Quantifying Spatial Patterns of Channel Geometry and Stream Incision in Urban Drainage Network

Digital topographic data derived from airborne light detection and ranging (LiDAR) provide high-resolution information for characterizing the morphology of channels and riparian zones, and can be used to map the spatial distribution of channel incision resulting from flashy runoff response in small urban streams. Here the researchers present a methodology for extraction and analysis of transects throughout the drainage network in order to quantify longitudinal patterns of bankfull channel geometry and to identify the spatial distribution of channel incision. Owing to local gaps in LiDAR coverage and anomalies in channel and riparian zone morphology, the set of extracted transects are censored to remove those that would otherwise yield spurious results. Two procedures are then utilized to identify the bankfull stage along individual transects: (1) an iterative spline procedure based on minimizing slope angle and distance from the channel for points that exceed a minimum elevation threshold above the thalweg, and (2) a slope-difference algorithm that identifies local maximum values of the change in slope by evaluating a fixed distance backward and forward from each point on the transect. Each of these approaches generates an independent estimate of bankfull stage, depth, and width for individual transects. These are supplemented by a third estimate by a human operator. All three approaches are compared along a study domain with a deeply incised channel. Longitudinal trends in bankfull depth and width derived for all three approaches reveal that the major trends are detectable regardless of which method is used. The method can be used to identify differences in channel geometry between incised urban streams and less-incised streams.