JJ.Jiao

Modification of regional groundwater regimes by reclamation

Jiu Jimmy Jiao

Department ofEarth Sciences, The University ofHong Kong, P. R.

Abstract Land reclamation has played a significant role in the urban development process in many coastal areas in . While reclamation provides valuable land, it also creates various , environmental and marine ecological problems. These problems directly caused by reclamation have been recognized and widely studied. However, it has not been recognized yet that reclamation will almost certainly change the regional groundwater regime, in tum causing similar problems. This paper represents the first attempt to address the impact of reclamation on groundwater regimes. It will be demonstrated that large-scale of reclamation will increase groundwater level, modify the groundwater divide, and alter submaririe groundwater discharge to the . The change of groundwater conditions will cause engineering and environmental problems by modifying the capacity, flooding pattern, stability ofslopes and foundations, interface between sea water and groundwater, and the coastal marine environment. This paper suggests that, for a major reclamation project, the change ofthe hydrogeological system in response to reclamation should be evaluated so that the disturbance to groundwater regimes and the damage to environment might be minimised.

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Introduction Reclamation ofland from the sea has played an coastal ecology (Yip, 1978) and marine environment important role in the development of coastal areas in (Poon, 1997). However, it has not been recognised yet many parts ofthe world, including China, Britain, , that reclamation will almost certainly have an impact Korea, the and the (Chen & on the regional groundwater regime, in tum causing Wang, 1999;Lee, 1998;Bouchardetal., 1998;Schofield coastal engineering and environmental problems. Since et al., 1992). While reclamation provides valuable land, the response ofgroundwater regime to reclamation may it also creates various engineering, environmental and be slow and not so obvious, the study from this ecological problems. These problems are widely perspective has been largely ignored. Mahmood & investigated. For example, there are studies about the Twigg (1995) appear to have been the first to notice impacts ofreclamation on coastal (Lee, 1998), that groundwater conditions may be altered by

29 Geologist, 2000, 6, 29-36

reclamation. They report that the water rises in, three are in the active construction stage. Much larger or immediately adjacent to, an area of reclaimed land in bays and coves are reclaimed for the creation of these Island of Saudi Arabia. Apart from that, there towns. With full development of these towns there has been little study to address the impact of would be a capacity of about 3.5 million. reclamation on regional groundwater regimes. The fill materials are mainly marine extracted from A research project has been initiated in the the seabed. University of Hong Kong to understand the The quality and nature of fill can be extremely modification ofthe natural groundwater regime caused variable, and there is nearly always a layer ofsoft marine by large-scale reclamation in coastal areas. In this short on the seabed beneath reclamation sites. An research communication, the background of land attempt is usually made to displace or dredge out the reclamation in Hong Kong will be briefly introduced. mud before reclamation, but very often a layer of mud Then, the ways in which reclamation may influence with thickness ranging from 5 to 10m remains in place groundwater regimes, including the water level, (Lumb, 1980). At some reclamation sites, such as the groundwater divide and submarine groundwater new town site, the mud was intentionally discharge (SGWD), will be discussed. Attempts will not displaced. At another site in , the mud was be made to link the modification of groundwater supposed to be progressively pushed off from the site, conditions to slope stability, flooding and other but in actuality, the m~d was reworked and very little possible coastal environmental and engineering was removed. This layer of mud is troublesome. It problems. gradually becomes less and less permeable and leads to differential ground settlement as consolidation Brief background on land reclamation in continues. Hong Kong Hong Kong is mountainous territory with little Submarine groundwater discharge natural level ground other than mud-flats, mangrove Coastal areas are usually the ultimate discharge and marshes. Since 1841 to the present, there zones of groundwater. The dynamic connection of the has been a continuous gain of developable land coastal groundwater and sea water has been studied through reclamation and levelling ofhills. Over 10% of by groundwater hydrologists since the 1950's (Jacob, the Hong Kong developed land area is reclaimed from 1950; Erskine, 1991; Jiao & Tang, 1999). Traditionally it the sea. was estimated that the amount of SGWD ranged from Before the mid-1950s, reclamation was O.Ol%to 100/oofsurface-waterrunoff. Recently, Moore essentially strip development along the coast near (1996) has discovered that SGWD could amount to as urban areas, straightening out the coast line by much as 40% of the total river flow into the , a constructing substantial sea-walls and filling in the surprisingly large percentage compared with the enclosed lagoons by public dumping, together with previous estimation. This finding has the potential to draining and filling in ofsome inland marsh land (Lumb, radically alter our understanding of oceanic chemical 1976, 1980). Some pre-war reclamation areas were mass balance and suggests that groundwater input nothing more than public rubbish dumps. may play a very significant role in the marine By 1955, a rapid increase in population and a environment. heavy demand for factory space led to a new SGWD can occur anywhere along the coastline reclamation policy-filling of shallow bays and coves or through the seabed in the form of distributed flow, remote from the traditional centres - to create large but it may also concentrate locally in the form of industrial areas and associated housing estates. Large submarine springs through fracture or fault zones or borrow areas had to be laid out in the nearby hills to other special geological structures, just like springs provide fill. By terracing or platforrning these borrow formed on hill slopes on the land. A recent study, areas, substantial amounts of levelled land could be based on diving research (Marui and Hayashi, 2000), formed at the same time that the reclamation was· being off a volcanic island in northern Japan indicates that won. The fill is generally the easily excavated some SGWD points occur 9 to 27 m below sea level. decomposed granite residual mantle, which is More comprehensive studies have been conducted on essentially silty coarse sand. SGWD on an island in Hawaii. Light stable isotope Since 1973, the Hong Kong Government has and carbon-14 data for the deepest fresh aquifer introduced the New Town Programme, the main showed that its recharge originated at an average objective of which is to provide land in the New elevation ofabout 2,000 m and that it had resided in the Territories in Hong Kong for public housing formation for about 1,800 years. Submersible surveys development. Six new towns have been created and conducted offshore of the island also showed that the

