Variable Density Flow and Transport in Tsunami Impacted Coastal Aquifers; Laboratory Investigations in Saturated Porous Media Meththika Vithanage1,2, Tissa Illangesekera3, Jayantha Obeysekera4, Peter Engesgaard1, and Karsten H Jensen1 1Department of Geography and Geology, University of Copenhagen, Denmark 2International Water Management Institute, Sri Lanka 3Colorado School of Mines, Golden, Colorado, USA 4South Florida Water Management District, West Palm Beach, Florida, USA

OUTLINE Introduction to mixed convective variable density flow Evolution of variable density flow concepts Variable density flow and tsunami in Sri Lanka Laboratory investigations Results from intermediate scale experiments Future work

Unstable Density Configuration Dense fluid Light fluid Hydraulically driven forced convection Buoyancy driven free convection Unstable Stable Instability is influenced by density difference, ambient velocity, heterogeneity and gravity Spatial scale for mixing are typically larger Simmons et al., 2001, 2002

EVOLUTION Fluid & heat (1900-1940) (Benard 1900, 1901; Rayleigh 1916) Fluid, porous media & heat (1940’s) (Lapwood 1948) Fluid, porous media and solute (1950’s) (Wooding 1959) Fluid, porous media, solute and heat (1960’s) (Wooding 1963, 1969;List 1965) Groundwater applications (After 1980’s)

ON GROUNDWATER…. Scale References 1980’s and 1990’s 2000’s Laboratory Schincariol and Schwartz (1990) Simmons et al., (2001) Oostrom et al., (1992a,b) Simmons et al., (2002) Liu and Dane (1996) Wooding et al., (1997) Silliman et al., (1998) Modeling Paschke and Hoopes (1984) Diersch and Kolditz (2002) Koch and Zhang (1992) Schincariol et al., (1994) Fan and Kahawita (1994) Simmons and Narayan (1997) Schincariol et al., (1997) Liu and Dane (1997) Simmons et al., (1999)

- the influence of plume density - and porous medium heterogeneity on the pattern of spreading of the plume They observed - Gravitational instabilities alter the plume shape and plumes are not reproducible - Lobe shape protuberances formed first along the bottom edge of the plume and later within the plume - Density as low as 0.0008 g/cu.cm produced instabilities

- Plumes are either stable or unstable and three dimensional - Importance of non-dimensional numbers to predict plume stability

- No gravitational instabilities in unsaturated zone Particular study evaluated plume behavior in unsaturated and saturated conditions with different densities but without horizontal flow - No gravitational instabilities in unsaturated zone - Fingering started to occur right at the water table

TSUNAMI DECEMBER 26TH 2004

TANK EXPERIMENTS OBJECTIVES Understand the fundamental processes behind unstable density dependant flow problem created by the tsunami Identify the factors which influence the fate and migration of the tsunami plume through freshwater aquifers

TANK EXPERIMENTS

HOMOGENEOUS Dense plume experiments Neutral plume experiments Effect different salt flux rates and times, hydraulic gradients were examined

Heterogeneous I Heterogeneous II

RESULTS

Homogeneous Effect of hydraulic gradient Increase of gradient from 0.0126% to 0.018%

PLUME MIGRATION Dense plume experiment Neutral plume experiment

Heterogeneous Dense plume experiments ~ 14 hrs ~ 17 hrs ~ 22.5 hrs Heterogeneous I Heterogeneous II

Neutral plume experiments ~ 5.5 hrs ~ 10.4 hrs ~ 28.5 hrs Heterogeneous I Heterogeneous II

MAJOR OBSERVATIONS All tsunami experiments showed unstable plume behaviour due to high density and low ambient flow Micro-scale heterogenieties and density difference likely create the instabilities in the homogeneous media Heterogeneous layers have significant effect on the migration and flushing time of the plume Neutral plume pushed the dense plume downward but no effect on the horizontal migration No instabilities or transition zone at the boundary between neutral and dense plume

Challange to the modeling experts!!! FUTURE WORK Use a variable density flow code to understand and model the behaviour of the tsunami Challange to the modeling experts!!!

Which code ??? Peclet number?? Rayleigh number?? Mixed convection??? Perturbation?? Fingering??? Heterogeneity?? HST3D ??? SEAWAT??? SUTRA??? MOCDENS??? FEFLOW???

Thank you!!!