Coastal geomorphology, dam construction's impacts and ecosystem based approach

On March 8, 2013, the training workshop “Coastal Geomorphology, Sediment Transit and Their Integration into Biodiversity Conservation Planning in the Mekong Delta” was held in Ben Tre by WWF – Greater Mekong Programme and Biodiversity Conservation Agency (BCA, MONRE). In the morning session, Professor Edward J. Anthony[1] gave lectures on coastal geomorphology and impacts of hydropower dams on coastal areas (Mekong Delta as a case study). In the afternoon session, Mr. Ngo Xuan Quy (BCA) gave an overview on biodiversity in the Mekong Delta and Ms. Tran Thi Mai Huong (WWF Vietnam) gave a brief introduction to ecosystem-based approaches to climate change. The workshop ended with the plenary discussion and conclusion.

1. Introduction to coastal geomorphology and coastal evolution

Sediment sources for coasts are from land (90%, mainly from river catchments), seabed, from the coast itself (coastal erosion), from marine and coastal plants and animals (corals, mangroves, salt marshes).

Sediment types can be boulders, blocks at high energy coasts (brought by tsunamis, extreme storms), gravel at high-energy-coasts in temperate to high latitude coasts, and sand or mud at all climate settings, but dominant in tropical settings due to chemical weathering.

There are erosional coasts (rocky coasts, cliffed coasts) and depositional/alluvial coasts (beaches, sandflats, mudflats, salt marshes, mangroves). Depositional coasts can become erosional in case of deficient sediment supply.

 

The energy sources for coasts are waves, tides, currents, wind flows, river flows, freshwater – saltwater interactions, exceptional events (storms, tsunami, earthquakes, landslides, volcanoes, etc) and also impacts of direct and indirect human interventions.

Longshore drift is a fundamental coastal process enabling sediment transport from sources (notably river mouths) to the rest of the coast. Although longshore sediment drift is essentially due to waves obliquely approaching the shore, this transport can also be generated by tidal currents and wind stress, especially where mud is available.

Sediment gain relates to coastal advance (accumulation, progradation) while sediment loss relates to coastal retreat (erosion). Sediment losses can be caused by perturbation of river sediment supply such as forestation, land use changes, dams, climate change, perturbation of longshore drift such as ports, coastal defence works, or extreme storms and tsunamis. Low eroding coasts are likely to be strongly impacted by sea level rise.

Estuaries are commonly net sediment sinks. High river flow, and ebb-dominated tidal flows, can lead to sediment transport from the estuary to the sea. The supply of sediment to coasts by rivers has, however, been strongly affected by humans through:

-       modification of catchment characteristics such as vegetation cover and soils mainly related to agriculture, mining, road construction, settlements;

-       river bank and channel engineering works, including waterway diversions, aimed at stabilizing flow, controlling floods and enhancing navigation;

-       and especially through dams and reservoirs for water storage, water control, hydroelectricity, irrigation.

The sediment input to the floodplains during the annual flood plays a crucial role in terms of nutrient supply to agriculture. Sedimentation in floodplain plays a key role for the economic and ecological sustainability of low lying deltas. Its values can be acknowledged for nutrient input for agriculture, but also in terms of compensation for delta subsidence and sea level rise.

The transition from the river to the coast involves complex interactions between sea, coastline and land.

 Humans have increased the sediment transport by global rivers through soil erosion by 2.3 ± 0.6 billion metric tonnes per year, but yet reduced the flux of sediment reaching the world's coasts by 1.4 ± 0.3 billion metric tonnes per year because of retention within reservoirs. Over 100 billion metric tonnes of sediment are now sequestered in reservoirs constructed largely within the past 50 years, especially in Africa and Asia (Syvitski et al., 2009).

Large-scale over-exploitation of riverbed sand, granulates together with dam construction all over the world have caused significant reduction of suspended sediment discharge which in turn can result in widespread coastal retreat. Engineering of delta shoreline for reclamation purposes can also reduce sediment supply to the coast. Moreover, river flow declines after dam construction.

Widespread erosion of the Mekong delta shoreline

Existing and planned mainstream dams in China would have large impacts in terms of decreasing sediment, given that more than 60% of the Mekong’s suspended sediment load originates from this part of the river. Models project that at least 50% of total basin sediment load will be trapped annually by the Chinese dams. Proposed dams in the lower Mekong would trap even more sediment, with substantial negative impacts expected in Cambodia and parts of the Mekong Delta in Vietnam.

Delta shoreline status shows that erosion dominates with more than 75% of the Mekong delta shoreline in erosion. Erosion rates of up to 10 m/year. Erosion is severe along the muddy wave-tide-dominated coast southwest of the delta mouths and most severe along north Ca Mau and south Bac Lieu provinces. Erosion ‘hot front’ appears to be migrating southwest.

