Homes, crops, and more in have been buried under fine sediment after Cyclone Gabrielle smashed through Esk Valley and other areas of the North Island.
The SMC asked experts to comment on what the sediment is, where it’s come from, what effect it could have on farming, aquatic life, and future earthquakes, and how we could stop it from happening in future floods.
Professor Troy Baisden, Principal Investigator, Te Pūnaha Matatini Centre of Research Excellence on Complex Systems, comments:
“The scale of sediment deposits from cyclone Gabrielle may at first seem impossible, but will also feel like déjà vu to those who remember or studied major past events like Bola and the 1938 cyclone. Scientists from all over the world have been interested in the North Island’s east coast because it is among the world’s leading hotspots in sediment uplift, erosion and deposition. In areas like the Waipaoa catchment above Gisborne, sediments from the 20th century are over 5 meters deep. Sediment tracing studies have confirmed their origin can be traced to three main sources, described below.
“Much of the silt and sediment is repeating a cycle, having been deposited some millions of years ago in thick marine sediments like those washed down rivers two weeks ago. It was then uplifted but only poorly consolidated. The result is the steep, soft-rock hill country that makes driving through the east coast difficult, yet spectacular.
“Where we are seeing deep deposits of silt and sediment, three soft-rock terrains are the main sources. Of these, steep mudstone hill country covers the largest area. The poorly consolidated mudstone makes for fertile pasture, until roughly the upper meter of soil and weathered rock becomes a slip (also known as a landslide). In events like Gabrielle, slips may cover roughly a quarter of the area. Further up the east coast in Tairāwhiti, are terrains where gullies expand at the top of stream channels in steep catchments. Finally, towards the tip of East Cape, and most notably in the Waiapu catchment near Ruatoria, are massive slopes composed of harder but highly fractured rocks that generate earthflows. These areas have amongst the highest sediment generation anywhere in the world – a result of vulnerable, rapidly uplifting mountainous exposed to extreme rainfall.
“After the 2004 floods across the lower North Island, satellite mapping of slips and their correspondence to land use and rock types was commissioned, showing that closed-canopy indigenous and exotic forests were effective at preventing slips because roots hold the soil together. Similar follow-up studies will likely be useful, to resolve two questions. First, can the more accurate elevation and slope data available today better resolve what land is at risk? And second, to what degree are vulnerable terrains planted into exotic forests at risk during the years after forest clearcutting?
“Followup studies may also be useful in gully and earthflow terrains. Observations from this cyclone, along with more isolated events severely impacting the Uawa catchment and Tolaga Bay, should begin to answer the concerns about whether clearcuts in exotic forests may still place landscapes and downstream settlements at general risk, or if the management of slash should be the primary concern.
“Studies of mudstone hill country around Lake Tutira suggest that indigenous reforestation can be effective, and mānuka honey can complement carbon credits as an income source from this land. Improved methods of planting mānuka and kānuka at similar costs to pine are now available.”
No conflict of interest. Note: Professor Baisden is also co-president of the New Zealand Association of Scientists; Honorary Professor at the School of Environment, University of Auckland; and Affiliate at Motu Research.
Associate Professor Peter Almond, Soil and Physical Sciences, Lincoln University, comments:
“The term ”silt” is being used as a generic term for the sediment that has ended up on floodplains in Hawkes Bay and Tairāwhiti. Silt refers to a particular size grade of sediment between 0.002 mm and 0.063 mm. Sand is between 0.063 mm and 2 mm in size. Below 0.002 mm the sediment is referred to as clay or mud.
“The sediment in these floods is made of a range of particle sizes from sand through silt to clay (mud). The distribution of the sediment types on the floodplains is determined by the energy of the water flows. Close to rivers and in channelized zones, only the larger-sized material (sand) can settle because of the strong current. In areas where flows are lower, dominantly silt and fine sand may settle. In ponded areas clay (mud) can settle.
“The sediment is derived from rocks dominated by the (silicate) minerals quartz and feldspar with some mica. These are naturally occurring minerals common to many of NZ’s rocks and sediments. The sediment is usually low in organic carbon and nitrogen, phosphorus and available potassium unless it was eroded from a nearby fertile soil and redeposited quickly.
“The sediment is not toxic unless 1) it dries and fine material (often silt-sized) is breathed in; this material can cause respiratory problems and with long-term exposure silicosis; or 2) where the sediment has interacted with stormwater and sewerage in settlements and urban areas. In the latter case, pathogens may be in the sediment, which could infect people if it is swallowed or inhaled.”
Where has the sediment deposited during the flooding come from?
“Sediment on the floodplains is coming from material eroded from hillslopes, or sediment in river channels or adjacent to them in river banks. In places flooding rivers have scoured floodplains themselves, which can provide sediment that is deposited elsewhere or carried out to sea.
