The South Dunedin Coastal Aquifer & Effect of Sea Level Fluctuations
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The South Dunedin Coastal Aquifer & Effect of Sea Level Fluctuations Prepared by Jens Rekker, Resource Science Unit, ORC Otago Regional Council Private Bag 1954, 70 Stafford St, Dunedin 9054 Phone 03 474 0827 Fax 03 479 0015 Freephone 0800 474 082 www.orc.govt.nz © Copyright for this publication is held by the Otago Regional Council. This publication may be reproduced in whole or in part provided the source is fully and clearly acknowledged. ISBN 978-0-478-37648-7 Published October 2012 Prepared by Jens Rekker, Resource Science Unit Sea Level Effects Modelling for South Dunedin Aquifer i Overview Background South Dunedin urban area, which is mainly residential, is generally low lying reclaimed land, having once been coastal dunes and marshes. The underlying area has a groundwater system (or coastal aquifer) with a water table very close to the surface. The water table is closely tied to the surrounding sea level at both the ocean and harbour margins. The Otago Regional Council has been monitoring groundwater levels at three bores since 2009; results from which indicate that water table height is under the direct influence of climate and mean sea level, plus the drainage provided by the area’s storm and wastewater drains. The low lying land and already high water table makes the area vulnerable to any future rises in sea level. If the water table did rise any further it would create further pressure on the current drainage system and also increase the chances of surface ponding. Groundwater modeling was used in this investigation to assess the effects of a range of different sea level rise scenarios. What this study found The water table height is strongly influenced by the mean water level in the adjoining ocean and harbour. The close association is highlighted by areas of the aquifer within 300m of the coast showing the effect of the tides by fluctuating water table levels. The sea level has risen slightly over the past 150 years (up to 0.2m), and the water table has probably responded in an equivalent fashion. Further rises in Mean Sea Level are highly likely to force further equivalent rises in the water table. Groundwater modeling of this effect suggests that even the mildest continuation of the current rise of sea level rise would create ponding in some parts of the South Dunedin urban area. Water table levels are also in part controlled by the fortuitous draining through the stormwater and wastewater pipe network, which allows surrounding groundwater to enter through cracks or breaks in the system. What should be done next? Groundwater levels will continue to be monitored to assess any changes to water table levels. The network of bores will also be extended to further improve the accuracy of the model predictions. The information from this report can be used for future planning requirements. ii Sea Level Effects Modelling for South Dunedin Aquifer Technical Summary The South Dunedin urban area is low-lying having been reclaimed from coastal dunes, marshes and intertidal deposits since European settlement. The water table lies close to the surface, typically 0.3 m to 0.7 m under the urban area. A coastal aquifer comprising mostly sands, estuarine muds and alluvial gravels makes up the basin infilling between the Dunedin mainland and the Otago Peninsula. The coastal aquifer is exposed to the sea at the Harbour Basin and Ocean Beach (St Kilda Beach, Middle Beach and St Clair Beach). Recent drilling investigations have characterised a sandy aquifer in hydraulic communication with the sea, including tidal fluctuations of the water table in proximity to the ocean. It is inferred that any long-term rise in the mean level of the sea would translate into a rise in the water table. Thus, groundwater modelling was used in this investigation to assess the magnitude of sea level rise effects on South Dunedin from groundwater causes. Scenarios or projections of future sea level rise encompassing the 21st Century were used in formulation model scenarios. The following bullet points outline the results of computer modelling: • Groundwater computer modelling suggests that even the mildest continuation of the measured rate of sea level rise would manifest in groundwater ponding of a few tens of centimetres in the parts of the South Dunedin urban area. • Based on the LiDAR land surface data and the groundwater model-derived water table surface, the greater potential for groundwater ponding at the surface is within the Bayview Road – Hargest Crescent – Forbury Park area. Sea Level Effects Modelling for South Dunedin Aquifer iii Table of Contents Overview ..................................................................................................................................... i Technical Summary .................................................................................................................... ii 1 Introduction .................................................................................................................. 1 1.1 Background .................................................................................................... 1 1.2 Purpose & Approach ...................................................................................... 1 1.3 Limitations ..................................................................................................... 2 2 Background & Setting .................................................................................................. 2 2.1 Location & Land Use ..................................................................................... 2 2.2 Geological & Marine Processes ..................................................................... 2 2.3 European Land-filling .................................................................................... 4 2.4 Climate ........................................................................................................... 6 2.5 Hydrology ...................................................................................................... 6 2.6 Water Services ............................................................................................... 6 2.6.1 Sewer Infiltration ............................................................................. 7 2.7 Coastal Aquifer .............................................................................................. 7 2.7.1 Soils & Foundations’ Investigations ................................................ 8 2.7.2 ORC Drilling Investigations ............................................................ 9 2.7.3 ORC Groundwater Monitoring ........................................................ 9 2.7.4 ORC Tidal Fluctuation Analysis ................................................... 10 2.7.5 DCC Tahuna WWTP Investigations ............................................. 11 3 Groundwater Modelling ............................................................................................. 12 3.1 Model Framework ........................................................................................ 12 3.2 Assignment of Model Properties ................................................................. 12 3.3 Aquifer Recharge ......................................................................................... 13 3.3.1 Urban Recharge Rate Quantification ............................................. 13 3.3.2 Recharge in Open Areas ................................................................ 13 3.4 Assignment of Model Boundary Conditions ............................................... 13 3.4.1 Fixed Head Boundaries .................................................................. 14 3.4.2 Drain Boundaries ........................................................................... 14 3.5 Model Calibration ........................................................................................ 14 3.5.1 Calibration Results ......................................................................... 15 iv Sea Level Effects Modelling for South Dunedin Aquifer 3.6 Scenario Modelling ...................................................................................... 16 3.6.1 Scenario 0: No Sea Level Rise ...................................................... 16 3.6.2 Scenario 1: 1.1 mm/y Sea Level Rise ............................................ 16 3.6.3 Scenario 2: 2.79 mm/y Sea Level Rise .......................................... 17 3.6.4 Scenario 3: 4 mm/y Sea Level Rise ............................................... 17 3.7 Results of Scenario Modelling ..................................................................... 18 3.7.1 Scenario 0: No Sea Level Rise ...................................................... 18 3.7.2 Scenario 1: 1.1 mm/y Sea Level Rise ............................................ 19 3.7.3 Scenario 2: 2.79 mm/y Sea Level Rise .......................................... 20 3.7.4 Scenario 3: 4 mm/y Sea Level Rise ............................................... 21 3.7.5 Results in terms of Fluxes and Flows ............................................ 22 3.8 Discussion of Model Results ....................................................................... 22 4 Summary & Conclusions ........................................................................................... 24 4.1 Summary ...................................................................................................... 24 4.2 Conclusions .................................................................................................. 24 References ...............................................................................................................................