The city of Los Angeles, CA, needed an alternative to the stainless steel automatic retractable screens (ARS) it had been using to protect storm drain inlets. Many ARS systems available have a high percentage of operational and maintenance problems during the rainy season. Clogging occurs often during these times and can be seen when the front or face of the ARS is covered with either trash (paper, plastic bags, styrofoam food container products) or debris (leaves, pine needles, grass clippings, twigs and branches, and silt buildup from water runoff by hoses or sprinklers). In addition, the old ARS had a high theft scrap value.
The city decided to test the newly patented Hydra TMDL (Total Maximum Daily Load) ARS, which has a highly reliable design, effective mechanism, and complies with the scrap value parameters. As a result, the city designated a special area to install the Hydra ARS as a proving ground for its effectiveness and functionality.
The Koreatown district was chosen for installation and testing of the Hydra ARS. Koreatown has a calibrated Continuous Deflection Separation (CDS) system installed to evaluate the effectiveness of ARS and connecting pipe screen designs. This area was selected because of the high volume of trash on the streets and the ability to monitor the efficiency of the Hydra TMDL system.
To calculate the effectiveness rate of the Hydra TMDL system, minimum rainfall of 0.25 inch per hour was vaulted to the CDS system. After six months of monitoring, the city found a 93% reduction of debris, compared to 85% for other ARS systems. City officials have been selecting Hydra TMDL systems to replace existing stainless steel gates that have suffered theft, failure, or damage.
The Hydra TMDL system requires less preparation and has a lower installation cost than the currently installed storm drain gates. Onsite adjustments for fitting are possible, unlike other systems.
Existing ARS systems do not allow water to flow through the holes in the front of the gate to activate the unlocking mechanism when they are clogged with trash or debris, resulting in a street flooding. These systems have an overflow bypass gap, or a space between the top of the gate and the top of the curb inlet opening. If the front of the gate is covered with trash or debris, stormwater starts to dam up or pool in the street until it reaches the overflow gap. The water then flows over the top of the gate to activate the unlocking mechanism and open the ARS; the design of these mechanisms does not allow them to unlock at all times.
Dennis Friezner, partner of Hydra TMDL Inc., designed the original ARS SurfGate in 2005. This gate is unique in that it has a horizontal pan on the backside of the gate to capture overflow water. The weight and flow of the water activates or tilts the unlocking mechanism to let the gate open up with the pressure of the stormwater dammed up or pooled in front of the gate.
All gates that use the overflow for unlocking their mechanism—which is a vertical or slightly tilted device mounted on the backside of the gate—use the flow of the falling water from the overflow gap to move the device and unlock the gate; they do not use the accumulation of the weight of the water to unlock the device the way Friezner’s ARS SurfGate did.
The problem with the currently installed gates is that the pressure applied to the front of the gate from the dammed or pooled water also applies pressure to the unlocking device. The flow of the falling water from the overflow gap is not strong enough at the beginning of a storm; but as the storm grows, increasing pressure is applied from the buildup of the water. The flow of water through the gap increases significantly, but fails to activate the unlocking device, leaving the gate closed. The resulting street flooding is not only inconvenient for vehicles and pedestrians, but can also be dangerous in certain situations. Unlike the currently installed gates, Hydra’s ARS allows for water to easily flow through the storm drains, keeping the trash out and preventing street floods.
