ASSESSMENT OF GROUNDWATER FLOODING RISKS POSED BY TURLOUGHS. Site Assessment Report Doughiska, Co. Galway

Transcription

1 ASSESSMENT OF GROUNDWATER FLOODING RISKS POSED BY TURLOUGHS Site Assessment Report Doughiska, Co. Galway Prepared for: The Office of Public Works Prepared by: Department of Civil, Structural and Environmental Engineering, Trinity College Dublin (TCD) Authors: Dr. Owen Naughton and Assoc. Prof. Paul Johnston March 2013

2 1. Introduction This report forms part of a study jointly funded by the Office of Public Works (OPW) and the Irish Research Council on the assessment of groundwater flooding risks posed by turloughs (Naughton, 2013). Turloughs are predominantly groundwater fed transient lakes and are a result of a combination of high rainfall and accordingly high groundwater levels in topographic depressions in the karst. Turloughs act as attenuation devices for regional groundwater and surface water flow, but sometimes they can flood beyond their normal limits causing localised flooding. The timing and extent of flooding within turlough basins can vary significantly between sites depending on the nature of and connections to the local karstic groundwater system. The primary objective of the turlough study was to investigate the causative factors influencing the range of flooding behaviours that occur in turlough basins using a variety of hydrological and eco-hydrological techniques. The purpose of this report is to assess the groundwater flood hazard area in the townland of Doughiska, Galway City, Co. Galway. This area was identified as a potential Area for Further Assessment (AFA) during the Preliminary Flood Risk Assessment (PFRA) (Mott McDonald, 2010). The objective is to collate available information on site hydrology and historic flood events, identify the key factors influencing flooding in the area and assess the validity of the Groundwater PRFA flood boundary. 2. Location Description The study area is located in the townland of Doughiska which is situated approximately 1.5km east of Galway city centre (Figure 1). The M6 Coolagh Roundabout and N6 National Road lie within the eastern extent of the study site. The Doughiska Road, a third class road running parallel to the N6, also bisects the site. The study site, as delineated during the groundwater PRFA, is the area within the 36.5mAOD contour and encloses an area of approximately 0.86km 2, as shown in Figure 1 (the PRFA boundary ). Figure 1: Doughiska Site Location Map Topographically the site lies in a relatively flat, low-lying basin running in a northeastsouthwest direction with a minimum elevation of approximately 26.5mAOD. There is

3 extensive development in the site with a total of 859 buildings lying inside the PFRA boundary (An Post/OSI GeoDirectory, August 2012). Development is primarily single and multiple-unit residential properties but there are also a number of commercial buildings including a retail centre and a temporary primary school (Merlin Woods Primary School) adjacent to the Doughiska Road. As the site contains a turlough, it was suggested that there was a potential flooding risk to these developments. 3. Site Description 3.1. Geology and Hydrogeology The Geological Survey of Ireland (GSI) national bedrock map shows the bedrock geology of the study area is entirely composed of undifferentiated Visean Limestone and is classified as a Regionally Important Karstified Aquifer (Appendix A: fig. 2). The purity (>90% CaCO 3 ) and well-bedded nature of this limestone makes it susceptible to dissolution processes and karstification, and as a result karst landforms and karst drainage form an important part of the landscape. Characteristic karst features include dolines, caves, bare rock exposures, turloughs, sinking and intermittent streams, dry valleys and springs. The GSI karst database indicates a single karst feature type within the site boundary: Doughiska Turlough (K1). The database shows Doughiska Turlough located on the eastern boundary of the study site. However, based on evidence from historic 25 maps it is likely that the marked location of the turlough in the database is incorrect. Instead, Doughiska Turlough is almost certainly sited in the depression labelled as Liable to Flood which currently lies between the Doughiska Road and N6. There are a variety of karst feature types in the surrounding area all indicating a developed karst system in the area and high levels of groundwater-surface water interaction, including springs to the north at Coolagh, enclosed depressions and superficial solutional features. GSI quaternary maps show the study site is dominated by shallow and deep well-drained soils, with deep well-drained soils becoming more common towards the centre of the study area (Appendix. A: fig. 3). Isolated areas of poorly drained and peaty soils also exist within the study site. GSI subsoil maps show the study area and surrounding vicinity dominated by limestone tills. There are also localised areas where subsoils are thin or absent and karst rock is close to or at the surface, particularly towards the outer extents of the study area Topography Topographically the site lies in a relatively flat basin surrounded by higher ground to the north, east and west which rises up to maximum elevations of between 40 and 60mAOD (Figure 2). The basin itself is orientated in a northeast-southwest direction and there is a general fall in towards Galway Bay to the south-west. Development in the area is concentrated along the slopes parallel with the NE-SW basin alignment. The groundwater PFRA boundary was defined as the area enclosed within the 36.5mAOD contour. As topographic contours were not confined to encircling the indicated turlough location, the PFRA boundary encompasses a much larger area, however, and it crosses multiple contours along the southern edge. Indeed, the boundary is not a closed contour and elevations along the boundary of the study site range from a high of 36.5mAOD along the north-eastern edge down to a low of 26mAOD along the south-western edge. The depression in the north-east of the site, the likely location of Doughiska Turlough and hereafter referred to as Doughiska Turlough, has a local minimum elevation of 27mAOD. The topography in this area rises steeply to the north and to east towards the N6 road embankment, while rising more gently towards the Doughiska Road and residential

