MONTANA DEPARTMENT OF NATURAL RESOURCES AND CONSERVATION Upstream of Cold Creek Rd. Bridge, 9/18/14 Hydrology Design Report Swan River Detailed Floodplain Study Missoula County, MT By the Montana Department of Natural Resources and Conservation May 2015 Hydrology Design Report, Swan River Detailed Floodplain Study May 2015 HYDROLOGY DESIGN REPORT SWAN RIVER Missoula County, MT Table of Contents 1.0 INTRODUCTION ........................................................................................................................ 1 1.1 LiDAR Collection ............................................................................................................................ 1 1.2 Watershed Description ................................................................................................................. 1 1.3 Effective FIS (Flood Insurance Study) Hydrology .......................................................................... 2 1.4 Historic Data .................................................................................................................................. 3 2.0 HYDROLOGIC ANALYSIS ............................................................................................................ 5 2.1 Stream Gage Analyses ................................................................................................................... 5 2.1.1 Systematic Estimation ........................................................................................................... 6 2.1.2 Two Station Comparison & MOVE.1 ..................................................................................... 7 2.1.3 Regional Regression Equations ............................................................................................. 8 2.1.4 Weighted Estimation ............................................................................................................ 9 2.1.5 Results Comparison ............................................................................................................... 9 2.2 Transfer to Ungaged Sites ........................................................................................................... 12 2.2.1 Streamflow Change Locations ............................................................................................. 12 2.2.2 Log Interpolation between 2 Gages .................................................................................... 14 3.0 HYDROLOGIC ANALYSIS SUMMARY & SELECTED DISCHARGES ................................................ 15 List of Tables Table 1: Flathead & Lake County Effective Floodplain Studies .................................................................... 2 Table 2: Peak Flows used in Effective Studies .............................................................................................. 3 Table 3: Highest Peaks Recorded at Swan River Stream Gages ................................................................... 5 Table 4: Stream Gage Summary Table ......................................................................................................... 6 Table 5: Stream Gage Analysis Results ‐ Systematic .................................................................................... 7 Table 6: Record Extension Methods Discharge Estimates ‐ Stream Gage No. 12369200 ........................... 8 Table 7: Regression Equation Estimates ‐ Stream Gage No. 12369200 ....................................................... 8 Table 8: Weighted Estimates ‐ Stream Gage No. 12369200 ........................................................................ 9 Table 9: Flood Discharge Estimates at USGS Gage 12369200 ................................................................... 10 Table 10: Gage 12370000 Trending Analysis Comparison ......................................................................... 11 Table 11: Hydraulic Model Stream Reaches .............................................................................................. 14 i Hydrology Design Report, Swan River Detailed Floodplain Study May 2015 Table 12: Log Interpolation between 2 Gages .......................................................................................... 15 Table 13: Selected Discharge Estimates ..................................................................................................... 15 List of Figures Figure 1: Study Reach Site Map ................................................................................................................... 4 Figure 2: Flood Discharge Estimates at USGS Gage 12369200 .................................................................. 10 Figure 3: Drainage Basin Area .................................................................................................................... 13 Appendices Appendix A: USGS Stream Gage Data Appendix B: Hydrologic Calculations ii Hydrology Design Report, Swan River Detailed Floodplain Study May 2015 1.0 INTRODUCTION As part of the Missoula County Memorandum of Understanding (MOU) (Reference 9) Agreement activities, DNRC is assisting Missoula County to complete a new detailed riverine floodplain study, with floodway, for a section of the Swan River beginning at the Lake/Missoula County boundary and extending approximately 19.1 miles upstream near the Beaver Creek confluence. Per the MOU, DNRC has completed a new hydrologic analysis to be utilized for the new study. The study reach is currently mapped by FEMA using approximate methods. This study will be completed satisfying current state and FEMA standards for Detailed/Enhanced floodplain studies. A Flood Insurance Study (FIS) has been completed for Missoula County and Incorporated Areas dated August 16, 1988; however, an update to the FIS has been completed and is slated to become effective on July 6, 2015. A summary of the Digital Flood Insurance Rate Map (DFIRM) panels covering this study reach is presented below (Reference 2): Community Community No. FIRM Panels Effective Date Missoula County, MT 300048 30063C0200D 30063C0075D 08/16/1988 30063C0050D 30063C0200E 30063C0075E 07/06/2015 30063C0050E This report summarizes the hydrologic analysis and results for the new detailed study stream reach described. The new study includes hydrologic analysis to estimate the 10-, 4-, 2-, 1-, and 0.2 percent-annual-chance flood discharges for the Swan River. 1.1 LiDAR Collection In 2012, Missoula County contracted with Watershed Sciences, Inc. to collect Light Detection And Ranging (LiDAR) data for four areas of interest for the purpose of supporting floodplain mapping projects: 1. Rock Creek floodplain, 2. Bitterroot River floodplain, 3. Swan River floodplain, and 4. Clearwater River floodplain. Accuracy of the topographic data meets FEMA standards for detailed level floodplain mapping. The LiDAR data covers the project reach for this study. 1.2 Watershed Description The Swan River originates from Gray Wolf Lake in the Mission Mountain Range and flows generally in a northwesterly direction before emptying into Flathead Lake at the town of Bigfork. 1 Hydrology Design Report, Swan River Detailed Floodplain Study May 2015 At its mouth, the river drains a watershed area of approximately 723 square miles. The Swan River valley bottom was formed by glacial melt waters and reworked by fluvial processes. The valley is bounded on the east by the Swan Mountain Range and on the west by the Mission Range. The vegetation in the valley is dominated by subalpine firs with slopes ranging from 0 to 20 percent. 1.3 Effective FIS (Flood Insurance Study) Hydrology The effective floodplain mapping for the Swan River in Missoula County stretches from the Lake County boundary upstream to the Beaver Creek confluence, which matches the extents of the study at hand. The level of detail of the effective mapping is approximate, as it has been carried forward from the historic flood hazard boundary maps (FHBMs) developed in the 1970s. In Flathead and Lake Counties, the Swan River floodplain has both approximate level and detailed level mapping; information about the effective detailed studies is shown in Table 1. Table 1: Flathead & Lake County Effective Floodplain Studies Approx. Effective Level of Author Study Title County Length Description Date Detail (mi) Beginning at the Swan River Rd. Bridge Simons Li & Lower Swan Lake March 12 Detailed upstream of Bigfork Dam and extending Associates, Inc. River Study County 1986 upstream to the outlet of Swan Lake. Beginning approx. 6.5 miles upstream of Simons Li & Upper Swan Lake March 20 Detailed the Swan Lake inlet and extending Associates, Inc. River Study County 1986 upstream to the Missoula Co. boundary. Lake September Limited Analysis of the Swan Lake floodplain PBS&J Swan Lake 9.5 County 2010 Detail including 23 miles of shoreline. Beginning at the river mouth and Swan River Flathead December DNRC 0.5 Detailed Flathead Lake confluence and extending at Bigfork County 2012 upstream 0.5 miles. Table 1 is a summary of the effective detailed level studies on the Swan River. There are also several reaches of the channel with approximate level mapping which are remnants of the FHBMs from the 1970s. Each of the effective detailed studies included a hydrologic analysis including estimation of peak flows. The following is a description of the hydrologic analysis method selected for estimating peak flows for each study: Lower Swan River Study – A Log-Pearson Type III analysis was performed on the annual maximum discharges to determine peak return flows (Reference 4). Upper Swan River