Yerba Buena Creek, in the Evergreen neighborhood of San Jose, has become deeply incised, likely beginning near 1968, which correlates to when suburban development began in the watershed. As part of a Project team including Jacobs Engineering and Callander Associates, Balance collaborated on the design of a pedestrian bridge replacement over the creek that would connect a residential neighborhood to the Evergreen Valley Community College as part of the Yerba Buena Trail system. Balance staff provide the geomorphic, hydraulic and hydrologic considerations for the bridge design, including estimated amounts of future incision, discussion of implications related to existing infrastructure (roads and storm-drain outfalls), span considerations, proposed bridge alignment, erosional time frames, and the condition of channel knickpoints.
The geomorphic challenge at this site is to estimate future channel dimensions in a creek that is actively incising and widening through the processes of migrating knickpoints (sharp changes in channel slopes), bottom-widening, and side-slope failures. These ongoing erosional processes are irregular and episodic (dependent on rainfall amount and intensity), thus there is uncertainty and a range of possible future channel evolution dimensions. Balance staff calculated a range of possible future channel top-widths (based on our analysis of survey data, historical aerial photographs, geotechnical borings, hydraulic modeling, previous flow data, and storm-drain outfall locations) and ultimately worked with the client and the rest of the project team to finalize a conceptual design that took both the geomorphic elements, as well as other bridge design factors, into consideration.
If you’d like more information about the project visit the City of San Jose’s project site:
https://www.sanjoseca.gov/Home/Components/FacilityDirectory/FacilityDirectory/3027/2028?npage=15
Public presentation (posted online) by Callander:
https://www.sanjoseca.gov/home/showdocument?id=20699
