Generalities on hydrodynamics and sediment transport

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Numerical studies of flushing in sewers

Sewer flushing has been commonly described using one-dimensional models because of the 1D nature of sewer systems. The hydrodynamic and sediment-transport characteristics of the flushing phenomenon have similarities with the dam-beak process. Predicting dam break flows and their consequential impacts over the movable river bed has been numerically investigated by many researchers (e.g., Zhang et al. 2011). The literature shows that the propagation of the flushing flow throughout sewer channels has been so far the subject of various numerical investigations during the recent decades. Numerical studies have been undertaken for the following main motivations:
o understanding the hydraulic boundary shear conditions and cleaning performance of flushes in terms of scoured distance.
o determining sediment removal efficiency of flushing operations
o evaluating the hydraulic behaviour of flushing to select the appropriate device for sewer systems under study.
o testing various sediment-transport formulas to develop or find a better relationship estimating the sediment-transport discharge under flushing operation (Creaco and Bertrand-Krajewski 2009; Shirazi et al. 2014).
o obtaining design and management indications for flush operatiob scheduling plans (Campisano and Modica 2003; Campisano et al. 2007; Tdeschini et al. 2010).

Aim of the flushing test and choice of the pilot channel

Based on information provided by the SAP’s (Section de l’Assainissement de Paris) information, the Parisian combined sewer network comprises a number of man-entry trunk sewers with a total length in the order of several tens of kilometres. Several of these sewers are affected by problems of sediment accumulation discussed in Chapter 2 of the thesis. Therefore, the Paris municipality decided to explore through experiments the potential of various flushing devices to perform the « automatic » cleaning of sewer channel and reduce human intervention in the high-risk cleansing procedures. Based on the available information (personal communication, June 6, 2016), almost 250 sewer workers attended yet in manual cleaning of about 130 km trunk sewer channels of the Parisian network. Thus, acknowledging the local particularities of each sewer channel, the SAP intended to consider the most performant and applicable flushing device to help the sewer management strategy. Different sites of Paris sewer network were initially proposed for the study. The proposed sites had specials common particularities: (i) they were subject to significant sedimentation due to the channel geometry; (ii) an online gate was already installed on these channels to reduce cost of installation. Moreover, a preliminary hydraulic study of the gates and their capacity to generate flush waves through downstream channel was carried out for each site. The results of the study led to choose the experimental site called Chemin Vert (Fig. 3.1).
In this regard, the municipality was interested in exploring the possibility to reconvert the existing mobile gate (used to derivate flow during storm flow events) as a flushing gate device to evaluate its removal efficiency on the existing deposits. The channel trunk downstream of the gate was selected as pilot system as it is prone to large sedimentation. To this end, the experimental campaign was designed and planned to obtain high-fidelity data able to characterize he flushing performance and to allow validating a reliable model with a novel approach.

Protocol of the flush experimentation

Due to the complexities of the sewers coming from a wide range of factors (e.g., difficulties of direct measurement, lack of appropriate devices, non-adapted method), the choice of measuring apparatus and techniques becomes a challenge in particular regarding to the experiment costs (Ashley et al. 1999; De Sutter et al. 2001). Indeed, measuring equipment is one of the important sources for operators and municipalities to improve sewer sediment management (Bertrand-Krajewski et al. 2006). That is why device factories aim to more and more develop/design/adapt measuring devices to better monitor sediments and study processes within sewers and consequently the sediment-transport models. The hostile conditions of the sewers mainly due to the aggressive gas and flow as well as high turbidity of the water do not allow applying measuring devices used for clear waters. Besides, the use of non-suitable classical devices in sewers can lead to errors in experimental data (flow and bed deposits). Therefore, in this study, efforts were made to reduce as much as possible, the errors coming from equipment and installations.

Phases of the flush test

A protocol was proposed to monitor the flushing experiment and to measure the effects of the flush in the pilot channel. The whole measurement campaign was carried out during five consecutive days since 07/07/2014 to 11/07/2014. The overall schedule is summarized in Table 3.1. In-sewer access time restriction was imposed by the Paris Municipality provided that the experimental procedure was developed over five days. During the first two days, the bed deposits before the flush was examined which was followed by device setup into the sewer channel during the third day. During the fourth day (10th July), the flushing procedure was carried out including the storage and releasing phase. Finally the bed deposited after the flush was examined during the last day by applying the same procedure done as before the flush. So, the measurement campaign of the sediments was carried out in two phases before the flush (called hereafter BF) and after the flush (called hereafter AF).
The day the flushing test was carried out, the sewer network flows were, modified to limit inlets into the trunk sewer. Furthermore, before the flushing test all devices that were in contact with the sewer flow, were cleaned to minimize the impacts on the quality of data. The gate closing procedure was launched to store inflows upstream the gate. The storage phase took almost 2 h. Then the gate was opened at 12:04:05 of the same day to release the flush in the downstream channel trunk. A sensor was installed on the gate to measure the crest position of the gate during the operation.

