Projekt

Zurück zur Übersicht

Lake Kivu - turbulence and double diffusion in permanent stratification

Titel Englisch Lake Kivu - turbulence and double diffusion in permanent stratification
Gesuchsteller/in Wüest Alfred
Nummer 140538
Förderungsinstrument Projektförderung (Abt. I-III)
Forschungseinrichtung Abteilung Oberflächengewässer EAWAG
Hochschule Eidg. Anstalt für Wasserversorgung, Abwasserreinigung und Gewässerschutz - EAWAG
Hauptdisziplin Hydrologie, Limnologie, Glaziologie
Beginn/Ende 01.04.2012 - 30.04.2015
Bewilligter Betrag 208'458.00
Alle Daten anzeigen

Alle Disziplinen (2)

Disziplin
Hydrologie, Limnologie, Glaziologie
Ozeanographie

Keywords (8)

Double diffusion; Permanent stratification; Diffusive-regime; Convective instabilities; Methane; Subaquatic springs; meromixis; unstable boundary-layers

Lay Summary (Englisch)

Lead
The 485 m deep Lake Kivu (Rwanda, DR Congo) is a fascinating and unique aquatic system: The water contains ~60 km3 methane and ~300 km3 carbon dioxide and the water column is strongly density-stratified. The aim of this project is to understand the evolution and the maintenance of this unusual natural stratification of the lake. Due to sub-aquatic inputs of salty and carbon dioxide-rich water, Lake Kivu is strongly and permanently stratified. This stratification is unique, as the lake water contains four components affecting water density. Whereas salt and carbon dioxide increase the density in deeper layers, the temperature and methane reduce density in the layers of greater depth. This close competition leads to a so-called double-diffusive layering regime, which causes an astonishing staircase of about 300 well-mixed layers (of ~m large scale), separated by thin but very stable interfaces (of ~dm large scale).
Lay summary

of carbondioxide and the water column is permanently density-stratified. Here, we aim (1)at explaining the build-up of this unusual stratification and its temporal evolutionand (2) at understanding the build-up of the enormous methane reservoir in LakeKivu over centuries. Our results will be of economic interest as the value ofthe methane is worth > 20 billion US dollars and it is important to betterquantify the methane production and the vertical fluxes.3 of methane and ~300 km3The 485 m deepLake Kivu (Rwanda, DR Congo) is a fascinating and unique natural water body: Itcontains ~60 km

 

Due to sub-aquaticinputs of salty and carbon dioxide-rich water, Lake Kivu is strongly andpermanently stratified. Salt and carbon dioxide increase the density in deeperlayers, the temperature and methane reduce the density with greater depth. Thisclose competition between increasing and decreasing density leads to aso-called double-diffusive layering, which causes an astonishing staircase ofabout 300 well-mixed layers (of ~1 m vertical thickness), separated by thin butvery stable interfaces (of ~1 dm vertical thickness).

 

In this project, wecould demonstrate that it is possible to resolve - with specialized microstructuresensors - the vertical structures of salinity down to 1mm and verticalstructures of temperature down to 1 cm. With this high resolution we could showthat the inner core of the interfaces are laminar whereas the outer boundary ofthe interface form an unstable boundary layer of a few mm to cm thickness. Thisunstable boundary layer is the result of the faster (and therefore broader)diffusion of temperature versus the slower (and therefore narrowed spread) ofsalinity. Those unstable boundary layers form the source of the convective mixingin the adjacent mixed layers above and below any of those interfaces. We havebeen able to simulate this production of turbulence with Direct NumericalSimulation for real conditions of Lake Kivu.

