An introduction to the geomorphology of the Huè area (Vietnam).

The Huè area is located at the interaction between different nat ural environment s that ar e the Song Huong and Song Bo Rivers, the Tam-Giang-Cau Hai lagoon and the coastal environment that include a long beach ridge. To p lan a sustainable utilisat ion of the area the main geomorphic features have been analysed using a photogeological investigation and a short field trip. In between the lagoon and the feet of the hills a series of beach ridges have be en recognised th at a llow to establish that during the maximum high stand of the t ransgression, possibly around 6-8 ka ago, there was no lagoon and the sea reached the foot of the hills. The following evolution is the result of the river deposition that led to the growth of a series of bar rier beach that had firstly a cuspate and later an arcuate shape and contained a tidal influenced de lta. Most probably thi s was the consequences of soil erosion following increased human deforestation of the catchment slopes. The genesis of the more external beach ridge seems to be associated to a peri od of enhanced marine ero sion and the landward reworkin g of the se dim ent s stor e d i n the submerged beach. It is suggested that this was the result of the decline of coral growth following increased amount of suspend l oad car ried b y riv e rs. In fact the pr e s ent -day barrier beach for the first time block the lagoon while in the past the interaction between river and coastal dynamics were always able to redistribute sideway its sediments. Although the agents at work have been outlined, in orde r to und erstand the evolutionary trend and the speed of the processes it is necessary to get information that are currently unavailable. They regards the type and amo unt of so lid lo a d c a rried by the rivers, the amount of sediments that enter or escape the lagoon area and the long-shore drif ting. In fact, all these agents play a role not only in the stability and the natural hazard affecting the town of Huè but also the lagoon that constitute the major resource for agriculture and fisheries.