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","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"]]