Impervious Cover by Watershed Basins is based on the Connecticut Drainage Basins layer and is 1:24,000-scale, polygon feature data that define natural drainage areas in Connecticut and contain values for calculated impervious cover (2012) within each basin. Drainage basins have been clipped to the Connecticut state boundary as well as each municipal boundary. The features are small basin areas that make up, in order of increasing size, the larger local, subregional, regional, and major drainage basin areas. Impervious Cover by Watershed Basins includes drainage areas for all Connecticut rivers, streams, brooks, lakes, reservoirs and ponds published on 1:24,000-scale 7.5 minute topographic quadrangle maps prepared by the USGS between 1969 and 1984. Data is compiled by the Connecticut Department of Environmental Protection (CT DEP) at 1:24,000 scale (1 inch = 2,000 feet). Basin delineation information is not updated. Each basin area (polygon) feature is outlined by one or more major, regional, subregional, local, impoundment, or river reach boundary (line) feature. These data include 9580 basin areas (polygons), clipped to the boundaries of Connecticut’s 169 municipalities. Impervious cover for each basin area has been calculated from 1-foot impervious cover data (link to metadata). Basin area attributes for imperviousness included total impervious area (acres), percent impervious area, building impervious area (acres and percent), roads (acres and percent) and other impervious area (acres and percent). Road impervious area has also been summarized for state (DOT) road area and non-state owned road area. Basin area (polygon) attributes include major, regional, subregional, local, (full) basin number, and feature size in acres and square miles. The full basin number (BASIN_NO) uniquely identifies individual basins and is up to 13 characters in length. There are 9580 unique basin numbers. Examples include 6000-00-1+*, 4300-00-1+L1, and 6002-00-2-R1. The first digit (column 1) designates the major basin, the first two digits (columns 1-2) designate the regional basin, the first 4 digits (columns 1-4) designate the subregional basin, and the first seven digits (columns 1-7) designate the local basin. Note, there are slightly more basin polygon features than unique basin numbers primarily because a few water supply watershed boundaries split a basin into two polygon features at the location of a small dam or point of diversion along a stream. Unique basin ID fields (BasinNoU and T_Basin_ID) have been added for the purposes of calculating impervious area. Connecticut Drainage Basins is the data source for other digital spatial data including the Connecticut Major Drainage Basins, Connecticut Regional Drainage Basins, Connecticut Subregional Drainage Basins, and Connecticut Local Drainage Basins.
The polygon features define the amount and type of impervious cover in contributing drainage areas for individual reservoirs, lakes, ponds and river and stream reaches in Connecticut. Please note that these drainage basins have been clipped to the municipal boundaries of Connecticut's 169 towns. Impervious area based on 2012 aerial imagery has been used to calculate area and percent of impervious cover (three major types: buildings, roads, and other impervious) for each drainage basin. The "roads" class has been broken down into town roads and DOT roads for the purposes of the State of Connecticut's General Permit for the Discharage of Stormwater from Small Municipal Separate Strom Sewer Systems (MS4) permit. All classes are calculated by total area (acres) and percent of the drainage basin. Drainage areas are hydrologic land units where precipitation is collected. Rain falling in a basin may take two courses. It may both run over the land and quickly enter surface watercourses, or it may soak into the ground moving through the earth until it surfaces at a wetland or stream. Use these basin data to identify where rainfall flows over land and downstream to a particular watercourse. Use these data to categorize and tabulate information such as amount of imperviousness according to drainage basin by identifying the basin number for individual reservoir, lake, pond, stream reach, or location of interest. Such information is useful for compliance with the State of Connecticut's MS4 permit. Due to the hierarchical nature of the basin numbering system, a database that records the 13-digit basin number for individual geographic locations of interest will support tabulations by major, regional, subregional or local basin as well as document the unique 13-digit basin number. To identify either all upstream basins draining to a particular location or all downstream basins flowing from a particular location, refer to the Gazetteer of Drainage Basin Areas of Connecticut, Nosal, 1977, CT DEP Water Resources Bulletin 45, for the hydrologic sequence, headwater to outfall, of drainage basins available at http://cteco.uconn.edu/docs/wrb/wrb45_gazetteer_of_drainage_areas_of_connecticut.pdf Not intended for maps printed at map scales greater or more detailed than 1:24,000 scale (1 inch = 2,000 feet.). Not intended for analysis with other digital data compiled at scales greater than or more detailed than 1:24,000 scale. Use these data with 1:24,000-scale hydrography data also available from the State of Connecticut, Department of Environmental Protection.