30 JJJiao

flow through this aquifer was discharged at about 300 the form of springs in the slope at the contact between m below sea level (Thomas, 2000). Although there has the original coastline and the reclaimed land. The not been any similar study in Hong Kong to date, this pressure build-up can be great if the slope is covered kind ofSGWD surely exists in Hong Kong considering by chunam or shotcrete. How significant the its large coastal areas with diverse geological modification due to reclamation will be depends on the conditions. If reclamation happens to block a major permeability of the fill materials and the distance submarine discharge zone, such as a large groundwater­ between the old and the new coastlines (or the scale of conductive fault zone, the impact on regional the reclamation). groundwater regimes can be profound and lasting. An important feature of the modification is that it is slow and becomes more significant as time goes by. This is because the permeability decreases Modification of groundwater regimes by progressively with time due to gradual consolidation reclamation offill materials and marine mud. Eventually the mud at Figure 1 shows an unconfined coastal the seabed may be so well consolidated that in places groundwater system before and after reclamation. it may essentially off the hydraulic connection Reclamation increases the groundwater flow path (or between the seawater and the groundwater behind the travel time) to the sea and reduces the groundwater reclamation site. Another reason for the time-variant discharge toward the coast. This will cause feature is due to gradual urbanisation over the site. groundwater pressure buildup behind the original More and more buildings and engineering structures coastline. Groundwater may be forced to outcrop in will be constructed at the site. Deep foundations of

Infiltration (a)

Water table Sea

Unconfmed aquifer Submarine groundwater discharge through +--+--­ fracture zone

Infiltration (b)

Heavily urbanized area

Retarded Unconfined aquifer submarine groundwater Reclamation discharge

Fig. 1 -An unconfined aquifer system in a hillside near the coast (a) before and (b) after reclamation.

31 Hong Kong Geologist, 2000, 6, 29-36

the structures will reduce significantly the horizontal unexpectedly, that more groundwater exists on the permeability. The buildings, and pavements, and right-hand side of the island and that there is more the shotcrete over the slopes near the reclamation site SGWD to the coast on the right. will reduce the over vertical permeability. For the simple unconfined aquifer systems in Figure 1 assumes that the groundwater divide Figures 1 and 2, the pore pressure build-up behind the is so far from the coast that the water divide remains slope may not be very significant if extra groundwater unchanged after reclamation. The reclamation has an can discharge as springs at the surface. For a coastal impact only on the groundwater regime of one side of slope consisting of weathered igneous rock in HK, the the hill. When the scale of the reclamation is flow system can be much more complicated. comparable or greater than the size of the original Conventionally, it is believed that the permeability groundwater catchment, as often occurs in Hong Kong, decreases progressively as the depth increases or as the modification can be more significant. As shown in the rock becomes less decomposed (GEO, 1996). Figure 2, since the SGWD to the left after reclamation Groundwater modeling studies in Hong Kong therefore is obstructed by the fill materials, more infiltrated water assumed that superficial soil is an unconfined aquifer will flow to the right-hand side of the hill. In this and that rockhead is an impermeable boundary (Lerner, situation, the reclamation on one side of the hill may 1986; GEO, 1996). Recently it has been demonstrated, cause displacement of the groundwater divide and however, that the permeability distribution in a eventually change the groundwater conditions on the weathered profile may include a relatively high other side of the hill. The process of modification may permeability zone associated with the jointed rock mass occur over years. One day it may be found, near rockhead (Jiao & Malone, 2000) and that in some

