Morphodynamic research challenges for braided river environments: Lessons from the iconic case of New Zealand

Pressures on braided river systems in New Zealand are increasing due to anthropogenic stresses such as demand for irrigation water, braidplain conversion to farmland and invasive vegetation, as well as extreme natural events associated with earthquakes and climate change. These pressures create issues around preserving braided river physical environments and associated ecosystems, and managing hazards such as floods, aggradation and erosion. A need for more robust understanding and quantification of braided river morphodynamic processes underpins many of these issues. Here, we present eight morphodynamic research challenges to service this need. The first four research challenges relate to managing aggradation-related flooding hazards; the last four address issues stem largely from recent dairy expansion, which has created huge pressure to take land and irrigation water from the alp-fed braided rivers and to alter flow regimes at their mouths. Hāpua, the freshwater lagoons found where most braided rivers meet the coast, show complex morphodynamic behaviour in response to the interplay of river and coastal processes, and their special ecosystems are sensitive to river flow and sediment load changes. We discuss how physical laboratory experiments and novel numerical modelling can help to understand the morphological processes braided rivers undergo, and we show how those research advances could inform planning and legal decisions to regulate land rights and irrigation water allocation on New Zealand's braidplains. We illustrate these environmental and engineering issues and research challenges with examples from the Kowhai, Waiho, Waiau, Rangitata and Hurunui Rivers. © 2020 John Wiley & Sons, Ltd.