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

Qu t = Qg t + !J.S f t + ,,5. 1 !J.Sj' t-1
, , , L..J= ,

(1)

is gaged discharge at Fair Oaks (below Folsom); !J.Sft is daily change in storage in
Folsom Reservoir; and !J.Sj,t is daily change in storage'in the five upstream reservoirs
(j = 1...5), then

The lagging of storage changes in upstream reservoirs is intended to reflect
flood wave travel times. Five significant upstream reservoirs have combined storage
in excess of 700,000 acre-feet and collectively account for 90% of upper basin
storage. Sources of error potentially associated with this procedure include errors in
gage discharge (Qg,V, errors in stage measurement (!J.Sf,t ), errors on the stage-
storage rating curve and errors in flood wave travel times.
The question of bias iri flood magnitude estimates following dam closure
was raised by Robert Meyer of the USGS at the July 1998 workshop hosted by the
committee in Sacramento (Meyer, personal communication, 1998). Meyer presented
results of double mass curve and regression analyses based on regional stream flow
and precipitation data that suggested that the American River flood record may be
non-homogeneous.
These observations, particularly given the concentration of large events in
the latter (post-dam) portion of the record, prompted further investigation into the
homogeneity of the American River flood record. The results of several independent
analyses, summarized below, provide convincing evidence that the apparent shift in
flood behavior commencing in the 1950s is most likely not, or at least not prirnarily,
an artifact of the methods used to estimate unregulated flood discharges in the post-
Folsom Dam period. These include David Goldman's (1998) double mass curve
analysis, analysis of discharge records in surrounding basins, order-of-magnitude
estimates of the impact of storage measurement errors, and statistical analysis of
long-term precipitation and temperature records from stations in or surrounding the
American River basin.
Goldman (1998) constructed an altemative double mass curve comparing
American River flood flows at Folsom with flood flows at North Fork Dam on the
American, where discharge is uncontrolled. The curve (Figure 2.5) is linear,
suggesting causes other than methodological error for Meyer's results. A similar
analysis was performed by the committee using annual three-day flood volume data
from six surrounding basins of comparable drainage area: the Feather (2 sites), Yuba,
Mokelumne, Stanislaus, Toulumne, and Merced Rivers. Like the American, these
basins were substantially regulated at some point in their periods of record, although
dams were constructed at different times. Visual inspection confirms a high degree
of similarity in the time series of three-day flood volumes across the seven basins,
with larger events concentrated in the post-1950 portion of respective records. The
bivariate test for statistically significant shift in mean (Maronna and Y ohi, 1978;
Potter, 1981) was applied using the American flood series as test series and the mean
of surrounding stations as regional series. (This test is similar in concept to double-
mass curve analysis, except that it provides an explicit measure of statistical
significance.) The only statistically significant (P<0.05) shift in American mean
flood magnitudes relative to regional values occurred in or around 1918, well before
-

26

Improving American River Flood Frequency Analyses