Modeling of sediment transport and self-cleansing of sea outfalls

Modeling of sediment transport and self-cleansing of sea outfalls

by Ina Ibro

Water and Environment

Department of Civil Engineering

Aalborg University

2011

Abstract

The present thesis describes the transport phenomena of non-cohesive sediment in sea outfalls from an experimental and numerical point of view. The non-cohesive sediment used during the course of the whole study was fine and artificial sand from a gravel pit near Horsens in Denmark denoted as Sand N0. 0000.

Outfalls as well as sewers have been designed on the concept of the self-cleansing criteria, where sediments are expected to move continuously without deposition. Nevertheless, due to erratic nature of flow deposition of solids in these systems occurs, particularly at low flows periods.

The two central elements of the project is the development of the numerical model and a matching physical model in the laboratory. The numerical model covers both sediment transport over bed accumulations as well as transport over clean bottom. The physical modeling emphasizes on measurement of the non-steady removal and transport of well-defined and limited accumulations along the pipe.

Experiments on sediment transport (especially bed load) of non-cohesive sediments following without deposition criteria were carried out in both a pipe of 240 mm dia. and a rectangular flume of 300 mm in width and 300 mm in depth, both running full.

A one dimensional numerical model was developed describing the sediment distribution along the pipe as well as predicting the transport time and velocity needed to erode and clean the pipe. Hence, numerical modeling was developed with the purpose to find an approach to formulate the self-cleansing criteria with a formula that included in the model, will give relatively similar results as experimental data. The transport of non-cohesive sediment is modeled by solution of the continuity equation for bed sediments. Ackers White sediment transport theory was implemented in the description of the non-cohesive sediment transport. The model was calibrated using the laboratory experiments.

As part of the project, a paper was written in describing modeling of sediment transport in outfalls, with focus on the self-cleansing problem occurring due to the daily flow variation seen in these water structures. The paper contains results from both the physical and the numerical modeling.

Dansk Rsum

I dette afgangsprojekt beskrives transportfnomenet for ikke-kohsivt sediment i marine udlbsledninger set fra en eksperimentel og numerisk synsvinkel. Det ikke-kohsive sediment anvendt i dette studie var fint sand (Sand No. 0000) fra en grusgrav nr Horsens i Danmark.

Svel udlbsledninger som kloakker har traditionelt vret designet efter konceptet om selvrensning, hvor sedimenter forventes at flytte sig kontinuert uden deposition. Ikke destromindre opstr der deposition af materiale i disse systemer, specielt under perioder med lav vandfring.

De to centrale elementer i projektet er udviklingen af en numerisk model og en tilsvarende fysisk model i laboratoriet. Den numeriske model dkker sedimenttransport over svel akkumuleret sediment som sedimenttransport over sidementfri bund.

Eksperimenter med sedimenttransport (specielt bundtransport) af ikke-kohsivt sediment der flger kriteriet for selvrensning blev udfrt i svel et 240mm fuldtlbende rr som i en 300mm bred og 300mm dyb strmrende.

En ndimensionel numerisk model blev udviklet til beskrivelse af sedimentfordelingen rret svel som forudsigelse af transporttiden og hastigheden der skulle til for at borterodere allerede opbygget sidement og holde rrer rent. Sledes blev den numeriske model udviklet med det forml at beskrive et selvrensningskriterie der stemmer overens med resultaterne af den fysiske model. Transporten af ikke-kohsivt sediment blev modelleret ved lsning af kontinuitetsligningen for bundsediment. Ackers-White sedimenttransportteori blev anvendt i beskrivelsen af den ikke-kohsive sedimenttransport. Modelen blev kalibreret med data far den fysiske model.

Som en del af projektet blev der udfrt en artikel omhandlende modellering af sedimenttransport i udlbsledninger med fokus p selvrensningsproblematikken p grund af de daglige vandfringsvariationer der ses i disse systemer. Artiklen indeholder resultater fra bde den fysiske og den numeriske model.