Measuring flow- and sediment-related parameters

A set of measurement devices was installed within the channel to monitor the flush experiment. Based on the results of the preliminary survey, the general idea was to measure as much as possible parameters related to the flow (e.g., shear stress) and to the sediment transport. Researchers have outlined that obtaining high-quality flow-related parameters allows estimating more accurate values shear stress that lead to better study the sediment-transport processes (Hughes et al. 1996; Staufer and Pinnekamp 2008; Lepot et al. 2013; Momplot et al. 2013; Lepot et al. 2016).
Therefore, the measuring devices were selected based on their capability to ‘catch’ the evolution of rapidly varying flows due to the flush (in particular at the beginning of the flush). The choice of the devices was also influenced by the need of using probes able to resist to the sewer harsh environment. The other important limitation that was taken into account for the selection of devices was the level of turbidity of the sewer flow in particular during the flush. It should be noted that resistance to corrosion in the sewer was not considered as a problem because of the short period of use of the measurement equipment for the experiment. During the experiments the devices were set to record with the maximum data acquisition frequency.
Five cross-sections of the pilot trunk sewer were identified to install measuring devices for monitoring the flush performance. Devices were place in the exact positions for each cross-section. Indeed, the importance of device position to limit experimental errors has been emphasized by Bonakdari and Zinatizadeh (2011) who concluded that measuring methods are unique and specific to each experimental site. Fig. 3.9 shows the position of the installation of the sensors in each sewer section. The figure illustrates also the locations of the five cross-sections along the sewer channel (located at S-50, S-5, S+5, S+50 and S+100). The location of the measuring sections was
determined to be almost close to the gate in both upstream and downstream of the flushing device to obtain more information about the flush waves. Moreover, Table 3.2 summarizes the information on the used measuring devices for the experimental test. Main flow parameters were measured by using a pulsed doppler velocimeter and an ultrasound (US) device. The first device manufactured by SonTek Company is able to measure the flow velocity [m/s]. The measure of the flow discharge [m3/s] is obtained based on the observed flow velocity and the associated wetted area at the given section. The device diffuses 5 signals towards different directions at a given an instance into the water column. According to the mode of operation the device provides both flow velocities and water levels separately and calculates time-averaged velocity over a lap of time.

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Table of contents :

0.1 État de l’art
0.2 Objectifs de l’étude
0.3 Étude expérimentale de la chasse hydraulique
0.3.1 Expérimentations d’une chasse hydraulique dans un collecteur parisien
0.3.2 Résultats des analyses expérimentales
0.4 Étude numérique de la chasse hydraulique
0.3.1 Présentation des modèles
0.3.2 Comparaison des résultats de deux modèles
0.5 Conclusions générales
1.1 General background
1.2 Aims and objectives
1.3 Synopsis and feature of current thesis
2.1 Sediment-transport mechanisms in open channels
2.1.1 Generalities on hydrodynamics and sediment transport
2.1.2General aspects of sediment transport in open channels
2.2 In-sewer solids
2.2.1 Generalities
2.2.2 Negative effects of sediments in sewer networks
2.2.3 Content of deposited load in sewers
2.2.4 Consolidation of deposits in combined sewers
2.3 Movement of solids in sewers
2.3.1 Deposition (settling) and accumulation of sediments in sewers
2.3.2 Erosion of sewer deposits
2.3.3 Transport of sediments
2.3.4 Available studies concerning modelling sewer sediment transport
2.4 Sewer cleaning methods
2.4.1 Sewer flushing
2.4.2General evaluation of flush performance
2.5 Experimental and Numerical investigations on flushing
2.5.1 Experimental analysis of flushing in sewers
2.5.2 Laboratory studies
2.5.3 Numerical studies of flushing in sewers
3.1 Generality
3.2 The experimental pilot sewer channel
3.2.1Aim of the flushing test and choice of the pilot channel
3.2.2Characteristics of the pilot system
3.3 Protocol of the flush experimentation
3.3.1 Phases of the flush test
3.3.2 Measuring flow- and sediment-related parameters
3.3.3 Video recording during the flush
4.1 Recorded data during the experiment
4.1.1 Collected data and missing information
4.1.2 Data preparation
4.2 Results of the experimental test
4.2.1 Gate operation during the test
4.2.2 Videos recorded from surface and cunette cameras
4.2.3 Analysis of the hydrodynamics of the flush operation
4.2.4 Deposit evolution due to the flush
4.2.5 Modification of the deposit grain-size distribution
4.2.6 Organic fraction in bed sediments
4.2.7 Complementary parameters
4.3 Performance of the experimental flushing test in Des Coteaux combined sewer
5.1 General overview
5.2 The unsteady flow model for uniform sediment transport
5.2.1 General introduction to the flushing mechanism
5.2.2 Governing equations for flow and sediment phases
5.2.3 Bed-load transport evaluation
5.2.3 Numerical scheme and solution
5.2.4 Domain discretization
5.2.5 Initial and boundary conditions
5.3 The unsteady flow model for non-uniform sediment transport
5.3.1 Governing equations for flow and sediment phases
5.3.2 Scheme, initial and boundary conditions in non-uniform model
6.1 Data preparation for simulation by both uniform and non-uniform sediment-transport models
6.1.1 Modelling the geometry of the sewer channel
6.1.2 Basic input data for the uniform sediment-transport model
6.1.3 Basic input data for the non-uniform sediment-transport model .
6.1.4 Initial, internal and boundary conditions for uniform and non-uniform sediment-transport approaches
6.2 Results from the simulations with the uniform sediment-transport model
6.2.1Results of the hydraulics of the flush
6.2.2Results concerning the sediment-bed evolution
6.3 Results from the simulation with the non-uniform sediment-transport model
6.3.1 Model consistency test
6.3.2 Hydraulics of the flush
6.3.3 Results of the sediment-transport processes
6.4 Sensitivity analysis of the model parameters
7.1 Summary and general conclusions
7.2 Future works and perspective


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