I am trying to calculate the volume of surface water runoff from an impermeable area. Can anyone recommend where I can obtain design rainfall data and what storm duation / return period should be assumed.

Thanks in advance

Met office can supply weather data – in the construction industry we usually use a ‘1 in 10 year’ event as a measure of whether something is a compensation event or not…..the reports can be pretty tailored to do what you want but I think most sewers are designed to cope with a 1 in 100 year event

Return period and duration will depend on what you need the volume for, and what are you going to do with it.

You’d generally get design rainfall from maps contained within the wallingford procedure.

to what end do you need the runoff?

for an approximation use 50mm/hr rainfall as recommended in building regs and Sewers for Adoption 6.

alternatively you need to do some maths based on hydrological intensity maps, rainfall rates and stom durations. there is some guidance in the building regulations.

a more expensive option is to buy modelling software (like microdrainage) that includes the rainfall data and maps and can do the above calcs in minutes rather than days.

if you are desining a housing estate or similar use microdrainage. if you are lookng at a new garage use 50mm/hr.

regarding return periods this again varies. the EA prefer 1 in 100 + 20% for climate change. Building regs is generaly 1 in 30 yr.

cheers,

mark

You can use 50mm/hr for a flow rate, but it won’t give you a volume. For that you’ll need the rainfall depth for the particular storm and duration you’re interested in, which you can derive from the M5-60 and r maps within the Wallingford procedure.

The Building Regs won’t really help – they just give you rainfall intensities (l/s/m2). What you need is rainfall depth, as above.

So, do you need volume or flow rate – you originally said volume? If you need an approximate flow rate, then 50mm/hr (or 0.014 l/s/m2) can be assumed, as per para 2.4 of Part H of the Building Regs.

Thanks for the advice chaps.

It is volume I require. I think I have the bones of the Wallingford procedure in the BRE DIGEST 365 ‘Soakaway design’, which gives me a range of volumes for various storm period for both M5 and M10 storms. At the moment the runoff discharges to an existing resevoir / pond and is pumped ad hoc by the existing site owner once the reservoir overflows to both existing foul and controlled water courses (dont ask!!). Trying to advise potential site purchaser on likely volumes we may be looking to discharge, so need a typical / worst case M10 period of storm.

Drainage is generally (in accordance with BS EN 752) designed for the 1 in 2 year storm, with a flooding check on the 30 year storm. Would the 30 year storm be more appropriate?

The volume will be dependant on duration – longer duration storms have greater rainfall depth and therefore volume. The MRM in the Wallingford Procedure has graphs to give you rainfall depth for the 5 minute to 48 hour storms. Your worst case will be your 48 hour storm, as this will have the greatest rainfall depth.

Can you reduce the volume going to the pond by using infiltration?

Current design parameters for sewers is based on 1 in 30 year storm conditions.

does the pond have any potential to soakaway? anyway you need to know at what level the pumps stop emptying the pond, or are you saying the pond just fills up then whatever overflows is pumped away?

not sure a volume of discharge is what you need, the stakeholders of the watercourse/combined sewer (i’m guessing it’s combined and not foul) will what to know the flow rate not a volume, also the pump capacity will be based on flow rate not volume.

for a fee i can look into it for you in detail 😉

You still working James?

yeah, still got a job for now