The accuracy of sediment transport models depends on identifying an appropriate sediment settling velocity. Determining this value for mud suspensions can be difficult because cohesive mud particles can aggregate, forming flocs whose sizes are a function of hydrodynamic and physiochemical conditions of the suspension. Here we present a new model refining the predicted floc size based on hydrodynamic conditions and inherited floc sizes, as well as on the salinity of the fluid environment. The improvements come from modifications made to the Winterwerp (1998) (W98) model. These improvements include: limiting floc size to the Kolmogorov microscale and including an initial salinity dependence. Limiting floc size in this way brings the model predictions more in line with flocculation theory and experimental observations. The salinity dependence was introduced based on a preliminary set of experiments that were conducted to examine floc growth rate and equilibrium size under different salinity conditions. In these experiments, increasing salinity from 2.5 to 10 PSU did not affect equilibrium floc size. However, the increases in salinity did result in longer times to equilibrium and an apparent increase in floc density or fractal dimension. The modified W98 model allows calibrated aggregation and breakup coefficients obtained under one set of concentration values (for both sediment and salinity) to be used to predict floc size under other concentration conditions. Comparing the modified W98 model with laboratory data shows more accurate predictive values, indicating that the modified W98 equation is a promising tool for incorporation into larger sediment transport models. / Master of Science / In order to assist in efforts to improve the environments of rivers, oceans, and estuaries, it is essential for scientists to have the ability to model all of the processes involved. One of the more difficult processes to model is sediment transport, specifically, cohesive sediment transport. How quickly a particle settles out of the water, and thus, how far a particle travels, is related to the density, size, and shape of the particle. With non-cohesive particles, this results in a relatively simple model as the particles stay the same size, shape, and density throughout time and space; however, cohesive particles are constantly changing as they can grow and shrink depending on the properties of the water at any given time. This process is called flocculation, and resulting particles are called flocs. This study aims to improve the modeling of cohesive sediment in the water column. Using existing data from [Tran et al., 2018], improvements were made to the existing Winterwerp [1998] model to account for the dependency of the particle size on sediment concentration. Tests were then run to collect data on how salinity impacts flocculation. This data was then used to further modify the Winterwerp [1998] model in order to account for salinity. These modifications resulted in predictions that better matched laboratory data, indicating that the modified Winterwerp [1998] equation is a promising tool for incorporation into larger sediment transport models.

The coagulation/flocculation process is an important part of surface water treatment. It has direct impact on the reliability of plant operations and final water qualities together with cost control. Low water temperature has a significant impact on the operation of drinking water treatment plants, especially on coagulation/flocculation processes.<p> A microscopic image technique has been used to study the coagulation and flocculation process in recent years, but it requires sample handling that disturbs the floc characteristics during measurement. A high resolution photographic technique was applied to evaluate flocculation processes in the present work. With this technique, the images of the flocs were obtained directly while the flocculation process was taking place. In combination with camera control software and particle size analysis software, this procedure provided a convenient means of gathering data to calculate size distribution. Once the size distribution was calculated, the floc growth and floc size change in the aggregation process could be analyzed. Results show that low water temperature had a detrimental impact on aggregation processes. A water temperature of 0 °C resulted in a slow floc growth and small floc size. Although the floc growth rates at 4 °C and 1 °C were less than those at 22 °C, they were higher than at 0 °C. To improve aggregation processes at low water temperature, adding the coagulant aid of anionic copolymer of acrylamide into the water was found to be effective when the temperature was not less than 1 °C. However, it made only a slight impact on aggregation when the temperature approached 0 °C. At water temperatures of 22 °C, 4 °C and 1 °C, the polymer caused the formation of large floc (larger than 0.5 mm2 in projected area). The polymer significantly shortened the required time of flocculation and sedimentation. Three minutes of flocculation and 20 minutes of sedimentation were sufficient for the polymer to achieve good treatment performance, while the flocculation time and sedimentation time had to be 20 and 60 minutes, respectively, without using the polymer. On the other hand, when the temperature was close to 0 °C, the polymer did not cause the formation of the large floc, nor did it shorten the time of flocculation and sedimentation.<p> The experimental results in this research agree with the model for flocculation kinetics given by Argaman and Kaufman (1970). With decreasing water temperature, the aggregation constant (KA) decreased and breakup constant (KB) increased. KA and KB with aluminum sulfate was close to those with ferric sulfate, respectively. <p> In treating the South Saskatchewan River water, an aluminum sulfate or ferric sulfate dosage greater than 50 mg/L resulted in marginal gains in treatment efficiency. Decreasing dosages of aluminum sulfate or ferric sulfate caused lower floc growth rates and smaller floc sizes. Extremely low dosages (5 mg/L or less) resulted in poor floc formation and extremely small sizes.

