The updated MS4 permit has several mapping components. A summary of those requirements and their deadlines is provided below. NEMO is working with 4 pilot communities to develop guidance, tools, and training to help towns meet these requirements.
|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
|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:
|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|
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:
For more information on DCIA, this tutorial covers the DCIA-related permit requirements and how to determine DCIA.
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. 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.
TIP: Statewide high resolution (1ft) impervious cover data is now available in our MS4 Map viewer! The data layer includes percent impervious cover for each basin which you can use to estimate DCIA using one of the methods below.
There are three methods you can use to determine the DCIA in each basin in your town. In increasing level of accuracy and effort, they are:
|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|
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. Towns that have planimetric data or higher resolution impervious data would likely choose to use that data instead.