In recognition of the environmentally sensitive nature of inlets and of a requirement to improve ... more In recognition of the environmentally sensitive nature of inlets and of a requirement to improve present knowledge of their function, the INDIA project has studied a small natural tidal inlet located in the Ría Formosa, Algarve, Portugal. The project has used state-of-the-art field equipment to study present day processes, and a range of numerical models to extend the spatial and temporal range of the measurements. Underpinned by knowledge of inlet evolution over several years, and by knowledge of other inlet systems, a conceptual ...
Suspended sand is sampled at several heights and positions on a beach and foredune, providing det... more Suspended sand is sampled at several heights and positions on a beach and foredune, providing detailed insight into the vertical and horizontal variation in sand content in the air during landward transport. Grain-size analysis is used to study the changes in grain-size distribution during landward transport. Mean grain size and sorting decrease during transport. Changes in textural parameters follow a gradual and regular path when the sediment is transported into the foredune. Sediment trapped on the seaward slope at a height of 30-50 cm above the surface closely resembles the sediment trapped landward, which implies that changes in the direction of transport are related to vertical changes within the sediment transport profiles. The movement of sand over the vegetated foredunes is induced by turbulent forces created by the air flowing across the vegetation and the foredune, leading to a change from saltation on the beach to modified saltation and suspension on the foredune. Small grains are lifted higher and fall more slowly than coarse grains and therefore are transported further landward, resulting in a gradual decrease in grain size of the landward-deposited sediment. Folk (1971) proposed a quantum model of aeolian deposition: wind deposits consist of a series of quantum bursts or swarms of well-sorted sand grains. Each quantum is carried by a separate gust and deposited as a thin layer of grains. The characteristics of the grains depend on the direction and strength of the gust. The resulting deposit is a mixture of quanta, which is reflected in the polymodal or skewed nature of the frequency curves . For these reasons, samples from beach and foredune are difficult to compare with respect to transport dynamics. Better results will be obtained if only the active layer is sampled . However, in the dune environment, sampling of the active layer is difficult to achieve when the surface is vegetated. In this case, trapping of the sediment when it is transported potentially renders good results.
Vegetation density on foredunes exerts an important control on aeolian sediment transport and dep... more Vegetation density on foredunes exerts an important control on aeolian sediment transport and deposition, and therefore on profile development. In a long-term monitoring field experiment, three plots were planted with regular grids of reed bundles in three different densities: 4, 2 and 1 bundles per m 2 . This study reports on the differences in profile development under the range of vegetation densities.
In recognition of the environmentally sensitive nature of inlets and of a requirement to improve ... more In recognition of the environmentally sensitive nature of inlets and of a requirement to improve present knowledge of their function, the INDIA project has studied a small natural tidal inlet located in the Ría Formosa, Algarve, Portugal. The project has used state-of-the-art field equipment to study present day processes, and a range of numerical models to extend the spatial and temporal range of the measurements. Underpinned by knowledge of inlet evolution over several years, and by knowledge of other inlet systems, a conceptual ...
Jacobs, A.F.G., Van Boxel, J.H. and Shaw, R.H., 1992. The dependence of canopy layer turbulence o... more Jacobs, A.F.G., Van Boxel, J.H. and Shaw, R.H., 1992. The dependence of canopy layer turbulence on within-canopy thermal stratification. Agric. For. Meteorol., 58: 247-256.
Fast-response wind and turbulence instruments, including sonic anemometers, are used more and mor... more Fast-response wind and turbulence instruments, including sonic anemometers, are used more and more in aeolian sediment transport research. These instruments give information on mean wind, but also on fluctuations and turbulent statistics, such as the uw covariance, which is a direct measure of Reynolds' stress (RS) and friction velocity. This paper discusses the interpretation of sonic anemometer data, the transformations needed to get proper results and turbulence spectra, and how they are influenced by instrument size, sampling frequency, and measurement height.
