GEOHYDROLOGICAL COMPENSATORY MEASURES TO PREVENT LAND SUBSIDENCE AS A RESULT OF THE RECLAMATION OF THE MARKERWAARD POLDER IN THE NETHERLANDS. G. Vos , F.A.M. Claessen , J.H.G. van Ommen** Public Works Department, Rijkswaterstaat, Directorate of Water Manage­ ment and Hydraulic Research, Northern District. International Water Supply Consultants IWACO B.V. ABSTRACT If the Markerwaard polder is reclaimed the water level in the polder will fall 5 to 6 metres over an area of 410 km2. This will cause a drawdown of the piezometric levels in the Pleistocene aquifers underneath the eastern part- of the province of North Holland. The spatial pattern of these drawdowns is calculated by a finite elements groundwater model. Without countermeasures to;compensate for the depletion of the piezometric level, settlement of the compressible Holocene clay and peat deposits will occur and resultant land subsidence may cause damage to buildings and infrastructure. Drawdown of the piezometric levels can be entirely or partially countered by means of artificial recharge of water into the Pleisto­ cene aquifers. There are two methods for this, viz. vertical recharge wells and infiltration grooves in the remaining western peripheral lake between North Holland and the Markerwaard polder. The amount of water necessary for the countermeasures is calculated with the same groundwater model. 1. INTRODUCTION The study area is situated in the north-western part of the Netherlands (fig. 1). The projected polder will have an area of 410 km2 and will be reclaimed in the south-western part of Lake IJssel. The phreatic groundwater level in the new polder will be some 5 to 6 metres lower than the present water level in Lake IJssel. As a result, a groundwater flow towards the new polder will be in­ duced. This will mean a considerable change in the groundwater regime underneath the new polder and its surrounding area. This change, particularly the drawdown of the piezometric level in the upper Pleistocene aquifer, could have a detrimental effect on agriculture and animal and plant life and may cause land subsidence, with resul­ tant damage to buildings and infrastructure. This would be most likely to occur in the eastern part of the province of North Holland (the shaded area in fig. 1). The province of North Holland can be characterized as an old polder area with a surface level varying from 1 to 4 metres below sea-level. The subsoil consists of Holocene clay and peat deposits 10 to 20 metres thick. (Westerhoff e.a. 1984). 915 The purpose of this investigation was firstly, to forecast the changes in the groundwater regime underneath the province of North Holland and secondly, to provide a feasibility study of countermeasures against the effects of such changes in the groundwater regime (Claessen, 1984). 2. GEOHYDROLOGICAL EFFECTS OF THE RECLAMATION OF THE MARKERWAARD POLDER 2.1. Introduction Depending on the various geological and geohydrological conditions, reclamation of the polder will influence the existing groundwater regime. Because of the complexity of the geohydrological structure of the subsurface (in both a horizontal and vertical direction) together with strongly varying boundary conditions, the calculation of the groundwater flow had to be carried out with the help of a numerical model. Simple preliminary analytical calculations were used to determine the extent of the study area (= model area). (figure 1). The geohydrological features of the model were derived from the geological conditions of the study area (Westerhoff e.a., 1984). After calibration and verification this numerical model was able to forecast the changes in the groundwater regime. 2.2. Çeohydrological_schematization For the geohydrological calculations it is necessary to schematize the subsurface into aquifers and aquitards, based on the geological conditions. As shown in figure 2 the subsurface can be schematized into three aquifers covered and partially separated by aquitards. On the basis of the geohydrological framework a number of geohydrological maps 916 Figure 2 Geohydrological profile have been prepared in which the preliminary values of the geohydro­ logical parameters needed for the model computations, such as values of transmissivity (T) and vertical hydraulic resistance (c), are given. The parameter values firstly used are readjusted by means of model calibration. 2.3. Calculations_with the finite element model 2.3.1. Description of the model To forecast the changes in the groundwater regime after reclamation of the Markerwaard, the FIESTA-model was used. This is a numerical physical finite element model which solves steady groundwater flow problems in multi-layered systems (Public Works Department, 1978). The finite element discretization is based on the regional character of this study and the expected effects of the reclamation of the Markerwaard polder (figure 3). The FIESTA-model calculates the piezometric levels and vertical groundwater flow for each nodal point of the three aquifers. In addition, water balances are calculated for the separate aquifers, the entire hydrological system and for those hydrological units that could be distinguished. 