Surface-water quality and flow Modeling Interest Group
Precipitation-Runoff, Suspended-Sediment, and Flood-Frequency
Characteristics for Urbanized Areas of Elmendorf Air Force Base, Alaska
by Timothy P. Brabets
USGS, Water Resources Division
4230 University Drive, Suite 201
Anchorage, AK 99508-4664
Internet: tbrabets@usgs.gov
Phone: (907) 786-7105
FAX: (907) 786-7150
Editor's note:
This precipitation-runoff modeling work is from a recently published USGS
report:
Brabets, T.P., 1999, Precipitation-runoff, suspended-sediment, and
flood-frequency characteristics for urbanized areas of Elmendorf Air Force
Base, Alaska: U.S. Geological Survey Water-Resources Investigations Report
98-4265, 34 p.
An electronic copy of the full report is available for download in
PDF format (1.55Mbytes). Paper copies of this
report can be obtained from any of the USGS
Earth Science
Information Centers.
Abstract
The developed part of Elmendorf Air Force Base near Anchorage, Alaska,
consists of two basins with drainage areas of 4.0 and 0.64 square miles,
respectively. Runoff and suspended-sediment data were collected from August
1996 to March 1998 to gain a basic understanding of the surface-water
hydrology of these areas and to estimate flood-frequency characteristics.
Runoff from the larger basin averaged 6 percent of rainfall, whereas runoff
from the smaller basin averaged 13 percent of rainfall. During rainfall
periods, the suspended-sediment load transported from the larger watershed
ranged from 179 to 21,000 pounds and that from the smaller watershed ranged
from 23 to 18,200 pounds. On a yield basis, suspended sediment from the
larger watershed was 78 pounds per inch of runoff and from the smaller basin
was 100 pounds per inch of runoff. Suspended-sediment loads and yields were
generally lower during snowmelt periods than during rainfall periods.
At each outfall of the two watersheds, water flows into steep natural
channels. Suspended-sediment loads measured approximately 1,000 feet
downstream from the outfalls during rainfall periods ranged from 8,450 to
530,000 pounds. On a yield basis, suspended sediment averaged 705 pounds per
inch of runoff, more than three times as much as the combined sediment yield
from the two watersheds. The increase in suspended sediment is most likely
due to natural erosion of the streambanks.
Streamflow data, collected in 1996 and 1997, were used to calibrate and
verify a U.S. Geological Survey computer model, the Distributed Routing
Rainfall Runoff Model--Version II
(DR3M-II). The model
was then used to simulate annual peak discharges and runoff volumes for 1981
to 1995 using historical rainfall records. Because the model indicated that
surcharging (or ponding) would occur, no flood-frequency analysis was done
for peak discharges. A flood-frequency analysis of flood volumes indicated
that a 10-year flood would result in 0.39 inch of runoff (averaged over the
entire drainage basin) from the larger watershed and 1.1 inches of runoff
from the smaller watershed.
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