4 development to the west and south respectively. If the Turlough were functional, flooding would potentially discharge to the southwest, the open end of the PFRA basin boundary. Figure 2: Doughiska digital elevation model (derived from LIDAR data) 3.3. Ecology The behaviour of a turlough as a wetland is fundamentally driven by its hydrology; the characteristic periodic flooding is the main driving force behind the ecological functioning of the turlough habitat, and plays a key role in determining the level of biological diversity (Bonacci et al., 2009; Bertrand et al., 2012, Naughton et. al., 2012). As a result there is a characteristic ecology associated with the pattern of groundwater inundation. Records of the vegetation in Doughiska area were collated and a walkover was carried out within Doughiska Turlough in October 2012 in order to identify any ecological indicators of recurrent flooding. Previous habitat surveys have been carried out in 2009 and 2012 within the boundaries of the Ardaun Local Area Plan (LAP) (Natura, 2009; Natura, 2012). This work, commissioned by Galway City Council, was undertaken to identify habitats of ecological significance within the site and areas of ecological sensitivity near the site that could be indirectly impacted by the LAP. These studies concentrated on the Ardaun area, which is located to the eastern side of the R446 and N6 dual carriageway and so did not include Doughiska Turlough. Some wet grassland areas were identified as potential Turlough habitat in the 2009 survey. The main species included creeping bent (Agrostis stolonifera), jointed rush (Juncus articulatus/acutiflorus), carnation sedge (Carex panacea) and localised patches of creeping willow (Salix repens) (Natura, 2009). The moss Calliergonella cuspidate, which is indicative of wet and basic conditions, was extensive in 2009 but was not present in 2012 (Natura, 2012). On reassessment of the area in 2012 the potential turlough habitat was confined to an area of only approximately 5m 2, and this is was thought likely to be due to retention of surface water rather than a fluctuation in ground water levels (RPS, 2012a; Natura, 2012). A site walkover was carried out in October 2012 to look for the presence of vegetation communities indicative of frequent flooding within Doughiska Turlough. The scrubby areas surrounding the central field contained plant species which are not usually found in

5 turloughs, such as Pteridium aquilinum (Bracken), Betula pendula (Birch), and Ulex sp. (Gorse). The centre of the field seemed to be a poor grassland/sedge community, with abundant Carex panicea and C. flacca. A local groundwater flush or spring was identified within the site, and the wetter areas fed by this spring had more lush, aquatic vegetation, with abundant Nasturtium officinale (Watercress) in the wettest parts (Figure 3). Figure 3: Groundwater Spring within Doughiska Basin The ground in the lower part of the basin has been disturbed and compacted during the construction of the Merlin Park Drainage Scheme. As a result the vegetation zonation does not appear to follow a flood gradient in the lower part of the basin. Instead, local ground conditions seemed to determine vegetation composition. Wetter vegetation communities occurred around the manholes in the basin, at elevations higher than adjacent areas which contained communities typically associated with drier conditions. Overall, while the vegetation identified could possibly be associated with the recurrent flooding typical of turloughs, the distribution indicates that local ground conditions, associated with the drainage works, may be the key driver in the vegetation distribution. Moreover, the presence of scrubby vegetation such as gorse throughout the site indicates that if there is recurrent flooding within the site it is at a very limited scale. 4. Hydrology 4.1. Meteorology Average annual reference rainfall ( ) for the site is estimated at 1168mm (Walsh, 2012). The general rainfall trend shows the lowest monthly average in April of 69mm followed by a gradual rise in average rainfall to a peak in October of 129mm. Average rainfall remains above 100mm for the months August to January before decreasing thereafter. The nearest available rainfall data is from the Met Eireann station in Galway City (National University of Ireland, Galway. Station ID 1921) located approximately 6km to the west of the site. Rainfall depth, duration and frequency (DDF) data for the site, derived from the FSU- DDF model developed by Met Eireann, is given in table 1 below (Fitzgerald, 2007).

