Belgian Coastal Plain Research Articles

Marginal Mounds and Maritime Shepherding. A Landscape Archaeological Survey of a Late Medieval Sheep Mound in the Belgian Coastal Plain

ABSTRACT The ‘Scaperie’ site in Knokke-Heist (Belgium), identified through a topographic analysis of a sixteenth-century map and located in a maritime sedimentary environment, serves as the focal point for this research. Employing a landscape archaeological approach, this study integrates multiple data sources including ceramics, historical maps, aerial photographs, LiDAR-data, geophysical surveys, and a coring survey. The dataset revealed the remnants of a anthropogenic sheep mound, evidenced by a localised microtopographic elevation, subsoil anomalies and surface artifacts associated with dairy production. More generally, this paper contributes to the understanding of the dynamic interplay between humans, sheep, and the coastal environment, offering new insights into historic land use and the archaeological characteristics of coastal mounded features. The study introduces a tripartite pattern of spatial organisation of animal husbandry, encompassing parent farms, droveways and satellite dwelling mounds. Encroached by the advancing embankments, this sheep-related taskscape was gradually marginalised both socio-economically and spatially.

Sedimentary and stratigraphic architecture in the geoarchaeological context of Late-Holocene deposits in the eastern coastal plain of Belgium: Zwin region – Hoeke

A sedimentary investigation was carried out into the rise and fall of mediaeval Bruges’ maritime access. Because the mediaeval metropolis had no direct connection with the North Sea, its maritime connectivity during the High- and Late-Middle Ages depended on a tidal channel, known as the ‘Zwin’. In order to reconstruct the mediaeval physical environmental situation correctly, the entire Holocene sedimentary record was studied through undisturbed mechanically-drilled cores and electrical cone penetration tests (CPT-e). A detailed sedimentary analysis was complemented with radiocarbon dating and datable archaeological finds to resolve a timeframe for the sedimentary evolution. Tidal channel systems with diverse morphologic and sedimentologic characteristics prevail and cover a period from at least the Roman period till the Late-Middle Ages. The presence of a single peat layer directly on top of the Pleistocene substrate and the stratigraphic position of the tidal channel deposits relative to this peat layer, proves that no seawater entered this part of the Belgian coastal plain before the Late-Holocene. The tidal channel system that was active during the heydays of Bruges (the Zwin) was the last natural one. It silted up due to a further reduction of the accommodation space as a consequence of new embankments, possibly expedited by storms.

Incised Pleistocene valleys in the Western Belgium coastal plain: Age, origins and implications for the evolution of the Southern North Sea Basin

The Belgian coastal plain occupies a key position as it is located at the transition between the Southern North Sea Basin and the Strait of Dover. It is characterized by thick sequences (>20m) of Pleistocene terrestrial and littoral sediments. Yet the wider stratigraphical and palaeo-environmental significance of these sediments received little attention. In this paper we draw on the results of a recent sedimentological study based on >100 drillings that spans the Pleistocene sequence, and present new biostratigraphical (pollen, foraminifera, ostracods) data, all revealing a complex history of deposition. The record includes evidence of the development of incised-valley systems that were initiated in the late Middle and Late Pleistocene. Five phases of fluvial incision can be identified. The majority of the infills are deposited in an estuarine environment that passes into a fluvial environment land inward, except the Weichselian infill which has a predominant fluvial origin. The greatest part of the most seaward located zone of the western coastal plain was free of valley incisions, there, shallow marine sediments built up the record. Local biostratigraphical investigations provide a timeframe. The result is placed in a regional context.

‘Productive’ Sites in the Polders? ‘Griffin brooches’ and Other Early Medieval Metalwork from the Belgian Coastal Plain

The present paper demonstrates how metal detectorists’ collections potentially form an important addition to the extant corpus of early medieval non-ferrous metalwork. Several sites in the Belgian coastal plain featuring rich metalwork assemblages dated to this period are presented. The important questions raised by these artefacts regarding the chronology, nature and socio-cultural affiliations of settlement in the early medieval coastal plain are discussed. In addition, a previously unrecognised, distinct type of openwork disc brooch is described, dated and framed within technical, typological and artistic developments of late tenth- to early twelfth-century north-western Europe, in particular the North Sea area.

