Private Stormwater Management Facilities
The requirement to detain stormwater runoff on-site has arisen from the increase of impervious coverage throughout the Village. This increase of impervious coverage through larger single family residences, swimming pools, tennis courts, patios, and more has reduced areas where stormwater drainage can absorbed naturally into the ground prior to entering into the Village's storm sewer infrastructure.
On-site stormwater detention ensures that new developments do not cause increased downstream flooding in either local drainage systems immediately or impact downstream rivers. With on-site stormwater detention, potential flooding problems are controlled at their source.
As subdivisions are developed, there is often a requirement to provide on-site stormwater detention for the entire subdivision. Generally, this is done through the construction of dry bottom detention basins or retention ponds and the maintenance often falls on the homeowners association. The Village has developed a map of the detention basins and retention ponds that depicts who is responsible for the maintenance of the facility.
Regular maintenance of stormwater management facilities helps ensure proper functioning, longevity, and public safety. Properly functioning private stormwater management facilities are a crucial component in mitigating flooding risks. In addition, routine maintenance is often required as a condition of developing a subdivision or a building permit.
General Surface Grading
Land grading is the reshaping of the ground in an effort to direct stormwater in a specific direction, typically toward storm sewer inlets or overland flow routes and away from structures. For drainage swales to function properly, it is required that they be a minimum width of 18 inches and a maximum depth of 12 inches. Maintenance is required when:
- Surface grading has changed in a way that redirects stormwater.
- Obstructions prevent the natural flow of water.
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 embankment.
- The embankment becomes denuded or otherwise presents a sediment erosion problem.
- Visible damage to any of the mechanical equipment is present (if applicable).
Dry PondsUnlike a wet pond, dry ponds are designed to retain stormwater runoff generally between 48 and 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 and rivers. Water retained on site too long can affect the walls of the basin or lead to algal problems. Maintenance is required when:
- Too much sediment accumulates and interferes with volume capacity.
- Trees or other shrub vegetation grown on the embankment.
- The embankment becomes denuded or otherwise presents an erosion problem.
- The low flow orifice, fore bay, or concrete trickle ditch is blocked by trash, debris, or sediment.
- Animal burrows are present on the embankment.
- Standing water remains longer than 72 hours after a rain event.
Underground DetentionConsisting 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 may be 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.
- If excessive sediment and/or debris has accumulated in the inlets, pipes, or outlets.
Green Infrastructure – Bio-Retention & Rain Gardens
Bio-retention is a stormwater management practice that comprises shallow depressions that incorporates soil amendments and native vegetation to temporarily store and filter stormwater runoff, increase soil porosity, and facilitate drainage. Bio-retention areas are suitable for residential areas. Bio-retention is not suitable in locations with continuous flow or a high water table, sites with slopes greater than 20 percent, locations less than 10 feet from a structure with a basement, locations less than 5 feet from a structure without a basement.
Green Infrastructure – Porous Pavement
Porous pavement consists of a permeable surface course (porous asphalt, porous concrete, or various porous structural pavers), which allow rapid infiltration of stormwater. The surface course is underlain by a uniformly graded stone bed, which provides temporary storage for peak rate control and promotes infiltration. Porous concrete systems require a very high level of construction workmanship to ensure that they function as designed.
Green Infrastructure – Impervious Area Disconnection
Impervious area disconnection aims to slow down the rate of stormwater runoff, using pervious areas to filter and infiltrate stormwater. This practice reduces the volume of stormwater draining into the municipal storm sewer by draining rooftops, driveways, sidewalks, patios, and other impervious areas to grass swales, bio retention areas, infiltration trenches or other infiltration devices. The two primary types of impervious area disconnection are downspout disconnection and pavement disconnection.
Green Infrastructure – Infiltration Trenches
An Infiltration Trench is a stone filled trench with a level bottom used to capture stormwater runoff and allow infiltration into the surrounding soils from the bottom and sides of the trench. Infiltration trenches are excavated areas typically filled with stone to create an underground reservoir for stormwater runoff. The runoff volume is stored in the void space between the stones within the trench and gradually exfiltrates through the bottom and sides of the trench into the surrounding soils.
