Interpretation of hydraulic model outputs in supporting ecologially-led river restoration.

Abstract:

In river systems, hydrodynamic forces are a major driver for geomorphic change, and a major influence on aquatic habitat. Representative modelling of channel hydraulics is therefore an invaluable tool in ecologically-led river restoration design, enabling a quantitative and objective assessment of complex processes that are essential to achieve biophysical objectives. Hydraulic modelling can form part of an iterative design process, utilised to indicate ‘design performance’ through a wide range of descriptors (e.g. physical heterogeneity, hydraulic micro-habitat) afforded by each stage of the design. However, it is important that the limitations of any computational fluid dynamic approach (e.g., 2D depth-averaged simulations) are well communicated to other specialists (e.g., geomorphologists, ecologists), managers, regulators and clients. One aspect of hydraulics where this is specifically important is in the use of vorticity and coherent vortex structures as indicators of habitat suitability and the probability of regions of scour or deposition. It has long been recognised that eddies structures provide important physical and ecological function in rivers but calculating vorticity changes using hydraulic models is relatively new in river hydraulics. The use of such calculations is growing in both academia and industry, especially as restoration approaches such as engineered log jams introduce significant vorticity into the flow. However, the introduction of vorticity needs to be modelled with very high resolutions near solid boundaries, and the convected vortex structures themselves are inherently three dimensional. Neither of these is routinely captured in river hydraulic models. Specifically, the interpretation of vorticity patterns or coherent vortex structures from 2D depth averaged river models must be treated with caution, or provided with further interpretation. We present work that demonstrates how coherent vortex structures and vorticity have been used to indicate the success of a number restoration designs in the UK, across a range of river environments and project objectives. We comment on how these 2D calculations must be further interpreted, based on a 3D understanding of the fluid mechanics, before conclusions and results can be presented.