2003-08-14_AgendaPacket - Laserfiche WebLink

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4.0 HYDROLOGIC MODELING AND RESULTS 4.1 Methodoio_w To assess the likelihood of flooding in Twin Lake, and to estimate the 100 year flood levels for the lake, the Barr Watershed Model was used. Watershed characteristics, including use, soil type, and slope were described. Culvert characteristics and capabilities were analyzed. Storage volumes - in the lake, in the basin southeast of the railroad tracks, and in the depression southeast of the trailer park - were computed and used as input for the model. Ail modeling runs were made with the assumption that Twin Lake would not be receiving overflow runoff from the Yadnais Lake watershed. This assumption is critical, anal is discussed further below. A range of storms and runoff events were analyzed in order to determine which events would be critical in causing extreme water elevations in Twin Lake. Two snowmelt events -the 100-year 30-day event, and the 100-year 10-day event, were considered; the 100-year rainfall events of 1, 2, 3, 6, and 12 hours, and 1, 2 and 4 days were also evaluated. Since the topographic map showed the existing low house low floor elevation at approximately 877 feet MSL, a maximum allowable flood elevation of approximately 875 feet was determined. With Elevation 875 feet as an upper Iimit, the model was run with the storm and snowmelt series described above to find a starting elevation for Twin Lake such that the lake Level would not exceed Elevation 875 feet as a result of the critical 100-year runoff event. 4.2 Modeling Assuming that all existing culverts and topography were unchanged, modeling shows the maximum allowable beginning lake level to be about 870.7 feet MSL (see Table 1). Throughout the modeling, rainfall events proved to cause Less lake level fluctuation than the snowmelt events; the model predicted that spring snowmelt ninoff would be of more ccncern than suri-.mer rainstorms. 231621264\TWINLAKE.RP'I'IKMH 7