5.1 Accelerated Erosion*
Background and Explanation
Erosion is a natural phenomenon which results in soil losses and water quality degradation. New Zealand is geologically young and active and, as a result, the natural level of erosion is high by international standards. Changes to the vegetative cover of the land brought about by activities such as farming, introduction of pests, burning, forestry, road construction and urban development reduce protection against erosive forces and lead to accelerated erosion. Soil is a finite resource. Once erosion has occurred, the productivity of the soil rarely returns to its former level.
Erosion Prone Areas
Although much of the Waikato Region’s 2.5 million hectares is relatively stable, the National Land Resource Inventory has identified over one million hectares affected to some degree by erosion, with almost 36,000 hectares ranked as severe to extreme. A further 400,000 hectares is classified as having severe erosion potential.
Soil erosion susceptibility is the result of a complex set of interactions between soil type, climate, vegetative cover, terrain characteristics (slope and aspect) and land management practices. Some land types are more susceptible to accelerated erosion than others. Susceptible land types generally include hill country and the banks of rivers and lakes. In addition, some soils such as pumice soils are more prone to erosion than others. In the Waikato Region, accelerated erosion occurs in different forms depending on the locality.
Erosion prone areas in the Waikato can be grouped as follows:
- The Central Volcanic Area
Includes the pumice land around Taupo, Kaingaroa and Mamaku. The soils lack coherence, making them prone to severe gully, sheet, rill, stream bank and wind erosion in situations where there is insufficient vegetative cover or water control. It is only when there is insufficient vegetative cover or water control that these soils are high erosion risk soils.
- The Western and Central Hill Country Includes the hill country extending along the Waikato’s west coast and becoming more dissected in the north. The central hill country consists principally of the Hapuakohe Range and the southern section of the Hunua Range. Overlain with volcanic ash, these hills are prone to sheet erosion, particularly where heavy livestock are grazed on steep land. This part of the Region also includes karst landscapes and caves. Cave systems are particularly vulnerable to disruption caused by sediment input or works that alter airflow characteristics.
- The Eastern Ranges Parts of the Kaimai, Coromandel and Hapuakohe Ranges. High rainfall in these areas exacerbates erosion effects. Downstream estuaries on the Coromandel Peninsula are vulnerable to accelerated infilling from sediment that threatens the ecological values of these areas.
- The Coastal Margins The dune systems of the east (Coromandel) and west coasts act as natural buffers against coastal hazards. These areas are prone to wind erosion.
- River and Lake Banks
Rivers across the entire Region are affected by erosion. River and lake banks are subject to the erosive force of water, which can be exacerbated by land use activities that damage or disturb the banks and beds of rivers and lakes. Erosion of the beds of rivers and lakes is addressed in Module 4 River and Lake Beds.
- Pukekohe, Bombay and Pukekawa Hills Cultivated soils around Pukekohe, Bombay and Pukekawa. These soils are prone to high rates of soil erosion due to frequency and extent of cultivation for food production. Open cultivated ground is prone to severe rill and sheet erosion during high intensity or prolonged rain events.
Water yields, and consequently, sediment yields from surface and stream bank erosion, have increased in catchments which have been largely cleared of forest, for example, the Waipa Catchment. Land use also has a major effect on the sediment loads in rivers. Erosion prone areas such as the weathered volcanics in the Coromandel Peninsula or the tertiary mudstones in the King Country yield higher levels of suspended sediment under agriculture than under forest.
Based on these erosion prone areas, Issue 5.1.1 identifies seven parts of the Region that are at particular risk from the adverse effects of land use activities. These areas are estuaries of Coromandel catchments, cave and karst systems, steep hill country, sand dune areas, areas adjacent to estuaries and river and lake banks.
Causes of Accelerated Erosion
Accelerated erosion is generally caused by activities that disturb or expose the soil to the erosive forces of gravity and rainwater. Climatic or weather conditions combined with human activity can accentuate soil erosion. For example, severe and intense storm events may increase the rate of accelerated erosion.
Different land uses have different effects. For example, forestry operations may have positive effects on land and water for the period that trees are growing and a period of adverse effects associated with land disturbance from harvesting. In contrast, erosion from pastoral activities may occur at reasonably constant rates for long periods, as there may be minimal protection against erosion.
Data from a wide variety of sources, including forestry industry research, shows the main land uses contributing to accelerated erosion are agriculture, earthworks, roading and tracking activities, establishment and harvesting of forests, and mining. In catchments undergoing land use changes, sediment yields increase as vegetation cover is reduced, exposing the soil surface.1
A wet winter period may saturate the ground increasing its susceptibility to accelerated erosion if used in an inappropriate manner. On land with high erosion potential, land managers need to be aware of, and take into account, the local weather conditions. The level of knowledge and experience of land managers is a major contributing factor to the severity of accelerated erosion arising from specific land disturbance activities. There are welldocumented land management procedures and good practices that can minimise accelerated erosion and subsequent adverse environmental effects.
The adoption of inappropriate land management practices, or the use of land for purposes to which it is not suited, are major contributors to erosion problems. For example, soil disturbance on steep slopes may lead to a high risk of accelerated erosion. Overgrazing by livestock and uncontrolled browsing by pests may damage vegetation to the extent that it no longer adequately protects the soil from erosion.