PERMFILE127679

• Permit No: C1981023
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: EXHIBIT 7 SEDIMENT CONTROL DESIGN CRITERIA
• Media Type: D
• Archive: No

above the Sediment Trap). Field observations that little or no deposition of sediment in the trap •
validate the SEDCAD+ model. The existing Sediment Trap (see Sediment Trap A Design
Criteria) is included as a Small Area Exemption Demonstration. Tr<<p capacity information and
curve (SEDCAD+ BASIN CAPACITY UTILITY Sediment Trap A) were developed from the
Stage Storage Data on the As-Built. To model the trap, a spillway (required by the SEDCAD
routine) was described. However, the trap has no spillway. Designs of ditches lA and 1B aze
presented under paragraph 7-7.
7-3 Pond 002
The SEDCAD+ Version 3 demonstrations appropriate to Pond 002 are C:\SEDCAD3\CR7\
PND2A9, Date 01-30-1996 Time 08:21:47 (10-yeaz, 24-hour pre~;ipitation event) and Time
08:24:22 (25-year, 24-hour precipitation event). Contributing watersheds drain, via ditches 2Na,
2Nb, 2S, 2S1, 2S2a, 2526, and 2S2c to Pond 2. Watershed definition is based primarily on
degree of slopes. The steep watersheds aze characterized by repetitive terraces and benches. The
west-facing slope is divided into two watersheds by the access road to the top of the reclaimed
area. Pond 002 configuration is taken from the Sedimentation Pond As-Built, Pond 002, date 7-
7-91, Rev. 1. Basin capacity information and curve (SEDCAD+ BASIN CAPACITY UTILITY
Pond 2) were developed from the Stage Storage Data on the As-Built. The as-built configuration
[baffle plate over the lower half of a 24-inch corrugated metal pipe (CMP) trickle tube] of the
primary spillway is not compatible with SEDCAD+ documentation. To simulate the as-built
configuration, a trickle tube of equal azea (17-inch CMP) was used, and the top of both tubes
placed at the same elevation to better match discharge flow characteristics. (As the actual invert
of the primary spillway is 97.0 in-lieu-of the 96.6 modeled, there would be no dischazge through
the primary spillway until the level of water in the pond exceeded 97.0.) Kaiser Steel Resources, •
Inc., proposes to maintain the water level of the pond below 94.7 (Maintenance Water Level) by
pumping. This operational procedure is simulated by a fixed siphon tube. With regard to the
10-year, 24-hour precipitation event, the design sediment storage volume is 0.07 acre feet,
between elevations 93.5 and 93.74. The Peak Settleable Concenb~ation of pond discharge is
demonstrated at 0.002 mUl (< 0.50 mUl criteria). The Peak Stag: is 97.39 (below the 98.0
elevation of the emergency spillway). There would be no dischazge over the emergency spillway.
The caution statement (SEDCAD+ Pond InpuUOutput Table) is generated due to the proximity
of the top of sediment storage and the simulated fixed siphon tube, azid is not applicable to actual
conditions. Cleaning of sediments from the pond is scheduled when 60% of the design sediment
storage (0.042 acre feet) is occupied, a maintenance sediment elevation of 93.64 (interpolation
between 93.5 and 93.74). Peak stage from a 25-yeaz, 24-hour precipitation event is 98.03,
providing 1.97 feet of emergency spillway freeboard to the top of embankment. Peak dischazge
over the emergency spillway is 1 cubic feet per second (cfs) (interpolation between 0 @ 98 and
16.6 @ 98.5 from the Elevation-Discharge Table). Riprap is not technically required to protect
the emergency spillway crest (SEDCAD+ ERODIBLE CHANNEL DESIGN). Riprap is required
to protect the emergency spillway exit channel (see SEDCAD+ RIPItAP CHANNEL DESIGN).
The emergency spillway crest is (good engineering practice and to provide additional protection
from scour) protected with riprap. The required depth with freeboard is 1.07 feet, two feet aze
provided. The required depth of the emergency spillway exit chamt:l, with 1 foot of freeboard,
is indicated as 1.01 feet. The SEDCAD+ model (Simons/OSM M<:thod) would not accept the
low flow of 1 cfs over the 20-foot-wide exit channel; therefore, a vridth of 10 feet was used in
the analysis. Thus actual depth would be close to 0.05 foot. Pond tonfiguration is summarized •
in the figure titled Sedimentation Pond 002 Supplemental Information, 1/96. Designs of ditches
2Na, 2Nb, 2S, 251, 2S2a, 2S2b and 2S2c are presented under Paragraph 7-7.
']-6 8 May 1996