PERMFILE107579

• Permit No: M1977526
• IBM Index Class Name: Permit File
• Doc Date: 11/4/1977
• Section_Exhibit Name: DESCRIPTION OF WATER RESOURCES
• Media Type: D
• Archive: No

by the USACE and the United States Gelogical Survey (USGS)
from gaging station measurements and calculations of flow
over Henderson Road to be approximately 30,000 cfs. A
special report on the flood was prepared by the USACE
in July, 1974, titled "Post Flood Report, Platte River and
Tributaries, Nebraska, layoming, and Colorado, Floods of
May-June, 1973". The extent of the flooded area was outlined
on aerial photographs taken duri. the flood. The flooded
area also is reproduced on Mates I, II, anti III. Examination
of the plates indicates that the l:ay 1973 flood was more
extensive than the FIA-UDFCD flood plain south of Henderson
Road and less ext~:nsive north of the road. Levees we
overtopped in several locations indicating interaction
between legit overbank and channel flows. The flooded areas
fir the Ma}' 6, 1973 flood represent a better appror.imation
of tt~e 100-year flood plain because it is based on documen-
tation of actual flooding which approximates the 100-year
event. The flood event a:as used to identify the 100-year
flood plain on the mining site. The west edge of the floodway
was estimated by id?ntifying the areas of apparent signifi-
cant. flow on aerial photographs of the flood of May 6, 1973
(Plates I, II, and III). A summary of all of the flood
hazard area delineatic,ns is contained in Table E.
In each of the flood studies, flood waters are shown
to back up behind Henderson Road. In the 1963 USACF. study,
the back-up occured behind a road which is now replaced by
Henderson Road. This creates an area of wider, deeper and
slower moving water south of Henderson Road. It is estimated
that depths of flooding south of Henderson Road range between
2 and 4 feet, and that velocities average 1 to 3 feet per
second. Flood depths north of Henderson Foad are expected to be
1 tc 3 feet and velociti~n 2 to 4 feet per second.
Computations of peak discharges over Henderson Road
for the flood of Clay 6, 1973 indicate that the overbank
flood was approximately 10,000-20,000 cfs in magnitude. This
is approximately 1/3-2/3 of the estimated 27,000-33,000 cfs
Dotal flow. These computations are basc~3 on the UDFCD detailed
topographic mappi;.7; a flood elevation established by the
USGS; and limits of flew established by the U51`.CE 1979
study. 15,OOP cfs is con~idercd an appropriate plan~iing
estimate for the overbank 100-year Food.
Site Drainage
The land between the Brantner Ditch, the South Platte
River, Henderson Road, ar.d the southern boundary of the
property is land which teas historically been irrigated
by the 'Brantner Ditch. Ditch laterals from the Brantner
Ditch co~-rol internal drainage. Storm runoff from the
property has historically flowed towards the laterals and
of _° the property into B_ant::cr Gulch, The first ditch
lateral uc the intersection of the Brantner Ditch ur.d the
property's sou*_herr boundary. The sc end lateral is approx-
ima'-ely 2,400 feet to tiie north. The laterals divert water
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