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# Sediment model for GESZ (Good Ecological Status in River Zenne)

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### Sediment model for GESZ (Good Ecological Status in River Zenne)

1. 1. Sediment Model for GESZ (Good Ecological Status in river Zenne) Shrestha N.K.; De Fraine B.; Bauwens W. Department of Hydrology and Hydraulic Engineering Vrije Universiteit Brussel nashrest@vub.ac.beJuly 14, 2011 Sediment Model for GESZ 1
2. 2. Presentation Layout  Introduction.  Objectives.  Theory.  Sediment module as OpenMI component.  Experiments.  References.July 14, 2011 Sediment Model for GESZ 2
3. 3. Introduction  Sediment has crucial role on Nutrient budget.  Sedimentation of suspended solids can be a major pathway for transfer of nutrients from surface to bottom and same applies for resuspension.  Sediments offers abundant surface area for the adsorption of various hydrophobic substances.  Modelling of sediment dynamic is essential to evaluate the ecological status of river Zenne.July 14, 2011 Sediment Model for GESZ 3
4. 4. Objectives  To model the transport, distribution, deposition and resuspension of suspended solid.  More specifically, Deposition of solid materials during dry weather flow (DWF) and subsequent scour during wet weather flow.July 14, 2011 Sediment Model for GESZ 4
5. 5. Theory (1) Shear Velocity: expresses the shear stress in a link as a velocity. With, u* = shear velocity g = gravity R = hydraulic radius S = slope of energy line v = cross-section velocity n = manning’s coefficientJuly 14, 2011 Sediment Model for GESZ 5
6. 6. Theory (2) Critical Diameter: dividing diameter between motion and no motion. Shield’s Criterion (1936): is based on an empirically discovered relationship between two dimensionless quantities.  θ = Ratio of shear stress and submerged weight of grain:  R* = Renoyld’s number: With, u* = shear velocity s = specific gravity g = gravity d = particle diameter ν = kinematic viscosity of waterJuly 14, 2011 Sediment Model for GESZ 6
7. 7. Theory (3) Shield’s Diagram in programming point of view:  Approximated using two straight line segments bound to a central polynomial approximation all in log-log plot.  This approach is not very practical to work with.July 14, 2011 Sediment Model for GESZ 7
8. 8. Theory (4) Soulsby and Whitehouse (1997):  Proposed an algebraic expression that fits Shields’ curve closely and passes reasonably well through the extended set of data that became available more recently.  This approach is used in this model.  Ordinate:  Abscissa (dimensionless grain size):  Relationship between θ and D*:July 14, 2011 Sediment Model for GESZ 8
9. 9. Theory (5) Soulsby and Whitehouse (1997) provides direct means to obtain θ and u* that corresponds to a given particle diameter.  For the inverse operation, i.e., to get dcr corresponding to u*, the equation u*(d) must be solved for d.  For this Newton-Rhapson iteration is used with bisection process (to refine possible interval for critical diameter; hence fast convergence).July 14, 2011 Sediment Model for GESZ 9
10. 10. Theory (6) Deposition and erosion calculations in the new model:  The sediment is divided into a number of classes. The number of classes is configurable.  Each single class is treated individually and behaves uniformly to erosion and deposition (i.e., a class erodes or deposits in its entirety).  Consider the class i of the sediment, bound on the lower side by diameter di and bound by diameter di+1 at the upper side.  Three situations can arise: 1) If di > dcr , all the sediment of class i that is in suspension is deposited to the bed: With, SSc = Suspended sediment concentration SSc(i)t = 0 BSm = Bed sediment mass BSm(i)t = BSm(i)t-1 + SSc(i)t-1 * Volume Volume = Volume of water in linkJuly 14, 2011 Sediment Model for GESZ 10
11. 11. Theory (7) 2) If di+1 ≤ dcr, all the sediment of class i that is on the bed will be eroded and enter suspension: SSc(i)t = SSc(i)t-1 + BSm(i)t -1 / Volume BSm(i)t = 0 3) If di < dcr< di+1, the state of the class i is not modified: SSc(i)t = SSc(i)t-1 BSm(i)t = BSm(i)t-1July 14, 2011 Sediment Model for GESZ 11
12. 12. Sediment model as OpenMI component (1) <?xml version="1.0"?> <LinkableComponent Type="GESZ.SimpleQualityComponent.DiscreteQualityComponent”Assembly="..OutputGESZ.SimpleQualityComponent.dll"> <Arguments> <Argument Key="InputFileSWMM" ReadOnly="true" Value="GESZ-8.inp" /> <Argument Key="InputFileTSS" ReadOnly="true" Value="TSS-GESZ-8.txt" /> <Argument Key="KinematicViscosity" ReadOnly="true" Value="1e-6" /> <Argument Key="SpecificGravity" ReadOnly="true" Value="1.45" /> <Argument Key="MaximumParticleDiameter" ReadOnly="true" Value ="3.0" /> <Argument Key="Resolution" ReadOnly="true" Value="20" /> <Argument Key="StorageUnitName" ReadOnly="true" Value="WWTP_Bxl_North" /> <Argument Key="TSSRemovalEfficiency" ReadOnly="true" Value="100.0" /> <Argument Key="SlopeRatingCurveForTSS" ReadOnly="true" Value="0.5749" /> <Argument Key="InterceptRatingCurveForTSS" ReadOnly="true" Value="16.93" /> </Arguments> </LinkableComponent>July 14, 2011 Sediment Model for GESZ 12
13. 13. Sediment model as OpenMI component (2) Input Exchange Items (Expects): Inflow (all nodes) Outflow (all nodes) Flow (all links) Volume (all links) Shear velocity (all links) Output Exchange Items (Provides): TSS (all links and nodes) Critical diameter (all links) Bed mass (all links)July 14, 2011 Sediment Model for GESZ 13
14. 14. Experiments (1)  Implemented in Non-navigable Zenne.  Distance over 20 km  Resolution = 20July 14, 2011 Sediment Model for GESZ 14
15. 15. Experiments (2)  Specific gravity (eg: 1.0 → no sedimentation,1.4 →slight sedimentation, 2.4 →heavy sedimentation)  Input of TSS (constant 100 mg/l for 2 days)  Fictitious particle size distribution (maximum particle diameter 3.0 mm) →July 14, 2011 Sediment Model for GESZ 15
16. 16. Experiments (3)  Results for S = 1.0 (no sedimentation) Flow Simulated TSS Concentration Profile plot of Simulated TSS ConcentrationJuly 14, 2011 Sediment Model for GESZ 16
17. 17. Experiments (4)  Results for S = 1.4 (slight sedimentation) Flow Simulated TSS Concentration Profile plot of Simulated TSS ConcentrationJuly 14, 2011 Sediment Model for GESZ 17
18. 18. Experiments (5)  Results for S = 2.4 (heavy sedimentation) Flow Simulated TSS Concentration Profile plot of Simulated TSS ConcentrationJuly 14, 2011 Sediment Model for GESZ 18
19. 19. References  Shields A. (1936): Anwendung der Ahnlichkeits-Mechanik und der Turbulenzforschung auf die Geschiebebewegung. Preus Versuchsanstalt Wasserbau Schifffahrt Berlin Mitteil 2b.  Soulsby RL., Whithouse R. (1997): Threshold of sediment motion in coastal environments. In: proc. Pacific Coasts and Ports Conf. 1, University of Canterbury, Christchurch, New-Zealand. pp 149-154.July 14, 2011 Sediment Model for GESZ 19
20. 20. Thank you for your Attention!!July 14, 2011 Sediment Model for GESZ 20