Mangroves and sediment dynamics along the coasts of Southern Thailand
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The data and observations discussed in this article illustrate 2.
Understand the hydrologic patterns in particular the that many mangrove-mud coasts do not form rigid, stable depth, duration and frequency of tidal inundation that coastlines. Instead, they are often highly dynamic, accreting control the distribution and successful establishment and retreating cyclically, in particular along coasts facing the and growth of targeted mangrove species. To understand how mangrove degradation and 3. Assess modifications of the original mangrove environ- other anthropogenic disturbances affecing these natural pro- ment that currently prevent natural regeneration recov- cesses, and to appreciate implications for mangrove rehabil- ery after damage.
Restore hydrology and other environmental conditions sediment dynamics across muddy coastlines and intertidal that encourage natural recruitment of mangrove propa- mudflats. The literature contains a wealth of publications on gules and successful plant establishment. Here we follow Friedrichs and Aubrey and 0. Continuing on the work by 0 mudflat Dean , Lee and Mehta predicted cross-shore The bed Spring tidal amplitude a0 amounts to 1 m. Lee and Mehta ; see also Lee favorably compared profile 1 with numerous mudflat profiles along i. In the tion of the continuity equation for water, assuming tidal absence of waves, Eq.
Lee ; Friedrichs x The resulting flow velocity at steepness of the mudflat profile. Indeed, these are typ- cross shore profile of intertidal mudflats under tidal condi- ical velocities over tidal flats.
Defining Eco-Morphodynamic Requirements for Rehabilitating Eroding Mangrove-Mud Coasts
The bed level Zb then the amount of water flowing towards the coast at location x. The line e. Note that Friedrichs and Aubrey also ment? The 1 m tidal range at spring tide a0 , a 0.
The Fig. Further MHW level, as sketched in Fig. For the shorter waves of 2 — 0. These higher waves erode the soil at the foot 0. When the waves are short and steep, cliff erosion may occur at the foot of the structure, 0 0 0. Figure 9 presents the time after LWS [hrs] results of a study on wave reflection by Klopman and van der Meer for a so-called Jonswap irregular wave spec- Fig. The reflection coefficient R varies between 0 and 1, may not seem a large reduction in onshore sediment flux, but for full dissipation and full reflection, respectively. Friedrichs However, when such asymmetries are taken into account, a reduction in tidal prism does result in While sedimentation in mangrove-mudflat systems is a reduction in sediment flux, even while the amount of water primarily governed by tidal processes, sediment trapping flowing towards the coast during rising tide equals the amount by the mangroves, and consolidation of freshly deposited of water flowing off the coast during falling tide.
Erection of a dam not only causes the on-shore sediment Note that mangrove-mud coasts are subject to erosive pro- flux to decrease, but also induces reflection of incoming cesses throughout the year, as even small, capillary waves waves. In principle, standing wave patterns are formed with can stir up the bed, owing to the fineness of the sediments. Dean and Dalrymple ; Svendsen of time. As mangroves roots anchor only about 0. As bed shear stress scale within the soil Tomlinson , a few decimeters of ero- with the wave height squared, eroding forces would increase sion is sufficient to destabilize the mangrove trees.
Because the incoming waves are irregular However, as argued in Winterwerp et al. These sediments are transported towards the mangrove-mud coast during rising water in response to the tidal filling of the upper parts of the mudflats, as discussed below. Hence, one can argue that larger waves give and take, whereas smaller waves, which do not stir up the foreshore, only take.
This is the reason that coast-parallel breakwaters work contra- productive along mud coasts, as the off-shore sediment source is reduced, e. Winterwerp et al. The simple coastline Eq. The initial response to this land- shrimp ponds use change is a reduction in sediment flux towards the coast, and a local increase in wave height, as discussed above. Then, Eq.
Their major finding is that the loss of are no longer compensated by onshore sediment transport. Fringes of mangroves that area, promoting net erosion of the sediments in between the remain in the disturbed area are slowly lost to the sea. This mangroves. This mechanism of reducing wave dissipation reduces sedimentation rates substantially, as less mangrove due to the loss of vegetation follows on our feed-back loop trunks and roots remain to capture sediments coming in with as described in Fig.