This paper discusses a model which simulates dune development resulting from aeolian saltation tr... more This paper discusses a model which simulates dune development resulting from aeolian saltation transport. The model was developed for application to coastal foredunes, but is also applicable to sandy deserts with transverse dunes. Sediment transport is calculated using published deterministic and empirical relationships, describing the influence of meteorological conditions, topography, sediment characteristics and vegetation. A so-called adaptation length is incorporated to calculate the development of transport equilibrium along the profile. Changes in topography are derived from the predicted transport, using the continuity equation. Vegetation height is incorporated in the model as a dynamic variable. Vegetation can be buried during transport events, which results in important changes in the sediment transport rates. The sediment transport model is dynamically linked to a second-order closure air flow model, which predicts friction velocities over the profile, influenced by topography and surface roughness.
The feasibility of assessing the sampling efficiency of aerosol samplers for large wind-borne par... more The feasibility of assessing the sampling efficiency of aerosol samplers for large wind-borne particles in the open air was determined in a pilot study. The experiments with 17 gm diameter monodisperse aerosols show the low sampling efficiency of widely used instruments such as the EPA high volume sampler and LIB sampler for a coarse aerosol. A new developed tunnel sampler shows a high sampling efficiency for this aerosol relative to the Rotorod as a reference sampler. Additional experiments with different aerosol sizes and under varying meteorological situations are planned.
The exchange process in the lower region of a maize canopy is analyzed for two nights. It appears... more The exchange process in the lower region of a maize canopy is analyzed for two nights. It appears that during calm nights a free convection state develops in the lower region of the canopy. Convective heat is released at the soil's surface and transported directly to the higher portion of the canopy. The released sensible heat at the soil's surface can be easily calculated by applying the Nusselt number for free convection. At night thermal energy is also released through cooling of the canopy. The released heat from the stored canopy heat is of the same order of magnitude as all other energy terms. Conversely during daytime for most agricultural crops this energy term is of minor importance in comparison to the other energy terms. The formation of dew at night is an important process. A maxima1 possible estimate of dew accumulation for a particular night can easily be made by using the potential dew. The potential dew can be deduced more accurately by taking into account the released heat stored in the plant canopy.
In recognition of the environmentally sensitive nature of inlets and of a requirement to improve ... more In recognition of the environmentally sensitive nature of inlets and of a requirement to improve present knowledge of their function, the INDIA project has studied a small natural tidal inlet located in the Ría Formosa, Algarve, Portugal. The project has used state-of-the-art field equipment to study present day processes, and a range of numerical models to extend the spatial and temporal range of the measurements. Underpinned by knowledge of inlet evolution over several years, and by knowledge of other inlet systems, a conceptual ...
Suspended sand is sampled at several heights and positions on a beach and foredune, providing det... more Suspended sand is sampled at several heights and positions on a beach and foredune, providing detailed insight into the vertical and horizontal variation in sand content in the air during landward transport. Grain-size analysis is used to study the changes in grain-size distribution during landward transport. Mean grain size and sorting decrease during transport. Changes in textural parameters follow a gradual and regular path when the sediment is transported into the foredune. Sediment trapped on the seaward slope at a height of 30-50 cm above the surface closely resembles the sediment trapped landward, which implies that changes in the direction of transport are related to vertical changes within the sediment transport profiles. The movement of sand over the vegetated foredunes is induced by turbulent forces created by the air flowing across the vegetation and the foredune, leading to a change from saltation on the beach to modified saltation and suspension on the foredune. Small grains are lifted higher and fall more slowly than coarse grains and therefore are transported further landward, resulting in a gradual decrease in grain size of the landward-deposited sediment. Folk (1971) proposed a quantum model of aeolian deposition: wind deposits consist of a series of quantum bursts or swarms of well-sorted sand grains. Each quantum is carried by a separate gust and deposited as a thin layer of grains. The characteristics of the grains depend on the direction and strength of the gust. The resulting deposit is a mixture of quanta, which is reflected in the polymodal or skewed nature of the frequency curves . For these reasons, samples from beach and foredune are difficult to compare with respect to transport dynamics. Better results will be obtained if only the active layer is sampled . However, in the dune environment, sampling of the active layer is difficult to achieve when the surface is vegetated. In this case, trapping of the sediment when it is transported potentially renders good results.
