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Estuarine barrages and their influence on groundwater
.• ELSEVIER
Journal of
Hydrology Journal of Hydrology 162 (1994) 247-265
[2]
Estuarine barrages and their influence on groundwater J.W. L l o y d Hydrogeology Research Group, School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Received 17 March 1994; accepted 27 April 1994
Abstract
The construction of a number of low crested barrage embankments across estuaries in the UK is under consideration. The structures will modify the hydrological regime to the advantage of certain infrastructural developments but the regime modifications will have repercussions for both surface water and groundwater that will impact upon the environment. Considering groundwater only, the main impact could be the effect of dampness in properties through groundwater head rise in urban areas. Because of the hydrogeological complexity of many of the urban related estuary areas coupled with the small anticipated rise in heads it is concluded that deterministic numerical modelling predictions of the head change cannot be realistically made and that schemes should proceed by staged impoundment, and compatible remediation where necessary.
1. Introduction
Over the past 30 years various types of engineering structure have been proposed in estuaries in the U K to act as fresh water storage facilities, for land reclamation, hydroelectric power, marinas, etc. As yet, apart from a barrage in Swansea, no major structure has been built, but a number have been subjected to feasibility studies and some are likely to be constructed before the end o f the century. The developments are principally to be in the form of estuary barrages. Those major schemes that have been subject to study or are currently being studied are shown in Fig. 1. The barrages will impound water locally within an estuary and allow some control of water movement. Although only small impoundment levels are envisaged, the subdued topographic setting of most estuaries may cause relatively widespread effects. Further, the position of an estuary at the downstream end o f a hydrological 0022-1694/94/$07.00 © 1994 - Elsevier Science B.V. All fights reserved SSDI 0022-1694(94)02534-1
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J. IV. Lloyd / Journal of Hydrology 162 (1994) 247-265
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Wash - bunding - water supply Fal - barrage Taw/Torridge - barrage S e v e m - barrage - tidal power Newporl - barrage Cardiff- barrage Swansea - barrage (constructed) Conwy - barrage Mersey - barrage Morecambe - barrage and bunding water supply Duddon - barrage Solway - barrage - water supply Stanford Lough - barrage
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catchment means that small water level changes can influence ecological functions back into the catchment, for example, the extent of fish migration. The impacts of the barrages are manifold and are being subjected to considerable scrutiny. Of particular importance is the amount of groundwater rise that will occur as a result of the estuary impoundment. The rise may have detrimental effects on land use, drainage and the integrity of property both in terms of structural safety and dampness. In cost terms, remediation related to groundwater rise may prove substantial, while a virtual blight on property prices and associated development may ensure in areas where the possibility of detrimental conditions developing is perceived prior to construction. In the discussion that follows, the style of impoundment structures envisaged is briefly described and some of the likely general impacts given. The main emphasis is placed on illustrating the complexity of estuary hydrogeology and the difficulty of making sufficiently accurate groundwater forecasts of head changes following impoundment.
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249
2. Types of impoundment structure The variable geometry of the UK estuaries and the various purposes of the proposed impoundment schemes have led to differing designs. Basically two types of impoundment have been considered: (i) bunded embayments (reservoirs) covering part of an estuary; (ii) barrages which completely span an estuary. In Fig. 2, a bunded scheme example considered as one alternative to a barrage in the Wash estuary on the east coast is shown. In Fig. 3, comparative schemes considered for Morecambe Bay on the west coast show combinations of barrages with high level bunded reservoirs. Both the Wash and Morecambe Bay examples relate to impoundment for water supply purposes, with some land reclamation, and are centred upon rural environments. The storage of good quality supply water in estuaries was very much a concern of the 1960s, but gradually lost favour to alternative water resources schemes because of high costs and contamination risks. The possible construction of bunded reservoirs in consequence, has given way to the consideration of barrages that facilitate nonsupply purposes of the type noted on Fig. 1. In Fig. 4, examples from the Cardiff and Merseyside show barrages designed to improve long-established urban-port estuary areas by raising mean estuary water levels permanently and covering unsightly mud-banks, thus attracting housing, light industry and marina developments. The general design concepts are for fill embankments with concrete gated sluices to allow operation and control estuary impounded water levels. Navigation, fish pass and roadway facilities are normally included. A simplified example of an embankment cross-section is given in Fig. 5.
3. General impact While obviously the proponents of estuary impoundments point to major environmental, transport, recreation, flood control and other benefits, there are manifold impacts that as usual are seen as advantageous or disadvantageous to the community and/or ecology. A long catalogue of possible impacts can be cited, but for the present purpose only the main non-hydrogeological general items of interest or controversy need be given. Groundwater impact features are discussed separately below. The construction of a barrage clearly changes the hydrological regime of an estuary and to varying degrees allows management of the regime. Most schemes will radically modify estuary tidal fluctuations and will reduce major flooding by negating the combining effects of high tides and major river runoff events. Impoundment can, however, result in lengthening flood periods for less extreme events and increase tidelock through reduced river head gradients, thus impeding side tributaries and land drainage. While the reduction in extreme event water levels upstream of barrages may be
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J.W. Lloyd / Journal of Hydrology 162 (1994) 247-265
predominantly seen as advantageous, the curtailment of the continual, moderate high tide influences could pose various ecological problems. For example, wetland habitats may change, food sources for certain birds may diminish, fish migration may be altered. Estuarine water quality may be adversely affected by impoundment, although in certain of the estuaries under consideration the current quality characteristics are poor because of agriculture and industrial effluent discharges. Existing pollution problems are principally related to the strong tidal currents and elevated suspended sediment concentrations which create high ambient turbidities. Associated with these are relatively high biological oxygen demands and low dissolved oxygen levels at the head of the tide which can be depressed during summer (McAlpine, 1989). Reduced tidal energy of post-barrage conditions may inhibit and reduce turbidity levels, however increased water residence time in an impoundment could adversely effect water and sediment oxygen demands. Algal carbon studies indicate that in certain impoundments persistent scums and discolouration of the impounded waters could occur. Obviously such adverse conditions may effect the ecology and also reduce recreational opportunities. To alleviate such problems flushing routines will be implemented, which in addition to creating viable pollution conditions will, for example, also control salinity distributions within an impoundment and siltation downstream (Rendel et al., 1991).
