AREA A/B ENGINEERING REPORT - Waste Management

Geosyntec Consultants
Design of the SWMS considers two primary rainfall factors: intensity (how hard it
rains) and duration (how long it rains). The combination of intensity and
duration defines how large a rainstorm is (how much rain will fall).
Clearly, the likelihood of a very large thunderstorm occurring is much
lower than that of a small everyday shower; therefore, the size of a storm
is defined in terms of its likelihood of occurring. In this way, a storm size
that is expected to occur only once per 100 years (a “100-year storm”) is very much less likely to
occur on any given day than a storm that is expected to occur at least every year (a “1-year
storm”). When designing a SWMS, each state typically has a “design” storm that must be
considered. The typical design storm for permanent stormwater flow control structures is a 100year
storm, a very large rainfall event, which means that the ponds are large enough to contain
stormwater runoff from all expected storms. The typical design storm for temporary stormwater
flow control structures (e.g., channels) is generally a 25-year storm.
Depending on the maximum design flow velocity for the SWMS components, erosion protection
features used to protect surfaces in contact with stormwater include grass and other vegetation,
“rip-rap” (carefully placed layers of rocks, boulders, and stones), “gabions” (rock-filled wire
baskets), erosion matting, and concrete. The landfill management team must understand and
properly interpret rainfall run-off management requirements and regulations pertaining to
compliance, proper SWMS maintenance, and protection of the environment.
The SWMS conveyance features typically include the following components:
• The landfill cover system, which is suitably vegetated, constructed, and graded to promote
runoff, prevent erosion, and controls infiltration;
• Erosion protected cover drainage features such as side slope “let-down structures” or
“downchutes”;
• Grass waterways, diversion ditches, other erosion protected channels, and culverts (i.e.,
pipes running beneath roadways and other obstructions); and
• Outlet control structures such as weirs, sluice gates, or overflow pipes for controlled
discharge to the natural surface water system.
Other specific stormwater controls include detention and retention ponds or basins. Detention
basins hold water for a limited period from a larger drainage basin area to prevent flooding.
These basins then release their water slowly, usually through an outlet pipe (or spillway in heavier
flow conditions), and are often used for de-silting runoff. Retention basins are used to contain
stormwater or rainfall run-off for longer periods. A retention basin provides an area to hold
water from a small surrounding drainage area that would otherwise flow into other areas. The
water is designed to remain in the local area with a limited outflow (e.g., a pond or lake).
MD10186.doc 64 29 March 2009