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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5323

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Summary

Duck and Jordan Creeks are small, urban streams that flow through closely-spaced engineered bridges and culverts in Juneau’s Mendenhall Valley before draining into salt water. Both streams drain areas along the steep eastern flank of the valley. However, Jordan Creek extends farther up the mountainside and collects more mountainous drainage than Duck Creek, which first becomes channelized in the valley floor and is more influenced by ground-water conditions. Jordan Creek’s drainage area is 2.6 square miles, twice the size of Duck Creek’s drainage area, and its peak flows are about 3 times larger than Duck Creek’s.

To provide information for updating Juneau’s most recent flood insurance study, this study determined flood magnitudes for floods with a 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year recurrence interval on Duck and Jordan Creeks. Computations from 11 and 8 years of streamflow data on Duck and Jordan Creeks, respectively, were weighted with results from regional regression equations for southeast Alaska. The flood discharges were used together with field surveys of channel geometry and details of culverts and bridges to construct a steady-state hydraulic model using the U.S. Army Corps of Engineers’ HEC-RAS software. Simulated reaches include 3.4 miles of main stem Duck Creek, 0.3 mile of an unnamed tributary to Duck Creek near McGinnis Street, and 1.1 miles of Jordan Creek downstream from Egan Drive. Additional work will be required to update Jordan Creek upstream from Egan Drive. The model was calibrated by adjusting Manning’s roughness coefficients so water-surface elevations more closely matched measured elevations from a small 2005 flood. Resulting flood profiles provide water-surface elevations for the 10-, 50-, 100-, and 500-year floods. These flood profiles are strongly affected by backwater from the 30 culverts and bridges on Duck Creek and the 14 culverts and bridges in the study area on Jordan Creek. Backwater conditions over much of the study area create a stair-step profile with water-surface elevations rising at the culvert or bridge and a flat water-surface profile extending upstream. Inter-basin flow was simulated in two locations, one leaving Duck Creek downstream from Egan Drive and entering Jordan Creek upstream from Jordan Avenue, and another leaving the Jordan Creek system entirely in a southeast trending depression along the upstream side of Egan Drive.

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