4. Groundwater impact Most of the barrages being considered have maximum retention levels of 3-5 m above current mean sea level. As such, the impounded levels are therefore less than the high tidal levels experienced in the estuaries. Impoundment, consequently, will nullify tide-induced groundwater head fluctuations currently experienced in areas immediately bordering the estuaries. The extent of these areas however, is usually small, probably less than 500m either side of an estuary, so that, although the reduction of periodic ground saturation is generally a positive feature, it is of only very local consequence. Obviously, with retained barrage water levels, groundwater heads in ground in hydraulic continuity with the impoundment will adjust to the new equilibrium base level and will rise. Also, the rise will diminish away from the impoundment under the influence of the appertaining groundwater gradient conditions. The groundwater head rise to a new equihbrium may have a number of possible adverse consequences which need hydrogeological consideration. These are discussed below in terms of rural and urban impact. The chief aspects of importance are shown in Table 1. A number of these possible impacts are intimately linked with surface
Fig. 2. Alternative proposals for bunded fresh water storage in the Wash estuary (Binnie and Partners, 1970).
J. IV. Lloyd/Journal of Hydrology 162 (1994) 247-265
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J. IF. Lloyd / Journal of Hydrology 162 (1994) 247-265
253
Table 1 Features that may be adverselyaffectedby groundwaterhead rise Rural
Urban
Ground saturationand groundwaterflooding Drainage Wetlandhabitat Tree growth Flood defences
Ground saturationand groundwaterflooding Drainage and sewage Foundationstability Dampness in buildings Pollutant mobilisation Underground services
water impacts that may be generated by impoundment, and are included in the discussions below. 4.1. Rural impacts
With respect to groundwater head rise, depth to groundwater is the important criterion that will dictate whether or not detrimental conditions may develop. Depending upon topography, therefore, rural areas may be subject to varying degrees of shallow ground saturation and the emergence of groundwater at the surface to cause localised flooding. Such saturation may result in general land use deterioration and cause periodic access difficulties. The condition may also be exacerbated by surface water flooding and reduction in any existing land drainage due to extended tidelock of outfalls. Further, although the increase in head does not relate to an overall increase in flow through the system, local hydraulic conductivity variations may promote groundwater flow concentrations that cannot be adequately accommodated by existing land drains. Alternatively, heads may not be dissipated through existing drainage under the elevated conditions because of local low hydraulic conductivity. Wetlands are sensitive to changes in the hydrological regime, although unfortunately the degree of sensitivity is very poorly understood in ecological terms. In estuaries, many of the wetlands are estuary-edge environments that will be lost, although it is anticipated that similar conditions will develop eventually with impoundment at new locations. Changes in wetland sites away from the estuaries may result in extended or reduced inundation, depending on location, while in some areas new wetlands may emerge. Allied to impact on wetlands is the general impact of increased groundwater head beneath vegetation, particularly trees. Many of the trees that occupy near-estuary positions do so to be close to water so that generally tree growth may not be adversely affected. Trees normally root principally in the capillary zone so that the depth to water can be critical. With rises in the position of this zone they will adjust their rooting depths unless very shallow ground saturation ensues, in which case they may die. As estuaries are naturally subject to periodic flooding, flood defences, normally in the form of levees, are commonplace in rural areas. Post-barrage tidal regimes
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J. IV. Lloyd/Journal of Hydrology 162 (1994) 247-265
will reduce flooding through over-topping of these structures, however, the higher persistent impounded levels may increase wave attack and promote erosion. Where embankments are founded on layered strata increased pore pressures may induce instability. Generally levee integrity is not seen as a problem following impoundment. 4.2. Urban impacts While the impact of groundwater rise is important in rural areas, the main stress in barrage hydrogeological studies is normally placed on urban impact, if significant urban areas are likely to be affected. As with rural areas, shallow ground saturation and groundwater flooding may occur, but prove potentially a more serious problem. Such conditions may develop simply in low topography areas or through the impedance of drainage outfalls, and sewers, which in many urban areas also influence land drainage. With shallow ground saturation and groundwater flooding, foundation stability may be impaired, dampness in buildings may occur, gardens and open areas may become untenable, and pollutants within the shallow ground may be mobilised. Wallace Evans (1988) list the following changes that would require assessment of their importance in estimating the effects on infrastructure with respect to impoundment of the Cardiff barrage: (i) reduction in bearing capacity under foundations and in skin friction of piles; (ii) settlement or swelling of foundation soils; (iii) structural distress due to increased water pressures on tunnels or basement floors and waUs; (iv) increased difficulty with dewatering excavations during construction; (v) surface blow outs if water in a water bearing horizon becomes artesian and the confining soil is too thin to resist uplift; (vi) reduction in slope stability; (vii) natural mineral dissolution. Calculations, however, tend to indicate that in most situations the structural deterioration will be minimal as head changes will be so small. Contaminated land and landfills are a feature of many urban areas so that increased pollution of groundwater and the transmission of such pollution to an estuary following impoundment is a concern. As the unsaturated ground has historically been subject to recharging waters with contaminant leaching to the groundwater, pollution increase may not be substantial. Increased gas generation from landfills could be a transitory problem however, depending upon the age of the landfill. Gas generation in refuse may be modified by the presence of water through confining pressure compressing material, water taking into solution a proportion of the gas produced and restricting decomposition
Fig. 4. (a) Plan of Cardiff barrage prepared by the Cardiff Bay Development Corporation. (b) Plan of a barrage proposed for the River Mersey provided by the Mersey Barrage Company, and Rendle Parkman consulting engineers.
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Fig. 6. Suggested methane generation response in a landfill subjected to groundwater rise following impoundment (after Wallace Evans, 1988).
that could normally occur in the unsaturated zone. Initially gas displacement by rising groundwater heads may increase gas production but with saturation the long-term generation is likely to proportionally decrease as shown in Fig. 6. The majority of the possible impacts identified above are likely to be localised with the exception of dampness in buildings and particularly in residential area basements. Because of the regionality of the groundwater head rise beneath long established urban areas, some dating from before the First World War, dampness may pose the main hydrogeological concern. However, as basement and footings depths vary greatly, dampness is likely to be very variably distributed and therefore very local. In Table 2, an indication of the extent of the possible risk to property is given for a preliminary basement assessment from a barrage study. Possible zones of groundwater rise have been identified and selected properties surveyed with adjacent property conditions estimated. As c a n be seen the number of basements that it is suggested could be affected is considerable.
