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<br />2.3 Principal Flood Problems <br /> <br />There is little definitive data available on past flooding in the <br />Hinsdale County and the Lake City area. Knowledge of past flooding <br />in Lake City is primarily based on newspaper records existing from <br />1884 to 1933 and after 1979. <br /> <br />The worst flooding ever experienced in the upper Lake Fork Valley <br />came in mid-June of 1921 (Reference 1). This occurred when <br />unusually high spring runoff caused Lake Fork and Henson Creek to <br />suddenly spillover their banks and threaten to engulf the Lake <br />City community. The high runoff during this flood event can be <br />attributed to a heavy snowmelt, along with an estimated rainfall of <br />seven inches in the Lake Fork Valley the week of June 3 to June 10. <br />Damage during the flood was primarily restricted to road and <br />railway beds in the immediate vicinity of Lake City. There was no <br />substantial building damage; however, severe erosion affected <br />streets, and the Gunnison Avenue Bridge over Henson Creek was <br />washed out. <br /> <br />Flooding in the study area can be aggravated by the steep <br />topography, particularly in tributary basins and at headwaters. <br />Tributaries may drop as much as 3,000 to 4,000 feet over 3 or 4 <br />miles. The grade below 10,000 feet in elevation is typically more <br />gentle. Obstructions to floodflow in the study area can include <br />vegetation, ice jams, bridges, buildings, and roadways. <br /> <br />2.4 Flood Protection Measures <br /> <br />There are no levees, diversions or dams of significance on Henson <br />Creek to promote formal flood protection. <br /> <br />However, in Hinsdale County in the Lake City area, Lake Fork and <br />Henson Creek have stayed within their banks during the last 20 <br />years. This is partially due to constant and continuous clearing <br />of channels and regular additions of heavy riprap to the banks. <br /> <br />Lake San Cristobal is a natural lake approximately 3.5 miles above <br />Lake City on the Lake Fork of the Gunnison River. This lake has a <br />dampening effect on potential floods such as those occurring from <br />high-intensity, short-duration rainfall and combination spring <br />snowmelt; however, the effect of the lake on peak floodflows is <br />insignificant. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the community, standard <br />hydrologic and hydraulic study methods were used to determine the flood <br />hazard data required for this study. Flood events of a magnitude which <br />are expected to be equaled or exceeded once on the average during any <br />100-year period (recurrence interval) have been selected as having <br /> <br />4 <br />