A solution (short-term fix perhaps) to prevent internal flooding used by some water companies, where the flooding mechanism is flow backing up from the surcharged main sewer up the property lateral and out from the property low point, can simply be a non return valve. This prevents flow from backing into the property (although flooding will of course appear elsewhere). However, when the lateral pipe connection is full, the residents of the associated property will then not be able to flush wastewater down their property sewer, and then it too will build up and flood, in particular during long duration storms. Therefore, a small amount of storage which takes flows from the impacted property only and then pumps it back into the system is required. This has been termed an Alternative Pumped Non Return Valve or can be referred to as localised pumped return storage.
Ascertaining the Required Storage Volume
To size the required storage we need to ascertain which storm (assuming we are using design storms) causes the greatest storage requirement, so we need to do the following for all storms we are assessing.
- Ascertain the surface water run-off flowrate from the property area.
- Ascertain the Dry Weather flowrate from the household.
- Decide upon a pump rate (1 l/s (0.001m3/s) is suitable for small areas) and subtract that rate from the flowing coming in.
Property Area Surface Water Run-off
Runoff = Property Area * rainfall intensity * runoff coefficient
Rainfall intensity can be taken from the simulation and should be presented in 1 minute intervals. It is usually provided in mm/hr so will need to be divided by 3600 (seconds in an hour) and also divide by 1000 (mm in an m).
This value will vary during the rainfall event as the rainfall intensity varies, so if your rainfall is divided into minute intervals, this value will change every minute.
Household Dry Weather Flowrate
DWF = population * usage
To convert from l/d to m3/s divide by (1000 (litres in a m3) x 86400 – (seconds in a day))
Subtract the pump-rate from the incoming flow
For each recorded rainfall timestep there will be an incoming flow value. Where this is greater than the pump rate, it becomes stored in the storage volume. We then add all the stored volumes throughout the storm which are grouped together. This gives us our required storage volume.
Confused? – we have a spreadsheet. It should clear things up a bit. And remember, this solution might not be suitable for your, so discuss it with your client before considering modelling it, and make sure you understand it’s limitations.
Lastly, we do this process for all design storms to ascertain which one causes the greatest storage requirement.
Things to consider…
- Can we even start pumping into the system while the sewer is surcharged? It may be possible to do this depending on how surcharged the main pipe is when the lateral is flooding. However, it may be more conservative and a better solution just to start pumping after the storm has completed subsided. However, in this scenario we would have excessively large storage during longer design storms? Could we us RTC? Then it’s getting overly complex perhaps….
- If there’s a lot of area surrounding the property which drains into the same lateral pipe we might need bigger storage or a higher pumping rate but what is feasible from the point of construction space and also flow limitations of the receiving pipe?
- Is it necessary to model the device? We know the required flow rate so perhaps it might not be necessary, however it may be beneficial for future modellers to understand what’s been done.