6 DURATION (days) RETURN PERIOD RAINFALL (mm) 2yr 5yr 10yr 20yr 30yr 50yr 100yr 150yr 250yr 500yr 1d d d d d d d d Table 1: Rainfall duration, intensity and frequency data for Doughiska 4.2. Surface Hydrology Regionally the study site is located in the Western River Basin District. Locally the site is mapped within the Ballynamanagh River catchment; however the study area and the surrounding vicinity drains directly towards Galway Bay via the Merlin Park drain rather than the Ballynamanagh River. At present there are no definite surface drains within or in the immediate vicinity of the study site. Historic 6 and 25 maps show a surface water channel running from the low-lying area adjacent to the Doughiska Road in a south-westerly direction towards Galway Bay approximately 2km away (Figure 4). Figure 4: Location map of springs and area liable to flooding (RPS, 2012a) The surface drain originated in an area indicated as Liable to floods. Areas labelled as Liable to floods commonly correspond to the turloughs and so this is the more likely location of Doughiska Turlough, rather than further to the east as indicated by the GSI Karst Database. The surface water channel is no longer present but has been culverted as part of the Merlin Park Drainage Scheme which runs directly through the likely location of

7 Doughiska Turlough (RPS, 2012a).The channel was originally fed by three springs, two of which now lie to the east of the N6 dual carriageway and in the vicinity of a surface-water attenuation pond. This pond drains into the Merlin Park Drainage scheme which runs directly through Doughiska Turlough (Figure 5). A small dry drainage channel was identified during the site visit leading from a small, partially blocked culvert. Surface channels of Merlin Park stream reappear in south-west of the site. Local residents indicated that since the construction of the Merlin Park Drainage Scheme the lower reaches of the stream have become ephemeral, with any flow sinking into the underlying karst formations. Figure 5: Location of subsurface drainage scheme and surface drain passing through Doughiska Turlough (Image derived from Microsoft Bing Maps, Cartography by Nokia. Retrieved 1st March 2013) 4.3. Groundwater Hydrology The study site lies on the boundary of the Galway East and Clarinbridge groundwater bodies. The inferred overall groundwater flow direction is to the south / southwest, with localised variations in flow are likely to occur within the area given the karstic nature of the bedrock. The primary mode of recharge is the area is diffuse autogenic input from rainfall. There are no springs indicated by the GSI karst database within the study area in the present day. Historical 25 maps identified three springs in the east of the site which demonstrates the shallow depth to groundwater in the area. Further springs were in evidence to the southwest, suggesting that the karst fissuring and associated groundwater flow direction in the Doughiska area runs parallel to the alignment of the ridges in the landscape (southwest-northeast). Extensive development of residential, commercial and transportation infrastructure with associated alteration of drainage, has substantially altered groundwater hydrology and reduced the zone of contribution to the original turlough. Given the large quantities of bedrock excavated from the turlough basin during the construction of the Merlin Park Sewage and Drainage Scheme, it is highly unlikely the much of the original karstic drainage system remains intact. However, there is still an active groundwater system within the site, as indicated by the presence of an active spring on the northern slope of the Doughiska basin (section 3.3). The topography surrounding the spring suggests that the recharge area feeding it is relatively limited in area relative to the size and volume of the site.

