Pájaro River Watershed Flood Protection Plan - The Pajaro River ...

where:
g adj is the adjusted skew
g s is the standardized skew
n is the sample size
For several exceedence probabilities (p) ranging from 0.99 (1.01-year
recurrence interval) to 0.01 (100-year recurrence interval), the values of the
standardized variate K were obtained using tables included in the Bulletin 17B report
for the adjusted skew value. The Log-Pearson Type III estimates are determined from
the following equation:
K
(g adj )
Y LT = Ŷ LT + KS LT
where:
Y LT is the log of the estimated discharge for the exceedence probability at
Y LT is the mean of the log-transformed sample
K is the Log-Pearson Type III variate determined using the adjusted skew
S LT is the standard deviation of the log-transformed sample
The antilog of the Y LT values determined is the estimate of discharge at
the specific exceedence probabilities or recurrence intervals. In addition, 90% upper
confidence intervals were set for all stations at each exceedence probability.
Smaller sub-sets of data from each station were analyzed for flood flow
frequency at intervals of 10 years. The sub-set analysis of the 100-year flood was
determined using the same methodology as the Log-Pearson Type III described
above, including skew adjustment. No confidence intervals were estimated in this
analysis.
Results
The results of the flood frequency analysis for the select gaging stations
in the San Benito River, Pájaro River and tributaries are summarized in Table 2, and
confidence intervals are graphed as shown in Figure 1.
Station Name
Station
Number
Years of
record
Calculated
100-year
flood Q cfs
Calculated 50-
year flood Q cfs
Calculated
25-year
flood Q cfs
Pájaro at Chittenden 11159000 62 30172.03 26759.25 22654.49
San Benito at 156 11158600 31 7157.91 7052.91 6813.80
San Benito near Hollister 11158500 33* 30234.73 21948.87 15034.67
DRAFT 7/22/03
22
Pajaro Watershed Flood Management