Lessons for understanding flood sediment deposition and management.
Alex Dickson1, Dan Bloomer1, Callum Rees2, presented by Peter Manson3
1 LandWISE, 21 Ruahapia Rd, RD10, Waipatu, Hastings
2 Massey University
3 AgFirst Pastoral
The East Coast of New Zealand’s North Island represents a dynamic landscape repeatedly shaped by extreme flood events. This presentation examines the documented history of significant floods, with particular focus on the Heretaunga Plains – a 300 km² alluvial region formed over 250,000 years by three major river systems (Ngaruroro, Tūtaekurī, and Tukituki). Today, 90% of the Heretaunga Plains is classified as highly productive land (LUC classes 1-3), supporting approximately 14,550 hectares of fruit and vegetable production.
Historical records reveal catastrophic flooding across the East Coast dating back to European settlement. The 1867 flood when the Ngaruroro River broke its banks near Fernhill, deposited 30-50 cm of sediment across much of the Heretaunga Plains and permanently changing the river’s course. This event prompted the first parliamentary response – the problematic 1868 Hawke’s Bay and Marlborough Rivers Act, which allowed individual property owners to construct stopbanks, often diverting water onto neighbouring properties.
The 1931 Napier earthquake, while not a flood event, profoundly influenced subsequent flooding patterns. The 7.8 magnitude earthquake caused land uplift of up to 2.7 meters and forced both the Esk and Tūtaekurī Rivers to change course. The earthquake triggered approval of comprehensive river control schemes in 1933, with work beginning in 1934 to embank the Ngaruroro and Tūtaekurī Rivers along their entire course over the Plains.
The ANZAC Day 1938 Esk Valley flood catalysed New Zealand’s modern soil conservation movement. Over three days (April 23-25), Puketitiri recorded over 1,000 mm of rainfall, with the Esk River experiencing an estimated peak flow of 1,347 m³/s. The floodwaters buried approximately 700 hectares of productive farmland under 1-3 meters of sediment, destroyed 12 bridges, and severely damaged 42 others. The immediate response involved distributing 24 tonnes of ryegrass seed and 3.6 tonnes of white clover. However, recovery was complicated when strong winds in January 1939 turned the valley into a “miniature desert,” destroying much of the newly established pasture. This catastrophe directly led to the passing of the Soil Conservation and Rivers Control Act 1941.
The 1948 Gisborne/Poverty Bay flood prompted the first rigorous scientific research on flood sediment revegetation. The storm caused significant damage across the Waipaoa catchment, with agricultural losses of £165,000 (approximately $15 million today). McKee and Graham’s (1952) groundbreaking research, conducted after both the 1948 and 1950 floods, established foundational principles about sediment classification, optimal timing for oversowing, and species selection that remained valid for decades. Their insight of a narrow “moisture window”, when surface water has drained but sediment remains sticky enough for successful oversowing, was validated repeatedly in later events. Their finding that Italian and short rotation ryegrass outperformed other species in flood sediments became a consistent theme in subsequent research.
Cyclone Bola (March 6-9, 1988) brought three days of torrential rain when the system stalled over the East Coast. With 916 mm at Tolaga Bay and maximum intensities of 85 mm/hour, the cyclone deposited sediment up to 1.5 meters deep across 8,000 hectares, with primary sector losses of $90 million ($210 million in 2023 dollars). Research by Gray and Korte following Bola confirmed McKee and Graham’s earlier findings, with Moata Italian ryegrass proving most vigorous even without fertilizer, outperforming perennial ryegrass.
The 2004 Southern North Island Storm left approximately 20,000 hectares underwater, with damages of $300 million. Comprehensive research found sediment deposits typically had high pH, low phosphorus and potassium, and minimal organic matter. Studies demonstrated that while oversowing provided quick initial cover, full cultivation with sediment incorporation produced better long-term outcomes, though it required waiting 30+ days for suitable moisture conditions.
This historical pattern of extreme weather events – with major floods recurring approximately every 10-40 years – provides essential context for understanding Cyclone Gabrielle (2023), which exceeded even these historical benchmarks in rainfall intensity and damage extent.
The presentation traces how European land clearance since the 1800s intensified erosion rates threefold in some catchments by replacing deep-rooted native forest (which reduces erosion by 70-90% during storms) with shallow-rooted pasture. It examines the evolution of flood protection from the problematic 1868 Rivers Act through to modern coordinated stopbank schemes.
Consistent themes emerge across 85+ years of documented events: the critical importance of timing for oversowing success, the need to classify sediment by texture and depth, the superior performance of Italian ryegrass, the connection between upland land use and downstream impacts, and the requirement for long-term commitment to recovery. These lessons, documented in research from 1952, 1990, 2004, and beyond, offer crucial guidance for building resilience in New Zealand’s valuable horticultural regions facing climate change and increasingly frequent extreme weather events.
This research was supported and funded by Page Bloomer Associates as part of master’s degree studies. It was completed in parallel with Cyclone Gabrielle research funded by MPI, Vegetables New Zealand and FAR.
Link to PowerPoint presentation