Rhine-Meuse Estuary Research Articles

The Dutch Rhine branches in the Anthropocene – Importance of events and seizing of opportunities

The Rhine branches in the Netherlands have been profoundly altered by human interventions in the past 2000 years. Three examples are elaborated to show that these alterations often result from specific events and the seizing of opportunities: the creation of a new Rhine bifurcation at Pannerden, the Room-for-the-River programme, and the mitigation of riverbed scour in the Rhine-Meuse estuary. This makes historical inquiry one of the keys to understanding geomorphological developments in the Anthropocene. For river restoration it holds the lesson that plans can be prepared in ordinary times and then be implemented at the right moment by seizing the opportunities offered by events.

Lithological control on scour hole formation in the Rhine-Meuse Estuary

River deltas commonly have a heterogeneous substratum of alternating peat, clay and sand deposits. This has important consequences for the river bed development and in particular for scour hole formation. When the substratum consists of an erosion resistant top layer, erosion is retarded. Upon breaking through a resistant top layer and reaching an underlying layer with higher erodibilty, deep scour holes may form within a short amount of time. The unpredictability and fast development of these scour holes makes them difficult to manage, particularly where the stability of dikes and infrastructure is at stake.In this paper we determine how subsurface lithology controls the bed elevation in net incising river branches, particularly focusing on scour hole initiation, growth rate, and direction. For this, the Rhine-Meuse Estuary forms an ideal study site, as over 100 scour holes have been identified in this area, and over 40 years of bed level data and thousands of core descriptions are available. It is shown that the subsurface lithology plays a crucial role in the emergence, shape, and evolution of scour holes. Although most scour holes follow the characteristic exponential development of fast initial growth and slower final growth, strong temporal variations are observed, with sudden growth rates of several meters per year in depth and tens of meters in extent. In addition, we relate the characteristic build-up of the subsurface lithology to specific geometric characteristics of scour holes, like large elongated expanding scour holes or confined scour holes with steep slopes. As river deltas commonly have a heterogeneous substratum and often face channel bed erosion, the observations likely apply to many delta rivers. These findings call for thorough knowledge of the subsurface lithology, as without it, scour hole development is hard to predict and can lead to sudden failures of nearby infrastructure and flood defence works.

Effects of discharge, wind, and tide on sedimentation in a recently restored tidal freshwater wetland

AbstractSediment deposition is one of the key mechanisms to counteract the impact of sea level rise in tidal freshwater wetlands (TFWs). However, information about sediment deposition rates in TFWs is limited, especially for those located in the transition zone between the fluvially dominated and tidally dominated sections of a river delta where sedimentation rates are affected by the combined impact of river discharge, wind, and tides. Using a combined hydrodynamic–morphological model, we examined how hydrometeorological boundary conditions control sedimentation rates and patterns in a TFW located in the Rhine–Meuse estuary in the Netherlands. The modelling results show that net sedimentation rate increases with the magnitude of the river discharge, whereas stronger wind increasingly prevents sedimentation. Sediment trapping efficiency decreases for both increasing river discharge and wind magnitude. The impact of wind storms on the trapping efficiency becomes smaller for higher water discharge. The spatial sedimentation patterns are affected by all controls. Our study illustrates the importance of evaluating both the separate and the joint impact of discharge, wind, and tides when estimating sedimentation rates in a TFW affected by these controls. Such insights are relevant to design measures to reactivate the sedimentation process in these areas.

Envisioning robust climate change adaptation futures for coastal regions: a comparative evaluation of cases in three continents

The paper reports on a comparative study of three different cases on vision and strategy development for climate change adaptation planning in (i) The South African Breede–Overberg Catchment, (ii) The Mississippi Estuary-New Orleans region and (iii) The Dutch Rhine-Meuse Estuary. The objective of the paper is twofold: to develop a better understanding of such processes and to further develop the Backcasting-Adaptive Management (BCAM) methodology. A framework for case evaluation is developed using six dimensions: (i) inputs and resources, (ii) future vision, (iii) stakeholder engagement, (iv) methodological aspects, (v) pathway development and (vi) impact. Major conclusions based on a cross-case comparison and testing propositions are (i) participatory vision development is a strong tool for climate change adaptation planning in different governance contexts and shows considerable diversity in its application in these contexts; (ii) a single, shared future vision is not a prerequisite for vision and pathway development and endorsement; (iii) broad stakeholder engagement enriches strategy development, but the involvement of marginal groups requires additional efforts and capacity building; (iv) multiple pathways and robust elements are useful but require novel expertise; and (v) more institutional embeddedness and support for participatory processes lead to better implementation of the outcomes of these processes.