8 5. Flooding There is historic evidence of flooding in the Doughiska area (Figure 4). 25 OSI maps developed between 1897 and 1913 show an area Liable to Floods located in the topographic depression which in presently lies between the Coolagh Roundabout and the Doughiska Road. Given the prevalence of active karst features such as springs in the area, it is likely that flooding was groundwater driven. However, since then the natural drainage system has been significantly altered due to extensive development of residential, commercial and transportation infrastructure. In addition, the Merlin Park stream which historically drained the area has been culverted and integrated into the Merlin Park Sewage System. The N6 carriageway bisects the northern section of Doughiska Turlough, and roadrunoff attenuation ponds have been constructed near the locations of the historic springs, with overflows from these ponds appearing to be routed into the Merlin Park Drainage system. In this context an investigation was carried out to identify any evidence of recent groundwater-related flooding within the Doughiska area. There are no specific incidents of flooding reported on the OPW national flood hazard website ( other than a mention of the recurring flooding associated with Doughiska Turlough. Details on historical flooding incidents contained within the SEA Environmental Report for Galway City lists Doughiska as an area affected by flooding, with turlough flooding given the main flooding mechanism (Galway City Council, 2011). The sources of these data were the OPW Floodmaps website along with consultation with the Transportation and Infrastructure Department of Galway City Council. Unlike the other areas mentioned within the SEA, there are no specific recorded flooding events given for Doughiska Turlough. Site visits on the 2 nd March 2012, 17 th October 2012 and 4 th March 2013 did not identify any floodwaters visible within the site. An aerial photograph taken on November 23 th 2009 of the Coolagh Roundabout is shown in figure 6 (RPS, 2012a). The edge of the Doughiska Turlough basin is visible in the bottom left of the photograph and does not show any signs of flooding in spite of significant rainfall. Figure 6: Aerial Photograph for the Coolagh Roundabout area on 23 rd November 2009 (RPS, 2012a)

9 Rainfall totals for November 2009 were exceptional and the highest on record at most stations. The Met Eireann climatological station located in Galway (NUIG) recorded a total of 26 rainfall days (daily rainfall >1mm) during the month of November, with heavy precipitation (daily rainfall >= 10mm) on 10 days. The total monthly rainfall for November 2009 was 329mm, or approximately 280% of normal rainfall. Given the prevailing groundwater conditions in the region at this time, substantial flooding would be expected in this area were the turlough to pose a significant risk. Based on GPS topographic data collected as part of this investigation it is estimated that the base elevation of the section of Doughiska Turlough visible in figure 6 may be as low as 27.2mAOD. As this is marginally above the lowest point in the basin, approximately 27mAOD, this suggests very limited flooding, if any, within the site at this time. This was corroborated by a local resident who stated that the adjacent Doughiska road was not inundated during the regional flooding which occurred in November A further factor that points to a limited groundwater flood risk in the area is the presence of the drainage scheme which passes directly under Doughiska Turlough. During the site survey, a series of manholes were identified corresponding to the culvert traversing the site. The elevation of the lowest manhole, at 27.3mAOD, was less than 0.3m above the base of the turlough. Thus if any hydraulic connection exists between the drainage system and the surface flooding it would limit the flood level within the surface depression. Any other flooding within the Doughiska PFRA can be mainly categorised as pluvial, which occurs when the capacity of surface drainage systems is inadequate following heavy and prolonged rainfall events and/or when soils are saturated and unable to take further infiltration. Video evidence of flood events within the PFRA boundary along the Fearann Ri and Doughiska Roads was identified on 21 st June and 16 th August 2008 (Desbonnet, 2008; Sebko87, 2008). Both flood events were associated with short-duration, high-intensity rainfall events during the summer and not related to groundwater flooding. The primary reason behind the delineation of the Doughiska groundwater PFRA boundary was the presence of Doughiska Turlough and the potential risk flooding posed to the surrounding area. Historic 25 maps suggest that this is the incorrect location of Doughiska Turlough. Instead, the turlough is located the depression between the N6 at the Coolagh Roundabout and the Doughiska Road. The PFRA boundary was defined as the area enclosed within the 36.5mAOD contour. This would indicate a potential groundwater level of 9.5m above the base level of Doughiska Turlough, which is extremely unlikely. There may remain a risk of localised groundwater related flooding within the original Doughiska Turlough basin, as indicated by the presence of an active spring within the site, but this flooding does not appear to pose a risk to any of the surrounding properties. Moreover, the sources of recharge to the spring are likely to have been significantly altered from local development. Due to the absence of significant flooding in the area indicated, together with site topography, it may be concluded that groundwater flooding in this area does not pose a significant risk.