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J. Itl. Lloyd / Journal of Hydrology 162 (1994) 247-265
Table 2 An example of a preliminary assessment of barrage-induced groundwater head rise affecting basements (after Wallace Evans, 1988)
Selected and surveyed Estimated more severe Estimated less severe
Basements at risk from increased dampness or wetness
Basements at risk from flooding
Zone
Zone
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81 0 590
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Head rise in Zone 1 > 2 > 3. Severeis relative to selected. 5. H y d r o g e o l o g i c a l a s s e s s m e n t difficulties
While the possible detrimental impacts o f any groundwater head rise can be defined in concept, the difficulty is to determine quantitatively the degree o f any o f the impacts. This fundamentally depends upon accurate prediction of the head rise. Estuarine barrage impoundments are unlike normal dam impoundments in that the impounded water will almost exclusively be contained within the long standing estuary/river banks. Head rise impact therefore relates to topography away from the estuary in terms of depth to groundwater, as noted above. A 3 m impounded level may give rise to a 3 m rise in groundwater head immediately adjacent to the estuary but the depth to water will diminish with rising land gradient to the valley sides. Further, the imposed impoundment level effect will decrease up the valley with increasing river gradient and consequently the adjacent groundwater head rise impact will also be significantly less. For the 3 m impoundment example therefore, most depth to water changes are unlikely to exceed 2 m and normally will be significantly less (i.e. < 1 m). In order to put this order o f groundwater rise into perspective the general hydrogeological environment also needs to be considered. 5.1. Types o f ground
At the conclusion of the various Pleistocene glaciations the global rise in sea level resulted in a submerged glacial-period coastline around the UK, typified by estuaries. Such estuaries form local discharge zones for the hinterland groundwater. In most of the estuaries, possibly with the exception o f the Mersey, the groundwater systems however, are complex because of the glacial-estuarine geology, and poorly defined because they are not important for groundwater resources. Imposed upon the natural complexity are man-made effects particularly, but not exclusively, in urban areas. As a representative illustration, the hydrogeological situation at Newport on the River Usk, where a 3.5m barrage impoundment is contemplated, may be cited.
J.W. Lloyd/Journal of Hydrology 162 (1994) 247-265
259
The hills surrounding the town and underlying the estuary at depth are composed of Triassic to Liassic marlstones with thin impersistent interbedded sandstones. Both the marlstones and the sandstones are variably deformed, fractured and weathered. Groundwater heads in the sequence are locally above mean sea level, directly below the estuary, indicating the 'sink' control. In response to Pleistocene base levels the Usk valley was eroded with a proto-valley aligned away from the current riverestuary position. Associated with the formation of this valley and subsequent base level controls are variable thickness scree deposits overlying the marlstone-sandstone sequence, but composed of variable grade fragments from that sequence, in a sandy, silty clayey matrix. With rising sea levels sedimentation occurred in the estuary initially resulting in localised impersistent gravel and sand infilling of the valley courses, but progressively finer grained materials were deposited under fluviatile meandering river conditions. The fine grained sediments dominate the current estuary extending some distance upstream into the main rural valley area, which is likely to be influenced by the impoundment. The estuarine alluvial sequence exhibits complex facies variations of clays, silts and sands with occasional peats. Continuing with Newport as the example, alluvial fiats evolved following the estuarine deposition. These fiats were traversed with small side streams to the river/ estuary and locally marshes developed. The relatively low elevation coupled with the overall low permeability of the estuarine deposits undoubtedly contributed to very wet shallow ground conditions, possibly permanently, but certainly during winter periods. The first man-made developments in the area pertinent to the hydrogeology, are believed to have been drainage ditches constructed to drain fields for cultivation. Many of these may have been realignments of naturally occurring water courses. These agricultural modifications took place prior to urban development but probably continued as the latter expanded. The urban expansion, from a small medieval town, accelerated in the industrial revolution with the development of the South Wales coalfields. To counter the wetground conditions, urban drainage was constructed and made-ground emplaced so that dry construction could be carried out. The fill emplaced was obtained from the coalfields and probably any other available wastes. While the natural units pose considerable definition problems, the dominating hydrogeological factor in the urban areas, and partly in rural areas, is the madeground conditions. For the purposes of this paper made-ground incorporates both the materials deposited and the utilities that tend to be incorporated in the manmade ground. As urban development continued into modern times more made-ground was emplaced using a variety of materials; mains water supplies and drainage were installed. Mains sewerage was also constructed, which undoubtedly acts as a secondary drainage path for groundwater. In Fig. 7, a conceptual diagram of the type of made-ground encountered and natural geology is shown. In rural areas the main comparable man-made influences on hydrogeology are stream canalisation, field drain installation and levees.
J. IV. Lloyd / Journal of Hydrology 162 (1994) 247-265
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Fig. 7. Conceptual diagram example from an estuary area showing geology and the features of madeground in which groundwater will rise following a barrage impoundment.
5.2. Heads, hydraulic conductivities and groundwaterflows Considering the type of natural and man-made ground described above, any groundwater head distribution within such a system is complex under a variety of influences, for example: (i) fades differences within the sequence; (ii) marked variations in both horizontal and vertical hydraulic conductivity of the sequence; (iii) loealised positive/negative head relations with the underlying gravels and sands; (iv) variable head relationships with the underlying bedrock-sequence; (v) variable influences of overlying made-ground. It must also be remembered that such a groundwater system is subject to the
J.W. Lloyd/Journal of Hydrology 162 (1994) 247-265
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Table 3 Hydraulic conductivities determined in the Newport barrage study (m s-~) Geological unit
Hydraulic conductivity
Made-ground Estuarine alluvium Gravel Marl
10-1 _ 10-1° 10-6-10 -9 10-4-I0-7 10-5-10 -7
transitory effects of the high groundwater gradient with its significant vertical component, related to the estuary 'sink' system and regional recharge. While clearly there are few problems in technically measuring groundwater levels in such a sequence as described, there are considerable problems in relating them coherently on an areal basis and therefore with respect to gradients. This is further complicated when groundwater flows are considered and hydraulic conductivity examined. Again techniques for the determination of the latter parameter are obviously available but for an accurate assessment an extremely large and economically unacceptable data set would be required to cover the range. Because of the nature of the ground, pumping test techniques can only be applied to a limited extent and often provide data that are difficult to analyse, so that 'slug-type' techniques and laboratory tests have to be relied upon. For any proper assessment both horizontal and vertical hydraulic conductivity should really be determined. To give an indication of the range found in this parameter in Newport the test results are included in Table 3. It is emphasised that the hydraulic conductivities listed for made-ground in Table 3 only demonstrate in part the hydraulic complexity. The large number of utilities of a range of sizes, such as drains and sewers (Fig. 7), provide a network of groundwater flow paths that locally approach infinite hydraulic conductivity. The influence of drains etc. on groundwater flows is illustrated by groundwater hydrographs located in their vicinity which show heads constrained to a 'cut-off' level irrespective of precipitation (Fig. 8). With the head difficulties and the range of hydraulic conductivities shown, the determination of groundwater flows other than on a very average basis poses problems. Of particular importance is the made-ground variability which tends to dominate some situations because it receives and discharges the bulk of the recharge. The bulk of the recharge in the rural areas can possibly be assessed using conventional soil moisture balance techniques. In the urban areas soil moisture balances can only be applied very locally, if at all, so that any assessment of naturally occurring recharge is completely compromised by man-made effects. Added to this natural recharge are losses from water supply mains and sewage mains. For supply mains losses very local pilot area assessment data may be available but usually only bulk loss figures for large urban areas can be obtained. Sewer losses are equally non-definitive. In consequence, any understanding of the total recharge in the urban areas on a proper distributed basis, that would assist in the evaluation of the groundwater flows, is unlikely to be forthcoming.
