Improving the Status Quo: Using Natural Channel Design to Add Value to River Restoration Approaches in New Zealand
Brandon Alderman
AECOM
Germantown, MD
River restoration and especially Rosgen Natural Channel Design (NCD) methods are unfamiliar and not yet wholly accepted in many countries outside of the United States and New Zealand is no exception. Fluvial geomorphologists and river engineers in New Zealand have conceded that achieving restoration success in urban watersheds is impossible due to the inability to restore the river to pre-development conditions. Restoration designers in New Zealand also face many design challenges unique to working in a seismically active region in which active plate-tectonic processes shape the present-day landscape. Beginning in April 2023, AECOM has collaborated in the RiverLink program located in Wellington, New Zealand. The RiverLink program includes over $400 million USD in design and construction works to improve floodplain connectivity, flood resilience, and urban livability in the Te Awa Kairangi, also known as the Hutt River.
The Hutt River historically was classified as an unconfined meandering and braided river channel which had a floodplain extending over 3,000 linear feet. Over the last 100 years, urban development and levee flood protection projects implemented in the valley have constrained the valley to a much smaller width of 400 linear feet. The river has also been channelized and straightened and undergone frequent heavy armoring maintenance that drives high bank erosion rates, rapid channel aggradation shifts in the river, and ultimately increasing near bank stress. Due to limited floodplain availability and constant sediment extraction the river continues to accumulate bedload sediment at an alarming rate. The accelerating rate of sediment aggregation necessitates even more frequent sediment extraction efforts to maintain flood capacity for the existing levee system and protect adjacent infrastructure, resulting in ever-increasing annual maintenance costs. Collaboration between local stakeholders and AECOM’s U.S. based stream restoration designers has allowed for a reconceptualization of river stability principles. NCD principles and techniques were implemented to provide a re-imagined channel dimension, pattern, and profile more conducive for sediment transport, providing grade control, and robust bank protection to restore the river not as it was pre-development unconfined river form but to function with characteristics common of reference reaches found in confined river valleys.
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