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Hydrology, Geomorphology, and Flood Profiles of Lemon Creek, Juneau, Alaska

U.S. Geological Survey Scientific Investigations Report 2005–5186

By Randy H. Host and Edward G. Neal

In cooperation with the Alaska Department of Fish and Game and City and Borough of Juneau

This report is available as a pdf.


Abstract

Lemon Creek near Juneau, Alaska has a history of extensive gravel mining, which straightened and deepened the stream channel in the lower reaches of the study area. Gravel mining and channel excavation began in the 1940s and continued through the mid-1980s. Time sequential aerial photos and field investigations indicate that the channel morphology is reverting to pre-disturbance conditions through aggradation of sediment and re-establishment of braided channels, which may result in decreased channel conveyance and increased flooding potential. Time sequential surveys of selected channel cross sections were conducted in an attempt to determine rates of channel aggradation/degradation throughout three reaches of the study area. In order to assess flooding potential in the lower reaches of the study area the U.S. Army Corps of Engineers Hydrologic Engineering Center River Analysis System model was used to estimate the water-surface elevations for the 2-, 10-, 25-, 50-, and 100-year floods. A regionally based regression equation was used to estimate the magnitude of floods for the selected recurrence intervals. Forty-two cross sections were surveyed to define the hydraulic characteristics along a 1.7-mile reach of the stream. High-water marks from a peak flow of 1,820 cubic feet per second, or about a 5-year flood, were surveyed and used to calibrate the model throughout the study area. The stream channel at a bridge in the lower reach could not be simulated without violating assumptions of the model. A model without the lower bridge indicates flood potential is limited to a small area.

Table of Contents

Abstract

Introduction

Description of Study Area

Hydrology of Lemon Creek

Geomorphology of Lemon Creek

History of Channel Changes

Current Channel Conditions

Upper Reach

Transitional Reach

Lower Reach

Flood Profiles

Hydrologic Analysis

Hydraulic Analysis

Calibration of the Hydraulic Model

Flood Profile Computations

Summary

References Cited



 

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This document is available in Portable Document Format (PDF) full report (6.9 MB)


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