CEMP-ET
TI 804-01
1 July 1908
(5) Consider porous surfaces that allow infiltration (e.g., porous asphalt and
concrete, gravel, open-cell paving systems, and turf) as paving alternatives.
b. Grading. Topography is the primary determinant in the amount, direction and rate of
runoff. Existing drainage patterns should be maintained where possible to preserve topsoil.
Facilities and parking areas should be sited to take advantage of existing topography (figure
4-11). Graded slopes should be gradual and avoid abrupt changes in gradient. Where graded
slopes meet the existing topography, they should blend into the existing slope. The AEI
provides guidance concerning slope gradients.
c. Positive Drainage. Positive drainage should be provided universally across the site.
Figure 4-12 illustrates the following basic principles:
(1) Direct water away from structures.
(2) Do not allow water to pond at low points or in low areas.
(3) Locate the finished floor elevations of buildings so that if drainage structures are
blocked, the water will not back up into the buildings.
(4) Direct water concentrated in parking lots and along curbs away from major
pedestrian areas and routes.
d. Drainage Control. Storm drainage from buildings and other impervious surfaces must
be directed to a storm drainage system. This water must be kept from infiltrating into the soils
and causing foundation problems for the buildings. It also must be directed away from
pedestrian walkways where it causes walking hazards. Runoff should be directed away from
landscaped areas not designed to accommodate Storm water runoff to keep from drowning
plant material and washing away mulch. Drainage can be controlled, captured and redirected
using various methods.
(1) Vegetated swales and ditches are effective in cost and function for moving
moderate amounts of runoff. It is recommended that these have a minimum gradient of 2.0%
to maintain positive drainage.
(2) Paved swales and ditches are recommended to have a minimum gradient of
1.0% to maintain positive drainage.
(3) Check dams or weirs are used to slow water movement and increase infiltration in
porous swales or ditches. Earth, stone, rip rap, gabions, and concrete are generally the best
materials for dam and weir construction.
(4) French drains are a cost-effective means of directing small amounts of runoff.
They can often be constructed by hand to correct post-construction drainage problems.
(5) Underground piping by way of sewer lines is the most expensive alternative in
terms of materials, construction and maintenance cost. Underground piping is completely
effective if the lines are sized correctly; however, changes to the system to suit future needs
are difficult and expensive.
4-8
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