10 6. Conclusions The primary groundwater flooding risk identified within the site was associated with Doughiska Turlough on the north-eastern edge of the groundwater PFRA boundary. Historic 25 maps suggest that this is the incorrect location of Doughiska Turlough. Instead, the turlough was likely located in the depression between the N6 at the Coolagh Roundabout and the Doughiska Road. The PFRA boundary was defined as the area enclosed within the 36.5mAOD contour. This would indicate a potential groundwater level of 9.5m above the base level of Doughiska Turlough, which is extremely improbable. No evidence of substantial groundwater flooding could be identified. This included during the regional flooding events which occurred in November Due to the absence of significant flooding in the area indicated, together with site topography, it may be concluded that groundwater flooding in this area does not pose a significant risk. While Doughiska turlough continues to exist as a topographic entity at its identified location, its functionality as a wetland has largely disappeared due to partial infill, infrastructural development and artificial drainage. There may remain a risk of localised groundwater related flooding within the original Doughiska Turlough basin, as indicated by the presence of an active spring within the site, but this flooding does not appear to pose a risk to any of the surrounding properties. Any other flooding within the Doughiska PFRA can be mainly categorised as pluvial. Evidence of flood events within the PFRA boundary along the Fearann Ri and Doughiska Roads was identified during the summer of 2008, but these were associated with short-duration, high-intensity rainfall events not groundwater flooding. 7. References Bertrand, G., Goldscheider, N., Gobat, J., Hunkeler, D., Review: From multi-scale conceptualization to a classification system for inland groundwater-dependent ecosystems. Hydrogeology Journal, 20: Bonacci, O., Pipan, T., Culver, D.C., A framework for karst ecohydrology. Environmental Geology, 56: Desbonnet, Doughiska road flash flood, online video, accessed 16 th December 2012, < Drew, D., Daly, D., Groundwater and Karstification in Mid-Galway, South Mayo and North Clare, RS 93/3 (groundwater). Geological Survey of Ireland, Dublin, Republic of Ireland Fitzgerald, D.L., Estimates of Point Rainfall Frequencies, Technical Note No.61, Met Eireann, Dublin. Galway City Council, SEA Environmental Report of Galway City Development Plan, prepared by the Development Plan and Policy Unit of Galway City Council Mott McDonald, Preliminary Flood Risk Assessments - Groundwater Flooding. Prepared for the Office of Public Works, Dublin Naughton, O., Johnston, P.M., Gill, L., Groundwater Flooding in Irish Karst: The Hydrological Characterisation of Ephemeral Lakes (Turloughs). Journal of Hydrology, Volume 470, p Naughton, Assessment of groundwater flooding risks posed by turloughs final report. Prepared for the Office of Public Works, Dublin

11 Natura, Ardaun Local Area Plan Habitat Assessment, August Prepared by Natura Environmental Consultants for Galway City Council Natura, Galway Ardaun LAP Habitat Survey Final Report, October Prepared by Natura Environmental Consultants for Galway City Council RPS, 2012a. Strategic Flood Risk Assessment (SFRA) for Three Local Area Plan Areas: Initial Flood Risk Assessment Stage I, September 2012, prepared for Galway City Council, Galway. RPS, 2012b. Strategic Flood Risk Assessment (SFRA) for Three Local Area Plan Areas: Initial Flood Risk Assessment Stage II, November 2012, prepared for Galway City Council, Galway. Sebko87, Doughiska (Galway) flooded after heavy rain on 16 AUG 2008, online video, accessed 16 th December 2012, < Walsh, S., A Summary of Climate Averages for Ireland , Climatological Note No.14, Met Eireann, Dublin. Acknowledgements This research was jointly funded and supported by the Office of Public Works and the Irish Research Council. The authors would also like to thank the Irish Meteorological Service (Met Eireann) for the provision of rainfall and evapotranspiration data, and Galway County Council for the provision of road flooding data and site reports. Ordnance Survey Ireland data reproduced under OSI Licence number EN (Office of Public Works).

