Effects of dynamically variable saturation and matrix‐conduit coupling of flow in karst aquifers

Abstract

[1] 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 dualhydraulic 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 conduitmatrix interaction during free-surface flow.

Process-based modelling to evaluate simulated groundwater levels and frequencies in a Chalk catchment in south-western England

“…Distributed karst modelling approaches are able to simulate groundwater levels on a spatial grid but their data requirements mostly limit them to theoretical studies (e.g. Birk et al, 2006;Reimann et al, 2011) or well explored study sites (e.g. Hill et al, 2010;Jackson et al, 2011;Oehlmann et al, 2015).…”

Representation of water abstraction from a karst conduit with numerical discrete-continuum models

“…For this reason, transient water storage can be considered by the continuum model only. Following this, Reimann et al (2011) presented a discrete-continuum modeling approach to simulate unsteady discrete flow in a variably filled pipe network employing the Saint-Venant equations, while de Rooij (2008) and de Rooij et al (2013) additionally considered surface flow and variably saturated matrix flow. The approach appears to be suitable for fundamental studies but high parameter demand and computational effort can be identified as potential drawbacks.…”

Simulation of saturated and unsaturated flow in karst systems at catchment scale using a double continuum approach

“…Flow through localized karst conduits might be highly transient, i.e. the application of open channel flow approaches would be appropriate (Reimann et al, 2011a). However, these approaches increase the number of calibration parameters and require additional knowledge about the geometry of a karst conduit system.…”

“…Accordingly, the final spring discharge is then a function of the individual flow contributions of each of these compartments (Smart and Hobbs, 1986), which makes the inverse analysis of spring discharge a major challenge, requiring elaborate modeling tools and a large spectrum of data to constrain the model. The simulation of coupled saturated and unsaturated flow is still a challenge in hydrogeology in particular in fractured (Therrien and Sudicky, 1996) and karstified systems (Reimann et al, 2011a). This is predominantly a result of the data scarcity respecting the hydraulic parameter field of real karst systems.…”