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

wet pond and constructed wetland operation during
cold weather is the freezing and clogging of inlet and
outlet pipes. To avoid these problems, the Center for
Watershed Protection (Caraco and Claytor, 1997) made
some general design suggestions, which are adapted as
follows:
• Inlet pipes should typically not be submerged, since
this can result in freezing and upstream damage or
flooding.
• Burying all pipes below the frost line can prevent
frost heave and pipe freezing. Wind protection can
also be an important consideration for pipes above
the frost line. In these cases, designs modifications
that have pipes “turn the corner” are helpful.
• Incorporate lower winter operating levels as part of
the design to introduce available storage for melt
events.
• Increase the slope of inlet pipes to a minimum of
one percent to prevent standing water in the pipe,
reducing the potential for ice formation. This design
may be difficult to achieve at sites with flat local
slopes.
• If perforated riser pipes are used, the minimum
opening diameter should be ½-inch. In addition, the
pipe should have a minimum eight-inch diameter.
• When a standard weir is used, the minimum slot
width should be three inches, especially when the
slot is tall.
• Baffle weirs can prevent ice reformation during the
spring melt near the outlet by preventing surface ice
from blocking the outlet structure.
• In cold climates, riser hoods should be oversized
and reverse slope pipes should draw from at least
six inches below the typical ice layer.
• Alternative outlet designs that have been successful
include using a pipe encased in a gravel jacket set
at the elevation of the aquatic bench as the control
for water-quality events. This practice both avoids
stream warming and serves as a non-freezing outlet.
• Trash racks should be installed at a shallow angle to
prevent ice formation.
Constructed wetland performance can be decreased in
spring months when large volumes of runoff occur in a
relatively short time carrying the accumulated pollutant
load from the winter months. Since constructed
wetlands are relatively shallow, freezing of the shallow
pool can occur.
Cost
Costs for detention facilities will vary depending on the
type as indicated below.
The construction costs associated with dry ponds can
vary considerably. One study evaluated the cost of all
pond systems (Brown and Schueler, 1997). Adjusting
for inflation, the cost of dry extended detention ponds
can be estimated with the equation:
C = 12.4V0.760 Where:
C = Construction, design and permitting cost
V = Volume needed to control the 10-year storm (cubic
feet)
Using this equation, typical construction costs are:
$41,600 for a one acre-foot pond
$239,000 for a 10 acre-foot pond
$1,380,000 for a 100 acre-foot pond
Dry ponds using highly structural design features
(riprap for erosion control, etc.) are more costly than
natural basins. An installation cost savings is associated
with a natural vegetated slope treatment, which
is magnified by the additional environmental benefits
provided. Long-term maintenance costs for processes
such as mowing and fertilizing are reduced when more
naturalized approaches are used due to the ability of
native vegetation to adapt to local weather conditions
and a reduced need for maintenance.
The construction cost of wet ponds varies greatly
depending on the configuration, location, site specific
conditions, etc. Typical construction costs in 2007
dollars range from approximately $30,000 to $60,000
per acre-foot of storage (based on USEPA, 1999).
Alternately, the construction cost of a wet pond can be
estimated as $6,000 per acre of contributing drainage
area. Costs are generally most dependent on the amount
of earthwork and the planting.
In addition to the water resource protection benefits of
wet ponds, there is some evidence to suggest that they
may provide an economic benefit by increasing property
values. The results of one study suggest that “pond
front” property can increase the selling price of new
properties by about 10 percent (USEPA, 1995). Another
study reported that the perceived value (i.e., the value
estimated by residents of a community) of homes was
increased by about 15 to 25 percent when located near a
wet pond (Emmerling-Dinovo, 1995).
LID Manual for Michigan – Chapter 7 Page 184