Estimating the magnitude of the 100-year peak flow in the Big Lost River at the Idaho National Engineering and Environmental Laboratory, Idaho

Accurate estimates of peak flows in the Big
Lost River at the Idaho National Engineering and
Environmental Laboratory (INEEL) are needed to
assist planners and managers with evaluating possible
effects of flooding on facilities at the INEEL.
A large difference of 4,350 cubic feet per second
(ft³/s) between two previous estimates of the magnitude
of the 100-year peak flow in the Big Lost
River near the western boundary of the INEEL
prompted the present study.
Regression models that compared annual peak
flows and attenuation of annual peak flows
between successive gaging stations for the same
flow event were used to estimate the magnitude of
the 100-year peak flow in the Big Lost River. The
100-year peak flow of 4,790 ft³/s at the Howell
Ranch gaging station was used as the starting point
for this analysis. This estimate was determined by
using a three-parameter log-Pearson Type III distribution
as outlined in “Guidelines for Determining
Flood Flow Frequency” (Bulletin 17B by the
Interagency Advisory Committee on Water Data).
The regression models indicated that, in the
reach of the Big Lost River between Howell Ranch
and Mackay Reservoir, downstream peak flows are
lower than upstream peak flows. Peak-flow attenuation
values for this reach of the river decreased
nonlinearly as the magnitude of the peak flow
increased. Extrapolation of the trend resulted in an
attenuation estimate of 13 percent for this reach
relative to the 100-year peak flow at the Howell
Ranch gaging station.
In the lower reach of the Big Lost River
between Mackay Reservoir and Arco, downstream
peak flows are also lower than upstream peak
flows. However, in contrast to the upper reach,
peak-flow attenuation values decreased linearly as
the magnitude of the peak flow increased. Extrapolation
of the data indicated that peak-flow attenuations
in this reach of the river approach zero for
flows approaching the 100-year peak-flow estimate
immediately upstream and downstream from
Mackay Reservoir.
A regression model of annual maximum daily
mean flows between Arco and the INEEL diversion
dam indicated that the attenuation values in
this reach of the river are nearly the same for all
flows of record. Extrapolation of the linear regression
of these values resulted in an attenuation estimate
of 10 percent. Seepage measurements made
during 1951–53 also resulted in a loss estimate of
approximately 10 percent. This attenuation value,
combined with the values from analyses of the
upstream reaches, resulted in an estimate of the
100-year peak flow for the Big Lost River immediately
upstream from the INEEL diversion dam of
3,750 ft³/s; upper and lower 95-percent confidence
limits were 6,250 ft³/s and 1,300 ft³/s, respectively.
Localized rainfall, even of high intensity, is
not likely to produce large peak flows at the
INEEL because of high loss rates (infiltration,
bank storage, and channel storage) along much of
the stream channel. The relatively short flow durations
resulting from rainstorms historically have
not provided sufficient volumes of water to satisfy
local storage demands (bank and channel storage).
Only after these storage demands are met do the
loss rates decrease enough for significant peak
flows to reach the INEEL site.
An uncertain component of the present analysis
is the effect of seismic activity on the 100-year
peak-flow estimate. Analysis of the effect of the
magnitude 7.3 Borah Peak earthquake in 1983 on normal flow conditions in the Big Lost River suggests
that the joint occurrence of a large earthquake
and a 100-year peak flow could significantly
increase the magnitude of the peak flow at the
INEEL.