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Chapter 29 Sediment Transport

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Chapter 29 Sediment Transport Chapter 29 Sediment transport 29.1 Introduction The following seven test cases have been developed to illustrate the various options implemented in the sediment transport module bedload Compares the different formulations for bed load transport by im- posed currents (bottom stress) values (see Section 7.4). totload Compares the different formulations for total load transport by im- posed current (bottom stress) values (see Section 7.5). wavload Compares formulations for bed and total load transport with wave effects using imposed values for currents and wave parameters (wave height, period and direction). sedvprof Simulates the diffusion and settling of sediments in a water column by a bottom stress imposed via an external pressure gradient (surface slope). sedhprof This test case, taken from Hjelmfelt & Lenau (1970), simulates the transition between a non-eroding and an eroding bed. This is modeled by applying a boundary condition with a concentration of zero at the upstream boundary. Note that the velocity field is supplied as an initial condition, rather than calculated by COHERENS. Both 2-D and 3-D simulations are performed and different model settings are applied to simulate the convergence to an equilibrium state. seddens Simulates the evolution of an initial sediment distribution in a closed channel, consisting of vertically uniform concentrations in a sec- tion at the left of the channel and zero concentrations to the right. 793 794 CHAPTER 29. SEDIMENT TRANSPORT The sediment is allowed to settle due to is own weight. A current is generated by the horizontal concentration gradient. thacker This test case, after Thacker (1981), simulates the oscillations in a rotating parabolic basin with moving boundaries (shores lines). The oscillations generate a bottom stress by which sediments are suspended in the water column. 29.2 Test case bedload 29.2.1 Model setup The aim of this test case is to compare the different formulations for bed load transport implemented in COHERENS. The simulations are performed in 0-D mode (i.e. iopt grid nodim=1, nz=1) without hydrodynamic calculations. A space and time independent current is imposed instead. The flow velocity takes values between 0.2 and 2 m/s with steps of 0.2 m/s. A bed roughness of 1 mm is taken. 29.2.2 Experiments and output parameters The following experiments are defined A : Meyer-Peter & M¨uller (1948) B : Engelund & Fredsøe (1976) C : Van Rijn (1984b) D : Wu et al. (2000) E : Soulsby (1997) F : Van Rijn (2003) The following output test parameters are defined umean current [m/s] bstres (non-normalised) bottom stress ρ0τb [Pa] qbed bedload mass transport ρsqb [kg/m/s] 29.2. TEST CASE BEDLOAD 795 Figure 29.1: Bed load mass transport (kg/m/s) as function of the bed shear stress (Pa), using six different formulations. Characteristic grain size used for all formulas: 100µm. 29.2.3 Results Results are shown in Figures 29.1{29.3 for simulations using the formula- tions by Meyer-Peter & M¨uller(1948), Engelund & Fredsøe (1976), Van Rijn (1984b), Wu et al. (2000), Soulsby (1997) and Van Rijn (2003). It can be seen that the computed transport rates seem to converge at some value of the flow velocity. For the cases with a grain size of 1000µm the convergence occurs near the maximum flow and shear stress, for the case with grain size 450µm near τb 3{4 Pa (except van Rijn 2003), but for the case with grain size 100µm that point is already reached near τb < sim2 Pa and consequently the transport rates for higher flow velocities diverge for different formula- tions. Near the converging point the different formulas deviate a factor 2 to 3, while for d = 100µm at τb = 8 Pa the range is a factor 50. The reason 796 CHAPTER 29. SEDIMENT TRANSPORT Figure 29.2: Bed load mass transport (kg/m/s) as function of the bed shear stress, using six different formulations. Characteristic grain size used for all formulas: 450µ. is the large range of exponents applied to the near-bed flow velocity: from 1 in the Engelund & Fredsøe (1976) formula to 4.4 in the Wu et al. (2000) formula. 