139736eo.pdf (20MB) - Japan Oceanographic Data Center

of unconsolidated sediment (assuming about 55 percent water) deposited yearly. Imagine what wouldhappen to the nearshore environment if this volume of sediment were cut off!One example of a river that has been recently diverted and dammed is the Lndus River, thelargest river emptying into the western part of the Indian Ocean, but the effects of its effects on thecoastal environment are still only poorly understood. Until the mid-l94Os, about 250 million tons ofIndus River sediment reached the Indus Delta. Beginning with wide-spread construction of barragesin the lower parts of the river valley in the mid and late 194Os, the sediment load dropped to less than50 million tons (Fig. 2). Since then, the annual load has averaged substantially less than 100 milliontons except for several years of large floods. Water discharge from the Indus, in contrast, remainedhigh until construction of upstream dams in the early 1960's, after which it dropped from more than100 km3/yr to less than 60 km3/yr. As hydroelectric and irrigation projects increase in the comingyears, sediment and water discharge will undoubtedly decrease even further. The impact ofdiminished river flow upon the mangrove ecosystem and the marine fisheries has been towardsdecreased fish yield, probably in part because of construction of the Kotri barrage in 1955 and in partbecause of dam construction in the early 1960's (Fig. 3). As of yet, the effect of decreased sedimentload upon the coastal environment has not been documented, but the effects should be apparent anddramatic. The Indus Delta is exposed to extremely high wave energy during the summer monsoonseason. This should cause large-scale erosion of the delta. The impact also will be felt ultimately innorthwest India, where coastal currents historically have transported Indus-derived sediment. It ispossible that the coast east of the Indus River is already eroding, but the area generally is unpopulatedso that human impact so far probably has been small.RISING SEA LEVELThe first two events discussed are primarily local events, related to local basin subsidence andto river flow. The third event, and the one that may have the greatest impact upon coastal states, is theworld-wide rise of sea level. With the end of the last glacial epoch about 15 thousand years ago,world sea level rose about 100 meters, in response to the influx of glacial melt water to the worldocean. Sea level more or less reached its modern level about 5000 years ago, and since then has risenonly a fraction of a mm per year. This natural situation changed, however, with the increased burningof fossil fuels in the early part of this century - initiating the so-called greenhouse effect.Presently sea level is (and has been) rising at about 1 mrn/year, and various scenarios predicttotal rises ranging from 25 cm to more than 2 meters by the end of the next century (HOFFMAN et al.,1983). This effect will be heightened by the use of chlorofluorocarbons within the world community(J. TITUS, 1985, oral communication). When added to regional subsidence, it is possible that IndianOcean deltaic areas may experience a relative subsidence of several meters or more. With the verylow gradients present on many deltas this would mean that coastal regression could be extensive inmany areas by the 22nd century. If one adds in the possible coastal erosion associated with thedecreased sediment influx from dammed or diverted rivers, the coastal regression may exceed severaltens of kilometers.Coastal erosion and regression, of course, are not the only problems. Rising sea level, whencombined with decreased river flow, will mean a landward migration of the salt water wedge withinthe ground water, which could severely affect the quality of drinking and agricultural waters. [Recentdamming of the Ganges River, for instance, has meant more than an order of magnitude increase insalinity of. groundwater within the Bangladesh delta (RAHMAN KHAN, 1983)l. Salt marshes andmangrove areas will be destroyed, thus adversely affecting the spawning areas of many fishes.Finally, the number of tropical storms may increase significantly with the slight climate changeassociated with rising sea level (D.G. AUBREY, 1985, oral communication), and thus furthering thepossibility of catastrophic coastal flooding in low-lying areas.19 7