J. IV. Lloyd / Journal of Hydrology 162 (1994) 247-265
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Table 4 An example of groundwater model results from an urban area of made-ground (m OD). Adjacent data set based on a 100m × 100m grid Ground level and measured head compared with modelled head
Example set of nodal areas from model grid
Minimum ground level Measured groundwater heada Modelled groundwater head
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aRepresentative. In conventional groundwater resources assessments, and indeed in m a n y other groundwater studies, distributed numerical modelling is a well-proved means of forecasting likely changes in regime. Unfortunately in most of the anticipated barrage locations such an approach is unlikely to be reliable. The problem clearly lies in the extreme difficulty of establishing an accurate distributed data base, as exemplified above for head, hydraulic conductivity and recharge, that will allow a sufficiently accurate model validation in the context o f the very small groundwater head rises that will occur. Without d o u b t most of the head rises will be well within model error. While in groundwater terms, the rises anticipated from barrage construction may be small, with respect to property even a small rise m a y be detrimental particularly in the urban made-ground where the largest inaccuracy in any modelling assessment will occur. To exemplify something of the difficulty, a data set from an urban barrage study area is given in Table 4. The data are related to a model of made-ground and illustrate firstly, the very small heads involved, and secondly, the degree of inaccuracy in model representation. The table also indicates the extent of data requirement for any model control. In a groundwater resources study the head representation in Table 4 m a y be acceptable, but for an urban property vulnerability assessment it clearly is not. For some model areas, groundwater heads are shown above ground level (in calibration before impoundment) while modelled head representation error ranges between - 1.5 and + l . 9 m with respect to the measured groundwater head. The errors are o f the same magnitude or greater than most o f the rise that can occur.
6. An assessment approach Unfortunately, it must be concluded that conventional distributed, deterministic,
264
J.W. Lloyd / Journal of Hydrology 162 (1994) 247-265
numerical modelling cannot realistically be used to adequately describe the groundwater rise anticipated following barrage impoundment in the type of complex hydrogeological environment being discussed. Further, the large range in hydrogeological parameter values (e.g. see Table 3), particularly where a dominating top-layer of made-ground is present, does not realistically permit confidence in a stochastic modelling evaluation, also in view of the very small head changes envisaged. For barrage schemes to progress therefore subjective hydrogeological judgement coupled with effective engineering is the practical way forward. The following course of action is proposed: (i) the classical ground geometry is established through drilling etc.; (ii) a minimum depth to groundwater assessment is made based upon at least one hydrological year's data, preferably longer; (iii) vulnerability criteria are established on the depth to groundwater under prebarrage conditions, such as: (a) all areas where the minimum recorded depth to groundwater is less than say 1 m are vulnerable; (b) all areas of low permeability ground where the minimum recorded depth to groundwater is less than say 2 m and made-ground is absent are vulnerable; (c) all areas where the minimum recorded depth to groundwater is less than say 2 m and hydrographs show no 'cut-off" constraint with recharge (precipitation) variation are vulnerable; (iv) the vulnerable areas are delineated and a compensational and remedial action agreed; (v) the impoundment is staged; (vi) a comprehensive monitoring of heads is carried out pre-barrage, during construction and post-barrage. Clearly, there will be some overlap in the vulnerability criteria suggested (iii, a-c) and naturally the type of criteria applicable will vary from site to site. The essential data are representative minimum depths to water pre-barrage, preferably for a major (extreme) recharge event. Because of the impossibility of adequately predicting the head rise, the staging, which is the norm in most dam commissioning, will permit the actual rise to be monitored under manageable conditions. The converse, of full impoundment followed by drawdown, should significant problems arise, is dangerous because heterogeneity in the made-ground may permit rising groundwater to enter a 'perched' feature (concave upwards) but not allow it to readily dewater. Staging importantly will permit progressive detailed identification of any vulnerable areas and time to implement remedial measures. The approach proposed above is very dependent upon comprehensive legislation and administrative support, and also upon sensible remedial measures. These aspects are outside the scope of the paper, although land drainage networks, both in urban and rural areas, are the preferred remedial action coupled in some rural areas with levees. Interestingly, for the Cardiff barrage a groundwater pumping scheme abstracting
J.W. Lloyd / Journal of Hydrology 162 (1994) 247-265
265
from gravels underlying estuarine alluvium and made-ground has been proposed as one possible remedial way of depressing post-barrage groundwater heads (Wallace Evans, 1992) in a vertically conductive system.
7. Conclusions
The construction of barrages in estuaries in the UK will cause environmental impacts which in groundwater and economic terms will probably mostly relate to the development of dampness in residential and industrial properties through small rises in groundwater head. The prediction of the likely amount and extent of the groundwater rise is fraught with difficulty because of the extreme hydrogeological complexity of many estuary areas, particularly when taken in the context of anticipated rises of less than 1-2 m. While the natural ground may be complex one of the overriding hydraulic influences in the urban areas is made-ground which is extremely heterogeneous. It is considered that the groundwater rise cannot be sensibly predicted using distributed modelling techniques and that for schemes to advance responsibly a staged impoundment policy should be invoked.
References Binnie and Partners, 1970. Wash desk study report. Report to Water Resources Board, Reading, UK, 153 pp. McAlpine, R., 1989. An initial review of possible changes in drainage and groundwater regimes and their implications associated with the construction of the Severn barrage. Severn Barrage Development Project, Hayes, UK, 26 pp. Rendle, Palmer and Tritton, 1991. River Usk barrage at Newport. Groundwater study. Report to Newport Borough Council, Newport, UK, 33 pp. Wallace Evans, 1988. Cardiff Bay barrage feasibilitystudy. Groundwater investigationsstage III. Report to South Glamorgan County Council, Cardiff, UK, 61 pp. Wallace Evans, 1992. Study into control of rise in groundwater by well dewatering. Cardiff Bay Barrage Groundwater Investigations. Report to Cardiff Bay Development Corporation, Cardiff, UK, 30 pp. Water Resources Board, 1966. Solway barrage. WRB Pub. No. 3. HMSO, London, 30 pp. Water Resources Board, 1972. Morecombe Bay: estuary storage• WRB Pub. No. 12. HMSO, London, 46 pp.
