UPDATING THE LANDSLIDE HAZARD ASSESSMENT SYSTEM WITHIN THE CITY OF THE GOLD COAST

Transcription

1 UPDATING THE LANDSLIDE HAZARD ASSESSMENT SYSTEM WITHIN THE CITY OF THE GOLD COAST Patrick Kidd Geotechnical Manager, SMEC Australia, Gold Coast, Queensland, Australia ABSTRACT The City of Gold Coast has a population of over 520,000 people and an area of 1400 square kilometres. The city has variable geology and geomorphology which includes significant amounts of sloping terrain. The project involved a review, revaluation and update of the city wide unstable soils and slope instability susceptibility zoning system and the detailed assessment of the existing planning scheme relating to the development on sloping sites of a medium or higher susceptibility of slope instability. The zoning was based on geology, landforms, climate and history of the project area using a GIS platform. The project incorporated current best practice guidelines for the assessment of slope instability risk and also included updating city wide susceptibility mapping showing numerous landslips that have occurred or been mapped since the original zoning was undertaken in 1999 by SMEC. 1 INTRODUCTION The Gold Coast City Council (GCCC) has had landslide susceptibility mapping across the shire since In 2010 the GCCC decided that the landslide susceptibility mapping should be updated along with an existing planning scheme relating to the development on sloping sites of a medium or higher susceptibility of slope instability. This paper discusses the updating of the mapping and planning scheme system. 2.1 GENERAL 2 BACKGROUND The City of the Gold Coast had landslide susceptibility mapping undertaken across the city in 1999 to provide full coverage of the city as a number of previous studies had been undertaken in only small portions of the shire in response to landslides that had occurred previously, predominantly following the 1974 floods. These studies were undertaken by Willmott in 1981 and 1983 and Amaral in This susceptibility mapping system predated the work undertaken by the Australian Geomechanics Society (AGS) in landslide risk management in 2000 and 2007(LRM) and the State Planning Policy 1/03 (SPP) The implementation of the 1999 study by Sandegh-Vaziri and Taylor was published in Since the 1999 mapping project had been undertaken significant development had occurred within the council area in areas that been identified as being of moderate or higher susceptibility of instability and a further susceptibility mapping project had been undertaken by Willmott and Hayne (2001) in another small section of the city. The GCCC engaged SMEC in 2010 to update the landslide susceptibility mapping that had been previously undertaken and revise the development assessment guidelines within landslide susceptibility management areas to the current AGS LRM guidelines, to comply with the SPP and to consider the development that had been undertaken across the city on sloping sites. 2.2 TOPOGRAPHY The topography of the City of Gold Coast is generally composed of coastal plains in the east and a hilly hinterland of increasing elevation and steeper slopes towards the west. Throughout most of the central and northern hinterland the ridges are often aligned just west of north, parallel to the regional geological structure. These ridges are typically underlain by metasediments. However prominent ridges with steeper valley sides and narrow ridge crests are often underlain by steeply dipping quartzite beds. Incised river valleys are oriented generally in an easterly to north easterly and occasional northerly direction. The general landform is of rounded convex slopes with locally incised drainage. However along the south western and southern boundaries, volcanic plateaus with a series of steep scarps and benches are found. Gullying into these benches is also common. Erosion gullies frequently have lateral concave extensions at the gully head due to a combination of erosion and landsliding. The lower section of the gully is usually relatively narrow. 2.3 GEOLOGY There are several main geological formations in the city: metasediments of the Neranleigh Fernvale Beds of Devonian to Carboniferous Age; 141