“In the big picture, the fertile plains of Hawkes Bay and Tairāwhiti owe their origin to the kinds of events we have witnessed with Cyclone Gabrielle. However, the frequency of the large sedimentation events building the plains has increased as a result of changes to land use, primarily the clearance of native forest from the hills. Climate change, bringing more frequent and more intense storms, will also speed up the frequency and rate of sedimentation on the plains. These events pose a greater hazard as populations grow and more people are put in harm’s way.”
What condition might topsoils be in after this storm?
“On hillslopes where soils have been eroded usually the topsoil and subsoil down to the bedrock (mudstone, siltstone or sandstone) are removed by the processes of land sliding. There will be other kinds of erosion involving flowing water such as rilling and gullying, which will remove all of the pre-existing soil. Eroded areas on hillslopes will have little or no soil left and soil formation will have to start anew. This stripping and reforming of soils is a common phenomenon in the rapidly eroding, steep soft-rock hill country of eastern North Island. It is common too in the Rangitikei, Manawatu and Whanganui regions.
“Where sediment has been deposited, the fate of the pre-existing soil will be determined by the depth of sediment deposited on it. Where sediment is thin (< 5 cm) existing plants will readily grow through it, albeit with some mortality, and the sediment will eventually get mixed into the organic-rich topsoil by biological processes. Where sediment is deeper a new topsoil will have to form. Where soils are buried by thick sediment and particularly where that sediment is fine (silt and clay) the zone near the buried topsoil becomes anaerobic (lacking oxygen) which can be fatal for plants rooted in that zone (i.e. pre-existing pasture or trees/vines), and the gas ethylene can be produced which is toxic to germinating seeds.”
What long-term implications could that have for working with the land?
“Where sediment is thin (< 5cm) it can be readily incorporated into the pre-existing topsoil or direct-drilled to re-establish pasture. With greater thicknesses (up to 25 cm) deeper cultivation can be used to achieve the same effect although there is a greater concentration of fresh sediment, which has a bigger impact on soil physical and chemical properties (lower fertility). In thick sediment where it cannot be removed, a new soil will have to form in the sediment. How that new soil develops will be influenced strongly by the nature of the sediment (sandy vs silty vs clayey). Depending on management approaches the new soil can be growing pasture somewhere more than 70% of its original productivity within 18 months. Fertiliser additions are essential especially when the sediment is thick because the sediment normally is of low fertility. The sediment will normally need capital applications of phosphorus and potassium and regular additions of N will be needed to support pasture. The sediment has a near-neutral pH (not acidic), so it will not need liming. Experience from the 2004 Manawatu floods showed that despite a relatively quick recovery to productive pasture, land affected by sedimentation tended to be affected by pugging and weed problems for a sustained period after sediment deposition.”
No conflict of interest.
Professor Amanda Black, Director, Bioprotection Aotearoa; and Rutherford Discovery Fellow, comments:
“With climate change these 1 in 100 years events such as cyclone Gabrielle, are likely to be more frequent. We may be looking at 1 in 20 years with these extreme events meaning a much shorter recovery period.
”What does this mean for our productive landscapes? The smothering of soils in these highly productive areas is arguably one of the biggest issues for continuation of plant based industries there. The silt, which is catch all description for the parent material (i.e. rock) contains no significant biological activity or nutrient availability to support any plant based industry, and has intombed the healthy productive soils and the crops they would support post cyclone Gabrielle.
”Re-stablishing and protection of healthy soils should be the long term priority, as well as ensuring we have adequate tree stock and seed to re-establish these industries. But the first priority I would think would be a plan of silt removal and disposal for any silt deeper than 20 cm, as conventional nutrient management to re-establish productivity would be ineffective.”
No conflict of interest declared.
Associate Professor Stephen Hartley, Director, Centre for Biodiversity and Restoration Ecology, Victoria University of Wellington, comments:
“In the wake of Cyclone Gabrielle, society is being forced to examine the trade-off between practices that maximise profits in the short-term versus building-in safety valves to cope with Mother Nature’s extremes. To paraphrase Roger Martin, there is a high price to pay for efficiency.
“Put bluntly, denuding steep hillsides of complex, multi-tiered forests has allowed millions of cubic metres of topsoil to slip off the face of our land, down raging, flashy rivers, only to be deposited across flat land, estuaries and ocean beds, smothering everything beneath it. Topsoil and subsoil we now call silt.
“Draining floodplain wetlands, so that water runs out of them quickly, has removed buffers that can soak up water during a downpour and then release it slowly downstream in the weeks and months that follow. Of course, we need agriculture and forestry for good reason, but these land-uses can and should be embedded in a matrix of diverse ecosystems that between them can regulate extremes. In a volatile and precarious system, too much of one thing is never a good thing.
