INFLUENCE OF COHESION ON DETACHMENT AND TRANSPORT OF CLAY-SAND-GRAVEL MIXTURES

Abstract

The knowledge on detachment and transport of sediments by overland and river flows is useful in planning for the sustainable use of natural resources such as the soil and the water. This knowledge is also necessary for solving various problems related to hydraulic engineering like soil erosion in catchments, reservoir sedimentation, stable channel design, river morphological predictions and effect of fine material deposition within riverbed on river environment. The condition for initiation of motion and process of detachment and transport of sediments by stream flows in the form of bed load and suspended load for cohesionless uniform and nonuniform sediments are well investigated and very well understood (Garde and Ranga Raju, 2000). Similarly several investigations are also available on erosion and transport of consolidated and unconsolidated uniform size cohesive sediments formed by clay material (Raudkivi, 1990). However the land surfaces and river bed materials frequently consist of mixture ofcohesive as well as cohesionless sediments like mixtures ofsand, gravel and clay etc. Therefore in the present thesis work it was intended to investigate the effect of presence of cohesive material such as clay on the condition for initiation of motion and the process of detachment and transport ofuniform and nonuniform sediments consisting ofclay-sand- gravel. Although several investigations for critical shear stress ofcohesive sediments e.g. Dunn (1959), Kamphis and Hall (1983), Julian and Torres (2006), Kothyari et al. (2006) etc. are available especially for cohesive sediments consisting ofmixture ofclay-silt-sand. No study has been carried out as yet on incipient motion characteristics ofthe sediment mixtures containing clay as well as gravel material whereas such sediment mixtures commonly exist in the nature. Likewise the phenomenon ofdetachment ofcohesive sediments mainly consisting ofmixture of clay-silt-sand has been studied in past by a large number of investigators viz; Kuti and Yen (1976), Hanson (1990), Mitchner and Torfs (1996), Aberle et al. (2004), Hanson and Hunt (2006) etc. However the study on the process of detachment of cohesive sediment mixtures consisting of clay-gravel and clay-sand-gravel has not yet been performed. Most of the analytical and the numerical models available for the estimation of river bed level variations deal with cohesionless sediment only. However in the nature river bed materials and catchment surfaces also contain cohesive material like clay. Thus there is a need to incorporate the aspects relating to cohesion in the numericalmodels for prediction of the stream bed variations. The present study was taken up in the above background. Extensive experiments were conducted on the condition of incipient motion of cohesive sediments, bed load transport and suspended load transport caused by detachment of cohesive sediments consisting of clay-gravel and clay-sand-gravel mixtures. The experiments were conducted in a tilting flume 16m long, 0.75 m wide and 0.5 m deep. A maximum discharge of 100 1/s could be obtained in this flume. The channel had a test section of 6.0 m length, 0.75 m wide and 0.13 m depth starting at a distance of 8.0 m from channel entrance. Observations were made at various slopes of flume ranging from 2.417xl0"3 to l.OxlO"2. For bed load and suspended load transport bydetachment, initial runs were conducted on cohesionless uniform and nonuniform sediments. Next, experiments were conducted on two types of cohesive sediments mixtures in first; fine gravel mixed with clay in proportion varying from 10% to 50% and in other; fine gravel and fine sand in equal proportion by weight mixed with clay in proportion varying from 10% to 50%. The cohesionless sediment and cohesive sediment used herein had the following characteristics: Sand had a median sized of 0.23 mm and ag of 1.53, while gravel had a median size dof 3.1 mm and crg of 1.23. Therelative density of sand and gravel was 2.65. Clay material had a median size d equal to 0.0039 mm, geometric standard deviation crg equal to 1.7. The other engineering properties of clay material were: liquid limit WL = 38%, plastic limit WP = 24%, plasticity index PI = 14%, maximum dry density (/rf)max= 18.27 KN/m3, optimum moisture content OMC = 16 %, cohesion at OMC, Cu = 54 KN/m2 and angle of internal friction at OMC, 0C = 21° and relative density = 2.6. As per Indian standard code (IS-1498, 1970) the clay was classified as CI, i.e. clay with intermediate compressibility. The percentage composition of various clay minerals was Kaolinite = 17.6%, Elite = 60.3%, Vermiculite = 15.3% and Chlorite = 6.8%. n Experiments were conducted by filling the cohesive sediments into test section in three different layers. Each layer was compacted by a dynamic compaction or a kneading method depending upon the antecedent moisture content and consistency of cohesive sediments. The flow parameters corresponding to incipient motion condition of cohesive sediments were measured. Measurement of bed load transport and suspended load transport caused by detachment of both cohesionless and cohesive sediment mixtures have also been carried out at regular time intervals throughout the duration ofeach experimental run. In order to study the stream bed variations in both cohesionless and cohesive sediments, for each run the bed elevations and water surface elevations were measured at alongitudinal interval of 0.5 malong the centre line of the flume. CONDITION FOR INCIPIENT MOTION OF COHESIVE SEDIEMENTS The process of initiation of motion of cohesive sediment mixtures containing gravel size sediment is noticed to be significantly different from that of cohesionless sediments as well as cohesive sediment mixtures without coarser size material. Analysis of the data collected in present study revealed