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

RISING SEA LEVEL AND CHANGING SEDIMENT INFLUXESREAL AND FUTURE PROBLEM FOR INDIAN OCEAN COASTALNATIONSJOHN D. MILLIMANWoods Hole Oceanographic InstitutionWoods Hole, MA 02543 USAABSTRACTWithin the next 100 years the combined effects of sea level rise, subsidence and the dammingand diverting of major rivers can effect a shoreline regression of 5 to more than 20 km in many deltaicareas bordering the Indian Ocean. Oceanographers may have little control in many of the policydecisions regarding the rising sea level or the damming of rivers, but by documenting the coastalenvironments we can at least estimate the potential damage as well as suggest ways in which the impactcould be lessened.INTRODUCTIONFew areas are as dependent upon the sea and their coastal areas as the countries bordering theIndian Ocean. In addition to the transportation and mariculture normally associated with coastal states,these deltas often house large human populations. A quick look at a map of western Asia shows thatthe large cities of Karachi, Calcutta, Dacca, Rangoon and Bangkok all lie on or near major deltas.Moreover, the continually renewed river sediments on the delta form the source for particularlyproductive soils. The Bengal Delta, for example, is one of the most productive agricultural areas inAsia. Yet because of its low elevations, it also is one of the most exposed to the whims of the ocean.A slight rise in sea level an increased rate of coastal erosion could have a major adverse effect on thepeoples in northwest India and Bangladesh. Similar arguments can be made for other deltas, bothlarge and small, that border the rim of the Indian Ocean.In the next century three events will occur, one natural and two man-made, which will affectmany coastal areas around the world, but perhaps particularly the deltaic areas around the IndianOcean. Indian Ocean nations will have no control over one of these processes (Basin Subsidence),some control over another (Rising Sea Level), and essentially complete control over the third(Damming and Diverting of Rivers). If all three events occur unchecked, the result will almostcertainly be disastrous for many coastal areas. Research into these problems should beginimmediately, so that the potential impact can be predicted and minimized.BASIN SUBSIDENCEDeltas subside at rates ranging from 1 mm to several cm per year; large deltas often subsidingat faster rates than smaller ones. This subsidence results partly from crustal processes (often causedby relative movement along continent-ocean crust boundaries) and partly reflects isostatic compemationand consolidation of the sediment deposited by rivers. Assuming that the delta subsides at a rateof 3 mm/yr, over the next 100 years the delta would subside 30 cm; that is, relative sea level wouldrise by that amount if accumulation of incoming river sediment did not offset the subsidence. Removalof ground water or petroleum from the underlying strata can increase local subsidence by as much asan order of magnitude.195