Preface and Acknowledgments

The present thesis is submitted to Aalborg University, Denmark as a final report in fulfilling the requirements for the Danish degree of Masters in Science (MSc). The project Modeling of sediment transport and self-cleansing of sea outfalls is written by Ina Ibro, Water and Environment student, Department of Civil Engineering at Aalborg University during the period of autumn 2010 till spring 2011.

As an outcome of the MSc study is the present MSc thesis and a scientific paper in modeling of sediment transport in unsteady conditions in outfall systems. The paper was written in collaboration with Professor Torben Larsen, which supervised also the present study, and was presented at International Symposium on Outfall Systems held in May in Mar Del Plata, Argentina. The report itself contains two main parts as well as background information on outfalls structures. The first half part describes the experimental work conducted at Aalborg University laboratory and the second half describes the set-up of the numerical model. Three different scenarios are assumed in both experimental and model work. Results and conclusions are shown in the end.

The author takes the opportunity to thank her supervisor, Torben Larsen for his scientific guidance throughout the whole process of study and Ole Munch Johansen for the time dedicated in helping to set up the numerical model and inspiration in critical time period.

I also express my gratitude to Emil Dietz Fuglsang for his help during experimental work, his inspirational ideas for this project and proofreading. Denitza Voutchkova and Niels Claes, with whom I have shared the office, are acknowledged for their support and friendship. A very special thanks goes to our laboratory technician, Niels Drustrup for his kind assistance in establishing the various experimental set-ups.

I am deeply in gratitude to my family for providing encouragement, moral and sometimes financial support during the course of study. Finally, I sincerely thank my boyfriend, Lasse Lehm without his tremendous patience, tolerance and help in written language this thesis would not have been readable and completed.

Aalborg, June, 2011

Ina Ibro

ContentsAbstract2Dansk Rsum3Preface and Acknowledgments4Contents51.Introduction71.1.Brief history and design of marine outfalls82.Scope of present study102.1.Diurnal and Seasonal variations in flow103.Sediment transport123.1.Properties of sediment in pipes123.2.Modes of sediment transport123.3.Incipient Motion133.3.1.Deposition process193.3.2.Side wall elimination203.4.Flow resistance223.5.Sediment transport formula233.5.1.Characteristics of sediment transport formula233.5.2.Ackers & White sediment transport model244.Laboratory Experiments274.1.Experimental Set Up274.1.1.Rectangular channel Set Up274.1.2.Circular pipe Set Up284.1.3.Experimental procedure294.2.Sediment properties325.Numerical Model345.1.Numerical model solution345.2.Boundary and Initial conditions365.3.Calibration376.Results and Conclusions406.1.Experimental results406.2.Modeling results437.Recommendations for further investigation458.References469.Appendixes47Appendix A Paper for in International Symposium on Outfall Systems47Appendix B Calculations during experimental work48

1. Introduction

The movement of sediment, mainly in sewers, has been the subject of studies in the latest years due to the fear of pollution in ditches. In recent years, attention has focused on studying complex sewer systems that cities have developed. The problems in such systems are mostly similar to those present in rivers or open channels, however, the only difference is that sewer systems are made from the man kind and the main problem is to design the systems in that way that both water and sediment presented can be transported satisfactory (Gonzales, 1991). Even though the majority of the experimental studies on sediment transport are mostly conducted in sewers the hydraulic principles upon which they are based can still be applied to outfall systems. In the 90-ties sewer sediment transport was included in the commercial computer packages for example in MOUSE (DHI software, Denmark) and Hydro Works / Info Works CS (Wallingford Software, United Kingdom). It seems well-argumented that these models can cover sea outfalls also.

Furthermore, any sewerage, storm drainage or sea outfall system should be dependent on the requirement that all pipes should be self-cleansing. This means that there will not be any progressive build-up of sediment deposition along the pipe. Hence, many sewerage systems have been traditionally build in the way so they can run by gravity. Nevertheless, most of the systems suffer from the problems of considerable variation in flow velocity. In this way, the necessary condition for all the sediments entering the system to be transported through, becomes a crucial element of hydraulic design (Ackers, 1991).