Flow and transport model of a polder area in the Belgian coastal plain: example of data integration

Tracer data can serve to derive parameters related to groundwater systems or can be used in the calibration of transport and flow models. Temperature, 3H/3He (age dating) and hydrochemical data, together with head measurements, borehole logs and an aquifer test, were used to obtain insight into the flow and transport of a Quaternary aquifer system located in the Belgian polder area. Flow and transport model code SEAWAT acted as the integration medium for the different data. Each type of data has its own interpretation technique and adds components to the model. Additionally, different types of data aid in verification of the results. For instance, fluxes from temperature logs (used with the SEAWAT model), and water quality and age dating all provide information on flow to and velocities in the vicinity of drainage ditches. Different data also provide information on different scales. Temperature logs and aquifer tests act on a small scale, groundwater age is influenced by larger-scale flow, and water quality is determined by the general flow of the area. Integration of different kinds of geological, hydrological, geophysical and geochemical data is shown to be an important way forward in the efforts to model real-world cases.

Groundwater chemistry patterns in the phreatic aquifer of the central Belgian coastal plain

The Holocene geological evolution of the Belgian coastal plain is dominated by a transgression of the North Sea, silting up of the coastal plain and human intervention (impoldering). This has led to a typical pattern in groundwater quality which is discussed here for the central part of the coastal plain. Therefore, a database with available groundwater samples is composed. Water type according to the Stuyfzand classification is determined and different hydrosomes and their hydrochemical facies are identified. Based on this, the origin and evolution of the water types is explained using Piper plots and geochemical calculations with PHREEQC. Before the impoldering, salinising and freshening conditions alternated with a general salinisation of the aquifer after about 3400 BP. This results in a dominance of brackish and salt NaCl subtypes which are still found in the deeper part of the aquifer. The subsequent impoldering resulted in an major freshening of the aquifer leading to NaHCO3, MgHCO3 and CaHCO3 subtypes. Overall, mixing, cation exchange, carbonate mineral dissolution and oxidation of organic matter are identified as the major processes determining the general water quality. The close link between geological evolution, water quality and what is still observable today is illustrated with this example of the Belgian coastal plain.

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.

Hydrogeological study for improved nature restoration in dune ecosystems–Kleyne Vlakte case study, Belgium

In dune slacks a close coupling exists between changes in the hydrology and changes in species composition and vegetation structure. Consequently, there is a need to underpin nature restoration projects not only with ecologically relevant knowledge but also with scientifically sound hydrogeological data. In this paper, this necessity is illustrated through a study of the Flemish Nature Reserve ‘The Zwindunes and Zwinpolders’ (Belgian coastal plain) as an example. The management plan for the nature reserve suggests rewetting part of it to enhance its ecological value. The groundwater aspect was studied by means of field observations and mathematical modelling. First, fresh water head observation showed a mean groundwater flow from the nature reserve to the adjacent polder. Secondly, groundwater quality was studied with borehole measurements and water samples, resulting in a map of the fresh-salt water distribution and of water types. All available information was then put together in a density dependent groundwater flow model. The aim of this model was the description of current flow and fresh-salt water distribution and to simulate the impact of three possible rewetting scenarios. Rewetting will be accomplished by the infiltration of water in a depression, different lay-outs for which are considered. A zoomed in flow model based on a regional model was used to incorporate both local scale, which is of importance to ecology, and the larger scale, which determines general groundwater flow and fresh-salt water distribution. This modelling indicated differences between scenarios and was used to decide on the best rewetting strategy.

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.