As the retreating coast then develops a large-scale second phase of coastal degradation, following the initial concave-up cross-sectional profile, wave effects start to effects described above. Upon interventions, e. Then, water depths in dynamics that are affected first, and only later the vegetation front of the coastline increase further, and waves can pene- apart, of course, from the vegetation that was removed.
The trate further towards the coast, enhancing erosion further. As the waves continue to do their job, the profile of the mudflat within the mangrove area, and in front changes, and then the wave-dominated processes described by Anthony and Gratiot start to play a role. These interactions explain why mangrove rehabilitation along eroding mangrove-mud coasts has proven so unsuc- cessful; the erosive forces prevented natural recruitment of mangroves, while planted seedlings literally washed to sea soon after they were put in place.
This does not mean it is impossible to restore eroding shorelines. Through a number of relatively simple measures, one may well be able to restore the morphodynamic requirements to facilitate man- grove growth and ensure that mangroves can resume their important role in protecting vulnerable coastlines.
These options are presented in the discussion section below. Discussion and Conclusions Fig. Our analyses are based on the results of studies measure several hundreds of meters, depending on the in Thailand, British Guyana and Suriname, and observations tidal range; in Indonesia, the Philippines and southern China, whereas 2. Enhance trapping of fine sediments on the mudflat in a the study by Winterwerp et al. In particular, of ditches to improve drainage and accelerate plant various authors highlight the need to i adopt improved establishment Bakker et al.
An additional ad- techniques for mangrove rehabilitation, and ii ensure rein- vantage of such artificial reclamation works is the local statement of appropriate hydrological conditions e. Here we add a third transport away from the rehabilitation site of fine sedi- argument on the basis of a morphodynamic analysis of ments by these long-shore currents; an example is eroding mudflats: iii restore morphodynamic conditions, shown in Fig. It is noted e. Restore hydrological conditions, if disturbed, for example This in-balance in fine sediment dynamics sets in motion through rehabilitation of creeks or removal of small dams an erosion process at a larger scale through which the shape further inland, to ensure appropriate tidal inundation char- of the mudflat becomes progressively concave-up, enhanc- acteristics and sufficient freshwater flow towards the ing the wave effects further.
Where human From our analyses and building on recommendations by modifications of the coast include large-scale conversion Winterwerp et al. This strategy contains the following elements: 1.
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Restore the onshore flux of fine sediment by restoring the intertidal area. This could be accomplished by cre- ating a well managed buffer zone green belt in which incoming tides can freely flow. Their timber benefits local people as construction material, firewood and charcoals and their marvelous root systems contribute to sediment deposition, mud flat formation and substrate stabilization.
Thus, these forests also function as a shelter belt during storms, cyclones and tsunamis. This function was evident when the mangroves in Southern Thailand helped to mitigate the recent tsunami's devastation on lives and properties of the villages situated behind them.emilycolemanportfolio.com/3670.php
Mangroves and sediment dynamics along the coasts of southern Thailand | Wda
The present status of these valuable resources is critical, particularly in Southeast Asia, where the mangrove forest area is estimated to have declined by more than 50 percent over the past 35 years. The major causes of this loss were encroachment due to population expansion, conversion to aquaculture ponds, coastal erosion, lack of awareness and obscure or poorly enforced regulations, In Thailand, for example, recent satellite images show ample evidence of established and widespread aquaculture ponds along the coast of the Gulf of Thailand due to lack of restriction.
The integrating models showed that water turbulence and seedling herbivory had the strongest impacts on mangrove colonization success. Newsletter Google 4. Help pages. Prothero Michael J. Benton Richard Fortey View All. Go to British Wildlife.
WP 1 Embankment and coastal protection
Conservation Land Management. Go to Conservation Land Management. Click to have a closer look. Select version. About this book Contents Customer reviews Biography Related titles. Images Additional images. About this book Mangroves inhabit tropical coastlines and are particularly abundant along deltas and bays where rivers bring freshwater and sediment to the sea.
Contents Preface Summary 1. Propagules, seeds and dispersal of mangroves and their potential sensitivity to sedimentation. Colonization success of common Thai mangrove species as a function of shelter from water movement 3.