Delineation of Small Flat Watershed with High-Resolution DEM from Terrestrial Laser Scanning

The digital elevation model (DEM) has been widely used in hydrological analysis and flood assessment. While DEMs, derived from airborne light detection and ranging (lidar) technology, have sufficient accuracy for large-scale floodplain management practices, their utility in supporting high-resolution hydrologic simulations is disputable due to their limitations in resolution. In contrast, terrestrial laser scanning (TLS) is capable of conducting very dense point measurements, especially in close distance, generating high-resolution point clouds that may support high-fidelity hydrologic modeling for small areas. The study takes a case study approach in which we delineate boundaries of a catchment area for a small-scale ($\mathrm{1,450}{\mathrm{m}}^2$) stormwater management measure (porous parking lot) under different DEM resolutions generated from TLS with the end goal of understanding the utility of TLS for high-fidelity hydrologic modeling. Results showed that the 0.03-m resolution provided an accurate representation of terrain surface. Larger raster cell–size DEMs generated greater uncertainties on boundaries and streamlines. DEMs with resolutions below $0.2\mathrm{m}$ have small differences of 1%, while larger cell–size DEMs ($0.96\mathrm{m}$) could have as high as 13%. Excluding the depressed parking lot of a constrained area, the remaining drainage area could be overestimated by 43% at a 0.96-m resolution. These findings indicate that a fine resolution DEM is necessary for quantifying the drainage area of small flat watersheds.