Radiocarbon-dated sediment sequences from the Belgian coastal plain: testing the hypothesis of fluctuating or smooth late-Holocene relative sea-level rise

The late-Holocene deposits of the coastal lowlands bordering the southern North Sea were formed by a renewed expansion of the tidal environment replacing freshwater peats. The sea-level history during the last 2500 years inferred from the post-peat deposits, containing little to no datable organic material, is characterized by two conflicting schools of thought. The first one supports a smooth sea-level curve. The second approach underpins fluctuating sea-level changes. The `fluctuating' approach also uses facies variations recorded in the post-peat deposits to provide a chronostratigraphical subdivision linked to transgressions and regressions. This paper examines the controlling factors responsible for the facies variations in the post-peat deposits in order to test the approach of the fluctuating sea level. The results of sedimentological investigation coupled with radiocarbon dates of intertidal shells from four shallow outcrops located in nearby sand-filled late-Holocene tidal channels in the Belgian coastal plain are reported. The integration with results from previous work has allowed the reconstruction of the mechanisms and processes of coastal evolution during the late Holocene. The changes in the coastal landscape reflected in the facies variations are caused by multiple factors. The major one is the dynamic nature of the tidal channels responding to changes in accommodation space and sediment budget, and finally, to storm incidence and human impact. The processes of the tidal channel networks, ie, their initiation and evolution, are similar, but the changes happened at different times in different places. The facies variations do not involve rises and falls of sea level, and therefore a fluctuating sea-level rise can not be considered as realistic.

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.

Potential Implications of Sea-Level Rise for Belgium

The Belgian coastal plain and the Schelde estuary are threatened by sea-level rise. While of great economic importance with a threatened population of some 0.8 million (of a total population of 10 million), assessments of these risks are limited. This article describes the physical characteristics of the coast and undertakes a qualitative interpretation of its vulnerability. Low-lying polders are the most vulnerable to sea-level rise where a major problem is water drainage during rainy periods; their varying vulnerability to sea-level rise and increase in rain intensity is assessed, including the relationship between drainage levels and saltwater seepage. Freshwater lenses developed within the dunes are also vulnerable to sea-level rise, leading to threats to drinking water supplies from saltwater intrusion. Belgian coastal defence structures and their effectiveness are discussed. Historical sea-level rise during the past century, wave and wind data, and the evolution of erosion and accretion along the coast are interpreted. For Antwerpen, a harbour city on the river Schelde, the effects of sea-level rise are far from clear. Included here are historical data on changes in tidal amplitude during the 20th century. Future research needs should focus on the quantitative interpretation of data to understand the effect of sea-level rise on beach erosion, flood risk, and fresh and salt groundwater distribution. Furthermore, a thorough socio-economic study should be undertaken to assess the vulnerability of the Belgian coast and the Schelde estuary.

Holocene relative sea-level change, isostatic subsidence and the radial viscosity structure of the mantle of northwest Europe (Belgium, the Netherlands, Germany, southern North Sea)

A comprehensive observational database of Holocene relative sea-level (RSL) index points from northwest Europe (Belgium, the Netherlands, northwest Germany, southern North Sea) has been compiled in order to compare and reassess the data collected from the different countries/regions and by different workers on a common time–depth scale. RSL rise varies in magnitude and form between these regions, revealing a complex pattern of differential crustal movement which cannot be solely attributed to tectonic activity. It clearly contains a non-linear, glacio- and/or hydro-isostatic subsidence component, which is only small on the Belgian coastal plain but increases significantly to a value of ca 7.5m relative to Belgium since 8cal.kaBP along the northwest German coast. The subsidence is at least in part related to the Post-Glacial collapse of the so-called peripheral forebulge which developed around the Fennoscandian centre of ice loading during the Last Glacial Maximum. The RSL data have been compared to geodynamic Earth models in order to infer the radial viscosity structure of the Earth's mantle underneath NW Europe (lithosphere thickness, upper- and lower-mantle viscosity), and conversely to predict RSL in regions where we have only few observational data (e.g. in the southern North Sea). A very broad range of Earth parameters fit the Belgian RSL data, suggesting that glacial isostatic adjustment (GIA) only had a minor effect on Belgian crustal dynamics during and after the Last Ice Age. In contrast, a narrow range of Earth parameters define the southern North Sea region, reflecting the greater influence of GIA on these deeper/older samples. Modelled RSL data suggest that the zone of maximum forebulge subsidence runs in a relatively narrow, WNW–ESE trending band connecting the German federal state of Lower Saxony with the Dogger Bank area in the southern North Sea. Identification of the effects of local-scale factors such as past changes in tidal range or tectonic activity on the spatial and temporal variations of sea-level index points based on model-data comparisons is possible but is still complicated by the relatively large range of Earth model parameters fitting each RSL curve, emphasising the need for more high-quality observational data.

