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

Detention considerations
For BMPs with a permanent pool, winter conditions can
create ice layers and reduce biological activity, oxygen
levels, and settling velocities. Ice layers can reduce the
permanent pool volume, act as an impervious surface
during rainfall, and potentially force incoming water
under ice layers and scour bottom sediments. Ice layers
can also reduce the oxygen exchange between the airwater
interface. If low oxygen levels extend to the
sediment-water interface, they can cause some adsorbed
pollutants, such as phosphorus and some metals to be
released back into the water column. Reduced settling
velocities will potentially result in lower pollutant
removal rates.
Minimizing the effect of ice cover can help address
these issues and can be accomplished by maintaining
design storage volumes. Installing a control mechanism,
such as a valve, weir, or stop-log, can reduce or eliminate
outflow for the normal water quality volume. This
volume is then made available for meltwater, which can
be held and slowly released.
It is important to recognize the potential for detention
facilities to incur a build up of pollutants (mostly chloride
applied to impervious surfaces) throughout the
winter. A balance needs to be considered in deciding
whether to adjust the detention level to pass pollutant-laden
runoff downstream or retain as much as
possible for later release when flows are higher. Retaining
polluted water all winter long only to discharge it all
at once in the spring is not in the best interest of receiving
waters, but this is what can happen in a detention
BMP not managed for seasonal conditions. In no case
should detention BMPs be drained in the spring after
a winter-long accumulation of under-ice contaminants.
If lowering is done, it should occur in late fall prior to
freeze-up.
Chloride-laden runoff can be denser than water already
in a basin, so it often pools at the bottom of the basin.
Without some level of mixing in the basin, the pool
can increase in chloride concentration over time. This
is especially important to consider during dewatering,
or if the pond will be used for irrigation and a pump is
placed in the bottom of the pond. Altering pump placement
or testing the bottom water before pumping are
two methods to avoid discharge or use of salty water.
BMP Fact Sheet and Detailed
Structural BMP Information
The remainder of the chapter focuses on individual
structural BMPs. As with the nonstructural chapter,
each BMP starts with a summary fact sheet. This fact
sheet provides a quick overview of the BMP, along
with a local case study. The fact sheets can be removed
from the manual and serve as stand-alone documents
for quick reference.
Following each fact sheet is detailed information on the
BMP which includes:
Variations
Discusses the variations to the BMP, if they are applicable.
Examples include alternatives in design that can
increase storage capacity or infiltration rates.
Applications
Indicates in what type of land use the BMP is applicable
or feasible.
Design Considerations
This section includes a list of technical procedures to
be considered when designing for the individual BMP.
This specific design criteria is presented, which can
assist planners in incorporating LID techniques into a
site design, as well as provide a basis for reviewers to
evaluate submitted LID techniques.
Stormwater Calculations
Provides specific guidance on achieving sizing criteria,
volume reduction, and peak rate mitigation, as applicable.
This section also references Chapter 9 which
discusses in detail how to achieve a specific standard or
implement measures that contribute to managing water
onsite in a more qualitative manner.
Construction Guidelines
Provides a typical construction sequence for implementing
the BMP. However, it does not specifically address
soil erosion and sedimentation control procedures.
Erosion and sediment control methods need to adhere
to the latest requirements of MDEQ’s Soil Erosion and
Sedimentation Control Program and local standards.
LID Manual for Michigan – Chapter 7 Page 129