Long term anthropogenic changes and ecosystem service consequences in the northern part of the complex Rhine-Meuse estuarine system

Around 0 AD, the Rhine-Meuse estuary in the southwest of the Netherlands was a typical coastal plain estuary. Drainage of peatland and land subsidence behind the dunes later caused the sea to penetrate into the land. Most of the peat was eroded, and by 1000 AD the so-called Delta area had turned into a landscape of large estuaries and intertidal zones. Rotterdam developed from a small fishing village on the banks of the tidal river “Nieuwe Maas” from the 14th century onwards into the largest seaport of Europe in 2013. The Rotterdam harbour area situated in the northern part of the Delta area includes the former Europoort harbour, and is nowadays known as Rijnmond. The hydrology of the area is controlled by the drainage regime of the sluices in the Haringvliet barrier that was constructed as part of the “Delta Works” project to protect the southwest of the Netherlands against storm surges. The sluices are opened at slack tide to discharge river water to the sea and are always closed at flood tide.As a baseline study for environmental and ecological reconstruction and development, we describe in detail the loss of intertidal soft sediment ecotopes due to land reclamation, harbour development and river training works (straightening of the navigational channel) in the tidal rivers, and the expansion of hard substrate ecotopes (quay walls, groynes, training walls, riprap, concrete, stones etc.) in the Rijnmond area in the 19th and 20th centuries. Within 135 years, more than 99% of the original 4 775 ha of characteristic pristine soft sediment estuarine ecotopes have disappeared. In the same period, 338 ha of hard intertidal substrate zone was constructed. Such trends can also be observed in harbour areas elsewhere, and have ecological and environmental consequences for estuarine areas in particular.Restoration of soft substrate estuarine ecotopes can be achieved by opening the Haringvliet Sluices at both ebb and flood tide, which would restore large-scale estuarine dynamics to the northern part of the Rhine-Meuse estuarine system. This will have a highly favourable effect on many ecosystem services. The Dutch division of the Word Wild Life Fund has launched a new proposal for a safer and more attractive South-West Delta area. It comprises the reopening of the sea inlets such as the Haringvliet by removing the barriers, and building climate-proof dikes in combination with natural wetlands. In case of storm surges, the hinterland could be protected with a new generation of barriers that do not hamper the free transport of sediment, tides and animals. Based on 30 ecosystem services or subservices, it was calculated that opening the Haringvliet inlet would lead to an increase in Total Economic Value (TEV) of at least 500 million Euro per year. The costs of removing old barriers and the construction of new ones was not included in the calculations.

Rapid range expansion of the invasive quagga mussel in relation to zebra mussel presence in The Netherlands and Western Europe

Since its appearance in 2006 in a freshwater section of the Rhine–Meuse estuary (Hollandsch Diep, The Netherlands), the non-indigenous quagga mussel has displayed a rapid range expansion in Western Europe. However, an overview characterising the spread and impacts of the quagga mussel in this area is currently lacking. A literature study, supplemented with field data, was performed to gather all available data and information relating to quagga mussel dispersal. Dispersal characteristics were analysed for rate and direction and in relation to hydrological connectivity and dispersal vectors. To determine ranges of conditions suitable for quagga mussel colonisation, physico-chemical characteristics of their habitats were analysed. After its initial arrival in the freshwater section of the Rhine-Meuse estuary and River Danube, the quagga mussel demonstrated a rapid and continued range expansion in Western Europe. Quagga mussels have extended their non-native range to the network of major waterways in The Netherlands and in an upstream direction in the River Rhine (Germany), its tributaries (rivers Main and Moselle) and the River Meuse (Belgium and France). The calculated average quagga mussel dispersal rate in Europe was 120 km year−1 (range 23–383 km year−1). Hydrological connectivity is important in determining the speed with which colonisation occurs. Dispersal to water bodies disconnected from the freshwater network requires the presence of a suitable vector e.g. pleasure boats transferred over land. Upstream dispersal is primarily human mediated through the attachment of mussels to watercraft. The relative abundance of quagga mussel to zebra mussel has greatly increased in a number of areas sampled in the major Dutch rivers and lakes and the rivers Main and Rhine and the Rhine–Danube Canal leading to a dominance shift from zebra mussels to quagga mussels. However, evidence for displacement of the zebra mussel is limited due to the lack of temporal trends relating to the overall density of zebra and quagga mussel.

