APPCOR12438

• Permit No: C1981014
• IBM Index Class Name: Application Correspondence
• Doc Date: 6/21/1982
• Doc Name: DORCHESTER 1 MINE FN C-014-81 SECONDARY ADEQUACY REVIEW
• From: MLR
• To: DORCHESTER COAL CO
• Media Type: D
• Archive: No

-6-
Concerns regarding spillway adequacy may be mitigated because extremely
conservative assumptions have been made in the determination of the
design flow. A desig:[ return period storm of 100 years has been used
and no attenuation of the flood peak as a result of storage in the ponds
has been considered. Dorchester Coal Company may wish to use a lesser
return period storm and take flood wave attenuation into account in
order to reduce the design flow for spillway design purposes.
In order to satisfy concerns regarding spillway designs, Dorchester
Coal Company should:
- Revise all design calculations using appropriate values for all
parameters of the design method used,
- Provide a discussion of available freeboard during design flow,
- Commit to improving all spillways found to be inadequate by the
design process in a timely manner following permit approval, and
- Design and install appropriate energy dissipating devices at
the outfall of alI culvert spillways.
6. Pond De[/a to ring Systems
From information provided by Dorchester Coal Company, it appears
that Pond 4 has no mechanism for dewatering the storm storage volume and
that all other sediment ponds have dewatering system descriptions which
are unclear.
The purpose of dewatering systems is to insure that an adequate storm
storage capacity exists shortly after other storm events have passed.
In ponds constructed without dewatering systems, a series of storms, none
of which exceeds the 10-year, 29-hour storm volume, may cause a discharge
through the emergency spillway, which would violate the requirements
of (4.05.6(3)(e)). Such a discharge would have a very short settling
or detention time and would likely not meet the effluent limitations
of (9.05.2(7)).
The Division suggests the use of open orifice dewatering systems,
located at the elevation of the sediment storage volume and sized to
allow between 24 and 36 hours of detention time. For existing ponds,
other systems such as gated outlets and automated pumps are acceptable.
However, some system of pond de[/ate ring must be included in pond designs
(9.05.6(3)(c)).
In order to satisfy the requirement for dewatering of aI1 sediment
ponds, Dorchester should pro vi de.designs and all supporting calculations
for dewatering systems, including:
- A showing that alI dewatering orifices or pump intakes are
located at an elevation which allows for the design se dimeht
volume below and the IO-year, 29-hour storm storage volume above
that elevation; and
- A showing that between 24 and 36 hours of detention time [/ill
be provided for the passage of the 10-year, 29-hour storm.