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By Peter Almond
Homes, crops and farmland have been buried under silt after Cyclone Gabrielle smashed through parts of the North Island. Associate Professor Peter Almond, from Lincoln University’s soil and physical sciences department, explains where the silt has come from and what the future holds for sediment-covered land.
What is the silt made of?
“Silt” is being used as a generic term for the sediment that has ended up on floodplains in Hawke’s Bay and Tairāwhiti. Silt refers to a particular size grade of sediment between 0.002mm and 0.063mm. Sand is between 0.063mm and 2 mm in size. Below 0.002mm 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 New Zealand’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 it dries and fine material (often silt-sized) is breathed in; this material can cause respiratory problems and, with long-term exposure, silicosis; or where the sediment has interacted with stormwater and sewage 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 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 Hawke’s 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 re-forming 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, Manawatū 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 (less than 5cm) 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 (that is, pre-existing pasture or trees/vines), and the gas ethylene can be produced; it is toxic to germinating seeds.
What long-term implications could that have for working with the land?
Where sediment is thin (less than 5cm) it can be readily incorporated into the pre-existing topsoil or direct-drilled to re-establish pasture. With greater thicknesses (up to 25cm), 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 Manawatū 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.