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The Origin of Stormwater and Its Characteristics
Stormwater pertains to water that has its origin from precipitation caused by rainfall. It can also come from snow that melts, or from excessive watering of gardens or lawns or other activities using water in large quantities. Normally rainwater or even snowmelt should soak in to the ground, but if the precipitation or rate of snow melt is very high, the capacity of the ground to absorb it being limited, this extra water becomes surface runoff which then enters the stormwater system that is in place in that area. Such systems can even be natural streams and waterways. The capacity of the ground to absorb runoff is also reduced by the built up nature of surfaces in urban areas that are made up of roads, pathways, and buildings.
Stormwater quantity is related to the intensity of the precipitation and where such quantity is in excess of the systems capacities can cause floods. Stormwater quality is linked to human activities and could enter the system from roads, roofs, and fields. The quality of stormwater becomes a concern when such runoff is allowed to enter natural water sources which may be sources of water supply for communities, even if this is not the community from which the stormwater runoff originates.
Image Source : Wikimedia : Stormwater drain
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Stormwater Pollution Plan Basics
A stormwater pollution plan would have to take into consideration both the quantity and quality of the stormwater in order to be an effective one.
Quantity considerations are taken into account by engineers planning the system that has to cater for the runoff of stormwater. Engineers estimate the maximum intensity of rainfall that occurs in the area. This is done by scanning rainfall records for at least three or more decades and finding out the highest intensity of precipitation. This translates into inches per hour and this allows the engineer to calculate the amount of water that he can expect to flow into a particular area for which he has been asked to design a stormwater system for. Allowances are made for infiltration into the ground and evaporation losses. It is estimated that urban areas would have a runoff of about 55 percent whereas rural areas would have one as low as 10 percent. Drains and other systems are then designed of adequate sizes and slopes so that the water runs off without causing any flooding. Bridges and other structures over culverts or drains that take care of the runoff have to be designed with sufficient freeboard that allows the bridge to be used at all times. Some planners prefer to take the help of holding ponds, which will store the excess water for a period and once the precipitation reduces this water is also then drained off. These ponds also assist evaporation of the stormwater.
Quality considerations are addressed by urban engineers by allowing some detention of stormwater in holding ponds or other devices to allow precipitation of dirt and pollutants before the water is allowed to runoff. Curb inlets are designed to reduce the sediment from stormwater getting into the system. They also monitor the runoff coming in from various sources and property owners who cause pollutants to come into the system are advised to take corrective action. Emergency control measures also have to be in place, involving health and other authorities in case of any breech of the stormwater system that can cause excessive pollutants into the system.
Regulations exist in most urban areas which require any new construction area to submit a stormwater management plan (SWMP) to the concerned authorities before starting work. The goal of such SWMPs is to see that pollutants are reduced to the maximum extent possible (MEP) and conform to standards set by the EPA.