Stormwater Planning Tool for Impervious Cover

Stormwater and Impervious Cover (IC)

Stormwater is rain or snowmelt that runs off hard surfaces such as rooftops, road, highways and parking lots. These hard surfaces are collectively called impervious cover. Stormwater that runs off impervious cover can pick up and transport contaminants including motor oils, gasoline, antifreeze, and brake dust (commonly found on pavements), fertilizers and pesticides (found on landscaped areas), and soil sediments (from construction and other sites). The stormwater eventually flows into a local stream, river or lake via a storm drain system.To learn more about storm water and water quality go to the Stormwater and Water Quality web page.

IC and Runoff

The runoff of water can occur anywhere there is impervious cover and stormwater. The figure below depicts the relationship between runoff and impervious cover. As areas are converted from the natural landscape to a developed environment, there is an increase in impervious cover. Areas with greater impervious cover result in greater amounts of stormwater pollution. Runoff picks up contaminants from impervious cover and impervious cover results in more pollutants in runoff. The quality of stormwater runoff is considered poor compared to local waterbodies. Since stormwater discharges to local waterbodies, the amount of impervious cover can impact the surface water quality because of stormwater pollution.

In order to restore and maintain water quality, tools are needed to provide information and guidance to implement management measures within the local watershed.

The figure below, from the Federal Stream Corridor Restoration Handbook (1998), depicts the increase stormwater runoff with the increase in hard surfaces.

Increased runoff with increased hard surfaces


What is a Watershed Response Plan? A Watershed Response Plan is a tool used to improve water quality in local waters. This type of plan offers water quality information and guidance for all members of the local community, including the public, municipal officials, businesses and watershed groups. The Watershed Response Plan for Impervious Cover (IC Response Plan) provides details on the local watershed conditions, impervious cover, and implementation measures. The local community can use the IC Response Plan to complement existing stormwater practices and infrastructure management. Improving stormwater quality and reducing runoff can reduce its negative effects and restore water quality in the local waterbodies.








The components of the IC Response Plan include:

Core document: Connecticut Watershed Response Plan for Impervious Cover 

 Appendix 1: Additional Resources for Implementation
 Appendix 2: Percent Impervious Cover as a Surrogate Target for TMDL Analyses in CT
 Appendix 3: Impervious Cover in Connecticut Municipalities
Appendix 4: Responding to an Impervious Cover-Based TMDL
Appendix 5: Case Studies: Examples of Action in Connecticut Watersheds 

Impaired Segment Summaries 

Appendix 6-1: Sub-Regional Basin CT2000: Southeast Shoreline
Appendix 6-2: Sub-Regional Basin CT4403: Trout Brook  
Appendix 6-3: Sub-Regional Basin CT4500: Hockanum River  
Appendix 6-4: Sub-Regional Basin CT5200: Quinnipiac River  
Appendix 6-5: Sub-Regional Basin CT5203: Misery Brook  
Appendix 6-6: Sub-Regional Basin CT5205: Sodom Brook   
Appendix 6-7: Sub-Regional Basin CT5206: Harbor Brook
Appendix 6-8: Sub-Regional Basin CT5207: Wharton Brook 
Appendix 6-9: Sub-Regional Basin  CT5302: Mill River   
Appendix 6-10: Sub-Regional Basin CT5306: Indian River    
Appendix 6-11: Sub-Regional Basin CT6000: Housatonic River 
Appendix 6-12: Sub-Regional Basin CT6600: Still River    
Appendix 6-13: Sub-Regional Basin CT7000: Southwest Shoreline 
Appendix 6-14: Sub-Regional Basin CT7105: Pequonnock River  
Appendix 6-15: Sub-Regional Basin CT7403: Noroton River 

 Where to target effortsimpervious cover in CT

Included in the IC Response Plan document is a detailed IC analysis of 16 watersheds. These watersheds all have impaired waterbodies and have IC of 12% or more. These watersheds were chosen as examples of waterbodies around the State impacted by stormwater and IC. The Watershed Response Plan is a resource guide to reduce stormwater pollution. It offers guidance on:
  • what can be done about IC
  • where to target regulating and non-regulating efforts 
  • case studies of reducing stormwater pollution
  • highlights of some impaired watersheds 

Fix what's broken 

Once you know the water quality problem, your community or town may be able to find and fix common sources that are polluting the local waters. The IC Response Plan has suggestions for common IC problems as well as implementing best management practices.Pollution reaching the waterbody is expected to be reduced as progress is made with these sources. Local education can improve water quality through efforts focused on preventing pollutants in runoff. Programs can also teach about the use and functions of storm drains. Regular inspections of stormwater drainage systems and wastewater sewers for leaks and making swift repairs will also have positive effects on water quality. The results will improve our natural resources, provide cleaner water, better habitat for fish and safer recreation for people. 
There are many stormwater system upgrade techniques that can be used as well as many different types.Most techniques involve disconnecting or reducing the effect of IC. These stormwater upgrades go by a number of names:
  • stormwater retrofits
  • low impact development (LID)

  • best management practices (BMPs) both structural and non-structural                                               

Grassed Swale

Tree Box Filter

While some techniques may seem novel, there are notable successes of proper installation, adequate maintenance, and stormwater pollution reduction. Examples of these practices include grassed swales, green roofs, tree box filters, rain gardens, and permeable asphalt. Many of these practices have been plotted within towns across the State by CT NEMO (Nonpoint Education for Municipal Officials) in their CT LID Atlas. No single device or practice can be expected to eliminate stormwater pollution, but in tandem, these practices significantly reduce impacts on the quantity and quality of stormwater runoff. Several practices have been studied including research at the University of New Hampshire and show significant improvements in runoff through pollution reduction practices. 


permeable asphaltrain garden

Other methods to reduce stormwater pollution are not necessarily structural. Some practices include public education programs such as campaigns to eliminate dumping in catch basins. Also town ordinances can be adopted to encourage techniques to reduce stormwater pollution from new and existing developed areas. Ultimately, the technique or structure regardless of scale has a goal to reduce pollution and impacts from stormwater.






For more information:

View the interactive maps to learn more about stormwater pollution management Plans in Connecticut.

To read about stormwater in a specific town, select the town from the map or choose from the drop-down list.

CT Town Map

Visit Total Maximum Daily Loads (TMDLs) to learn more about the tool Water Quality Managers in Connecticut use to address water quality problems.

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Content last updated on February 13, 2020.