Stormwater Management

What is Stormwater?
Stormwater runoff is generated from rain and snowmelt events that flow over land or impervious surfaces, such as paved streets, parking lots, and building rooftops, and does not soak into the ground. The runoff picks up pollutants like trash, chemicals, oils, and dirt/sediment that can harm our rivers, streams, lakes, and coastal waters.To protect these resources, communities, construction companies, industries, and others, use stormwater controls, known as best management practices (BMPs). These BMPs filter out pollutants and/or prevent pollution by controlling it at its source.

Types of Best Management Practices
Dry Ponds:
Unlike a wet pond, dry ponds are designed to retain Storm water runoff generally between 48-72 hours after a rain event. Water that drains too quickly from a dry pond does not have adequate time to filter and can damage the banks of downstream creeks. Water retained on site too long can impact the dam or lead to algal problems (particularly during the summer). Maintenance is required when:

  • too much sediment accumulates and interferes with volume capacity,
  • trees or other shrub vegetation grow on the dam embankment,
  • the dam embankment becomes denuded or otherwise presents an erosion problem,
  • visible damage to any of the mechanical equipment is present,
  • the low flow orifice, forebay or concrete trickle ditch is blocked by trash, debris or sediment,
  • animal burrows are present on the dam embankment, or
  • standing water remains longer than 72 hours after a rain event.

stormwater-2Wet Ponds or Wetlands:

Much like a lake, wet ponds are designed to hold water permanently. When the amount of water entering the pond exceeds capacity, water is released into the nearest creek at a controlled rate. Maintenance is required when:
  • too much sediment accumulates and interferes with volume capacity,
  • trees or other shrub vegetation grows on the dam embankment,
  • the dam embankment becomes denuded or otherwise presents a sediment erosion problem, or
  • visible damage to any of the mechanical equipment is present.

Sand Filters:

Used typically in parking lots or in areas of high density with a high percentage of impervious surfaces, usually sand filters are located underground with a series of manholes present above ground for maintenance access. Sand filters contain a series of sand baffles that filter pollutants, such as nutrients, oil and grease, before allowing the water to discharge into a storm drain. Maintenance is required when:

  • the facility accumulates too much sediment to properly treat and drain Stormwater runoff,
  • excessive oil and debris has accumulated in the system; standing water is present, or
  • when the suggested maintenance interval has elapsed.

stormwater-3Rooftop Detention:

In order to control the volume of Stormwater entering the storm drain system during a rain event, rooftop detention retains a certain volume of water (depending on the size of the roof), then slowly discharges runoff into the storm drain system. Rooftop detention is maintenance intensive and must be regularly checked to ensure proper functioning. Maintenance is required when:
  • the rooftop drain is blocked by leaves and other debris,
  • leaks are visible, or
  • standing water is present 17 hours after a rain event.

Underground Detention: 

Consisting of a series of large underground pipes or chambers, underground detention facilities detain Stormwater runoff and slowly release it into the storm drain system. Underground detention facilities are usually coupled with other treatment facilities to remove pollutants while the runoff is detained. Maintenance is required when:

  • there is visible damage present to any of the inlets, pipes or outlets, or
  • if excessive sediment and/or debris has accumulated in the inlet, pipes or  outlets.

stormwater-4Infiltration Trenches:

Usually composed of gravel, these trenches are designed to give Stormwater runoff an opportunity to seep into underlying soil. Maintenance is required when:
  • trash, vegetation or other debris is present on the gravel surface,
  • woody vegetation begins to grow in the trench,
  • visible damage to any of the mechanical equipment is present,
  • standing water is  present 48 hours after a rain event, or
  • if runoff no longer infiltrates into but flows across the trench.

Manufactured Facilities:

Also called manufactured Best Management Practices and often difficult to locate, these facilities appear to be nothing more than manholes. Manufactured facilities are subterraneous chambers that use various techniques to filter pollutants from Stormwater runoff. Maintenance is required when:

  • trash and/or other debris is present at or inside the inlet or outlet,
  • the accumulation of sediment is greater than the manufacturer’s recommendation, or
  • too much oil is present for proper water filtration.

stormwater-5Rain Gardens:

Mimicking natural vegetative cover, rain gardens absorb and treat runoff from pavement or yard areas through mulch, layers of soil and certain microbes. Rain gardens are maintenance intensive and require regular mulch replacement. Maintenance is required when:

  • vegetation becomes discolored, wilted or dies,
  • erosion is present on the berms or slopes,
  • the overflow riser or grate is blocked with debris, or
  • standing water is present in the basin 72 hours after a rain event.

stormwater-6Permeable Pavement:

Made of pervious materials like bricks and/or gravel, permeable pavement is typically used in parking lots or driveways to provide Stormwater runoff a chance to percolate into the underlying soil, while still maintaining the structural integrity of pavement. Maintenance is required when:
  • standing water is present 48 hours after a rain event, or
  • if too much sediment has accumulated inside the pavers so that the Stormwater runoff absorption rate is affected.