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

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

[email protected]

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Presentation Layout

Introduction.

Objectives.

Theory.

Sediment module as OpenMI component.

Experiments.

References.

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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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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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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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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

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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)

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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.

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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.

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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:

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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

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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)

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<?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)

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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)

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Implemented in Non-navigable Zenne.

Distance over 20 km

Resolution = 20

Experiments (2)

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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)

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Results for S = 1.0 (no sedimentation)

Flow

Simulated TSS

Concentration

Profile plot of

Simulated TSS

Concentration

Experiments (4)

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Results for S = 1.4 (slight sedimentation)

Flow

Simulated TSS

Concentration

Profile plot of

Simulated TSS

Concentration

Experiments (5)

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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.

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Thank you for your Attention!!

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