Sediment transport capacity in a large braided river: integrating substrate mapping with flow scenarios

Abstract. Large braided rivers offer a challenging field environment and while our ability to map these complex environments through remote sensing has advanced significantly, even detailed surveys represent only a moment in time. In this study we have linked the remotely-sensed observations of substrate, including deposited fine sediment, at reach-scales to a hydraulic model of the braided Rangitata River, New Zealand. Excess fine sediment can alter fluvial form, ecosystem health and groundwater recharge. The advent of large-scale mapping technologies has enabled reconstruction of fluvial substrate facies over broad scales, and now spurs an examination of what can be learned with the greater fidelity of data from these surveys compared to simpler renditions of the riverbed. Here we propose a hybrid methodology that links spatially-distributed models of bed substrate with river hydraulics reconstructed from a ‘model library’ of steady state flows to better infer the controls on sediment transfer and deposition in braided rivers.

In this paper, we map potential sediment transport and depositional environments over a range of naturalised and modified flows. Our findings show that spatial bed data is the key to longitudinal consistency in sediment transport. The ‘model library’ method allowed us to efficiently test the tendency and magnitude of deficit, equilibrium, or surplus sand capacity in any location and the sensitivity of the result to bed composition, hiding and exposure formulations. The results indicate that the bed is sensitive to contemporary changes in hydrologic regime, particularly in the side channels accessible during the common ‘moderate’ flows that occur for tens of days per year. Simulations comparing the river’s sand transport capacity under the present hydrologic regime with a naturalised hydrologic regime indicate that the impacts of the flow abstractions are comparable to a 10–15 % change in the bed sand fraction. Maps of potential sand deficit align well with observed depositional areas, and highlight the critical importance of the ‘moderate’ flows.