Owen NAUGHTON, Paul JOHNSTON, Laurence GILL
Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Dublin 2, Ireland, e-mail: [email protected]
Abstract: Turloughs are one of the characteristic features of the Irish karst landscape. They are transient lakes resulting from a combination of high rainfall and accordingly high groundwater levels in topographic depressions in the karst. A turlough is effectively a hydrogeological feature defined as “A topographic depression in karst which is intermittently inundated on an annual basis, mainly from groundwater, and which has a substrate and/or ecological communities characteristic of wetlands” (Tynan et al. 2005). The hydrological regime in a turlough results in a characteristic ecology associated with the pattern of groundwater inundation. The behaviour of a turlough as a wetland is fundamentally driven by its hydrology. Discharges through turloughs are typically difficult to assess because of the often uncertain location and nature of the inflow and outflow points. The only realistic hydrological measures are based on water level. This study quantifies the hydrological regime of a set of turloughs and suggests a conceptual model to explain turlough operation. Semi-permanent water level monitoring stations were established in twenty-two selected turloughs using pressure transducers at or near the lowest point of each turlough. Time series datasets of water levels were then developed using hourly intervals over a 30 month period from November 2006 to May 2009. Stage–Volume relations were derived using digital terrain models generated from detailed topographic surveys. These were then combined with water level data to give volume time series. A range of turlough response and recession characteristics were observed with some having multiple flood events in the course of a year whereas others show a single event with a slow recession. Maximum flow capacities were derived through analysis of the volume recessions. The observed hydrological behaviour has been used to develop a conceptual hydrological model for the functioning of turloughs. Together with ecological and land-use data, this model will aid in the evaluation of turlough conservation status as groundwater dependent terrestrial ecosystems as defined in the Water Framework Directive (2000/60/EC). For the first time the role turloughs occupy within a karst groundwater system have been defined; risks posed to these protected ecosystems may now be evaluated and quantified. Whether through abstraction/drainage or through hydrochemical pressures on trophic status, these potential risks are assessed in terms of appropriate hydrological indicators relevant to the characteristic ecology of a turlough.
Keywords: turloughs, hydrology
1 Introduction 1.1 Definition
Turloughs are one of the characteristic features of the Irish karst landscape. They are transient lakes resulting from a combination of high rainfall and accordingly high groundwater levels in topographic depressions in the karst. A turlough is effectively a hydrogeological feature defined as “A topographic depression in karst which is intermittently inundated on an annual basis, mainly from groundwater, and which has a substrate and/or ecological communities characteristic of wetlands” (Tynan et al. 2005).
1.2 Ecological Importance
By their nature, turloughs support many characteristic flora and fauna species (Reynolds
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1996). Under the Water Framework Directive (2000/60/EC), turloughs are classified as Groundwater Dependent Terrestrial Ecosystems (GWDTEs) and as a Priority Habitat in Annex 1 of the EU Habitats Directive (92/43/EEC). Consequently, under national legislation, many have been designated Special Areas of Conservation (SAC), that is, areas of ecological importance which are afforded the highest level of protection as ‘natural’ sites. Both EU directives necessitate the monitoring and management of these habitats to ensure favourable conservation and groundwater status is achieved. In particular, the Water Framework Directive requires a good understanding of the hydrological linkage between the turlough wetland, its ecological functioning and the connected groundwater body. Development of a conceptual model for the hydro-ecology and hydrochemistry of turloughs is currently the subject of a major research project being carried out by Trinity College Dublin (Republic of Ireland) on behalf of the National Parks and Wildlife Service (NPWS) of the Department of the Environment, Heritage and Local Government in aid of a management strategy for these wetlands.
1.3 Hydrology
Turloughs are at the interface between groundwater and surface water. They fill mainly by rising groundwater levels through estavelles and springs together with surface runoff; they ultimately empty through estavelles and swallow holes (Coxon 1986). Filling normally occurs in late autumn due to periods of intense or prolonged rainfall; with emptying typically occurring from April onwards. The karst flow system, of which a turlough is a surface expression, possesses a flow capacity which is defined by the size and connectivity of the flow paths present within the rock (Drew and Daly 1993). Rainfall of insufficient duration or intensity can be accommodated by subsurface flow paths; hence no surface flooding is visible in the turlough basin during these dry periods. However once the required combination of rainfall intensity and duration occurs the storage of the system is exceeded and flooding begins.
A range of turlough response and recession characteristics exist with some having multiple flood events in the course of a year whereas others show a single event with a slow recession as shown in Figure 1. The level, duration and extent of flooding vary greatly among turloughs with maximum flood depths of 3-14 metres and flooded areas of over 60 hectares recorded during the monitoring period.
