Step 1: figure out your outfall location.
Very briefly, the hierarchy of outfall is as follows: infiltration > watercourse > surface water sewer with outfall to a combined sewer being the lowest and least preferable option.
To check infiltration capability for your site use this map... landis.org.uk/soilscapes
If you have a free draining site, you will need to carry out infiltration tests (if you're doing this for planning you'll need to hire a company who can carry out infiltration testing in accordance with BRE365).
If you have "impeded drainage" (think, clay)… infiltration is not likely to be your outfall solution.
Next, check the watercourses/ditches in your area... you can use the surface water (pluvial) flood map for this check-long-term-flood-risk.service.gov.uk/map
This is a more likely solution for larger sites and greenfield development sites, where ditches for land drainage will already exist.
Lastly - for all sites, buy your sewer records from your wastewater company. Check who that is here... water.org.uk/advice-for-customers/find-your-supplier/
For extensions to existing property or new builds within an already developed area, you will most likely outfall to a surface water or combined sewer. Run-off rates for any new (or redeveloped) areas of impermeable hardstanding will require restriction so that flood risk is not increased to downstream properties. This is discussed in the next step.
Step 2: Figure out your runoff rates
For greenfield sites (this includes brownfield sites that have not had any development on them for 5 or more years) you can work out the greenfield runoff rate using this site here... uksuds.com/tools/greenfield-runoff-rate-estimation or you can use a standard rate of 2 l/s/ha.
For brownfield sites (that have an active, existing connection to an outfall), you existing runoff rates can be calculated using the rational method.
Q (flow in litres/second) = A (area in hectares) x 50 mm/hr x 2.78
50mm/hr is a standard rainfall intensity that approximates to the runoff received in a 100 year, 6-hour storm event. So for a site area of 350m2, that's 0.035 ha, would generate a flow rate of 4.865 l/s.
Depending on your site/situation, one of three things happens next.
You are outfalling to a public sewer who states that the minimum restricted runoff rate is 5 l/s (this is because it's approximately equal to the rate that goes through a minimum accepted orifice size of 100mm). So in the example, your runoff rate is less than this, so no restriction is required. This method is getting phased out as flood risk/SuDS become more relevant, but it's worth knowing, especially for small sites.
You have to provide 50% betterment on brownfield rates. So your design run-off rate is now 2.43 l/s and you can achieve this by installing an orifice chamber and providing stormwater storage (see next step).
The LLFA is determined that you should provide greenfield equivalent runoff rates (either 2 l/s/ha as standard or calculated based on your specific site). For this example, you'd get a runoff rate of 0.07 l/s which is tiny. The absolute minimum orifice size is 20mm (the 100mm minimum orifice size mentioned earlier was specific to what wastewater companies find acceptable). So based on your design, you will likely have to argue that whatever rate 20mm generates will be acceptable (the outflow rate will depend on your drainage design) but a minimum design flow rate of 1 l/s is a good rule of thumb to offer.
Step 3 - Figure out your stormwater volumes and provide SuDS
If you restrict runoff rates, you'll get a backlog of stormwater in a heavy rainfall event. To figure out these volumes you can use this link: uksuds.com/tools/surface-water-storage-volume-estimation
Using our previous example, for a site area of 350m2, let's assume 80% will be made up of impermeable area and the design rate is 1 l/s. I picked a random location in Cheshire East, leave the rest of the figures as default and the calculator returns a stormwater storage value of 8m3. So this is our design storage.
SuDS are sustainable drainage systems (previously called sustainable URBAN drainage systems but no-one has used that terminology in years so you can always tell whether someone is up-to-date with the current guidance by keeping an ear out for that).
In a nutshell, traditional drainage is massive pipes, tanks and concrete, put in deep (min 0.6m) underground, whereas SuDS is rainwater gardens, green roofs, permeable paving, swales, ponds, tanks (yes, we still like some tanks) and tend to be either above ground or shallow systems, which reduces dig effort and makes them easier to access for maintenance.
Using our example, we need to provide 8m3 of storage. This is easier to illustrate in a drawing but I'll try my best.
The example area is 350m2, let's say 100m2 is a car park. Let's make that car park permeable and put in a subbase storage layer of 300mm using Type 3 gravel which has minimum 30% voids. 100m2 x 0.3m x 0.3 = 9m3. Sufficient storage. That was too easy.
Let's say the car park can't be permeable for some unknown reason. 8m3 volume could be split across a green roof, which outfalls to a shallow rainwater garden which outfalls to a swale. Of course in an example like this we can make anything up that we want! The real fun happens on a real site, where constraints often limit what we can do, and our imagination has to create new ideas of how to best use SuDS to achieve our rainwater goals.
So that's it... how to write your own drainage strategy in 3 easy steps. We covered the basics, and we didn't cover the other three considerations of a good SuDS design which is water quality, biodiversity and amenity. We also didn't cover interception design (and the first 5mm). That will have to be a topic for a future post.
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