Sea-Level Rise and Coastal Subsidence

No environmental issue in the past 20 years has raised as much scientific and public interest and debate as the greenhouse effect. Because the problem is global, no reaction of a single nation or group of nations can necessarily solve its many ensuing problems.

This paper reviews observationally based estimates of past global-mean temperature change and sea-level rise and compares them with model-based estimates. The climate model used is a simple upwelling-diffusion, energy-balance model, which is coupled to a set of simple ice-melt models to give total sea-level change. For best-guess model parameter values there is reasonable agreement between observed and modelled results. The same models are used to estimate future temperature changes and sea-level rise for the standard IPCC95 set of emissions scenarios, updating earlier work. Projected warming over 1990–2100 ranges between 1.4 and 2.9^°C for the central emissions scenario (1595a), while sea-level rise ranges between 20 and 86 cm. Mid-value estimates for a climate sensitivity of 2.5^°C for a CO₂ doubling are 2.0^°C and 49 cm. Temperature and sea-level rise estimates are also given (out to 2500) for five standard (IPCC) CO₂ concentration scenarios in which CO₂ levels stabilize at 350, 450, 550, 650 and 750 ppmv. The sea-level-rise commitment after stabilization is very large: for stabilization levels of 550 ppmv or above, sea-level rise continues for many centuries at rates similar to those occurring at the stabilization point in spite of the constancy of radiative forcing. Finally, the sensitivity of these results to changes in the ocean’s thermohaline circulation is examined. The effects of a thermohaline slowdown are reduced warming rate and increased rate of sea-level rise.

Causes and ranges of coastal lowland subsidence are varied, but in general natural subsidence amounts to centimeters to a few meters per century, while human-induced subsidence can amount to meters in a few decades. As a result, coastal subsidence must be taken into account when predicting relative sea-level rise over the next century.

River sediments are distributed unevenly in space and time, and they are markedly susceptible to human influences. Half the world’s river sediment is derived from the Himalayan region and its environs. Most of the remainder is derived from other tectonically active regions such as the western Pacific islands, the Andes, and southern Alaska. River-sediment loads are variable at many time scales: seasonal, annual, decadal, and longer. The storage of sediment in river systems confounds our ability to predict the delivery of sediment to coastal zones. Natural river-sediment loads are increased by deforestation and crop farming, and decreased by dams and reservoirs.

Coastal erosion is facilitated by rising sea level, but it can also occur on coastlines where the sea level is stable or even falling. The modern prevalence of beach erosion on the world’s coastlines is not, therefore, an indication of global sea-level rise. However, if such a sea-level rise develops, perhaps as a consequence of the greenhouse effect, there will be an acceleration of existing beach erosion, and erosion will begin on many beaches that are now stable or growing. Accelerated erosion also will be seen on cliffs, deltas, coastal swamps, and developed coastlines. The changes predicted globally are already in evidence on sectors of the world’s coastline where land subsidence has produced a relative sea-level rise.

More than 4,550 km² of Bangkok was affected by land subsidence between 1960 and 1988; maximum local subsidence exceeded 160 cm. Most subsidence was due to soil compression at 10–200 m depth in response to the reduction of the aquifers’ piezometric levels. Using aquifer characteristics, ground-water hydrochemistry and subsoil geotechnical properties, mathematical models have been developed to predict further subsidence and recharge response. To inhibit this subsidence, proper planning of land use, flood protection, waste water treatment, pollution control, and sound management of surface and ground-water resources are recommended.

Shorelines in the Gulf of Thailand that have undergone extreme change (such as, the Chao Phraya River mouth and the east coast of the peninsula) have been studied through the analysis of available navigation and topographic maps, aerial photographs and satellite images to determine areas of erosion and accretion. Severe shoreline erosion can be found on the west of the Chao Phraya River mouth (−500 m), Phetchburi (−200 m) and Hua Hin (−100 m), while accretion can be found on the east of Chao Phraya River mouth and south of Songkhla Lagoon inlet. Active sand-spit migration at Bang Nara River mouth poses severe problem for navigation of fishing boats.

No generalizations can be made from the existing data about the sediment load transported by the Ganges-Brahmaputra River systems due to wide diurnal, seasonal and annual variations in the sediment-carrying capacity of these rivers. Estimates of the sediment load are highly variable, ranging from 402 to 710 × 10⁶ tonnes/year for the Brahmaputra River and from 403 to 660 × 10⁶ tonnes/year for the Ganges River. Both these rivers carry predominantly coarse silt to sand-size particles. Human impact on the natural processes of sediment erosion and deposition are evident in both river systems although at different levels. Detrital sands and illitic clays dominate the mineral composition of the river loads. In comparison to the global averages, the denudation rates in the Himalayan river system are four-fold higher.

