Geyer, Tobias

[["dc.bibliographiccitation.artnumber","W11503"],["dc.bibliographiccitation.journal","Water Resources Research"],["dc.bibliographiccitation.volume","47"],["dc.contributor.author","Reimann, Thomas"],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Shoemaker, W. Barclay"],["dc.contributor.author","Liedl, Rudolf"],["dc.contributor.author","Sauter, Martin"],["dc.date.accessioned","2018-11-07T08:49:58Z"],["dc.date.available","2018-11-07T08:49:58Z"],["dc.date.issued","2011"],["dc.description.abstract","Well-developed karst aquifers consist of highly conductive conduits and a relatively low permeability fractured and/or porous rock matrix and therefore behave as a dual-hydraulic system. Groundwater flow within highly permeable strata is rapid and transient and depends on local flow conditions, i.e., pressurized or nonpressurized flow. The characterization of karst aquifers is a necessary and challenging task because information about hydraulic and spatial conduit properties is poorly defined or unknown. To investigate karst aquifers, hydraulic stresses such as large recharge events can be simulated with hybrid (coupled discrete continuum) models. Since existing hybrid models are simplifications of the system dynamics, a new karst model (ModBraC) is presented that accounts for unsteady and nonuniform discrete flow in variably saturated conduits employing the Saint-Venant equations. Model performance tests indicate that ModBraC is able to simulate (1) unsteady and nonuniform flow in variably filled conduits, (2) draining and refilling of conduits with stable transition between free-surface and pressurized flow and correct storage representation, (3) water exchange between matrix and variably filled conduits, and (4) discharge routing through branched and intermeshed conduit networks. Subsequently, ModBraC is applied to an idealized catchment to investigate the significance of free-surface flow representation. A parameter study is conducted with two different initial conditions: (1) pressurized flow and (2) free-surface flow. If free-surface flow prevails, the systems is characterized by (1) a time lag for signal transmission, (2) a typical spring discharge pattern representing the transition from pressurized to free-surface flow, and (3) a reduced conduit-matrix interaction during free-surface flow."],["dc.description.sponsorship","Deutsche Forschungsgemeinschaft (DFG) [LI 727/11-1, SA 501/24-1]"],["dc.identifier.doi","10.1029/2011WR010446"],["dc.identifier.isi","000296622300002"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/21580"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Amer Geophysical Union"],["dc.relation.issn","0043-1397"],["dc.title","Effects of dynamically variable saturation and matrix-conduit coupling of flow in karst aquifers"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","W09503"],["dc.bibliographiccitation.journal","Water Resources Research"],["dc.bibliographiccitation.volume","47"],["dc.contributor.author","Reimann, Thomas"],["dc.contributor.author","Rehrl, Christoph"],["dc.contributor.author","Shoemaker, W. Barclay"],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Birk, Steffen"],["dc.date.accessioned","2018-11-07T08:51:50Z"],["dc.date.available","2018-11-07T08:51:50Z"],["dc.date.issued","2011"],["dc.description.abstract","Karst aquifers exhibit highly conductive features caused from rock dissolution processes. Flow within these structures can become turbulent and therefore can be expressed by nonlinear gradient functions. One way to account for these effects is by coupling a continuum model with a conduit network. Alternatively, turbulent flow can be considered by adapting the hydraulic conductivity within the continuum model. Consequently, the significance of turbulent flow on the dynamic behavior of karst springs is investigated by an enhanced single-continuum model that results in conduit-type flow in continuum cells (CTFC). The single-continuum approach CTFC represents laminar and turbulent flow as well as more complex hybrid models that require additional programming and numerical efforts. A parameter study is conducted to investigate the effects of turbulent flow on the response of karst springs to recharge events using the new CTFC approach, existing hybrid models, and MODFLOW-2005. Results reflect the importance of representing (1) turbulent flow in karst conduits and (2) the exchange between conduits and continuum cells. More specifically, laminar models overestimate maximum spring discharge and underestimate hydraulic gradients within the conduit. It follows that aquifer properties inferred from spring hydrographs are potentially impaired by ignoring flow effects due to turbulence. The exchange factor used for hybrid models is necessary to account for the scale dependency between hydraulic properties of the matrix continuum and conduits. This functionality, which is not included in CTFC, can be mimicked by appropriate use of the Horizontal Flow Barrier package for MODFLOW."],["dc.identifier.doi","10.1029/2010WR010133"],["dc.identifier.isi","000294619100002"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/22030"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Amer Geophysical Union"],["dc.relation.issn","0043-1397"],["dc.title","The significance of turbulent flow representation in single-continuum models"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","227"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","HYDROLOGY AND EARTH SYSTEM SCIENCES"],["dc.bibliographiccitation.lastpage","241"],["dc.bibliographiccitation.volume","18"],["dc.contributor.author","Reimann, Thomas"],["dc.contributor.author","Giese, Marcus"],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Liedl, Rudolf"],["dc.contributor.author","Marechal, J. C."],["dc.contributor.author","Shoemaker, W. Barclay"],["dc.date.accessioned","2018-11-07T09:46:33Z"],["dc.date.available","2018-11-07T09:46:33Z"],["dc.date.issued","2014"],["dc.description.abstract","Karst aquifers are characterized by highly conductive conduit flow paths embedded in a less conductive fissured and fractured matrix, resulting in strong permeability contrasts with structured heterogeneity and anisotropy. Groundwater storage occurs predominantly in the fissured matrix. Hence, most mathematical karst models assume quasi-steady-state flow in conduits neglecting conduit-associated drainable storage (CADS). The concept of CADS considers storage volumes, where karst water is not part of the active flow system but hydraulically connected to conduits (for example karstic