3.6 Subsurface Piracy and Karst Drainage Basins

The conduit networks of many karst aquifers are characterized by a hierarchical tributary pattern of underground drainage. In simple terms, these conduit networks develop and continuously evolve by a process of subsurface piracy. The growth and propagation of conduits occurs by way of a complex flow-and-dissolution feedback mechanism. The larger initial conduit flow paths, having greater hydraulic capacity, develop preferentially and enlarge most rapidly. The largest conduits act as master drains that create localized zones of greater discharge and lower hydraulic head in the aquifer. Consequently, this alters the hydraulic flow field (changes the hydraulic heads) so as to increasingly capture ground water from the surrounding aquifer matrix, fractures, and smaller nearby conduits (Palmer, 1991, 1999; White and White, 1989). As the process continues over geological time with hydraulic gradients continually changing, discrete conduit flow paths link together. The larger, more efficient conduits capture flow from nearby incipient and developed small conduits and enhanced fractures, forming the characteristic branching drainage networks that converge in the downgradient direction, diverging and propagating in the headward (upgradient) direction (Figure 28).

Diagrams showing hypothetical stages in conduit network development

Figure 28  Diagrams showing plan view (top panel) and cross-sectional view (bottom panel) of hypothetical stages in conduit network development by the alteration of hydraulic gradients and subsurface conduit piracy. Open light blue circles indicate sinkholes where water is recharged. Lines extending from them represent the beginning formation of conduits along fractures. Filled light blue circles are springs where water is discharged. Wavy dark blue lines are surface streams. Heavy purple lines are conduits. For diffuse flow, the thin black lines are equipotential contours of a typical potentiometric map and the thick black line is the groundwater trough. a) Early-stage development with one master conduit connecting drainage from a surface stream into a sinkhole to the discharge from a karst spring with multiple small inputs of concentrated recharge from other sinkholes and incipient conduits. Flow is predominantly diffuse with equipotential and flow lines indicating the diffuse flow field. b) Growth of master conduit and linking of smaller nearby conduits results in a mix of diffuse and conduit flow that alters the equipotential field such that potentiometric trough shifts. c) Late-stage linking of all conduits into a subsurface tributary drainage network such that concentrated-conduit (alternatively: conduit-controlled) flow dominates the aquifer. Diffuse flow is negligible and occurs as local leakage from the rock matrix into conduits with only intermittent overland flow during storm events. Modified from Mull and others (1988a).

Palmer (1999) notes that in many karst-aquifer systems, flow convergence is present even in the pre-conduit openings because of differences in hydraulic efficiency created by zones of enhanced intergranular porosity and fractures, and that the branching pattern that develops as conduits grow is largely inherited. The dynamics of this process have been simulated repeatedly using both physical and numerical models, and recent advances in numerical models have been able to effectively simulate the development and linking of conduits to form complex drainage networks that are representative of karst aquifers as delineated by field data, and to forecast their continued evolution in response to projected changes of hydrologic conditions (Perne et al., 2014; de Rooij and Graham, 2017).

In many karst areas, conduit development short-circuits surface stream drainage by providing alternative subsurface flow paths (White, 1999). Conduit piracy of surface stream flows often initiates the formation of sinking or disappearing streams, whose flows abruptly end via subsurface diversion into a streambed swallow hole, terminal cave, sinkhole, losing stream reach, or “dry” stream channels (Brahana and Hollyday, 1988; Ray, 2012). In “dry” stream channels, surface flows occur intermittently during seasonally-high water-table conditions or when storm events overwhelm the drainage capacities of streambed swallow holes and underlying conduits. This hydrologic mechanism is sometimes responsible for localized, often damaging, recurrent flooding in karstic watersheds (Bayless et al., 2014).

From field observations in the Mammoth Cave area, Kentucky, USA, where conduit drainage originated by leakage or base-flow piracy beneath stream channels, Ray (1999) hypothesized that the hydrologic characteristics of stream valleys in karstic watersheds change along an evolutionary sequence as erosion progressively exposes more limestone bedrocks to weathering and karstification, and as conduit piracy of surface drainage progressively increases. He described three significant phases of development by classifying watersheds as either: overflow allogenic (type 1), underflow allogenic (type 2), or local autogenic (type 3) as shown in Figure 29. In the overflow allogenic basin, the drainage capacity of subsurface conduits is initially too limited to capture and discharge more than low-to-base flow runoff from the entire watershed. Storm flows and higher runoff are discharged from the watershed through streambeds of downstream surface reaches that are usually dry or losing streams. These intermittent higher surface flows continue to erode and maintain the downstream surface channels. As the conduit networks evolve over time and increase their drainage capacity and piracy, more of the surface flow is diverted underground, upstream surface tributaries disconnect from downstream reaches, becoming sinking streams, and all watershed runoff, including storm runoff, is discharged through conduits and karst springs. These characteristics define an underflow allogenic basin. Eventually, as all or the majority of surface runoff is diverted underground, sinkhole catchments increase in number and grow in size, surface stream channels and intervening topographic divides erode, and internal drainage through conduits and karst springs dominate the hydrology of the watershed. These are the characteristics that define a local autogenic basin.