The Ganges—Brahmaputra Delta is one of the most densely populated areas of the world. The delta occupies most of the Bengal Basin and is slowly subsiding as a result of isostatic adjustment of the crust due to rise of the Himalayas and dewatering of the Proto-Bengal Fan sediments which is now buried under thick Mio-Pliocene-Pleistocene deltaic sediments. Well-log data from northwest of Dhaka indicates that at least a part of the basin is subsiding at a rate of 2.2 cm/year. Three areas of the basin — the Hatiya Trough, Faridpur Trough and Sylhet Trough — may be subsiding at similar or higher rates. Engineering projects that do not consider the subsidence component in planning and designing may produce results detrimental to the environment.

The subsidence and the relative sea-level rise could cause serious drainage and sedimentation problems in the Ganges-Brahmaputra Delta. With higher sea level, more areas will be affected by cyclonic surge; inland fresh water lakes, ponds and aquifers are likely to be affected by saline and brackish water intrusion. The present limit of tidal influence is expected to extend further north. Expected sea-level rise will cause soil salinity, as well as surface water and ground water salinity for a large part of the coastal area. The above conditions, together with lack of dry-season stream flow, may cause serious ecological and economic problems for the country.

The present shape of India is due to a combination of events that took place over geological time, including the breaking away of the Indian Plate from Australia and Antarctica, rifting and drifting between India and Madagascar along the west coast, and collision of the plate with the Asian continent. Following these tectonic events, eustastic changes have played a major role in modifying the east and west coasts of India. Another factor responsible for both past and present regional changes is isostatic movement, resulting in uplift and subsidence, especially in deltaic areas.

The Yangtze Delta, with an area of about 40,000 km² and a population of about 40 million, is one of the largest and most heavily populated deltas in the world. The low-lying delta includes a considerable amount of reclaimed lands with elevations less than 2 m above mean sea level. Natural rates of subsidence throughout the delta appear to be less than 5 mm/year, but locally rates have exceeded 100 mm/year where groundwater pumping has been excessive.

Future sea-level rise will directly threaten the safety of the delta lowlands, particularly if subsidence accelerates in response to groundwater removal. The impact of rising sea level also may be exacerbated if water and sediment from the Yangtze River is prevented from reaching the East China Sea. A great part of the delta could be affected by salt-water intrusion, causing damage to agriculture, industry, water supply, and inland navigation.

The northern Nile Delta is particularly vulnerable to a rise of sea level and subsidence. Due to its high population, key economic activities, agricultural and reclaimed lands, as well as lagoonal fish production, rising sea level will have major implications on Egypt’s economic future. A 1-m rise in relative sea level, for example, may affect 15% of Egypt’s current gross domestic product (GDP).

The northeastern region of the delta is more likely to be affected by a rise of sea level than other regions due to more rapid subsidence. Due to scanty or inaccessible information, however, the actual impact is difficult to quantify. Recommendations are outlined to overcome some of the information gaps.

Prevailing conditions and socio-economic realities in Egypt need to be taken into consideration in any future studies or development plans for the northern delta. Instead of imposing highly developed capital-intensive industrial and agricultural systems, for example, development should aim at controlling erosion, water conservation and engineering measures to protect this region from rising sea level. This approach should preferably last until a more meaningful prediction model is available for the region, one based on more concrete data.

The Italian lowland in the NW Adriatic Sea includes the Po River Delta and the Lagoon of Venice, both of which experienced rapid subsidence in the 1950’s and 60’ in response to the excessive withdrawal of groundwater. Even though overpumping has mostly ceased, the large area of coastal lowland lying below sea level suggests that any rise in local sea level could have severe environmental and economic consequences. Exacerbating the problem is the erosion of local beaches, largely the result of decreased fluvial sediment input. Unless defense structures are built or “accommodation” is practiced, one can expect considerable loss of coastal land and the corresponding impact on local and regional socio-economic well being.

The Niger Delta is arcuate in plan form and low lying, with a maximum elevation about 3m above mean sea level on the sandy barrier islands that border the sea. The barrier islands, approximately 2,000 km² in area, are cut by tidal channels through which oceanic waters and tides gain access to an extensive mangrove swamp (7,000 km²). Presently, freshwater swamps are flooded for 3 to 5 months of the year while the barrier islands are in retreat, with rates of erosion reaching several tens of meters annually, the result of both natural and man-induced factors. Coastal settlements are continually displaced, agricultural and recreational grounds destroyed, harbour and navigational structures disrupted, and oil-producing and export-handling facilities dislodged.

