Daniel Alstead, associate director of Hydrology at Enzygo, talks about the importance of getting your site drainage right and what to look out for when doing the planning for new site developments.
What do you do at Enzygo?
Enzygo Ltd are an independent, multi-disciplinary environmental consultancy with a proven track record of delivering creative, integrated and cost-effective solutions that maximise the potential of development sites. We use a collaborative approach to devise solutions that that are respectful of technical and budgetary constraints, comply with industry guidance, whilst also fulfilling planning and commercial aspirations.
We have substantial experience in the delivery of all types of developments, including major infrastructure projects, residential housing schemes and renewable energy programs, across a broad range of environmental disciplines, including Planning, Hydrology and Drainage, Permitting and Regulation, Landscape, Ecology, Transport, Geo-Environmental and Hydrogeology, Noise and Vibration, and Arboriculture.
More detail required at outline stage
Over recent years there has been a notable increase in the level of details required at outline stage, whether that includes an allowance for long-term storage and urban creep, or consideration for maintenance/management and exceedance routes. The Lead Local Flood Authorities are increasing their scrutiny of drainage designs and picking up on incorrect inputs into drainage calculations.
Whilst rainfall values are unlikely to have much spatial change within the boundaries of your site, underlying soils and geology can have more localised variations and affect input values, both of which can be critical in design a robust surface water drainage strategy.
How can getting it wrong impact on your site
With current industry standard software packages and online calculation tools it is all too easy to click on a map, export your descripts, and run your calculations without a second thought. This approach can have significant impacts on your surface water drainage strategy where one minute you have a site underlain by presumably moderate permeable sandstone, however onsite soakaway testing identified low permeability or perhaps the presence of shallow groundwater.
Using incorrect descriptions will affect the calculated runoff rates and required attenuation volumes. The knock-on effect is a considerably larger or even smalls SuDS feature and the loss or gain of developable area. Not good if your client ends up needing to go through a re-design when a detention basin eats into the developable area or not losing units at reserved matters!
My advice is to undertake a thorough desktop study, consultation with the water regulators, and where possible undertake soakaway testing before developing a drainage strategy. This allows an understanding of the working constraints through a knowledge of local policy and an understanding of onsite baseline conditions. Although this may appear to be more upfront work, it can have significant benefits for the project in terms of timescales and reducing costly iterations.
What advice would you give?
- Initial descriptors kicked-out by software and packages and online calculation tools can give incorrect results. Soils soils and geology mapping, as well as nearby borehole records should be treated as indicative. There is no substitute for known onsite conditions.
- Make sure your drainage engineer is using a justifiable parameter so that your drainage calculations are representative
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