Work Package 3

Data synthesis and upscaling

The final component of this research will synthesize the results from WP1 and WP2, allowing the new, novel understanding to be upscaled. In WP3 we will apply a numerical modelling framework to upscale our results in four ways;

1. to perform model tests across multiple channel scales (e.g. pool-riffle amplitude, grain size diameter; stress history);
2. work with project patners Environment Agency and Halcrow to integrate the model into the 1D industry standard ISIS-Sediment framework, and apply to a new field site with existing data;
3. work with visiting researcher Joseph Wheaton to integrate the results (i) into his MORPHED reduced complexity model, and apply it to the braided River Feshie (existing morphodynamic data);
4. provide visiting researcher Maarten Kleinhans and project partner Deltares with the code from (i) for inclusion within the Delft 3D morphodynamic model. Modelling using the latter will not be a project output but by sharing and integrating our findings into the most widely used open-source 3D morphodynamic model we ensure lasting global impact.

SEDROUT

The approach in (i) will be to quantify the effect of different stress history, grain size diameters and pool-riffle morphology on the development of sediment structure, critical shear stress and consequent morphological development using the numerical model SEDROUT. SEDROUT is a 1D cellular model that reproduces the transport and storage of water and sediment (of multiple grain sizes) along a river channel. SEDROUT is low risk, but also well suited to the upscaling and synthesis of the results from WP1 and WP2 because:

• it is a simple and established process-based modelling framework and has been independently tested;
• each model cell contains multiple sediment layers, enabling us to represent vertical differences in sediment populations and structure in response to morphological evolution of the bed; and,
• it has already been integrated into ISIS-Sediment developed with EPSRC funding and industrial support.

In this application, SEDROUT will be developed under the guidance of project partner Trevor Hoey to reproduce a riffle-pool morphology and to incorporate fine sediment. Results from WP1 and WP2 will be used to produce relationships that predict local critical shear stress as a function of grain size, stress history, fine sediment infiltration and local channel morphology. NERC-funded (NE/H020993/1) flume data from project partner Stephen Rice will be used to further develop and validate these relationships. These data will quantify the 2.5D surface structure of sediment beds and the interactions between structure development, near-bed hydraulics and bed load transport under contrasting transport intensities and sediment supply regimes. The finalised relationships will be implemented in SEDROUT.

The modified version of SEDROUT will be run to evaluate the evolution of the plane bed and riffle-pool sequence under a range of flows and inputs of fine sediment observed in WP2. The results from model runs will be the spatial pattern of local critical shear stress and channel morphology. Initial runs will test the model under simple conditions (e.g. constant flow and/or no fine sediment) to assess the model behaviour. These runs will be paired with another run with the same boundary and initial conditions, but without the new relationships for predicting local critical shear stress. Comparison between these results will demonstrate the effect of local variations in critical shear stress on the development of channel morphology. A second set of model runs will reproduce the flow and sediment supply conditions experienced in WP1. These results will be assessed to see if the emergent pattern of sediment structure and morphological development is consistent with that observed in WP1. The final set of runs will extend the model application to a range of flow and sediment supply regimes outside that observed in the field, and will be used to determine the extent to which the relationships derived from WP2 control the morphological development of the channel.