12 GROUNDWATER FLOODING SITE REPORT: DOUGHISKA APPENDIX A

13 Title: Site Location Map Site: Doughiska Figure Number: 1 Sheet Size: A4 Scale: 1:50000

14 K4 K1 K1 K7 K6 K3 K2 K4 K2 K1 K1 K5 K3 K5 Title: Bedrock Geology Site: Doughiska Figure Number: 2 Sheet Size: A4 Scale: 1:25000

15 K1 K2 K4 K2 K1 K5 K3 Title: Soils Map Site: Doughiska Figure Number: 3 Sheet Size: A4 Scale: 1:25000

16 Title: Subsoils Map Site: Doughiska Figure Number: 4 Sheet Size: A4 Scale: 1:25000

17 GROUNDWATER FLOODING SITE REPORT: DOUGHISKA APPENDIX B

18 Definitions from "A Lexicon of Cave and Karst Terminology with Special Reference to Environmental Karst Hydrology" EPA/600/R-02/003, 2002, EPA: Washington, DC. This is available to download at Anisotropy: The condition of having different properties in different directions Aquiclude: A formation which, although porous and capable of storing water, does not transmit it at rates sufficient to furnish an appreciable supply for a well or spring Aquifer, karst: An aquifer in which the flow of water is or can be appreciable through one or more of the following: joints, faults, bedding-plane partings, and cavities any or all of which have been enlarged by dissolution. Backwater: The accumulated water above the normal level of a water course due to impoundment at a point downstream Calcareous: Containing calcium carbonate Clastic: Pertaining to a rock or sediment composed principally of broken fragments that are derived from pre-existing rocks or minerals and that have been transported some distance from their places of origin Conduit; karst conduit: Relatively large dissolutional voids, including enlarged fissures and tubular tunnels; in some usage the term is restricted to voids that are water-filled. Conduits may include all voids greater than 10mm in diameter, but another classification scheme places them between arbitrary limits of 100mm to 10m. Dissolution (or solution): The change of matter from a solid or gaseous state to a liquid state by combination with a liquid Doline: A basin- or funnel-shaped hollow in limestone, ranging in diameter from a few meters up to a kilometer and in depth from a few to several hundred meters. Epikarst: The portion of bedrock that extends from the base of the soil zone and is characterized by extreme fracturing and enhanced solution. It is separated from the phreatic zone by an inactive, relatively waterless interval of bedrock that is locally breached by vadose percolation. Significant water storage and transport are known to occur in this zone. Estavelle: An intermittent resurgence or exsurgence, active only in wet seasons, which may act alternatively as a swallow hole and as a rising according to ground-water conditions Heterogeneity: characteristic of a medium in which material properties vary spatially or from point to point Phreatic zone: Those parts of the earth s crust in which all voids are filled with water under pressure greater than atmospheric Recharge, allogenic: Recharge derived from runoff of neighbouring or overlying non-karst rocks that drains into a karst aquifer. Diffuse allogenic recharge is used to describe the slow percolation of recharge when runoff into direct input points is reduced in magnitude while concentrated allogenic recharge is used to describe the concentrated recharge that occurs by runoff into large fractures, sinkholes, and sinking streams. Recharge, autogenic: Recharge derived from precipitation directly onto the karst landscape. Diffuse autogenic recharge is used to describe the slow percolation of recharge through a myriad of small openings while concentrated autogenic recharge is used to describe the concentrated recharge that occurs by flow into large fractures, sinkholes, and sinking streams. Siliciclastic: Clastic non-carbonate sedimentary rocks that are almost exclusively silicabearing, either as forms of quartz or other silicate minerals.

19 Stratigraphy (bedrock): The sequence of rock types in an area Solutional Planation: The process whereby bedrock is removed by dissolution down to a surface controlled by the water table. Once this base level is reached, the plain expands by the gradual retreat of adjoining karst uplands (Ford & Williams, 2007). Distal Catchment: A catchment area supplying recharge to a groundwater system which lies outside the bounds of its topographic catchment Vadose Zone: The zone between the land surface and the water table. Generally, water in this zone is under less than atmospheric pressure, and some of the voids may contain air or other gases at atmospheric pressure.