0
Oct-06 Nov-06 Jan-07 Mar-07 Apr-07 Jun-07 Aug-07 Sep-07
Date
Water Depth (m)
Figure 1 Contrasting turlough hydrological regimes of Lough Aleenaun and Termon Termon
Lough Aleenaun
149 1.4 Geology
Turloughs are found in areas of thin, relatively permeable subsoil on well-bedded, pure grey calcarenite. They occur predominantly on Dinantian pure bedded limestone due to its purity and well developed bedding (Coxon 1987). Turloughs were originally considered hollows in glacial drift with underlying karst drainage systems (Williams 1964). However, Drew (1976) asserted that turloughs invariably lie in bedrock hollows and were solutional features requiring a far longer period to develop than has passed since the last glaciation.
Coxon and Coxon (1997) later suggested that turloughs are polygenetic with both processes playing a part in their formation. The presence of lacustrine marl in the basin of many turloughs also provides evidence that the flood regime has altered over time (Coxon 1994).
2 Methodology 2.1 Hydrology
The behaviour of a turlough as a wetland is fundamentally driven by its hydrology, essentially groundwater but with some surface water interaction that includes direct rainfall.
Discharges through turloughs are typically difficult to assess due to the often uncertain location and nature of the inflow and outflow points. The only realistic hydrological measures are based on water level. Semi-permanent water level monitoring stations were established in twenty-two selected turloughs (Figure 2) using proprietary pressure transducer-based instruments (“Diver”). Time series of water levels were collected using Divers installed at or near the lowest point of each turlough and recorded on an hourly basis. Sites were instrumented in early November 2006 and monitoring is still ongoing at selected sites.
Figure 2 Turlough monitoring site locations
To supplement long term rainfall records obtained from synoptic stations operated by the Irish Meteorological Service (Met Eireann), three rain gauges were installed in Kilchreest and Francis Gap in Co. Galway and Ballintober in Co. Roscommon. Rainfall was measured using an ARG100 tipping bucket rain gauge which recorded cumulative rainfall at 15 minute intervals.
150 2.2 Topography
In the absence of direct measurement of flows in or out of a turlough, the approach was taken to estimate net flows by determining the volume of the turlough and deriving flows from a combination of time changes in stage related to the relevant volume at that stage. Thus a depth-volume relationship for each turlough was essential in determining flow. A detailed topographic survey was carried out on each turlough using Trimble differential GPS equipment. An average of over 900 points was taken per turlough with a mean horizontal point spacing of 12 m.
Using this extensive topographic dataset, digital terrain models (DTM) were generated (Fig. 3). Statistical methods from relevant literature (Yanalak 2003) were used to assess this effect on turlough topographic models by calculating and comparing the standard deviation of derived surfaces using various gridding methods for a range of grid spacing. Kriging and multi-quadratic radial basis function were found to produce the most accurate results, with radial basis consistently showing lower values. However at high grid resolution radial basis generated unrealistic physical features and so kriging with a 2m grid resolution was selected as the preferred method for all DTM work.
Figure 3 Digital terrain model for Termon Turlough, Co. Galway (Maximum extent of flooding highlighted in bold)
To determine turlough volume and net flow rates the relationship between stage and volume in each turlough basin was established. This was achieved by calculating the volume between the lower surface of the turlough and an upper horizontal surface representing a specific water level at 20mm intervals over the recorded range. Applying this relation allowed time series of volumes and associated flow rates to be obtained from recorded water levels for each turlough.
151 3 Discussion
3.1 Regime Quantification
The behaviour of a turlough as a wetland is fundamentally driven by its hydrology.
Discharges through turloughs are typically difficult to assess because of the often uncertain location and nature of the inflow and outflow points. The only realistic hydrological measures are based on water level.
x Flood depths of 3-14 metres were measured during the monitoring period with a mean flood depth of 5 metres determined.
x Maximum flood volumes of 400,000-4,000,000 m3 were found during the monitoring period with a median maximum volume of 750,000 m3 determined.
3.2 Drainage Capacity
Upon applying stage – volume relationships to recorded hydrographs a common characteristic was observed across the set of turloughs. During the emptying phase the net outflow did not vary with decreasing head. Instead net outflow remained constant for a large part of the recession, the outflow only reducing at low water levels. This implied that there exists a maximum rate at which each turlough can drain, a rate which is independent of water level within the turlough. By applying linear regression to the recession curves the maximum drainage capacity was derived.
