Evaluating Morphology Changes of Designed Rivers with a Geomorphic Form Variation (GFV) Approach
Abstract
Process-based approaches for river restoration are encouraged in the literature so designed rivers can remain resilient in the long-term. However, the practice of river restoration is largely form-based with stabilization objectives resulting in symmetrical planform channel patterns and rigid protection structures proposed for erosion control. In the last decade, many form-based projects have failed to remain stable in the face of changing flow regimes so novel river design strategies and quantifiable geomorphic analysis methods are required for future restoration success.
My thesis proposes a Geomorphic Form Variation (GFV) approach which exploits high-resolution topographic data and complements automated feature mapping applications used in fluvial geomorphology. The GFV applies variety statistics to raster-based surface metrics as a measure of natural complexity, proposing a repeatable and data-driven approach to river design planning and analysis. The aim was to provide a multi-scaled analytical method for quantifying topographic surface variety as an additional component for the decision-making phase of more process-based river restoration approaches.
GFV variety statistics were tested for their response to controlled and quasi-controlled morphology changes of high-resolution digital elevation models (DEM). Experimental flume channels were comparatively analyzed to determine where variety clusters were most concentrated while River Builder software was used to derive synthetic channels of systematically adjusted planform geometry changes. Real-world river restoration projects were evaluated by comparing topographies of designed rivers relating to project objectives and proposed feature elements. The GFV was complemented with the Geomorphic Unit Tool (GUT) and Hot Spot Analysis mapping methods to determine how variety values responded to morphological feature changes and suggest potential process-based design strategies in practice.
Morphological feature margins of curved slopes were major topographic variety components while planar surfaces showed low variety values. Increased sinuosity, higher bend radius, width variability, and streambank roughness contributed to greater variety values resulting from general planform geometric irregularity. Restoration projects aimed at channel stabilization resulted in simplified surface geometry while habitat enhancement projects showed more complex topographies resulting in greater GFV variety values. With further GFV research and testing, variety values may be defined as statistically significant and proposed as a quantitative restoration objective for more resilient river designs.