Journal of
Hydrology Journal of Hydrology 162 (1994) 247-265
[2]
Estuarine barrages and their influence on groundwater J.W. L l o y d Hydrogeology Research Group, School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Received 17 March 1994; accepted 27 April 1994
Abstract
The construction of a number of low crested barrage embankments across estuaries in the UK is under consideration. The structures will modify the hydrological regime to the advantage of certain infrastructural developments but the regime modifications will have repercussions for both surface water and groundwater that will impact upon the environment. Considering groundwater only, the main impact could be the effect of dampness in properties through groundwater head rise in urban areas. Because of the hydrogeological complexity of many of the urban related estuary areas coupled with the small anticipated rise in heads it is concluded that deterministic numerical modelling predictions of the head change cannot be realistically made and that schemes should proceed by staged impoundment, and compatible remediation where necessary.
1. Introduction
Over the past 30 years various types of engineering structure have been proposed in estuaries in the U K to act as fresh water storage facilities, for land reclamation, hydroelectric power, marinas, etc. As yet, apart from a barrage in Swansea, no major structure has been built, but a number have been subjected to feasibility studies and some are likely to be constructed before the end o f the century. The developments are principally to be in the form of estuary barrages. Those major schemes that have been subject to study or are currently being studied are shown in Fig. 1. The barrages will impound water locally within an estuary and allow some control of water movement. Although only small impoundment levels are envisaged, the subdued topographic setting of most estuaries may cause relatively widespread effects. Further, the position of an estuary at the downstream end o f a hydrological 0022-1694/94/$07.00 © 1994 - Elsevier Science B.V. All fights reserved SSDI 0022-1694(94)02534-1
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J. IV. Lloyd / Journal of Hydrology 162 (1994) 247-265
1 2
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catchment means that small water level changes can influence ecological functions back into the catchment, for example, the extent of fish migration. The impacts of the barrages are manifold and are being subjected to considerable scrutiny. Of particular importance is the amount of groundwater rise that will occur as a result of the estuary impoundment. The rise may have detrimental effects on land use, drainage and the integrity of property both in terms of structural safety and dampness. In cost terms, remediation related to groundwater rise may prove substantial, while a virtual blight on property prices and associated development may ensure in areas where the possibility of detrimental conditions developing is perceived prior to construction. In the discussion that follows, the style of impoundment structures envisaged is briefly described and some of the likely general impacts given. The main emphasis is placed on illustrating the complexity of estuary hydrogeology and the difficulty of making sufficiently accurate groundwater forecasts of head changes following impoundment.
J.W. Lloyd/Journal of Hydrology 162 (1994) 247-265
249
2. Types of impoundment structure The variable geometry of the UK estuaries and the various purposes of the proposed impoundment schemes have led to differing designs. Basically two types of impoundment have been considered: (i) bunded embayments (reservoirs) covering part of an estuary; (ii) barrages which completely span an estuary. In Fig. 2, a bunded scheme example considered as one alternative to a barrage in the Wash estuary on the east coast is shown. In Fig. 3, comparative schemes considered for Morecambe Bay on the west coast show combinations of barrages with high level bunded reservoirs. Both the Wash and Morecambe Bay examples relate to impoundment for water supply purposes, with some land reclamation, and are centred upon rural environments. The storage of good quality supply water in estuaries was very much a concern of the 1960s, but gradually lost favour to alternative water resources schemes because of high costs and contamination risks. The possible construction of bunded reservoirs in consequence, has given way to the consideration of barrages that facilitate nonsupply purposes of the type noted on Fig. 1. In Fig. 4, examples from the Cardiff and Merseyside show barrages designed to improve long-established urban-port estuary areas by raising mean estuary water levels permanently and covering unsightly mud-banks, thus attracting housing, light industry and marina developments. The general design concepts are for fill embankments with concrete gated sluices to allow operation and control estuary impounded water levels. Navigation, fish pass and roadway facilities are normally included. A simplified example of an embankment cross-section is given in Fig. 5.
3. General impact While obviously the proponents of estuary impoundments point to major environmental, transport, recreation, flood control and other benefits, there are manifold impacts that as usual are seen as advantageous or disadvantageous to the community and/or ecology. A long catalogue of possible impacts can be cited, but for the present purpose only the main non-hydrogeological general items of interest or controversy need be given. Groundwater impact features are discussed separately below. The construction of a barrage clearly changes the hydrological regime of an estuary and to varying degrees allows management of the regime. Most schemes will radically modify estuary tidal fluctuations and will reduce major flooding by negating the combining effects of high tides and major river runoff events. Impoundment can, however, result in lengthening flood periods for less extreme events and increase tidelock through reduced river head gradients, thus impeding side tributaries and land drainage. While the reduction in extreme event water levels upstream of barrages may be
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J.W. Lloyd / Journal of Hydrology 162 (1994) 247-265
predominantly seen as advantageous, the curtailment of the continual, moderate high tide influences could pose various ecological problems. For example, wetland habitats may change, food sources for certain birds may diminish, fish migration may be altered. Estuarine water quality may be adversely affected by impoundment, although in certain of the estuaries under consideration the current quality characteristics are poor because of agriculture and industrial effluent discharges. Existing pollution problems are principally related to the strong tidal currents and elevated suspended sediment concentrations which create high ambient turbidities. Associated with these are relatively high biological oxygen demands and low dissolved oxygen levels at the head of the tide which can be depressed during summer (McAlpine, 1989). Reduced tidal energy of post-barrage conditions may inhibit and reduce turbidity levels, however increased water residence time in an impoundment could adversely effect water and sediment oxygen demands. Algal carbon studies indicate that in certain impoundments persistent scums and discolouration of the impounded waters could occur. Obviously such adverse conditions may effect the ecology and also reduce recreational opportunities. To alleviate such problems flushing routines will be implemented, which in addition to creating viable pollution conditions will, for example, also control salinity distributions within an impoundment and siltation downstream (Rendel et al., 1991).
