Stopbanks or Space for the River?
The Lower Matai River drains a 320 km² catchment. Since 1990, 41% of its upper terraces have been converted from sheep to dairy, and the river carries a heavy sediment load. Stopbanks built in the 1970s protect the town of Matai (population 4,200) from a 1-in-50-year flood. In July 2022, a 1-in-120-year event overtopped the banks at two bends, flooding 63 homes and closing the only bridge for nine days. Climate projections show the 1-in-100-year flow could increase by 20% by 2080. The regional council has $18 million. Option A raises the stopbanks by 1.2 m along 6 km, protecting the town but narrowing the channel and speeding flow downstream. Option B buys 14 riverside farms for floodplain restoration ($14 million) and relocates 20 homes, but leaves some town edges exposed. Iwi hold customary interests in the river mouth and oppose further channel narrowing.
1. Explain how natural processes and human actions have together increased flood risk in the Lower Matai catchment.
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Natural processes set the context: the catchment receives high-intensity rainfall, the river has limited channel capacity and meanders, and sediment is deposited on inner bends, raising the bed. Human actions have altered these processes. Conversion of 41% of upper terraces to dairy increased runoff because compacted soils and drainage reduce infiltration, and it increased sediment delivery. Stopbanks built in the 1970s narrowed the channel, so the same flow is forced through a smaller area and moves faster downstream. Urban growth on the floodplain placed 63 homes in the 2022 flood path.
The interaction matters: neither the rainfall nor the stopbanks alone explains the flood. Sediment raises the bed, stopbanks confine flow, and dairy conversion accelerates water and sediment delivery. The 2022 event was a 1-in-120-year flood, but the damage was also produced by land-use change and channel management. This is human modification of a natural system increasing hazard exposure and vulnerability, not a purely natural event.
2. Compare Option A and Option B using risk, sustainability, and affordability. Recommend one option.
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Option A has high upfront cost ($18m) but protects the town quickly. It reduces risk in the short term but does not remove it: the 2080 projection shows a 20% increase in the 1-in-100-year flow, so a higher bank may later be overtopped. It narrows the channel, speeds flow downstream, and can increase risk for downstream farms. Maintenance is ongoing.
Option B is cheaper upfront ($14m) and more sustainable because it gives the river space, reduces sediment, and improves ecology. But it requires relocating 20 homes, which has social and cultural costs, and leaves some town edges exposed. Affordability: B leaves $4m for compensation and monitoring, but relocation costs may exceed that.
Recommendation: a modified B - buy the 14 farms and restore floodplain, but use part of the $18m to raise the stopbank at the town bend and relocate the 20 homes. If a single option must be chosen, choose B because it is more sustainable under climate change, but only with a targeted stopbank upgrade. The trade-off is short-term protection versus long-term resilience.