertake a surface water design for a development of 14 houses. The pre-existing conditions were a yard and storage area which was already generating some surface water runoff, which was included in our assessment. Soil infiltration, in accordance with CIRIA 156, was undertaken and the soakaways were designed in accordance with the 1 in 100 year return event plus 30% for climate change. Underground pipework was then designed to convey the rainwater to indivdual soakaways including going underneath some of the properties which was signed off by Building Control. The soakaways could not be positioned at a depth greater than 1.8m due to the location of an impermeable clay layer identified on site during the site investigations.