Sediment management by jets and turbidity currents with application ...

Journal of Hydraulic Research Vol. 47, No. 3 (2009), pp. 340–348 doi:10.3826/jhr.2009.3497 © 2009 International Association of Hydraulic Engineering and Research

Sediment management by jets and turbidity currents with application to a reservoir for flood and pollution control in Chicago, Illinois Gestion de sédiment par jets et courants de turbidité: application à un réservoir pour l’inondation et la lutte contre la pollution à Chicago, Illinois OCTAVIO E. SEQUEIROS, (IAHR Member), Graduate Research Student, Ven Te Chow Hydrosystems Laboratory, Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews Ave., Urbana, IL 61801, USA. E-mail: [email protected] (author for correspondence) MARIANO I. CANTERO, (IAHR Member), Post-Doctoral Researcher, Ven Te Chow Hydrosystems Laboratory, Department of Geology, University of Illinois at Urbana-Champaign, 205 North Mathews Ave., Urbana, IL 61801, USA. E-mail: [email protected] MARCELO H. GARCIA, (IAHR Member), Chester and Helen Siess Professor, and Director, Ven Te Chow Hydrosystems Laboratory, Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews Ave., Urbana, IL 61801, USA. E-mail: [email protected] ABSTRACT Management of fine sediments presents an important engineering and environmental problem. The active dredging of large volumes of sediments from harbors and reservoirs involves the use of expensive equipments with large operational costs. In several situations, however, passive systems can afford an efficient and low operational cost alternative. This work assesses the feasibility of eroding fine bed sediment by jet discharges and the subsequent transport in suspension by an ensuing turbidity current. A calibrated numerical model is applied to study the flow transport capacity under field conditions. Results show that large amounts of sediment can be eroded from the bed in the near-field region of the jet discharge, and that part of this eroded sediment can be transported in suspension further downstream by turbidity currents. The flow transport capacity depends strongly on the initial conditions of the jet discharge. This work has implications on sediment management by passive methods and presents a simple setting that could result in resource savings by reducing operational costs. RÉSUMÉ La gestion des sédiments fins représente un important problème d’ingénierie et d’écologie. Le dragage actif des grands volumes de sédiments des ports et des réservoirs utilise des équipements chers avec des coûts opérationnels élevés. Dans plusieurs situations, cependant, les systèmes passifs peuvent constituer une alternative efficace et à bas coût opérationnel. Cette étude évalue la possibilité d’éroder le sédiment fin de lit par des jets et de le transporter en suspension par un courant de turbidité. Un modèle numérique calibré est appliqué pour étudier la capacité de transport de l’écoulement dans des conditions réelles. Les résultats montrent que de grandes quantités de sédiment peuvent être érodées du lit dans la région du champ proche du jet, et qu’une partie de ce sédiment érodé peut être transportée en suspension plus loin en aval par des courants de turbidité. La capacité de transport par l’écoulement dépend fortement des conditions initiales du débit du jet. Ce travail a des applications dans la gestion de sédiments par des méthodes passives et présente un cadre simple qui épargne des ressources tout en réduisant les coûts opérationnels.

Keywords: Cohesionless sediment, deposition, numerical model, scour, sediment dredging, sediment erosion, submerged jet, suspended sediment, turbidity current 1 Introduction

several environments (Van Dorn et al. 1975, Jenkins et al. 1981, Bailard and Camperman 1983, Dellaripa and Bailard 1986, Hotchkiss and Huang 1995, Jenkins et al. 1993, Odgaard and Wang 1991). Many of the proposed technologies rely on high jet discharges producing enough scour to maintain the desired depth; however, in these cases the dredged sediment is deposited in nearby regions. In cases in which the dredged sediment must be transported for long distances the proposed technologies are based on the active mechanical dredging of sediment with high operational cost.