2 volcanic rocks of the Chillingham Volcanics of Triassic Age; sedimentary rocks of the Woogaroo Sub-Group of Triassic Jurassic Age; volcanic rocks of the Lamington Group, Springbrook, Mount Gillies and Binaburra Rhyolites, Hobwee, Beechmont and Albert Basalts of Tertiary Age and alluvial, marine and colluvial deposits of Recent Age. The metasediments are found in most parts of the City of Gold Coast and comprise interbedded argillite (recrystallised mudstone), greywacke (recrystallised sandstone), quartzite (recrystallised chert) and greenstone (recrystallised basalt). These rocks are well foliated and have been sheared and folded by several tectonic episodes. They are steeply inclined and have a regional geological structure towards the north west. The younger Triassic and Jurassic age sedimentary rocks are generally limited to a number of small areas and comprise sandstone, conglomerate, shale and siltstone. Small areas outcrop in the extreme north east part of the City of Gold Coast which forms part of the river plains of the Albert and Pimpama Rivers. The volcanic rocks are mainly found in the elevated areas in the south western and southern parts of the City of Gold Coast. Due to their resistance to erosion, volcanic rocks form the hilltops of many areas including Mt. Tamborine, the Beechmont Plateau, Numinbah Valley, Tallebudgera Valley and Currumbin Valley. They mainly comprise interbedded rhyolite and tuff which have near horizontal bedding. These rocks are fractured often by near vertical and horizontal joint patterns. The combination of the bedding and fracturing results in relatively flat topped hills with several steep scarps separating benches. Several isolated hilltops with volcanic caps beyond the main volcanic areas also are present including Currumbin Hill and Burleigh Heads. 2.4 LANDSLIDES Natural landslides in the City of Gold Coast are most commonly found: In volcanic areas and On prominent ridges of quartzite and greenstone. These slides may be rotational, translational or debris flows. There is also an association between instability and steep slopes at the heads of erosion gullies and with the accumulation of soil downhill of these drainage features. In general wherever colluvium is found several metres thick on sloping ground, slope instability may be expected. These failures may not be deep seated, but they can still cover large areas particularly where there is a combination of thick low strength material and high groundwater pressures. This combination may occur on relatively gentle slopes. The natural slope angles in the metasediments are generally between about 5 degrees and 15 degrees on the lower slopes and along ridge tops. The middle and upper slopes are predominantly between about 15 degrees and 30 degrees with small areas in the western and southern hinterland where the natural slope exceeds 30 degrees generally at the upper slopes. In the volcanics the natural slope angles are generally between about 15 degrees and 45 degrees on the middle and upper slopes with some slopes greater than 45 degrees particularly along scarps around the plateau edge. On the top of the plateau slope angles are usually less than 15 degrees. There is no obvious evidence of natural slope instability initiated from the gentle slopes less than 11 degrees except for the volcanic areas with colluvium deposits particularly on benches below steep scarps. The majority of instability noted within the City of the Gold Coast occurs on slopes steeper than 15 0, instability within the metasediments and alluvium has been noted down to 11 0 at a number of locations within the City of the Gold Coast and across South East Queensland (AGSO 2009). Deforestation is a contributing factor to slope instability as reported by Willmott (1981 and 1983)The loss of vegetation may be caused by disease, bushfires or more likely, activities by man. Deforestation can result in the elevation of a near surface groundwater table. Studies in the Gold Coast area have found a significant correlation between deforestation and slope instability on volcanic benches (Willmott 1981). The previous studies undertaken across the City and this study have identified a total of just over 1000 landslips. 2.5 RAINFALL Rainfall records have been kept in the area generally for more than 100 years. Six rainfall stations with good records over that period provide a reasonable geographical spread over the City of Gold Coast and distribution 142

