Oceanography The Official Magazine of
The Oceanography Society
Volume 30 Issue 03

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Volume 30, No. 3
Pages 34 - 47

A Question of Scale: How Turbulence Around Aerial Roots Shapes the Seabed Morphology in Mangrove Forests of the Mekong Delta

Julia C. Mullarney Stephen M. HendersonBenjamin K. NorrisKarin R. BryanAaron T. FrickeDean R. Sandwell Daniel P. Culling
Article Abstract

Mangrove forests are highly productive ecosystems that provide many physical, societal, and ecological services in tropical and subtropical regions. Accurate prediction of the morphological evolution for these areas, in the face of global sea level rise and changes in sediment supply, requires understanding of interactions between vegetation growth, water flows, and sediment transport. Data presented from a wave-exposed mangrove forest in the Mekong Delta, Vietnam, include unique measurements that resolved water flows in and around the aerial mangrove roots (known as pneumatophores) over scales from a few millimeters to hundreds of meters. Flows were highly turbulent, with turbulence values sometimes as large as those measured in surf zones. These energetic processes appear to stir up sediments, with small scour troughs observed around individual pneumatophores, and larger-scale scour around clusters of pneumatophores. The vegetation fringe (the boundary between forest and mudflat) was a particularly dynamic area, with elevated turbulence levels, greater vegetation densities, coarser sediments, and occasional wave breaking. Intense turbulent dissipation at the fringe then reduces the energy of shoreward-propagating waves, sheltering the forest interior. The small-scale processes appear to be linked with forest-wide patterns of sediment transport and deposition. We discuss these links in the context of the biophysical interactions that control the changing shapes of deltas worldwide.

Citation

Mullarney, J.C., S.M. Henderson, B.K. Norris, K.R. Bryan, A.T. Fricke, D.R. Sandwell, and D.P. Culling. 2017. A question of scale: How turbulence around aerial roots shapes the seabed morphology in mangrove forests of the Mekong Delta. Oceanography 30(3):34–47, https://doi.org/10.5670/oceanog.​2017.312.

References

Allison, M.A., C.A. Nittrouer, A.S. Ogston, J.C. Mullarney, and T.T. Nguyen. 2017. Sedimentation and survival of the Mekong Delta: A case study of decreased sediment supply and accelerating rates of relative sea level rise. Oceanography 30(3):98–109, https://doi.org/​10.5670/oceanog.2017.318.

Anthony, E.J., G. Brunier, M. Besset, M. Goichot, P. Dussouillez, and V.L. Nguyen. 2015. Linking rapid erosion of the Mekong River delta to human activities. Scientific Reports 5, https://doi.org/10.1038/srep14745.

Bear, J. 1979. Hydraulics of Groundwater. Dover, New York, 567 pp.

Bouma, T.J., L.A. van Duren, S. Temmerman, T. Claverie, A. Blanco-Garcia, T. Ysebaert, and P.M.J. Herman. 2007. Spatial flow and sedimentation patterns within patches of epibenthic structures: Combining field, flume and modelling experiments. Continental Shelf Research 27:1,020–1,045, https://doi.org/10.1016/j.csr.2005.12.019.

Bryan, K.R., K. Black, and R. Gorman. 2003. Spectral estimates of dissipation rate within and near the surf zone. Journal of Physical Oceanography 33:979–993, https://doi.org/​10.1175/1520-0485(2003)033<0979:SEODRW>​2.0.CO;2.

Bryan, K.R., W. Nardin, J.C. Mullarney, and S. Fagherazzi. In press. The role of cross-shore tidal dynamics in controlling intertidal sediment exchange in mangroves in Cù Lao Dung, Vietnam. Continental Shelf Research, https://doi.org/10.1016/​j.csr.2017.06.014.

