Encyclopedia of Environmental Science and Engineering, Volume I and II

1188 URBAN RUNOFF

Table 9. Costs include regulator stations, central monitoring

and control systems, and miscellaneous hardware.

Off-Line Storage

Off-line storage facilities can be located at overflow points

or near dry-weather treatment plants. Typical storage facili-

ties include lagoons and covered or uncovered concrete tanks.

Tunnels are also used where land is not available. Costs for

basin storage facilities are presented in Table 10, and construc-

tion cost curves are shown in Figure 8. Note that these curves

do not include pumping facilities, so these curves are applicable

to in-line basins; the costs for earthen basins include liners.

Innovative Storage Technology

In-receiving Water Flow Balance System Karl Dunkers,

an independent research engineer from Sweden, has devel-

oped, under the auspices of the Swedish EPA counterparts,

an approach to lake protection against pollution from

stormwater runoff. Instead of using conventional systems

for equalization (i.e. concrete tanks or lined ponds), which

are relatively expensive and require a lot of land area, the

flow balanced method uses a wooden pontoon tank system

in the lake, which performs in accordance with the plug-

flow principle. The tank bottom is the lake bottom itself.

The tank volume is always filled up, either with polluted

stormwater runoff or with lake water. When it is raining,

the stormwater runoff will “push” the lake water from one

compartment to another. The compartment walls are of

flexible PVC (polyvinyl chloride) fiberglass cloth. When

not receiving runoff, the system reverses by automati-

cally flowing back and the lake water fills up the system.

Thus, the lake water is utilized as a flow balance medium.

These units are sized to yield an effective in-water volume

equal to the storage required for the storm size selected

for design. Added benefits are the ease of construction and

the flexibility to expand the volume if deemed necessary

after initial installation; initial storage-volume estimates

need not be as exact. Costs have been estimated to be one-

fifth to one-tenth the cost of conventional land-side storage

without real estate costs.

Sweden has invested in three of the installations so far.

Two have been in operation for eight to nine years, and a third

for seven years. The systems seem to withstand wave action

up to 0.9 m (3 ft) as well as severe icing conditions. If a wall

is punctured, patching is easily accomplished. Maintenance

has been found to be inexpensive. The in-receiving water-

flow balance system has been successfully demonstrated

with urban runoff in freshwater lakes only. If used with CSO,

consideration would have to be given to sludge handling

and disposal. EPA’s Storm and Combined Sewer Pollution

Control Program is demonstrating this unique system with

CSO in a much harsher marine/estuarine environment in

New York City. Testing for seven storms indicates effective-

ness as both a floatables trap and a temporary storage of CSO

volumes. The estimated cost of this system is $1,641/lin m

($500/lin ft).

10 100 1000

1,000

10,000

100,000

STORAGE CAPACITY

CONSTRUCTION COST, $1,00

EARTHEN

CONCRETE

UNCOVERED

CONCRETE COVERED

FIGURE 8 Storage basin construction costs: ENR 
 5,000 (EPA-600/8-77-014).

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