Discharge of land drainage

[J0hnny]

Dear All,

It's a common problem regarding the discharge of land drainage (from retaining walls) in areas where soakaways are not suitable. Obviously local authorities do not accept ground water/ land drainage discharges into their systems so how can we deal with these flows and volumes? is the correct way via drainage fields to re-charge the ground? This is sometimes not possible on site that are 100% developed.

Any advice woudl be gratefully received.
Johnny

 

[civilman72]

Local authorities do accept land drainage discharges - their storm sewer systems should be designed to accommodate off-site flows. However, any land owner that increases the amount of runoff from their property would be expected to detain this increased runoff. If a development did not previously consider detention, and there is no room for detention now, then your job of solving their issues may prove to be very difficult.

With regards to "ground water" that is discharged from the bottom of a retaining wall, and whether this water can be "discharged" to the surface and eventually to a storm sewer system is a more complicated issue. If the site has significant retaining walls the site is probably steep. If the site is steep, then very often the water you encounter when excavating out the retaining wall my be "underground" at the point of the retaining wall, but more than likely this water eventually discharges to the surface further down the slope (i.e. a spring). In this instance I would argue that you are not altering the "groundwater."

If you are truly discharging groundwater through a retaining wall and to the surface (the retaining wall must be very tall?) then you should recommend that your client hire a water attorney. At least in Colorado, there are not only issues with the local municipality accepting this groundwater, but your client could also be potentially effecting existing water rights, including individual surface and groundwater water rights. FYI- "water rights" is basically a known quantity of water that is owned by someone, it has a lot of monetary value and altering someone's water rights is not taken lightly around here, but may not be an issue where you live.

Also, depending on where you live, you should anticipate having to apply for a Groundwater Discharge Permit and the requirements of this permit may require that your client "re-charge" the ground. If there is no room on the site to do this, again, your client is in a difficult situation.

If you are dealing with an existing development, that's at least 20 years old, then I believe the detention requirements, water rights issues, and permit requirements I discussed above are probably not pertinent.

 

[J0hnny]

Thanks Civilman for your advice. You're right the site is very steep. we are demolishing existing units and replacing with larger. We are attenuating the surface water associated with the new units to a greenfield run off rate in order to meet latest codes and also reduce the burden on the existing combined system. Our local authorities do not accept ground water or land drainage discharges and as you say one way around this is to discahrge to surface and allow it to find it's way into the system. however I'd like to install something a little more formal and acceptable.As you say I think my options are limited! I had considered discharging to the bed and surround of the new (long) drainage system to allow it to attenuate and infiltrate back into the ground via whatever percolation is available.