The 2017 MS4 permit has several mapping related requirements. A summary of those requirements is below with deadlines. NEMO is working with 4 pilot communities to identify approaches to meeting these requirements and will be providing options, tools, and training to help towns meet these requirements.

Tasks & Timelines

Mapping Task Details Deadline
Permit Registration Map Click for details April 1, 2017
(submit with registration)
Electronic or paper copy of the relevant portion (or a full-sized original) of a USGS quad map (1:24,000 scale) showing permitee's boundaries and limits of its separate storm sewer system
Map and develop database for MS4 outfalls and interconnections Click for details Existing MS4s: June 30, 2019

New MS4s: priority areas by June 30, 2020 and townwide by June 30, 2022
  • All outfalls within and owned/operated by municipality or institution and interconnections with other MS4s that discharge into waters of the state
  • Develop map or series of maps (ideally using GIS) within scale range of 1": 2000' to 1": 100' showing all points of stormwater discharge from pipes or conduits
  • Inventory shall include:
    • Unique identifier
    • Receiving water name, waterbody ID and Surface Water Quality Classification
    • Type, material, shape and size (e.g. 24” PVC pipe)
    • Location (lat and long +/- 30 feet accuracy)
    • Date of last inspection
    • Physical condition
    • Any indicators of potential non-stormwater discharges
      • Presence/evidence of suspect flow, odor, color, turbidity, floatables, or oil sheen
Detailed map of MS4 infrastructure Click for details Existing MS4s: June 30, 2020

New MS4s: June 30, 2022
Map the following elements within priority areas, at a minimum:
  • outfalls and receiving waters,
  • pipes,
  • open channel conveyances (diches, swales),
  • catch basins,
  • manholes,
  • interconnections with other MS4s,
  • MS4-owned stormwater treatment structures (ex. detention basins, oil/water separators),
  • catchment delineations (to be used for assessment and priority ranking),
  • waterbodies identified by name and any use impairments
Where available, also include the following data layers/attributes:
  • storm sewer material/size/age,
  • sanitary sewer material/size/age,
  • properties known/suspected to be served by septic system,
  • areas with potential to receive flow from septic system discharges (ex. poor soils, high groundwater tables),
  • seasonal high water table elevations impacting sanitary alignments,
  • topography,
  • orthophotography,
  • locations of suspected, confirmed, and corrected illicit discharges

Calculate Directly Connected Impervious Area (DCIA) for each outfall Using percent impervious cover, estimate the percentage of directly connected impervious area (see section on DCIA below) June 30, 2020
Track DCIA disconnection Click for details Existing & New MS4s: Begin July 1, 2017
  • Track total acres of DCIA disconnected annually
    • Document area of DCIA disconnected for a given redevelopment or retrofit project (public and private)
    • Do not count new developments
  • Include any disconnections since July 1, 2012 (5 year look back)

Directly Connected Impervious Area

There is wide scientific agreement that increasing levels of impervious cover in a watershed lead to degraded water quality. Under the new permit, towns and institutions must:

  1. estimate the amount of directly connected impervious area (DCIA) for each outfall to a water body, AND
  2. estimate a townwide DCIA baseline number and begin reducing that by 1% per year in 2021.

DCIA is the impervious area that discharges stormwater directly into a surface waterbody or into stormwater drainage infrastructure that discharges directly into a surface waterbody. Any areas above 11% DCIA that overlap an urbanized area are considered "priority areas" and should therefore be a focus of activites under the permit.

Is your IC connected?

Watch this brief video to get a better understanding of the difference between impervious area and directly connected impervious area.

Area of Measurement

The permit requires that the town/institution calculate the DCIA "that contributes stormwater runoff to each of its MS4 outfalls (i.e. catchment area)." (page 30) The question then becomes how do you 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." (page 4)

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 for the area of measurement. That basin then becomes the catchment area. NEMO's MS4 Map includes this basin layer. Note: 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 often required significant field verification.

TIP: We are acquiring high resolution (1m) impervious cover data for the whole state and expect to have it available in the summer of 2017. With that we will be providing the IC percentage for each basin in our MS4 Map viewer. You can then use that to estimate DCIA using the methods indicated below.

Calculating DCIA

There are three methods you can use to determine the DCIA in each basin in your town. In increasing level of accuracy and likely increasing level of effort, these are:

  1. Assume that the Impervious Cover (IC) (soon to be available in the MS4 map viewer) is 100% connected, and therefore is your DCIA. Note that if you choose this method you'll likely overestimate the amount of DCIA.

  2. Use the following equations to estimate DCIA based on the development density within each basin. This requires categorizing each basin into one of the five categories (1 – 5) in the table below and inserting the IC value for each basin to the relevant equation to estimate DCIA. No one knows your town better than you, so this may be an easy exercise to complete by looking at the high resolution aerial imagery and IC in each basin (coming summer 2017) using the MS4 map viewer or your own maps.

  3. 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, for additional information see the EPA technical support document: Estimating Change in Impervious Area (IA) and Directly Connected Impervious Areas (DCIA) for Massachusetts Small MS4 Permit (

  4. 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.

Determining Impervious Cover Percentage

Through a grant from CT DEEP, NEMO is acquiring statewide high resolution (1M) impervious cover data. The data will be available by September 2017 and can be used to determine the percent impervious cover for a given area. It will be based on 2012 imagery to reflect the permit's 5 year look back period for tracking disconnections of impervious cover. Towns that have planimetric data or higher resolution impervious data would likely choose to use that data instead.