Optimization of Design and Operation of Torrance Airport Storm Water Infiltration Project
The Torrance Airport Storm Water Infiltration Project is to be installed at the east end of the Torrance Municipal Airport. The infiltration system which will receive runoff from a 640 acre drainage area via two storm sewer inlets was originally sized assuming no infiltration would be achieved. The system was sized to provide temporary storage and infiltration of runoff from storms up to the 85th Percentile 24 hour storm event. This study discusses innovative designs incorporated into the Torrance Airport Storm Water Infiltration Project to optimize the storage volume through improved soil infiltration. The study also uses an unsaturated and saturated flow model for the purpose of evaluating mounding and particle transport beneath the infiltration system. Mound formation can reduce the thickness of the soil available for pollutant removal, and reduce the infiltration rate of the soil if the mound intersects the bottom of the infiltration system.
The characteristics of the native subsoil and underlying geology encountered during geotechnical investigations suggest low levels of permeability. Native subsoil at the site is silty sand which underlies the entire 6 acre site, followed by sandy clay, alluvium, and clay deposits. At depths ranging from 40 to 45 feet below surface, a sand layer is present. This layer would be most suitable for infiltration of stormwater. However, substantial excavation would be required to install the subsurface infiltration system at the sand layer at this site. This will result in higher cost and difficult access for maintenance. Innovative designs that optimize storage volume through improved soil infiltration were, therefore, explored. The optimized system will consist of two hydrodynamic units for pretreatment, infiltration chambers, and the Energy-Passive Groundwater Recharge Product (EGRP®).
The EGRP® is a new technology that can be classified as a vertical capillary driven drainage device, but in reality, it is more complex than most vertical drainage systems. In the optimized design for the Torrance Airport Project, the top of the EGRP® is positioned about 3 feet below the gravel bed in which the infiltration chamber will be embedded and continues down for a depth of about 40 feet. Multiple EGRP® devices will be installed in groups across the entire site in a pattern designed to maximize soil infiltration. The pattern allows the EGRP® to join separate soil layers and promotes connectivity across the soil matrix, allowing both mass flow and diffuse water movement. An EGRP® is a unique five chamber 1 1/4" tube varying in length from 5’ - 40’.
The optimized design was evaluated using unsaturated and saturated flow model. Two storm events were modeled using the three-dimensional saturated numerical model MODFLOW. Recharge used in MODFLOW was taken from the seepage flux of the unsaturated one-dimensional model HYDRUS.
Based on the simulation results the incorporation of EGRP® will reduce the storage volume of the infiltration chamber from 22 ac-ft to about 11.4 ac-ft, which is a 48% reduction. Seasonally, high water table elevations in the vicinity of the infiltration chamber system were estimated by the model to be between 30 and 45 feet below ground surface. Without the EGRP® perched groundwater is likely to occur in some places during wet seasons, which could periodically affect drainage times. While results from this study of whether or not a perched water table will be created without EGRP® are preliminary, they highlight the importance of careful consideration of the underlying stratigraphy and predevelopment water table elevation when deciding on suitability of the site for storm water infiltration practices.