Bullock, E.L., S. Fagherazzi, W. Nardin, P. Vo-Luong, P. Nguyen, and C.E. Woodcock. In press. Temporal patterns in species zonation in a mangrove forest in the Mekong Delta, Vietnam, using a time series of Landsat imagery. Continental Shelf Research, https://doi.org/10.1016/j.csr.2017.07.007.

Chen, Y., Y. Li, T. Cai, C. Thompson, and Y. Li. 2016. A comparison of biohydrodynamic interaction within mangrove and saltmarsh boundaries. Earth Surface Processes and Landforms 41:1,967–1,979, https://doi.org/10.1002/esp.3964.

Coco, G., Z. Zhou, B. van Maanen, M. Olabarrieta, R. Tinoco, and I. Townend. 2013. Morphodynamics of tidal networks: Advances and challenges. Marine Geology 346:1–16, https://doi.org/10.1016/​j.margeo.2013.08.005.

Darby, S.E., C.R. Hackney, D.R. Parsons, J.L. Best, A.P. Nicholas, and R. Aalto. 2016. Fluvial sediment supply to a mega-delta reduced by shifting tropical-cyclone activity. Nature 539:276–279, https://doi.org/10.1038/nature19809.

Fagherazzi, S., K.R. Bryan, and W. Nardin. 2017. Buried alive or washed away: The challenging life of mangroves in the Mekong Delta. Oceanography 30(3):48–59, https://doi.org/​10.5670/oceanog.2017.313.

Fagherazzi, S., M.L. Kirwan, S.M. Mudd, G.R. Guntenspergen, S. Temmerman, A. D’Alpaos, J. van de Koppel, J.M Rybczyk, E. Reyes, C. Craft, and J. Clough. 2012. Numerical models of salt marsh evolution: Ecological, geomorphic, and climatic factors. Reviews of Geophysics 50, https://doi.org/10.1029/2011RG000359.

Feddersen, F. 2012. Observations of the surf-zone turbulent dissipation rate. Journal of Physical Oceanography 42:386–399, https://doi.org/10.1175/JPO-D-11-082.1.

Fricke, A.T., C.A. Nittrouer, A.S. Ogston, and H.P. Vo-Luong. In press. Asymmetric progradation of a coastal mangrove forest controlled by combined fluvial and marine influence, Cù Lao Dung, Vietnam. Continental Shelf Research, https://doi.org/​10.1016/j.csr.2017.07.012.

Henderson, S.M., B.K. Norris, J.C. Mullarney, and K.R. Bryan. In press. Wave-frequency flows within a near-bed vegetation canopy. Continental Shelf Research, https://doi.org/10.1016/j.csr.​2017.06.003.

Hong, P.N., and H.T. San. 1993. Mangroves of Vietnam. Vol. 7 of the International Union for Conservation of Nature and Natural Resources Wetlands Programme, 173 pp.

Horstman, E.M., C.M. Dohmen-Janssen, T.J. Bouma, and S.J.M.H. Hulscher. 2015. Tidal-scale flow routing and sedimentation in mangrove forests: Combining field data and numerical modelling. Geomorphology 228:244–262, https://doi.org/​10.1016/j.geomorph.2014.08.011.

Horstman, E.M., C.M. Dohmen-Janssen, and S.J.M.H. Hulscher. 2013. Flow routing in mangrove forests: A field study in Trang province, Thailand. Continental Shelf Research 71:52–67, https://doi.org/10.1016/j.csr.2013.10.002.

Horstman, E.M., C.M. Dohmen-Janssen, P.M.F. Narra, N.J.F. van den Berg, M. Siemerink, and S.J.M.H. Hulscher. 2014. Wave attenuation in mangroves: A quantitative approach to field observations. Coastal Engineering 94:47–62, https://doi.org/​10.1016/j.coastaleng.2014.08.005.

Jadhav, R.S., Q. Chen, and J.M. Smith. 2013. Spectral distribution of wave energy dissipation by salt marsh vegetation. Coastal Engineering 77:99–107, https://doi.org/10.1016/j.coastaleng.2013.02.013.

