TMDL Masthead


Watershed Characterization

1. Watershed Land Cover

The project team collected datasets on the watershed from both the University and the Town of Mansfield. These data can be viewed on the interactive map.  A detailed land cover map was created by digitizing recent high resolution aerial imagery (below).  To give the team a better idea of retrofit opportunities, impervious cover features were also classified by type (Section 2, below).

View Eagleville Brook Land Cover Map

Eagleville Brook Watershed Land Cover Pie
Land Cover Type Area (acres) % of Watershed
agriculture 16.8 1.4%
barren 15.8 1.3%
forest 682.9 56.9%
grass 17.7 1.5%
turf 243.6 20.3%
water 2.5 0.2%
wetland 1.7 0.1%
impervious cover 218.4 18.2%
Total 1199.2 100%


2. Watershed Impervious Cover by Type

View Eagleville Brook Impervious Cover Map

Eagleville Brook Watershed Land Cover Pie
Impervious Cover Type Area (acres) % of Watershed % of Impervious Cover
building 65.3 5.4% 29.9%
parking lot 53.5 4.5% 24.5%
road 34.1 2.9% 15.7%
driveway 30.0 2.5% 13.7%
sidewalk 23.41 2.0% 10.7%
other 12.1 1.0% 5.5%
Total 218.4 18.2% 100%


3. Watershed Impervious Cover Estimates and Targets

In July 2009, the project team conducted a four-day field analysis of the watershed. Field work identified important features that could not be determined from the mapping exercise alone. First, the team identified discrepancies in the original watershed boundary as contained in the state hydrography data layer; the revised watershed boundary was about 26 acres less than the original. Second, the team estimated that about 51 acres of the impervious cover in the watershed were effectively disconnected via sheet flow to a large forested area, undetected diversion to another watershed, or through treatment by a recently constructed stormwater practice. Revised impervious cover disconnection targets were made using these adjustments (below).

Eagleville Brook Watershed TMDL Estimated Adjusted with updated mapping Field Adjusted
Watershed drainage area (acres) 1225 1225 1199
Watershed IC (acres) 145 218 167
Watershed IC (%) 11.8% 17.8% 13.9%
11% IC TMDL target (acres) 111 111 132
IC to disconnect/manage to reach target (acres) 34 107 35

Table 1. Existing conditions in Eagleville Brook. The original estimates were based on modeling using 2002 land cover data with 30m resolution. The second column shows additional impervious cover resulting from updates and improvements using 2008 high resolution satellite imagery. The last column includes field adjustments that decreased the watershed area by 26 acres, and "removed" 51 acres of disconnected IC.