Intiltration

Water divide in the middle

Infiltration (b)

Reduced submarine groundwater discharge , · Water divide moves to left

Figure 2 An unconfined aquifer system in an island (a) before and (b) after reclamation.

32 J.J. Jiao

places the rockhead is chair-shaped (Jiao, 2000), as Hong Kong. Some slides are rather shallow and show shown in Figure 3(a). If the toe of such a fracture zone an immediate response to rainfall. This is probably is exposed at the ground surface, or is in good hydraulic caused by quick and local saturation of the superficial communication with seawater in the form ofsubmarine soil in response to heavy rainfall. To understand these discharge, then the pore pressure may dissipate fairly types of shallow requires study of the quickly and there will be no significantly pressure build movement of infiltrated water in unsaturated zones up even during heavy rainfall. If the discharge at the (Anderson, 1983). Some landslides in Hong Kong, toe is significantly obstructed by reclamation, as shown however, exhibit movements well after the peak of the in Figure 3(b), then the groundwater in such a permeable storm, and many are delayed by a few days. The risk zone may be well confined, resulting in high pore levels associated with such delayed failures are high pressures during exceptionally heavy rainfall periods. because delayed failures are unexpected. These kinds oflandslide are usually deep-seated and oflarge scale. Impact ofmodification ofgroundwater regimes Most of the deep-seated landslides are, speculatively, on coastal engineering and environment associated with high pore pressure and complex hydrogeological conditions (GEO, 1993; Jiao eta/., Reclamation and slope stability 1999; Jiao & Malone, 2000). Malone (1999) identified If reclamation is believed to have an impact on ten deep-seated landslides and discussed several the regional groundwater level, it is natural to speculate features of their slope movement. that it may have an adverse effect on slope stability. A re-examination ofthe ten landslides compiled Rainfall is the apparent cause of most landslides in by Malone (1999) shows an interesting aspect: seven

(a)

Grade I Sea (Fresh rock)

(b)

Groundwater level in fracture zone during heavv rainfall , - \ , -- ...... Reclamation ,, +

Fig.- 3 A hillside with a confined.fracture aquifer zone in a slope ofweathered igneous rock (a) lxfore and (b) qfterrec/omation.

33 Hong Kong Geologist, 2000, 6, 29-36

ofthem are located in slopes immediately behind major Evaluation should be made of these environmental and reclamation sites. This suggests that there may be a engineering impacts before a major reclamation project connection between reclamation and deep-seated is carried out, especially when such a project is next to landslides. It appears that some of the failures can be an environmentally sensitive area such as a , explained by Figure 3. For the hydrogeological setting or near . shown in Figure 3, reclamation can affect the Figure 4 illustrates the possible engineering groundwater level in slopes tens of meters above the and environmental impacts of reclamation. After sea level. This will generate hydraulic conditions reclamation, the groundwater will be redistributed and detrimental to slope stability. This hypothesis, the groundwater divide will move toward the reclaimed however, requires more detailed studies. land. Special attention should be paid to the stability of slopes near area "A", since the build-up of the water Reclamation and flooding level may be the most significant of all the slopes It is broadly accepted that reclamation increases around the bay. After reclamation the coastline around the possibility of flooding by modifying the pattern of the Dollar Bay is much shorter and there is only a very surface runoff, but it is not so well known that the small amount of SGWD to the bay. However, the alteration ofgroundwater conditions due to reclamation groundwater discharge to the coast near areas B and C can also have the same effect. Reclamation obstructs will be increased. This will push the interface between groundwater flow to the sea and a part of the sea water and groundwater towards the coast. The groundwater becomes surface water in the form of wetland on the east may become wetter since it has springs at the contact between the reclamation site more groundwater input from its west bank due to and the original soil.

