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Last modified
1/25/2010 6:27:14 PM
Creation date
10/4/2006 11:45:18 PM
Metadata
Fields
Template:
Floodplain Documents
County
Pitkin
Community
Pitkin County, vols. 1-3
Stream Name
Roaring Fork River
Basin
Colorado Mainstem
Title
Flood Insurance Study - Pitkin County Incorporated Areas, Volume 1 of 3
Date
6/4/1987
Prepared For
Pitkin County
Prepared By
FEMA
Floodplain - Doc Type
Historic FEMA Regulatory Floodplain Information
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<br />The soils in and around Basalt are variably textured, water-de- <br />posited, sandy, and loamy, and with varying ~ounts of cobble and <br />gravel occurring in the valley bottoms and benches. The mountains <br />vary from colluvial to the no~th, basalt lava to the southwest, <br />and sandstone and shale to thE! east and west. <br /> <br />The watercourses in Aspen have steep, well-defined channels with <br />heavily vegetatedoverbanks. 'Channel bottoms are covered with <br />cobbles and boulders. <br /> <br />Average annual precipitation for Pitkin County is approximately <br />19.25 inches (Reference 4). On the average, 11.8 inches of this <br />precipitation is snowfall and 7.5 inches is rainfall. Average <br />temperatures range from approximately 200p during the winter to <br />600p to 700p during the summe~. <br /> <br />2.3 principal Flood Problems <br /> <br />Flooding fram streams in Pitkin County usually occurs during May <br />through August, with the principal cause of flood-flows being snow- <br />melt runoff. Summer rains do not cause major floods in this area. <br />Thunderstorm activity creates ,major floods only on small tributary <br />streams because of the storms~ short duration. <br /> <br />Major floods on the Roaring FQrk and Fryingpan Rivers result from <br />rapid melting of mountain sn~pack during the period from late May <br />through early July. These snOwmelt floods are characterized by <br />moderate peak flows, large voiumes, long durations, and marked <br />diurnal fluctuations in flow. Rainfall on melting snow may acceler- <br />ate the rate of the snowmelt, 'thus augmenting floodflows. <br /> <br />The largest discharge recorded on the Roaring Fork River was in <br />July 1957, with a peak of 19,QOO cubic feet per second (cfs). A <br />ODE letter report (Reference $) describes the 1957 flood as follows: <br /> <br />Floods on the Roaring Fo~k result fram snowmelt and occur <br />, <br />principally during the mQnth of June. The flood of July 1, <br />1957 had an instantaneous peak discharge of 18,700 cfs <br />at the Glenwood Springs gage. This flood was the maximum <br />of 49 years of record, and its magnitude has an estimated <br />frequency of occurrence ~f once in approximately 60 years. <br /> <br />The river in the problem! area is characterized by low <br />banks, braided channels, land a considerable amount of <br />gravel, cobbles, and sna9s deposited on gravel bars. <br /> <br />T~e river carries a large bedload of gravel and cobbles, <br />same of the latter being'more than 6 inches in diameter. <br />The capacity of the channel has been reduced by this <br />sediment. In same instances, where channel changes took <br />place during the flood, ~he original channels were so <br /> <br />6 <br />
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