System Maintenance & Repair

Underground Detention

As sites become more difficult to develop in both rural and urban areas, developers are constantly looking to maximize buildable area, leaving them with the decision to put stormwater detention below grade.  These systems use a network of underground chambers or pipes with a control structure to detain stormwater volume for a given period of time and release it slowly.  This poses a series of concerns when it comes to operation and maintenance.  These systems are often “out-of-sight-out-of-mind” and when the problem cannot be seen it is likely to be ignored until it becomes visible.  When an underground detention system becomes fouled with sediment and debris causing a clog, the problem can become visible very quickly in the form of flooding.  SCS can manage that problem that problem before it happens, and monitor the stormwater management system periodically.  Periodic inspections can identify a problem before it reaches the level of an emergency repair.

Underground Basins need to be inspected for debris and sediment accumulation and cleaned out whenever either is present.  UG basins utilize control structures to regulate flow, these structures should always be accessible by a grate or manhole and they should be monitored regularly.

Example of an underground detention basin:
Underground Detention Basin

Filtration Systems

The use of space on a site is generally of the greatest importance to making the development function properly.  Many sites are constrained and cannot incorporate a large above ground water quality system so the only choice is to go underground.  In order to meet water quality standards in accordance with the best management practices (BMPs), engineers often choose to use a Manufactured Treatment Device (MTD).  Filtration systems are used to achieve up to 80% or 90% removal of total suspended solids (TSS) in these applications.  There are a wide variety of filtration systems to choose from, produced by different manufacturers.  It is important to choose a system based not only on the initial installation cost but one which will have the greatest long term value with respect to maintenance costs.

Filtration can be accomplished through a gravity separation system where the water is screened through filter media , or an up flow filter where the head pressure is used to push the water up through the filtration system.  Both types of systems need appropriate designs to ensure the flow of water through the filters at a given rate.

Filtration systems need to inspected either quarterly, or annually depending on the site usage and size of the drainage area.  Filters need to be either cleaned or replaced every 2-5 years generally.  SCS can service all types of stormwater filtration systems, repair any structural components, and clean the system thoroughly.

Hydrodynamic Separation

Like filtration, hydrodynamic separation is used to achieve stormwater quality treatment by means of separating out the solids from the water.  The difference is there is no filter media to achieve this separation.  The highest level of treatment that a hydrodynamic separator is certified to treat is 50% of the TSS from the stormwater across the State of New Jersey.  Treatment is performed by swirling the water around the chamber, lengthening the detention time of the water and allowing larger solids to fall out of suspension.  Oil and grease is retained through a series of baffle walls.

Hydrodynamic separation requires maintenance just like any filtration system or other type of stormwater treatment system.  The maintenance tends to be easier and does not require replacement of filters.  Maintenance is recommended when debris and sediment on the bottom of the tank reaches a level where the manufacturer states it should be cleaned.  This is generally between 12” and 24”.

hydrodynamicseparation

CDS® Unit by Contech Engineered Solutions.  www.conteches.com

 

Bio-Retention

A bio-retention systems come in many shapes and sizes, but they all do two things; they treat the water quality storm (1.25”) and they detain large water volumes up to the 100 year storm.  Bio-retention basins are stormwater conveyance and storage structures that are engineered and constructed out of native material, soil, rock and vegetation.  They treat stormwater through a natural method of vegetated filtration and infiltration into the native soil.  The volume for retention of peak flows and the water quality treatment occur simultaneously in one system.  These systems are the preferred method of stormwater treatment by environmental agencies, watershed groups and developers.  They are relatively less expensive to construct than underground detention with filtration systems, and they can be maintained without the use of underground confined space equipment.  The reason these systems are not used everywhere is their size and many small urban sites do not have the space to accommodate a large basin.

Bio-retention basins need to be maintained for vegetative growth and invasive plant species.  The inlet areas need to be stabilized and the outlet control structure should be regularly monitored for clogging.  After large storm events of 1” or more, the basins should be inspected for erosion, and clogging.

bio-retentionbasin

Conventional schematic of a bio retention system. Photo by the New Jersey Department of Environmental Protection – BMP Manual, April 2004.