Introduction - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 15

likely contributed to at least some of these incidents. This report deals with both
of these issues in detail (Chapters 4 and 5 ).
IMPOUNDMENT HAZARD RANKING SYSTEMS
MSHA bases its hazard potential rating system on the height of the
embankment, the volume of material impounded, and the downstream effects of
an impoundment failure (MSHA, 1974, 1983). The resulting three
classifications are:
• Low Hazard Potential —Facilities in rural areas where failure would cause
only slight damage, such as to farm buildings, forest, agricultural land, or
minor roads.
• Moderate Hazard Potential— Facilities in predominately rural areas where
failure may damage isolated homes or minor railroads, disrupting services
or important facilities.
• High Hazard Potential —Facilities whose failure could reasonably be
expected to cause loss of human life, serious damage to houses, industrial
and commercial buildings, important utilities, highways, and railroads.
The MSHA guidelines indicate that design criteria become more
conservative as the hazard potential increases. For example, design criteria for
the maximum precipitation (flood) event increase as the hazard classification
moves from low to high. Thus, storm design criteria for a long-term high-
hazard-potential impoundment require that the impoundment be designed to
contain the probable maximum precipitation that is reasonable for the region
(MSHA 1974, 1983). In addition, piezometers are generally required to monitor
and verify the water saturation conditions within the embankment for moderate
and high hazard sites.
MSHA guidelines further state that the stability of an embankment should
normally have minimum static and seismic factors of safety of at least 1.5 and
1.2, respectively, under maximum anticipated phreatic conditions. The
guidelines require extra attention to seismic events for high hazard
impoundments in certain regions (MSHA, 1974, 1983).
On December 1, 1997, after two unintentional releases of slurry in Virginia
in a two-month period, MSHA introduced a second classification system that
addresses the potential for the unintentional release of water or slurry from
impoundments into active or abandoned mines (Sidebars 1.9 and 1.10 ). This
classification system allows the coal mine operator to evaluate