FLOOD04413

• County: Statewide
• Basin: Statewide
• Title: Improving American River Flood Frequency Analysis
• Date: 1/1/1999
• Prepared By: National Research Council
• Floodplain - Doc Type: Educational/Technical/Reference Information

Executive Summary

5

TABLE ES.I Swnmary of Three-Day Flood Quantile Estimates for the American River at Fair
Oaks Using the Expected Moments Algorithm (EMA) a
Data and Assumptions:
Systematic Observations:
Historical Period:
Historical Flood
Upper Bound for Remainder of Historical Period:
Paleoflood Observations:

1905 - 1997
1848 - 1904
1862; 147,000 cfsb
147,000 cfsb
not included

Estimated Disnibution Moments:
Log(lO) Mean:
Log(lO) Std. Deviation:
Log(lO) Skewness Coefficient:

4.3329
0.4149
-0.1000

Estimated Three-Day-Mean Flood Quantiles and 90% Confidence Limitsc:
QIO (P ~""" = 0.10) 72,500 cfs (60,000 cfs; 88,000 cfs)
Q20 (P ~""" = 0.05) 101,000 cfs (81,000 cfs; 126,000 cfs)
Q50 (P ~""" ~ 0.02) 145,000 cfs (109,000 cfs; 192,000 cfs)
QIOO (P ~""" = 0.01) 185,000 cfs (131,000 cfs; 257,000 cfs)
Q200 (P """" = 0.005) 230,000 cfs (154,000 cfs; 338,000 cfs)

Associated Recurrence Interval ofPMF:
USBR 1996 (401,000 cfs)
USACE 1997 (485,000 cfs)

1,500 years
3,400 years

a Flood quantile estimates are based on rainfloods only.
b Corresponds to estimated 1862 three-day mean Q
c Based on the LP III using a log skew of -0.1 to the systematic record and the historical record
from 1848 that included the hisroricall862 flood.

The committee did not have time to develop a recommendation regarding
extrapolation of the frequency distribution beyond the flow with an annual
exceedance probability of I in 200. This is clearly an area in need of analysis. One
complicating factor is the observed post-1950 increase in large floods. This increase
may reflect structural changes in the flood generation process wrought by human
activity (e.g., atmospheric composition changes or global land use changes) or by
natural factors that have always been present

IMPliCATIONS FOR FLOODPLAIN CERTIFICATION

Based on the USACE 1998 100-year flood estimate, the Federal Emergency
Management Agency (FEMA) issued new floodplain maps for Sacramento. As
result of these new maps, most of the floodprone areas of Sacramento were classified
as being in the so-called AR zone (area of special flood risk). Generally, this
designation would have resulted in building restrictions and higher flood insurance
rates. In this case, FEMA waived the increases in flood insurance rates, but enforced

-