An aquifer behaves like an underground Groundwater reservoir and can receive a certain amount of rainwater. .· · (li;ide ·. After reclamation, the groundwater table is higher and consequently infiltration capacity may be significantly reduced. More rainwater may therefore runoff directly. Meanwhile, the reclamation area is usually heavily urbanized and the surface is covered by concrete materials such as buildings, roads and pavements. The land surface impermeability will lead to weak infiltration (Foster et a/., 1994) and a large portion of rainfall will become surface water directly. Large-scale reclamations may increase the risk of flooding not only in the area around the reclaimed South China Sea site, but also on the other side of a hill by elevating the water level over the entire island, as discussed earlier (a) 2000m

Reclamation and other possible engineering and environment problems Modification of groundwater regimes due to reclamation may cause other engineering and environmental problems. For example, the rise ofwater level will lead to reduction of of foundations. Groundwater may also penetrate the underground concrete and cause corrosion of the steel reinforcement. The increase of water level may cause damp surfaces and superficial damage to the floor of residential buildings (Mahmood & Twigg, 1995). A significant modification of SGWD will surely have an impact on coastal water quality and the marine environment since coastal water and some species may be sensitive to groundwater input. Although the (b) coastal water quality depends on both the surface runoff and SGWD, so far the latter has been ignored in Fig. 4 - Reclamation near environmentally sensitive areas (a) before and (b) after reclamation (the size of arrows studies of coastal water quality in Hong Kong. represents schematically the magnitude ofgroundwater flux).

34 J.J. Jiao

reclamation in the Dollar bay. Whether the impact on its environmental and engineering impacts. The the wetland is positive or negative depends on the concepts and speculations discussed in this paper form groundwater quality. If the groundwater in this region the basis for ongoing research carried out in the is significantly contaminated, the extra groundwater University of Hong Kong. input to the coastal water in areas B or C will surely damage the water quality or ecology ofthe wetland. In Hong Kong there are many poorly-designed Acknowledgement old landfills located near the coast. Some are now under The study is supported by the Committee on restoration. If there is a near area B, it may Research and Conference Grants (CRCG) at the unexpectedly generate more leachate several years University of Hong Kong. after reclamation of the Dollar Bay. It is easy to understand the direct impact ofreclamation around the bay, but it may not be so easy to appreciate the impact References in the neighborhood areas beyond the bay. It may take Anderson, M.G. (1983) Prediction ofsoil suction for years for the modification in the neighborhood areas slopes in Hong Kong, Geotechnical Control to become noticeable. How much time it will take Office, Hong Kong, GCO publication No 1/84, depends on hydrogeological conditions, such as the pp.243. permeability ofthe reclaimed site, the aquifer structures, Bouchard S., Moran, K., Tiwari, M., Wood, D., Bolten, and the process of consolidation of fill and mud. It is A., Eliazar, P., Bjomdal, K. (1998) Effects of highly possible that by the time the impact of the exposed pilings on sea turtle nesting activity at reclamation becomes significant in areas beyond the Melbourne , . Journal ofCoastal bay, people will not link the effects to the reclamation, Research. 14(4), 1343-1347. having forgotten about its construction. ChenZ.Y. & Wang, Z.H. (1999) Delta, China: Taihu -level variation since the 1950s, response to sea-level rise and human impact. Summary Environmental . 37(4), 333-339. This paper discusses the impact of land Erskine, A. D. ( 1991) The effect oftidal fluctuation on reclamation on coastal groundwater regimes, an issue a coastal aquifer in the U. K.. Ground Water. 29 largely ignored in the literature. It is believed that the (4), 556-562. degree of modification of groundwater conditions by Forster, S.S.D., Morris, B. L. & Laurence, A.R. (1994) reclamation depends on the scale of reclamation, the Effects of urbanisation on groundwater permeability of the fill materials, and the recharge. In: Groundwater Problems in Urban hydrogeological structures of the hill slope. Areas (Ed. W B. Wilkinson), Thomas Telford, If reclamation is of a large scale and the London, p. 43-63. permeability ofthe fill is low, reclamation will not only Office ( 1993) Interim Report increase water level in the immediate area around a on Investigation ofthe Failure ofSlopes I 1SE­ reclamation site. It will also move the groundwater DIC 182 & 11 SE-DIC 183 Siu Sai Wan, Hong divide and eventually modify the groundwater Kong, SPR 3/93, Geotechnical Engineering conditions through an entire island or area. Although Office, Civil Engineering Department, Hong the impact of reclamation on the groundwater regime Kong. near a reclamation site may be easily understood, the Geotechnical Engineering Office ( 1996) Report on the influence on coastal areas opposite reclamation sites Shumwan of13'h August 1995, may be ignored. Findings of the Landslide investigation. This paper also suggests that the change of Geotechnical Engineering Office, Hong Kong. groundwater conditions will in turn cause engineering Geotechnical Engineering Office ( 1997) Geotechnical and environmental problems by modifying the Manual for Slopes, Geotechnical Control Office, infiltration capacity, the flooding pattern, the stability Hong Kong, 2rd Edition. of slopes and foundations, the interface between sea Jacob, C. E. (1950) Flow of groundwater. In: water and groundwater, and the SGWD. It is also Engineering Hydraulics (Ed. H. Rouse), John pointed out that the modification of groundwater Wiley, , p. 321-386. regimes due to reclamation is a slow process and may Jiao, J. J. (2000) A confined groundwater zone in occur over years due to the time-dependent soil weathered igneous rocks and its impact on slope consolidation. It is suggested that for a large stability, Submitted to International Symposium on reclamation site, the response of groundwater regime and the Environment to be held in to reclamation should be studied in detail to evaluate , China, Oct 17-20,2000, p. 602-608.