 

For background information on Lake Kivu

http://www.eawag.ch/forschung/surf/gruppen/kivu/methane_harvesting/index_EN

 

For an introduction to double diffusion

http://www.eawag.ch/forschung/surf/gruppen/kivu/publications/2012_carpenter.pdf

Direktlink auf Lay Summary Letzte Aktualisierung: 15.05.2014

Verantw. Gesuchsteller/in und weitere Gesuchstellende

Mitarbeitende

Name Institut

Publikationen

Publikation
Minimal model for double diffusion and its application to Kivu, Nyos, and Powell Lake
(2015), Minimal model for double diffusion and its application to Kivu, Nyos, and Powell Lake, in Journal of Geophysical Research - Oceans , 120(9), 6202-6224.
Double diffusion in saline Powell Lake, British Columbia
(2014), Double diffusion in saline Powell Lake, British Columbia, in Journal of Physcial Oceanography, 44(11), 2893-2908.
Double-diffusive interfaces in Lake Kivu reproduced by direct numerical simulations
(2014), Double-diffusive interfaces in Lake Kivu reproduced by direct numerical simulations, in Geophysical Research Letters, 41(14), 5114-5121.
Interface structure and flux laws in a natural double-diffusive layering
(2013), Interface structure and flux laws in a natural double-diffusive layering, in JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS, 118(11), 6092-6106.
Revisiting Microstructure Sensor Responses with Implications for Double-Diffusive Fluxes
(2013), Revisiting Microstructure Sensor Responses with Implications for Double-Diffusive Fluxes, in JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY, 30(8), 1907-1923.
Diffusive-type of double diffusion in lakes-a review
(2012), Diffusive-type of double diffusion in lakes-a review, 271-284.
Simulations of a double-diusive interface in the diusive convection regime
(2012), Simulations of a double-diusive interface in the diusive convection regime, in J of Fluid Mechanics, 711, 411-436.
Stability of a Double-Diffusive Interface in the Diffusive Convection Regime
(2012), Stability of a Double-Diffusive Interface in the Diffusive Convection Regime, in JOURNAL OF PHYSICAL OCEANOGRAPHY, 42(May 2012), 840-854.
Stratification, mixing and transport processes in Lake Kivu
(2012), Stratification, mixing and transport processes in Lake Kivu, 13-29.
Temperature steps in salty seas
(2012), Temperature steps in salty seas, in PHYSICS TODAY, 65(3), 66-67.

Zusammenarbeit

Gruppe / Person Land
Formen der Zusammenarbeit
Institut Supérieur Pédagogique de Bukavu; Muvundjia Kongo, Demokratische Republik (ex-Zaire) (Afrika)
- vertiefter/weiterführender Austausch von Ansätzen, Methoden oder Resultaten
- Austausch von Mitarbeitern
University of Trento; Toffolon Italien (Europa)
- vertiefter/weiterführender Austausch von Ansätzen, Methoden oder Resultaten
- Austausch von Mitarbeitern
Kigali Institute of Science and Technology Ruanda (Afrika)
- vertiefter/weiterführender Austausch von Ansätzen, Methoden oder Resultaten
- Austausch von Mitarbeitern
Helmholtz Institut, Geesthacht; Carpenter Deutschland (Europa)
- vertiefter/weiterführender Austausch von Ansätzen, Methoden oder Resultaten
- Publikation

