FLOOD07296

• Designation Number: 569
• County: Mesa
• Community: Grand Junction
• Stream Name: Bosley Wash
• Basin: Colorado Mainstem
• Title: Bosley Wash Drainage Master Plan - Phase 1: Watershed Floodplain Delineation
• Date: 11/1/2002
• Prepared For: Bosley Wash Watershed
• Prepared By: GMS
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Bosley Wash Drainage Master Plan Phase I: Watershed Floodplain Delineation
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3.1 Precipitation Summary

There is a lack of a quantitative historical precipitation record within the Bosley Wash Basin,
Precipitation values used within the Grand Valley are often based on the isopluvials published
within the NOAA Atlas or depth-duration-frequency curves developed at the Walker Field
Airfield gaging station in Grand Junction, These show a large amount of variability throughout
the basin,

The NOAA Atlas publishes a lOO-year, 24-hour value of PIOO = 2,6 inches, Grand Junction area

gages show a wide range of estimated lOO-year, 24-hour rainfall depths, ranging from 1.54

inches at Walker Field to 2,7 inches at the Colorado National Monument and 2,1 inches at an

historical gage within Palisade. HDR has chosen to use the NOAA Atlas values as being the

most representative of the conditions within the Bosley Wash Basin, The precipitation values

used in this study are as follows:

· P2 = 1.1 inches

· Ps = 1.4 inches

· PIO = 1.7 inches

. Pso = 2.3 inches

· PlOo = 2,6 inches

· Psoo = 3.2 inches

An SCS Type II, 24-hour storm distribution was used for this study, Type II storms are typical
of interior, continental thunderstorms, and are representative of the location of this basin,

3.2 Storm Event Reconstruction

There are no formal precipitation gages within the Bosley Wash Watershed, so the extreme

storm events of 1999 lacked quantification, HDR reconstructed these precipitation events using

information from the Doppler radar system located on Grand Mesa, Based on this evaluation, it

was determined that the July 24, 1999 storm generated 1.7 inches of rain in approximately 90

minutes, The August 11, 1999 storm generated 1.3 inches of rain in less than 30 minutes, These

events had relatively low total rainfall amounts when compared to the 24-hour storms listed

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