2020-08-07_PERMIT FILE - C1981010 ‎(117)‎

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 8/7/2020
• Doc Name: MIDDLE PYEATT HYDROLOGIC MODEL
• Section_Exhibit Name: APPENDIX Q SECTION VII
• Media Type: D
• Archive: No

Middle Pyeatt Ponds
The Middle Pyeatt Pond system consists of three sediment ponds in series at the locations
shown on Figure 1, "Middle Pyeatt Area As-Built Ponds." The ponds are designed to operate in
series to treat the runoff from the 10 year -24 hour storm to meet the 0.5 ml/I settleable solids
standard, and to safely pass the 25 year — 24 hour storm runoff through each embankment's
spillway. The SEDCAD computer program was used to model the performance in both storm
events.
The 10 year— 24 hour and 25 year/ 24 hour storms (hereafter 10/24 and 25/24) are 1.40" and
1.80" respectively. The combined watershed of all three ponds is 383.91 acres, 361.1 of which
report to the uppermost pond, with an additional 8.76 acres reporting to the middle pond and an
additional 14.05 acres reporting directly to the lowermost pond. The effluent quality exiting the
lowermost pond is the parameter of interest in the 10/24 storm, while spillway adequacy for
each of the three ponds is the parameter checked for the 25/24 storm.
Runoff is calculated using the SCS Curve Number method which assigns a curve number based
on land use condition. As shown on Figure 1, most of the watershed (226.82 acres) is
reclaimed land which has been assigned a Runoff Curve Number of 67 in accordance with the
SCS Curve Number procedure. The next largest land use category is undisturbed land,
assigned a runoff Curve Number of 49. Finally there are 72.2 acres of active disturbed land,
mainly along the eastern boundary of the watershed, assigned a Runoff Curve number of 87.
The sediment yield from the entire watershed is determined by the SEDCAD program using the
MUSLE (Modified Universal Soil Loss Equation). Input parameters used in this part of the
analysis are presented on the results pages (pages 13 and 14) of the model output for the 10/24
storm.
All three ponds have trapezoidal earthen channel spillways, with control section elevation,
bottom width and side slopes as described in the SEDCAD output. Area-Capacity tables for
each pond are presented on Figure 1 as well as included in the tables in the SEDCAD output.
Each pond is assumed to be full to the spillway elevation at the start of the 10/24 and 25/24
events. Spillway details and the stage storage curve are presented on the original design
drawings.
The runoff inflow into each pond is then routed through the pond to determine the peak pool
elevation while passing the storms. Because the ponds are in series, the inflow into the middle
pond includes the runoff from its direct watershed plus the routed outflow from the uppermost
pond. Similarly, the inflow to the lowermost ponds includes the routed outflow from both upper
pond plus the runoff from its directly tributary watershed.
As shown in the SEDCAD output, the 24-hour average settleable solids concentration released
from the lowermost pond is 0.00 ml/l, well below the 0.5 ml/I settleable solids standard.
In the 25/24 event, the ponds spillways flow at 0.69, 0.13 and 0.33 feet deep for the uppermost,
the middle and the lowermost ponds respectively, leaving freeboard to the dam crest of 1.31
feet, 2.87 feet and 2.67 feet respectively. In conclusion, the existing pond network is adequate
under these conditions.
The following materials accompany this submittal: Revision:
Approved:
1 August, 2018