Pole and pontoon hulas: An effective way of ecological engineering to increase productivity and biodiversity in the hard-substrate environment of the port of Rotterdam

Underwater environments in ports are designed for harbour activities solely. However, by simple and cost-effective measures, suitable habitat for underwater flora and fauna can be created. This is expected to have positive effects on higher trophic levels, such as fish, and improve water quality, by enlarging filter feeder biomass. In this study we developed ‘pole hulas’ and ‘pontoon hulas’, consisting of hanging ropes of different materials. The pole hulas are made up of many 6mm thick and 55cm long strings just above and below the mean low water level (MLWL) around poles. The pontoon hulas resemble raft like structures with 12mm thick and 150cm long ropes within the open space of mooring pontoons. The first experimentation with these structures was executed in the polyhaline harbours of the port of Rotterdam. The pole hulas were rapidly colonised by a variety of organisms. Above MLWL a seaweed community dominated on the strings. Below MLWL Mytilus edulis (the Blue mussel) was found to be the dominating species after a few months. In the dense layer of M. edulis on both pole hulas and pontoon hulas many mobile soft-bottom amphipods and young ragworms occurred, which means that colonisation on these structures compensate for biodiversity loss of bottom fauna due to dredging and disturbance by propellers of ships. Settlement of the exotic Crassostrea gigas (Pacific or Japanese oyster) did not occur on the strings of the pole hulas, the ropes of the pontoon hulas and not on the poles with hulas.Wet biomass (including shells) on pole hulas was positively correlated with depth and on average 4.4–11.4 times higher compared to biomass on reference poles. Colonization of the pontoon hula ropes was similar to colonization of the pole hulas below MLWL. Biomass production per rope was density dependent and optimal density of ropes was estimated at 4–8ropesm−2. Biomass (mainly M. edulis) on the ropes of pontoon hulas decreased to a half from the edge to the heart of the hula demonstrating the limitation of food by competition.It was concluded that ecological engineering in the port of Rotterdam with simple structures such as pole and pontoon hulas strongly enhances sessile biological production and biodiversity. This is likely to result in a positive impact on local water quality, and, if applied at larger scales, may have positive influences on the remains of Rhine–Meuse estuary.

Patterns in macrozoobenthic assemblages indicate the state of the environment: insights from the Rhine-Meuse estuary

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 436:29-50 (2011) - DOI: https://doi.org/10.3354/meps09210 Patterns in macrozoobenthic assemblages indicate the state of the environment: insights from the Rhine-Meuse estuary Sander Wijnhoven*, Herman Hummel Monitor Taskforce, Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology (NIOO-CEME), Korringaweg 7, PO Box 140, NL-4401 NT, Yerseke, The Netherlands *Email: s.wijnhoven@nioo.knaw.nl ABSTRACT: During the 1960s and early 1970s, the open system of the Rhine-Meuse estuary with a continuous salinity gradient was converted into a series of stagnant fresh water lakes separated by sluices from the North Sea coastal region ‘Voordelta’. For the last 50 yr, the macrozoobenthic communities have been monitored intensively, and a large dataset is available on the responses of macrozoobenthic communities to strong disturbances (e.g. transitions in salinity, currents and dynamics) and gradual changes (e.g. changes in pollution and nutrient levels). The current study analyzes patterns and developments in macrozoobenthic compositions in terms of community descriptors (e.g. density, richness, evenness and diversity) and proportional distributions over feeding guilds (suspension feeders, deposit feeders, surface deposit feeders, herbivores, omnivores and predators). We compare historic and present situations and show that communities respond in similar ways (viz. peaking densities co-occurring with decreasing richness and diversity) to large disturbances and that the distribution over the feeding guilds indicates the state of the environment. Although the historic communities showed signs of system deterioration (mainly due to pollutants and nutrients), and at present, exotic species are abundant in the region, historic data were found to be valuable as a reference for what can be expected on the macrozoobenthic level following estuary restoration. This study showed the value of an undisturbed estuary in terms of total macrozoobenthic diversity compared to the series of fresh water lakes now separated from the Voordelta system and raises questions about future plans for a regulated salt water inlet that will be restricted to only 11.5 km inland. KEY WORDS: Benthic communities · Species diversity · Feeding guilds · Quality indicators · ­Reference state · Salinity gradient · Multi-dimensional scaling Full text in pdf format Supplementary material PreviousNextCite this article as: Wijnhoven S, Hummel H (2011) Patterns in macrozoobenthic assemblages indicate the state of the environment: insights from the Rhine-Meuse estuary. Mar Ecol Prog Ser 436:29-50. https://doi.org/10.3354/meps09210 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 436. Online publication date: August 31, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