Wissenschaftliche Veranstaltungen

Aktiver Beitrag

Titel Art des Beitrags Titel des Artikels oder Beitrages Datum Ort Beteiligte Personen
Eawag internal seminar Einzelvortrag All quiet on the Lake Kivu Front 16.03.2015 Kastanienbaum, Schweiz Sommer Tobias;
Kigali Lake Kivu Monitoring Team Einzelvortrag Lake Kivu Expedition: 23.1.-11.2.2015 06.02.2015 Kigali, Ruanda Wüest Alfred; Sommer Tobias;
Eawag internal seminar Einzelvortrag Biomixing in Lago di Cadagno 22.09.2014 Kastanienbaum, Schweiz Sommer Tobias;
17th Workshop on Physical Processes in Natural Waters Vortrag im Rahmen einer Tagung Double Diffusion in Lake Kivu and Direct Numerical Simulations 01.07.2014 Trento, Italien Sommer Tobias; Wüest Alfred;
17th Workshop on Physcial Processes in Natural Waters Vortrag im Rahmen einer Tagung Double-diffusive convection in lakes - Nyos, Kivu and Powell Lake compared 01.07.2014 Trento, Italien Wüest Alfred; Sommer Tobias;
UBC Seminar Einzelvortrag Double Diffusion at Lake Kivu, the Afracan Brother of Powell Lake 17.06.2013 Vancouver, Kanada Sommer Tobias; Wüest Alfred;
Seminar Einzelvortrag Double diffusion and energy from methane - the fascinating interior of the unusual Lake Kivu 03.06.2013 University of Innsbruck, Oesterreich Wüest Alfred; Sommer Tobias;
Colloquium in Climatology, Climate Impact and Remote Sensing; Universität Bern Vortrag im Rahmen einer Tagung Double diffusion and energy from methane - the fascinating interior of the unusual Lake Kivu 24.04.2013 Universität Bern, Schweiz Wüest Alfred; Sommer Tobias;
IBP Congress Vortrag im Rahmen einer Tagung Climbing the Lake Kivu staircase 13.04.2013 Zürich, Schweiz Sommer Tobias; Wüest Alfred;
Bodenseephysikertreffen Vortrag im Rahmen einer Tagung Double Diffusion: A paradox phenomenon in a stratified water column 24.01.2013 Langenargen, Deutschland Sommer Tobias; Wüest Alfred;
Eawag internal seminar Einzelvortrag Sensor responses and their effect on heat and salt fluxes in Lake Kivu 05.11.2012 Kastanienbaum, Schweiz Sommer Tobias;
Institute Seminar Einzelvortrag Double diffusion and methane extraction in Lake Kivu (East Africa) 20.09.2012 Université de Genève, Versoix, Schweiz Wüest Alfred; Sommer Tobias;
Seminar Series Einzelvortrag Double diffusion and methane extraction in Lake Kivu (East Africa) 03.09.2012 Notre Dame, Indiana, Vereinigte Staaten von Amerika Wüest Alfred; Sommer Tobias;
Workshop on double-diffusive systems, Santa Cruz Vortrag im Rahmen einer Tagung Resolving salinity and temperature interfaces of double-diffusive staircases in Lake Kivu 27.08.2012 Santa Cruz, Vereinigte Staaten von Amerika Sommer Tobias; Wüest Alfred;
Workshop on Double-Diffusive Systems, Santa Cruz Vortrag im Rahmen einer Tagung Simulations of diffusive convection with application to Lake Kivu 27.08.2012 Santa Cruz, CA, USA, Vereinigte Staaten von Amerika Wüest Alfred; Sommer Tobias;
Workshop on double-diffusive system, Santa Cruz Vortrag im Rahmen einer Tagung Comparison of two diffusive-type thermohaline staircases with diverging residence time scales. 27.08.2012 Santa Cruz, Vereinigte Staaten von Amerika Wüest Alfred; Sommer Tobias;


Veranstaltungen zum Wissenstransfer

Aktiver Beitrag

Titel Art des Beitrags Titel des Artikels oder Beitrages Datum Ort Beteiligte Personen
Review LKMP Workshop 05.05.2015 Kigali, Rwanda, Ruanda
Seminar, Lake Kivu Expedition: 23.01.2015- 11.02.2015, Vortrag 06.02.2015 Kigali, Rwanda, Schweiz


Auszeichnungen

Titel Jahr
ETH Medaille Sommer T (2013). Double diffusion in Lake Kivu. PhD thesis ETH nr 21’463, ETH Zürich medallion for exceptional thesis. 2014

