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

The construction cost of constructed wetlands varies
greatly depending on the configuration, location, site
specific conditions, etc. Typical construction costs
in 2004 dollars range from approximately $30,000 to
$65,000 per acre (USEPA Wetlands Fact Sheet, 1999).
Costs are generally most dependent on the amount of
earthwork and planting. Annual maintenance costs have
been reported to be approximately two to five percent of
the capital costs (USEPA, 2000).
The construction cost of underground detention can
vary greatly depending on the design, configuration,
location, storage volume and media, and site specific
conditions, among other factors. Typical construction
costs are approximately $8 to $10 per cubic foot for
proprietary high capacity storage systems. Systems
using uniformly graded aggregate as the primary storage
media will typically be less expensive but require
additional area and/or depth for an equivalent storage
volume.
Annual maintenance costs for dry ponds and wet ponds
have been reported to be approximately three to five
percent of the capital costs, though there is little data
available to support this. Alternatively, a community
can estimate the cost of the maintenance activities
outlined in the maintenance section. Ponds are longlived
facilities (typically longer than 20 years). Thus,
the initial investment into pond systems may be spread
over a relatively long time period.
General Specifications
The following specifications are provided for information
purposes only. These specifications include
information on acceptable materials for typical applications,
but are by no means exclusive or limiting. The
designer is responsible for developing detailed specifications
for individual design projects in accordance
with the project conditions.
Dry detention and underground
structures
Site preparation
All excavation areas, embankments, and structure locations
should be cleared and grubbed as necessary, but
trees and existing vegetation should be retained and
incorporated within the dry detention basin area where
possible. Trees should not be removed unless absolutely
necessary.
Where feasible, trees and other native vegetation should
be protected, even in areas where temporary inundation
is expected. A minimum 10-foot radius around the inlet
and outlet structures can be cleared to allow room for
construction.
Any cleared material should be used as mulch for
erosion control or soil stabilization.
Care should be taken to prevent compaction of the
bottom of the reservoir. If compaction should occur,
soils should be restored and amended.
Earth fill material & placement
• The fill material should be taken from approved
designated excavation areas. It should be free
of roots, stumps, wood, rubbish, stones greater
than six inches, or other objectionable materials.
Materials on the outer surface of the embankment
must have the capability to support vegetation.
• Areas where fill is to be placed should be
scarified prior to placement. Fill materials for the
embankment should be placed in maximum eightinch
lifts. The principal spillway must be installed
concurrently with fill placement and not excavated
into the embankment.
• Control movement of the hauling and spreading
equipment over the site.
Embankment core
• The core should be parallel to the centerline of
the embankment as shown on the plans. The top
width of the core should be at least four feet. The
height should extend up to at least the 10-year
water elevation or as shown on the plans. The side
slopes should be 1:1 or flatter. The core should be
compacted with construction equipment, rollers,
or hand tampers to assure maximum density
and minimum permeability. The core should be
placed concurrently with the outer shell of the
embankment.
• Construction of the berm should follow
specifications by the project’s geotechnical
engineer.
Structure backfill
• Backfill adjacent to pipes and structures should be
of the type and quality conforming to that specified
for the adjoining fill material. The fill should be
placed in horizontal layers not to exceed eight
inches in thickness and compacted by hand tampers
or other manually directed compaction equipment.
The material should fill completely all spaces under
and adjacent to the pipe. At no time during the
LID Manual for Michigan – Chapter 7 Page 185