Management of fine deposited sediment raises significant engineering and environmental problems. For example, in combinedsewer-overflow reservoirs, bottom sediments present a source of high biochemical oxygen demand. The depositon of fine sediment in harbors and navigation canals demands the dredging of many tons of material per year. Other examples that could benefit from new sediment management technology can be found in literature mainly related to dredging and depth-maintenance in

Revision received January 12, 2009/Open for discussion until December 31, 2009. 340

Journal of Hydraulic Research Vol. 47, No. 3 (2009)

Sediment management by jets and turbidity currents with application to a reservoir

Previous work on density and turbidity currents by García (1994), García and Parker (1993), Parker et al. (1986), Fukushima et al. (1985), and Parker (1982) suggests that a jet-induced density current could be an effective mechanism to transport sediment for long distances. The conceptual idea of the model presented herein consists of a wall jet discharging over an erodible sloping bed. The jet-induced flow may resuspend sediment in a way that a self-sustained, buoyancy-driven turbidity current might “ignite” with enough transport capacity to move the resuspended material for long distances (Parker 1982). However, this process is not always feasible since it strongly depends on the characteristics of the jet discharge, the extent and slope of the domain, as well as on the erosion and settling properties of the bottom sediment. The proposed McCook reservoir in Chicago, Illinois, for instance, to be built as part of the Chicago area Tunnel and Reservoir Plan (TARP) of the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) is an example where this kind of technology could be applied. This large urban reservoir will provide 27 × 106 m3 of storage capacity with a surface area of 395 000 m2 for the storage of Combined Sewer Overflows (CSO) to prevent the release of mixed runoff and wastewater to the environment during extreme rainfall events. Figure 1 shows a schematic of how the described system would work in McCook Reservoir. Given the large dimensions of the surface area that will have to be washed mechanically to clean up the reservoir after large storms, the possibility of using jets and turbidity currents to clean the bottom before emptying the reservoir is attractive and has provided the motivation for this work. Wall jet initial conditions and sediment characteristic do not only set the near field scour pattern (Rajaratnam and Berry 1977, Rajaratnam 1981, Rajaratnam 1982, Lim and Chin 1993, Stein et al. 1993, Ade and Rajaratnam 1998, Chiew and Lim 1996, Aderibigbe and Rajaratnam 1998, Sequeiros et al. 2007) but also influence the far field flow and scour development. The scour pattern corresponding to sediments with different mean size presents similarities, but the downstream evolution of the suspended sediment is significantly different. For a given wall jet, the near-field region is dominated by momentum and the scour is related to

jet discharge water entrainment

sediment entrainment

jet discharge

341

the bed shear stress. In the far-field the jet momentum dissipates and buoyancy effects are responsible for maintaining the flow by initiating a turbidity current flowing down the sloping bottom (Parker et al. 1987). This, however, depends on the ability of the flow to maintain sediment in suspension by net erosion or weak deposition (Garcia and Parker 1993). The purpose of this work is to study the feasibility of transporting fine sediment, originally eroded from the bed by a jet discharge, for long distances by means of a developing turbidity current. First, the near-field erosive pattern and sediment transport created by different settings of wall jets for a wide range of densimetric particle Froude numbers is studied to validate and calibrate a numerical model. The model is then used to assess the flow transport capacity in the far-field region and the conditions leading to the development of self-sustained, weakly depositional turbidity currents capable of tranporting sediment in suspension to a sump or drainage channel as shown in Fig. 1.

2 Numerical model The model problem is shown in Fig. 2, involving a twodimensional (2D), clear-water jet discharging over a movable bed of constant slope S. The model considers only noncohesive sediment having a settling velocity vs and a submerged specific gravity R. The mathematical model used herein is the four-equation, depth-averaged model of Parker et al. (1986), together with the closure relations proposed by García and Parker (1993). In this model flow is steady and fully turbulent, and the sediment concentration is assumed to be small in order for the Boussinesq approximation to be valid. The layer-averaged quantities are flow velocity U, volumetric concentration of suspended sediment C, layer thickness h, and the layer-averaged value of turbulent kinetic energy K. The equations governing are read as follows: dUh = we dx

(1a)

dU 2 h 1 dCh2 + RgChS − u2∗ = − Rg 2 dx dx

(1b)

dUCh = vs (Es − cb ) dx

(1c)

water entrainment sediment entrainment drainage channel

Figure 1 Schematic of implementation of bottom sediment management system proposed for McCook Reservoir in Chicago Ill. A clear-water wall jet discharges and resuspends sediment accumulated along bottom after a rainfall even resulting in combined-sewer-overflows captured by reservoir. If enough sediment is resuspended by jets, a turbidity current develops transporting the sediment toward a submerged drainage channel from where it can be pumped for treatment and disposal

Figure 2 Schematic of jet-induced density current over erodible bed. The jet discharge induces an initial sediment resuspension that may develop a self-sustaining density current

342

O.E. Sequeiros et al.


