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

2. Introduction Lake Folsom Sacramento River American River
Sacramento, CA

3. 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.

4. 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.

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

6. 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

7. Watershed Base Map
NHDplus
Watershed Shapefile
Flow Lines
Waterbodies

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

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

10. 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)

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

12. Two Models
Precip – ET = Runoff
Precip = Runoff

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

14. Behind the Scenes
Precip Raster
ET Raster
Runoff Raster

15. 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

16. 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

17. 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

18. 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

19. 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

20. What is flooded?

21. 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

22. 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)

23. Data Sources http://landcover.usgs.gov/
viewer.htm

24. Questions?