The updated MS4 permit has several mapping requirements which generally involve mapping stormwater infrastructure under the IDDE control measure and estimating, tracking and disconnecting Directly Connected Impervious Area.

Relevant terms

Directly Connected Impervious Area (DCIA): impervious area which drains stormwater runoff into catch basins or directly into waterbodies. Impervious areas that discharge to an area or system that retains the appropriate Water Quality Volume are not considered DCIA.

Outfall: is the point where the MS4 discharges to waters of the state. It's defined as a point source in the permit - 'any discernible, confined and discrete conveyance, including but not limited to, any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock, concentrated animal feeding operation, landfill leachate collection system, vessel or other floating craft from which pollutants are or may be discharged'.


Stormwater infrastructure mapping

The Connecticut Department of Transportation (DOT) worked with the Standards Committee of the CT GIS Network to develop a standard stormwater infrastructure mapping schema (i.e. the way the data is organized). CT DOT will be using this format for their infrastructure mapping. Towns may find it useful to use or modify this format for their own stormwater system mapping. It may save time in developing your own system and make it easier to exchange stormwater mapping information with CT DOT and other towns and institutions.

The new schema can be downloaded as either a spreadsheet or geodatabase.

Tools for mapping your stormwater system

We're testing two different field data collection applications that can be used to map stormwater infrastructure. There will be a Stormwater infrastructure mapping workshop offered to towns this September (2018) and we'll provide presentations on both options there. In the meantime, this comparison chart highlights the key capabilites and requirements of each application.

Epicollect Collector for ArcGIS

What is it?

Form-based application (no visual aid or map)

Map-based application with ability to cache base maps and imagery

Compatible operating systems

IOS, Android

IOS, Android, Windows

Required software

open source

ArcGIS Desktop + ArcGIS Online organization (subscription)

Data format


File geodatabase and map service



Variable (depends on type of organization, population size, etc.)

Ability to collect pictures?



Ability to collect point data (ex. outfalls, manholes, catch basins, etc)

Yes Yes

Ability to collect line data? (ex. pipes)



Ability to collect polygon data (ex. bioretention areas)



Allows for multiple users / editors



Offline data collection



Requires data plan



Requires phone or tablet to have integrated or external GPS



Requires GIS expertise to deploy



Expertise required for field data collection



What is DCIA?

Directly Connected Impervious Area (DCIA) is the impervious area that transports stormwater directly into a waterbody or into stormwater drainage infrastructure that transports runoff directly into waterbodies. Because there is wide scientific agreement that increasing amounts of impervious cover in a watershed lead to degraded water quality, the new permit addresses DCIA in a few different ways. It requires towns to:

  1. Estimate the amount of DCIA for each outfall to a water body;
  2. Estimate a townwide DCIA baseline number;
  3. Track additions and subtractions of DCIA; and
  4. Implement a Retrofit program to reduce DCIA by 2% by the end of the permit term.

Is your impervious cover connected?

Watch this brief video to get a better understanding of the difference between impervious area and Directly Connected Impervious Area.

Impervious cover data for CT

Through a grant from CT DEEP, NEMO acquired statewide high resolution (1ft) impervious cover data in September 2017. This data includes the percent impervious cover for each town and each basin. The data is based on 2012 imagery to reflect the permit's 5 year look back period for tracking disconnections of impervious cover. You can see this data on the MS4 Map viewer or download it from CT ECO. Towns that have planimetric data or higher resolution impervious data would likely choose to use that data instead.

Calculate DCIA

There are three methods you could use to estimate the DCIA in each basin* in your town. In increasing level of accuracy and effort, they are:

  1. Assume Impervious Cover (IC) is 100% connected and is equal to DCIA. If you choose this method you'll likely overestimate the amount of DCIA but the advantage is this information is already available in the MS4 map viewer (use the 'IC by basin' layer).

  2. Use the following equations to estimate DCIA based on the land use in each basin. First, categorize each basin into one of the five categories (1 – 5) in the table below then insert the IC value for each basin into the relevant equation to estimate DCIA.

    No one knows your town better than you, so categorizing your basins may be an easy exercise to complete by looking at the high resolution aerial imagery and IC in each basin using the MS4 map viewer or your own maps.

