Sacramento: Flood Risk CEE 6440 – GIS in Water Resources 6 December 2007 Matthew Hill Chris Webb

Introduction Lake Folsom Sacramento River American River Sacramento, CA

Introduction 1944 Folsom Dam Authorized 1956 Dam is completed. Folsom Dam was originally designed to provide in excess of a 500-year level of flood protection. http://www.spk.usace.army.mil/projects/civil/folsom/photos.html

Introduction 1997 Record Flood: The fifth record flood in 46 years occurs over the New Year's holiday. 2005 Improvements to American River levees, including deep under-seepage cutoff walls and erosion protection and operational improvements for Folsom Dam provide 100-year protection for much of the American River Floodplain 1986 Record Flood: The February 1986 storm dumps 10 inches of rain on Sacramento in 11 days. Folsom performance downgraded to about a 60-year storm. 1964 Record Flood: The third record flood in less than 15 years causes engineers to re-evaluate storm frequency. They conclude the storm Folsom is designed to handle is a 120-year storm not a 500-year storm. 1956 Record Flood: Though engineers had been predicting it would take a year to fill the nearly completed Folsom Dam, the second record storm filled the dam in a week and Sacramento is saved from flooding. 1951 Record Flood: Just after ground is broken on Folsom Dam, the American River watershed experiences the first of five record storms. http://www.spk.usace.army.mil/projects/civil/folsom/photos.html

Hydrology and Hydraulics Inflow Outflow Baseflow Rainfall Evapo-transpiration Infiltration Inflow Reservoir operations Flood routing using HEC-RAS

Predicting Runoff Goal Steps Predict the runoff for a forecasted storm 1. Create GIS Base Map of Watershed 2. Develop GIS Model to Calculate Runoff

Watershed Base Map NHDplus Watershed Shapefile Flow Lines Waterbodies

GIS Runoff Model Runoff Model Runoff = Precip – ET – Infiltration from weather forecast ET from spreadsheet calcs Infiltration assumed to be negligible

Weather Station Spreadsheet Forecast for each location is input is spreadsheet Estimated ET is calculated for each location using Shuttleworth (1993) Weather Station Inputs

ET Estimation Shuttleworth (1993) Rn = net radiation for reference crop (mm/d) U = average wind speed measured at 2 m height (m/s) D = average vapor pressure deficit measured at 2 m height (kPa) G = estimate of soil heat flux (MJ/m2d) T = average temperature (°C)

Create XY Points Weather Station Spreadsheet is loaded into ArcMap XY points are created from the file Run Model

Two Models Precip – ET = Runoff Precip = Runoff

Model Outputs The model output is a table showing the total runoff in cfs

Behind the Scenes Precip Raster ET Raster Runoff Raster

Runoff Results Historical Data Model Predictions Actual Stream Flow 2286 cfs (Oct 29, 81) Base flow 674 cfs (Oct 27, 81) Actual Runoff 1612 cfs (Oct 28-29, 81) Model Predictions Runoff (precip only) 1561 cfs 3.2 % Error Runoff (precip - ET) 1392 cfs 13.6 % Error

Model Conclusions Infiltration and ET not significant Good estimation for storms not involving snow Able to process storms longer than one day Automations possible through programming

Reservoir Release Estimated PMF inflow of 1,000,000 cfs Pre-release 115,000 cfs Give evacuation orders Ramp up to 160,000 cfs Maintain as long as possible Release up to 800,000 cfs to save dam http://doctorflood.rice.edu/bedient/Handouts/HEC-HMS.ppt

Flood Routing and GIS Download HEC-RAS and HEC-GeoRAS Cross sections created in ArcMap using 10-m DEM Import sections into HEC-RAS Some processing by hand required Run HEC-RAS

Flood Mapping Export HEC-RAS file to ArcMap Water Surface Generation Grid Intersection Engineering judgement (interpretation) to draw innundated areas 175 sq. mi. submerged For Probable Maximum Flood of 808,585 cfs

What is flooded?

HEC-RAS A few troubles “losing water” Mannings n Channel slope Normal depth downstream? Bridges(10)/constrictions neglected Lake Natoma Number and size of cross sections

Conclusions Floodplain mapping is difficult! Creating good flood maps takes some practice and good training Sacramento is an area of high risk for flooding (similar to New Orleans)

Data Sources http://landcover.usgs.gov/ http://seamless.usgs.gov/website/seamless/ viewer.htm http://www.horizon-systems.com/nhdplus http://waterdata.usgs.gov/nwis http://www.daymet.org

Questions?