Wind forces and related saltation transport
Leenders, J.K. ; Boxel, J.H. van; Sterk, G. - \ 2005
Geomorphology 71 (2005)3-4. - ISSN 0169-555X - p. 357 - 372.
atmospheric surface-layer - boundary-layer - sediment transport - sea
The effect of several wind characteristics on sand transport was studied in three experiments in north Burkina Faso, West Africa. The first experiment is used to analyse the relation between wind speed and shear stress fluctuations across height. The second experiment is used to study the relation of these wind characteristics with saltation transport for fourteen convective storms, registered during the rainy seasons of 2002 and 2003. The effect of sampling time is studied for two of these convective storms. The third experiment relates the turbulent structures of four convective storms to saltation transport. Wind speed measurements were undertaken with two sonic anemometers and sediment transport was measured by two saltiphones. The sampling frequency was either 8 or 16 Hz. The sonic frame of reference was rotated according to a triple rotation. Horizontal fluctuations showed a (fairly) good correlation with height because the wind speed at both sensors was affected by the same vortices. The correlation coefficients ranged from 0.42 (when the distance between the sensors was 1.75 m) to 0.92 (when the distance was 0.25 m). The instantaneous Reynolds' stress had the weakest correlation (correlation coefficient of 0.05 at 1.75 m between the sensors and 0.56 at 0.25 m between the sensors), because the momentum at 2 m above the soil surface is transported by different eddies than those close to the ground. This also explains the fairly good correlation coefficients between the horizontal components of the wind and saltation compared to the poor correlations between instantaneous Reynolds' stress and saltation. An increase in sampling time did not have much impact on these correlation coefficients up to sampling periods of about 30 s. However, this sampling interval would be too coarse to describe the vertical wind component adequately. The classification of the moments of shear stress into the turbulent structures, sweeps, ejections, inward and outward interactions, showed that the mean saltation flux is higher at sweeps and outward interactions than at ejections and inward interactions. Also, saltation occurred more often during sweeps and outward interactions than during ejections and inward interactions.
Late-Holocene evolution of the Mahakam delta, East Kalimantan, Indonesia
Storms, J.E.A. ; Hoogendoorn, R.M. ; Dam, R.A.C. ; Hoitink, A.J.F. ; Kroonenberg, S.B. - \ 2005
Sedimentary Geology 180 (2005)3-4. - ISSN 0037-0738 - p. 149 - 166.
brahmaputra river system - ganges-brahmaputra - sediment transport - subaqueous delta - shelf - ocean - quaternary - record
The late-Holocene Mahakam delta, located along the tropical eastern shore of Kalimantan, Indonesia, is considered to be a textbook example of a mixed tide-fluvial dominated delta system. The delta prograded about 60 km during the past 5000 years, which led to the development of a distinct network of distributary and tidal channels. Wave action is low due the limited fetch in the narrow strait of Makassar. Mahakam River discharge is about a quarter of the Mississippi River discharge and is characterized by absence of flood surges. Therefore, natural levees, crevasse splays and avulsions are absent in the delta plain. For the past four decennia, both modern and ancient Mahakam delta deposits have been studied in detail in order to better understand subsurface Miocene and Tertiary Mahakam deposits, which host large volumes of hydrocarbons. This study focuses on the dynamics and stratal patterns of delta plain, delta-front platform deposits and suspended sediments. Due to the predominance of semi-diurnal tides and the associated flow reversals, depositional patterns are highly variable which has resulted in the formation of characteristic sand-mud couplets. The distribution of the sand-mud couplets found in this study differs from previously proposed conceptual models. They are limited to the fluvial domain and form in the distributary channels (lateral channel bar) or at the fluvial dominated delta-front platform, which flanks the mouth bar deposits in offshore direction. The sand-mud couplets which formed as delta-front platform and lateral channel bar deposits are similar and can only be identified based on their 14C age. The sand content decreases significantly towards the tidal dominated areas due to limitation in transport capacity. Turbidity measurements taken in front of the river mouth also show rapid settlement of river plume sediments. Some 22 new AMS 14C dates show that late Holocene sea level history resembles the eustatic sea level curve giving a first approximation of the Late Holocene sea level history for East Kalimantan. The dates suggest that the progradational delta system evolved under conditions of slowly rising sea level, which compares well to the eustatic sea level curve. In addition, calculated averaged deposition rates of the sand-mud couplets indicate that deposition is driven by the spring-neap tide cycles instead of the daily tidal cycle
Nutrient losses by wind and water, measurements and modelling
Visser, S.M. ; Stroosnijder, L. ; Chardon, W.J. - \ 2005
Catena 63 (2005)1. - ISSN 0341-8162 - p. 1 - 22.
northern burkina-faso - sediment transport - farming systems - erosion - soil - productivity - balances - fields - flows - niger
In the Sahelian zone of West-Africa, erosion by both wind and water causes a serious decline in fertility of the already low fertile soils. Despite the fact that the flow of nutrients has been intensively investigated by the use of nutrient balances, little attention has been paid to the contribution of the soil erosion to the nutrient balance. Two physically based models (WEPS and EUROSEM, both written in PCRaster) were extended with nutrient modules to investigate the role of wind and water erosion in tire loss and gain of nutrients at the scale of a Sabetian field. The models are applied at three geomorphic units in the Katchari catchment in northern Burkina Faso. WEPS can predict spatial patterns of erosion and deposition due to wind-blown particle transport. Depending on wind direction, crusting and vegetation cover net erosion or deposition can occur. When erosion occurs considerable amounts of nutrients are lost, but when deposition occurs, most of these nutrients may be regained. Soil loss by water erosion is closely related to the crust type present, which regulates infiltration and thus runoff. Nutrient losses by water erosion are small compared with those by wind erosion, but are forever lost for the area. Sediment transport by wind in saltation mode results in the largest soil and nutrient loss at the time scale of an event. (c) 2005 Elsevier B.V. All rights reserved.