An evaluation of semidistributed-pipe-network and distributed-finite-difference models to simulate karst systems

Abstract

Several different approaches have been developed to model the specific characteristics of karst aquifers, taking account of their inherent complex spatial and temporal heterogeneities. This paper sets out the development of a semidistributed modelling approach for applications in an Irish karst context using urban drainage software. The models have proven to be very useful for different studies, with examples given for the ecohydrology of ephemeral karst lakes, extreme groundwater-flood alleviation, karst network investigation, submarine groundwater discharge, and quantification of different recharge and flow components. The limitations of the approach are also highlighted, in particular not being able to simulate diffuse infiltration and flow paths explicitly across the groundwater catchment. Hence, a more distributed, finite-difference modelling approach using MODFLOW Unstructured Grid (USG) with the newly developed Connected Linear Network (CLN) process is then compared against the semidistributed approach on the same karst catchment. Whilst it has proven difficult to achieve the same levels of model performance in simulating the spring flows in the distributed model compared to the semidistributed model, the ability to interrogate the flow paths at any point on the three-dimensional aquifer is demonstrated, which can give new insights into flows (and potential contaminant transport) through such complex systems. The influence of the proximity of highly transmissive conduits on the flow dynamics through the much-lower transmissive matrix cells in which the network is embedded has been particularly investigated.

Quantitative analysis of Cenozoic faults and fractures and their impact on groundwater flow in the bedrock aquifers of Ireland

“…Continuum-based approaches include the use of Darcy's Law for groundwater flow modelling and estimation of hydraulic conductivity and transmissivity, and in some instances incorporate the effects of pipes and turbulent flow e.g. the MODFLOW Conduit Flow Process (Freeze and Cherry 1979;NRC 1996;Neuman 2005;Quinn et al 2012;Gill et al 2013;McCormack et al 2017;Medici et al 2019b;Quinn et al 2020); Schuler et al 2020b;Gill et al 2020). The stratabound nature of joint systems suggests that their hydraulic properties can, in principle, be estimated using REV or continuum methods (NRC 1996;Zimmerman and Bodvarrson 1996;Odling et al 1999) and, in that sense, these fracture systems are more predictable than Cenozoic faults (e.g.…”

“…Depth-related joint structure and karstification effects can, in principle, be parameterised and upscaled for incorporation into hydrogeological models as near-surface zones of diffuse flow (Gill et al 2013;McCormack et al 2017;Schuler et al 2020b;Gill et al 2020); however, the impact of older structures on jointing and karstification is more difficult to deal with. Joints immediately adjacent to, and controlled by stress release of, older structures (such as normal faults, Variscan veins and Cenozoic faults) can penetrate to well beyond 10 m depth and to some extent inherit the scale-independent geometries and spatial distributions of preexisting structures.…”

“…In this area, the prevalence of karst has meant that hydrological modelling has involved the use of water levels in karst lakes (Turloughs) for the calibration of flows in groundwater catchments of discrete conduit (i.e. pipe) networks at depth and diffuse 'epikarstic' flow at shallower levels (McCormack et al 2017;Gill et al 2013;Schuler et al 2020b;Gill et al 2020). This type of approach is similar to fracture and pipe flow models used elsewhere where longer and wider conduits are modelled using laminar to turbulent flow equations (Hill et al 2010;Medici et al 2019b;Gill et al 2020) and a contributing fracture system or 'epikarstic' flow is represented as a continuum using Darcy's Law (laminar flow; McCormack et al 2017;Gill et al 2013;Medici et al 2019b;Schuler et al 2020b;Gill et al 2020).…”

“…pipe) networks at depth and diffuse 'epikarstic' flow at shallower levels (McCormack et al 2017;Gill et al 2013;Schuler et al 2020b;Gill et al 2020). This type of approach is similar to fracture and pipe flow models used elsewhere where longer and wider conduits are modelled using laminar to turbulent flow equations (Hill et al 2010;Medici et al 2019b;Gill et al 2020) and a contributing fracture system or 'epikarstic' flow is represented as a continuum using Darcy's Law (laminar flow; McCormack et al 2017;Gill et al 2013;Medici et al 2019b;Schuler et al 2020b;Gill et al 2020). Whichever methods are employed, however, the parameterisation and modelling of complex fracture systems will always benefit from local geological constraints, maps and conditioning, combined with more detailed hydraulic monitoring and analysis similar to the flow response type approach employed for Galmoy.…”

“…By contrast, hydrogeological applications within fractured aquifers do not often have recourse to seismic imaging, aquifer stimulation and multiple/horizontal wells and are, to a greater extent, encumbered by the notion of unique or measurable transmissivities for given volumes of rock. The application of continuum through to discrete fracture flow modelling and simulation should provide an improved predictive capability, for directly estimating flow volumes and velocities for assessing resources and contaminant transport potential (Quinn et al 2012;Maldaner et al 2018;Parker et al 2019;Medici et al 2019b;Gill et al 2020;Quinn et al 2020). The analysis performed here, however, reinforces the requirement for multiple scenario modelling, with much improved structural and hydrogeological parameterisation from future studies, particularly given the sensitivity of groundwater flow and connectivity to fault/fracture density, geometry, aperture and related karst.…”