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Scientific Investigations Report 2006–5323

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

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Introduction

Duck and Jordan Creeks flow through a low-lying, urban to suburban environment in Juneau’s Mendenhall Valley, presenting flood hazards to traffic, structures, and properties. Flood plain mapping provides a means to mitigate and manage such flood hazards, but agencies must revise maps to reflect new data and natural or engineered environment changes. The U.S. Geological Survey (USGS), in cooperation with the City and Borough of Juneau, Alaska, began a study in 2004 to update hydrologic and hydraulic analyses in support of flood plain mapping updates. This study took advantage of newly available streamflow data and assessed the present configuration of culverts and bridges.

The two streams flow several miles through the Mendenhall Valley, a broad, recently deglaciated valley, and continue another mile over a broad, low-lying plain before discharging into salt or brackish water near Fritz Cove. Duck Creek and the lower part of Jordan Creek lie within an urban to suburban environment, including residential and commercial properties and the Juneau International Airport. Culverts, road bridges, and pedestrian bridges cross the streams an average of every 600 ft in the study area. Flooding along both streams periodically overtops roadways and encroaches on streamside properties and structures. To manage and mitigate these flood hazards, the City and Borough of Juneau is participating in a process to revise flood plain maps for many local streams, including Duck and Jordan Creeks.

Many cities formalize flood plain mapping by participating in the Federal Emergency Management Agency National Flood Insurance Program. A Flood Insurance Study (FIS) documents an investigation of the extent of flooding expected during floods of various magnitudes. The most recent FIS for the City and Borough of Juneau, updated in 1990 (Federal Emergency Management Agency, 1990), maps flood plain areas along Duck and Jordan Creeks. The FIS indicates the streams’ flood plains may merge in part of their lower reaches. New USGS streamflow data, channel changes, and engineered structure changes prompted the City and Borough of Juneau to seek a hydrologic and hydraulic analysis of Duck Creek and the lower reaches of Jordan Creek to help update flood plain maps.

Purpose and Scope

The purpose of this study was to provide hydrologic and hydraulic analyses to assist the City and Borough of Juneau with a re-analysis of the flood plains adjacent to Duck Creek and the lower reaches of Jordan Creek. This report describes the methods for and results of a flood frequency analysis using data from stream-gaging stations and regional regression equations. The report also describes the methods for and results of development of a flood profile from a one-dimensional hydraulic model using the U.S. Army Corps of Engineers’ computer program Hydrologic Engineering Center River Analysis System (HEC-RAS). The study focuses on Duck Creek, including Duck Creek and that part of Jordan Creek where its flood plain could merge with Duck Creek’s at higher flows. Additional work will be required to update Jordan Creek upstream from Egan Drive.

Previous Studies

The existing City and Borough of Juneau FIS (Federal Emergency Management Agency, 1990) pre-dated the USGS stream-gaging stations on Duck or Jordan Creeks. Recognizing the potential for inter-basin transfer of flood flows between Duck and Jordan Creeks and the potential for storage of flood flows, that report includes a customized hydrologic analysis to modify discharges initially modeled from precipitation. The analysis identified several major control areas spanning both streams, in particular Berners Avenue/Glacier Highway and Egan Drive. Rating curves relating water-surface elevation to discharge were developed for each limiting engineered structure—bridge, culvert, or roadway where weir flow could occur—within the control areas. Combined rating curves determined the outflow from each control area. The effects of storage for these control areas and for other storage areas were simulated, and a Corps of Engineers computer program was applied to route the flows and determine the effect on the hydrograph (Federal Emergency Management Agency, 1990). This effort resulted in reducing the estimated combined 100-year flood for the two streams from 1,200 to 699 ft3/s. The tidal elevation used as a starting water surface for Duck and Jordan Creeks was 20 ft, stated as the high tide that would occur during a month when the selected floods would occur.

An updated hydrologic and hydraulic analysis for the Mendenhall River (Neal and Host, 1999) indicates that channel incision is associated with high regional rates of land-surface uplift. Although uplift equally affects the Mendenhall Valley, Duck and Jordan Creeks have a much more limited capacity for incision. The gentle gradients, low velocities, and locally dense in-stream vegetation along these smaller streams may limit channel erosion to local scour.

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