Economic & Floodplain Assessment
  San Diego Creek | FEMA Support | 2D Flood Modeling | American River Watershed Floodplain Studies

American River Watershed Floodplain Studies
Sacramento, CA

The limitations of the existing flood control system in the Sacramento area and the urgent need to increase the level of flood protection have recently received increased public attention in the aftermath of the 2005 Gulf Coast hurricanes. The Corps of Engineers, Bureau of Reclamation, Sacramento Area Flood Control Agency (SAFCA), the Department of Water Resources (DWR), and the Reclamation Board seek to integrate planning, design, and implementation of enhanced flood protection measures with dam safety hydrologic risk reduction under a single Joint Federal Project (JFP) at Folsom Dam. Tetra Tech, Inc. is assisting the Corps’ Sacramento District by performing multiple tasks for this project including: Development of hydraulic and economic models to evaluate expected flood damages and flood damage reduction benefits in the American River floodplain which includes downtown Sacramento.  Tasks Tetra Tech is or has accomplished are as follows:

Hydraulic Analysis: Tetra Tech developed two FLO-2D hydraulic models; one model for the area south of the American River and one for the area north of the river. There are 44,415 individual (400-foot by 400-foot) grid elements in the South Bank Model, and 9,685 grid elements in the North Model.  These models provide for a total of 310 square miles of potential floodplain coverage.  Seven individual storm frequency events have been analyzed for both without project and with project events for each of four alternatives. The hydraulic modeling team included 2 primary modelers (with one person as a replacement/back up technician), 1 field tech, one GIS person plus 3 quality control reviewers. 

Finite Grid Hydraulic Model FLO-2D readily computes flow splits in floodplains.

The models include major embankments that could potentially inhibit flow such as levees, highways, and railroad grades. The model was applied to develop floodplains for seven event frequencies under both without project and with project conditions.  A technical engineering appendix is being developed as part of the American River Re-Evaluation that includes documentation of the FLO-2D model development, analysis, and results.

Levee Breakout locations were selected based on review of topographic data and field inspections of the levees.

Floodplain Inventory:  Tetra Tech is conducting a floodplain inventory to document floodplain development for use in flood damage economic modeling. Tetra Tech developed a 100% structural inventorying utilizing county spatial data, aerial photography and field inspection. This effort resulted in gathered data for over 200,000 structures. Data collected for sampled residential structures includes characteristics such as class, type and first floor foundation. Data was also collected for all non-residential structures in the floodplain. Using parcel data as the base, field teams assigned characteristics to be used in both the valuation and categorical assignment of the structural inventory. Data collected is coded and assigned to a floodplain structural database.

The floodplain inventory and valuation data are used in developing stage damage functions for each study reach. The Corps’ HEC-FDA model is being used to integrate results of hydraulic modeling, geotechnical data, and floodplain inventory data to estimate expected annual flood damages under both without and with-project conditions. These hydrologic, hydraulic and geo-technical input data are being used to develop uncertainties in probability-discharge relationships, regulated inflow-outflow relationships, and stage-discharge and levee failure probability functions. In addition, we have been able to utilize 2-dimensional floodplain analysis data (FLO-2D) for direct entry for stage damage evaluation in HEC-FDA.  One of the unique components of this analysis was the direct use of FLO-2D output flood data by spatial grid instead of the Corps traditional one-dimensional HEC-RAS data in the HEC-FDA model. This allows for more streamlined modifications and application of with project residual floodplain data as direct inputs into the risk-based model. This is the first time that this exceptionally large quantity of FLO-2D output data have been directly input into HEC-FDA.  A technical appendix is currently being developed as part of the American River re-evaluation that includes documentation of the economic survey, modeling, and results.

GIS Analysis: Tetra Tech is using GIS for the inventory database to: assign assessor’s parcel data to the floodplains, identify and analyze spatial relationships in the structural inventory, determine building footprints, assign grid locations to buildings, and determine flood depths.