PERMFILE128415

• Permit No: C1996084
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 15 POND DRAINAGE DESIGN
• Media Type: D
• Archive: No

• Exh~bi! l5 ~ Pond Drainage Design
shrub communities at 20-35% live cover, respectively), SEDCAD+ calculated the peak dischazge of
a 2-yr 24-hr storm immediately above the P1/2 portal azea to be in excess of 2000 cfs. This
SEDCAD+ calculated flow is much larger than that measured by USGS gages in the Purgatoire
River downstream of its confluence with the Lorencito drainage. For example, USGS gage data for
the Purgatoire River at Madrid above Trinidad Reservoir, with a contributing watershed of 505
squaze miles, reports summer monthly flows which razely rise over 300 cfs (USGS, 1995, Colorado
Water Resources Data). Furthermore, the non-erosive channel capacity flow estimated using [he
channel morphology of Lorencito River at LC3 (see Exhibit 8) showed the average storm to range in
size from 25 - 100 cfs, and first year crest flow measurements at LC-3 and LC-4 ranged from 100-
200cfs.

A review of the literature (Barfield and Haan, 1994) suggested that total cover values should be used
in assessing the curve numbers. The subsequent reassessment of the vegetation community cover
data yielded much lower curve numbers than originally assumed (Table 3).
TABLE 3
LORENCITO CANYON MINE-VEGETATION COMMUNITY
GROUND COVER and CURVE NUMBERS
CN* CN*
Vegetation Bare Live Cover Total 40% total 80% total
Community Litter Rock Ground (vegetation) Cover cover cover
Ponderosa Pine 72.4 9.6 6.4 11.6 93.6 71-83 61-72
- Ref
Mountain Shrub 42.7 13.5 7.9 35.9 92.1 65-73 44-57
- R3
Grassland 49.I 7.1 16.5 27.3 83.5 74-80 50-61
- Ref
"Range in CN relates to that between C and D soils, respectively.
As a consequence, curve numbers in SEDCAD+ runs designed to estimate the flows ofmajor creek
crossings (Table 4) have been modified to provide more realistic peak dischazge estimates. In
Lorencito Canyon, Ponderosa Pine communities are now run at a CN = 66 (average of C & D soils,
80% cover); Mountain Shrub communities are now run at a CN = 51 (average of C & D soils, 80%
cover). Additionally, in an effort to better characterize the influence of a storm on a large
undisturbed watershed, the hydrograph response in both Lorencito and Cow Canyon major crossings
has been changed to 'Slow'.
Similarly, the curve numbers in SEDCAD+ runs for major crossings along Cow Canyon were
reduced. However, the reduction is not as great: Mountain Shrub communities are now run at 61;
• and Grassland communities are run at 56. This is due to the fact that while it is unlikely that a single
summer storm event could encompass the entire Lorencito Canyon watershed, it could potentially
(revised 4/20/01)