1991-05-16_REVISION - C1981035 ‎(2)‎

• Permit No: C1981035
• IBM Index Class Name: REVISION
• Doc Date: 5/16/1991
• Doc Name: Hydrologic Design of East Drainage Channel and Sediment Ponds
• From: Don May
• To: DMG
• Type & Sequence: TR2
• Media Type: D
• Archive: No

Total storm depths were obtained from the National Oceanic and
Atmospheric Administration (NOAA) rainfall atlas (Miller, 1973) .
Standard Soil Conservation Service methods, available within the
HEC-1 package, were used to determine excess rainfall and unit
hydrograph routing for the watershed. The previous table lists the
watershed parameters for the 10yr and 25yr events.
Flows were routed through the pond using the Modified Puls method
assuming level pool conditions. As stated the modeling scenario
consisted of the pond, initially empty, with subsequent inflow of
the 10yr and 25yr events, back to back. The volume of the 10yr-24hr
event is determined to be 2 . 1 AF and the annual sediment load is
estimated at 0. 1 AF, resulting in a total required pond volume of
2 . 2 AF. The existing pond geometry will be modified to attain this
volume. The HEC-1 computer run to support the flood volume appears
in the appendix 2, and the sediment yield is discussed in the next
section. The peak inflow from the 25yr event is 7 cfs (appendix 3) .
In turn the peak outflow through the combined spillway/low level
outlet is 7 cfs. Most of the flow passes over the 8 ft. sharp edged
weir/spillway. The maximum stage is 98 ft leaving the required 1 ft
of freeboard to the top of the embankment, elevation 99. The crest
elevation is 97.7 ft. Elevations associated with the pond are based
on an assumed datum used in developing the topographic map supplied
by National King Coal Company.
In order to achieve a 48 hr drainage time for a full pond, 10yr-
24hr event, to empty, the size of the low level outlet was varied
until the desired result were obtained. Documentation of the
results can be found in the computer runs in the appendix 4. A
circular orifice with a 3 .2 inch diameter is required.
Watershed Sediment Yield
Sizing criteria for the sedimentation ponds must include the
effect of the annual sediment yield from the watershed as well as
the volume of the 10yr-24hr event. Numerous empirical equations
have been developed for estimating annual sediment yield. Probably
the most popular are the variations of the Universal Soil Loss
Equation (USLE) . This method was originally developed for
agricultural lands and is dependent on at least 6 parameters which
describe the physical, land use, and management characteristics of
the basin. For watersheds with no observed data on these
parameters, such as those at the National King Coal Mine site, the
use of this type of equation requires guestimating most of the
parameters. The errors in this could be excessive. Furthermore,
most of the sediment yield in the National King Coal Mine
watersheds is most likely is derived from channel erosion which in
not considered in the USLE formulation.
' As an alternative to empirical estimates of sediment yield, this
study uses measured values from around the country to bracket the
3