Limiting Potential Failures - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 100

Overtopping
A coal refuse impoundment must be designed so that inflow does not
exceed storage and outflow capacity. If inflow exceeds storage plus designed
outflow (including that handled by spillways, decant facilities, diversion
structures, and evaporation), the dam will be overtopped. Overtopping can
cause substantial erosion of the embankment crest, which, if left uncontrolled,
will usually work progressively downward, releasing water and coal refuse
downstream.
Inflow includes: direct precipitation onto the impoundment area; runoff
from the contributing drainage basin due to precipitation or snowmelt;
groundwater inflows to the basin; outflow from other ponds in the basin; mine-
water disposal; and preparation-plant slurry. Especially important are natural
floods produced by major storms. The designed precipitation event used for a
coal refuse impoundment will vary depending on the hazard classification of the
facility, but in nearly all cases in the Appalachian region will be the probable
maximum precipitation event. This criterion is conservative in that designers
are obliged to provide sufficient storage in the facility to contain direct
precipitation from the probable maximum precipitation event plus all other
influent fluids from processing and runoff and still maintain 3 feet of freeboard.
In determining the probable maximum precipitation event, designers rely on
precipitation records and storm recurrence intervals to predict severe storm
events such as hurricanes, as well as the effect of a reasonable foreseeable rain
or snow runoff event. As discussed in Chapter 3 , diligent slurry and water
management is critically important to an effective coal waste impoundment
system.
BASIN FAILURE MODES
Basin integrity is a routine consideration in all impoundment design.
However, mining near a basin introduces special problems. In the evaluation of
the basin of an impounding structure, the hydrogeological parameters are key in
the determination of potential failure. The control and understanding of the
leakage pathways—such as subsidence, excessive seepage, or internal erosion—
are essential to determine the stability of the impoundment basin and the effect
on the water balance and to comply with regulatory issues of these discharges.
These are common considerations in all impoundment designs. It is essential
that attention be paid to the identification, evaluation, and mitigation of
potential failure modes in the basin.