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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
July 14, 2011 1Sediment Model for GESZ
Presentation Layout
Introduction.
Objectives.
Theory.
Sediment module as OpenMI component.
Experiments.
References.
July 14, 2011 2Sediment Model for GESZ
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 3Sediment Model for GESZ
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 4Sediment Model for GESZ
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
July 14, 2011 5Sediment Model for GESZ
Theory (2)Critical Diameter: dividing diameter between motion and no motion.
With,
u* = shear velocity
s = specific gravity
g = gravity
d = particle diameter
ν = kinematic viscosity of water
July 14, 2011 6Sediment Model for GESZ
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:
Theory (3)
July 14, 2011 7Sediment Model for GESZ
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.
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.
July 14, 2011 8Sediment Model for GESZ
Ordinate:
Abscissa (dimensionless grain size):
Relationship between θ and D*:
This approach is used in this model.
Theory (5)Soulsby and Whitehouse (1997) provides direct means to obtain θ
and u* that corresponds to a given particle diameter.
July 14, 2011 9Sediment Model for GESZ
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).
Theory (6)Deposition and erosion calculations in the new model:
July 14, 2011 10Sediment Model for GESZ
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:
SSc(i)t = 0
BSm(i)t = BSm(i)t-1 + SSc(i)t-1 * Volume
With,
SSc = Suspended sediment concentration
BSm = Bed sediment mass
Volume = Volume of water in link
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
July 14, 2011 11Sediment Model for GESZ
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
Sediment model as OpenMI component (1)
July 14, 2011 12Sediment Model for GESZ
<?xml version="1.0"?>
<LinkableComponent
Type="GESZ.SimpleQualityComponent.DiscreteQualityComponent”Assembly="..\Output\GESZ.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>
Sediment model as OpenMI component (2)
July 14, 2011 13Sediment Model for GESZ
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)
Experiments (1)
July 14, 2011 14Sediment Model for GESZ
Implemented in Non-navigable Zenne.
Distance over 20 km
Resolution = 20
Experiments (2)
July 14, 2011 15Sediment Model for GESZ
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)
→
Experiments (3)
July 14, 2011 16Sediment Model for GESZ
Results for S = 1.0 (no sedimentation)
Flow
Simulated TSS
Concentration
Profile plot of
Simulated TSS
Concentration
Experiments (4)
July 14, 2011 17Sediment Model for GESZ
Results for S = 1.4 (slight sedimentation)
Flow
Simulated TSS
Concentration
Profile plot of
Simulated TSS
Concentration
Experiments (5)
July 14, 2011 18Sediment Model for GESZ
Results for S = 2.4 (heavy sedimentation)
Flow
Simulated TSS
Concentration
Profile plot of
Simulated TSS
Concentration
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 19Sediment Model for GESZ
Thank you for your Attention!!
July 14, 2011 20Sediment Model for GESZ