Lanckriet, T., and J.A. Puleo. 2013. Near-bed turbulence dissipation measurements in the inner surf and swash zone. Journal of Geophysical Research 118:6,634–6,647, https://doi.org/​10.1002/2013JC009251.

Le Bouteiller, C., and J.G. Venditti. 2015. Sediment transport and shear stress partitioning in a vegetated flow. Water Resources Research 51:2,901–2,922, https://doi.org/​10.1002/2014WR015825.

Liénard, J., K. Lynn, N. Strigul, B.K. Norris, D. Gatziolis, J.C. Mullarney, K.R. Bryan, and S.M. Henderson. 2016. Efficient three-dimensional reconstruction of aquatic vegetation geometry: Estimating morphological parameters influencing hydrodynamic drag. Estuarine, Coastal and Shelf Science 178:77–85, https://doi.org/10.1016/j.ecss.2016.05.011.

Liu, J.P., D.J. DeMaster, T.T. Nguyen, Y. Saito, V.L. Nguyen, T.K.O. Ta, and X. Li. 2017. Stratigraphic formation of the Mekong River Delta and its recent shoreline changes. Oceanography 30(3):72–83, https://doi.org/10.5670/oceanog.2017.316.

MacDonald, I.T., and J.C. Mullarney. 2015. A novel “FlocDrifter” platform for observing flocculation and turbulence processes in a Lagrangian frame of reference. Journal of Atmospheric and Oceanic Technology 32:547–561, https://doi.org/10.1175/JTECH-D-14-00106.1.

Massel, S., K. Furukawa, and R. Brinkman. 1999. Surface wave propagation in mangrove forests. Fluid Dynamics Research 24:219–249, https://doi.org/10.1016/S0169-5983(98)00024-0.

Mazda, Y., E. Wolanski, B. King, A. Sase, D. Ohtsuka, and M. Magi. 1997. Drag force due to vegetation in mangrove swamps. Mangroves and Salt Marshes 1:193–199, https://doi.org/10.1023/​A:1009949411068.

Mullarney, J.C., and S.M. Henderson. 2013. A novel drifter designed for use with a mounted acoustic Doppler current profiler in shallow environments. Limnology and Oceanography: Methods 11:438–449, https://doi.org/10.4319/lom.2013.11.438.

Mullarney, J.C., and S.M. Henderson. In press. Flows within marine vegetation canopies. In Advances in Coastal Hydraulics. V. Panchang, and J. Kaihatu, eds, World Scientific.

Mullarney, J.C., S.M. Henderson, J.A.H. Reyns, B.K. Norris, and K.R. Bryan. In press. Spatially varying drag within a wave-exposed mangrove forest and on the adjacent tidal flat. Continental Shelf Research, https://doi.org/10.1016/j.csr.2017.06.019.

Nardin, W., and D.A. Edmonds. 2014. Optimum vegetation height and density for inorganic sedimentation in deltaic marshes. Nature Geoscience 7:722–726, https://doi.org/10.1038/NGEO2233.

Nardin, W., D.A. Edmonds, and S. Fagherazzi. 2016a. Influence of vegetation on spatial patterns of sediment deposition in deltaic islands during flood. Advances in Water Resources 93:236–248, https://doi.org/10.1016/j.advwatres.2016.01.001.

Nardin, W., S. Locatelli, V. Pasquarella, M.C. Rulli, C.E. Woodcock, and S. Fagherazzi. 2016b. Dynamics of a fringe mangrove forest detected by Landsat images in the Mekong River delta, Vietnam. Earth Surface Processes and Landforms 41:2,024–2,037, https://doi.org/10.1002/esp.3968.

Nardin, W., C.E. Woodcock, and S. Fagherazzi. 2016c. Bottom sediments affect Sonneratia mangrove forests in the prograding Mekong Delta, Vietnam. Estuarine, Coastal and Shelf Science 177:60–70, https://doi.org/10.1016/j.ecss.2016.04.019.

