THE WORLD DELTA DATABASE

THE RED DELTA

Delta ID     # 34.

The Red River system is located in the country of Vietnam and the drainage basin has an area of 164,600 sq km. The main channel originates in the north-south trending Triassic and Jurassic fold belts (Sulongshan Fold Belt) and the northeastern part of the basin is composed of Paleozoic sediments of the South China Fold Belt. The main channel is some 760 km in length and flows through narrow gorges within the basin. The average elevation in the basin is 1,409 m, while the maximum and minimum average elevations are 2,243 and 263 m, respectively. Annual average rainfall is quite high, some 1,122 mm per year. Maximum rainfall is 1,417 mm while minimum rainfall is 928 mm. The rainy months are associated with the annual monsoon and occur during the months of May through September when average monthly rainfall rarely falls below 150 mm. The dry months, from October through April, has a monthly average of approximately 20 mm. Most of the drainage basin is densely vegetated with tropical and subtropical broadleaf forests. The drainage density of the tributaries is quite dense, averaging 0.31 km stream length per 500 sq km (Figure 78).

The alluvial valley is well-defined and has a width that ranges from less than 3 km to 10 km and has a length of 599 km. During most of length, the channel flows through narrow gorges. The average annual river discharge is 3,913 cu m/sec. The modern delta, some 12,073 sq km in size, protrudes significantly into the Gulf of Tonkin. Progradation has been active in this delta lobe, with average rates of progradation up to 100 m per year. The most recent active delta is well defined and several older delta lobes are apparent on the satellite image [34-i01]. The subaerial delta is nearly four times the size of the subaqueous delta. The delta plain has nearly been completely modified by man and little of the original wetlands that once existed remain. With a population of almost 17 million people living in the delta, the Red River Delta is one of the most densely populated rural areas in the world, with about 1,000-persons/sq km. This is well illustrated by the satellite image in Plate 33; the cultivated areas being represented by the pink and blue reflections. The tidal range at the river mouth is 1.86 m and parts of the older abandoned deltas display tidal intrusion. Wave action along the delta front is quite high and the root mean square wave height is 1.92 m. This high wave energy has resulted in most of the delta shoreline being fronted by barrier spits. To the south of the presently active delta, the wave energy is resulting in shoreline erosion and erosion rates are as high as 8 m retreat per year. Stranded beach ridges are found throughout the delta plain, especially those that were associated with former active river mouths. The Holocene delta sediments are relatively fine-grained, up to 30 m thick, and represent rapid progradation during the current high sea level stand (Mathers and Zalasiewicz, 1999). The Holocene delta sediments are relatively fine-grained, up to 30 m thick, and represent rapid progradation during the current high sea level stand. The wave-dominated portion of the shoreline appears to have advanced by leapfrogging seaward, perhaps under the influence of episodic high-discharge fluvial flood events, producing lateral alternations of stacked sandy beach ridges and intervening fine-grained lagoonal deposits in chenier-like architectures (Mathers and Zalasiewicz, 1999).