Future stability of the Mekong delta shoreline and assurance of the continuity of its ecosystem services will strongly depend on dam effects on the sediment balance, in a context of exacerbated vulnerability from sea-level rise and delta sinking.

2. Biodiversity conservation in the Mekong Delta

 

The national policy for adaptation to climate change requires assessment of climate change impacts to related aspects, including biodiversity. The draft national strategy on biodiversity conservation recognises climate change as one of the threats to biodiversity and suggests actions for biodiversity conservation in the context of climate change.

Planning is considered as a useful tool to conserve and identify priority zones for conservation, particularly relevant in the context of climate change. Among 12 provinces in the Mekong Delta, Soc Trang, Ben Tre and Ca Mau have developed their biodiversity conservation planning.

3. Introduction to ecosystem based approaches to climate change

An ecosystem approach is a strategy for the integrated management of land, water and living resources that promotes conservation and sustainable use in an equitable way. Mainstreaming an ecosystem based approach to biodiversity conservation plan is crucial.

Ecosystems provide a variety of services to people and economies that range from provisionary services such as water and food to regulatory services such as regulating local climate. Ecosystem-based approaches address the crucial links between climate change, biodiversity, ecosystem services and sustainable resource management which have the potential to simultaneously contribute to the avoidance and reduction of greenhouse gas emissions while maintain and increase resilience, reduce vulnerability of ecosystems and people, help to adapt to climate change impacts, improve biodiversity conservation and livelihood opportunities and provide health and recreational benefits.

Ecosystem-based adaptation (EbA) is the use of biodiversity and ecosystem services as part of an overall adaptation strategy to help people to adapt to the adverse effects of climate change.

Ecosystem based mitigation (EbM) is the use of natural ecosystems as the major carbon stores and sinks to mitigate the causes of climate change  (mitigating and reducing GHG emissions from energy production or land use changes).

Final decision of using one approach instead of the other one needs to be carefully weighted, considering local situations and scientific evidences, since in many cases the best strategy might be the combination of the two. It may be appropriate to combine EBA and infrastructure solutions in some cases.

Excursion on March 9, 2013

In the tentative agenda, the half-day fieldtrip on March 9 was supposed to visit the erosion site in Ben Tre. However, since they could not get permission for the two foreign experts (Prof. Edward Anthony and Dr. Marc Goichot, senior adviser of WWF), the plan was changed to visit Vam Ho Bird Sanctuary and the Ba Lai sluice gate.

Vam Ho has long become a favourite destination for many animals, especially birds. Vam Ho Bird Sanctuary is home to thousands of storks, herons and other types of bird. We arrived at the entry to Vam Ho Bird Sanctuary but could not move deeper inside the forest. Again unfortunately, the visit to Vam Ho Bird Sanctuary had to cancell for safety reason because of huge amount of mosquitos and because people were not well prepared (wearing shorts).

On the way back, we had a quick look at the Ba Lai sluice gate, which was built in 2000 and has been operated since 2002. The aim of this Ba Lai sluice gate construction is to serve for salinisation prevention, freshwater retention, flooding drainage and soil reclamation. However, currently salinization, reduction of sediment, erosion at the two banks of the estuary are some problems in the area. 

 

Some key remarks at the plenary discussion

·         Dams construction and overexploitation of sand cause sediment deficits which in turn affects significantly to coastal stability because this creates more erosion downstream.

·         Reduction of sediment is just one legacy of dams construction, there are many more negative consequences, including the reduction of fish resources.

·         Mekong Delta are facing risks of erosion, shrinking and sinking.

·         Putting coastal barriers can affect downstream, especially for muddy coast. Therefore, we need to see the whole picture, looking the whole coast as a system.

·   “Hard” solutions or structural measures such as sea dyke are costly and only create temporary sense of safety. After some years it can be collapsed and washed away out to the sea. Moreover building sea dyke can block the water exchange which is needed for mangroves (as mangroves also need freshwater), resulting in mangrove death.

·       Ecosystem based approaches or “soft” measures are environmental friendly and can bring multiple benefits at the same time.

·         Restoration of coastal mangroves can facilitate rehabilitation of biodiversity through creating habitats for aquatic resources and other animals, birds.

A video clip in Vietnamese made by VTV Can Tho about building wavebreaker in Vam Ray, Hon Dat, Kien Giang province to reduce wave energy (reduce 63% wave energy) and stimulate sedimentation (deposition rate of 20 cm sediment/year) for mangrove planting. Results after 3 years of implementation this model (2010-2013) show that no more erosion, increasing deposition, decreasing of salinization, restoration of habitat with more aquatic resources, birds returning to the area.