A model for the estimation of water filtration and nutrient regeneration by exposed sandy beaches

Exposed sandy beaches filter huge volumes of sea water daily as a result of wave and tide action. A regression model is developed which predicts the volume of water filtered daily by a beach as a function of tide range, beach slope and sand particle size. From these parameters estimates are also made of intertidal distance, the degree of expansion of the intertidal above the still water tide range by wave action and the average path length through the sand interstices percolated by filtered sea water. An equation is derived relating the degree of mineralisation of organic matter in the filtered sea water to the distance filtered through the sand and the sand particle size. Employing all this information, a simple model is used to estimate the volumes of sea water filtered and the amounts of inorganic nitrogen regenerated by intertidal sandy beaches over a range of physical conditions. The implications of this nutrient generation for the inshore zone are discussed.     

Design of sediment detention basins: Scaled model experiments and application

Sediment detention basins are implemented on mountain rivers to trap solid material that may aggravate the flooding of downstream settlements.However,retention structures built in the past may unnecessarily retain sediment during non-hazardous flood events,resulting in high maintenance costs and sediment deficit downstream.In addition,the so-called spontaneous self-flushing of previously retained sediment during floods has occasionally been observed.Recent research suggests to design sediment detention basins for controlling sediment passage with a guiding channel across the deposition area upstream of a hybrid barrier.Such barriers consist of a check dam with a slot orifice and an upstream bar screen with a bottom clearance in order to benefit from a combined mechanical-hydraulic retention control.The present paper enhances this pioneering research with the help of new experimental data,including a wide range of sediment mixtures and large wood,as well as variable barrier heights.Improved design criteria are provided regarding the bar screen and the basin storage capacity.The functionality of the enhanced concept for sediment detention is illustrated by a case study on a physical model:the protection of the Villard-Bonnot village(France)against torrential hazards.     

Modelling mean annual sediment yield using a distributed approach

In this paper a spatially distributed model for the calculation of sediment delivery to river channels is presented (SEDEM: SEdiment DElivery Model). The model consists of two components: (1) the calculation of a spatial pattern of mean annual soil erosion rates in the catchment using a RUSLE (Revised Soil Erosion Equation) approach; and (2) the routing of the eroded sediment to the river channel network taking into account the transport capacity of each spatial unit. If the amount of routed sediment exceeds the local transport capacity, sediment deposition occurs. An existing dataset on sediment yield for 24 catchments in central Belgium was used to calibrate the transport capacity parameters of the model. A validation of the model results shows that the sediment yield for small and medium sized catchments (10–5000 ha) can be predicted with an average accuracy of 41 per cent. The predicted sediment yield values with SEDEM are significantly more accurate than the predictions using a lumped regression model. Moreover a spatially distributed approach allows simulation of the effect of different land use scenarios and soil conservation techniques. Copyright © 2001 John Wiley & Sons, Ltd.     

OH defects in forsterite

Polarized FTIR spectra of near endmember forsterite single crystals from Pamir, Tadzikistan show the existence of sharp strongly pleochroic absorption bands in the region of the OH stretching fundamental. Bands centered at 3674/3624, 3647/3598 and 3640/ 3592 cm^-1 are attributed to OH dipoles oriented parallel to [100]. An OH band doublet at 3570/3535 cm^-1 shows both, a strong absorption parallel to [100] and a strong component parallel to [001]. On the basis of the pleochroic scheme and under the assumption of vacancies on Si- and M-sites it is proposed that O1 is partially replaced by OH defects pointing to the vacant Si-site. O3 is donator oxygen of OH dipoles lying near the O3-O1 tetrahedral edge or roughly pointing to a vacant M2-site. Also O2 can act as donator oxygen of an OH group oriented along the O2-O3 edge of a vacant M1 octahedron. The splitting of the bands is explained by the presence of Fe²⁺ in cation sites surrounding the OH defects.     

Seismic codas on the Earth and the Moon: a comparison

The phenomenon of the seismic coda, which is composed of seismic energy delayed by scattering, is seen on both the Earth and the Moon. On the Moon the scattered coda is very large relative to body wave arrivals with a delay of the time of maximum energy, whereas on Earth scattered codas are relatively small and show no delay of the energy maximum. In both cases the form of the coda is controlled by three distance scales, the mean free path L, which is the average distance seismic energy travels before it is scattered, the attenuation distance $\text{x1}$, which is the average distance seismic energy travels before it is attenuated, and the source-receiver distance R. Two coda models are discussed based on these parameters; a strong scattering (diffusion) model, and a weak scattering (single scattering) model. A discussion of the diffusion scattering model indicates that if $\text{x1/L ? 1}$, diffusion scattering is an appropriate model, but if $\text{x1/L ? 1}$, single scattering is the appropriate model, within the appropriate range of R. A survey of the literature indicates that for the frequency range 0.5–10 Hz, diffusion scattering is important in lunar codas, but for the frequency range 1–25 Hz single scattering is important in terrestrial codas. Another important effect of attenuation is the elimination of scattering paths much longer than $\text{x1}$. On the Moon, this means that seismic energy in the coda can only propagate directly in the near-surface strong scattering zone between surface sources and the seismometer for source-seismometer separations of the order of $\text{(x1L)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$; otherwise, scattering is limited to regions near the source and the receiver. On Earth, this effect probably prevents multiple scattering.