Overcoming The Design Challenges of The Torrance Airport Regional BMP Project
The Torrance Airport Basin is located in the Machado Lake Watershed. This basin is about 60 percent impervious with a concentrated impervious configuration and moderate road density. To meet Machado Lake TMDLs, various sites in the basin were investigated for installation of BMPs. Using satellite imageries, aerial photos, site investigations, detailed modeling, and anopportunity to partner with other agencies, a vacant lot located along Crenshaw Blvd was selected for the implementation of a regional BMP. There is an opportunity for the Peninsula Cities to "financially partner" with the City of Torrance on the proposed Airport Project. The cities include Rolling Hills Estate, Rancho Palos Verdes, Palos Verdes Estates, and Los Angeles County unincorporated.
The proposed project consists of pretreatment units and an underground infiltration chamber. Hydrodynamic systems are proposed as the pretreatment units. The chamber system, including the gravel bed in which it will be embedded, covers an area of about 11 acres and will store up to 88 ac-ft of stormwater. At least two inlets will be on the south and west sides of the chamber system. The chambers will be embedded in a gravel bed filled with 20 to 50 mm diameter clear crushed stone with an assumed void space ratio of 40%, to provide storage and structural support to the chamber system and overlying pavement. The infiltration chamber is eight feet deep and will receive stormwater runoff through 21- and 24-inch diameter gravity pipes along Crenshaw Blvd. The main challenges of this project are ensuring adequate infiltration rate, depth of the chamber, and the ability to maintain at least 3 feet between the base elevation of the chamber and seasonally high table elevation.
Due to concern regarding infiltration rates at the Torrance Airport, a geotechnical investigation of this site was conducted using three soil borings. It was concluded from the boring logs that the top layer below the surface at the Airport consists of a thin layer of silty sand followed by sandy clay, alluvium, and clay deposits. At depths ranging from 25 to 45 feet below surface, a sand layer is present. This layer would be most suitable for infiltration of stormwater. Through a detailed evaluation process, it was concluded that the silt and clay layers will have to be excavated and filled with engineered soil. Even though substantial excavation would be required under this scenario, the improved performance justifies the cost.
Performance of the Torrance Airport infiltration chamber system was evaluated with regard to the volume of water infiltrated, post-construction infiltration rate of the underlying native soil and water storage capacity. Precipitation and pertinent hydrologic parameters were used to estimate the total volume of stormwater infiltrated over the simulated period to evaluate the degree to which the design objective of maintaining pre-development infiltration volume is being achieved. It was assumed that the flow of stormwater from the infiltration chamber system into the underlying native soil can be predicted by Darcy’s Law. Darcy’s Law predicts that as the volume of water in the chamber system and contributing storm sewers decreases and exerts less pressure at the interface between the gravel bed of the chamber system and the underlying native soil that the rate of flow into the native soil will decrease. Therefore, it can be expected that infiltration rate will be highest when the chamber system and contributing storm sewers are full, as this will be when the pressure difference is greatest. It can also be expected that infiltration rate will decrease as water levels decline in the system, reaching a minimum when they are close to the bottom of the gravel bed.
The evaluation procedure used to overcome the challenges at this project site will benefit other agencies facing these types of problems.