Umesh C. Kothyari

Estimation of soil erosion and sediment yield using GIS

Abstract A Geographical Information System (GIS) based method is proposed and demonstrated for the identification of sediment source areas and the prediction of storm sediment yield from catchments. Data from the Nagwa and Karso catchments in Bihar (India) have been used. The Integrated Land and Water Information System (ILWIS) GIS package has been used for carrying out geographic analyses. An Earth Resources Data Analysis System (ERDAS) Imagine image processor has been used for the digital analysis of satellite data for deriving the land cover and soil characteristics of the catchments. The catchments were discretized into hydrologically homogeneous grid cells to capture the catchment heterogeneity. The cells thus formed were then differentiated into cells of overland flow regions and cells of channel flow regions based on the magnitude of their flow accumulation areas. The gross soil erosion in each cell was calculated using the Universal Soil Loss Equation (USLE) by carefully determining its various parameters. The concept of sediment delivery ratio (SDR) was used for determination of the total sediment yield of each catchment during isolated storm events.

Artificial neural networks for daily rainfall—runoff modelling

Abstract The application of artificial neural network (ANN) methodology for modelling daily flows during monsoon flood events for a large size catchment of the Narmada River in Madhya Pradesh (India) is presented. The spatial variation of rainfall is accounted for by subdividing the catchment and treating the average rainfall of each subcatchment as a parallel and separate lumped input to the model. A linear multiple-input single-output (MISO) model coupled with the ANN is shown to provide a better representation of the rainfall-runoff relationship in such large size catchments compared with linear and nonlinear MISO models. The present model provides a systematic approach for runoff estimation and represents improvement in prediction accuracy over the other models studied herein.

Scour around spur dikes and bridge abutments

Realistic estimation of scour depth around spur dikes and bridge abutments in alluvial rivers is important for safe and economic design of their foundations. Procedures have been developed by previous investigators for determination of design scour depth in steady flows at abutments and spur dikes by making use of the design discharge. However, the time required by the design discharge to scour to its full potential is generally much larger than the time for which it runs. Therefore, computations on temporal variation of scour depth are also important for design purposes. Scour processes at bridge piers, abutments and spur dikes have been found to be similar except that the boundary layer effect induced by the channel wall upstream of the abutment or spur dike causes less scour around these as compared to the case of piers. In the present study, therefore, the concept of an analogous pier is developed. The analogous pier would have the same equilibrium scour depth as the given abutment or spur dike under similar hydraulic conditions. The parameters relating to drag due to flow around abutment/ spur dike and bridge pier have been found to be useful in establishing a relationship for the diameter of the analogous pier. The temporal variation of scour depth and the equilibrium scour depth at the spur dike and the abutment are then computed using pier scour equations with size of the analogous pier being taken as the pier diameter. Results obtained are verified using laboratory data of several investigators for both clear-water and live-bed scour conditions.

Sediment yield estimation using GIS

Abstract A method has been developed in the present study for the determination of the sediment yield from a catchment using a GIS. The method involves spatial disaggregation of the catchment into cells having uniform soil erosion characteristics. The surface erosion from each of the discretized cells is routed to the catchment outlet using the concept of sediment delivery ratio, which is defined as a function of the area of a cell covered by forest. The sediment yield of the catchment is defined as the sum of the sediments delivered by each of the cells. The spatial discretization of the catchment and the derivation of the physical parameters related to erosion in the cells are performed through a GIS technique using the Integrated Land and Water Information Systems (ILWIS) package.

Drag coefficient of unsubmerged rigid vegetation stems in open channel flows

Results of an experimental study on the drag force measurement involving a single stem kept in a channel flow stem array are presented. The data collected herein and those from literature indicate that the stem drag coefficient logarithmically increases with the areal stem density. The stem Reynolds number is noticed to have only a small effect on the stem drag coefficient which was however found to depend on the stem staggering pattern. The drag coefficient is less influenced by the Froude number in subcritical flows but it decreases with the Froude number in supercritical flows. New relationships are proposed for the stem drag coefficient which appear useful in partitioning the total flow resistance of vegetated bed streams into the stem and the bed particle resistances. The bed particle resistance applies to sediment transport through such vegetated flows for which the average flow velocity is available.

Influence of cohesion on scour around bridge piers

Experimental results on temporal variation of scour around circular bridge piers founded in cohesionless and cohesive sediments under steady clear water flows are reported. The difference between scour patterns in cohesionless and cohesive sediments is brought out. Considering the horse shoe vortex to be the prime agent causing scour, a procedure is developed for computing the temporal variation of scour depth in cohesive sediments. Empirical relationships have also been obtained for maximum scour depth around bridge piers in cohesive sediments.

Estimation of temporal variation of sediment yield from small catchments through the kinematic method

Abstract A method is proposed for estimation of temporal variation of the sediment yield for single storm-events in small catchments. The kinematic wave equations are used for simulation of overland flow whereas continuity equation for sediment and expressions for sediment detachment and transport are used to compute the sediment yield. The method is based on numerical solution of these equations through a non-uniform spatial grid formed by time-area segments of the catchment. Carefully collected data of 12 experimental catchments from varying climates and ranging in size from 0.002 km2 to 92.5 km2 are used for calibration and verification of the proposed method. The observed sediment yields during the storm events studied hererin varied from about 0.003 tonnes to 800 tonnes. The method developed in the present study depends on calibration against a record of past conditions. Therefore, it can not be used directly to predict the future changes in catchment land use or climate. However, the present method can be used for estimation of sediment yield in other such ungauged catchments which have similar hydro-meteorological conditions and for which other information are available.

Live-bed scour around cylindrical bridge piers

Considering the primary vortex in front of the pier to be the prime agent causing scour at bridge piers, a scheme has been proposed for computation of the temporal variation of scour depth during live-bed condition. An equation has also been developed for the computation of maximum scour depth around circular bridge piers in uniform sediments during live-bed condition. The equation follows logically from the scheme proposed for the computation of the temporal variation of scour. The effect of unsteadiness of flow on scour depth has been also studied.

Effect of tall vegetation on sediment transport by channel flows

Results of an experimental study on sediment transport by channel flows with tall rigid stems are presented. The experiments were conducted for various stem areal densities, channel slopes and sediment sizes with uniformly distributed unsubmerged cylindrical rigid stems forming a regular square staggering pattern in plan. The rates of sediment transport in the presence of simulated vegetated surfaces were observed to be significantly smaller than those without the vegetation. A method is proposed to determine the effective (grain) shear stress in presence of vegetation under the condition that the apparent shear stress is available from easily measurable flow parameters. The present data along with literature data are used for quantification of the reduction in the rate of sediment transport by the rigid tall stems. The grain shear stress rather than the apparent shear stress is observed to better describe the variations in the rate of sediment transport by vegetated flows.

Estimation of temporal variation of sediment yield using GIS

Abstract A GIS-based method is proposed for computation of temporal variation of sediment yield during isolated storm events. Data from three Indian catchments, namely Karso and Nagwa in Jharkhand and Kharkari in Rajasthan, have been used. The Integrated Land and Water Information System (ILWIS) GIS package was used for (a) catchment discretization into cell areas using grid networks, (b) evaluation of the spatial variation in catchment topographical characteristics and land use, and (c) presentation of the results obtained. The process of sediment delivery from grid cells to the catchment outlet is represented by the topographical characteristics of the cells. Unit sediment graphs for the catchments are derived by translation of the sediment yield from the grid cells and routing through a linear storage reservoir. The proposed method is found to provide satisfactory estimates of the temporal variation of sediment yield during isolated storm events. The total sediment yield of a storm event may also be computed using the proposed method.