On the other hand the design practice regarding self-cleansing is not based fully on rational causal considerations. As mentioned, because of the daily variations in the flow, it is difficult to satisfy a minimum standard for velocity or shear stress all 24 hours of the day. Most often the criterion is formulated as a minimum which has to be fulfilled at least 1 2 hours of the day. This implies that an accumulation of sediments can built up during the night and then be flushed out during the day. In order to sustain self-cleansing in a long-term perspective it is necessary that all sediments are removed every day in the high-flow hours.

Most of the sewers have been designed mainly on a single critical criterion which could be either velocity or shear stress, with the aim of keeping the sewers free of any sediment deposition. Given the fact that not many researches are made concerning sediment transport in outfall systems, one should bear in mind that, there is a big difference between sewer systems and sea outfalls which consist on the running conditions. Sewer systems run mainly half full and under steady conditions, while outfall systems run always full bore and unsteady and non-uniform conditions. The figures 1 and 2 below show the differences in flow velocity and wall shear stress between ordinary part-full running sewers and full-running sea outfalls in respect to variations in the flow.

Figure 1, Relative flow velocity (V/Vmax) in circular pipeline as function of relative flow (Q/Qmax.). The blue trendline corresponds to part full pipe where Qmax occurs when pipe is running half full. The red trendline is full running pipe

Figure 2, Relative wall shear stress (/max) in circular pipeline as function of relative low (Q/Qmax.). The blue trendline corresponds to part ful pie where Qmax occurs when pipe is running half full. The red trendline is full running pipe

It is seen that the variations in sea outfalls are considerably larger than in sewers. This indicates that experiences from sewer systems only with some caution can be transferred to sea outfalls.

From many research works (Mayerle et al 1991, May 1982, Ackers 1978) has come into light that the limiting criterion gives conservative results mainly for small sewers however, these results has not yet given enough evidence to prevent the sediment deposition in larger sewers (Ghani, (November 1993)).

1.1. Brief history and design of marine outfalls

A sea outfall is a long pipeline or a tunnel placed more or less perpendicular to the shore line and reaches a length of 0.5 km to 10 km out of the coast. The outfall ends with a section called diffuser, which consist of a number of openings denoted ports. From here the sewage coming out of the outfall pipeline, is discharge in the horizontal direction. Nowadays the diffuser can be placed also vertically to avoid sedimentation in that part of the pipeline. The design of the sea outfall covers a number of stages corresponding to various objectives that should be fulfilled; however one of those with main interest to the subject is the internal hydraulics design. (Larsen, 2000). The aim is to ensure that this structure has the sufficient hydraulic capacity to transport the predicted design flow. Insufficient flow means impossibilities for flushing out the sediments, creation of air pockets and saline intrusions.

Figure 3, Schematic view of a marine outfall placed on the sea bed

The purpose of building the first marine outfall was related to the concern of water quality in relation to the discharge of sewage into the sea. The apprehensions for the bathing waters made human kind to think again about when and where to discharge the urban water. The solution was basically to send the waste water so far away from the coast that any interaction with human activities was prevented. The origin of marine outfall comes before the Second World War, where the sea was looked upon as an infinite recipient which could receive unlimited amounts of waste (Larsen, 2000).

2. Scope of present study

The present study was intended for marine outfall systems carrying preliminary treated urban water. Only non-cohesive sediment was used even though it is recognized that cohesion itself would be an independent part of the process and another factor influencing the outfall transport capacity. All experiments were conducted under full uniform flow conditions with sediment transported as bed-load in smooth pipes and flumes.

The aim was to gain a better understanding on the sediment transport process in outfall systems and to find a possible way to provide an improved relationship for self-cleansing criteria. This is achieved by modeling sediment transport including deposition as well as erosion in a 24-hour flow cycle in order to clarify when and why self-cleansing can be insufficient. Further, finding the limiting velocity of sediment motion by experimental and numerical way and compare these results with those suggested by other authors.

