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

Stormwater Functions and
Calculations
Volume
Any portion of a site that can be maintained in its presettlement
state by using this BMP will not contribute
increased stormwater runoff and will reduce the amount
of treatment necessary. In addition, trees protected
under this requirement can get a “credit” by receiving
a curve number reflecting a woodlot in “good” condition.
Calculation methodology to account for this BMP
is provided in Chapter 9.
Peak rate
Runoff from the minimized disturbed area may be
excluded from peak rate calculations for rate control,
provided that the runoff from the area is not conveyed to
and/or through stormwater management control structures.
If necessary, runoff from the minimized disturbed
area should be directed around BMPs and stormwater
pipes and inlets by means of vegetated swales or low
berms that direct flow to natural drainageways.
Water quality improvement
Water quality is benefited substantially by minimizing
the disturbed area.
Maintenance
Minimizing site disturbance will result in a reduction
of required maintenance of a site in both the short- and
long-term. Areas of the site left as intact native plant
communities do not typically require replacement with
hard surfaces or additional vegetation to retain function.
On the other hand, artificial surfaces such as pavement or
turf grass require varying levels of maintenance throughout
the life of a development. Higher levels of disturbance
will also typically require significant maintenance of
erosion control measures during the active development
of a parcel, thus adding to short-term development costs.
While intact natural areas may require small amounts
of occasional maintenance (typically through invasive
species control) to maintain function, levels of maintenance
required for hard surfaces or turf grass will
remain static or, in most cases, increase over time.
Avoiding disturbance to natural areas benefits the short
term developer and the long-term owner by minimizing
time and money needed to maintain artificial surfaces.
Cost
The reduced costs of minimized grading and earthwork
should benefit the developer. Cost issues include both
reduced grading and related earthwork as well as costs
involved with site preparation, fine grading, and seeding.
Calculation of reduced costs is difficult due to the
extreme variation in site factors, (amount of grading,
cutting/filling, and haul distances for required trucking,).
Some relevant costs factors are as follows (as based on
R.S. Means, Site Work & Landscape Cost Data, 2007):
Site clearing
• Cut & chip light trees to six-inch diameter $3,475/acre
• Grub stumps and remove $1,600/acre
• Cut & chip light trees to 24-inch diameter $11,600/acre
• Grub stumps and remove $6,425/acre
Strip topsoil and stockpile
• Ranges from $0.52 to $1.78 / yard3 because of
Dozer horse power, and ranges from ideal to
adverse conditions
• Assuming six inches of topsoil, 500 ft haul
$2.75 - 9.86 per yard3 • Assuming six inches of topsoil, 500 ft haul
$9,922 -16,746 per acre
Site preparation, fine grading, seeding
• Fine grading w/ seeding $2.91 /sq. yd.
• Fine grading w/ seeding $14,084 /acre
In sum, total costs usually range from $29,000 - $49,000
per acre and could certainly exceed that figure substantially
at more challenging sites.
LID Manual for Michigan – Chapter 6 Page 79