September 24, David Waterman and Marcelo Garcia. In collaboration with:

Transcription

1 September 24, 2012 David Waterman and Marcelo Garcia In collaboration with:

2 PRESENTATION OUTLINE: 1. Mo%va%on and Background 2. Field Experiments 3. Implica%ons for Flow Augmenta%on 4. Conclusions 2

3 Mo%va%on and Objec%ves: Ø If we increase the flow rate in a DO- limited stream (e.g., Flow Augmenta.on), what will happen to the water column DO concentra%ons? Ø We shall simplify the problem to only consider the surface and boqom DO fluxes [per Dr. Xiaofeng Liu and Davide MoQa]. u* : shear velocity [L/T]; a surrogate measure of interfacial shear stress; at bed: C DO : Concentra%on of Dissolved Oxygen [M/L 3 ] ΔC DO : Change in C DO across the diffusive sublayer [M/L 3 ] 3

4 Conceptual Framework: v Adapted from Nakamura and Stefan (1994); Parkhill and Gulliver (1997) Ø With the right SOD sampling apparatus, we could define an en%re curve of boqom DO flux at a sampling sta%on. 4

5 2+000 Study Area: TURNING BASIN SOUTH BRANCH CHICAGO RIVER BUBBLY CREEK Source: MWRDGC (1992) N GRAPHICAL SCALE IN METERS RACINE AVENUE PUMPING STATION (RAPS) 5

6 Experimental Apparatus: U of I Hydrodynamic SOD Sampler Side View (Schema%c) Total Volume: 51.2L Exposed Sediment Area: m 2 Photo: 3D view of sampling chamber during construc%on 6

7 Ø Not much fluid shear stress required to resuspend sediment Sample Sta%on 4: Fine Sandy Muck sediment 1 4 Sample Sta%on 1: Muck sediment Ini%al: C TSS < 100 mg/l u*=0.60 cm/s C TSS = 148 mg/l u*=0.82 cm/s C TSS = 368 mg/l u*=0.83 cm/s C TSS = 2984 mg/l 7

8 2-Slide Side Note: When sediment is resuspended, the volumetric DO sink associated with resuspended BOD dominates the DO flux at the bed Standard Sink Term for DO expenditure due to BOD in water column: dcdo C = K Θ C dt K C T 20 DO D D BOD BOD + DO Temperature-dependent term Rate Constant Concentration of BOD DO-dependent term (Monod formulation with half-saturation constant) Coupled with a BOD sink term: 8

9 Assume: (1) (2) Parallel 4- part sink term for BOD derived from resuspended sediment: from (Waterman et al, 2011) Ø 4 components isolated, and best fits to parameters determined across all samples Ø K D,TSS = mgo 2 /mgtss/day; K BOD,TSS = 2.44 mg/l; and Λ = 2.22 Simula%ons of the Oxygen Sink term on Field Experiments: Trial 1B C TSS = 2984 mg/l T = 26.5 C 1 9

10 Now let s return to considera%on of boqom flux (SOD NR ): Trial ID u * (cm/s) T ( C) SOD NR20 (g/m 2 /day) 2A A A A Ø Due to limita%ons of the experimental apparatus, only 1 non- resuspension sample could be obtained at 4 of the 7 sample sta%ons Ø Note that the subscript NR refers to Non- Resuspension condi%ons; Ø The subscript 20 indicates that the value is normalized to T=20 C. X v Therefore we only have 1 point on the curve at each sta%on, and we don t know what region of the curve it is in. 10

11 To meet our objec%ves, we need to express fluxes in terms of C DO and mean velocity U Flux 1: The bo6om flux, SOD NR Ø We can express the SOD NR flux using a standard diffusion formula%on: where: D is the diffusion coefficient [L 2 /T]; and dc DO /dz is the gradient across the diffusive sublayer Ø Or equivalently, we can use a mass transfer coefficient approach: (see O Connor et al, 2009) v In the diffusion- limited regime, C DO at the base of the diffusive sublayer layer is very close to 0 11