dUKh 1 = u2∗ U + U 2 we − εh − Rgvs Ch dx 2 −

1 1 Rgwe Ch − Rgvs h(Es − cb ), 2 2

(1d)

where u∗ , Es , we , cb , and ε are the bed shear velocity, the sediment entrainment coefficient, the water entrainment velocity, the near-bed sediment concentration, and the layer-averaged viscous dissipation of turbulent kinetic energy, respectively. The closure relations are as follows: u2∗ = α K Es =

(2)

A z5u A 5 zu 1 + 0.3

(3)

A = 1.310−7 , u∗ zu = f(Rp ), vs 0.6 Rp f(Rp ) = 0.586 R1.23 p

3 Validation and calibration

3.5 ≤ Rp Rp ≤ 3.5

Here Rp = (RgDs )1/2 Ds /ν is a Reynolds number defined with the characteristic sediment size Ds , the acceleration of gravity g, the submerged specific gravity of the sediment R, and the kinematic viscosity of the water ν (Garcia and Parker, 1991). Further we = ew U ew =

(4a) 0.075

(1 + 718Ri2.4 )1/2

,

(4b)

where Ri = RgCh/U 2 is a bulk Richardson number, cb = r0 C

(5a)

r0 = 1 + 31.5

vs u∗

1.46 (5b)

and K3/2 h 1 ew 1 − Ri − 2 CαD∗ + CD∗ β= 2 , CD∗ 3/2 ε=β

zero net erosion of material from the bottom. The initial value for K0 is set to CD∗ U02 /α, namely U02 /K0 = α/CD∗ (Parker et al. 1986). For the present case the boundary conditions are assumed to be those of a wall jet, that is a jet of clean water with no suspended material. Jet flows are dominated by momentum (inertial forces) whereas buoyancy forces are negligible near the nozzle. Bottom sediment is immediately entrained from the bed, as the jet has a high erosion capacity. This is a realistic boundary condition, as water used in the jet system will most probably be taken from the top of the reservoir itself, and will be deprived of suspended material. Furthermore, inflow conditions were also set for clear-water jets in the experiments used to calibrate the model, which are described below.

(6a) (6b)

α

where CD∗ is a bed friction coefficient. This model takes into account the coupling between sediment in suspension and the state of turbulence by setting the bottom shear stress proportional to the layer-averaged turbulent kinetic energy shown in Eq. (2). It is important to mention that even though these equations were developed for turbidity currents, they can also be applied to analyze the evolution of plane wall jets like these considered herein (Parker et al. 1986). From an analysis of the equations and closure relations it is seen that the dimensionless parameters governing the flow are Ri0 = RgC 0 h0 /U02 , U0 /vs , α, S, U02 /K0 , CD∗ , and Rp . U0 , h0 and K0 are initial values, and C0 is computed from Eqs. (1c), (5a), and (5b) for equilibrium conditions, i.e. as the value resulting in

The mathematical model problem described above was validated focusing on the near-field region of the jet discharge. The experiments performed by Sequeiros (2004), and Sequeiros et al. (2007) were used for calibration. Two kinds of experiments were considered: Multiple circular wall jets and Plane wall jets. The former consisted of an array of submerged turbulent circular wall jets parallel to and resting on a fixed boundary and flowing upon a layer of sediment resting along the same fixed boundary (Fig. 3(a)). The plane wall jet tests were carried out in a channel resting on a plate, located inside the water tank used for the circular wall jet tests (Fig. 3(b)). The experiments were run until an equilibrium or approximate asymptotic state for the scour condition was reached (Rajaratnam and Berry 1977, Dey 1995). Three kinds of bed material were used to cover a wide range of cohesionless sediment sizes. Table 1 shows the characteristic sizes and the geometric standard deviation σg for each of the materials used. Various discharges were used to determine the scour pattern for a wide range of densimetric particle Froude numbers F0 defined at the nozzle as U0 , (7) F0 = gdi ρ ρ where U0 is jet velocity at the nozzle, di a representative size of the bed material, and ρ the difference between the bed material density ρs and fluid density ρ. For circular jets, the scour pattern created by a single jet is characterized by measuring the maximum (subscript m) scour length rm amongst other parameters. Considering the boundary of the final or equilibrium scour pattern to be an isoline of constant bottom shear stress equivalent to the threshold value for sediment motion (van Dorn et al. 1975), the scour induced by the jets will tend to the equilibrium state asymptotically, as long as enough time is given to develop. At the asymptotic state, the jet can no longer transport sediment because its momentum is dissipated through friction and the scour boundaries remain constant. At this point rm tends to rm∞ . For the plane wall jet experiments the erosion is characterized by the variable xm . At the asymptotic or steady state it tends to xm∞ . The densimetric Froude number, a measure of the ratio of the tractive force on a submerged grain to its resistive