4. Groundwater impact Most of the barrages being considered have maximum retention levels of 3-5 m above current mean sea level. As such, the impounded levels are therefore less than the high tidal levels experienced in the estuaries. Impoundment, consequently, will nullify tide-induced groundwater head fluctuations currently experienced in areas immediately bordering the estuaries. The extent of these areas however, is usually small, probably less than 500m either side of an estuary, so that, although the reduction of periodic ground saturation is generally a positive feature, it is of only very local consequence. Obviously, with retained barrage water levels, groundwater heads in ground in hydraulic continuity with the impoundment will adjust to the new equilibrium base level and will rise. Also, the rise will diminish away from the impoundment under the influence of the appertaining groundwater gradient conditions. The groundwater head rise to a new equihbrium may have a number of possible adverse consequences which need hydrogeological consideration. These are discussed below in terms of rural and urban impact. The chief aspects of importance are shown in Table 1. A number of these possible impacts are intimately linked with surface
Fig. 2. Alternative proposals for bunded fresh water storage in the Wash estuary (Binnie and Partners, 1970).
J. IV. Lloyd/Journal of Hydrology 162 (1994) 247-265
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J. IF. Lloyd / Journal of Hydrology 162 (1994) 247-265
253
Table 1 Features that may be adverselyaffectedby groundwaterhead rise Rural
Urban
Ground saturationand groundwaterflooding Drainage Wetlandhabitat Tree growth Flood defences
Ground saturationand groundwaterflooding Drainage and sewage Foundationstability Dampness in buildings Pollutant mobilisation Underground services
water impacts that may be generated by impoundment, and are included in the discussions below. 4.1. Rural impacts
With respect to groundwater head rise, depth to groundwater is the important criterion that will dictate whether or not detrimental conditions may develop. Depending upon topography, therefore, rural areas may be subject to varying degrees of shallow ground saturation and the emergence of groundwater at the surface to cause localised flooding. Such saturation may result in general land use deterioration and cause periodic access difficulties. The condition may also be exacerbated by surface water flooding and reduction in any existing land drainage due to extended tidelock of outfalls. Further, although the increase in head does not relate to an overall increase in flow through the system, local hydraulic conductivity variations may promote groundwater flow concentrations that cannot be adequately accommodated by existing land drains. Alternatively, heads may not be dissipated through existing drainage under the elevated conditions because of local low hydraulic conductivity. Wetlands are sensitive to changes in the hydrological regime, although unfortunately the degree of sensitivity is very poorly understood in ecological terms. In estuaries, many of the wetlands are estuary-edge environments that will be lost, although it is anticipated that similar conditions will develop eventually with impoundment at new locations. Changes in wetland sites away from the estuaries may result in extended or reduced inundation, depending on location, while in some areas new wetlands may emerge. Allied to impact on wetlands is the general impact of increased groundwater head beneath vegetation, particularly trees. Many of the trees that occupy near-estuary positions do so to be close to water so that generally tree growth may not be adversely affected. Trees normally root principally in the capillary zone so that the depth to water can be critical. With rises in the position of this zone they will adjust their rooting depths unless very shallow ground saturation ensues, in which case they may die. As estuaries are naturally subject to periodic flooding, flood defences, normally in the form of levees, are commonplace in rural areas. Post-barrage tidal regimes
254
J. IV. Lloyd/Journal of Hydrology 162 (1994) 247-265
will reduce flooding through over-topping of these structures, however, the higher persistent impounded levels may increase wave attack and promote erosion. Where embankments are founded on layered strata increased pore pressures may induce instability. Generally levee integrity is not seen as a problem following impoundment. 4.2. Urban impacts While the impact of groundwater rise is important in rural areas, the main stress in barrage hydrogeological studies is normally placed on urban impact, if significant urban areas are likely to be affected. As with rural areas, shallow ground saturation and groundwater flooding may occur, but prove potentially a more serious problem. Such conditions may develop simply in low topography areas or through the impedance of drainage outfalls, and sewers, which in many urban areas also influence land drainage. With shallow ground saturation and groundwater flooding, foundation stability may be impaired, dampness in buildings may occur, gardens and open areas may become untenable, and pollutants within the shallow ground may be mobilised. Wallace Evans (1988) list the following changes that would require assessment of their importance in estimating the effects on infrastructure with respect to impoundment of the Cardiff barrage: (i) reduction in bearing capacity under foundations and in skin friction of piles; (ii) settlement or swelling of foundation soils; (iii) structural distress due to increased water pressures on tunnels or basement floors and waUs; (iv) increased difficulty with dewatering excavations during construction; (v) surface blow outs if water in a water bearing horizon becomes artesian and the confining soil is too thin to resist uplift; (vi) reduction in slope stability; (vii) natural mineral dissolution. Calculations, however, tend to indicate that in most situations the structural deterioration will be minimal as head changes will be so small. Contaminated land and landfills are a feature of many urban areas so that increased pollution of groundwater and the transmission of such pollution to an estuary following impoundment is a concern. As the unsaturated ground has historically been subject to recharging waters with contaminant leaching to the groundwater, pollution increase may not be substantial. Increased gas generation from landfills could be a transitory problem however, depending upon the age of the landfill. Gas generation in refuse may be modified by the presence of water through confining pressure compressing material, water taking into solution a proportion of the gas produced and restricting decomposition
Fig. 4. (a) Plan of Cardiff barrage prepared by the Cardiff Bay Development Corporation. (b) Plan of a barrage proposed for the River Mersey provided by the Mersey Barrage Company, and Rendle Parkman consulting engineers.
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Fig. 6. Suggested methane generation response in a landfill subjected to groundwater rise following impoundment (after Wallace Evans, 1988).
that could normally occur in the unsaturated zone. Initially gas displacement by rising groundwater heads may increase gas production but with saturation the long-term generation is likely to proportionally decrease as shown in Fig. 6. The majority of the possible impacts identified above are likely to be localised with the exception of dampness in buildings and particularly in residential area basements. Because of the regionality of the groundwater head rise beneath long established urban areas, some dating from before the First World War, dampness may pose the main hydrogeological concern. However, as basement and footings depths vary greatly, dampness is likely to be very variably distributed and therefore very local. In Table 2, an indication of the extent of the possible risk to property is given for a preliminary basement assessment from a barrage study. Possible zones of groundwater rise have been identified and selected properties surveyed with adjacent property conditions estimated. As c a n be seen the number of basements that it is suggested could be affected is considerable.