35 Hong Kong Geologist, 2000, 6, 29-36

Jiao, J. J. & Malone, A. (2000) An hypothesis Malone, A.W. (1998) Slope movement and failure: concerning a confined groundwater zone in evidence from field observations oflandslides slopes ofweathered igneous rocks, Symposium associated with hillside cuttings in saprolites on Slope Hazards and Their Prevention, 8-10 in Hong Kong. Proceedings 13th South East May, 2000, Hong Kong, P. R. China, p. 165-170. Asian Geotechnical Conference, Taipei (Eds. Jiao, J. J., Nandy, S. & Malone, A. ( 1999) Siu-Mun Woo and Yu-Tung Chung), Southeast Hydrogeological studies of the slope failure Asian Geotechnical Society, Volume 2, p. 81-90. at Tuen Mun Highway Chainage 550, Hong Marui, A. & Hayashi, T. (2000) Submarine groundwater Kong, in Proceedings of the 2rd International discharge and seawater/freshwater interface in Conference on Landslides, Slope Stability and a volcanic island, north Japan, Western Pacific Safety of Infra-Structures , 26-28 July, 1999 in Geophysics Meeting in , 2000, EOS, . Transactions, American Geophysical Union, Jiao, J. J. & Tang, Z. (1999) An analytical solution of 81(22), WP45-45. groundwater response to tidal fluctuation in a Moore, W. S. (1996) Large groundwater inputs to leaky confined aquifer, Water Resources coastal waters revealed by 226Ra enrichment, Research, 35(3), 747-751. Nature, 380,612-614 Lee, H. D. (1998) Economic value comparison between Poon, S. M. ( 1997) and reclamation impact preservation and agricultural use of coastal on marine environment in Deep Bay, M.Phil. wetlands. Ocean Research, 20(2), 145-152 thesis, University of Hong Kong. Lerner, D.N. ( 1986) Predicting piezometric levels in steep Schofield, J. M., Labern, M.V., Lord, R.M., Massey, M.E. slopes, Groundwater in Engineering Geology, (1992) , England and a developing Geological society, Engineering Geology special nature conservation strategy. Netherlands publication, No.3, London, p. 327-333. Institute for Sea Research, Publication Series, Lumb, P. (1976) Land reclamation in Hong Kong, 20, 105-111. Residential Workshop on Materials and Thomas, D. & Team, H. (2000) Ocean island Methods for Low Coast Road, Rail and as revealed by the Hawaii scientific drilling Reclamation Works, Leura, Australia, project, Western Pacific Geophysics Meeting September 6010, pp 299-314. in Tokyo, 2000, EOS, Transactions, American Lumb, P. (1980) Thirty years of soil engineering in Geophysical Union, 81(22), WP45-45. Hong Kong. In: Selected Topics in Geotechnical Yip, S. Y. (1978) The Ecology ofCoastal Reclamation Engineering (Lumb Volume) (Ed. K. S. Li), in Hong Kong, M.Phil. thesis, University of UniversityofNew South Wales, Canberra, 1991, Hong Kong. pp 18-38. Mahamood, H.R. & Twigg, D. R. (1995) Statistical analysis of water table va