Urbanized stream restoration as designed adaptation: Planning using synthetic digital rivers and form variation assessment

Abstract Restoring urbanized streams is challenging because of the absence of natural reference reaches and major changes in discharge and sediment supply as a consequence of catchment urbanization. In many cases the risk to urban infrastructure leads to form‐based restoration with less opportunity for process‐based approaches. Restoration involves designed adaptation to the new catchment state, rather than true restoration, but more‐natural designs are still feasible based in principles of fluvial geomorphology. An important element of this designed adaptation is planform design which is a part of urban channel restoration that is often simplified and which, in turn, simplifies the in‐stream morphology and habitats. More variable in‐stream morphology develops in irregular planforms which are also more common in nature. Tools and methods for designing complex planforms and associated bed morphology have developed, but further innovation is needed to meet interdisciplinary objectives in the planning phase of design. Here, we present a flexible approach for assessing surface‐form complexity of planform and in‐channel morphology as part of a move to geomorphically‐based urban riverscape design to accommodate urbanized channel changes within the overall interdisciplinary framework and to increase overall riverscape amenity. Our approach uses River Builder software for deriving 3D rivers based on hydro‐morphodynamics and applies the Geomorphic Form Variation (GFV) approach as a measure of geomorphic complexity for collaborative urbanized stream planning. Assessment of planform variable inputs showed a strong control on in‐channel morphology, and GFV showed that form complexity increases with higher width variability, stream bank roughness, and meander bend frequencies and curvatures.