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| Use of Catch Basins to Control Floatables in New York City |
| William M. Leo, P.E. HydroQual, Inc. Les Kloman, P.E. New York City Department of Environmental Protection, Corona, New York |
| This article originally appeared in Wet Weather published by the Water Environment Research Foundation, Winter 1997, Volume 1, Issue 4. |
| New York City and other coastal metropolitan areas have been experiencing the wash up of floatable debris onto beaches for many years. A program conducted by New York City between 1989 and 1993 identified various modes of floatables transport and potential methods for floatables control. One major finding of the study was that street litter being washed into New York Harbor from combined and storm sewers was the predominant source of floatables. The study also identified catch basins as one possible control option. A follow-up project was conducted to investigate catch basin design and operation. The project developed information on numbers and types of catch basins in New York City and on the cleaning practices in use. A literature review and survey of 15 major cities revealed general informatin of this nature for other areas of the country. Designs employed by different cities were found to vary considerably, but tended to include common key features, such as a sump volume, a water seal and a curb inlet to provide hydraulic relief when the grating becomes clogged. The study revealed that cleaning frequency ranged from twice per year to once every 5 or 6 years, but annual cleanouts were most common. Two of the 15 cities surveyed did not have a formal schedule; instead they cleaned individual units as specific needs arose. According to the most recent (1994) survey, New York City has an estimated 99,719 catch basins consisting mainly of two standard design types (Figure 1). A total of 63,000 basins were cleaned during the 45 month period ending March 1994, reflecting a six-year average cycle.
The effictiveness of certain catch basin design modifications on the reduction of combined sewer overflows (CSOs) and floatables discharges was evaluated as part of this effort. Field studies were conducted to monitor floatables retention in catch basins. A total of 38 catch basins distributed throughout the city were monitored: a) to determine floatables loadings that are presently discharged to sewers from catch basins; b) to assess the effect of enhanced catch basin cleaning programs; and c) to assess the benefit of design modifications. Measured discharges to sewers via the catch basins had considerable variability, attributed partly to the cleanliness of the contributing street. The most significant basin design feature impacting floatables retention, however, proved to be the presence of a basin hood, which also provides a seal for controlling sewer gas (Figure 1). Hoods were observed to produce a 50 to 70 percent reduction in floatables passing through the catch basin (Figure 2). Special tests designed to confirm this performance level showed reduction efficiencies of 60 to 80 percent. An analysis of vortex valve flow restriction devices and alternative hood designs showed no significant floatables retention improvement over the basic hood design.
A limited survey of the city's catch basins suggests that many are currently without hoods. Replacement costs per catch basin were estimated to range from $195 for simple installation of a new hood on existing fittings, to $776 when mounting hardware must also be installed. These costs do not include the additional expenses of cleaning and flushing catch basins to permit entry and proper hood installation. Alternative devices that could substitute for the standard hood, such as vortex valve flow restriction devices, piping elbows, and the Littertrap and Hydrovex devices (at unit costs in the $100-$200 range, depending construction materials), are estimated to have overall installation costs of (material and installation) of approximately $300 to $350 per basin. The design of these units makes them more difficult to accidentally dislodge than the standard hoods. Pilot studies on catch basin cleaning operations provided operational costs estimates. Cleaning costs ranged from about $67 per basin for a smaller, more maneuverable vac-truck, to about $110 per basin for the standard New York City clamshell catch basin cleaning vehicle. Overall costs would depend on the cleanout frequency established for individual catch basins. New York City is in the process of inventorying each of its catch basins. The inventory program is also linked to a major capital program to ensure that hoods are installed on every catch basin. Authors:
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In the catch basin pit, the orientation of the
outlet pipe with respect to the direction of inflow appeared to affect retention. For the
limited tests conducted, one catch basin with an outflow opposite to the direction of
inflow exhibited higher floatables retention. Five other basins with the outlet at a 90
degree orientation related to the inlet showed intermediate retention. Three basins with
the outflow in the direction of inflow showed the lowest average retention.