343

0.35 m 5.4 m jets overflow

(a)

supply tank 2.3 m pump

2.6 m

7.3 m

supply line

2.7 m

(a)

(b) 5.0 m

0.40 m

5.4 m jet

overflow 0.30 m

supply tank

2.3 m pump

2.6 m

7.3 m

supply line

2.7 m

(c) (b) Figure 3 Set-up for (a) circular wall jet experiments, (b) plane wall jet experiments

Table 1 Characteristic diameters of sediments and geometric standard deviation Material

d50 [µm]

d95 [µm]

d84 [µm]

d16 [µm]

σg

Material 1: Sil-Co-Sil 106 Material 2: Sil-Co-Sil 250 Material 3: Silica Sand 60-80

19 45 250

86 196 360

56 118 325

5 7 196

3.28 4.26 1.29

force, is defined using d95 as the effective diameter, rather than d50 (Aderibigbe and Rajaratnam 1998, Sequeiros et al. 2007). A more detailed test description provides Sequeiros (2004), and Sequeiros et al. (2007). Figure 4(a) shows the final scour pattern for a plane jet. Figure 4(b) and 4(c) show the final scour pattern for 4 and 7 multiple circular jet tests, where the scour corresponding to each single jet merges with its neighbor’s turning the flow and erosion patterns two dimensional. Numerical solutions involving materials 1 (d95 = 86 µm) and 2 (d95 = 196 µm), and a slope of S = 0.015, as described by

Figure 4 Final scour pattern for (a) plane jet, (b) 4 circular wall jets, (c) 7 circular wall jets. Note initial expansion angle and transition toward 2D flow and scour pattern downstream in (b) and (c)

Sequeiros et al. (2007) and α = 0.1 (Parker et al. 1986), were computed to determine the scour predicted by the model. The estimated scour is considered to be the distance from the nozzle to the point where net erosion ends and deposition starts. The friction coefficient for smooth boundaries was estimated to 0.0025 ≤ CD∗ ≤ 0.0065 (Sigalla 1958, Rajaratnam and Pani 1974), despite Law and Herlina (2002) report values as high as 0.01. The friction coefficient CD∗ is known to decrease with increasing Reynolds number, but it also dwindles away from the nozzle in the downstream direction (Myers et al. 1963, Rajaratnam 1976, Pani and Dash 1983). The experiments of Sequeiros (2004), and Sequeiros et al. (2007) used to calibrate the model correspond to smooth boundaries. For rougher boundaries, Wu and Rajaratnam (1990) developed an equation to estimate the coefficient of skin friction as a function of the relative roughness, defined as the ratio between the equivalent sand roughness and the diameter of the nozzle. Figure 5 shows the experimental results compared to the model predictions, characterized by the dimensionless maximum scour length rm∞ /b0 and xm∞ /b0 versus F0 . The numerical predictions

344



1000

10

Sequeiros et al. (2007) circular wall jets Sequeiros (2004) plane wall jets 2 1/4

CD* R0 x10

rm ∞ /b 0, xm ∞ /b 0

numerical model

100

numerical model Myers et al. (1963) Schwarz and Cosart (1961) 1

10 10

100 F0

1

1000

Figure 5 Dimensionless values of maximum scour lengths as function of densimetric Froude number F0 . Scour predicted by numerical model is compared to experimental results corresponding to circular and plane wall jets

10 x m∞ /b 0 R0-1/6

100

Figure 6 Calibrated friction factor used in model versus nozzle Reynolds number R0 and distance from nozzle compared with previous researchers expressed following Myers et al. (1963) 188 F 0=178 F0=178

