W 32nd Detention Basin Wet Ponds
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MEMORANDUM
September 21,2005
TO:
The Honorable Mayor and City Council Members
FROM:
Michael C. Van Milligen, City Manager
SUBJECT: W. 32nd Street Detention Basin Design With Wet Ponds
City Engineer Gus Psihoyos recommends City Council approval of the proposed design
for the West 32nd Street Detention Basin that includes two wet ponds. Wet ponds are
the most effective way to prevent sediment from dispersing throughout the entire
detention basin, thereby reducing the cost to remove the sediment.
I concur with the recommendation and respectfully request Mayor and City Council
approval.
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Michael C. Van Milligen
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Attachment
cc: Barry Lindahl, Corporation Counsel
Cindy Steinhauser, Assistant City Manager
Gus Psihoyos, City Engineer
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MEMORANDUM
September 20,2005
TO:
Michael C. Van Milligan
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FROM:
Gus Psi hoyos, City Engineer
SUBJECT: W. 32nd Street Detention Basin Design With Wet Ponds
INTRODUCTION
The purpose of this memorandum is to seek the endorsement of the proposed design
for the W. 32nd Street Detention Basin that includes two wet ponds.
BACKGROUND
A Fiscal Year 2005 appropriation established funding to design the improvements to the
W. 32nd Street Detention Basin conceptually presented in the Drainage Basin Master
Plan. The Fiscal Year 2006 Capital Improvements Program budget includes $1,573,000
for construction of the improvements.
In December of 2004, the City Council endorsed an RFP and authorized its release to
the public. Project objectives include a detention basin that incorporates features to
minimize or ease maintenance activities required to keep the facility operating at an
optimum level. It must include an area to collect sediment and debris for easy removal.
Moreover, the detention basin should blend into its surroundings resulting in an amenity
for the neighborhood.
In February of 2005, the City Council approved the selection of MSA Professional
Services, Inc. to provide engineering and design services for the W. 32nd Street
Detention Basin project.
DISCUSSION
With the ability to remove pollutants from stormwater together with the aesthetic,
wildlife, and environmental functions they provide, stormwater ponds are one of the
most prevalent best management practices throughout the Midwest. According to the U.
S. Environmental Protection Agency, tens of thousands exist nationally (K. Christen,
Water Environment & Technology, April 2003). They are effective at removing sediment,
heavy metals, phosphorus, nitrogen, and pathogens from stormwater. To meet the
design requirement for a "detention basin that incorporates features to minimize
maintenance activities required to keep the facility operating at an optimum level" and
"include(s) an area to collect sediment and debris for easy removal," a 3-acre wet pond
is recommended within the proposed 10-acre W. 32nd Street Detention Basin. Figure 1
and Figure 2 show existing wet ponds in Wisconsin.
Figure 1. Existing wet pond with vegetation buffer.
Figure 2. Another example of an existing wet pond with wild flower vegetation buffer.
Approximately 2,700 cubic yards of sediment is delivered to the detention basin each
year. The preliminary design of the detention basin shown in Figure 3 includes a wet
pond with capacity sufficient to store the entire annual sediment load in the lowest two
feet of the pond, below the water surface. As a result, the appearance of the detention
basin will not change due to sediment deposition. In addition, sediment deposition will
not reduce the capacity of the detention basin as the accumulated sediment only
displaces water in the pond. The preliminary design includes an outlet structure for the
wet pond that would allow draining of the pond by gravity to facilitate cost effective
removal of sediment. Other methods of managing sediment were considered but the
wet pond is the most effective way to prevent the sediment from dispersing throughout
the entire detention basin, thereby reducing the cost to remove the sediment.
In addition to the wet pond designed to capture sediment, Figure 3 depicts a second,
smaller wet pond at the eastern end of the detention basin. It is proposed because the
area may be wet as it has been in the past (see Figure 4) and the second pond will
reduce the likelihood of standing water.
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Figure 3. Proposed design of the W. 32nd Street Detention Basin that includes two wet ponds.
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Both wet ponds are designed with the following features:
o Safety shelf
A typical wet pond feature, a 10-foot long safety shelf with a 10-foot horizontal to 1-
foot vertical slope is proposed just under the water surface along the perimeter of
the pond to provide a safe exit for anyone who accidentally enters into the pond.
o Bank Slopes
As shown in Figures 1 and 2, the sloped banks (4-foot horizontal to 1-foot vertical) of
the pond will be planted with wild flowers and other vegetation.
The recent increase in mosquito-borne diseases, such as West Nile and encephalitis,
has prompted the design of stormwater facilities that deter mosquito habitat.
Collaborating with the Health Department, several design features are included for the
proposed W. 32nd Street Detention Basin to prevent mosquito propagation. These
features include:
Figure 4. Aerial photograph looking west at the existing W. 32n Street Detention Basin
o Pond Depth
Studies have shown that permanent pool depths in excess of 4 feet are
unsuitable for mosquito larvae due to the continual water surface disturbances.
Besides the 1 to 2 foot deep safety shelf around the pond perimeter, the wet
pond is designed to be 6 feet deep. This depth will also prevent emergent
vegetation near the shore that could provide mosquito larvae refuge from natural
predators.
o Water Circulation
To prevent stagnant water, a condition that facilitates mosquito breeding, the
preliminary design includes the use of a solar powered water-circulating device.
The SolarBee@, would thoroughly mix the entire pond, reaching all stagnant
areas. Solar powered, it would have little or no energy cost, a 25-year expected
life, and virtually no maintenance. Figure 5 shows a SolarBee@ in use.
o Veqetation
The vegetation surrounding the wet ponds and throughout the rest of the
proposed detention basin was chosen because it would provide habitat for
natural mosquito predators such as dragon flies, aquatic beetles, and
amphibians.
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Figure 5. Residential pond with a SolarBee@ water circulation deviceJsee center of photo and inset) like
that proposed for the wet ponds associated with the W. 32 Street Detention Basin.
Besides these measures, mosquito populations will continue to be monitored by the
Health Services Department. If an increase in the mosquito population is observed, an
inexpensive larvacide available in a slow-release pellet form will be utilized. Over time,
the operation of the Carter Road Detention Basin and other ditch improvements planned
upstream of the detention basin may result in reduced sediment loads to the W. 32nd
Street Detention Basin. Ultimately, the need for the wet pond may diminish allowing for
its elimination. The cost to eliminate the wet pond, if deemed necessary, , would involve
only a minor maintenance cost.
RECOMMENDATION
I recommend including the two wet ponds in the design of the W. 32nd Street Detention
Basin Project.
BUDGET IMPACT
It is currently anticipated that the construction of the wet ponds in conjunction with the
W. 32nd Street Detention Basin improvements will not exceed the project budget.
ACTION REQUESTED
I respectfully request endorsement of the use of two wet ponds to work in conjunction
with the operation of the W. 32nd Street Detention Basin project.
Prepared by Deron Muehring
cc: Dawn Lang, Budget Director
Mary Rose Corrigan, Public Health Specialist
Deron Muehring, Civil Engineer II