3 over low and high elevations were selected for review. Although there are some gaps in the information these records provide a good indication of the rainfall trends. Intense rainfall is considered the most common trigger for landslides. Review of the rainfall records indicate that all areas of the City of the Gold Coast regularly receive rainfall intensities of over 50 mm per day, which was considered by Braybrooke (1998) to be a trigger point for landslides depending on geology and slope condition. Consequently no areas of the City were weighted differently for landslide susceptibility on the basis of rainfall predicted intensities. 2.6 EARTHQUAKES From reference to the Geoscience Australia Natural Hazard Mapping website it appears that no significant earthquakes (magnitude >2.1) have occurred in the Gold Coast area since Within the region there have been five earthquakes recorded at west Burleigh, South Stradbroke Island, Beenleigh area, Coolangatta (offshore) and Fingal Head just over the border in New South Wales. On this basis earthquake hazard has not been considered further during this study. However, It should be noted that earthquakes could occur on the Gold Coast in the future and if they do structures sited over areas with deep alluviual soils, deep fills or deep colluvial soils will typically experience greater levels of damage compared to structures founded on rock. 3 METHODOLOGY The methodology undertaken in assessing potential Landslide Susceptibility Management (LSM) Areas in this study was based on a review of the available information which included: the existing 1999 susceptibility zoning by Sadegh-Vaziri and Taylor (2002) which used a GIS platform to produce a zonation based on a combination of the slope, geology, history of instability, topography and geomorphology; the slope susceptibility zoning undertaken in parts of the city by Willmott in 1981, 1983 and Willmott and Hayne in 2001; historical landslide information from GCCC and from Geoscience Australia and literature review of pertinent reports, publications and approximately 4000 development application records. Following the review of the above mentioned items the landslide hazard management areas were reassessed with the following modifications to the previous model: 1 m contour information was used instead of the previous 10 m (at best) contour interval to determine slope angles; Slope trigger criteria for various zones were modified downwards slightly to result in the trigger value for moderate susceptibility complying with the 11 0 in the GCCC Constraint Code for Sloping Sites; aerial photographic interpretation of the entire city, supplemented by review of available imagery of the www, identified substantial additional areas apparently affected by slope instability; Use of a 25 m by 25 m grid in the GIS platform instead of the previous 50m by 50m grid substantially refined the zonation by the reducing the extent of the averaging across the grid cells for the calculation of slope angles; The site specific information available from site specific stability assessments undertaken for development applications allowed updating of the zoning where available and Modifications to the boundaries of the zoning undertaken by Willmott and Hayne were undertaken due to the appreciably better terrain model available. The Mapinfo GIS system was used to generate the modified LSM areas map based on the modifications outlined above. The study undertaken was regional and not site specific. The assessment has been restricted to an evaluation of susceptibility rating (i.e. a medium scale, Regional Landslide Susceptibility Zonation, as defined in AGS2007a). In addition to the production of a new LSM area map, development of a new constraint code for use in the assessment of developments in LSM areas was undertaken. Modifications to the site specific system of landslide risk assessment were also made. 143

4 4 CHANGES TO THE INCORPORATION OF THE HAZARD ZONING INTO THE PLANNING SCHEME Current practice within the City where a development is proposed, uses the existing overlay maps Areas of unstable soils and areas of potential landslip hazard, as a tool to trigger the requirement for a Slope Stability Assessment to be undertaken by an appropriately experienced registered professional engineer in Queensland (RPEQ) on sites that have an existing zoning of medium, high or very high. The stability assessment required needs to use the currently available frequency calculation to demonstrate that the hazard of the site is currently low or very low or can be reduced to low. As part of the 1999 study a method for site specific evaluation of Hazard Frequency, using a standard form, was developed as a tool to assess the landslide frequency at the site. The Landslide Frequency Analysis Form developed rated various topographical, geological and hydrological factors for a specific site to determine the potential hazard frequency. This methodology was developed by MacGregor and Taylor and published in The following issues / shortfalls with the system were identified in this current study are listed below: The frequency analysis calculation (in effect a susceptibility analysis) being used by GCCC since 1999, doesn t include provisions for modifications to the site as a consequence of the proposed development or previous works on the site. It is also somewhat unclear regarding the impact of soil creep and the definitions of existing instability on the site. The system did not consider that different parts of the site or elements of the development may have different susceptibility ratings. The 1999 system also required use of the frequency analysis calculation to demonstrate that a low hazard was present. This did not allow consideration of the actual risk to the existing or proposed development by giving due consideration of the actual elements at risk, vulnerability and consequences plus the hazard. The system also did not incorporate the provisions of the AGS LRM Guidelines This study recommended the following changes to the assessment system: The susceptibility analysis be updated to include modifications to the site and improved definitions of instability. Figure 2 is the proposed updated susceptibility analysis assessment form. A frequency analysis, or number of analyses (depending on the size and uniformity of the site and the development proposed) be undertaken as part of the slope stability assessment. If the results of this assessment or assessments for multiple scenarios within a proposed development, all produce results of low or very low hazard of instability then no further assessment is required. The low or very low hazard which includes consideration of development modifications indicates the risk would be acceptable. If the result of initial site specific frequency analysis is moderate or higher, then a slope stability assessment would need to be more detailed to include a risk assessment in accordance with the method outlined in AGS The detailed risk assessment would need to consider the implications of the proposed development on the site, its proposed infrastructure, occupants and adjoining properties. The risk assessment would need to consider elements at risk, vulnerability, consequences and the effects of any planned mitigation measures. Providing the resulting risk is acceptable, development could be potentially approved subject to conditions imposed by GCCC. 144

