Continuous Simulation Modelling to Support Healthy Waterways

Frequent storm events contribute to the majority of the stream erosive effects compared to larger, rare events. Developing appropriate solutions to manage erosion requires a good understanding of the associated flows for frequent storm events. Long-term continuous simulation modelling is suited to represent the complex hydrological processes and to predict low magnitude stream flows.

Continuous simulation hydrological models for five gauged catchments in the Auckland Regions were developed using EPA-SWMM modelling software. Three infiltration models were used – Horton’s method, Green-Ampt method and the Curve Number (SCS) method. Each of the models were calibrated against the stream flow gauge in the catchment. Calibration of the hydrological models considered methods other than just matching peak flows and the receding limb of individual events. This included calculation of the Nash-Sutcliffe model efficiency coefficient, matching of the peak flow frequency and flow duration curves from the gauge and the model. This ensures better overall flow replication and thus allows for better prediction of frequent events.

Calibration resulted in a good match for >99.5% of the stream flows. Four out of the five catchments calibrated provided at least a satisfactory match, based on the Nash-Sutcliffe results, with the Whau catchment providing a very good calibration and the Hoteo catchment providing a good calibration.

The analysis undertaken across the five gauged catchments enables a suitable set of continuous hydrological modelling parameters to be established. These parameters could be adopted for ungauged catchments across the Auckland Region and used to better understand stream erosion processes in lieu of observed data. The understanding of the stream flows can then be used to calculate stream flow velocities and shear stress acting on the stream bank to predict which streams may erode and where, and to estimate the quantity of streambank erosion and sediment in the receiving environment. The resultant models can also be used to assess the effects of future development and the benefits of potential erosion mitigation interventions. This is critical in protecting and restoring stream health and attaining healthy waterways.