xm∞ /b 0

184 180 176 172

(a) 122 F =68

0 F0=68

xm∞ /b 0

120

118

116

(b) 36

F =15

0 F0=15

xm∞ /b 0

are in good agreement with test observations. Note that for a given F0 , plane wall jet scour lenghts tend to be larger than circular wall jet scour lengths. This is expected because plane jet settings restrain spanwise spread of momentum opposite to single circular wall jets where the momentum can be diffused laterally (Sequeiros et al. 2007). As expected, the agreement between the 2D numerical model and test observations is better for plane than circular jets. Downstream of the scoured area, a turbidity current was created due to sediment entrained into suspension by the jets. The currents evolved up to the end of the plataform located approximately 6 m downstream (Fig. 3(a) and 3(b)). As long as the jets suspended fine sediment from the scour region, the turbidity currents generated by this process were sustained all over the domain. The use of Fig. 5 for practical purposes may require an iterative process. Assuming the characteristics of the sediment are known, in particular grain size and density, F0 is first guessed to estimate the inlet velocity U0 from Eq. (7). Then, using the equation proposed by Sequeiros et al. (2007) for rm∞ /b0 = 3.51F00.75 , the dimensional scour length rm∞ can be estimated depending on the commercial pipe diameter b0 available for the jet nozzle. This process is repeated until the inlet velocity U0 and commercial pipe diameter b0 satisfy scour requirements and pump design discharge. Figure 6 shows the variation of the friction factor CD∗ with the Reynolds number and the distance from the nozzle to the maximum scour length. The plot includes values from model calibration, the classical equation of Myers et al. (1963) and the relation by Schwarz and Cosart (1961), showing that the calibrated friction coefficient has the same trend as both relations, but correspond better with the observations by Schwarz and Cosart (1961).

34

32 1.0E-05

1.0E-04

1.0E-03 S

1.0E-02

1.0E-01

(c)

Figure 7 Effect of slope S on scour predicted by numerical model for F0 = (a) 178, (b) 68, (c) 15. Note increment in scour for slopes steeper than approximately 1%

4 Effect of bottom slope As mentioned above the bottom slope S in the experiments and the numerical simulations was 1.5%. Steeper slopes, however, are expected to increase the scour. The effect of S was studied

by running numerical simulations for different inlet boundary conditions and a wide range of slopes, from 0.001% to 10%. Figure 7 shows results for F0 = 178, 68, and 15 covering the experimental range of studies. The model predicts an increment



in scour for S > 1%. For milder slopes its effect is negligible, and the scour then can be considered to be reference or base scour. The increment of scour with slope with respect to the base scour was found to be up to 6% when S = 10%. Note, however, that the range of slopes in practical applications such as for the planned McCook reservoir is usually not steeper than 1% or 2%. These simulations are consistent with the friction factor relationship shown in Fig. 6. Furthermore, the effect of slope on the evolution of the turbidity current created downstream the scour is also limited. Steady supercritical turbidity currents, usually occurring for S > 1% (Sequeiros 2008), have an intrinsic self-regulating mechanism for deceleration, in that the faster they are, the more ambient water they incorporate through mixing at their interface, thus becoming more dilute and flowing slower.

III III

B B

1000 Sel f - Sust aining self–sustaining d ensit y cur r ent s density currents

U0/vs 800

IIII

600 Jet - likeflows f lows jet-like

400 0

1

A A

II 2

3 Ri0

4

5

6

Figure 8 Curves A and B delimit three regions, each one corresponding to values of jet discharges leading to flows with different characteristics 0.6

5 Numerical prediction for prototype scale

v s=1000 U0/vUo/ s =1000 900 900 800 800 700 700

0.5

600

U/U0

600

555

555

0.4

500

500

400

400

0.3 0

300

600

900

1200

1500

x/h0

Figure 9 Dimensionless velocity for Ri0 = 2. Solutions for U0 /vs = 555 correspond to jet discharges over Curve A, solutions for U0 /vs < 555 to jet discharges in region I, and solutions for U0 /vs > 555 to jet discharges in region II 1.5E-3 U0Uo/ /vsv s=1000 = 1000 1.2E-3