2.3.2. Calibration and verification of the model For the calibration of the model (=parameter evaluation) the average piezometric levels over the period October/November 1978 were used (these being representative of a long term steady state situation), as well as infiltration and seepage values measured in the field. The output of the model calculations, in terms of piezometric levels and vertical groundwater flows, should agree, within certain limits, with the measured data. The.geohydrological parameters in the model (T- and c-values) have been adjusted to an optimum value by a selective trial and error method. During the calibration process it became increasingly clear that the vertical hydraulic resistance of the Holocene 917 figurt 3-. Model area; part of grid spacing aquitard is the most sensitive parameter for the groundwater flow of the area involved. Figure 4 shows the spatial distribution of- this c-value as determined by the calibration of the model for the existing hydrological situation. After calibration, the forecasting ability of the model was verified with data, different from those used with the calibration. In 1967 the Southern Flevoland polder (in the southern part of the model area) was reclaimed. This resulted in a drawdown of the piezometric levels in that particular region. The actual extent of the depletion was then compared with the predictions of the numerical model with fairly similar results. 2.3.3. Results of the calculations To indicate the final situation (steady state) after the reclamation of the Markerwaard polder the input data of the model were adjusted as follows: - in the new polder area the phreatic level drops 5 to 6 metres; - in the same area the vertical hydraulic resistance of the Holocene aquitard decreases through ripening of the tap soil and reclamation works. It was estimated, based on experience in other polders,- that 918 Legend; j Abtonco of Molocono 12.500 - 30.000 dayt Hlllll < mill) °ayt > 30.000 days 1000. 2000 dayt Futt tntBriBction of the Holocano cover- layer 2000. 5000 dayt by canatt. c-valua It 1500 dayt. S000- 12.500 day. figura t VerHcal hydraulic resisfance (days) of the Holocene cover-layer. (Situation biforo raclamation of tht Markorwaard poldarl the c-values would decrease from about 27000 days to 4000 days. After these adjustments, the new piezometric levels and vertical groundwater fluxes in the study area were calculated. Subtraction of these values from the corresponding values at present,results into the drawdowns of the piezometric levels in the three aquifers and the changes in infiltration and seepage rates. The drawdowns in the upper Pleistocene aquifer (figure 5) is especially important, because these data are important for the geotechnical calculations necessary to forecast land subsidence and subsequent damage. 919 figure 5 : Ultimate drawdown in upper aquifer. 2.3.4. The speed of piezometric drawdown Calculations have been made using an analytical non-steady groundwater flow model (Barends, 1984) to estimate the speed of the piezometric drawdown in relation to the speed of pumping dry the new polder. In this model the consolidation of the compressible Holocene deposits has been taken into account as a function of time and place. It appears from these calculations that under the Markerwaard and North Holland the piezometric level will recede at the same rate as ascertained under the north-western part of Southern Flevoland during and after the reclamation of this polder. In Southern Flevoland and its surroundings measurements showed that one year, respectively five to seven years after the start of pumping, more than 50%, respectively almost 100% of the final drop of the piezometric head was reached. If the Markerwaard is pumped dry at the same rate as Southern Flevoland, it is to be expected that after 5 to 7 years about 90% of the final drop under and around the Markerwaard will be reached. (Hannink e.a., 1984) 3. COUNTERMEASURES TO PREVENT DRAWDOWN OF PIEZOMETRIC LEVELS 3.1. Introduction Without countermeasures settlement of the compressible Holocene deposits will occur. Due to resulting land subsidence, buildings and infrastructure may be damaged (Hannink and Talsma, 1984; van Bruchem, 1984; Carrée and Hulsbergen, 1984). 920 IH i i j ! i i 921 3.2. Countermeasures_studied Drawdown of piezometric levels can be countered by means of artificial recharge of water into the Pleistocene aquifers. Various measures such as gas(air) injection, injection of chemicals, construc­ tion of slurry walls, sandpiles, infiltration galleries, recharge wells and infiltration grooves, have been briefly considered and analysed provisionally with regard to their technical, practical and economic feasibility. Two solutions were found feasible for this specific problem and have been studied more in detail: - injection of water by means of vertical recharge wells (fig. 6). These wells can be located along the east coast of North Holland (fig. 7) or clustered near those urban areas most vulnerable to damage (fig.