New threats of an old enemy: The distribution of the shipworm Teredo navalis L. (Bivalvia: Teredinidae) related to climate change in the Port of Rotterdam area, the Netherlands

The effects of four climate change scenarios for the Netherlands on the distribution of the shipworm upstream of the Rhine–Meuse estuary are described. Global warming will cause dry and warmer summers and decreased river discharges. This will extend the salinity gradient upstream in summer and fall and may lead to attacks on wooden structures by the shipworm. Scenarios including one or two degree temperature increases by 2050 compared to 1990 with a weak change in the air circulation over Europe will lead to an increased chance of shipworm damage upstream from once in 36years to once in 27 or 22years, respectively; however, under a strong change in air circulation, the chance of shipworm damage increases to once in 6 or 3years, respectively. The upstream expansion of the distribution of the shipworm will also be manifested in other northwest European estuaries and will be even stronger in southern Europe.

Distribution, settlement, and growth of first-year individuals of the shipworm Teredo navalis L. (Bivalvia: Teredinidae) in the Port of Rotterdam area, the Netherlands

During the period 2004–2008 the distribution, settlement, and growth of first-year shipworms ( Teredo navalis L., 1758) was studied by exposing fir and oak panels in the Port of Rotterdam area, which is situated in the Rhine-Meuse estuary in the Netherlands and covers the complete salinity gradient. Shipworms were found yearly in the western large polyhaline harbours. On only a few occasions were they were found in harbours that showed large seasonal and daily fluctuations in salinity. In 2006 the shipworm was found in fir panels 20 km upstream from the polyhaline harbours, demonstrating their ability to travel with the tidal currents over considerable distances and to settle once the abiotic conditions become favourable. Although the water temperatures allowed them to breed from April until November, infestations were not found before September, and from the size of the animals in the panels it was concluded that in the Port of Rotterdam area they spawned from August until the end of November. The settlement height was negatively correlated with the distance of the panels to the sea floor. In the first season after settlement they showed a substantial growth rate of 0.18 cm day −1. The longest shipworm found measured 36.8 cm after 4–5 months of growth after settlement. Infestations and growth were lower in oak than in fir wood. In 2006 the maximum consumption of wood by individuals settled in the same year in panels at the bottom accounted for 12.4%. Shell size and body length of the animal after the first season of growth showed a significant positive logarithmic relation. In both 2006 and 2007 a similar relation between the average boring tube diameter and the length of the animals was found. Lower river discharges leading to salinisation of the eastern part of the Port of Rotterdam area create conditions favourable for the shipworm, with serious consequences for the piles upon which the quays are built.

Toxicity profiling: An integrated effect‐based tool for site‐specific sediment quality assessment

A toxicity profile is a toxicological "fingerprint" of an environmental sample, obtained by testing its extract in a battery of bioassays. Each represents a different mode of action. The present work explores the applicability of in vitro toxicity profiles as an effect-based tool for sediment quality assessment. For this purpose, a previously published dataset was used, in which sediment extracts from 15 different locations in the Rhine-Meuse estuary were tested in 5 different bioassays. Three useful approaches could be distinguished for applying toxicity profiles in sediment quality assessment. In the first approach, toxicity profiles are translated into hazard profiles, indicating the relative distance to the desired or acceptable sediment quality status for each toxic mode of action. Hazard profiles can be considered as location-specific characteristics; sampling locations with similar hazard profiles can be classified into clusters. This approach seems directly applicable but requires a very careful selection of a reference toxicity profile that is either measured at a reference location or is designated as a desirable or acceptable toxicity profile for that particular location. In the second approach, toxicity profiles are translated into ecological risk profiles indicating for each toxic mode of action the ratio between the actual measured bioassay response and the bioassay response level that is considered safe for environmental health. This approach has a high ecological relevance but is only feasible for a few modes of action for which toxicity data are available at the ecological level of population or higher that allow derivation of ecologically safe bioassay responses for sediment extracts. In the third approach, toxicity profiles and their derived hazard profiles are used to select samples with unusually or unexpectedly high bioassay responses for further in-depth effect-directed analysis (EDA). EDA is a powerful strategy to identify emerging compounds that contribute significantly to the toxic load on the environment. EDA is an expensive and laborious strategy, however, making it currently suitable only for investigative monitoring on a limited scale and not for routine monitoring. Future perspectives in toxicity profiling include expansion of the battery of bioassays with test methods that cover other toxic endpoints or multiple endpoints, are high throughput, and improve the ecological relevance.

