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<br />Green River Floodplain Inundation <br /> <br />15 <br /> <br />June 2005 <br /> <br />dam) peak flows at the Jensen gage is about 447 m3/s (15,800 cfs) (Figure 4). A flow of 396 m3/s <br />(14,000 cfs) had about a 60% probability of exceedance and a flow of 368 m3/s (13,000 cfs) had <br />about a 70% probability of exceedance under historical regulated conditions. All 1,917 ha (4,737 <br />ac) of the total priority depression floodplain area become inundated at about 716 m3/s <br />(25,300 cfs), which historically would have occurred in about 44% of years (i.e., a 44% <br />probability of exceedance) under unregulated conditions and in about 10% of years (i.e., a 10% <br />probability of exceedance) under regulated flow conditions (Figure 4). <br /> <br />4.3 Area ofInundation <br /> <br />In general, the total area of inundated depression floodplain habitats increases as peak- <br />flow levels increase (Figure 4). However, the increase in inundated area does not increase <br />continuously, but in discrete increments as flows reach levels that are high enough to pass <br />through water-control gates or levee breaches or overtop levees. In the middle Green River, a <br />flow of 527 m3/s (18,600 cfs) is not adequate to overtop most levees. As shown in Figure 4, the <br />amount of priority depression floodplain inundation that occurs at 396 m3/s (14,000 cfs) is not <br />different from the amount of inundation that occurs at 527 m3/s (18,600 cfs). Figure 5 shows the <br />cumulative area of priority floodplain depression habitats (Valdez and Nelson 2004) that would <br />be inundated at flows of 396 m3/s (14,000 cfs) or 527 m3/s (18,600 cfs) in relationship to <br />distance downstream of the spawning bar. The following summarizes the amount of inundation <br />that would occur at key flow levels: <br /> <br />· At a flow of 368 m3/s (13,000 cfs), approximately 891 ha (2,201 ac) or 55% of the <br />total of priority depression floodplain area would be inundated. <br /> <br />· At a flow of396 m3/s (14,000 cfs) approximately 1,027 ha (2,537 ac) or 63% of the <br />total priority depression floodplain area would be inundated. This additional area <br />results from connecting 136-ha (336-ac) Old Charley Wash-Main which is located <br />97 km (60 mi) downstream of the razorback spawning bar. <br /> <br />· At a flow of 527 m3/s (18,600 cfs), about 1,027 ha (2,537 ac) or 63% of the total <br />priority depression floodplain area would be inundated. This is the same amount of <br />habitat inundated by flows of396 m3/s (14,000 cfs). <br /> <br />· At a flow of 566 m3/s (20,000 cfs), about 1,080 ha (2,669 ac) or about 66% of the <br />total priority depression floodplain area would be inundated. This additional area <br />results from connecting 53-ha (132-ac) Sportsman's Lake. Flows of this magnitude <br />occurred in about 30% of years under historic regulated conditions. <br /> <br />· At a flow of 716 m3/s (25,300 cfs), the Sheppard Bottom levee is overtopped and an <br />additional 546 ha (1,350 ac) of floodplain area is inundated. At this flow, 100% of the <br />total priority depression floodplain area identified by Valdez and Nelson (2004) is <br />inundated. Flows of this magnitude occurred in approximately 10% of years under <br />historic regulated operations. <br />