GENERAL53493

• Permit No: M1989065
• IBM Index Class Name: General Documents
• Doc Name: CASTLE ROCK QUARRY APPLICATION TO DOUGLAS CNTY FOR PHASE III DRAINAGE PLAN
• Media Type: D
• Archive: No

' C. Hydrologic Criteria
~' 1. Design rainfall. This site is located in Douglas County Zone I as shown on Figure 501 of
the Douglas County Storm Drainage Design and Technical Criteria (DCSDDTC) manual.
Please fmd a copy of this figure included with this report.
In the pre-mining runoff prediction, the SCS's Worksheet 3 (from the TR-55 program)
was used to compute times of concentration. Worksheet 3 requires the two-year 24-hour
rainfall depth. Since this is not provided in the DCSDDTC manual, rainfall data was
gathered from the NOAH Atlas 2Precipitation-Frequency Atlas of the Western United
States, Volume III -Colorado. For the two-yeaz 24-hour storm event, a precipitation
value of 1.9 inches was estimated.
The peak depths of the 10-yeaz and 100-year 24-hour storm events was estimated at 2.9
inches and 4.4 inches, respectively, from the NOAA Precipitation Atlas. The County has
requested that the Castle Rock Quarry site be capable of retaining storm water from
consecutive 24-hour 100-yeaz events. Therefore, the 4.4 inch rainfall depth was doubled
to 8.8 inches.
' These rainfall depth estimates are considered conservative because the Castle Rock
Quarry site is located in a poorly defined azea of the NOAA rainfall isopluvial maps.
Figures C and D are the maps from the atlas, with the location of the Castle Rock Quarry
pin-pointed to verify the previous statement.
2. Runoff calculation method.
Pre-minine
1 The Sedcad drainage modeling program was used to predict the peak dischazges and
volumes for the 10- and 100-yeaz 24-hour storm events. The following steps were taken
to predict runoff for the pre-mining sub-basins:
a) The sub-basins were delineated on the plan maps and their areas calculated.
b) Runoff design points were selected at the property line (Winkler lease boundary)
for each of the sub-basins.
c) The longest runoff travel paths were delineated and drawn on the plan maps.
l
d
i
th
CS'
TR
5
h
d
f
d) concentration were ca
culate
us
ng
s
-5
T
e travel time an
time o
e S
Worksheet 3.

e) Area weighted SCS Curve Numbers were calculated. The upland areas (where the
mining will occur) have a Curve Number of 81 and the minor drainageways
below the mine site have a Curve Number of 57 (See SCS's TR-55 Table 2.2d in
Appendix B).
f) The compiled data was put into the Sedcad program and output results reviewed.