Ψ /C0 U0 h0

The set of Eqs. (1a) to (1d) was solved numerically for prototype conditions focusing the far-field development of the turbidity current with the following parameters (Cantero 2002): S = 0.02, CD∗ = 0.01, α = 0.1, ρs = 1500 kg/m3 , Ds = 50 microns (Rp = 0.783). Values for the sediment density and the characteristic diameter are typical of combined-sewer overflow reservoirs. Solutions were computed to determine the flow regime for different initial conditions, represented by the dimensionless numbers Ri0 and U0 /vs . A clear-water jet discharging over a movable bed induces highly erosive flow conditions. The suspended sediment produces a lower layer of denser-fluid that becomes the driving force for the underflow to develop, and at the same time is a source of turbulence. However, the sediment is kept in suspension if turbulence is high enough, being the work needed to keep sediment in suspension a sink of turbulent kinetic energy. This competitive process may, or may not, lead to a self-sustained flow, which is fully governed by the initial jet conditions, the bottom slope, and the characteristics of the sediment being resuspended. Note that even if the turbidity current is not in a self-sustained mode, it can still transport sediment in suspension for considerable distances as long as it is only weakly depositing along its path. It is also possible that if the flow does indeed resuspend more sediment than it deposits over long distances, it could self-accelerate in a self-reinforcing cycle (Parket et al. 1986). However, such conditions are not expected to occur for the McCook Reservoir. Figure 8 shows three different regions, each corresponding to values of jet discharges that lead to flows with different characteristics. Region below curve A (region I) is characterized by jets with low inertia, which do not induce enough sediment resuspension to ignite a self-sustaining process. After an initial adjustment length, the turbidity current starts to deposit the sediment in suspension, and the sediment flux ψ = UCh and U eventually vanish. The density current behaves as a jet-like flow. Solutions in Figs. 9 and 10 for U0 /vs = 400 and 500 are typical of these flows. All near-field experiments described above with 250 µm sand, and some low inertia experiments with finer sediment (d50 = 19

Sub - cr i t ical sub–critical d ensit y cur r ent s density currents

1200

345

9.0E-4

900 900

6.0E-4 800

800 3.0E-4

700 700 600 600

555 555

500 500

400 400

0.0E-0 0

300

600

900 x/h0

1200

1500

Figure 10 Dimensionless sediment flux for Ri0 = 2. Solutions for U0 /vs = 555 correspond to jet discharges over Curve A, solutions for U0 /vs < 555 to jet discharges in region I, and solutions for U0 /vs > 555 to jet discharges in region II

and 45 µm), also belong to this region. For jet discharges near curve A (Fig. 8), the flow reaches pseudo equilibrium conditions. After an initial adjustment length, resuspension balances deposition, and ψ as well as U reach approximately constant values. This behavior of the solution is observed in Figs. 9 and 10 for U0 /vs = 555.

346


3.0E-5

UUo/0/vvs=1000 s = 1000 900 900 800 800

1.5E-5 (dΨ /dx)/C0U0


700 700 600 600 555 555

0.0E+0

500 500 400 400

-1.5E-5 -3.0E-5 0

300

600

900 x/h0

1200

1500

Figure 11 Dimensionless local sediment resuspension dψ/dx/(U0 C0 ) for Ri0 = 2. Positive value indicates resuspension and negative value deposition. Solutions for U0 /vs = 555 correspond to jet discharges over Curve A, solutions for U0 /vs < 555 to jet discharges in region I, and solutions for U0 /vs > 555 to jet discharges in region II

validated with experimental data from laboratory observations. For a given bottom slope, the key parameters influencing the evolution of the turbidity current are the initial conditions of jet-like flow, grain size of the suspended sediment and its corresponding settling velocity. The calibrated model is then used to study the conceptual model of jet-induced turbidity currents at field scale using as the prototype the McCook Reservoir to be built in Chicago, Illinois for flood and pollution control. It is found that even if a jet-induced turbidity current might not resuspend much bottom sediment along its path, it can still transport sediment in suspension for considerable distances. The present findings have important implications on sediment management in the environment by passive methods and present a simple setting which could imply large resource savings by reducing operational, maintenance, and energy costs.

Acknowledgments Jet discharges between curves A and B (region II) induce a self-sustained process leading to the development of downslope flowing turbidity currents. As U0 /vs increases the density current accelerates faster and induces higher sediment resuspension. Curves for U0 /vs > 555 in Figs. 9 and 10 are typical of this behavior. If a turbidity current is generated with a jet discharge in the region above curve B (region III), it eventually becomes sub-critical. Usually, subcritical turbidity currents are net-depositional (Garcia 1994), indicating that the current is losing suspended sediment as it moves downstream. This leads to a velocity decrease, which in turn leads to a further capacity decrease to transport sediment in suspension. Yet, the current can transport considerable suspended material before vanishing. Some near-field experiments described above with fine sediment (d50 = 19 and 45 µm), also belong to this region. Figure 11 shows the dimensionless local sediment resuspension (dψ/dx/(U0 C0 )) for Ri0 = 2 and several values of U0 /vs , with a positive value indicating resuspension and deposition otherwise. Figure 11 also shows that for each flow regime there is high sediment entrainment at the very beginning, x = x/ h0 < 50. Higher sediment deposition (flows related to region I) or smaller resuspension (flows related to region II) occurs at a distance 80 ≤ x ≤ 110.

