The document describes a sediment transport model developed for the river Zenne to evaluate ecological status. The model simulates the transport, deposition and resuspension of suspended solids. It uses equations from Shields and Soulsby and Whitehouse to calculate critical shear stress and particle motion. The sediment module is implemented as an OpenMI component providing outputs of total suspended solids, critical diameter and bed mass. Experiments were run on a 20 km section of the Zenne using different specific gravities showing varying levels of sedimentation.

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.be
July 14, 2011 Sediment Model for GESZ 1

2. Presentation Layout
 Introduction.
 Objectives.
 Theory.
 Sediment module as OpenMI component.
 Experiments.
 References.
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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.
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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.
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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 coefficient
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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 water
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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.
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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*:
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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).
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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 link
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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-1
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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>
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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)
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14. Experiments (1)
 Implemented in Non-navigable Zenne.
 Distance over 20 km
 Resolution = 20
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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)
→
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16. Experiments (3)
 Results for S = 1.0 (no sedimentation)
Flow
Simulated TSS
Concentration
Profile plot of
Simulated TSS
Concentration
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17. Experiments (4)
 Results for S = 1.4 (slight sedimentation)
Flow
Simulated TSS
Concentration
Profile plot of
Simulated TSS
Concentration
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18. Experiments (5)
 Results for S = 2.4 (heavy sedimentation)
Flow
Simulated TSS
Concentration
Profile plot of
Simulated TSS
Concentration
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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.
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20. Thank you for your Attention!!
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