“A few billion “new” trees could certainly make a difference to our country’s carbon balance, but the right trees in the right places can do so much more: improving the soil’s water-absorbing capacity and stability, reducing flood risk and erosion, improving water quality, providing habitat for insects and birds, and shelter for stock, the list goes on. Now, more than ever, is the time to recognise the value of green infrastructure and the services it provides. Let us heed the UN Decade on Ecosystem Restoration. Let us listen to the people who know the land best: indigenous and local communities, farmers and environmental scientists. Let us challenge our politicians and economists to take a multi-generational perspective in preparation for the next once-in-a-generation event.”
Note: Associate Professor Hartley has been involved in monitoring the benefits of wetland restoration and tree-planting initiatives across the Wellington and Wairarapa region for the past 12 years.
No conflict of interest.
Professor Conrad Pilditch, Programme Leader for Degradation and Recovery, Sustainable Seas National Science Challenge; and Professor of Marine Science, University of Waikato, comments:
“I see a big issue with the sheer volume of sediment that’s entering into the coastal environment from this year’s weather events. Cyclone Hale did a little bit, but Gabrielle just really smashed it. We can see the devastation on land, in the volumes of silt in the Esk Valley that the poor people in Hawke’s Bay are cleaning up – but eventually all of that ends up in our coastal estuaries and in our coastal zones. And although the water may get clear after a little bit of time, the silt will still be around in these environments for a long, long period to come.
“I have colleagues who are out surveying the Waikato estuaries, and some of the estuaries north of Auckland after the cyclone. For some of those estuaries, there’s an immediate effect, with a lot of silt already accumulating. This stuff landing on the seafloor or on rocky reefs can smother animals, and smother plants.
“Then as that disperses away, there could be a background increase the turbidity, which creates a longer term effect of silt being resuspended and dispersed around and generally browning those coastal waters. All of these ecosystems are dependent on plant growth – microscopic phytoplankton or large kelps and seaweeds. When you put these suspended sediments in the water column, it stops some light from hitting the seafloor. This cuts down the amount of food at the base of the food web, and then all the animals that feed on that, it has devastating consequences for them as well.”
No conflict of interest declared.
Associate Professor Martin Brook, School of Environment, University of Auckland, comments:
“The origin of the sediments that have been deposited are likely to be from (1) a range of rock types in upland areas, and (2) reworking of existing younger sedimentary deposits by floodwater flowing over lower slopes and floodplains. Thus, the “silts” could consist of a range of minerals and grain-sizes. Flood deposits will tend to exhibit graded bedding, usually fining upwards.
“Regarding grain-size, while the deposited material is termed “silt” by many, flood deposits are better characterised as sedimentary soils. The deposits may actually be a wide range of grain-sizes, from “coarse soils” such as gravel (2-60 mm) or sand (0.06-2 mm), to “fine soils” such as silt (0.002–0.06 mm) or the finest sediment, clay (less than 0.002 mm). These size ranges are from the New Zealand Geotechnical Guidelines (2005), but broadly follow international standards.
“The grain-sizes deposited are important because they control how well the soil will drain during future rainfall (i.e. permeability), and engineering properties such as strength and bearing capacity. Fine, tightly-packed sediment, with minimal pore spaces will drain poorly, compared with a coarser deposit of sand or gravel. The finer materials will likely have a broad range of strength properties, such as cohesive strength, friction angle, and penetration resistance. The way the soils consolidate over time will also affect their strength properties and permeability, which may evolve. Soils have 3 components: solids (grains), water, and air in the pore spaces between grains. The soils may change in volume over time (consolidate), causing compaction settlement (“subsidence”). Primary consolidation is a more rapid consolidation due to loss of excess water pressure, while secondary consolidation is a slower “creep” process that can persist for years.
“Another important factor is mineralogy. The finer-grained sediments may include swelling clays such as smectite. These have a high shrink-swell capacity, and this causes seasonal cracking of soils in parts of New Zealand (wetting=swelling; drying=shrinking). The Tolaga Group sedimentary rocks in the Gisborne area, exhibit this process, and smectite is in the hills that drain into northern Hawke Bay, for example.
“A further factor to consider about a new sedimentary soils is its liquefaction potential during future earthquakes, which affect the eastern North Island. As we saw in Christchurch, soil liquefaction potential can be very site-specific and grain-size is one of the many factors that is important. So, the new “silts” may be too problematic to be built on.
“Finally, recent work by Landcare, Massey and Auckland Universities has shown the ability to geochemically “fingerprint” river deposits to determine the approximate area they were sourced from, but it’s very broad scale. Perhaps in our AI-driven utopian future, this geochemical fingerprinting could be sharpened up – poor land management that leads to a hillside being eroded and transported downstream onto someone’s property, could then yield a fine for the hillslope’s landowner.”
No conflict of interest declared.