(a.y. 2012/13, 9 credits – 90 hours)

Environmental Fluid Mechanics – Hydropower Plants

Transport processes and impactsMarco Toffolone-mail: marco.toffolon@ing.unitn.itLaboratorio Didattico di Modellistica Idrodinamica(2nd floor, central corridor)tel.: 0461 28 2480

Hydrology and water resourcesprof. Alberto Belline-mail: alberto.bellin@ing.unitn.it

Constructionsprof. Maurizio Righettie-mail: maurizio.righetti@ing.unitn.it

Part II: Transport processes in the environment

II-1. Introduction (10 hours)Basic concepts: definition of concentration, mass balance, diffusion. Turbulent mixing. Gaussian model for diffusion processes: basic solution and typical scales. Advection-diffusion equation and analytical solutions in the one-dimensional context. Phases of mixing: near field, intermediate field, far field. Dispersion resulting from non-uniform advection. Dynamics of reactive tracers (including temperature): zero- and one-dimensional models. II-2. Transport processes in rivers and effects of hydropower production (9 hours)Review of basic hydraulic concepts. Estimates of turbulent diffusion and dispersion coefficients. Flood waves due to sudden releases from hydropower plants (hydropeaking). Temperature waves due to the temperature differences between rivers and hydropower releases (thermopeaking). Introduction to river morphology. Hints on biological effects of hydro- and thermo-peaking. Modification of habitats in impacted rivers. Numerical models for longitudinal dispersion: examples. II-3. Thermal dynamics of reservoirs (9 hours)Heat budget in closed basins. Stratification cycle and implications on vertical mixing. Effect of withdrawals and inflows on the temperature profile. Hints on biological aspects and water quality. Numerical models for hydro-thermodynamics of reservoirs: examples. Application to a real case: reservoir management and impact on downstream river. ~28 hours

Lecture notes.

Suggested textbook (transport processes):S.A. Socolofsky & G.H. Jirka, dispense del corso Special Topics on Mixing and Transport in the Environment, Texas A&M University, 2005.

Further reading on environmental fluid mechanics:Fischer H.B., Koh J., List J., Imberger J., Brooks H., Mixing in Inland and Coastal Waters, Academic Press, New York, 1988.Rutherford J.C., River Mixing, John Wiley & Sons, Chichester, 1994.

J.L. Martin, S.C. McCutcheon, Hydrodynamic and transport for water quality modeling, Lewis Publishers CRC Press

About HP impacts on the environment:Journal papers

Main references

link on website: http://www.ing.unitn.it/~toffolon/ (“Materiale didattico”)

Environmental fluid mechanics: An emblematic

case

21/04/2010

http://earthobservatory.nasa.gov/NaturalHazards/event.php?id=43733

Deepwater Horizon oil spill

http://fastfreenews.com/wp-content/uploads/2010/06/gulf-oil-spill1.jpg

25/04/2010

01/05/2010

09/05/2010

17/05/2010

24/05/2010

12/06/2010

19/06/2010

Impacts of hydropower production

Reservoirs

Rivers Ecosystems

Thermal structure

Macro-benthosFishes

Sediments

Infilling

CloggingCoasts

Eco-hydraulics in Trento: a multi-disciplinary research group

Guido ZolezziNunzio Siviglia

M. Cristina BrunoBruno Maiolini

Department of Civil and Environmental Engineering

University of Trento, Italy

Typical medium-term behaviour:daily cycle + weekly cycledue to the production of peaks of electricity.

Sund

ay

Mon

day

Frid

ay

Thur

sday

Wed

nesd

ay

Tues

day

Satu

rday

Sund

ay

Stage variations are very rapid (order of cm/min or m/h) both in the rising and in the decreasing phase travelling waves.

Simplifying assumption: waves have approximately a square shape.

Hydropeaking: qualitative description

http://www.racine.ra.it/europa/uno/esame2003/terzaf/vcv/html/due.htm

… and what is thermopeaking?

temperature of reservoir

Hydropeaking thermal alterationIntensity changes during the year

river

temperature of river≠

Main concepts

Transport in the environment

1. mass is conserved (non-reactive tracers)

2. concentration tends to become spatially homogeneous

1 2 3

passive tracer

flow field

0dtdM

VMC

concentration:

“diffusion”

(exceptions: reactive tracer oxygen, nutrients, and temperature)

DiffusionDiffusive flux works against concentration gradient Fick law

(1855)CD

200 “balls”, probability of movement 0.2, single boxes

Phenomenological explanation: random displacement rightward or leftward

N steps (time)

Main features of diffusive processes

1 2 3Characteristic dimension of the cloud

L(t1)

L(t3)L(t2)

DttL )(

Self-similar Gaussian solution

2

2

21 2exp

2

xMxC D

with variance Dt22

(1D, infinite domain)

± 68.3%±2 95.5%±3 99.7%

“mass” between extreme points:

How an advective process becomes diffusive…

Turbulence (“random” advection)Turbulent diffusion(property of the flow field,

and not of the tracer+fluid)for times long enough

(longer than the integral scals of turbulence)

Non-uniform advective motion+ diffusionorthogonal to the flow

Dispersion(combined mechanism)

for times long enough(longer than the characteristic scale of orthogonal diffusion)

Thermal oscillations Molecular diffusion (property of tracer+fluid)

typical values in water ~ 10-5 cm2/s = 10-9 m2/sin air ~ 10-5 m2/s

Dispersion: phenomenological description

Lagrangian model: following particles

deterministic component(assigned flow field)

random component(turbulence or thermal oscillation)

y

u(y)

non-uniform advective motion cloud distortion along x

xorthogonal diffusion “compacts” the cloud along y

dispersion enhanced “diffusion” along x

concentration C(x)

particles in the x,y domain

C(y) zoom

zoom

x

y

particles

Numerical simulation

x

y

River mixing

hp. shallow water, large width (B>>Y)

z

y

B

Yvertical mixing is much faster than transverse mixing

Mixing phases

sourcecompleted vertical mixing

completed transverse mixing

near field: 3D model, turbulent diffusion (+ molecular)

intermediate field: 2D model (depth-averaged), dispersion + turbulent diffusion (+ molecular)

far field: 1D model (cross-section-averaged), dispersion (+ turbulent diffusion + molecular)

Gallery of images

Point source in a river 1/2

flow direction

Tracciante rilasciato in un fiume. Il mescolamento verticale viene raggiunto molto velocemente (a distanza di circa 10 volte la profondità); il mescolamento trasversale è molto più lento.

Point source in a river 2/2

Le curve incrementano fortemente il mescolamento trasversale a causa delle correnti secondarie.

Confluence

Confluenza di tre fiumi: a sinistra, con una concentrazione molto alta di particolato; al centro con una concentrazione intermedia; a destra (più scuro), più pulito. Contorni ben definiti separano di diversi flussi. [Inn a sinistra, Danubio al centro, Passau DE]

Un caso concreto: Scarico accidentale in un corso d’acqua

Fasi del problema

rio Sorne

fase 2:confluenza

fase 1:mixing nel rio Sorne

fase 3:mixing nell’Adige

fase 4:cosa succede a valle?

fiume Adige

scarico massa M