Simulating changes to the sediment transport regime of the Waipaoa River, New Zealand, driven by climate change in the twenty-first century

Abstract

A one-dimensional sediment transport model ( TUGS) and a climate-driven hydrological model ( HydroTrend) are used to predict changes to the sediment transport regime of the Waipaoa River that may occur in response to forecast 21st Century variations in climate. Climate change may reduce the mean flow in the Waipaoa River at Matawhero by an average of 13% in the 2030s and 18% in the 2080s. In the 2030s, the maximum simulated change in the mean annual suspended sediment discharge of ± 1 Mt y − 1 may be difficult to discern because of the large variation in the contemporary suspended sediment load (13.4 ± 7.3 Mt y − 1). Depending on the climate change scenario, in the 2080s the suspended sediment discharge may either decline by 1 Mt y − 1 or increase by 1.9 ± 1.1 Mt y − 1. Adverse impacts have the potential to be offset or ameliorated by a modest (35%, ~ 12000 ha) increase in forest cover across the basin headwaters. Size-selective transport and deposition throughout the lower reaches of the Waipaoa River currently limit the amount of bed load exported at the coast to 10.2 ± 24.3 Kt y − 1. In the 2030s this may decline to 6.3 ± 16.1 Kt y − 1, but in the 2080s it may rise to 9.4 ± 20.1 Kt y − 1 as aggradation reduces the amount of accommodation space and modifies the long profile of the simulated river. The bed in the lower 27 km of the river could aggrade by an average of 0.31 m in the 2030s, and 0.85 m in the 2080s. This is likely to be the most costly consequence 21st Century climate change, because rising bed levels have the potential to cause a loss of capacity throughout 75% of the Waipaoa River flood control scheme.