Western Belgian Coastal Plain Research Articles

Heat transport in a coastal groundwater flow system near De Panne, Belgium

Temperature distribution and heat transport are studied in a coastal aquifer at De Panne in the western Belgian coastal plain. Field observations include temperature profiles of groundwater in the dunes and temperature measurements at the water table in a profile on the shore. Freshwater–saltwater distribution is known from previous studies. These are used to constrain a density-dependent model simulating the freshwater–saltwater distribution and heat transport using the SEAWAT code. The yearly fluctuation of the groundwater temperature in the phreatic aquifer under the dunes, shore and sea, and the influence of a tidal inlet in the dunes are simulated. The observations show that seawater temperature variations determine the temperature variations on the shore whereas atmospheric temperature changes determine this in the dunes. Yearly temperature fluctuations imposed at the water table propagate mainly vertically in the aquifer with only limited lateral influence. Heat transport is mainly convection dominated. Thickness of the surficial zone is determined by the amplitude of the groundwater temperature at the water table and the groundwater flow. Establishment of a tidal inlet in the dunes results in asymmetric temperature profiles under and in the vicinity of it.

Water quality changes in the dunes of the western Belgian coastal plain due to artificial recharge of tertiary treated wastewater

Since July 2002, tertiary treated waste water has been used to artificially recharge the phreatic aquifer of the dunes in the Belgian western coastal plain. The purpose of this project is to develop a sustainable water source, and thereby minimize lowering of the water table while maintaining production capacity. The recharged water is extracted via production wells located south and north of two ponds used for the recharge. The uniqueness of this system is that the total dissolved solids in this recharge water is low (50 mg/L) in comparison with natural dune water (550 mg/L). In this paper, water quality of recharge water, groundwater between ponds and extraction wells, extracted water and dune water is analysed. The extraction water quality is determined by mixing of recharged water and dune water. Mineralisation of the recharge water occurs mainly by carbonate dissolution. Further O2 consumption, NO3- reduction and oxidation of Fe sulfide minerals are distinguished. At the beginning of 2007 clogging of the ponds began to occur and it was observed that extraction rates and temperature of the recharge water influenced the amount of water which could be recharged. Using Cl− data, it is confirmed that all the recharged water is recovered by the extraction wells. Finally, it is concluded that the quality of the extracted water does not change significantly as a function of time.

Salt water infiltration in two artificial sea inlets in the Belgian dune area

In the dune area of the Westhoek Nature Reserve, situated in the western Belgian coastal plain, two artificial tidal inlets were made aiming to enhance biodiversity. The infiltration of salt water in these tidal inlets was carefully monitored because a fresh water lens is present in the phreatic dune aquifer. This forms an important source of fresh water which is for instance exploited by a water company. The infiltration was monitored over a period of two years by means of electromagnetic borehole measurements (EM39) and by measurements of fresh water heads and temperature using a large number of observation wells. EM39 observations point to aquifer heterogeneity as a determining factor in the movement of the salt infiltration water. It is shown that part of the infiltration water moves further in the dunes instead of towards the sea. On the long term run, possibility exists that salt water enters the extraction’s capture zone. This issue needs further monitoring and study. Fresh water head and temperature data illustrate that the main period of infiltration is confined to spring tide when large amounts of salt water enter the tidal inlets.

Sustainable groundwater extraction in coastal areas: a Belgian example

Water extractions in coastal areas have to deal with salt water intrusion and lowering of hydraulic heads in valuable ecosystems. Therefore, sustainable management of fresh water resources in these areas is crucial. This is illustrated here with two water extractions in the western Belgian coastal plain which extract groundwater from a phreatic dune aquifer. One water extraction faced problems with salt water intrusion, while lowering of hydraulic heads was an issue for both. To remedy the salt water intrusion, it was found that decreasing the extraction rate was the only solution. To offset this and to increase hydraulic heads around both extractions, it was decided to artificially recharge the aquifer of the second extraction with tertiary treated wastewater. By taking these interventions, the combined production capacity of the water extractions was increased with 56% whereas 27% less water was extracted from the dune aquifer itself. Extraction history and the effects of interventions are illustrated for both water extractions with water quality data and fresh water head observations. A more detailed insight in groundwater flow and fresh-salt water distribution in the aquifer is provided by simulating the evolution of the water extractions with a 3D density dependent groundwater flow model.