5 UPDATING THE LANDSLIDE HAZARD ASSESSMENT SYSTEM WITHIN THE CITY OF THE GOLD COAST Legend Very Low Susceptibility Low Susceptibility Medium Susceptibility High Susceptibility Very High Susceptibility Landslide Figure 1: LHA Map Example 145

6 Figure 2: Landslide Susceptibility Analysis Assessment Form The results of the susceptibility calculation/s undertaken would be assessed against the modified correlation between Relative Susceptibility and Susceptibility Rating established by Taylor and MacGregor (2001) as shown in Table

7 Table 1: Correlation between Relative Suseptability and Suseptability Rating Relative Susceptibility Susceptibility Rating < 0.2 Very Low Low Moderate High >6.0 Very High The revised constraint code was developed for use by the council to enforce the assessment of sites that have been zoned in the LSM areas of moderate or higher susceptibility. 5 CONCLUSIONS AND RECOMMENDATIONS The updating of the LSM mapping and slope assessment system for GCCC has resulted in an improved mapping and a slope assessment system that complies with the AGS 2007 LRM Guidelines for those sites within the City assessed as having a moderate or higher susceptibility to landslide. The improved digital terrain model utilised in the study, along with the 25 m grid system, review of available additional information including development application data (Consultants reports) and recent publications resulted in mapping that demonstrated significantly better agreement with the detailed mapping undertaken by Willmott (1981, 1983) and Willmott and Hayne (2001) in parts of the city. The modification of the slope angle trigger angles has resulted in compliance with the trigger slopes in the GCCC Constraint Code for development on sloping sites, thereby removing an inconsistency with the current system of sloping site assessment. The changes to the site specific hazard assessment process now considers consequences, vulnerability and overall risk where the initial site specific assessment indicates a moderate or higher hazard is present. These changes provide a significantly more robust assessment mechanism, without imposing substantial additional compliance costs on those sites that have only a low or very low hazard. The assessment of risk in accordance with the AGS 2007 LRM guidelines brings the Gold Coast City Council in line with the National Landslide Risk Management Framework. 6 REFERENCES AGSO-Geoscience Australia 2009, Natural Hazards and the risk they pose to South-East Queensland, AGSO Cat. No ,2001/29. Australian Geomechanics Society Sub-Committee on Landslide Risk Management, Landslide Risk Management Concepts and Guidelines, Australian Geomechanics Journal, Volume 37 No. 2, May Australian Geomechanics Society, Landslide Risk Management Concepts and Guidelines, Australian Geomechanics Journal, Volume 42 No. 1, March Braybrooke, J Written Communication, Douglas Partners Pty Ltd. Gold Coast City Council, Guidelines for Control of Slope Instability within the City of Gold Coast MacGregor and Taylor A Method of Zoning Landslide Hazards, Australian Geomechanics Journal, Volume 36 No. 3, Sept Queensland Government, State Planning Policy 1/03 Guideline Mitigating the Adverse Impacts of Flood, Bushfire and Landslide (SPP 1/03), June 2003 Sadegh-Vaziri, M. & Taylor, B. Implementation of Landslide Risk Management on the City of Gold Coast, Australian Geomechanics Journal, Volume 37 No. 2, May Willmott, W.F Slope Stability and its constraints on closer settlement on Tamborine Mountain, Southeast Queensland. Geological Survey of Queensland, Record 1981/14 Willmott, W.F Slope Stability and its constraints on closer settlement in the Canungra-Beechmont- Numinbah Area, Southeast Queensland. Geological Survey of Queensland, Record 1983/64 Willmott, W.F.1992 Rocks and landscapes or the Gold Coast Hinterland. Geological Society of Australia, Queensland Division. Second edition. Willmott, W.F. & Hayne 2001, Slope Stability and its constraints on closer settlement on Springbrook Plateau and Upper Tallebudgera and Currumbin Valleys, Southeast Queensland. Queensland, Geological Record 2001/1. 147