12 Flux 2: The top flux, Reaera>on Ø Start with a depth- averaged Reaera%on Rate source term: where: k a is the reaera%on rate constant; θ a is the temperature correc%on coefficient; C DO,sat is the value of C DO at satura%on O Connor and Dobbins (1958) for H > 1 m and U < 1 m/s where: U is the mean flow velocity in [m/s]; H is the mean flow depth in [m] Ø Convert to the flux value by mul%plying by flow depth H: v Note that U and H are dependent on each other through hydrodynamic equa%ons containing the fric%on coefficient C f ; and C DO,sat is a parameter that is a f(t) v Therefore both flux terms contain only 2 unknowns: C DO and U. 12

13 Ø SOD NR is commonly modeled as a constant; let s model a point in space to steady state under that assump%on o U = 0.1 m/s o Ini%al C DO = 2.0 mg/l o T = 20 C o SOD NR = 3.0 g/m2/day Steady state solu%on for C DO 13

14 Ø The following are the steady state solu>ons for a range of condi%ons under the constant SOD NR assump%on o Vary U as a parameter (ie, simulate an induced flow) o Solve for C DO C DO (mg/l) SODnr= 8.5 SODnr= 6.7 SODnr= 5.0 SODnr= 3.0 Onset of major resuspension U (m/s) In the flat por%on of the curve, SOD NR flux exceeds Reaera%on flux (Q 10 m 3 /s 228 MGD) Ø In this op%mis%c approach to the problem (ie, no SOD NR velocity- dependence), inducing flow generally does indeed improve DO levels Ø The maximum C DO that can be achieved will s%ll ul%mately be limited by the onset of resuspension 14

15 Ø Now let s consider the more realis%c case where SOD NR is U- dependent; let s allow the system to evolve to steady state under that assump%on o U = 0.1 m/s o Ini%al C DO = 2.0 mg/l o T = 20 C Steady state solu%on for C DO Ø Fluxes again evolve to equivalence Ø The SOD NR flux is ini%ally much higher than the reaera%on flux; as a consequence, DO concentra%ons actually decrease. 15

16 Ø The following is the steady state solu>on for the U- dependent formula%on of SOD NR (at equilibrium) Ø This picture of the effect of flow augmenta%on is not as rosy; an addi%onal aera%on source would be needed to improve the DO content of Bubbly Creek Ø Ul%mately the SOD NR curve will asymptote (due to sediment- side control), but probably not before reaching resuspension 16

17 Conclusions 1. Before implemen%ng any WQ purifica%on scenario that involves flow- augmenta%on, it is crucial to characterize the following in the field: a. u* versus SOD NR flux curve b. u* crit (cri%cal value when sediment is first resuspended) c. Sediment entrainment rela%onships in the regime where u* > u* crit 2. Any numerical modeling to support a WQ purifica%on plan must take into account the above factors, or the benefits of the plan may be grossly over- es%mated. 3. Although assump%ons were required in this analysis, the conceptual framework should be valid for analyzing future field experimental data sets. 4. In the case of flow- augmenta%on only, the ini%al experiments suggest that the increase in boqom flux may actually outpace the increase in surface reaera%on flux, leading to lower DO concentra%ons; the opposite of the inten%on. 5. If the flow- augmenta%on causes sediment resupsension, the impact to DO levels could be quite severe, and could be transmiqed substan%al distances downstream. 17

18 Acknowledgements MWRDGC: Heng Zhang, Stickney WRP Laboratory Staff USGS: Ryan Jackson and Jim Duncker VTCHL: Andrew Waratuke, Davide Motta, Yovanni Catano-Lopera, Xiaofeng Liu, and many others References Motta, D., Abad, J. D., and García, M. H. (2010). Modeling framework for organic sediment resuspension and oxygen demand: Case of Bubbly Creek in Chicago. J. Environ. Eng., 136(9), Waterman, D.M., Waratuke, A.R., Motta, D., Cataño Lopera, Y.A., Zhang, H., and García, M.H. (2011). In situ characterization of resuspended sediment oxygen demand in Bubbly Creek, Chicago, Illinois. J. Environ. Eng., 137(8), O Connor, B. L., Hondzo, M., and Harvey, J. W. (2009). Incorporating both physical and kinetic limitations in quantifying dissolved oxygen flux to aquatic sediments. J. Environ. Eng., 135(12),