29.2.4 Conclusion For medium to coarse sands and for fine sands under relatively low shear stress, the different formulations for bed load transport are in the same range (minimum and maximum values within factor 3 from mean). For fine sands under larger shear stress a wider range of values is obtained. This might be due to the different definition of bed load transport used during measure- ments of bed load and the development of bed load transport in the sediment model (e.g. maximum level above the bed). This becomes more apparent for 29.2. TEST CASE BEDLOAD 797 Figure 29.3: Bed load mass transport (km/m/s) as function of the bed shear stresss, using six different formulations. Characteristic grain size used for all formulas: 1000µm. fine materials under high shear stress. It is advised to use a either compatible suspended load equation (or total load formulation) or the combination with an advection-diffusion solution for suspended load in this case. The validity range of transport formula is an important issue, for exam- ple, the Engelund & Fredsøe (1976) was calibrated on sands with minimum diameter of 190µm. Indeed, we see that this formula deviates strongly for fine sands. The only formula for experiments on fine sands (d50 ≤ 100µm) is the one by Wu et al. (2000). Even though the Meyer-Peter & M¨uller(1948) formula was developed for gravel only, its results are always in the centre of the range of values computed by the different formulae. 798 CHAPTER 29. SEDIMENT TRANSPORT 29.3 Test case totload 29.3.1 Model setup This test case compares different formulations for bed load transport. Setup is the same as for test case bedload using an imposed current which varies between 0.2 and 2 m/s and a bed roughness of 1 mm. 29.3.2 Experiments and output parameters The following experiments are defined A : classic form of the Engelund & Hansen (1967) formula B : Engelund & Hansen (1967) using the Chollet & Cunge (1979) expression for θ∗ C : Ackers & White (1973) D : Wu et al. (2000) E : Van Rijn (2003) F : Van Rijn (2007a) The following output test parameters are defined vmean current [m/s] bstres (non-normalised) bottom stress ρ0τb [Pa] qtot total load mass transport ρsqt [kg/m/s] 29.3.3 Results From Figures 29.4{29.6, showing computations of total load transport with the formulas of Engelund & Hansen (1967), Chollet & Cunge (1979), Ackers & White (1973), Wu et al. (2000), Van Rijn (2003) and Van Rijn (2007a), it is clear that for total load the different formulae give a more narrow band of transport rates compared to the bed load only formulations, except for the Chollet & Cunge (1979) formula. The widest variation is still computed for the very fine sands of d50 = 100µm. Due to the addition of suspended load to the bed load component, the Van Rijn (2003, 2007a) formulations differ slightly due to the different for- mulation of turbulence damping. This effect is more pronounced in the fine sands computations since the lower Rouse number leads to more suspension. 29.3. TEST CASE TOTLOAD 799 Figure 29.4: Total load mass transport (kg/m/s) as function of the bed shear stress (Pa), using six different formulations. Characteristic grain size used for all formulas: 100µm. A remark should be given about the original Engelund & Hansen (1967) formulation and the one using the Chollet & Cunge (1979) extension. In the former only sheet flow conditions have been considered, while in the latter the different transport regimes have been taken into account. As can be seen from the results, the Chollet & Cunge (1979) relationship deviates from the former in conditions where sheet flow is least expected: on coarse grained beds and at low bed shear stress. For coarse sand (d50 = 1mm) it is clear that the Chollet & Cunge (1979) formula results in transport rates up to a magnitude higher. For medium sands (d50 = 450µm) it still shows higher transport rates, up until a bed stress of 8 Pa where both curves converge. For fine sands (d50 = 100µm) both equations are identical for a bed stress of 2.5 Pa or above. The user should be aware of these differences. 800 CHAPTER 29. SEDIMENT TRANSPORT Figure 29.5: Total load mass transport (kg/m/s) as function of the bed shear stress (Pa), using six different formulations. Characteristic grain size used for all formulas: 450µm. 29.3.4 Conclusion The user should be aware that the formulation of Chollet & Cunge (1979) predicts considerably higher transport rates than all other formulae for coarse sediment and/or low flow velocity. Considering the six different equations, the minimum and maximum transport rates obtained are within a factor 2 larger or smaller than the average, except for the original Chollet & Cunge (1979) equation. It is strongly recommended to keep the switch iopt sed eha at the value of one. It should be considered by the user that the formulation of Wu et al. (2000) makes of use a fixed formulation for critical shear stress (inherent to the formula) which cannot be changed by the user. However, when using the recommended values based on the Shields curve in the other total load 29.4. TEST CASE WAVLOAD 801 Figure 29.6: Total load mass transport (kg/m/s) as function of the bed shear stress (Pa), using six different formulations. Characteristic grain size used for all formulas: 1000µm. formula results are very similar.
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