1993-10-08_REVISION - C1981017

• Permit No: C1981017
• IBM Index Class Name: Revision
• Doc Date: 10/8/1993
• Doc Name: Bid Documents ‎(IMP)‎ Revision MR-45
• Type & Sequence: MR45
• Media Type: D
• Archive: No

Peak Flow Calculations for the the Dutch Creek Drainage
The draiange basin of Dutch Creek is approximately 2816 acres of
steep pine and aspen forest with some grass pasture areas and some
steep bare rock slopes. The CN number for the basin is calculated
as follows :
Area CN Number Weighted Average
15% Bare Rock 85 12 . 75
20`K Grass 72 14 . 40
65% Aspen & Pine 68 44 . 20
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Avg. = 71 . 35
The hydraulic length of the watershed is approximately 24000 feet ,
mostly in Dutch Creek itself . The Storm program, developed by OSM,
has been used to predict the peak flow for Dutch Creek . Other input
parameters of the watershed are shown in the computer printout on
the following page. The SCS Manual "Procedures for Determining Peak
Flows in Colorado" was used to obtain the 100 year - 24 hour peak
precipitation of 2 . 8 inches .
As can be seen from the printout , the peak flow for the 100 year
event is 268 . 6 cfs . Past experience with the drainage has shown
that high runoff events are normally accompanied by debris flows ,
therefore, the design for any structures at the mouth of Dutch
Creek will be based on a peak flow of 500 cfs .
Drawing II -13-7 - ATT 6-21 of the permit shows the plan view design
for the existing flume in Dutch Creek . This flume is a concreted
trapezoidal trough of 11 . 7 feet width and 4 . 0 feet height . The
length of the diversion is approximately 500 feet . it is more than
adequate to handle the peak flow of the 100 year - 24 hour event .
At the outlet of the flume, a concrete vertical apron was installed
to deliver the Dutch Creek flow into Coal Creek , lying
approximately 55 feet below. This apron is cracking and the
immediate area of the trough leading to it is also showing wear .
For this reason , it has been decided to replace the last 180 feet
of trough with a 9 ft. diameter steel pipe with a concrete pillar
support placed under the pipe outlet . The design for this
installation is included on the following pages .
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