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.

Deriving TDS values in coarse sediments from long normal and electromagnetic logs.

This paper presents an empirical relationship of quantitatively linked electromagnetic (EM) borehole recordings of the total dissolved solids (TDS) in pore water in the Quaternary deposits of the Belgian coastal plain. First, the long normal (LN) logs are linked to EM logs, then the already developed relationships between LN resistivity measurements and the TDS values are rewritten for EM recordings. The main parameter in these equations is the formation factor, which is derived from ground water analyses and LN logs through Archie's law. The EM recording has several advantages compared to the LN logs. The EM analysis allows measuring in PVC-cased wells and is not hindered by the invasion zone around the well. Furthermore, it has a high vertical resolution. LN logs can be measured only once, after drilling a well; EM recordings can be repeated several times in monitoring wells, which allows the gathering of time-dependent data over a complete vertical cross section. Such data could be obtained with LN logs only in wells with screens over the full-depth interval, which causes a hydraulic short circuit. This short circuit can result in a large artificial flow through the well between different levels, resulting in a salinity profile, which is no longer representative for the studied site. Remediation against short circuiting is a reduction of the screened interval, which strongly reduces the gathered information. The application of the derived equations is one of setting up a monitoring network along the Belgian coast to monitor the trend in salinity levels and comparing present salinity levels with older LN recordings to investigate the salinity changes in the last 30 years. Deep wells already present in the Belgiancoastal plain can then be used to monitor both the fresh water head changes and the salt water evolution. The technique has also been used for parameter identification for which real concentration measurements were needed.

Numerical modelling and hydrochemical characterisation of a fresh-water lens in the Belgian coastal plain

The distribution of fresh and salt water in coastal aquifers is influenced by many processes. The influence of aquifer heterogeneity and human interference such as land reclamation is illustrated in the Belgian coastal plain where, around A.D. 1200, the reclamation of a tidally influenced environment was completed. The aquifer, which was filled with salt water, was thereafter freshened. The areal distribution of peat, clay, silt and sand influences the general flow and distribution of fresh and salt water along with the drainage pattern and results in the development of fresh-water lenses. The water quality in and around the fresh-water lenses below an inverted tidal channel ridge is surveyed. The hydrochemical evolution of the fresh water lens is reconstructed, pointing to cation exchange, solution of calcite and the oxidation of organic material as the major chemical reactions. The formation and evolution of the fresh water lens is modelled using a two-dimensional density-dependent solute transport model and the sensitivity of drainage and conductivities are studied. Drainage level mainly influences the depth of the fresh-water lens, whereas the time of formation is mainly influenced by conductivity.

Changes in coastal zone processes at a high sea‐level stand: a late Holocene example from Belgium

AbstractThis paper presents the results of an investigation of late Holocene deposits on the Belgian coastal plain. The upper clastic deposits overlying the uppermost intercalated peat bed were studied on the basis of lithology, radiocarbon dates and foraminiferal assemblages to determine why silting‐up phases or peat beds are lacking, and to establish the evolution of the sedimentary environments as well as the age of the deposits. Two contrasting sites, in quite different locations and showing a different stratigraphy, were studied. It appears that the coastal evolution was not gradual and continuous, but characterised by short periods with high‐energy conditions alternating with long periods where low‐energy conditions and little sedimentation prevailed. The evolution is determined mainly by tidal channel activity. At about 3400 cal. yr BP, the peat was eroded in narrow zones by the main tidal channels and this until the landward limit of the plain. The peat was only partly flooded and mudflats fringed the channels. With time, new tributaries formed and the channel network progressively enlarged. The channels were rapidly filled, followed by a period of little sedimentation, which lasted at least until 1400–1200 cal. yr BP. The following channel activity is characterised by lateral sedimentation, rather than vertical erosion, together with increased sediment reworking. The mechanisms of coastal change in the late Holocene might be similar over the entire plain, but the timing of the changes is certainly not. Copyright © 2002 John Wiley & Sons, Ltd.

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.