258
J. Itl. Lloyd / Journal of Hydrology 162 (1994) 247-265
Table 2 An example of a preliminary assessment of barrage-induced groundwater head rise affecting basements (after Wallace Evans, 1988)
Selected and surveyed Estimated more severe Estimated less severe
Basements at risk from increased dampness or wetness
Basements at risk from flooding
Zone
Zone
Total
1
2
3
1
2
3
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81 0 590
43 57 102
568 979 48
590 671 0
102 345 0
1641 2056 1148
Head rise in Zone 1 > 2 > 3. Severeis relative to selected. 5. H y d r o g e o l o g i c a l a s s e s s m e n t difficulties
While the possible detrimental impacts o f any groundwater head rise can be defined in concept, the difficulty is to determine quantitatively the degree o f any o f the impacts. This fundamentally depends upon accurate prediction of the head rise. Estuarine barrage impoundments are unlike normal dam impoundments in that the impounded water will almost exclusively be contained within the long standing estuary/river banks. Head rise impact therefore relates to topography away from the estuary in terms of depth to groundwater, as noted above. A 3 m impounded level may give rise to a 3 m rise in groundwater head immediately adjacent to the estuary but the depth to water will diminish with rising land gradient to the valley sides. Further, the imposed impoundment level effect will decrease up the valley with increasing river gradient and consequently the adjacent groundwater head rise impact will also be significantly less. For the 3 m impoundment example therefore, most depth to water changes are unlikely to exceed 2 m and normally will be significantly less (i.e. < 1 m). In order to put this order o f groundwater rise into perspective the general hydrogeological environment also needs to be considered. 5.1. Types o f ground
At the conclusion of the various Pleistocene glaciations the global rise in sea level resulted in a submerged glacial-period coastline around the UK, typified by estuaries. Such estuaries form local discharge zones for the hinterland groundwater. In most of the estuaries, possibly with the exception o f the Mersey, the groundwater systems however, are complex because of the glacial-estuarine geology, and poorly defined because they are not important for groundwater resources. Imposed upon the natural complexity are man-made effects particularly, but not exclusively, in urban areas. As a representative illustration, the hydrogeological situation at Newport on the River Usk, where a 3.5m barrage impoundment is contemplated, may be cited.
J.W. Lloyd/Journal of Hydrology 162 (1994) 247-265
259
The hills surrounding the town and underlying the estuary at depth are composed of Triassic to Liassic marlstones with thin impersistent interbedded sandstones. Both the marlstones and the sandstones are variably deformed, fractured and weathered. Groundwater heads in the sequence are locally above mean sea level, directly below the estuary, indicating the 'sink' control. In response to Pleistocene base levels the Usk valley was eroded with a proto-valley aligned away from the current riverestuary position. Associated with the formation of this valley and subsequent base level controls are variable thickness scree deposits overlying the marlstone-sandstone sequence, but composed of variable grade fragments from that sequence, in a sandy, silty clayey matrix. With rising sea levels sedimentation occurred in the estuary initially resulting in localised impersistent gravel and sand infilling of the valley courses, but progressively finer grained materials were deposited under fluviatile meandering river conditions. The fine grained sediments dominate the current estuary extending some distance upstream into the main rural valley area, which is likely to be influenced by the impoundment. The estuarine alluvial sequence exhibits complex facies variations of clays, silts and sands with occasional peats. Continuing with Newport as the example, alluvial fiats evolved following the estuarine deposition. These fiats were traversed with small side streams to the river/ estuary and locally marshes developed. The relatively low elevation coupled with the overall low permeability of the estuarine deposits undoubtedly contributed to very wet shallow ground conditions, possibly permanently, but certainly during winter periods. The first man-made developments in the area pertinent to the hydrogeology, are believed to have been drainage ditches constructed to drain fields for cultivation. Many of these may have been realignments of naturally occurring water courses. These agricultural modifications took place prior to urban development but probably continued as the latter expanded. The urban expansion, from a small medieval town, accelerated in the industrial revolution with the development of the South Wales coalfields. To counter the wetground conditions, urban drainage was constructed and made-ground emplaced so that dry construction could be carried out. The fill emplaced was obtained from the coalfields and probably any other available wastes. While the natural units pose considerable definition problems, the dominating hydrogeological factor in the urban areas, and partly in rural areas, is the madeground conditions. For the purposes of this paper made-ground incorporates both the materials deposited and the utilities that tend to be incorporated in the manmade ground. As urban development continued into modern times more made-ground was emplaced using a variety of materials; mains water supplies and drainage were installed. Mains sewerage was also constructed, which undoubtedly acts as a secondary drainage path for groundwater. In Fig. 7, a conceptual diagram of the type of made-ground encountered and natural geology is shown. In rural areas the main comparable man-made influences on hydrogeology are stream canalisation, field drain installation and levees.
J. IV. Lloyd / Journal of Hydrology 162 (1994) 247-265
260
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Fig. 7. Conceptual diagram example from an estuary area showing geology and the features of madeground in which groundwater will rise following a barrage impoundment.
5.2. Heads, hydraulic conductivities and groundwaterflows Considering the type of natural and man-made ground described above, any groundwater head distribution within such a system is complex under a variety of influences, for example: (i) fades differences within the sequence; (ii) marked variations in both horizontal and vertical hydraulic conductivity of the sequence; (iii) loealised positive/negative head relations with the underlying gravels and sands; (iv) variable head relationships with the underlying bedrock-sequence; (v) variable influences of overlying made-ground. It must also be remembered that such a groundwater system is subject to the
J.W. Lloyd/Journal of Hydrology 162 (1994) 247-265
261
Table 3 Hydraulic conductivities determined in the Newport barrage study (m s-~) Geological unit
Hydraulic conductivity
Made-ground Estuarine alluvium Gravel Marl
10-1 _ 10-1° 10-6-10 -9 10-4-I0-7 10-5-10 -7
transitory effects of the high groundwater gradient with its significant vertical component, related to the estuary 'sink' system and regional recharge. While clearly there are few problems in technically measuring groundwater levels in such a sequence as described, there are considerable problems in relating them coherently on an areal basis and therefore with respect to gradients. This is further complicated when groundwater flows are considered and hydraulic conductivity examined. Again techniques for the determination of the latter parameter are obviously available but for an accurate assessment an extremely large and economically unacceptable data set would be required to cover the range. Because of the nature of the ground, pumping test techniques can only be applied to a limited extent and often provide data that are difficult to analyse, so that 'slug-type' techniques and laboratory tests have to be relied upon. For any proper assessment both horizontal and vertical hydraulic conductivity should really be determined. To give an indication of the range found in this parameter in Newport the test results are included in Table 3. It is emphasised that the hydraulic conductivities listed for made-ground in Table 3 only demonstrate in part the hydraulic complexity. The large number of utilities of a range of sizes, such as drains and sewers (Fig. 7), provide a network of groundwater flow paths that locally approach infinite hydraulic conductivity. The influence of drains etc. on groundwater flows is illustrated by groundwater hydrographs located in their vicinity which show heads constrained to a 'cut-off' level irrespective of precipitation (Fig. 8). With the head difficulties and the range of hydraulic conductivities shown, the determination of groundwater flows other than on a very average basis poses problems. Of particular importance is the made-ground variability which tends to dominate some situations because it receives and discharges the bulk of the recharge. The bulk of the recharge in the rural areas can possibly be assessed using conventional soil moisture balance techniques. In the urban areas soil moisture balances can only be applied very locally, if at all, so that any assessment of naturally occurring recharge is completely compromised by man-made effects. Added to this natural recharge are losses from water supply mains and sewage mains. For supply mains losses very local pilot area assessment data may be available but usually only bulk loss figures for large urban areas can be obtained. Sewer losses are equally non-definitive. In consequence, any understanding of the total recharge in the urban areas on a proper distributed basis, that would assist in the evaluation of the groundwater flows, is unlikely to be forthcoming.