voids and large fractures). The disregard of conduit storage can be inappropriate when direct water abstraction from karst conduits occurs, e. g., large-scale pumping. In such cases, CADS may be relevant. Furthermore, the typical fixed-head boundary condition at the karst outlet can be inadequate for water abstraction scenarios because unhampered water inflow is possible. The objective of this work is to analyze the significance of CADS and flow-limited boundary conditions on the hydraulic behavior of karst aquifers in water abstraction scenarios. To this end, the numerical discrete-continuum model MODFLOW-2005 Conduit Flow Process Mode 1 (CFPM1) is enhanced to account for CADS. Additionally, a fixed-head limited-flow (FHLQ) boundary condition is added that limits inflow from constant head boundaries to a user-defined threshold. The effects and the proper functioning of these modifications are demonstrated by simplified model studies. Both enhancements, CADS and FHLQ boundary, are shown to be useful for water abstraction scenarios within karst aquifers. An idealized representation of a large-scale pumping test in a karst conduit is used to demonstrate that the enhanced CFPM1 is able to adequately represent water abstraction processes in both the conduits and the matrix of real karst systems, as illustrated by its application to the Cent Fonts karst system."],["dc.identifier.doi","10.5194/hess-18-227-2014"],["dc.identifier.isi","000331123300014"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/11690"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/34895"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Copernicus Gesellschaft Mbh"],["dc.relation.issn","1607-7938"],["dc.relation.issn","1027-5606"],["dc.rights","CC BY 3.0"],["dc.rights.uri","https://creativecommons.org/licenses/by/3.0/"],["dc.title","Representation of water abstraction from a karst conduit with numerical discrete-continuum models"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","3909"],["dc.bibliographiccitation.issue","10"],["dc.bibliographiccitation.journal","HYDROLOGY AND EARTH SYSTEM SCIENCES"],["dc.bibliographiccitation.lastpage","3923"],["dc.bibliographiccitation.volume","16"],["dc.contributor.author","Kordilla, Jannes"],["dc.contributor.author","Sauter, M."],["dc.contributor.author","Reimann, Thomas"],["dc.contributor.author","Geyer, Tobias"],["dc.date.accessioned","2018-11-07T09:14:50Z"],["dc.date.available","2018-11-07T09:14:50Z"],["dc.date.issued","2012"],["dc.description.abstract","The objective of this work is the simulation of saturated and unsaturated flow in a karstified aquifer using a double continuum approach. The HydroGeoSphere code (Therrien et al., 2006) is employed to simulate spring discharge with the Richards equations and van Genuchten parameters to represent flow in the (1) fractured matrix and (2) conduit continuum coupled by a linear exchange term. Rapid vertical small-scale flow processes in the unsaturated conduit continuum are accounted for by applying recharge boundary conditions at the bottom of the saturated model domain. An extensive sensitivity analysis is performed on single parameters as well as parameter combinations. The transient hydraulic response of the karst spring is strongly controlled by the matrix porosity as well as the van Genuchten parameters of the unsaturated matrix, which determine the head dependent inter-continuum water transfer when the conduits are draining the matrix. Sensitivities of parameter combinations partially reveal a non-linear dependence over the parameter space. This can be observed for parameters not belonging to the same continuum as well as combinations, which involve the exchange parameter, showing that results of the double continuum model may depict a certain degree of ambiguity. The application of van Genuchten parameters for simulation of unsaturated flow in karst systems is critically discussed."],["dc.identifier.doi","10.5194/hess-16-3909-2012"],["dc.identifier.isi","000310474300028"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9516"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/27516"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Copernicus Gesellschaft Mbh"],["dc.relation.issn","1607-7938"],["dc.relation.issn","1027-5606"],["dc.rights","CC BY-NC 3.0"],["dc.rights.uri","https://creativecommons.org/licenses/by-nc/3.0"],["dc.title","Simulation of saturated and unsaturated flow in karst systems at catchment scale using a double continuum approach"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","41"],["dc.bibliographiccitation.issue","1-2"],["dc.bibliographiccitation.journal","Carbonates and Evaporites"],["dc.bibliographiccitation.lastpage","46"],["dc.bibliographiccitation.volume","28"],["dc.contributor.author","Geyer, Tobias"],["dc.contributor.author","Birk, Steffen"],["dc.contributor.author","Reimann, Thomas"],["dc.contributor.author","Doerfliger, Nathalie"],["dc.contributor.author","Sauter, Martin"],["dc.date.accessioned","2018-11-07T09:25:13Z"],["dc.date.available","2018-11-07T09:25:13Z"],["dc.date.issued","2013"],["dc.description.abstract","Because of the small radius of investigation of hydrogeological standard testing methods, the characterization of karst aquifers is still a challenge. The development of a karst conduit system introduces an element of large contrast in hydraulic conductivity in the hydraulic parameter field of a karst aquifer. It leads to complex flow patterns and transport phenomena that differ significantly from those observed in porous and fissured media. While on a local, i.e., borehole scale, the fissured matrix of karst aquifers can be regarded as a continuum, on a regional, i.e., catchment scale, the drainage of the aquifer system is controlled by the conduit system, which may have a highly anisotropic geometry. Therefore, characterization of karst aquifers requires a differentiated approach by the combination of various hydrogeological field methods or the application of large-scale tests, which cover the scale of dominant aquifer heterogeneities. Existing numerical modeling approaches can be applied for integral data interpretation on catchment scale."],["dc.identifier.doi","10.1007/s13146-013-0150-9"],["dc.identifier.isi","000317969600006"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/30014"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Springer"],["dc.relation.issn","0891-2556"],["dc.title","Differentiated characterization of karst aquifers: some contributions"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dspace.entity.type","Publication"]]