the following relationship to hold good for critical shear stress of cohesive sediments. — =0.9<l+pJV1/6(l+0.001£/CST/20 wto T*cc=-~-, r.c=-^ and UCS* = UCS Here r.ccis the dimensionless critical shear stress of cohesive sediments, r is critical cc shear stress ofcohesive sediments, Ays is submerged specific weight ofsediment which is equal t0 (/, -Yf\ r, is specific weight of sediment and yf is specific weight of fluid. rc is critical in shear stress of cohesionless sediment, da is arithmetic mean size of cohesive sediments, da is the arithmetic size of the cohesionless sediment (base sediment) mixture, UCS is unconfined compressive strength ofcohesive sediments, Pc is clay content in sediment bed and e is void ratio. In absence of cohesion the proposed relationship for critical shear stress of cohesive sediments reduces to the critical shear stress of cohesionless sediment. BED LOAD TRANSPORT The bed load transport caused by detachment ofcohesionless sediments has been found to very well follow the method of Misri et al. (1984) for cohesionless uniform sediment whereas the bed load transport ofnonuniform cohesionless sediment very well followed the method ofPatel and RangaRaju (1996) perfectly. The transport ofcohesive sediments as bed load through the process of detachment was observed to be considerably different from that of cohesionless sediments. Two types of conditions were found to evolve over the channel bed surface during bed load transport representing the rill and gully erosion over the catchment surfaces. Mathematical formulations for the computation of active bed layer composition are suggested in case of the degrading nonuniform sediment bed at different time periods since start of the degradation process. The bedload transport rate of the cohesionless sediment size fractions of the cohesive bedmaterial were found to decrease with an increase in clay content in sediment bed and it was also found to decrease with the increase in unconfined compressive strength of sediment bed. The non-dimensional parameters affecting the bed load transport rate are identified. Following form of relationship has been proposed to compute the bed load transport by detachment ofcohesive sediment mixtures. it., <1b (( O ^ 1 + — ,(1+UCS ) J J IV Here qBc is me bed load transport rate ofgiven size fraction present in cohesive sediment bed, qB is the bed load transport rate of same size fraction in cohesionless sediment bed under the same flow conditions. Here C. - P'C" 0 _Pc tan^c+(1-Pc)tan^ (r,-rw)da tan^ ucs' =- ucs' , ucs' =^-ucs (rs-rw)da p Also in above Pco is the initial percentage of clay in bed material, Pc is the clay percentage in bed material at aparticular time, Cu is the value ofcohesion at optimum moisture content OMC, £ is angle of internal friction for clay at OMC, and ^ is angle of internal friction of cohesionless sediments. The relationship for computation of bed load transport rate has been proposed in graphical form to compute the transport rate of gravel and sand present in cohesive sediment mixture caused by the detachment due to the flow. In the absence of cohesion the proposed relationship for rate ofbed load transport reduces to that ofcohesionless sediment. SUSPENDED LOAD TRANSPORT The suspended load transport caused by detachment of cohesionless sediments has been found to very well follow the method of Samaga et al. (1986 b). In case of cohesive sediments, based on dimensional considerations the functional relationship is derived for estimation of the suspended load transport rate ofclay in clay-gravel and clay-sand-gravel mixtures is given as 9m m/(( C^ 1 + — ,(1 + UCS ) J ) Here qsc is the suspended load transport rate ofclay, qs is the suspended load transport rate of cohesionless sediments under the same flow conditions. The graphical relationship for suspended load transport rate computation has been proposed herein that is resulted by the detachment of cohesive sediment consisting of clay-gravel and clay-sand-gravel mixtures. In absence of cohesion the proposed relationship for suspended load transport reduces to that of cohesionless sediment. MATHEMATICAL MODELING FOR COMPUTATIONS OF STREAM BED VARIATIONS The mathematical model proposed in thepresent study is based on the following coupled form of governing partial differential equations for one-dimensional, unsteady flow in rectangular prismatic channel with no lateral discharge ofwater and sediment. (i) Flow continuity equation dh dq Pdz . — + -3- + = 0 dt dx B dt (ii) Flow momentum equation dq d f„q2* 11 > • + — dt dx Kh 2 j dz ?-+-gh2 +gh—+ghSf=0 dx (iii) Sediment continuity equation B dt ys dx yt dx dt Here B is top width of flow, P is flow perimeter, q is discharge per unit width of channel, h is depth of flow, z is bed elevation, g is acceleration due to gravity, Sf is energy slope, CflV is average volumetric concentration of suspended load, Xis porosity of bed material, x is distance along the flow direction and t is time. vi The predictor-corrector based essentially non oscillatory finite difference numerical scheme proposed by Nujic (1995) is utilized for the solution of the above governing equations through the use of appropriate initial and boundary conditions. Data for cohesionless sediment of Soni et al. (1980), Mehta (1980) and Yadav (1992) were used for model validation. The proposed relationships for computing the rate of bed load transport, and suspended load transport for cohesive sediments and mathematical formulations for computations of active bed layer composition have been incorporated into the proposed model. The model has been tested for wide range of flow conditions in channel made up of bedsediment consisting of clay-gravel and clay-sand-gravel mixtures. The comparison of predicted and corresponding observed values showed that proposed model well predicted the transient bed and water surface profiles in cohesive sediment bed channels. vn