Nepf, H.M. 2012a. Flow and transport in regions with aquatic vegetation. Annual Review of Fluid Mechanics 44:123–142, https://doi.org/10.1146/annurev-fluid-120710-101048.

Nepf, H.M. 2012b. Hydrodynamics of vegetated channels. Journal of Hydraulic Research 50:262–279, https://doi.org/10.1080/00221686.2012.696559.

Nittrouer, C.A., J.C. Mullarney, M.A. Allison, and A.S. Ogston. 2017. Introduction to the special issue on sedimentary processes building a tropical delta yesterday, today, and tomorrow: The Mekong System. Oceanography 30(3):10–21, https://doi.org/​10.5670/oceanog.2017.310.

Norris, B.K., J.C. Mullarney, K.R. Bryan, and S.M. Henderson. In press. The effect of pneumatophore density on turbulence: A field study in a Sonneratia-dominated mangrove forest, Vietnam. Continental Shelf Research, https://doi.org/10.1016/​j.csr.2017.06.002.

Nowacki, D.J., A.S. Ogston, C.A. Nittrouer, A.T Fricke, and P.D.T Van. 2015. Sediment dynamics in the lower Mekong River: Transition from tidal river to estuary. Journal of Geophysical Research 120:6,363–6,383, https://doi.org/​10.1002/2015JC010754.

Pratolongo, P.D., G.M.E. Perillo, and M.C. Piccolo. 2010. Combined effects of waves and plants on a mud deposition event at a mudflat-saltmarsh edge in the Bahia Blanca estuary. Estuarine, Coastal and Shelf Science 87:207–212, https://doi.org/10.1016/​j.ecss.2009.09.024.

Quartel, S., A. Kroon, P.G.E.F. Augustinus, P. Van Santen, and N.H. Tri. 2007. Wave attenuation in coastal mangroves in the Red River Delta, Vietnam. Journal of Asian Earth Sciences 29:576–584, https://doi.org/10.1016/j.jseaes.2006.05.008.

Sumer, B., and J. Fredsøe. 2002. The Mechanics of Scour in the Marine Environment. Advanced Series on Ocean Engineering, vol. 17, World Scientific, 552 pp.

Syvitski, J.P.M., A.J. Kettner, I. Overeem, E.W.H. Hutton, M.T. Hannon, G.R. Brakenridge, J. Day, C. Vörösmarty, Y. Saito, L. Giosan, and R.J. Nicholls. 2009. Sinking deltas due to human activities. Nature Geoscience 2:681–686, https://doi.org/10.1038/NGEO629.

Thorpe, S. 2005. The Turbulent Ocean. Cambridge University Press, 484 pp.

Tinoco, R.O., and G. Coco. 2016. A laboratory study on sediment resuspension within arrays of rigid cylinders. Advances in Water Resources 92:1–9, https://doi.org/10.1016/j.advwatres.2016.04.003.

Trowbridge, J., and S. Elgar. 2001. Turbulence measurements in the surf zone. Journal of Physical Oceanography 31:2,403–2,417, https://doi.org/10.1175/1520-0485(2001)031​<2403:TMITSZ>2.0.CO;2.

van Maanen, B., G. Coco, and K.R. Bryan. 2015. On the ecogeomorphological feedbacks that control tidal channel network evolution in a dynamic mangrove setting. Proceedings of the Royal Society A 471, https://doi.org/10.1098/rspa.2015.0115.

Wiles, P.J., T.P. Rippeth, J.H. Simpson, and P.J. Hendricks. 2006. A novel technique for measuring the rate of turbulent dissipation in the marine environment. Geophysical Research Letters 33, L21608, https://doi.org/10.1029/​2006GL027050.

Yager, E.M., and M.W. Schmeeckle. 2013. The influence of vegetation on turbulence and bed load transport. Journal of Geophysical Research 118:1,585–1,601, https://doi.org/10.1002/jgrf.20085.