EPA-8 VI -F-8 9-100
United States
Environmental Protection
Agency
Office of Water Publication U-1
Nonpoint Source Branch August 1989
(WH 585)
&EPA
Retrofitting Storimwater
Management Basins for
Phosphorus Control
The Problem
n the early 1970s, nuisance blue-green
algae blooms and low dissolved oxygen
problems began to plague the Loch
Raven Reservoir, a 21 billion gallon
impoundment near Baltimore, Maryland.
The cause was an overload of
phosphorus generated by
agricultural activities and
urbanization.
Baltimore County had several
phosphorus control programs, but of limited
effectiveness. In urban areas, the programs
focused mainly on retaining stormwater,
primarily through some 36 "dry" ponds that
become inundated only during very large
storms. Because the hydraulic controls were
designed to accommodate only large flows,
most storm flows and the sediments they
carried passed through the basin unimpeded
by a low-flow pipe. Thus, the basins did little
to enhance water quality — and,
consequently, the reservoir suffered.
To combat this problem, the Water
Quality Management Office of the Baltimore
City Department of Public Works started a
program, funded in part by a Clean Lakes
Program grant, to modify these stormwater
structures. The goal: change the structures so that
they could d etain flows from small storms without
compromising their ability to control larger storm
flows.
Baltimore County
Prettyboy Reservoir
Loch
Raven
Reservoir
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Project Design and Construction
efore modification, the 36 structures
were able to control the flows from
storms that occurred every two years,
10 years, and 100 years. But since
most storm events were less intense
than even the two-year variety,
engineers and planners needed to
adjust the structures to accommodate the smaller
storms.
To modify the structures, engineers took a
two-pronged design approach. First, they needed to
needed to know how small the basins could be
without restricting their capacity to control the
runoff from larger storms. Through computer
modelling, it was determined that the basins could
be modified to accommodate one-year storms, and
that even smaller storms would not be
short-circuited.
Second, the engineers needed to design the
actual retrofits, which would vary the size of the low
flow release structure to handle smaller storms. This
was accomplished by designing a special
attachment and installing a trash debris guard.
So far, five retrofits have been built. The first,
Dulaney Gate, was installed during the summer of
1984; the most recent, Huntridge, was completed in
March 1988.
Each retrofit was tailored to individual outlet and
site conditions. Three of the retrofits were dry ponds
and installation took less than three days.
Retrofitting the wet pond was more complicated and
expensive because the pond had to be pumped and
the bottom dredged because the low flow release
was covered by sediment.
A schematic of a detention basin that has been
retrofitted appears on the last page.
Retrofit Diagram
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Costs
he cost of the retrofits ranged from
$2,000 to $13,000 per basin. The dry
ponds cost approximately $2,000 and
the wet pond $13,000. The added cost
of dredging bottom sediment and
pumping the standing water accounts for the higher
cost associated with the wet basin. In addition to the
costs, it took an average of one day to install the dry
basin and one week for the wet basin.
Maintenance and Liability
ny modification made to an existing
pond could increase liability and
maintenance. The county government
agreed to work with private basin
owners and, if necessary, relieve them
of these responsibilities. Because of
legal and political issues, the county could guarantee
maintenance for the retrofit only on ponds already
owned by the county, thus limiting the available
coverage of this effort.
Water Quality Monitoring and Results
reliminary results indicate the retrofits
remove over 90 percent of all
paniculate material and between 30
and 40 percent of the total phosphorus.
These high removal efficiencies were
fairly consistent for all storms. All of the
storms successfully monitored had less than a
one-year recurrence interval and detention times
ranged from 3 to 5 hours.
Not one of the retrofits has clogged,
demonstrating the effectiveness of the trash-debris
guards. Sediment in-filling of the basins does not
appear to be appreciable despite their estimated
high trap efficiency. This is most I ikely due to the fact
that all of the retrofits drain stabilized urban areas
that characteristically have low sediment export
rates. Sediment in-filling is expected to be a concern
in areas with construction activity.
Retrofit Design Criteria
Retrofit
Detention Times (hours)
112 year 1 year
Drainage Area
(acre)
DulaneyGate
Oakhampton
Mays Chapel
Loveton
Huntridge
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..^sSSOS^^ L°""sta<">
Extended
Detention
Control
Device
10 Year Water Surface Elevation
2 Year
Illustration from Controlling Urban Runoff: a Practical Manual for Planning and Designing Urban BMPs,
by Thomas R. Schueler, Department of Environmental Programs, Washington
Metropolitan Council of Governments; published July 1987.
If you have any questions concerning this project, please contact Randall G. Waite, Clean
Water Coordinator, Region HI, Philadelphia, PA, (215/597-3425) or William P. Stack, Tech-
nical Manager, Baltimore City Water Office, Baltimore, MD (301/396-0732).
Printed on Recycled Paper
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