Variations In Sediment Discharge Research Articles

Application of GIS to Evaluate Long-Term Variation of Sediment Discharge to Coastal Environment

This paper presents a GIS-based method to calculate the total sediment discharge from river basins to coastal areas. This method uses Revised Universal Soil Loss Equation (RUSLE) to calculate the rate of soil erosion and Gross Erosion-Sediment Delivery method (GESD) to calculate the total sediment discharge in a GIS modeling environment. The model is tested using the data of Abe River then applied to four river basins in Asia. Global data sets are used as the input to the current model. The result shows that there are significant variations of sediment discharges due to the precipitation change in these river basins.

PREDICTION OF SEDIMENT RUNOFF FROM A MOUNTAIN WATERSHED

In this paper, a simulation method is developed to predict sediment runoff from a mountain region into a reservoir. In the model, the watershed is constituted by several slope units and channel network. A flood runoff in each slope unit is calculated by using kinematic wave runoff model, and sediment runoff in the channel network is simulated by using governing equations which correspond to each flow regime such as a debris flow, a immature debris flow, and bed load. The model is applied to the actual mountainous watershed, the Fudosawa and Nigorisawa River, Nagano Prefecture. The spatial and temporal variation of sediment discharge, bed elevation, and grain size distribution are calculated. Calculated sediment volume inflowing into the Takase reservoir approximately agrees with observed one.

Exhumation of the Pyrenean orogen: implications for sediment discharge

Apatite fission track analyses of 21 samples from the central and eastern Pyrenees are modelled to generate time–temperature plots for the post 110±10 °C cooling history over the 40–10 Ma time interval. Modelled thermal histories have been converted into exhumation plots through the application of the present‐day geothermal gradient in the Pyrenees. The documented geology of the Pyrenees allows us to assume no significant extensional unroofing and subvertical exhumation trajectories, thus enabling exhumation to be translated into erosional denudation. Maps of denudation have been constructed for six, 5‐Myr time intervals. Denudation varied with a 20–50‐km length scale, and does not appear to have been related to the major structural zones of the mountain belt. Spatially averaged denudation rates for the six time intervals ranged from 34 to 61 mm kyr−1 assuming the present‐day geothermal gradient. Maximum rates of 240 mm kyr−1 occurred in the interval 35–30 Ma, in the region of the Querigut‐Millas massif.Assuming the denudation resulted primarily from erosion, the denudation maps can be used to calculate sediment discharge through time to the neighbouring foreland basins. Using a series of rectangular drainage basins with a 2:1 aspect ratio (based on modern linear mountain belts) and a location of the main drainage divide based on the mean present‐day position, it is possible to evaluate the potential for spatial and temporal variations in sediment discharge as a function of denudation. The results show along‐strike variations in sediment discharge between drainage basins of 500%, and temporal variations from individual basins of >300%. A comparison of total sediment discharge per year to the Ebro and Aquitaine basins, assuming a fixed drainage divide, shows that the discharge to the south is likely to have been between 1.5 and 2.8 times greater than to the north.

PREDICTION OF BRAIDED CHANNELS PATTERN AND SEDIMENT TRANSPORTATION

In this study, we propose a simulating method to predict channel pattern in braided reach. Stream length which is defined as the distance from a channel bifurcating point to next bifurcating point, can be predicted by the channel width and the Froude number of the stream channel. We calculate the channel pattern using the channel length, separating rates of flow discharge and angles of channel bending at bifurcating paints.The channel pattern calculated by the simulation model agree well with the experimental data. And also, the temporal variation of sediment discharge through braided channels can be estimated by this model.

Degradation downstream from a sluice gate; variation of bed and sediment characteristics with time and discharge

Results observed from a laboratory study concerning the degradation of an alluvial channel owing to the flow of clear water are reported. Two sizes of sand, of median diameter 0.47 mm and 0.79 mm and geometric standard deviation 4.65 and 3.54, respectively, are used as a bed material. This paper focuses on studying the variation of the surface layer size with time and distance, the variation of sediment discharge with time, the sediment size of the armoured layer, and the time required for stabilization of the channel bed. Some useful equations for predicting the sediment size of the armoured layer and the total time for the degradation are also given.

Variation of Sediment Discharge in Braided channels

In braided channels which often form in mountain streams, sediment discharge varies spatially and temporally with the variation of channels. Results obtained from flume tests show that merging and diverging phases of channels occur alternately and repeatedly in space and time, which bring about the periodic change of sediment discharge. In order to estimate the sediment discharge averaged temporally, the average width of the channel should be known. We propose a new method predicting the width of channel in terms of the concept of the self-formed channelwhich is similar to the criterion of alternating bars formation.

Dynamics of turbidity currents measured in Katsurazawa Reservoir, Hokkaido, Japan

The movement of turbid underflows in a reservoir is examined using three recording current meters. The density underflows, called turbidity currents, are induced by sediment discharge in the inflow system during the spring thaw. The continuous measurement near the reservoir bottom at two longitudinally located stations shows the characteristic diurnal variations of flow velocity and temperature, and decreasing temperature in the downslope direction. The former corresponds to the diurnal variations of sediment discharge and its temperature in the inflow system, and the latter is explained by the ‘entrainment’ of upper, cold water which turbidity currents induce. Assuming the upper water temperature to be 0°C, two mixing coefficients and turbidity-current discharge at the stations are numerically obtained. Applying Manning's equation to the critical, low turbidity-current discharge, a value of 1.51 is obtained for the roughness coefficient, n. Hence, assuming n to be constant, Reynolds number, Re, and densimetric Froude number, Fd, for the turbidity currents are evaluated at 7.1 × 10 4–5.8 × 10 5 and 1.5–4.4, respectively. The entrainment coefficient. Ee, from the equation of continuity holds a significant correlation with ‘overall Richardson number’, Ri 0 (= 1 Fd 2 ) . This shows decreasing Ee with increasing 1 Ri 0 . The decreasing entrainment is due to decreasing friction at the interface with the relative increase of kinetic energy. This could be applied to turbidity currents with Re of the order of 10 5. The downslope movement of turbidity currents with the diurnal variations of discharge and temperature is also explained by the longitudinal propagation of long waves. These could change from kinematic to intermediate waves with increasing discharge.

SEDIMENTOLOGICAL ASPECTS OF THE 1988 ORANGE RIVER FLOODS

SUMMARY Daily sampling of suspended sediment in the flooding Orange River was carried out at Alexander Bay from late February to the end of May 1988. Coupled with simultaneous measurements of water run off, the suspended-sediment data have been used to calculate daily variations in sediment discharge from just before the floods started, through four distinct pulses of varying magnitude and duration, namely the Bloemfontein, Fish River, Kroonstad and Upington floods. During the 3-month period, 24,3 km3 of water, 118,6% more than the mean annual runoff of 11,1 km3, was discharged, varying from 15,5 km3 in March (a record) to 5,3 km3 in April and 3,5 km3 in May. The amount of sediment, transported chiefly in suspension, was substantial but was not in proportion to the amount of water. During March 64,2 × 106t was carried out to sea, the load declining sharply to 9,4 × 106t in April and 7,3 × 106t in May. The total sediment discharge for the three-month period amounted to 80,9 × 106t, which exceeds the mean a...