8181 (2) - Laserfiche WebLink

Home Browse Search

• Executive Summary Ten Green River study reaches containing bottomlands were selected for floodability assessment. The ten bottomland areas were located near Vernal, Utah, stretching over 40 river miles. Five of the ten sites were located on Bureau of Land Management (BLM) sites and the other five sites were located on the Ouray National Wildlife Refuge (ONWR). The objectives of the study were to determine: 1) The discharge at which each of the bottomlands flood; 2) Areas of inundation for each bottomland for different flood levels, and 3) Levee removal (natural or artificial) strategies in order to flood the bottomlands at a more historical frequency and magnitude. These objectives were met by performing the following: • Implementation of a cross section monitoring program for each reach. • Collecting hydrographic data, including cross section surveys, discharge measurements, and measuring numerous water surface elevations at various discharge levels in order to develop stage/discharge relationships. • Identifying bankfull flooding discharges for each bottomland. • Performing topographic surveys in order to create mapping, estimate and show areas of inundation at different flood levels, and identify levee removal strategies to enhance floodability. • Relate historical flow magnitudes, frequency, duration to existing flooding discharges and inundation conditions for the bottomlands. • Design restoration alternatives to enhance the floodability of the bottomlands while avoiding the overwintering of non-native fishes. The presentation of the results of the study can be split into two categories: 1) BLM Bottomland sites and 2) ONWR Bottomland sites. BLM Bottomland Sites The BLM sites are characterized by high, heavily vegetated natural levees. The BLM bottomland sites partially fill and drain on an annual basis through seepage connections as the river rises during runoff season. However, overbank bottomland flooding does not occur at most sites due to the natural levees and lower peak flows than were experienced before Flaming Gorge Dam and other water projects upstream were developed. Flooding discharges, discharges necessary to develop a significant surface flow connection to fill the bottomlands, for the sites ranged from 12,900 cfs at Above Brennan to 26,500 cfs at Baeser Bend. The average flooding discharge for the BLM bottomland sites was 20,500 cfs. This average discharge occurs approximately every 2 out of 7 years with an average duration of 2.6 days. A discharge of magnitude above the flooding discharge would need to occur for several days to a week in order to significantly fill a bottomland. For example, the flows over 19,700 cfs, the flooding discharge at the Bonanza Bridge Bottomland, occurred for approximately 4 days. This duration was not long enough to fill the bottomland or provide a well defined fish passage. Above Brennan appeared to be the only bottomland which had a functional inlet, which would develop a hydraulic connection frequently. Based on post-1963 hydrology, this bottomland would develop a hydraulic connection approximately every 2 out of 3 years. Restoration alternatives have been proposed for the BLM sites to allow flooding to occur at 13,000 cfs, a discharge with a return period of 1.5-years based in post-1963 hydrology. This flow is equivalent, in terms of frequency, to a approximately a 20,000 cfs flow based on pre-1963 hydrology. After restoration, the duration of connection to the river would range from no flooding during dry years, to 1-3 weeks in average years, or 4-8 weeks during wet years. Restoration designs ranged from a 20 feet channel cut from the river bank to match an existing drain at the Stirrup bottomland, to a large scale 1000 foot of levee removal at