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

Stormwater Functions and
Calculations
Quantifying impervious areas at a proposed development
site, pre- to post-development continues to
dominate stormwater calculations. Stormwater calculations,
as discussed in Chapter 9, are sensitive to
pervious areas and their contribution to total volume
of runoff, increased peak rate of runoff, and increased
generation of nonpoint source pollutants. A reduction in
imperviousness achieved through reduced street widths
and lengths and reduced paved parking areas automatically
reduces the volume and peak rate of runoff. To
the extent that water quality is linked to runoff volume,
reduction in imperviousness translates into a reduction
in water quality management requirements as compared
with standard design.
Maintenance
A reduction in impervious area results in decreased
maintenance. For example, whether publicly or privately
maintained, reducing roadway or parking lot imperviousness
typically translates into reduction in all forms
of maintenance required, from basic roadway repair to
winter maintenance and snow removal.
Cost
Street width
Costs for paving are estimated to be approximately
$15 per square yard (Center for Watershed Protection,
1998), which would be considerably higher in current
dollars. At this cost, for each one-foot reduction in
street width, estimated savings are $1.67 per linear foot
of paved street. For example, reducing the width of a
500-foot road by five feet would result in a savings of
over $4,100, which would be considerably higher in
current dollars. This cost is exclusive of other construction
costs including grading and infrastructure.
Street length
Factoring in pavement costs at $15 per square yard (as
above), a 100-foot length reduction in a 25-foot-wide
road would produce a savings in excess of $4,000 (much
higher in current dollars).
In addition to pavement costs, costs for street lengths,
including traditional curb and gutter and stormwater
management controls, are approximately $150 per linear
foot of road (Center for Watershed Protection, 1998),
which would be considerably higher in current dollars.
Decreasing road length by 100 feet would save an additional
$15,000, for a combined total of $19,100.
Parking
Estimates for parking construction range from $1,200
to $1,500 per space (Center for Watershed Protection,
1998), which would be significantly higher in current
dollars. For example, assuming a cost of $1,200 per
parking space, reducing the required parking ratio for
a modest 20,000 square foot shopping strip from five
spaces per 1,000 square feet to four spaces per 1,000
square feet would represent a savings of $24,000.
LID Manual for Michigan – Chapter 6 Page 113