Development of a mid-Holocene estuarine basin, Rhine–Meuse mouth area, offshore The Netherlands

A proper understanding of coastal development during periods of rapid sea-level rise is a prerequisite for the prediction of future coastal response to the expected acceleration in sea-level rise. However, the development of back-barrier basins, especially in river-mouth areas, during such periods is still not well understood. Here we show the response of back-barrier basins adjacent to the Rhine–Meuse river-mouth area, The Netherlands during the mid Holocene. A combination of high-resolution seismic data and cores was used to describe and explain late-Weichselian to early Holocene terrestrial and mid-Holocene back-barrier sequences. Along with dating and micropalaeontological analyses, these descriptions and explanations form the basis of a reconstruction of river-mouth and adjacent basin behaviour under conditions of rapid SLR. The nearby presence of the Rhine–Meuse estuary had a significant influence on the development of the adjacent back-barrier basin and especially its tidal channels. The back-barrier channels started to fill in between 8.3 and 7.4 ka BP due to decreasing rates of both sea-level rise and tidal-amplitude increase. During this time the Rhine relocated its mouth to the study area and the increased sediment delivery contributed to tidal-prism decrease and channel infill. Close to the Rhine–Meuse river mouth the back-barrier channels lay embedded in thick clayey, estuarine deposits and lateral migration of the tidal channels was limited. At greater distances, lateral migration rates increased as the thickness of cohesive back-barrier deposits decreases. The spatial pattern and stratigraphic setting of the northern channel fills suggests an episode of barrier overstepping between 7.5 and 6.6 ka BP. The mode and spatial limits of this overstepping are still not understood.

Historic developments in macrozoobenthos of the Rhine–Meuse estuary: From a tidal inlet to a freshwater lake

Water works during the 1960s and 1970s changed the northern part of the Rhine–Meuse estuary in the south-west of the Netherlands into a freshwater lake, from west to east divided into three basins called the Haringvliet, the Hollands Diep and the Biesbosch. Concurrently water quality parameters (e.g. nutrients and pollutants) changed drastically during the last 50 years. This study combines macrozoobenthic monitoring data from the region from 1960 to 2001 with trends in abiotic parameters to evaluate historic developments of the communities, including densities, species numbers and diversity, and assess future developments as a first step to a rehabilitation of the estuary as planned for January 01, 2008. During the 1960s, the macrozoobenthic densities of Oligochaeta and/or Polychaeta dominated communities increased with a gradual decrease of saltwater intrusion and salinity variability. The first years after the basins became stagnant, the species numbers per sample and the Shannon diversity were high due to the coexistence of salt and freshwater species. An increase in nutrient and pollutant loads led to a decrease in the macrozoobenthos densities. As water and sediment quality gradually improved, nowadays the former estuary contains high diversity and high density macrozoobenthic communities, whereas Oligochaeta and/or Polychaeta were dominant in the 1960s, and Bivalvia and Gastropoda were more abundant during the 1970s. Macrozoobenthic communities moved from the east to west with a time-lag, which may primarily be attributed to changing salinities, salinity variances and oxygen levels. Therefore, the current communities of the Haringvliet show similarities with the communities that occurred already during the 1960s in the Biesbosch. This study shows the value of macrozoobenthos monitoring data over longer periods. The possible impact of a new saltwater inlet in the west of the Haringvliet, allowing in the near future saltwater to enter 11.5 km eastward, yet alternated by frequent flushing with freshwater to ascertain that the salt intrusion does not reach further, on the development of the macrozoobentic communities is discussed.

