New Techniques for Stormwater Management Low Impact Development Techniques

New stormwater management techniques have been developed that minimize and prevent adverse stormwater effects from land disturbance. These techniques are referred to by the NJDEP as Low Impact Development techniques (LIDs) and include both nonstructural and structural Best Management Practices (BMPs). LID-BMPs first minimize quantitative and qualitative changes to a site’s pre-developed hydrology (i.e., employ nonstructural techniques first) and then provide stormwater management through smaller sized structural techniques distributed throughout the site. The link to the NJDEP website to download the BMP Manual is:

Nonstructural LID-BMPs include such practices as minimizing site disturbance, preserving important site features, reducing and disconnecting impervious cover, flattening slopes, utilizing native vegetation, minimizing turf grass lawns and maintaining natural drainage features. It may be possible at some sites to satisfy all stormwater management requirements through nonstructural LID-BMPs. Structural BMPs are considered LIDs, if they are located close to the source of runoff. Structural LID-BMPs include various types of basins, filters, devices and permeable surfaces located within residential lots and otherwise throughout residential, commercial, industrial or institutional development.

Because LIDs rely on nonstructural or relatively small structural BMPs distributed throughout a land development site, ownership and maintenance may be similarly distributed to an array of property owners. The new Stormwater Management rule requires the use of deed restrictions for LID-BMPs to ensure that property owners fully recognize, understand and support the continuing use of LID-BMPs for stormwater management.

Nonstructural LID-BMPs
The NJDEP’s new Stormwater rule’s design and performance standards require the maximum possible use of nine nonstructural strategies.

1. Protect areas that provide water quality benefits or areas particularly susceptible to erosion and sediment loss.
2. Minimize impervious surfaces and break up or disconnect the flow of runoff over impervious surfaces.
3. Maximize the protection of natural drainage features and vegetation.
4. Minimize the decrease in the pre-construction time of concentration.
5. Minimize land disturbance including clearing and grading.
6. Minimize soil compaction.
7. Provide low maintenance landscaping that encourages retention and planting of native vegetation and minimizes the use of lawns, fertilizers, and pesticides.
8. Provide vegetated open-channel conveyance systems discharge into and through stable vegetated areas.
9. Provide preventative source controls.

The nonstructural LID-BMPs have been grouped by the NJDEP into four general categories:

Vegetation and Landscaping – reduces runoff volumes and peaks through infiltration, surface storage, and evapotranspiration, provides pervious surface for groundwater recharge and removes pollutants from stormwater. Key techniques include:

1. Preservation of Natural Areas – preserve areas with significant hydrologic functions including forested areas, riparian corridors and soils/geology with high recharge potential.
2. Native Ground Cover – reduce the use of turf grass and preserve areas that naturally minimize runoff.
3. Vegetative Filters and Buffers – provide native ground cover and grass areas to filter stormwater runoff from pervious areas and to provide locations for runoff to infiltrate.

Minimizing Land Disturbance – reduces runoff volume and pollutant loads and maintains existing recharge rates and other hydrologic functions. Key techniques include:

4. Planning and design to fit the development to the terrain, limiting clearing and grading.
5. Evaluating site conditions and constraints including soil types, geology, topography, slopes, drainage areas, wetlands, and floodplains to maintain high recharge areas and provide runoff storage areas.
6. Utilizing construction techniques that limit disturbance and soil compaction.
7. Restricting the future expansion of buildings and other improvements that will adversely affect runoff volumes and rates or recharge rates.

Impervious Area Management – reduces water quality impacts, runoff volume and peak rates, runoff velocity, erosion and flooding. Key techniques include:

8. Streets – use minimum acceptable pavement widths and incorporate pervious vegetated medians and islands with curb cuts for runoff access.
9. Sidewalks – use pervious pavement with infiltration storage beneath and disconnect from the street drainage system.
10. Parking and Driveways – use pervious pavement wherever practical and reduce parking space requirements by sharing requirements in mixed uses and by reducing parking space lengths by allowing for overhang into pervious areas.
11. Pervious Paving Materials – Use pervious materials in parking spaces, driveways, access roadways and sidewalks, including pavers, porous pavement and gravel.
12. Unconnected Impervious Areas – Disconnect impervious areas and runoff form the site’s drainage system allowing the sheet flow to cross pervious areas through curb cuts or by eliminating curbing and using shoulders and swales.
13. Vegetated Roofs – install lightweight vegetative planting beds on new or existing roofs.

Time of Concentration Modification – minimize reductions to the time of concentration caused by changes in hydrologic characteristics in order to minimize the peak runoff rate. Key techniques include:

14. Surface Roughness Changes – increase surface roughness through the use of land cover and decrease the amount of connected smooth surfaces in order to increase runoff travel time throughout the drainage area.
15. Slope Reduction – reduce slopes in graded areas and/or provide terraces and reduced slope channels to increase runoff travel length and time.
16. Vegetated Conveyance – use vegetated channels and swales to increase roughness and runoff travel time and to provide opportunities for runoff treatment and infiltration.

Structural LID-BMPs
There are 11 structural stormwater management techniques described by the NJDEP in the BMP Manual. These include:

1. Bioretention Systems
2. Constructed Stormwater Wetlands
3. Dry Wells
4. Extended Detention Basins
5. Infiltration Basins
6. Manufactured Treatment Devices
7. Pervious Paving Systems
8. Rooftop Vegetated Cover (reserved – no information provided)
9. Sand Filters
10. Vegetative Filters
11. Wet Ponds

For more detailed information on a specific structural LID-BMP, go to the NJDEP BMP Manual website and download Chapter Nine.

Mitigation Plans
Land development projects that have unique, site-specific conditions preventing strict compliance with the new Stormwater Rules may request a waiver or exemption from strict compliance with the New Jersey Stormwater BMP Manual and the groundwater recharge and stormwater runoff quality and quantity requirements and may submit a mitigation plan in accordance with the MSWMP.

The mitigation plan must identify the measures required to offset any potential impact created by granting the variance or exemption to the performance standards. Several strategies can be used for mitigation. Applicants must identify, design, and implement a compensating measure to mitigate impacts; that is, complete a project identified as equivalent to the environmental impact created by the exemption or variance; or, provide funding for municipal projects that would address existing stormwater impacts. Preference is to be given to a mitigation project within the drainage area that directly compensates for the projected impact of the variance or exception.

The mitigation plan may include property owned by the municipality or secured through easements, as a condition of planning and zoning board approvals, and that would allow implementation of future mitigation measures. A list of projects to be implemented that compensate for stormwater quality and stormwater quantity impacts must be developed by the municipality. If direct mitigation for the projected environmental impact is not feasible, nonequivalent project mitigation may be used that targets a specific nonpoint source stormwater management problem. The mitigation may permit funding of specific stormwater projects within the municipality that address existing stormwater quality or quantity issues. The mitigation funding must result in projects that provide adequate protection and compensate for the impact created by failing to strictly comply with the performance standards in the new Stormwater Rules.

All mitigation plans and reviews will consider the location of the mitigation project in relation to the major development. Mitigation Plans can be simple or complex, provided they afford sufficient protection of water resources. Mitigation will not be an option until it is clearly demonstrated that compliance is not practical. The mitigation plan and approval must ensure that long-term maintenance is achieved by clearly assigning responsibility for maintenance and by securing the funding and resources required to perform it.

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