Simulating the effect of flow path roughness to examine how green infrastructure restores urban runoff timing and magnitude

Impervious land cover was the choice for many urban development projects in order to accelerate runoff and reduce the depth and duration of local flooding, however this led to increases in downstream runoff characterized by large, flashy peak flows. Urban ecosystem restoration now involves slowing down urban runoff to restore local hydrology with green infrastructure, which can be installed as single parcels along runoff flow paths; green infrastructure essentially increases the surface roughness, delaying the speed of runoff compared with relatively smooth impervious surfaces. In this study, we present a runoff routing model that inter-compares runoff from different land parcel surfaces with different roughness to help communities quantify how land parcel restoration impacts runoff timing and peak magnitude. Our modeling experiments contrast the timing and magnitude of floods for discrete runoff pathways composed of parcels with different cover types, converting rough rural land cover to smooth impervious cover as well as the restorative impact of relocating impervious area further from the water body or installing green infrastructure along the flow path. We demonstrate how the model is calibrated to match observed runoff for an entire watershed, and how the model can then be used to estimate how land cover changes will affect runoff timing and magnitude. This study introduces how a simple model to represent land parcel surface roughness and stormwater runoff timing and magnitude, allowing for informed watershed restoration planning, urban planning and placement of green infrastructure.