Abstract
Coastal wetlands are dynamic environments that provide a range of high value ecosystem services. Salt meadow wetlands are rare and not well studied in southern New Zealand. Pounawea Wetland (also referred to as the ‘Hungerford Point Saltmarsh’) provides a unique opportunity to examine how environmental change has contributed to lateral erosion of the wetland edge; and the potential for erosion mitigation strategies. This research aims to (1) document environmental change and the morphodynamics of Pounawea wetland over the last century; (2) examine this change in relation to changing boundary conditions; (3a) determine the spatial patterns and rates of sediment deposition during inundation events; and (3b) examine how sea-level rise will affect future wetland evolution.
Aerial photographs, satellite imagery, and UAV (drone) imagery, from 1948−2022, is used to examine estuary and catchment morphodynamics, shoreline erosion, and vegetation zonation. Shoreline erosion rates were calculated using the Digital Shoreline Analysis System (DSAS). The more exposed margins of the wetland (eastern and western margins) have eroded 60−93 m since 1948; while the more sheltered areas have eroded between 3−30 m over the same period. The average yearly shoreline retreat is -0.67 ± 0.05 m/yr. The rate of erosion increased from 1948−1985, following the erosion of Cabbage Point, a vegetated sand spit. The erosion of this spit likely increased ocean wave propagation into the estuary during periods of north-easterly winds and easterly swells. Wetland species migration from lower to upper marsh was identified through historic aerial images. The podocarp forest receded by 10−50 m between 1967 and 1985.
The severity and frequency of weather events did not increase in the period 1972−2022. Wind and pressure data from Nugget Point climate station, 10 km north-east of Pounawea did not illustrate an increase in potential erosion events (defined with respect to wind speeds > 7 m/s; wind direction 180°-270°; and pressure < 990 hPa).
Deforestation upwind of Pounawea wetland has resulted in an increase in wind speed across the estuary during southerly to south-westerly wind events and consequent increase in significant wave heights along the wetland margins, resulting in an increase in the rates of lateral erosion of the wetland edge. Computational fluid dynamics (CFD) modelling was used to demonstrate the reduction in wind speed across the estuary in the lee of an upwind forest. Scenarios using different porosities (Cd = 0.005, 0.05 and 0.15) were modelled and all results demonstrated that a forest upwind of the wetland (on the Hinahina side of the estuary) would reduce wind speed across Catlins Estuary by 50−80%.
Sediment, comprised of mineral sand and rock fragments, is accumulating on the surface of the wetland, however, the wetland is eroding faster than the rate of vertical accretion. Sediment accumulation was measured using artificial grass mats during six inundation events that encompassed both high and low energy wave environments. The spatial distribution of sediment deposited on the wetland during inundation events is not evenly distributed. Over 60% of the sediment that accumulated on the eight artificial mats was deposited within 2 m of the wetland edge. Consequently, the margins of the wetland have a higher elevation than the wetland core. Sediment accumulation increases with an increase in wind speed and significant wave height; The wetland edge is accreting faster than SLR, however, this area is periodically eroding at a rate of -0.67 ± 0.05 m/yr and sediment deposited at the edge is eroded.
CFD modelling was also used to examine the effects of rising water-levels on the wall shear stress experienced at the wetland edge. Seven water levels were modelled (0.2-1.4 m, at 0.2m increments) to illustrate different tidal stages and future sea-levels. Results illustrated an increase in the amount of shear stress on the wetland edge up to the level of wetland inundation (0.8 m), beyond this point, wall shear stress decreased. The modelled decrease in wall shear represents waves breaking across the surface of the wetland rather than against the wetland edge. Pressure is exerted on the wetland edge as water level rises, resulting in turbulence and erosion; upon inundation, wall shear stress decreases and the potential for sediment accumulation on the wetland increases.
Pounawea Wetland is a vulnerable, low-lying, and eroding salt meadow. Processes of accretion have been identified, however, the wetland is eroding faster than it is accreting. Two key environmental drivers of wetland change are identified; land use change and sea-level rise. Short, medium and long-term mitigations strategies are proposed to reduce wetland edge erosion and increase ecosystem services. Nature-based solutions such as oyster reefs can attenuate waves to mitigate shoreline erosion, stabilise sediment, improve water quality and provide habitat for fisheries. The combination of oyster reefs, sand renourishment of Cabbage Point and an upwind shelterbelt would reduce wind speed and wave height across the estuary thereby reducing the potential for lateral erosion. The simultaneous use of multiple strategies has the potential to provide the most effective strategy for erosion mitigation. However, sea-level rise will continue to accelerate, and inundation of the wetland will become more frequent by 2100 and by 2150 the wetland will be inundated every day. Sediment accumulation may increase during this period, but lower marsh species are not suited to spend extended periods of time inundated. Pounawea wetland is eroding faster than it is accreting and if this trend continues (it may slow or accelerate) then the wetland will cease to exist by 2150, and possibly much earlier.