Philippe Dussouillez

Linking rapid erosion of the Mekong River delta to human activities

As international concern for the survival of deltas grows, the Mekong River delta, the world’s third largest delta, densely populated, considered as Southeast Asia’s most important food basket, and rich in biodiversity at the world scale, is also increasingly affected by human activities and exposed to subsidence and coastal erosion. Several dams have been constructed upstream of the delta and many more are now planned. We quantify from high-resolution SPOT 5 satellite images large-scale shoreline erosion and land loss between 2003 and 2012 that now affect over 50% of the once strongly advancing >600 km-long delta shoreline. Erosion, with no identified change in the river’s discharge and in wave and wind conditions over this recent period, is consistent with: (1) a reported significant decrease in coastal surface suspended sediment from the Mekong that may be linked to dam retention of its sediment, (2) large-scale commercial sand mining in the river and delta channels, and (3) subsidence due to groundwater extraction. Shoreline erosion is already responsible for displacement of coastal populations. It is an additional hazard to the integrity of this Asian mega delta now considered particularly vulnerable to accelerated subsidence and sea-level rise, and will be exacerbated by future hydropower dams.

Acoustic measurements of the spatial distribution of suspended sediment at a site on the Lower Mekong River

The Mekong River spans thousands of kilometers, flows through six countries, and its basin is one of the world’s richest in terms of biodiversity. However, land-use changes, dredging of the river bed, and the construction of dams are changing its sediment dynamics and morphology. The resultant increases in bank erosion and reduction in sediment supply to floodplains may have adverse effects on the economical and biological productivity of the region. In order to monitor these changes, the current conditions must be well understood. Comprehensive measurements of the spatial distribution of sediment (both suspended and bed load) were made at three locations in different physiographic regions of the Lower Mekong at the end of the 2012 rainy season. Acoustic Doppler Current Profilers and a multifrequency acoustic backscatter system, the AQUAscat, were used in combination with water sampling to provide high resolution measurements of concentration and grain size. The AQUAscat consisted of four monostatic trans...

Alongshore variations in morphology and incident wave energy on a human-impacted coast: Agadir, Morocco.

ABSTRACT Aouiche, I.; Daoudi, L., Anthony, J.A., Sedrati, M., and Dussouillez, P., 2016. Alongshore variations in morphology and incident wave energy on a human-impacted beach: Bay of Agadir, Morocco. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No.75, pp. 1027 - 1031. Coconut Creek (Florida), ISSN 0749-0208. Beaches in developing countries are increasingly affected by human impacts, notably the construction of harbors and tourist infrastructure. In Morocco, the city of Agadir is a fine example of this situation. Agadir was destroyed by an earthquake in 1960 (5.7 on the My scale and 60,000 dead). The Agadir city front corresponds to a semi-sheltered bay that has been strongly impacted by economic development and rapid (> 10% a year) demographic growth in the course of its recovery following this earthquake. The northern sector of the bay is sheltered by a commercial harbor constructed in 1988. In order to understand how this harbor has affected sediment circulation, the morphology and hydrodynamics of the beach have been monitored since 2012. This study discusses beach morphological variations between the harbor-sheltered northern sector and the exposed southern sector. The offshore and nearshore wave regime, analysed using the MIKE 21 coupled model Fm, is largely dominated by waves approaching from the north. The modeling results show that wave energy attenuation between the inner shelf and the shore in the northern sector ranges from 30% to 80% as a result of diffraction caused by the harbor breakwater, whereas the southern sector is exposed to much higher waves. Two digital elevation models obtained in March 2012 and April 2015, confirm these expected results from the alongshore wave-energy gradient, and show that the northern sector of the beach is largely dominated by accretion, whereas the southern sector is undergoing erosion.