8181 (2) - Laserfiche WebLink

Home Browse Search

• CHAPTER VI. CONCLUSIONS Conclusions Bureau of Land Management Sites During the 1996 runoff season for the Green River near Vernal, Utah, several objectives were met regarding potential floodplain habitat restoration activities. Across section monitoring program for reaches adjacent to five selected BLM bottomland sites was established. Stage/Discharge relationships were developed for each site. From these relationships flooding discharges which will significantly fill the bottomlands were discerned at each site. The flooding discharges ranged from 12,900 cfs at the Above Brennan bottomland to 26,500 cfs at the Baeser Bend Bottomland site. An average flooding discharge of 20,500 cfs was determined. In order to restore historical flooding frequencies and durations, levee removal strategies were designed for each bottomland site. Topographic surveys were conducted at each site. Mapping with 2 foot contours were created which identified levees, bottomland configurations, and inlets or low spots which could be modified to increase flooding frequencies. At each site, an existing inlet was located and/or the feasibility of a large scale levee removal was assessed. Unique levee removal strategies were proposed for each site with varying widths and lengths, locations relative to the bottomlands, and channel morphology characteristics. Each site was designed to flood at 13,000 cfs, which will occur 2 out of every 3 years, for 11 days based on post-1963 hydrology. The duration of flooding should range from no flooding in dry years to 1-3 weeks during average years, or 4-8 weeks during wet years. Under existing conditions, on the average, the sites will only flood approximately 2 out of 7 years on average and the duration of flooding would be only 3.5 days • statistically. The flooding return periods ranged from a 1.4-year return period at the Above Brennan bottomland to a 15-year return period at the Baeser Bend bottomland. In order to significantly fill the bottomlands, a discharge above the flooding discharge would have to occur for several days to a week. For example, flows over 19,700 cfs, the flooding discharge at Bonanza Bridge, were experienced during the 1996 runoff season for approximately 4 days. However, the bottomland did not completely fill and fish passage was limited due to a short duration. • Levee removal designs have been proposed for each of the five BLM sites. Favorable flooding locations were identified in the field, and the sites were designed to flood at 13,000 cfs utilizing different design parameters for each site. The frequency and duration of flooding will be returned to an assumed historic frequency and duration as a result of the designs. The areas of inundation for the BLM bottomlands were determined utilizing the stage/discharge relationships. Table 14 gives a comparison of the areas of inundation under existing and proposed conditions. Under existing conditions, flooding with a hydraulic connection is limited to approximately 50 acres for flows up to 18,000 cfs (all in Above Brennan). Under proposed conditions, approximately 140 acres at five different sites would occur when flows exceeded 13,000 cfs. For comparison, that area would be increased to 173 acres at 18,000 cfs after levee removal versus 50 acres under existing conditions. Under proposed conditions, these sites will flood more frequently and the value of these bottomland sites as floodplain habitat can more readily be assessed. These sites can be monitored to evaluate sedimentation, rates of filling and/or draining, and temperature. These parameters need to be evaluated in relation to biological concerns and how they relate to endangered fish species. 59