The Effects of Exchange Flow on the Karst Spring Hydrograph under the Different Flow Regimes: A Synthetic Modeling Approach

Malihe Shirafkan,Zargham Mohammadi,David Labat,Vianney Sivelle

doi:10.3390/w13091189

Abstract

In this study, a synthetic modeling approach is proposed to quantify the effect of the amount and direction of the exchange flow on the karstic spring discharge fluctuations under different hydrologic conditions corresponding to high and low flow conditions. We hypothesis that the spring discharge fluctuations constitute a valuable proxy to understand the internal processes of the karst system. An ensemble of spring hydrographs was synthetically produced to highlight the effect of exchange flow by exploring the plausible range of variability of coefficients of exchange flow, conduit diameter, and matrix hydraulic conductivity. Moreover, the change of the rate of point recharge through the karst conduit allows for the quantifying of the sensibility of the spring hydrograph to the directions of exchange flow. We show that increasing the point recharge lies to a remarkable linear recession coefficient (β) as an indication of the conduit flow regime. However, a reduction in and/or lack of the point recharge caused the recession coefficient to change to exponential (α) due to the dominant effect of the matrix restrained flow regime and/or conduit-influenced flow regime. The simulations highlight that the exchange flow process from the conduit to the matrix occurred in a short period and over a restricted part of the conduit flow regime (CFR). Conversely, the exchange flow dumped from the matrix to the conduit occurs as a long-term process. A conceptual model is introduced to compare spring hydrographs’ characteristics (i.e., the peak discharge, the volume of baseflow, and the slope of the recession curve) under the various flow conditions with the directions of the exchange flow between the conduit and the matrix.

Highlights

Karst aquifers can be considered as a “dual-porosity” or “dual-permeability” media (Figure 1) that include (1) matrix porosity comprising intergranular pores and the small joints with high storage capacity and low velocity, and (2) conduit porosity comprising enlarged, dissolved, and unclogged fractures with low storage capacity and high flow velocity [1]
These conditions imply an inherent duality in all processes governing the hydrological response of the karst system, including (1) the infiltration and recharge processes which range from diffusive and slow recharge into the matrix to rapid and concentrated recharge into the conduit through features such as sinkholes, sinking streams, and ponor caves, (2) the flow regime changes between the two endpoints with slow velocity in the matrix and fast flow velocity in the karst conduits, and (3) mechanism of discharge that ranges from low and continuous discharge during dry periods to high and rapid discharge with high temporal variability during the rainfall events [3]
Considering scenario A1, the flow regime is mainly controlled by the conduit-influenced flow regime (CIFR) and/or the matrix restrained flow regime (MRFR), as well as by the direction of the exchange flow being from the matrix to the conduit (QMC+ discharge)

Summary

Introduction

Karst aquifers can be considered as a “dual-porosity” or “dual-permeability” media (Figure 1) that include (1) matrix porosity comprising intergranular pores and the small joints with high storage capacity and low velocity, and (2) conduit porosity comprising enlarged, dissolved, and unclogged fractures with low storage capacity and high flow velocity [1]. Karst aquifers are characterized by a strong physical heterogeneity and may present strong anisotropy in terms of hydrodynamic properties such as hydraulic conductivity and specific storage due to a complex and three-dimensional dissolution development [2] These conditions imply an inherent duality in all processes governing the hydrological response of the karst system, including (1) the infiltration and recharge processes which range from diffusive and slow recharge into the matrix to rapid and concentrated recharge into the conduit through features such as sinkholes, sinking streams, and ponor caves,. The matrix may be alternatively recharged or drained by conduits depending on the occurrence of recharge events and the respective flow dynamics in the conduit and matrix This process is known as the hydraulic gradient inversion [5]. Due to the inherent heterogeneity and anisotropy of karst aquifers, the exchange flow is often variable in both temporal and spatial spaces [6]

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