With the continuing subsidence of the delta and the decreased sediment input into the coastal area from fluvial sources, the projected acceleration of eustatic rise in sea level will accentuate wave erosion and acute flooding, leading to the devastation of vast areas of coastal land. Additional effects may include loss of wetlands, salinisation of coastal aquifers and rivers, and modification of the fragile ecosystems that make up the delta. When these effects are viewed against the background of the Niger Delta as home of an oil industry, which contributes over 90% of Nigeria’s export and foreign exchange income, as well as the nation’s new industrial “heartland” and established ‘bread basket’ of the south, Nigeria’s response to rising sea in this low-lying and subsiding terrain must be addressed.

Chains of sand barrier islands are common morphologic features along many shores, occupying about 8% of the world’s coastlines. The Wadden Sea, which stretches along the eastern side of the North Sea, has one of the best studied barrier island systems in the world (Fig. 1).

Since late Cretaceous, depocenters with oscillating deltas and migrating shorelines have provided a fundamental geologic rhythm to the coast of Louisiana. Sites of deltaic sedimentation have shifted, sea level has risen and fallen by more than 100 m, and sequences of preserved deltas have been vertically stacked in the geologic record. This paper summarizes, in the form of a case history, recent changes in the modern Mississippi River Delta with special emphasis on the causes for geometrically increasing rates of wetland loss that have been experienced since the turn of the century. Rates of relative sea-level rise and discharge of freshwater down the main stem of the Mississippi River (north of Louisiana) appear to have been constant throughout the 1900s, indicating that the demise of the Mississippi Delta is probably a result of an inadequate sediment supply and an inefficient sediment delivery network. The combined effects of levees that prevent overbank flooding and funnel sediments to deep water, upstream dams that trap sediments in the Missouri and Arkansas River basins, and formation of a new delta lobe 150 km to the west have had a profound effect on sediment supply. This loss of sediment load is occurring as the Mississippi Delta is nearing the end of its natural 1000-yr life cycle, and has overwhelmed the ability of fragile wetlands, already in a state of delicate balance, to survive the combined effects of global sea-level rise and subsidence. Mitigation through creation of an extensive network of artificial diversions will slow the rate of delta deterioration but will not be able to rejuvenate a dying delta lobe.

Potential economic impact and required human adjustments due to land subsidence and rising sea level in low-lying coastal areas are explained in order to help guide public response, and to anticipate the ways in which people will affect and be affected by these problems. A form of benefit-cost analysis to estimate the expected cost of an event in the absence of mitigation is an economic impact assessment. For example, rough estimates of the scale of potential impact of land subsidence and sea-level rise in Bangladesh and Egypt are reported on the basis of inundation scenarios up to the year 2050. Using strong assumptions about economic growth rates, land rent as a segment of the national product, intertemporal discount rate, and rate of inundation, a method is formulated for extending this characterization to an even cruder estimate of potential economic loss. This is a certainty-equivalent value confined to land inundation. Aside from obvious sources of imprecision, it underestimates several important phenomena: lost capital structures, increased exposure to storm damage and interior flooding, and such secondary effects as saline intrusion, crowding and factor reallocation costs. Even so, it is probably an overestimate because it is not based on probability, assumes a linear rate of land loss, and, most importantly, ignores cost reductions arising from human responses. Five economic topics involved in such responses receive special attention. These are: (1) the advantages of incremental responses to gradual change; (2) principles for managing uncertainty; (3) the ‘retrofit’ problem and capital durability; (4) economic discounting of future values; and (5) the nature and implications of common property, spillover, transboundary and informational effects.

Coastal areas in many countries could suffer the combined effects of sea-level rise and land subsidence. In this paper the consequences of local relative sea-level rise on man-made projects will be discussed. Some general principles are presented and the example of the the Dutch situation are discussed. The main concerns for The Netherlands will be: strengthening of our dunes and dikes against the threat of overtopping during storm surges and the adaptation of our water management system to ensure adequate drainage and the ability to cope with salt water intrusion. A method to study these combined problems and some of the first results are also shown

Long years of flood damage in Bangkok, Thailand have shown that flooding is not only a natural occurrence but also has resulted from urbanization and the utilization of natural resources. The steady rise in the mean sea water level, caused mainly by land subsidence, poses a threat for investment, operation costs and the safety level of the flood control system. The prevailing flood control scheme relies mostly on the protection from the rising estuarine and sea levels, and the estimated annual pumping costs for the Bangkok may reach US$ 20 million per meter rise.

Clearly more attention must be paid to this phenomenon and the ways and means to mitigate its effects. The exchange of experience and knowledge obtained in other countries may help minimize the problems and costs of“trial and error” in the developing countries such as Thailand.

The change in sea level at any location represents the combination of global sea-level change (e.g., due global warming) and local/regional sea-level change due to subsidence or uplift. Because of the wide variations in the latter component, the rate of future sea-level rise and its implications must be assessed separately for each coastal zone. To plan a meaningful response strategy, risk assessment for shoreline regression must be based on local empirical data, preferably as an integral part of the coastal zone management activities.