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Design Report Big Wood River Restoration Project

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Design Report Big Wood River Restoration Project DESIGN REPORT BIG WOOD RIVER RESTORATION PROJECT BROADFORD FISHERMAN’S ACCESS AREA BLAINE COUNTY, IDAHO 1.0 INTRODUCTION Biota Research and Consulting, Inc. (Biota) has been retained by Blaine County to complete a site assessment, restoration design, and permitting for the Broadford Fisherman’s Access Area of the Big Wood River. This project was completed as follow-up work to the Emergency Stream Alteration Permit (ESAP 17-067) issued to Blaine County for the Broadford Road Fisherman’s Access Area. This project was completed to identify specific treatments to be implemented in order to protect proximate land uses and recreational opportunities from severe erosion while maximizing ecological values of the fluvial system. The primary strategy to meet project objectives was to restore function through application of in-channel treatments that enable the river to convey hydrologic and sediment inputs without severe erosion or deposition. The design report presents analyses completed during the design development process including hydrologic investigations, geomorphic assessment, hydraulic modeling, sediment transport analyses, and restoration treatments and strategies. Hydraulic modeling has been conducted to demonstrate regulatory compliance. The design plans for the Broadford Fisherman’s Access Area include construction-ready drawings, construction implementation details, and specifications. 1.1 PROJECT AREA The project area includes an approximately 1,100-foot segment of the Big Wood River near Bellevue, Idaho, adjacent to the Fisherman’s Access on Broadford Road (design drawing Sheet 1). The project reach encompasses private properties and a 4-acre parcel owned by Blaine County that provides public access to the river corridor. 1.2 PROJECT BACKGROUND The Big Wood River mainstem has been directly altered by anthropogenic activities that include development encroachment on the floodplain; clearing of riparian vegetation; removal of instream woody debris; construction and maintenance of flood control levees; installation of rip rap and bank armoring; and the establishment of transportation crossings. Related resource concerns include unstable channel braiding, widening or enlargement, and severe lateral instability (bank erosion). The large magnitude flood experienced in the Big Wood River in 2017 resulted in widespread changes in the river corridor, and dramatic alteration of sub-reaches that were previously impaired by land use activities. The US Geological Survey (USGS) has maintained a stream gauge in Hailey (#13139510) for more than 100 years, and flow data obtained from the gauge reveal that the Big Wood River experienced a flood in 2017 that had a greater than 50-year return interval. Mean daily flow data recorded at the gauge indicate that 2017 peak flows were approximately 3 times higher than average, and that the 2017 runoff period experienced multiple distinct flood peaks (Figure 1). The multiple, prolonged, large peaks in flood Broadford Fisherman’s Access Design Report 1 Biota Research and Consulting, Inc. waters caused extensive flooding, river bank and bed erosion, and widespread gravel deposition within the project reach. Private lands and facilities were impacted by erosion, site access and public infrastructure were destroyed, flood protection levees were compromised, and there was concern that Broadford Road itself could be impacted. During the 2017 flood event, emergency actions were undertaken by Blaine County in response to rapid localized lateral river migration, and the destruction of the Fisherman’s Access parking area. Emergency actions included the discharge of rock rip rap to harden the river right bank. This restoration design was completed to identify a long-term solution to local river instability. 7000 6000 5000 4000 3000 2000 Mean Daily Discharge Daily Mean (cfs) 1000 0 1/1 3/3 4/2 5/3 6/2 7/3 8/2 9/2 1/31 Date 10/2 11/2 12/2 2017 Mean Daily Flow Period of Record Mean Daily Flow Figure 1. Mean daily flows from the period of record and from 2017 at the USGS Big Wood River gauge in Hailey (#13139510). Conceptual restoration and enhancement designs have been developed for the entire Broadford Reach of the Big Wood River (from Broadford Road Bridge to Star Bridge). This project will refine those conceptual designs into a final design plan set for the Fisherman’s Access project area. Design treatments are intended to protect infrastructure and facilities proximate to the river while restoring fluvial processes of sediment transport and flood water conveyance. The project was informed by site observation and assessment, local hydrologic and topographic data, including LiDAR flown in the fall of 2017 (QSI 2017), and