Stormwater device brings Caltrans closer to statewide compliance

Caltrans receives patent for a space-saving device that makes stormwater flow cleaner and require less maintenance

Trash and debris accumulate along roadsides and often then get carried into drainage systems with rainfall runoff. The refuse can end up in streams, rivers, or lakes, where it can adversely affect the use of those resources for humans, fish and wildlife.

Removal of garbage and debris from roadway runoff is a challenging problem for Caltrans. Until recently, the trash-capture systems that are available have had large footprints and required considerable right of way. Installing and operating them in urban and ultra-urban areas has posed many difficulties, including tricky access, high costs, disruptive construction and persistent maintenance issues.

Traditional trash-capture devices that can be installed in upstream locations such as drain or curb inlets are of limited use in highway environments due to potential flooding, hydroplaning, safety and maintenance-personnel exposure issues. There is a need for a trash-capture system that could be easily retrofitted in existing highway drainage infrastructure or included in newer projects. Such a system is designed to reduce frequency of maintenance and be easier to maintain without compromising safety of the travelling public.

To address this issue proactively, Caltrans Division of Environmental Analysis (DEA) initiated a trash-capture device development and testing project in early 2020. The DEA Stormwater staff Cornelis Hakim  developed the initial idea for a “capture housing” device.

To address this issue proactively, Caltrans Division of Environmental Analysis (DEA) initiated a trash-capture device development and testing project in early 2020. The DEA Stormwater staff Cornelis Hakim  developed the initial idea for a “capture housing” device.

Computational fluid dynamics modelling was used first to assess the feasibility and performance. Next the hydraulic capacity, trash-capture characteristics, operation and ease of maintenance was tested on a full-scale physical model. The test results helped guide development of several device configurations suitable for various field conditions. After the successful testing, Standard Plans and Design Guidance were developed. Caltrans also submitted an application to the U.S. Patent Office, and a patent for the innovative trash-capture device was granted in December.

The device is essentially a concrete structure that consists of two vaults. Vault 1 houses a screen weir that divides Vault 1 into a trash-capture approach/storage area and a bypass channel inlet. Vault 2 houses the trash-capture box (screen box) and bypass channel outlet.

The accompanying figure shows typical flow paths through the capture housing when it is in place – typically under a roadway curb’s storm grate. Rainfall runoff drains through the grates into the capture housing trash-storage area and screen box. The recommended flows up to 0.7 cubic feet per second for the 1year/ 1hour intensity  are contained within the screened trash storage area. Larger storms are bypassed over the screen weir in the trash storage area, over the screen box.

The capture housing is designed so that all particles greater than or equal to 5 millimeters (0.197 inches) are captured within a stainless-steel screened trash-receiving area. The screen for the weir and trash box consists of uniformly spaced 4.8-millimeter openings that prevent particles larger than that from passing through. Untreated stormwater runoff enters Vault 1 either from top through the roadway grates, from an inlet pipe, or both. Roadway grates over Vault 2 mainly provide maintenance access and are also screened to smaller than 5 millimeters.

This innovation demonstrates Caltrans active and continuing commitment to initiatives that enhance California's natural environment. This device could help Caltrans to meet the 2030 statewide trash compliance and 2026 trash discharge Cease & Desist Order enforcement obligations.  

Source: Bhaskar Joshi, supervising transportation engineer, Division of Environmental Analysis-Office of Stormwater Program and Development