The experiments were carried out at the Hydraulics and Costal Laboratory of Aalborg University, Department of Civil Engineering. The author knowledge that this is the first attempt to study the sediment transport in either sewerage or outfalls systems.

2.1. Diurnal and Seasonal variations in flow

For preventing the build up of the sediment deposits it is necessary that the total daily transport capacity exceeds the total daily load during almost all times. One possible way to deal with this phenomenon is to base the design on the average daily flow during the year.

Figure 4, Daily variation in flow

Nevertheless this would bring to an underestimation of the daily transport capacity because of the non linear function of the transport. In full bore case, it is known that the flow velocity is proportional to the discharge. Ackers (Ackers, 1991), showed that in range of interest, as seen in Figure 5 from his measurements and calculations, the concentration is proportional to the fifth power of discharge. Moreover, for a particular diurnal variation he found that the curve was approximately linear.

Figure 5, Sediment transport in 299 mm pipe, full bore (H R, Wallingford data (3)), limit of deposition and various mean depths of deposit compare with development of Ackers and White function with several WE values (Ackers, 1991)

3. Sediment transport 3.1. Properties of sediment in pipes

The movement of sediment in pipelines involves different processes known as erosion, transport and deposition which will not necessarily happen in the order mentioned. There are considerable spatial variations in the source and origins of sediments where the sediment inputs may differ not only between catchment types but also between contiguous pipe inlets.

Some of the most important reasons for the presence of sediments in sea outfalls:

Initial sediments left from construction (or repair) works

Sediments carried by the sewage

Intrusion of sediments from the sea caused by waves, currents and density differences

Intrusion of sediments from the sea because of transients caused by pump operations, surge towers etc.

Intrusion of sediments because of damage of diffuser from anchors and fishing gears etc.

Flocculation due to salinity gradients.

A classification of sediment present on the pipes is made based on a visual appearance, location, physical and chemical analysis (Ghani, (November 1993)):

Classes: A: Coarse granular material

B: partly class A deposits and greases and mineral cements

C: mobile fine grained

D: organic wall slimes (biofilms)

E: deposits found in tanks

In sewers or outfall systems it is possible to find all the classes and mixtures of sediment and that will depend on the source of the sediment in the system.

3.2. Modes of sediment transport

Insufficient information exists about the precise way of sediment movement in the pipes. For simplicity the sediment movement could be classified into two categories:

1. Transport with clean invert

2. Transport over loose sediment deposits (Ghani, (November 1993))

In either cases three type of solid transport are distinguished:

1. The bed load transport: where the particles are sliding, rolling and saltation without leaving the bed. If insufficient sediment present in the pipe, the bed load transport will often form dunes or other bed forms, as well as this process can bring an increase in the flow resistance. The bed load can itself be classified in two types:

Flow with a continuous dunes that moves along the pipe

Flow with separated dunes moving along the clean bottom

2. The suspended transport: the particles from the bed material or from other sources remain in suspension in the flow without definitive deposition, nevertheless temporary deposition is possible. The suspended transport process is possible when the turbulent eddies have large vertical velocities (Mark, May 1995).

3. The wash load transport: very fine particles which are permanently transported in the flow without any deposition. While the suspended load is part of erosion and deposition process of sediment deposits in pipes, the wash load is a process which contribute in the exchange of the particles with the bed load is very small and mostly present no interest with respect to flow resistance. The wash load has a very small settling velocity.

The total sediment transport is the sum of all the above loads.

This succession phenomenon corresponds to uniform sediments with the same grain size. For the transport of mixed sediments more regimes can exist simultaneously.

Experiences from sewers systems show that depositions of sediment significantly increase the flow resistance primarily because of the influence of the bed forms.

