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<br />drainageways and creeks can also occur from the leaching of the <br />tailing piles (Reference 5). <br /> <br />2.3 Principal Flood Problems <br /> <br />Floods in the mountainous areas of the Front Range can result <br />from rainfall or rainfall in combination with snowmelt. Generally, <br />the smaller the watershed, the more likely it is that floods will <br />be caused primarily by intense thunderstorms. <br /> <br />Floods in the area are generally caused by intense cloudbursts <br />or thunderstorms during the period of May through August. The <br />steep and narrow nature of the topography combined with the high- <br />intensity, short-duration storms results in floods with high peaks <br />that last only a short time. with development concentrated immedi- <br />ately adjacent to the waterways, little time is left for flood <br />warning and no opportunity to avoid flood damage once rainfall <br />begins. <br /> <br />The peak snowfall month is April. While spring snowmelt undoubtedly <br />contributes to full-flowing streams, it is not the primary cause <br />of flooding. The snowmelt runoff is characterized by periods <br />of high flow and marked diurnal fluctuation. <br /> <br />Black Hawk is typical of many small mountain communities located <br />along streams or gulches in Colorado, in that it experiences almost <br />annual flooding due to very localized intense thunderstorms, some- <br />times in combination with snowmelt. <br /> <br />Each of the three waterways studied in detail is very steep, resulting <br />in very high channel and overbank water velocities when floods <br />occur. Future development of flood plain areas must consider <br />depth of floodwater as well as possibly destructive high velocities. <br />with the limited area available for development, it is likely <br />that the older properties and few remaining lots on which building <br />is possible will receive pressure for development. Therefore, <br />the flood hazard to the community will continue to increase unless <br />adequate flood plain regulations are adopted. <br /> <br />Chase and Gregory Gulches were enclosed in underground conduits <br />below the respective streets in order to make more room for devel- <br />opment. This in turn has resulted in a unique flooding problem, <br />with floodwater flowing in the streets once these conduits reach <br />their capacity. <br /> <br />Obstructions to floodflow within the study reaches include natural <br />obstructions, such as rock, brush, and other vegetation growing <br />along the streambank, and manmade obstructions, such as bridges <br />and buildings. In the two gulches where flow can occur in the <br />roadway, curb stops, mailboxes, automobiles, walls, fences, and <br />other similar features become unintended floodflow obstructions. <br /> <br />5 <br />