Quagga mussels Dreissena rostriformis bugensis (Andrusov, 1897) in the Main River (Germany)

The first record of the quagga mussel Dreissena rostriformis bugensis (Andrusov, 1897) in Germany is described. This species has expanded its distribution area in Europe at a slower rate than the zebra mussel Dreissena polymorpha (Pallas, 1771). Recent records from the Danube in Romania and from an enclosed Rhine-Meuse estuary in The Netherlands, suggest that the Main-Danube canal and River Rhine have functioned as the dispersal pathway of the quagga mussel to The Netherlands. The record of this species in the River Main supports this hypothesis.

Alternative management of the Haringvliet sluices: first step towards major rehabilitation of the Rhine-Meuse estuary.

In 1970 the Haringvliet estuary of the rivers Rhine and Meuse was closed off from the sea by a dam with outlet sluices to protect the land from storms at sea. This resulted in (he creation of a freshwater lake, suitable for agricultural water supply and drinking water. However, the change of an open estuary into a semi-stagnant lake had many negative ecological effects: loss of natural habitats, bank erosion, accumulation of polluted sediments and the disappearance of a natural transition zone between the estuary and the sea. Hence many characteristic species disappeared. In the 1980s public demand for nature in the Netherlands grew and agricultural importance declined. The ecological degradation was more and more recognized as a problem. In 1988 the government decided to investigate whether it was possible to change the management of the Haringvliet sluices. The conclusion was that alternative management, can lead to a major rehabilitation of the Rhine-Meuse estuary since natural processes will return. In 1998, the government decided to strive for an alternative management regime. The first step will be to put the 'sluices ajar' in 2008. This article gives an overview of questions arisen and learning points encountered while implementing the alternative management of the Haringvliet sluices.

From nature conservation towards restoration of estuarine dynamics in the heavily modified Rhine-Meuse estuary, The Netherlands.

The Rhine-Meuse estuary is potentially one of the most important wetland areas in the Netherlands, but since the Middle Ages it has been subjected to excessive influence from human activities. Marshes have been convened into agricultural land, harbours, and cities. Estuarine dynamics were minimized for storm surge protection, and the estuary became heavily polluted. Since the late seventies water quality has improved, and therefore habitat rehabilitation has become effective. To achieve a healthier ecosystem a program of remedial action has been launched. Some elements, such as the protection of valuable wetlands against erosion, have been almost completely realized. In the nineties, the management strategy changed direction towards the restoration and enhancement of ecological functions, rather than being confined exclusively to the protection of existing values. The main issue became the restoration of estuarine processes. This can be achieved through the modification and management of the Haringvliet sluices, and by creating conditions that assist ecological rehabilitation. Greater emphasis is given to the restoration of tidal and river dynamics in polders by replacing the dike further inland. These projects are in many respects different from the nature conservation works, and therefore require a different, more process oriented approach. The main challenge is to find a compromise between ecological values and multiple human uses and demands upon the water system, such as safety from flooding, recreational use, agriculture, and housing projects on the waterfronts.

Radiometric sand–mud characterisation in the Rhine–Meuse estuary: Part A. Fingerprinting

The composition of the underwater sediment bed is often defined in terms of the mud (<63 μm) or sand (>63 and <2000 μm) content. Mud and sand differ considerably in their content of (natural) radionuclides. Radionuclide concentrations are highest in the clay minerals that form a significant part of the mud fraction. A study was initiated in the Hollandsch Diep and Haringvliet, two semi-stagnant fresh water basins in the southwestern part of Rhine–Meuse Estuary (The Netherlands), to investigate whether radionuclides can be utilised to determine the sand and mud content of aquatic sediments. The radiometric fingerprint was assessed based on sediment samples with different composition, provenance and age. Differences in provenance (Rhine versus Meuse) could be discriminated by the 40K content of the sediment. Differences in age or composition, in terms of mineralogy, organic matter or carbonate content, only slightly affected the radiometric characteristics based on 232Th and 238U. On the other hand, no reliable correlation between 40K and the mud content was obtained; therefore, 40K is excluded from the fingerprint. The fingerprints based on the 232Th and 238U contents characterise mud (<63 μm) and sand (>63 μm) as: 46.2±1.9 Bq/kg 238U and 9.3±0.9 Bq/kg 238U and 45.6±1.9 Bq/kg 232Th and 9.7±0.9 Bq/kg 232Th, respectively. The assessed radiometric fingerprint allows a quantitative interpretation of the mud and sand content through the total 238U+ 232Th activity. This interpretation is confirmed by the high correlation ( R 2=0.96) for the mud percentages obtained from radiometric analysis and those by laser diffraction in the laboratory. It can be concluded that radiometric sedimentology provides a tool for reliable sediment characterisation. New perspectives are opened when radiometric fingerprints are combined with in situ radiometric characterisation of sediments, which will be presented in part B of this paper (this issue).