6 Conclusions This work introduces the concept of sediment management by a combination of water jets and the subsequent triggered turbidity current. Conceptually, the system consists of clear-water, wall jets eroding sediment from the bed in the near-field region and ignite a gravity flow moving downstream into the far-field region, which - under proper conditions - transport the eroded sediment for long distances. This concept presents a passive mechanism for erosion and transport of fine sediment with large applicability in many engineering and environmental problems. The complete set of mathematical equations is presented and implemented in a numerical model, which is calibrated and

The support of the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) and the Chicago District of the US Army Corps of Engineers (USCOE) is gratefully acknowledged. We are particularly grateful to Tom Fogarty (retired, USCOE) and Dick Lanyon (General Superintendent, MWRD) for their interest in and support of this research.

Notation

d50

b0 = Jet diameter bs = Thickness of sediment layer C = Layer-averaged volumetric concentration of suspended sediment CD∗ = Friction coefficient cb = Near-bed sediment concentration Ds = Characteristic sediment size used in numerical model (or di ) = Bed material diameter, 50% (or i%) of which is finer by weight dj = Distance between centers of nozzles in multiple jet tests Es = Sediment entrainment coefficient F0 = U0 /(gdi ρ/ρ)1/2 = Densimetric particle Froude number at nozzle g = Acceleration due to gravity h = Layer thickness K = Layer-averaged level of turbulent kinetic energy Q = Water discharge R = ρ/ρ = Submerged specific gravity R0 = U0 b0 /ν = Reynolds number at nozzle Ri = RgCh/U 2 = Bulk Richardson number Rp = (RgDs3 )1/2 /ν = Reynolds particle number rm = Maximum scour length from nozzle (circular jet) rm∞ = Maximum scour length at asymptotic state (circular jet) S = Bed slope U = Layer-averaged velocity



u∗ = Bed shear velocity vs = Settling velocity we = Water entrainment velocity x, y, z = Cartesian coordinates xm = Maximum scour length measured from nozzle (plane jet) xm∞ = Maximum scour length at asymptotic state (plane jet) α = Coefficient ρ = Difference between density of bed material and fluid ε = Layer-averaged viscous dissipation φ = Scour angle formed by jet downstream of nozzle ν = Kinematic viscosity of water ρ = Density of fluid ρs = Density of bed material σg = Geometric standard deviation of bed material size = (d84 /d16 )0.5 ψ = Sediment flux Subscript 0 = Initial value (at nozzle) Superscript = Dimensionless parameter in numerical model References Ade, F., Rajaratnam, N. (1998). Generalized study of erosion by circular horizontal turbulent jets. J. Hydr. Res. 36(4), 613–635. Aderibigbe, O., Rajaratnam, N. (1998). Effect of sediment gradation on erosion by plane turbulent wall jets. J. Hydr. Engng. 124(10), 1034–1042. Bailard, J.A., Camperman, J.M. (1983). A design for a test bed scour array for Mare Island Naval Shipyard. Technical Report R-899. Naval Civil Engineering Laboratory, Port Hueneme CA. Cantero, M.I. (2002). Theoretical and numerical modeling of turbidity currents as two-phase flows. Master Thesis. University of Illinois, Urbana-Champaign USA. Chiew, Y.M., Lim, S.Y. (1996). Local scour by a deeply submerged horizontal circular jet. J. Hydr. Engng. 122(9), 529–532. Dellaripa, F., Bailard, J.A. (1986). Studies of scour patterns produced by rotating jets in a flow field. Technical Note N-1753. Naval Civil Engineering Laboratory, Port Hueneme CA. Dey, S. (1995). Three-dimensional vortex flow field around a circular cylinder in a quasi-equilibrium scour hole. Sadhana, Proc. Indian Academy of Science 20(6), 871–885. Fukushima, Y., Parker, G., Pantin, H. (1985). Prediction of ignitive turbidity currents in scripps submarine canyon. Marine Geology 67, 55–81. Garcia, M.H., Parker, G. (1991). Entrainment of bed sediment into suspension. J. Hydr. Engng. 117(4), 414–435.

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