The constant flow rate during recession is only clearly defined when there is little or no rainfall during the recession period. Recharge from rainfall events causes water levels in the catchment to rise thus slowing the rate of emptying of the turlough. It also enters the turlough directly via surface flow and direct rainfall. In an attempt to limit this effect, regression analysis was carried out on data from mid March to early April 2007, a period where little or no rainfall fell. Sites found to empty before this period or having multiple recessions had regression analysis carried out on all available recessions and the highest recorded rate taken as the maximum flow rate (Figure 4).
3.3 Hydro-ecological Indicators
From a turlough’s water level record and DTM appropriate measures for hydroecological risk are being established. Frequency-duration curves for different stages in the turlough are uniquely related to characteristic ecological communities (Tynan et al. 2005), while the period of inundation or hydroperiod has been shown to influence mean abundance and taxon richness of macroinvertebrates (Porst and Irvine 2009). As such depth – duration – frequency curves have been generated over the monitoring period.
Disturbance has an important effect on macroinvertebrate community structure in turloughs, with high disturbance generally supporting lower faunal diversity (Porst et al.
2009). The areal rate of change, defined as the average rate of change of area between the time of maximum areal inundation and the emptying of a turlough, has been used to represent turlough disturbance (Porst et al. 2009).The higher the areal reduction rate the more rapid the changes in water levels and, thus, disturbance. The rate at which soil nutrients within the turlough basin are released to the water column could potentially be dependent on the areal rate of change.
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Straight Line Recession: Turloughmore
R2 = 0.9992
R2 = 0.9985 R2 = 0.9972
0 50000 100000 150000 200000 250000 300000 350000 400000 450000
19-Nov 29-Nov 09-Dec 19-Dec 29-Dec 08-Jan 18-Jan 28-Jan 07-Feb Date/Tim e
Volume (m3)
Figure 4 Recession analysis for Turloughmore Turlough, Co. Clare
3.4 Conceptual Model
The recession behaviour observed from 2 years of hydrological data led to the development of a conceptual model for the operation of turloughs. As the turloughs are typically empty during summer months despite rainfall events, there exists a flow system capable of carrying this recharge without surface flooding. When the rainfall on the catchment exceeds the capacity of this system, it becomes surcharged and flooding occurs. This process is represented by the conduit flow system shown in Figure 5.
Figure 5 Schematic for conceptual turlough model
The single conduit represents an actual conduit or a system of interconnected fractures or conduits. The turlough is represented by a pond with depth – volume characteristics
Q = 0.37m3/s Q = 0.42m3/s
Q = 0.40m3/s
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derived from the digital terrain model (DTM) fitted to the topographic survey data. Two catchments are defined in the model. The first is the greater catchment area which drains via the conduit system beneath the turlough. The second is a smaller local catchment which supplies water to the turlough via direct rainfall, surface runoff and shallow groundwater flow.
Rainfall on the greater catchment enters the turlough via the conduit flow system; the capacity of which is controlled by the restriction. During recession periods flow through the conduit system does not enter the turlough. Instead it controls the rate of release of water from the turlough by varying the pressure in the conduit.
The hydrological data collected to date also confirmed the basic operation of a turlough as a ‘surge tank’ in a hydraulic sense and that most turloughs operate through a limited number of entry points rather than behaving as ‘flow through’ devices where the surrounding boundaries are all permeable. Once a turlough fills, it has very little mixing association with the underlying groundwater. This model has implications for interpretation of potentially polluting pressures and on the management of associated risks.
4 Conclusion
The behaviour of a turlough as a wetland is fundamentally driven by its hydrology.
Discharges through turloughs are typically difficult to assess because of the often uncertain location and nature of the inflow and outflow points. The only realistic hydrological measures are based on water level. This study quantifies the hydrological regime of a set of turloughs and suggests a conceptual model to explain turlough operation, thus allowing hydro-ecological indicators to be defined.
For the first time the role turloughs occupy within a karst groundwater system have been defined; risks posed to these protected ecosystems may now be evaluated and quantified.
Whether through abstraction/drainage or through hydrochemical pressures on trophic status, these potential risks are assessed in terms of appropriate hydrological indicators relevant to the characteristic ecology of a turlough.
Acknowledgements
This research is funded by a grant from the National Parks and Wildlife Service (Department of the Environment, Heritage and Local Government). This research forms a part of the large interdisciplinary project on Assessing the Conservation Status of Turloughs, being carried out in Trinity College Dublin in 2006-2010, funded by the National Parks and Wildlife Service of the Department of the Environment. We are also grateful to Patrick Veale for his field assistance.
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