Radiometric sand mud characterisation in the Rhine–Meuse Estuary: Part B. In situ mapping

Traditionally, obtaining accurate spatial information about the textural composition of heterogeneous aquatic sediments requires extensive sediment sampling. To avoid a costly and time-consuming operation, a new in situ technique has been investigated. This technique characterises sediment components by the activity concentrations of natural radionuclides. In situ activity concentrations are continuously measured with the Multi-Detector system for Underwater Sediment Activity (MEDUSA), a detector trailed over the bottom by a towing vessel, containing a highly sensitive gamma-ray BGO-detector, a water-depth sensor and a microphone. From the collected γ-ray spectra, the activity concentrations of 40K, 238U and 232Th are derived. During two separate field surveys of 5 days each, MEDUSA was applied to the Hollandsch Diep and Haringvliet, two fresh water basins, each approximately 20 km long and 2–3 km wide in the Rhine–Meuse Estuary, the Netherlands. In part A of this paper (this issue), it has been illustrated how the radiometric fingerprint of sand and mud in the area was determined. This fingerprint was then used to calculate the sand-mud ratio from the 238U and 232Th activity concentrations of about 25,000 MEDUSA data points. The interpolated sand-mud distribution of the top layer of the aquatic sediment shows a distinct correlation with bathymetry: the deeper channels are mud-rich, whereas the shallow zones are predominantly sandy. In general, the mud content decreases in the seaward direction. This corroborates well with the morphological development of the area in which the deposition of mud is highest near the rivers' outflow in the east. The absolute total random error in the sand map varies between 6% and 18%, where the largest errors are caused principally by spatial variability of the sediment composition. At present, the radiometric results underestimate the mud content by 10–30% due to a higher water content of sediment with high mud contents (>60%). The simultaneously recorded friction sound levels provide qualitative information with respect to the sediment composition. Sandy areas show a higher sound level than muddy areas; with the sound and sand map showing a striking similarity.

A two-dimensional model for suspended sediment transport in the southern branch of the Rhine-Meuse estuary, The Netherlands

A two-dimensional model for suspended sediment transport in the southern branch of the Rhine-Meuse estuary, The Netherlands

Helophyte Zonation in Two Regulated Estuarine Areas in the Netherlands: Vegetation Analysis and Relationships with Hydrological Factors

The relationship between hydrological conditions and riparian helophyte vegetation was studied in two freshwater estuaries that differed in tidal regulation in order to assess the effects of large-scale hydrological regulation on the fringe vegetation. Vegetation and environmental variables were sampled for 206 sites in the Rhine-Meuse estuary (146 sites) and the IJsselmeer region (60 sites) in the Netherlands. These samples were classified into 10 vegetation types, all of which were dominated by tall helophytes. The most common vegetation types were dominated byPhragmites australis andTypha angustifolia, which formed both monospecific stands and mixtures. Tall Cyperaceae dominated three vegetation types (dominated individually bySchoenoplectus lacustris, Bolboschoenus maritimus, andSchoenoplectus tabernaemontani).Acorus calamus. Principal components analysis of the species composition of vegetation fringing open-water areas and associated environmental data revealed complex gradients incorporating differences in water depth, water-level fluctuation, were exposure, and sedimentation and/or erosion. The composition of the helophyte belts varied among the areas as the result of the differing times at which regulation occurred. Based on historical data, hydrological regulation of the estuaries has resulted in deterioration of intertidalSchoenoplectus andBolboschoenus stands, due to erosion and predation.P. australis stands have been invaded by terrestrial plant species or have been replaced byT. angustifolia. A scheme is presented of helophyte vegetation development under the influence of changes in the hydrological regime.