Low Impact Development Manual for Michigan - OSEH - University ...

• Protect the infiltration area from sediment by
ensuring erosion and sediment control practices
are implemented until the surrounding site
is completely stabilized. Methods to prevent
sediment from washing into BMPs should be
clearly shown on plans. Where geo-textile is used
as a bed bottom liner, this should be extended
several feet beyond the bed and folded over the
edge to protect from sediment wash into the bed
during construction, and then trimmed.
Runoff from construction areas should never be
allowed to drain to infiltration BMPs. This can
usually be accomplished by diversion berms and
immediate vegetative stabilization. The infiltration
area may be used as a temporary sediment trap
or basin during earlier stages of construction.
However, if an infiltration area is also to be utilized
as a temporary sediment basin, excavation should
be limited to within one foot of the final bottom
invert of the infiltration BMP to prevent clogging
and compacting the soil horizon, and final grade
removed when the contributing site is fully
stabilized.
All infiltration BMPs should be finalized at the end
of the construction process, when upstream soil
areas have a dense vegetative cover. In addition,
do not remove inlet protection or other erosion
and sediment control measures until site is fully
stabilized. Any sediment which enters inlets during
construction is to be removed within 24 hours.
• Provide thorough construction oversight.
Long-term performance of infiltration BMPs is
dependent on the care taken during construction.
Plans and specifications must generally be followed
precisely. The designer is encouraged to meet with
the contractor to review the plans and construction
sequence prior to construction, and to inspect the
construction at regular intervals and prior to final
acceptance of the BMP.
• Provide quality control of materials. As with
all BMPs, the final product is only as good as the
materials and workmanship that went into it. The
designer is encouraged to review and approve
materials and workmanship, especially as related
to aggregates, geotextiles, soil and topsoil, and
vegetative materials.
Additional Construction Guidelines for
Infiltration Berms
The following is a typical construction sequence for
an infiltration berm without a subsurface infiltration
trench, though alterations will be necessary depending
on design variations.
• Lightly scarify (by hand) the soil in the area of
the proposed berm before delivering soil to site (if
required). Heavy equipment should not be used
within the berm area.
• Bring in fill material to make up the major
portion of the berm (as necessary) as soon as
subgrade preparation is complete in order to avoid
accumulation of debris. Soil should be added in
eight-inch lifts and compacted after each addition
according to design specifications. The slope and
shape of the berm should graded out as soil is
added.
• Protect the surface ponding area at the base of the
berm from compaction. If compaction of this area
does occur, scarify soil to a depth of at least
8 inches.
• After allowing for settlement, complete final
grading within two inches of proposed design
elevations. Tamp soil down lightly and smooth
sides of the berm. The crest and base of the berm
should be level along the contour.
• Seed and plant berm with turf, meadow plants,
shrubs or trees, as desired. Water vegetation at the
end of each day for two weeks after planting is
completed. (Appendix C).
• Mulch planted and disturbed areas with compost to
prevent erosion while plants become established.
Additional Construction Guidelines for
Subsurface Infiltration
• Where erosion of subgrade has caused
accumulation of fine materials and/or surface
ponding, this material should be removed with light
equipment and the underlying soils scarified to a
minimum depth of six inches with a York rake (or
equivalent) and light tractor. All fine grading should
be done by hand. All bed bottoms are to be at level
grade.
• Earthen berms (if used) between infiltration beds
should be left in place during excavation.
• Geotextile and bed aggregate should be placed
immediately after approval of subgrade preparation
LID Manual for Michigan – Chapter 7 Page 212