The coagulation/flocculation process is an important part of surface water treatment. It has direct impact on the reliability of plant operations and final water qualities together with cost control. Low water temperature has a significant impact on the operation of drinking water treatment plants, especially on coagulation/flocculation processes.<p> A microscopic image technique has been used to study the coagulation and flocculation process in recent years, but it requires sample handling that disturbs the floc characteristics during measurement. A high resolution photographic technique was applied to evaluate flocculation processes in the present work. With this technique, the images of the flocs were obtained directly while the flocculation process was taking place. In combination with camera control software and particle size analysis software, this procedure provided a convenient means of gathering data to calculate size distribution. Once the size distribution was calculated, the floc growth and floc size change in the aggregation process could be analyzed. Results show that low water temperature had a detrimental impact on aggregation processes. A water temperature of 0 °C resulted in a slow floc growth and small floc size. Although the floc growth rates at 4 °C and 1 °C were less than those at 22 °C, they were higher than at 0 °C. To improve aggregation processes at low water temperature, adding the coagulant aid of anionic copolymer of acrylamide into the water was found to be effective when the temperature was not less than 1 °C. However, it made only a slight impact on aggregation when the temperature approached 0 °C. At water temperatures of 22 °C, 4 °C and 1 °C, the polymer caused the formation of large floc (larger than 0.5 mm2 in projected area). The polymer significantly shortened the required time of flocculation and sedimentation. Three minutes of flocculation and 20 minutes of sedimentation were sufficient for the polymer to achieve good treatment performance, while the flocculation time and sedimentation time had to be 20 and 60 minutes, respectively, without using the polymer. On the other hand, when the temperature was close to 0 °C, the polymer did not cause the formation of the large floc, nor did it shorten the time of flocculation and sedimentation.<p> The experimental results in this research agree with the model for flocculation kinetics given by Argaman and Kaufman (1970). With decreasing water temperature, the aggregation constant (KA) decreased and breakup constant (KB) increased. KA and KB with aluminum sulfate was close to those with ferric sulfate, respectively. <p> In treating the South Saskatchewan River water, an aluminum sulfate or ferric sulfate dosage greater than 50 mg/L resulted in marginal gains in treatment efficiency. Decreasing dosages of aluminum sulfate or ferric sulfate caused lower floc growth rates and smaller floc sizes. Extremely low dosages (5 mg/L or less) resulted in poor floc formation and extremely small sizes.

