USING LIDAR TO MEASURE URBANIZATION'S ADDED HYDROLOGIC CONNECTIVITY AND ITS IMPLICATIONS FOR FLOODING

The increase of surface runoff due to urbanization’s increase in impervious area is well documented. However, an overlooked impact of urbanization is the increase in hydrologic connectivity of the land surface. This increased hydrologic connectivity is accomplished during urbanization by the smoothing of the landscape and the installation of infrastructures (e.g., storm sewers, sidewalks, and roofs) that efficiently convey water. This increase in hydrologic connectivity would be especially significant in landscapes that, pre-urbanization, are poorly drained with low stream densities. Such a landscape is often found underlain by carbonate lithologies. These carbonate landscapes frequently have subtle topographic basins that function as small, closed, internally-drained watersheds. After urbanization these basins are eliminated and the effective drainage area of watersheds is increased, which adds to the larger peak flows post-urbanization.

These small basins are often small enough that LIDAR is necessary to identify them from a digital elevation model (DEM). The improved spatial resolution (~1 m grid cells) of LIDAR allows these subtle landscapes to be identified and measured. DEMs with larger cell sizes, such as the National Elevation Database (NED) smooth out these landscapes and the basins are “lost”. Furthermore, frequently-used analysis tools in Geographic Information Systems (GIS) such as “Fill” also serve to smooth out these landscapes. By using LIDAR DEMs and putting limits on the GIS “Fill” function it is possible to measure the extent of these internally-drained areas and how urbanization affects them.

Analysis of several small watersheds in central Pennsylvania show that there are significant areas that effectively act as closed drainage areas. These small (~10 km²) watersheds have up to 8% of their area hydrologically disconnected on the surface from the larger drainage network. These areas become connected post-urbanization, and the implications could be significant for those watersheds that have large areas that are only hydrologically connected post-urbanization and increase the effective drainage areas of the local streams. These implications include flooding, erosion control, and biologic impacts.