Olive Ave Drainage Plan: Sideways Lot, 7% Slope

We received a new project on Olive Ave in Altadena for a single-family residential fire rebuild. We completed the drainage plans for this 0.1685-acre sideways lot with the entrance on the side street. The lot presented an interesting drainage challenge: it slopes from east to west at approximately 7%, while the north-south elevation change is minimal at only about 2 feet.

Existing Conditions

The existing conditions showed a relatively simple slope pattern. The 7% east-to-west grade meant stormwater would naturally want to flow toward the western edge of the property. The existing driveway apron would remain as part of the final design, which gave us a constraint to work around.

Our approach was to minimize cross drainage throughout the site. Instead of trying to fight the natural slope, we designed a system that works with it.

Proposed Design

The existing driveway apron remains, and we regraded it slightly to widen it, a small change that improved the transition for the new construction. The front of the house was designed to slope 10 feet at 5% to a flowline that splits the drainage into two directions: North to the sideyard and South to the driveway.

Neither sideyard requires retaining walls, a significant cost savings for the project. The driveway itself was matched into existing grades to minimize disturbance. The rear of the property slopes behind the garage at the Southwest corner.

Sheet Flow by Gravity

The entire site sheet flows by gravity through landscape areas to the low point of the lot. This approach avoids the need for any sump pumps or mechanical drainage systems, which can cost tens of thousands of dollars to install and require ongoing maintenance.

The proposed model shows how we threaded the drainage through the existing constraints while maintaining the natural grade as much as possible. The building finished floor is 8” minimum above finished grade, and the garage is tied into the driveway and rear landscape.

If you have a sideways lot in Altadena that needs drainage plans, get in touch. We can help you work through the existing conditions and find a drainage solution that works with your site’s natural slope.