It is often difficult for planners to generate accurate imperviousness values for highly urbanized watersheds.  Often, these values are most easily calculated by employing Digital Elevation Models that show how water moves across the topography.  However, in a small, urban watershed, such as Malletts, this will not produce realistic results.  By under laying the below digital orthophotograph and individually digitizing areas, using Arcview GIS, for a sample of rooftops, driveways, pathways, and sidewalks (excluding roads), we found the following breakdowns and imperviousness value for the 2.2 acre block:
 

Type	Avg. Area (Sq.Ft.)	Area in Acres	% Impervious
Sidewalks	125.14	.05	2.34
Roofs	2077.05	.86	38.76
Paths	46.80	.02	0.87
Drives	346.89	.14	6.47
TOTAL	2595.88	1.07	48.45

    A general rule of thumb used by planners:  if more than 10% of the creekshed is impervious surfaces, the water quality of the creek starts deteriorating.  Since 40% of Malletts Creekshed is residential, and assuming that all the residential communities are around 50% impervious, then the creekshed - just considering this land use type -  is already about 20% impervious.