Academic literature on the topic 'Upper deltaic plain'

The 4800 km Mekong (known as the Lan Tsan Chiang or Lancang in its upper reaches in Yunnan Province, China) rises at 5100 m elevation on the eastern edge of the Tibetan (Xizang) Plateau where the Yangtze (Chang Jiang) and Salween also rise. With a drainage basin covering 795 000 km2, the river ranks as the ninth largest and twelfth longest in the world and discharges some 475 billion m3 of water to the South China Sea annually. The mean annual flow at Kratié in Cambodia (where the catchment area upstream is 646 000 km2) is 14 700 m3 s−1 with a maximum of 67 000 m3 s−1 and a minimum of 1250 m3 s−1 (Committee for Coordination of Investigations of the Lower Mekong Basin 1966; Volker 1983). The river flows from the Tibetan Himalayas southward through China receiving tributaries from a small part of Myanmar. The drainage basin also encompasses nearly all of Lao PDR, northeast Thailand, most of Cambodia, and part of the Central Highland and the delta of south Viet Nam. In the heart of Cambodia, where the river is joined by the Tonlé Sap or Great Lake River, it rises from 1 or 2 m above sea level in May to 8 or 10 m above sea level in August. The Mekong Basin embraces some of the most diverse scenery in the world, with landforms ranging from deep gorges, to spectacular karst features, great lakes, and a huge delta. These varied landscapes support one of the most biologically diverse river systems in the world, surpassed only by the Amazon and possibly the Nile. The high biodiversity varies greatly across the following distinct landform and biogeographic provinces: 1. the eastern edge of the Tibetan Plateau (here termed the Chinese upper reaches); 2. the highlands of Myanmar, northern Thailand, and the northern Lao PDR; 3. the Annamite Mountains of eastern Lao PDR and western Viet Nam; 4. the plains around the central Mekong in Lao PDR, Thailand, and Cambodia; 5. the Tonlé Sap Basin; 6. the Mekong Delta and coastal mangroves (MacKinnon and MacKinnon 1986).

Erosion is a common problem at pipeline watercourse crossings. Watercourses are naturally vulnerable to erosion but the risk is particularly acute after sub-soil and armour materials have been disturbed by trenching and backfilling during pipeline construction. The process of pipeline exposure at watercourse crossings can be grouped into these types. One common type of erosion phenomenon is episodic exposure resulting from general and local scour. This is associated with temporary river scour during flood events. It includes general scour involving temporary lowering or the entire river bed during high flows, as well as local scour which involves development of a scour hole during high flow events at a predictable location on the river bed. River engineers have adopted various design methods to ensure sufficient pipeline burial depth which minimizes pipeline exposure due to such periodic occurrences. A second type of erosion phenomenon causing pipeline exposure is progressive river channel bed and bank erosion. This is not a function of a single event but occurs periodically, resulting in progressive removal of pipeline cover. Progressive erosion at pipeline crossings includes riverbed degradation, bank erosion and growth of gullies. River bed degradation (progressive river bed lowering) is a complex phenomenon associated with the stage of geomorphic development of the drainage basin. Its prediction is based on a sound understanding of sediment supply, river hydraulics and river outlet conditions. Bank erosion is a common occurrence and is readily observed. It may be a continuous or episodic occurrence and is often related to the river’s tendency to change its meander pattern, cross sectional shape or bed level. Growth of gullies is a very common cause of erosion at pipeline crossings and results from changes in land use, soil composition, and landscape drainage networks. Techniques for predicting progressive erosion are not well developed and widely understood. As a result, progressive erosion is a common cause of erosion and even pipeline exposure at pipeline crossings of rivers, streams, and gullies. A third mechanism of pipeline exposure is river avulsion. This is often associated with a tortuously meandering channel, a steep braided channel with a wide flood plain or an immature channel on a delta. Methods of avoiding or controlling erosion are based on a sound understanding of causal factors. Each river crossing location is unique and the local risks of pipeline exposure must address specific local conditions. Methods of estimating the risk of local and general scour, progressive erosion, and river avulsions are discussed. Methods of mitigating erosion at pipeline crossings include proper siting of pipeline crossings, deep burial, conventional armouring and a combination of bank toe protection and upper bank vegetation cover.