Wait, The Bed Moves? - Necessity and Implications of Mobile-Bed Hydraulic Modeling For Flood Control Purposes, The San Lorenzo River, Ca

The City of Santa Cruz, California, partnered with the USACE, constructed a project to meet or exceed FEMA standards and they are now seeking certification of the San Lorenzo River levee systems. The San Lorenzo River bed is comprised of mostly sand and gravel. Observed high water marks from historic flood events (e.g., January 1982) were significantly lower than predicted by previous static-bed hydraulic modeling, which suggested that channel scour is a significant process at high flows and should be considered when calculating flood profiles. However, mobile-bed modeling is not typically applied in the context of FEMA levee certification and flood hazard mapping. Building on previous work, ESA developed a mobile-bed hydraulic model (HEC-RAS) for contemporary conditions to evaluate hydraulics and sediment transport in the channel and establish annual chance exceedance event (ACE) base flood elevation profiles for levee certification. This process first involved converting an HEC-6T model to HEC-RAS, which presented some unique challenges as well as valuable lessons learned and insight into sediment transport modeling with the latest version of HEC-RAS.

The model integrates the fluvial processes of sediment transport and flow and the influence of coincident coastal processes (i.e., an elevated downstream boundary condition that includes regional non-tidal residual and local wave setup). We calibrated and compared the predicted water surface elevations and sediment changes to historic flood events spanning over three decades, including the record wet winter of WY 2017. The paper will highlight the challenges as well as the necessity of a mobile-bed model in this case, its robustness in replicating both historic and contemporary observations of flow and sediment transport, and its ability to quantify the relative sensitivity of WSE and sediment flux to downstream boundary conditions. Through example, we further demonstrate that the San Lorenzo River model is-well suited for assessing the effects of climate change (e.g., sea-level rise and future extreme rainfall-runoff) and watershed-scale changes in sediment transport and delivery processes in a coastal river, and how these processes may ultimately affect water surface elevations during floods.