This dissertation aims to better understand how floc aggregate characteristics and behaviors are modified under different local conditions and how such alterations impact the floc settling velocity, which is one of the most crucial parameters influencing sediment transport modeling. A series of laboratory experiments were conducted to examine the impact of suspended sediment concentration, mixes of clay and silt, and resuspension process to equilibrium floc size and floc settling velocity. In order to observe floc size evolution, a new floc imaging acquisition was first developed. This new method allows flocs in suspended sediment concentration up to C = 400 mg/L can be imaged non intrusively. This new method was applied in all three individual studies, which are composed of this dissertation. The first chapter investigates the behaviors of flocs under constant and decay suspended sediment concentrations within a steady turbulent suspension. In the constant-concentration set of experiments, floc size time series were measured for 12 h for each of the concentration C = 15, 25, 50, 100, 200, 300, and 400 mg/L. In the decay-concentration experiments, clear water was introduced to the mixing tank, simultaneously the suspension was drained out of the mixing tank at the same rate to make the suspended sediment concentration reduce while the turbulent shear was remained unchanged. The data shows that the equilibrium floc size is a weak, positive function of concentration. For example, in order to increase 20% of floc size (approximate 22 um) the concentration needs to be increased by 700% (going from 50 to 400 mg/L). The data also illustrates that during the decrease of concentration from C = 400 to 50 mg/L, the floc size responses to the changes of concentration in the order of 10 min or less. The second chapter examines how silt particles and clay aggregates interact in a turbulent suspension. Floc sizes and settling velocity of three different suspensions, i.e., pure clay, pure silt, and a mixture of clay and silt, were monitored. The floc size data show that the presence of silt particles does not have significant impacts on clay aggregate sizes. Silt particles, however, get bound up within floc aggregates, which in turn increase the settling velocity of the floc by at least 50%. The third chapter examines whether any changes in floc properties during the deposition and resuspension processes. The floc sizes and shapes in a set of experiments with different consolidation times, concentrations, and shear patterns were measured. The conditions at which the flocs deposited or resuspended were maintained the same. The data reveal that floc size and shape of freshly deposited and after resuspended are unchanged. The erosion rate and concentration is a function of consolidation time and the applied shear stress during the deposition phase. Hence, there is a small reduction in resuspended concentration resulting in a slight decrease in resuspension floc size since floc size is also a function of concentration. / Ph. D. / Sediment transport is a narrative poem from mother nature telling us about the evolution of ancient and modern rivers, deltas, and estuaries. For thousands of years, mankind has been examining the coarser part of the poem, the gravel and sand. The finer part, the mud, has not been systematically investigated until the last 60 years. The key difference between sand and mud is the capability of mud to aggregate and form flocs which have sizes, densities, and shapes that are vastly different from the original constitutive particles. This flocculation process adds a layer of dynamics to the erosion, deposition, and transport of mud that is not present in the transport of sand. Therefore, the primary motivations for this dissertation are 1) to better understand the behavior of floc size under different conditions, e.g., in the estuaries, and 2) to provide high-quality data of floc characteristics and size evolution for model development, testing, and calibration purposes. Laboratory studies are conducted to measure the floc size and in some cases settling velocity, as a function of time under different turbulent, concentration, and sediment mixture. The findings in this dissertation help to fill the gaps of knowledge in cohesive sediment transport processes. This dissertation also suggests how floc behaviors should be accounted for under different conditions. Such information is valuable for projects such as management of sediment supplies, mitigation of land loss, restoration, and land-building diversions, e.g., on the Mississippi and Atchafalaya Rivers. Data associated with this dissertation are also available on GitHub under https://github.com/FluidSedDynamics.

