Caltrans HSU Guidance Document
Trash and road debris clogging storm drains is a major problem within California’s municipalities. This phenomenon affects numerous systems that rely on the effective filtering of stormwater, as well as creating a blight on aesthetics.
In response to this need, the State Water Resources Control Board adopted an amendment to the Water Quality Control Plan for Ocean Waters of California to Control Trash and Part 1 Trash Provision of the Water Quality Control Plan for Inland Surface Waters, Enclosed Bays, and Estuaries. Enacted in 2015, and collectively referred to as the “Trash Amendments,” the objective is to provide statewide consistency of approach to protect aquatic life and public health beneficial uses, and reduce environmental issues associated with trash in state waters.
The Amendments require the creation of trash collecting Treatment Best Management Practices (TBMPs). cbec is working with Brown and Caldwell to develop Hydrodynamic Separation Unit (HSU) TBMPs. A HSU is the particular type of Gross Solids Removal Device (GSRD) that filters by swirling concentration and indirect screening. This effectively separates and traps debris down to a size of roughly two and a half millimeters, and are often used to ensure compliance with trash total maximum daily loads. Typically used as the first step in a multilayered storm water treatment system, one benefit of using a HSU is that it can be sized to fit in areas other Gross Solids Removal Devices just simply could not; which is incredibly important in an already developed landscape. While effective at removing floatables and neutrally buoyant materials, HSUs are not effective at removing soluble pollutants. These devices also can also have standing water when not in use, which can breed mosquitoes if not dealt with properly. By reviewing the current HSU literature to gain a complete grasp of this type of GSRD, cbec was able to assist in the assembly of Hydrodynamic Separation Unit (HSU) TBMPs for Caltrans.