30 Sep 01
other grasses, turf reinforcement mats, and rock armor. Other design considerations are described in
Paragraph B-3b Erosion Control Layer.
(h) Floodplain Considerations. Construction of the cover near a waterway may require
placement of material within a floodplain. This may increase river stages and could result in greater
flood damage to surrounding properties. Consequently, it may be necessary to design diversion
channels, levees, or provide other flood protection measures to minimize impacts to surrounding
properties. Erosion control materials may also need to be placed on potentially submerged areas of the
landfill cover system.
(i) Storm Water Runoff Calculations. Selection of an appropriate method of calculating
runoff is based on the size of the drainage area and the required output. Typically a 25-year 24-hour
storm event is used to design landfill cover drainage systems. Other storm events may be used,
depending on site specific regulatory requirements. Commonly used methods to determine runoff
include: the Rational Method; the Peak Discharge Method; the Tabular Method (TR-55); and the Unit
Hydrograph Method. Additional information for each of these methods can be found in the text
Hydrologic Analysis and Design (McCuen 1989). The TR-55 program can be obtained at
National Weather Service (NWS) Technical Paper No. 40 entitled Rainfall Frequency Atlas of the
B-5. Civil Design. There are many other features that must be addressed during design that are
integral components of a landfill cover construction project. These features are discussed in the
a. Site Access Routes. Both public and private access routes to the site need to be investigated to
ensure that they can handle anticipated construction traffic. The contractor should be required to
maintain any access routes including post-construction rehabilitation, if necessary. Aggregate surfaced
roads may need to be built to allow access of vehicles or for operation and maintenance of the landfill.
Access roads are often located around the landfill cover perimeter and may also be located on the
cover system as well.
b. Staging Areas and Phasing Requirements. The contract documents
should identify acceptable
locations for support facilities, storage areas, and parking. Construction phasing requirements should
also be identified in the contract documents to prevent the uncontrolled migration of contaminants,
surface runoff, and dust.
c. Decontamination Facility.
(1) General. The contractor must decontaminate all vehicles and equipment that enter the
exclusion zone or contamination reduction zone. The contract documents should address the re-
quirements for a decontamination facility and for the treatment and disposal of rinsate water. The final