Abstract This study focused on web forming at a 5–10% consistency range, termed Ultra High Consistency (UHC). The study continued work done by Gullichsen with his research groups (1981–2007) and combined it with the HC forming research done by Valmet (HC, 1999–2004). The hypothesis was that by utilizing a rotor to fluidize suspension and a wedge to eliminate the free jet and thus prevent reflocculation, web forming at UHC is feasible at commercial speeds. The research method was experimental. The bulk of the research was conducted at pilot scale. A new UHC headbox was designed and mounted on a pilot former. The key elements of the headbox are the rotor and the wedge. As fluidization forms the base for UHC forming, this was evaluated at the pilot former using image analysis. In addition, fluidization was studied using a laboratory-scale device. Besides basic paper analysis, X-ray microtomography and sheet splitting methods were utilized to analyze the sheet structure. The results show that forming is possible within the focus area, 5–10% consistency and machine speeds of 150–600 m/min, although the operation potential of the UHC former is even wider. The results demonstrate that the wedge is needed for successful UHC forming, but the rotor is not required, providing the flow rate is sufficiently high. This indicates that various forces induced by the flow itself can be adequate to fluidize suspension for forming. The critical Reynolds number of full fluidization was found to be 200–250. The Reynolds numbers were estimated utilizing the linear dependencies found between the apparent viscosity and consistency, using the maximum mean flow velocities inside the headbox, and neglecting the possible rotation of the rotor. The corresponding critical flow velocities at 10% consistency are 12 and 19 m/s for a eucalyptus and pine pulp. The velocities are on average 70 and 60% lower than those given in the literature (40–50 m/s). The results reveal that the fiber orientation of UHC sheets is planar, the floc size of the web increases with consistency, the internal bond increases linearly with the floc size, and the tensile strength appears to decrease with increasing floc size. In consequence, it is postulated that the increase in the out-of-plane strength at the expense of in-plane strength with the consistency increase results from a more flocculated structure. / Tiivistelmä Tutkimus keskittyi rainanmuodostukseen 5–10 % sakeudessa. Sakeusalue nimitettiin ultra korkeaksi (lyhenne UHC). Tämä työ jatkoi Gullichsenin ja hänen tutkimusryhmiensä tutkimustyötä (1981–2007) ja samalla yhdisti sen Valmetin tekemään suursakeusrainaustutkimukseen (HC, 1994–2004). Työn hypoteesina oli, että käyttämällä roottoria massan fluidisoimiseen sekä ns. wedgeä eliminoimaan vapaa suihku ja estämään jälleenflokkaantuminen, rainanmuodostus UHC-sakeudessa on mahdollista kaupallisissa nopeuksissa. Tutkimusmetodi oli kokeellinen. Pääosa tutkimuksesta suoritettiin koekonemittakaavassa. Uusi UHC-perälaatikko suunniteltiin ja asennettiin koeformeille. Perälaatikon pääelementit ovat pyörivä roottori ja wedge. Koska fluidisointi muodostaa UHC-rainauksen perustan, fluidisointia evaluoitiin koekoneella käyttäen kuva-analyysiä sekä tutkittiin lisäksi käyttäen röntgenmikrotomografia ja arkin halkaisu -metodeja. Tulokset osoittavat, että rainaaminen on mahdollista määritellyllä fokusalueella, 5–10 %sakeudessa ja konenopeudella150–600 m/min, joskin UHC-formerin toimintapotentiaali on vieläkin laajempi. Tulokset osoittavat, että wedge tarvitaan onnistuneeseen UHC-muodostamiseen, mutta roottoria ei tarvita, mikäli virtausnopeus on riittävän suuri. Tämä tarkoittaa, että virtauksen aikaansaamat voimat voivat itsessään olla riittäviä massan fluidisoimiseksi rainaamista varten. Täyden fluidisaation kriittisen Reynoldsin luvun havaittiin olevan välillä 200–250. Reynoldsin luvut arvioitiin käyttäen löydettyjä viskositeetin ja sakeuden välisiä lineaarisia riippuvuuksia, päävirtauksen maksiminopeuksia perälaatikossa ja jättäen huomioon ottamatta mahdollinen roottorin pyöriminen. Reynoldsin lukuja vastaavat kriittiset virtausnopeudet 10 % sakeudessa ovat eukalyptus- ja mäntymassalla 12 ja 19 m/s. Nopeudet ovat keskimäärin 70 ja 60 % pienempiä kuin kirjallisuudessa annetut (40–50 m/s). Tulokset osoittavat, että UHC-arkeissa kuituorientaatio on tasomainen, rainan flokkikoko kasvaa sakeuden kasvaessa, palstautumislujuus kasvaa lineaarisesti flokkikoon kanssa ja vetolujuus näyttäisi laskevan flokkikoon kasvaessa. Näin ollen esitetään, että sakeuden kasvaessa tapahtuva palstautumislujuuden kasvu tasolujuuksien kustannuksella johtuu flokkaantuneemmasta rakenteesta.