Its been an experience, but we are finally getting the latest batch of Altadena drainage plans approved by LA County. Our technique is to keep the existing drainage pattern so we don’t have to install stormwater sump pumps. This is special for the Altadena fire rebuilds only. The County then wants us to verify the pre-fire and post-fire hydrology drainage patterns, which we are also submitting a Hydrology Study like report to verify these numbers. Any excess stormwater we are handling using lined rain gardens offset 5 feet from the property lines and 15 feet from structures.
The existing topography is typical of the now cleared and foundations excavated out. The next one to submit is similar to many of the lots we are doing Altadena drainage plans for. We keep the existing driveway apron, and redo a portion of that driveway all the way back to a proposed garage.
The rest is pretty typical and we connect the downspouts and area drains to a stormwater pipe system that will outlet to a plastic lined rain garden. This will attempt to keep stormwater from cross draining, which is better than the pre-fire drainage pattern, and anything the plastic lined rain garden can’t handle will go to its previous drainage pattern. We are also trying to keep water from crossing by building a 6inch curb behind the rain garden.
The overall site has been resloped to get stormwater into area drains and a pipe system. The driveway was a bit tricky as we still need to connect into the existing contours on the neighboring lot, so the grade gets a few grade breaks to follow the neighbor. The other idea is to try to not use any form of retaining walls even if short 1 foot walls. Overall the site is keeping a similar drainage pattern, and brings the water away from the structures.