Vegetation density on foredunes exerts an important control on aeolian sediment transport and dep... more Vegetation density on foredunes exerts an important control on aeolian sediment transport and deposition, and therefore on profile development. In a long-term monitoring field experiment, three plots were planted with regular grids of reed bundles in three different densities: 4, 2 and 1 bundles per m 2 . This study reports on the differences in profile development under the range of vegetation densities.
In recognition of the environmentally sensitive nature of inlets and of a requirement to improve ... more In recognition of the environmentally sensitive nature of inlets and of a requirement to improve present knowledge of their function, the INDIA project has studied a small natural tidal inlet located in the Ría Formosa, Algarve, Portugal. The project has used state-of-the-art field equipment to study present day processes, and a range of numerical models to extend the spatial and temporal range of the measurements. Underpinned by knowledge of inlet evolution over several years, and by knowledge of other inlet systems, a conceptual ...
Jacobs, A.F.G., Van Boxel, J.H. and Shaw, R.H., 1992. The dependence of canopy layer turbulence o... more Jacobs, A.F.G., Van Boxel, J.H. and Shaw, R.H., 1992. The dependence of canopy layer turbulence on within-canopy thermal stratification. Agric. For. Meteorol., 58: 247-256.
Fast-response wind and turbulence instruments, including sonic anemometers, are used more and mor... more Fast-response wind and turbulence instruments, including sonic anemometers, are used more and more in aeolian sediment transport research. These instruments give information on mean wind, but also on fluctuations and turbulent statistics, such as the uw covariance, which is a direct measure of Reynolds' stress (RS) and friction velocity. This paper discusses the interpretation of sonic anemometer data, the transformations needed to get proper results and turbulence spectra, and how they are influenced by instrument size, sampling frequency, and measurement height.
This paper discusses a model which simulates dune development resulting from aeolian saltation tr... more This paper discusses a model which simulates dune development resulting from aeolian saltation transport. The model was developed for application to coastal foredunes, but is also applicable to sandy deserts with transverse dunes. Sediment transport is calculated using published deterministic and empirical relationships, describing the influence of meteorological conditions, topography, sediment characteristics and vegetation. A so-called adaptation length is incorporated to calculate the development of transport equilibrium along the profile. Changes in topography are derived from the predicted transport, using the continuity equation. Vegetation height is incorporated in the model as a dynamic variable. Vegetation can be buried during transport events, which results in important changes in the sediment transport rates. The sediment transport model is dynamically linked to a second-order closure air flow model, which predicts friction velocities over the profile, influenced by topography and surface roughness.
The feasibility of assessing the sampling efficiency of aerosol samplers for large wind-borne par... more The feasibility of assessing the sampling efficiency of aerosol samplers for large wind-borne particles in the open air was determined in a pilot study. The experiments with 17 gm diameter monodisperse aerosols show the low sampling efficiency of widely used instruments such as the EPA high volume sampler and LIB sampler for a coarse aerosol. A new developed tunnel sampler shows a high sampling efficiency for this aerosol relative to the Rotorod as a reference sampler. Additional experiments with different aerosol sizes and under varying meteorological situations are planned.
The exchange process in the lower region of a maize canopy is analyzed for two nights. It appears... more The exchange process in the lower region of a maize canopy is analyzed for two nights. It appears that during calm nights a free convection state develops in the lower region of the canopy. Convective heat is released at the soil's surface and transported directly to the higher portion of the canopy. The released sensible heat at the soil's surface can be easily calculated by applying the Nusselt number for free convection. At night thermal energy is also released through cooling of the canopy. The released heat from the stored canopy heat is of the same order of magnitude as all other energy terms. Conversely during daytime for most agricultural crops this energy term is of minor importance in comparison to the other energy terms. The formation of dew at night is an important process. A maxima1 possible estimate of dew accumulation for a particular night can easily be made by using the potential dew. The potential dew can be deduced more accurately by taking into account the released heat stored in the plant canopy.
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Papers by John Van Boxel