the previously completed Geomorphic Assessment Report, Big Wood River, Blaine County, Idaho (Biota, 2016) which studied the associated sub-reach of the Big Wood River and presented localized predictions related to channel stability and sediment transport. Broadford Fisherman’s Access Design Report 2 Biota Research and Consulting, Inc. 1.3 PROJECT GOALS AND OBJECTIVES Project objectives were identified based upon existing site conditions, previous geomorphic assessment, land use constraints, and fluvial system potential. Project objectives include the following: 1. Identify the stable functional channel form appropriate under the current (anthropogenically altered) hydrologic regime; 2. Restore sufficient capacity to transport the sediment load while providing opportunities for storage of surplus sediment; 3. Increase lateral channel stability; 4. Reduce aggradation, or severe localized deposition; 5. Reduce channel enlargement potential; 6. Reduce sediment input to the watershed resulting from bank erosion; 7. Implement fluvial enhancement treatments that reduce, or leave unaltered, the flood hazard proximate to development; 8. Identify self-maintaining treatments that maximize the ecological and recreational values of the Big Wood River. 2.0 FLUVIAL CONDITIONS The project area has a catchment, or watershed, characterized by the following: drainage area of 695 square miles, mean basin elevation of 7,550 ft, maximum basin elevation of 12,000 ft, average annual precipitation of 27.9 inches, about 40% forested land cover, and land slopes of 30% or greater across 75% of the basin. Hydrologic and geomorphic conditions within the project reach are summarized in the following sub-sections. 2.1 HYDROLOGIC REGIME The 2016 Geomorphic Assessment identified the project area bankfull discharge, which was the design discharge used for site assessment and analysis. Bankfull discharge is the flow rate, and bankfull stage is the corresponding water surface elevation, at which instream water escapes the active channel and inundates the floodplain (when incipient flooding occurs). There is natural variability in the recurrence interval of bankfull discharge between sites, but a reasonable estimate of bankfull discharge recurrence interval in the project area is 1.1-1.5 years. Bankfull discharge was selected as the primary hydrologic parameter for assessment purposes because it can be identified and corroborated through field investigations, as opposed to potential alternate parameters of dominant discharge (e.g., the flow rate responsible for the stable morphology) or effective discharge (e.g., the flow rate that transports the greatest fraction of the annual sediment load) that are primarily derived through analytical processes, without empirical corroboration. Estimation of bankfull discharge was performed using hydraulic modeling of open channel flow conditions based upon field-measured morphologic and sediment data (floodplain elevation, bankfull indicators, channel dimension and profile, sediment size class distribution, hydraulic roughness). Analyses indicate that the bankfull discharge in the project area is 560 cfs. Peak flow characteristics within the project area were quantified in order to inform analysis of channel function and sediment transport. The closest USGS gauge on the Big Wood River is approximately 2 miles upstream of the project area at the Bullion Bridge in Hailey (#13139510). A geomorphic channel Broadford Fisherman’s Access Design Report 3 Biota Research and Consulting, Inc. survey was conducted through the stream gauge reach, and included measurement of local gauge datum, riffle cross sectional geometry, bankfull indicators, and channel profile (slope). The active stage-discharge rating curve was obtained from the USGS and was used to determine the discharge corresponding to the local bankfull indicators. Analysis of the gauge period of record indicates that the bankfull discharge has a recurrence interval of 1.2 years within the project reach. The USGS gauge at Hailey (#13139510) was also used to calculate project area peak flows. There are currently over 100 years of peak flow data for the gauge, which has been in operation since 1915. Peak flow recurrence intervals for the gauge were calculated using the Log-Pearson Type III technique and adjusted for drainage area differences between the gauge location and project area following the methods recommended by the USGS (Wood et al. 2016). Table 1 contains the bankfull and peak flow discharges that were used during hydraulic modeling and design development. Table 1. Peak flow recurrence intervals for the project area. Discharge Recurrence Interval (cfs) Bankfull 560 10-Year 4,949 50-Year 6,989 100-Year 7,835 2.2 GEOMORPHIC ASSESSMENT A geomorphic assessment of the project reach was completed in order to establish baseline conditions
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