Figures showing a conceptual model of the evolution of some karstic watersheds

Figure 29  A conceptual model of the evolution of some karstic watersheds, resulting from progressive piracy of surface drainage by subsurface conduits. a) Overflow allogenic (type 1) where nearly all outflow from the basin occurs via surface stream channels. The karst window is an opening in the stream where some of the surface-water flow may move underground. If the karst window is not large there is an upper limit to the amount of flow diverted underground, but here the arrow indicates all surface flow goes underground. The estavelle is an open ground orifice that can either be a sinking stream (like a karst window), but under conditions when the groundwater table rises high enough, flow will reverse (so flow goes either direction). The main difference between the losing stream and sinking stream is that all the flow is not lost in a losing stream and the sinks aren’t specifically known, whereas at a sinking stream usually the sink is a known karst window and all flow moves underground into the conduit network. b) Underflow allogenic (type 2) where conduit networks have evolved to the extent that nearly all outflow is from conduits and karst springs in lower portions of the basin. Note the now three karst windows that divert surface-water flow underground towards the springs. c) Local autogenic (type 3) where outflow is from conduits and karst springs at the lowest part of the basin. The surface is devoid of perennial streams and most of the rainfall moves underground through surface depression sinks. Modified from Ray (2001).

Within a karst aquifer or aquifer system, multiple discrete karst groundwater basins are often present. Each basin receives recharge from a specific area of land surface through infiltration and point sources such as sinkholes and sinking streams and drains to a specific spring or group of springs, by way of an integrated network of subsurface conduits (White, 1993). Identification and delineation of karst basins is challenging because their characteristics differ considerably from conventional conceptual models of porous media aquifers and cannot be determined using only conventional methods that rely on water-level data from wells and topographic mapping to identify recharge and discharge boundaries and groundwater flow directions (Groves, 2007; Taylor and Doctor, 2017). Moreover, the term “groundwater basin” is somewhat of a misnomer in that it misrepresents both the highly interconnected nature of surface and groundwater in most karst aquifers and the role of concentrated stormwater runoff as a significant, often dominant, source of recharge. The boundary of a karst basin includes surface catchments for all contributing sources of allogenic recharge and sinkholes (sinkholes usually provide autogenic, but may provide allogenic, recharge).

Karst basins differ from conventional groundwater basins as defined by Toth (1963) in major ways. Karst basin boundaries may not coincide with topographic drainage divides that define the hydrologic boundaries of surface water drainages. Groundwater recharge at karst basin divides may flow in multiple directions, following conduits in a radial or semiradial pattern and flow into one or more adjacent basins. Moreover, the position of recharge areas and basin divides may shift under different hydrologic conditions, for example under flood or storm conditions groundwater levels rise and flow follows higher level conduits. Subsurface conduits and karst basin boundaries may extend well beyond boundaries indicated by topographic drainage divides (Figure 30), giving rise to so-called “misbehaved” drainage patterns in karstic watersheds (Ray, 2001). Often, discharge from a karst basin does not occur over a widely distributed seepage zone along a surface stream channel but is concentrated at local points, for example, at one or more springs that form major headwaters that are perennial tributaries of nearby surface streams. Finally, because water movement is largely concentrated within discrete conduit-controlled flow routes or preferential flow layers, groundwater flow directions and discharge locations at springs do not always conform with those anticipated or predicted from hydraulic gradients inferred by water-level measurements in wells. The concept of a karst water table has been debated and considered problematic, in part because of the extreme heterogeneities, discontinuities of flow and hydraulic head fields in the aquifer (White, 1993; Ewers, 2006; Taylor and Doctor, 2017).

Map showing “misbehaved” drainage patterns

Figure 30  Section of a map showing examples of so-called “misbehaved” drainage patterns in karstic watersheds where surface water divides and groundwater divides do not coincide. These are indicated where karst basin boundaries (green polygons) as determined by dye tracer tests (red lines link from injection to detection of dye) extend across topographic basin boundaries (blue polygons). Blue dots represent karst springs, red triangles represent sinkholes, swallets, or wells used for dye-tracer injection sites. Modified from Currens and others (2002).


Introduction to Karst Aquifers Copyright © 2022 by Eve L. Kuniansky, Charles J. Taylor, and Frederick Paillet. All Rights Reserved.