How subsoil morphology and erodibility influence the origin and pattern of late Holocene tidal channels: case studies from the Belgian coastal lowlands

This paper aims at a better understanding of the late Holocene re-entrance of the tidal system in the Belgian coastal plain after a 2–3 ka years long period of peat growth. The re-entrance was associated with the development of deeply incised tidal channels. The initial cause of the re-entrance, still a missing link in the understanding of the late Holocene coastal change, is investigated by focusing on the specific location of the young tidal channels. The research, though based in the western Belgian coastal plain, is relevant to the lowlands of the southern North Sea and English Channel. The investigation combines stratigraphic, radiocarbon and sedimentological data, together with maps showing the morphology of the pre-Holocene surface, the distribution of the Holocene deposits and the erosional surface produced by the late Holocene channels. It appears that the young channels re-occupied the same position as their early and mid Holocene predecessors. The re-entrance of the tidal system began by a removing of the upper part of the older channels which originated from the mainland. Once the channels were cleaned of sediment, tidal waters could re-enter and rework the easily erodable sand of the older channels and Pleistocene deposits.

Sequence mapping of Holocene coastal lowlands: the application of the Streif classification system in the Belgian coastal plain

A geological map of the Holocene deposits of a portion of the western Belgian coastal plain is presented. The map differs from a conventional geological map which is of little use for coastal lowlands because of the high lateral and vertical variability in lithology in the subsoil. The method of sequence maps was chosen because these maps allow the representation of the entire vertical sequence of the Holocene deposits. A sequence map is constructed on the basis of profile types consisting of the vertical succession of the various mapping units. In order to produce a user-friendly map, preference was given to lithogenetic units instead of formal lithostratigraphic units for the determination of mapping units. The organisation of the mapping units was established according to the Streif classification system. The application of the system is demonstrated with two different sequence maps each constructed on a different level of the hierarchic system: a general sequence map and a special sequence map showing further differentiation.

Vroeg en midden holocene kustevolutie van de westelijke Belgische kustvlakte - een synthese

A large-scale pattern of Holocene coastal evolution of the western Belgian coastal plain has been reconstructed by means of a series of palaeogeographical maps at 9 time slices between 9.5 and 6 cal. ka BP together with schematical cross-sections of the vertical sediment accumulation. The time-depth estimate and the spatial extension of the palaeo-environments is based on the relative sea-level (RSL) curve and radiocarbon dates of (basal and intercalated) peats and shells. This paper describes the methodology of reconstructing the pre-Holocene surface and asserts that the morphology of the flooded surface is a controlling factor in the distribution of the Holocene sediments. For a better visualization of the rather weak relief, a 3D terrain model is presented. The coastal evolution is further controlled by the changes in the rate of RSL rise and sediment budget. The period before ca. 7.500 cal BP is marked by a rapid RSL rise and consequently by a progressive rapid landward migration of all depositional environments as well as a major vertical sediment accretion. In the period following the first substantial decrease in the rate of RSL rise, the direct impact of the latter is subordinate to the effect of sediment budget which is in balance with the creation of accommodation space. This resulted in the sedimentary infilling of the tidal basin and the deposition of tidal clastic sediments with peat accumulation.

The Holocene depositional history of the IJzer palaeovalley (Western Belgian coastal plain) with reference to the factors controlling the formation of intercalated peat beds

Detailed stratigraphic and sedimentological work, together with radiocarbon datings of the Holocene coastal deposits in the IJzer palaeovalley was performed in order to document the formation of the sedimentary sequence in relation to the controlling factors, to reveal the stratigraphical significance of intercalated peat beds, and to reconstruct the depositional history. Because of the many controversies relating to these topics, the Belgian literature has been reviewed. An initial rapid sea-level rise prior to ca. 7800 cal BP resulted in a rapid shift of the facies belts across the palaeovalley and was associated with rapid filling with no formation of intercalated peat beds. A deceleration in the rate of the sea-level rise with sufficient sediment supply produced rapid facies changes registered as frequent alternations of mud, peat and gyttja in the sedimentary sequence deposited between ca. 7800 and 6000 cal BP. The facies changes are determined by a sedimentological control related to the tidal channel and creek network, and not by sea-level fluctuations. Periods of peat growth lasted longer and the lateral extension became more widespread as deceleration of the sea-level rise and filling of the plain continued. This was associated with progradation of the shoreface from ca. 6000 cal BP onwards. Tidal conditions returned to the area from ca. 4000 cal BP onwards. This return was accompanied by erosion and landward migration of the shoreface and channel network. This study demonstrates that the tripartite stratigraphy using Dunkerque and Calais as units should be abandoned in favour of lithological and sedimentological descriptions and age determinations together with environmental interpretation.

