Bart Schultz
The Red River rises in China, but about half of its basin lies in Vietnam. The Mekong also rises in China, but flows through Laos, Myanmar, Thailand and Cambodia before discharging into the sea through the Mekong Delta. Only eight percent of its basin lies in Vietnam, and forms a significant part of the delta, that also lies partly in Cambodia.
Over the years, an extensive system of dikes has been built in the Red River Delta to prevent flooding as much as reasonably possible. This also involved the problem that sedimentation in the delta had caused the bed of the Red River to rise locally to several metres above the level of the adjacent land. Over the years, both in China and in Vietnam, several dams have been built in the Red River and its tributaries, mainly for hydropower, so that extreme discharges are now stored more in the reservoirs and the risk of flooding in the delta has decreased. A special contribution for Hanoi is made by the Day River system, which allows water to be diverted in the event of extreme discharges in the Red River to protect Hanoi from flooding. See my contribution about the Day River system that was posted on Flevolands Geheugen on April 14, 2020.
In the Mekong Delta, dike construction has been less common and in large parts of the delta, flooding has been used much more to capture the fertile silt that has been brought in. Here too, the construction of no fewer than eleven dams in the upstream parts of the river and its tributaries has led to a significant change in the discharge to the delta in recent decades. Because most of the sediment in the river water now remains in the reservoirs, virtually no sediment is discharged to the delta anymore. Although all these reservoirs have reduced the risk of major damaging flooding, the risk of flooding is increasing again due to soil subsidence in the delta and, to a certain extent, sea level rise. In the downstream part of the delta, there is a risk of flooding with salt or brackish water.
In addition to the dikes with a probability of failure of 0.5 to 4 percent per year, the polders themselves have a water management system with irrigation canals and drains, that can also be combined. In a number of cases, the excess water can be discharged through discharge sluices, but increasingly, drainage by pumping is also becoming necessary, especially due to subsidence. The problem here is that drainage is a costly affair given the amounts of precipitation in the deltas. In the Red River Delta, the annual precipitation is approximately 1,500 millimetres. In the Mekong Delta, it is approximately 1,250 millimetres. By way of comparison: the annual precipitation in the weather station Hoofddorp in the Netherlands is approximately 900 millimetres, with the precipitation being much more evenly distributed over the year than is the case in Vietnam.
In Vietnam, however, the annual evaporation is greater than in the Netherlands, so that the question for pumping will be how much more water needs to be pumped out in Vietnam than in our country. In addition, the amount of irrigation water that is let into the polders during dry periods also plays a role. In short: it is not that easy to make a good comparison.
In the Mekong Delta there is also the problem of acid sulphate soils (cat clay), which often develop after land reclamation. This problem requires sophisticated water management in the first decades after land reclamation, in which the water levels must be kept high, especially during the growing season. The problem then decreases very gradually, but it can take years before it is really solved.
