30 Sep 01
and conduct surface runoff to a stable outlet or drop structure. Terraces are normally trapezoidal in
shape and have a minimum depth of 300600 mm (12 ft). Terraces should have enough capacity to
control the design runoff event, which is typically the result of a 25-year, 24-hour rainfall event. Flow
velocity depends on channel slope, friction, discharge depth, and flow volume. The flow velocity must
be analyzed for all terraces, ditches, and drop structures to ensure that erosion will not occur. If grass-
lined channels are not adequate, riprap, grout bags, geosynthetic erosion control materials, or gabions
can be used to armor the side slopes and bottoms of terraces. The length of drainage terraces is
controlled by flow capacity and non-erosive velocity requirements. Drainage terraces can also function
as both an anchor for geosynthetics and a buttress for cover soils to improve cover stability.
(c) Drop Structures. Terraces normally discharge into collection ditches or drop structures
that descend down the steep slopes of a cover. Depending on the slope, a drop structure may be
constructed of vegetation, erosion control mats, riprap, or gabions. As with the terraces, the drop
structures are usually trapezoidal in shape and must be hydraulically sized. A stilling basin at the toe of
the cover is sometimes required to dissipate flow velocities prior to discharging the water off-site.
(d) Perimeter Drainage Control. Surface runoff from the cover should be controlled to
prevent adverse impacts to adjoining properties and receiving waterways. Collecting the water in lined
or unlined perimeter channels or storm sewers should control surface runoff.
(e) Off-site Discharge Control. Depending on the final cover's design, there may be an
increase in both the total volume and the peak discharge of surface runoff leaving the site. The effect on
the receiving stream from this increased runoff volume should be a design consideration. In addition, the
potential downstream impacts of increased sedimentation both during construction and during normal
project operations should be assessed. Off-site discharge can be regulated with detention ponds, which
store water and trap sediments. State and local regulations must be assessed when discharge patterns
are changed. National Pollution Discharge Elimination System (NPDES) requirements for storm water
discharges must also be assessed.
(f) Temporary Erosion Control Measures. During construction, waste and soil will be
exposed to the elements. Temporary erosion control measures such as mulches, hay bales, erosion
control mats, and silt fences may be used until permanent measures are established. Temporary
vegetative covers, using appropriate rapidly growing annual plants, can be applied to finish grades as a
project progresses. It may be necessary for some temporary measures to be applied on the same site
several times over the course of the project. Requirements for temporary erosion control should be
specified in the contract documents.
(g) Permanent Erosion Control Measures. The selection of permanent erosion control
measures is based on the location of the site, agronomic principles, availability of construction materials,
and future maintenance requirements. Permanent erosion control methods include seeding of native and