3.3. Incipient Motion

Both sewers and outfalls are mainly designed to be self-cleansing. Mostly, all the sediments entering these kinds of structures are expected to move continuously without deposition. Nevertheless, due to the diurnal variations of flow, deposition is to be expected. Hence, under these conditions, it is estimated that the deposition would be flushed out by high flow happening mostly during the day.

As mentioned previously (see Introduction), the self-cleansing criterion is based either on minimum mean velocity or shear stress. The tables below show criteria given by CIRIA (1987) (Ghani, (November 1993)).

Table 1. Constant Velocity Criteria (Ghani, (November 1993))

Reference Source

Country

Pipe condition

Minimum Velocity [m/s]

American Society of Civil Engineers (1970)

USA

Full/Half Full

0.6

British Standard (1987)

UK

Full

0.75

EESCRITT (1979)

UK

Full

0.76

BIELECKI (1982)

Germany

Full

1.5

Table 2. Constant Shear Stress Criteria (Ghani, (November 1993))

Reference Source

Country

Minimum Shear Stress [N/m2]

Pipe Conditions

MAGUIRE RULE

UK

6.2

Full/ Half Full

YAO (1974)

USA

3.0 4.0

LYSNE (1969)

Norway

2.0 3.0

ASVISNINGAR (1976)

Sweden

1.5

Studies and experiments shows that a minimum flow is required before particles laying on the bottom start to move. There are two possible definition of threshold; the first one is based upon a minimum transport rate (such as works by Shields) and the second definition consists of the visual observation of particle motion on the bed.

The most widely used approach in determining the initiation of the sediment motion is based on the critical shear stress c. The critical bed shear stress is one of the most important parameters in the process of sediment transport which describes whether the sediment will be eroded or not. Put simply, shear stress describes the force of water that is trying to drag the channel surface downstream with it (americanexcelsior)

The initiation of motion has been widely studied and critical bed shear stress has been incorporated in the various transport parameters. This concept represents the minimum flow resistance which is exerted by the particles at the moment before the motion starts (Gonzales, 1991).

In some sewer systems, where the sediment may be noncohesive, the critical bed shear stress can be found by Shields parameter. Shield has put a standard to the studies of incipient motion and sediment transport. He defined the threshold as a zero transport rate from interpolation of transport data. The critical bed shear stress was found to be a function of the grain Reynold number Re.

Results of the interpolation of the data were illustrated in a plot (Figure 6) of dimensionless entrainment parameters, Fr2d,cr = versus shear Reynolds number, Recr = u*cr*d/.

Numerical Model

39

Figure 6, Shields Diagram (after Graf 1984)

Where, c is the critical shear stress,

is the density of water,

s is the specific gravity of sediment

d is the diameter of sediment,

g is the acceleration due to gravity,

u*cr is the critical shear velocity (),

is the kinematic viscosity of water (temperature dependent).

When a fluid is currently at rest, to be possible to have transport of sediment, the boundary (or bed) shear stress 0 must exceed the critical shear stress c for the initiation of motion of grains at the bed.

0 > c Bed load motion

Based on different experiments critical Shields parameter was formulated as see; (chanson, 1999):

(eq. 1)

0 = (s-1) g*ds cr(eq. 2)

It is estimated that the sediment starts to move for a Shields critical parameter cr = 0.05. The critical shear stress will be then presented as below:

0 = 0.05*(s-1) g*ds(eq. 3)

Van Rijn (1984) expressed Shields Fr2d,cr in terms of dimensionless grain diameter (Figure 7)

D* = D50* [(s-1)*g/2]1/3(eq. 4)

where D50 is the particle diameter for which 50% of the sample is finer.

Figure 7, Shields Curve (from Van Rijn (1984)) (BERTRAND-KRAJEWSKI, 2006)

Novak Nalluri (1975) performed theoretical analysis by associating the forces acting on a smooth bed and obtained a single equation for both rectangular and circular channels, by only introducing a relative roughness (d/R), where R is the hydraulic radius.

(eq. 5)

for a range 0.01