Green Infrastructure – Rainwater Harvesting
Rainwater harvesting systems treat stormwater as a resource rather than a waste product. It is a more sustainable urban drainage infrastructure that attempts to minimize the use of drinking water for irrigation purposes. Cisterns, rain barrels, vertical storage, and similar devices have been used to capture stormwater from the roofs of buildings.
Green Infrastructure – Vegetated Swales
Vegetated Swales are broad, shallow, trapezoidal or parabolic channels, densely planted with a variety of trees, shrubs, and/or grasses. They are designed to capture and infiltrate stormwater runoff from adjacent impervious surfaces, allowing some pollutants to settle out in the process. Check dams may be used to improve filtration and infiltration opportunities.
Grading – Barriers
When properly designed and constructed, berms and levees can be effective in reducing structural damage from overbank flooding. The sides of a levee or berm are sloped to provide stability and resist erosion; thus, the width is usually six to eight times its height. As a result, taller levees require more land. A floodwall is an engineered structure made of reinforced concrete or reinforced concrete block and varies in height from 1-to 20-feet. Similar to berms and levees, a floodwall can surround a structure or a portion of a structure. Barriers are not typically used to resolve structural flooding in urban areas due to the potential impacts on adjacent properties; however, there are some situations where this flood mitigation strategy may be used. Some appropriate applications of barriers include areas outside the regulatory floodplain where the barrier can be constructed without adverse impacts to adjacent properties, and structures with a low opening that can be protected without adverse impacts to adjacent properties. The local floodplain management ordinance must be reviewed for restrictions on the use of barriers.
Grading – Driveway Berms
Reverse sloped driveways are often used in high density neighborhoods, where there is not sufficient area for detached garages. This type of driveway creates a significant flood risk when it directs overland stormwater flows into homes. Water that enters homes through reverse sloped driveways can cause structural damage and contribute to sewer backups, if this water enters basement floor drains. One solution is to construct a driveway berm. This can be achieved by either raising the sidewalk and/or reconstructing the entire driveway. This can reduce the chances that overland flooding will enter the structure through the reverse sloped driveway.
Grading – Overland Flow Swales
In newer subdivisions, residential lots are designed for surface water to flow away from the structure towards the lot lines. An overland flow swale is provided along the property lines within an easement, which is designed to keep the area open. This swale allows stormwater runoff to flow towards drainage infrastructure, such as detention basins.
Dry Floodproofing – Flood Shields for Exteriors
Removing basement windows and doors that are the first entry point for floodwaters, and incorporating them into the wall system can seal a building from floodwaters and alleviate structural flooding. The decision to eliminate an opening depends on the use of the opening, location and the ease with which the opening can be filled and sealed. Sealing an opening is dependent on the wall or foundation’s ability to resist the loads. If the walls or foundation are structurally insufficient to carry these loads, they must be reinforced prior to sealing the opening. Sealants used to seal openings in walls or floors should be able to withstand being submerged for the anticipated duration of flooding. During flood conditions, doors typically present the largest openings requiring protection from water intrusion into the building. Flood shields or panels are watertight structural systems that bridge the openings in walls to prevent the entry of floodwaters.
Dry Floodproofing – Glass Block Basement Windows
An alternative to a raised window well is to remove the glass from the window and replaced it with glass blocks. When installed properly, glass blocks can withstand the pressure of shallow ponding floodwaters. The glass blocks will reduce the occurrence of seepage through a lower level window.
Dry Floodproofing – Raised Window Wells
Properties that do not have adequate protection of their low opening can effectively raise the low opening height with a window well. Window wells can prevent water from entering the basement and prevent rotting of window sills, which may reduce the ability of the windows to hold back flood water.
Plumbing - Overhead Sewer Systems
An overhead sewer is generally viewed as the most cost effective sewer backup protection measure for residential basements. A sump is installed under the basement floor to intercept sewage flowing from basement fixtures and the basement floor drain. An ejector pump in the sump pushes sewage up above the flood level. From there it can drain by gravity into the sewer service line. Plumbing fixtures on the first floor continue to drain by gravity to the service line. If the water level in the municipal sewer system reaches an elevation of the overhead sewer where it exits the structure, a check valve in the pipe from the ejector pump keeps the water within the pipes.