Conference proceeding
Impacts of mangrove removal on hydrodynamics and sediment transport over decadal timescales in a quiescent, mesotidal, New Zealand estuary; in-situ observations and numerical modeling
American Geophysical Union Fall Meeting, Vol.2020
American Geophysical Union 2020 fall meeting (Online , 01/12/2020 - 17/12/2020)
12/2020
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Abstract
Mangroves are often perceived as “ecosystem engineers” that significantly alter estuarine sediment dynamics and morphology, especially through sediment retention. In many New Zealand estuaries, expanding mangrove forests are cleared in an effort to remove fine sediment and increase bivalve habitat. However, the decadal-scale impacts of mangrove removal on sediment dynamics are poorly understood, limiting the capacity for stakeholders to make effective ecosystem management decisions. Mangroves began expanding in Waikaraka Estuary in Tauranga Harbor, New Zealand in the 1960s. The 2005 partial clearing of mangroves from the estuary now provides an opportunity to examine long-term changes to hydrodynamics and sediment dynamics. The estuary is mesotidal with waves typically <0.08 m. Water velocity, wave height, and turbidity were measured in June 2019 along with surface sediment grain size. The bed sediment coarsened from predominantly sandy in the lower estuary to muddy in the upper estuary. Flow in the lower estuary was ebb dominant resulting in net sediment export, while flow in the upper estuary was weakly flood dominant resulting in sediment retention. Due to these hydrodynamic patterns, fine sediment is unlikely to be flushed out of the upper estuary despite mangrove removal. A Delft3D numerical model calibrated with this in-situ data was used to examine the impact of mangrove extent on hydrodynamics. Tidal asymmetry, velocity skew, and peak ebb-tide shear stress were not significantly altered by varying the mangrove extent between the 2005 maximum and complete removal. Mangroves were not the primary drivers of flow in this system; instead, the hydrodynamics were controlled by bathymetric interactions with the tide. When mangrove extent was expanded to fill all intertidal surfaces, the water velocity on the flats decreased while the peak ebb-tide velocity in the main channel increased. This indicates that the capacity for the estuary to flush fine sediment may actually decrease with mangrove removal. Overall, mangrove removal is an ineffective method for flushing fine sediment out of quiescent, infilled estuaries. Mangroves appear to act as opportunistic colonizers not ecosystem engineers.
Details
- Title
- Impacts of mangrove removal on hydrodynamics and sediment transport over decadal timescales in a quiescent, mesotidal, New Zealand estuary; in-situ observations and numerical modeling
- Creators
- Hannah Glover (Author) - University of Washington, School of Oceanography Seattle, WA USA United StatesDebra Stokes (Author) - Southern Cross UniversityAndrea Ogston (Author) - University of WashingtonKarin R. Bryan (Author) - University of WaikatoConrad A. Pilditch (Author) - University of Waikato
- Publication Details
- American Geophysical Union Fall Meeting, Vol.2020
- Conference
- American Geophysical Union 2020 fall meeting (Online , 01/12/2020 - 17/12/2020)
- Publisher
- American Geophysical Union
- Identifiers
- 991013019916302368
- Academic Unit
- Faculty of Science and Engineering; Science
- Language
- English
- Resource Type
- Conference proceeding