J. IV. Lloyd / Journal of Hydrology 162 (1994) 247-265
262
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Table 4 An example of groundwater model results from an urban area of made-ground (m OD). Adjacent data set based on a 100m × 100m grid Ground level and measured head compared with modelled head
Example set of nodal areas from model grid
Minimum ground level Measured groundwater heada Modelled groundwater head
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aRepresentative. In conventional groundwater resources assessments, and indeed in m a n y other groundwater studies, distributed numerical modelling is a well-proved means of forecasting likely changes in regime. Unfortunately in most of the anticipated barrage locations such an approach is unlikely to be reliable. The problem clearly lies in the extreme difficulty of establishing an accurate distributed data base, as exemplified above for head, hydraulic conductivity and recharge, that will allow a sufficiently accurate model validation in the context o f the very small groundwater head rises that will occur. Without d o u b t most of the head rises will be well within model error. While in groundwater terms, the rises anticipated from barrage construction may be small, with respect to property even a small rise m a y be detrimental particularly in the urban made-ground where the largest inaccuracy in any modelling assessment will occur. To exemplify something of the difficulty, a data set from an urban barrage study area is given in Table 4. The data are related to a model of made-ground and illustrate firstly, the very small heads involved, and secondly, the degree of inaccuracy in model representation. The table also indicates the extent of data requirement for any model control. In a groundwater resources study the head representation in Table 4 m a y be acceptable, but for an urban property vulnerability assessment it clearly is not. For some model areas, groundwater heads are shown above ground level (in calibration before impoundment) while modelled head representation error ranges between - 1.5 and + l . 9 m with respect to the measured groundwater head. The errors are o f the same magnitude or greater than most o f the rise that can occur.
6. An assessment approach Unfortunately, it must be concluded that conventional distributed, deterministic,
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numerical modelling cannot realistically be used to adequately describe the groundwater rise anticipated following barrage impoundment in the type of complex hydrogeological environment being discussed. Further, the large range in hydrogeological parameter values (e.g. see Table 3), particularly where a dominating top-layer of made-ground is present, does not realistically permit confidence in a stochastic modelling evaluation, also in view of the very small head changes envisaged. For barrage schemes to progress therefore subjective hydrogeological judgement coupled with effective engineering is the practical way forward. The following course of action is proposed: (i) the classical ground geometry is established through drilling etc.; (ii) a minimum depth to groundwater assessment is made based upon at least one hydrological year's data, preferably longer; (iii) vulnerability criteria are established on the depth to groundwater under prebarrage conditions, such as: (a) all areas where the minimum recorded depth to groundwater is less than say 1 m are vulnerable; (b) all areas of low permeability ground where the minimum recorded depth to groundwater is less than say 2 m and made-ground is absent are vulnerable; (c) all areas where the minimum recorded depth to groundwater is less than say 2 m and hydrographs show no 'cut-off" constraint with recharge (precipitation) variation are vulnerable; (iv) the vulnerable areas are delineated and a compensational and remedial action agreed; (v) the impoundment is staged; (vi) a comprehensive monitoring of heads is carried out pre-barrage, during construction and post-barrage. Clearly, there will be some overlap in the vulnerability criteria suggested (iii, a-c) and naturally the type of criteria applicable will vary from site to site. The essential data are representative minimum depths to water pre-barrage, preferably for a major (extreme) recharge event. Because of the impossibility of adequately predicting the head rise, the staging, which is the norm in most dam commissioning, will permit the actual rise to be monitored under manageable conditions. The converse, of full impoundment followed by drawdown, should significant problems arise, is dangerous because heterogeneity in the made-ground may permit rising groundwater to enter a 'perched' feature (concave upwards) but not allow it to readily dewater. Staging importantly will permit progressive detailed identification of any vulnerable areas and time to implement remedial measures. The approach proposed above is very dependent upon comprehensive legislation and administrative support, and also upon sensible remedial measures. These aspects are outside the scope of the paper, although land drainage networks, both in urban and rural areas, are the preferred remedial action coupled in some rural areas with levees. Interestingly, for the Cardiff barrage a groundwater pumping scheme abstracting
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from gravels underlying estuarine alluvium and made-ground has been proposed as one possible remedial way of depressing post-barrage groundwater heads (Wallace Evans, 1992) in a vertically conductive system.
7. Conclusions
The construction of barrages in estuaries in the UK will cause environmental impacts which in groundwater and economic terms will probably mostly relate to the development of dampness in residential and industrial properties through small rises in groundwater head. The prediction of the likely amount and extent of the groundwater rise is fraught with difficulty because of the extreme hydrogeological complexity of many estuary areas, particularly when taken in the context of anticipated rises of less than 1-2 m. While the natural ground may be complex one of the overriding hydraulic influences in the urban areas is made-ground which is extremely heterogeneous. It is considered that the groundwater rise cannot be sensibly predicted using distributed modelling techniques and that for schemes to advance responsibly a staged impoundment policy should be invoked.
References Binnie and Partners, 1970. Wash desk study report. Report to Water Resources Board, Reading, UK, 153 pp. McAlpine, R., 1989. An initial review of possible changes in drainage and groundwater regimes and their implications associated with the construction of the Severn barrage. Severn Barrage Development Project, Hayes, UK, 26 pp. Rendle, Palmer and Tritton, 1991. River Usk barrage at Newport. Groundwater study. Report to Newport Borough Council, Newport, UK, 33 pp. Wallace Evans, 1988. Cardiff Bay barrage feasibilitystudy. Groundwater investigationsstage III. Report to South Glamorgan County Council, Cardiff, UK, 61 pp. Wallace Evans, 1992. Study into control of rise in groundwater by well dewatering. Cardiff Bay Barrage Groundwater Investigations. Report to Cardiff Bay Development Corporation, Cardiff, UK, 30 pp. Water Resources Board, 1966. Solway barrage. WRB Pub. No. 3. HMSO, London, 30 pp. Water Resources Board, 1972. Morecombe Bay: estuary storage• WRB Pub. No. 12. HMSO, London, 46 pp.