Verbundene Projekte

Nummer Titel Start Förderungsinstrument
120128 Turbulence and Fluxes in Stratified Natural Waters 01.04.2008 Projektförderung (Abt. I-III)
123923 Lake Kivu: Learning from the past for managing its future 01.06.2009 Resource not found: '20438a61-de74-41e3-9fb1-8132973c4a1f'
122183 Lake Kivu - turbulence and double diffusion in permanent stratification 01.03.2009 Projektförderung (Abt. I-III)
160726 A Flexible Underwater Distributed Robotic System for High-Resolution Sensing of Aquatic Ecosystems 01.08.2015 Sinergia
157779 Léman exploration (LÉXPLORE) 01.05.2015 R'EQUIP
160114 Managing Lake Kivu: moving from a steady-state to a dynamic modelling approach 01.12.2015 Projektförderung (Abt. I-III)

Abstract

A two year prolongation (“Verlängerung”) on the project “Lake Kivu - turbulence and double diffusion in permanent stratification” (NF-Grant 200021 - 122183) is proposed for the period of 1 March 2012 to 28 February 2014. In the first year the focus is on finishing the PhD thesis of Tobias Sommer and for the second year, we seek for post-doc salary and finances to (i) complete the data base in Lake Kivu and (ii) exploit the findings in an integrative and comparative analysis. The project aims at explaining the build-up and maintenance of the unusual density strati¬fication, and its role in the unique accumulation of methane (CH4) in Lake Kivu. The water column is density stratified due to subaquatic inputs of salty and carbondioxide-rich (CO2) water. This strati¬fication is unique insofar, as the lake water contains four relevant components: Salt and CO2 increa¬se stratification while temperature and CH4 reduce stratification. This competition leads to a so-called double-diffusive layering regime, which causes an unprecedented staircase of more than 250 well-mixed layers, separated by thin but extremely stable interfaces. To understand how such a stable structuring can establish and evolve is the primary goal of this project.As a result of the permanent stratification, the continuous supply of CO2 and decomposition of organic matter (plankton), led to the accumulation of astounding volumes of CH4 (~60 km3) and CO2 (~300 km3). The lake is therefore (i) of great economic interest (>20 billion $ in value for CH4) and (ii) a potential risk for a catastrophic out-gassing. Hence, the two riparian governments decided to use the CH4 for power production and thereby reduce the risk. It is of great interest to quantify the CH4 production and related vertical fluxes of bio-geochemical constituents (such as nut¬rients). The secondary goal of this proposal is the integration of our field observations on turbulent and double-diffusive mixing to the parallel SNF/SDC project “Lake Kivu: Learning from the past for managing its future” (Grant IZ70Z0_123923) in order to constrain the carbon/CH4cycling in Lake Kivu and contribute to a sustainable harvesting plan. So far we have acquired an unprecedented data set of more than 55 km of sub-mm scale Temperature-Salinity profiles, containing more than 10,000 layers (~250 per full profile) and interfaces, by using the micro¬structure profiler VMP-500 and by collecting several months of thermistor data in various layers at 5 seconds reso¬lution. This prolongation will build on this data and its preliminary analysis. We define five objectives for continuing the current project to be completed after two years: (1) Estimate the baroclinic energy content (internal waves) in the permanent stratification(2) Quantify rates of merging of layers and interface spitting from thermistor records (3) Linking turbulence, heat fluxes and temporal evolution within zones of the staircase(4) Complete the microstructure data set on the Congolese part of Lake Kivu(5) Complete and publish the following manuscripts in peer-reviewed journals: - Comparison between Direct Numerical Simulations and in-situ observations - Homogeneity and coherency of the double-diffusive staircase - Baroclinic energy and staircase dynamics - Comparative analysis between Lake Kivu and Lake Nyos - Synthesis: relation between overall vertical structure, lateral gradients and staircase dynamics. The project will make a significant contribution to research into double-diffusive regimes on the scales of real natural systems. In addition, the results of this project will be made available for the benefit of optimized planning of the CH4 extraction.
-