A diatom and radiocarbon perspective of the palaeoenvironmental history and stratigraphy of Holocene deposits between Oostende and Nieuwpoort (Western Coastal Plain, Belgium)

Diatom and radiocarbon data of two cores from the north-eastern part of the western Belgian coastal plain are discussed. The palaeoenvironmental reconstruction of these sites and additional stratigraphic and geochronological information, allow an assessment of the general development of this region throughout the Holocene. Prior to c. 7500 yrs cal. BP a rapidly transgressing coast exists. Basal peat development may have been fairly widespread. Extensive erosion channels form towards the interior once the volume of the developing tidal basin has increased sufficiently. In spite of very high rates of sediment accumulation, high-intertidal environments remain poorly developed due to the high rate of RSL rise and the relatively steep slope of the transgressed terrain. Diatom taphocoenoses of this period consist primarily of coastal tychoplankton (mainly Cymatosira belgica), deposited in association with flood transported sediment, and some epipsammon. Hereafter, the incidence of negative tendencies in marine influence increases, culminating between c. 6300 to 5600 yrs cal. BP. Diatom assemblages are now more varied with an improved representation of tidal flat and salt marsh communities. The marine species Paralia sulcata and Pseudopodosira westii become abundant in high intertidal deposits, which may involve selective transport, sedimentation and preservation mechanisms. In general, the environmental changes reflect the development of a broad intertidal sedimentation zone in which sediment supply remained high, combined with a gradual slowing down of the RSL rise. Coastal progradation finally leads to sufficiently sheltered conditions for long-lasting peat growth in the intracoastal area up to perhaps c. 2100 yrs cal. BP. Presumably a coastal barrier, which remains intersected by some inlets, also develops during this period. At places, peat growth is interrupted at about 4200 yrs cal. BP by a marked but short-lived period of marine sedimentation. Within the general framework of a return to a transgressive setting and a destabilisation of the barrier system due to a decreasing sediment supply, climatic conditions and human interference may have played a role in the final termination of peat growth and the resumption of tidal activity. So far, the exact chronology of the return of tidal sedimentation in the study area remains unclear. No general sedimentation cycles, or so-called Dunkerque transgressions, are identified during the deposition of the upper tidal deposits, which are again characterized by a high representation of Cymatosira, reflecting a more open coastal setting. A tentative chronology of regional tendencies in the marine influence is derived from both bio- and lithostratigraphical indications and compared with the assumed chronology of transgressive and regressive phases in the northern Netherlands. Points of agreement are discussed in terms of possible supra-regional processes.

Results Of An Artificial Recharge Test And A Double Pumping Test As Preliminary Studies For Optimizing Water Supply In The Western Belgian Coastal Plain

In the western Belgian coastal plain, a major part of the water demand is supplied by catchments in the dune belt, where water is extracted from fresh groundwater lenses in the unconfined aquifer. Because of overexploitation, mainly in summer, the water catchments are threatened by salt-water encroachment. Artificial recharge of the catchments could be considered since, in winter, a large amount of fresh surface water of the polder area is evacuated to the sea. A proposal for an improved water management of the region is formulated. Preliminary studies consisted of two tests: an artificial recharge test in the dune area and a double pumping test in a sandy ridge near a canal in the polder area. The tests are interpreted by means of inverse models. Results of the artificial recharge test demonstrate that recharge is feasible for the water catchment area of Koksijde. Results of the double pumping test show that the Avekapelle creek ridge in the polder area is very well suited as an intake area.

Diatom analysis of an Atlantic-Subboreal core from Slijpe (western Belgian coastal plain)

Diatom analysis of a clayey Atlantic-Subboreal sequence from the western part of the Belgian coastal plain revealed the existence of intertidal conditions throughout, with the exception of a swampy lagoonal stage at the top of the basal peat. Several transgressive events were shown to have influenced the local palaeoenvironment. The possible relationship of these events to the Calais II and Calais III transgressions is discussed.