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.
|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|
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:
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.
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.
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:
|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 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.