    NOTE: %IC = percent impervious cover
    Connectivity Level Description of Contributing Area Land use type Equation Example for a watershed with 20% impervious cover (IC)
    1. Fully Connected (default) 100% storm sewered with all IC High density mixed use, commercial None. DCIA% = IC% 20% DCIA
    2. Wicked Connected Mostly storm sewered with curb and gutter, residential rooftops connected to MS4 High density residential, commercial, industrial, institutional DCIA%=0.4(%IC)^1.2 0.4(20)^1.2 = 14.6% DCIA
    3. Moderately Connection Mostly storm sewered with curb and gutter, residential rooftops NOT connected to MS4 Medium density residential, commercial, industrial, institutional, open land DCIA%=0.1(%IC)^1.5 0.1(20)^1.5 = 8.9% DCIA
    4. Sorta Connected 50% storm sewered with some infiltration and residential rooftops not connected to MS4 Low density residential, open land DCIA%=0.04(%IC)^1.7 0.04(20)^1.7 = 6.5% DCIA
    5. Slightly Connected Small % of urban area storm sewered or mostly infiltration Agricultural, forested, natural areas DCIA%=0.01(%IC)^2 0.01(20)^2 = 4% DCIA
    This table was adapted by CT DEEP from EPA guidance on DCIA.
  3. Calculate DCIA through map analysis and field checking. This requires the most effort but will result in the most accurate DCIA determination.

Map Analysis

You can get a good sense of where stormwater drains are using publically available data on the CT ECO website. See NEMO’s website for details on conducting a stormwater drainage map analysis using this data.

Field Checking

In some areas, rural roadways for example, a ‘windshield survey’ from the car will be enough to confirm the status of IC. In more urban areas prepare for sticking your head down a lot of storm drains! As you walk the watershed, note the drains and confirm, when possible, where they take the water—by looking. Where there are no drains, note which way the land slopes and where the stormwater goes—this can best be confirmed by going out on a rainy day. The NEMO website has some videos about this too.

*Basins as catchment areas

The permit requires that the town or institution calculate the DCIA that contributes stormwater runoff to each of its MS4 outfalls (i.e. catchment area). The question then becomes how to determine the 'catchment area' which is defined as "the land area from which stormwater runoff is collected by a permitee's MS4 and discharges through a single outfall to surface water".

There are methods for identifying the catchment area for an outfall. However, these methods can be cumbersome, time consuming, and often inaccurate. In recognition of this, DEEP has agreed that towns and institutions can use the smallest watershed 'basin' unit that an outfall is in as the catchment area. NEMO's MS4 Map viewer includes this basin layer. If you prefer determining the specific catchment area for each outfall, this method developed by a regional planning agency in Massachusetts may help. Just be warned that in our testing of this method, it was time consuming, did not return great results and required significant field verification.

DCIA tutorial

For more information on DCIA, this tutorial covers the DCIA-related permit requirements and how to determine DCIA.

Task list

Where the deadline for new MS4 permittees differs from the deadline for exisiting permittees, the second deadline in parenthesis applies to new permittees.

Activity Deadline Permit Page

DCIA tasks

Track DCIA

Jul 1 2017


Determine townwide DCIA baseline

Jul 1 2020


Develop retrofit plan

Jul 1 2020


Begin implementing projects from retrofit plan

Jul 1 2020


Goal to disconnect 2% of baseline DCIA

Jul 1 2022


Stormwater system mapping

Map all MS4 outfalls

Jul 1 2019 (Jul 1 2020)

23 & Appendix B p.2

Complete detailed storwater system mapping

Jul 1 2020 (Jul 1 2022)

Appendix B p.3

CT MS4 Mapping Workshop Materials

The NEMO program presented a workshop on the Mapping portions of the MS4 Permit on 10/19/18 at Rocky Hill Town Hall. All of the presentations were recorded. The presentation slides and recordings can be found below.


MS4 Mapping Overview, presented by David Dickson, UConn CLEAR

Mapping Impervious Cover, presented by Chet Arnold, UConn CLEAR

Estimating Directly Connected Impervious Area and Identifying Priority Areas, presented by David Dickson, UConn CLEAR

Identifying Opportunities for Disconnects, presented by Chet Arnold, UConn CLEAR

Western Council of Governments (WestCOG) DCIA Tools, presented by Nick Trabka, WestCOG

City of Manchester Stormwater Infrastructure Mapping, presented by Rich Gallacher and Elizabeth DaRos, Town of Manchester

Mapping Stormwater Infrastructure with Collector and iForms, presented by Kristen LaBrie, New England Geosystems

City of Stamford Stormwater Infrastructure Mapping, presented by Tyler Theder, City of Stamford

DIY System Mapping on the Cheap, presented by Cary Chadwick, UConn CLEAR

Mapping CT DOT’s Stormwater System and Interconnections, presented by Jeremy Willcox and Kevin Carifa, CT DOT