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Geological Processes Causing Increased Flood Vulnerability

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Geological Processes Causing Increased Flood Vulnerability REPORT Chapman Tripp on behalf of the Earthquake Commission (EQC) Increased Flood Vulnerability: Geological Processes Causing Increased Flood Vulnerability Confidential and legally privileged REPORT ChapmanTripponbehalfofthe EarthquakeCommission(EQC) IncreasedFloodVulnerability: GeologicalProcessesCausing IncreasedFloodVulnerability Confidentialandlegallyprivileged Reportpreparedfor: Chapman Tripp on behalf of the Earthquake Commission (EQC) Reportpreparedby: Tonkin & Taylor Ltd Distribution: Chapman Tripp on behalf of the Earthquake Commission (EQC) PDF Tonkin & Taylor Ltd (FILE) 1 copy August2014 T&T Ref: 52010.150 Final Tableofcontents 1 Introduction 1 2 Geography and Geology of the Greater Christchurch area before the Canterbury Earthquake Sequence 2 2.1 Ground levels 2 2.2 Geology of the Greater Christchurch area 2 2.3 Catchment areas 3 2.3.1 Avon River 3 2.3.2 Heathcote River 3 2.3.3 Styx River 4 2.3.4 Coastal catchments 4 2.3.5 Waimakariri to Ashley River catchment 4 3 Geological processes of the Canterbury Earthquakes affecting ground elevations 5 3.1 Introduction 5 3.2 Tectonic effects 5 3.3 Liquefaction effects 6 4 Changes in ground level caused by the Canterbury Earthquake Sequence 11 4.1 Total change in ground elevation 11 4.2 Contributions from tectonic and liquefaction effects 12 5 Effects of changes to ground levels on flooding vulnerability 15 5.1 Flooding in Christchurch 15 5.2 How Flooding has changed as a result of the Canterbury Earthquake Sequence 18 5.3 Flood modelling using ground levels before and after the Canterbury Earthquake Sequence 19 5.4 Flood events and rainfall frequency 20 5.4.1 Introduction 20 5.4.2 Rainfall station analysis 20 5.4.3 Temporal distribution of storm rainfall 21 6 References 24 7 Applicability 25 Increased Flood Vulnerability: Geological Processes Causing Increased Flood Vulnerability T&T Ref. 52010.150 Final Chapman Tripp on behalf of the Earthquake Commission (EQC) August 2014 Executivesummary This report responds to the Earthquake Commission’s (EQC) request for a summary of the geological processes giving rise to increased flooding vulnerability in Canterbury as a result of the Canterbury Earthquake Sequence. We understand that this report may be used by EQC for the purposes of the declaratory judgment proceedings brought by it in the High Court. Canterbury experienced four major earthquakes (Magnitude 5.9 to 7.1) during 2010 and 2011. The four major earthquakes and associated aftershocks are known as the Canterbury Earthquake Sequence. These earthquakes caused significant changes to the land in some locations due to a combination of tectonic and liquefaction effects. Christchurch is a low lying city situated on the Canterbury Plains. The city has a history of flooding because of its proximity to both rivers and the coast. The geological setting is also responsible for the soils and consequently the susceptibility to liquefaction of the land. The main causes of subsidence leading to increased flooding vulnerability (IFV) in some parts of Christchurch as caused by the Canterbury Earthquake Sequence are tectonic effects (Item 1 below) and liquefaction effects (Items 2 to 4 below): 1. Tectonic ground movements (plate movements that may result in vertical tilting or horizontal movement); 2. Ejection of liquefied soil from the ground; 3. Lateral spreading; and 4. Volumetric consolidation of liquefied soil. The changes in ground levels due to the earthquake sequence in Canterbury have changed flooding in Christchurch. Although, flooding for the 1% AEP event was predicted to be wide-spread prior to the Canterbury Earthquake Sequence, it has worsened in some areas by the earthquakes. Changes in ground elevation due to the Canterbury Earthquake Sequence (including that caused by tectonics) have in some places had the effect of changing flood depths and changing overland flow paths. In addition, lateral spreading has the effect of narrowing watercourses and in some cases uplifting the beds. This in-turn has the effect of reducing the hydraulic capacity of watercourses. To provide context for the recent flood events during 2013 and 2014 analysis of rainfall records was undertaken. The 48 hour rainfall experienced in Christchurch at Botanical Gardens in 4/5 March 2014 is the highest rainfall since 1974. Generally, across rain gauges the recently observed rainfalls (48 hour depths) have not been observed since events in 1974, 1979 and 1980 (depending on the rain gauge). The gauge at the Firestone Factory shows large rainfall events occurring in 1998 and 1993. Increased Flood Vulnerability: Geological Processes Causing Increased Flood Vulnerability T&T Ref. 52010.150 Final Chapman Tripp on behalf of the Earthquake Commission (EQC) August 2014 1 1 Introduction This report responds to Earthquake Commission’s (EQC) request for a summary of the geological processes giving rise to increased flooding vulnerability in Canterbury as a result of the Canterbury Earthquake Sequence. We understand that this report may be used by EQC for the purposes of the declaratory judgment proceedings brought by it in the High Court. Increased Flooding Vulnerability is a physical change to residential land1 as a result of an earthquake which adversely affects the use and amenity that could otherwise be associated with the land by increasing the vulnerability of that land to flooding events. Previous reports have been supplied to EQC from Tonkin & Taylor (T&T). These reports are listed below and are referred to in the remainder of this report. x Volume 1: T&T (April 2014), Increased Flooding Vulnerability: Assessment Methodology; x Volume 2: T&T (August 2014), Increased Flooding Vulnerability: River Modelling and Coastal Extensions Report; x Volume 3: T&T (August 2014), Increased Flooding Vulnerability: Overland Flow Model Build Report; x Peer Review: x Benn et al, EQC Increased Flooding Vulnerability Damage Peer Review JOINT REPORT OF THE EXPERT PANEL, Final Report - Part 1 (Flood Modelling) April 2014; and x Benn et al, EQC Increased Flooding Vulnerability Damage Peer Review JOINT REPORT OF THE EXPERT PANEL, Final Report - Part 2 (Flood Modelling), 13 August 2014. This report summarises how these geotechnical processes have caused changes to the land which has resulted in Increased Flooding Vulnerability (IFV) in some locations. The report is organised into the following sections: x Section 2 describes the geography and geological setting of the greater Christchurch area; x Section 3 describes the geological process of the Canterbury Earthquake Sequence; x Section 4 describes the changes to ground levels caused by the Canterbury Earthquake Sequence; and x Section 5 describes the effects of changes to ground levels on flooding vulnerability. 1 “Residential land” is used in this assessment methodology as it is defined in the Earthquake Commission Act 1993, s2(1). Increased Flood Vulnerability: Geological Processes Causing Increased Flood Vulnerability T&T Ref. 52010.150 Final Chapman Tripp on behalf of the Earthquake Commission (EQC) August 2014 2 2 GeographyandGeologyoftheGreater ChristchurchareabeforetheCanterbury EarthquakeSequence 2.1 Groundlevels Greater Christchurch is situated at the coastal eastern edge of the Canterbury Plains. The land is generally low-lying with a gentle overall slope towards the coast, as shown in the ground elevation map in Figure 2.1. This ground elevation map was derived from aerial survey measurements (called “LiDAR”) undertaken across the region in several stages between 2003 and 2008. Figure2.1ȂLiDARgroundelevationmodelofChristchurchandKaiapoipriortoCanterburyEarthquake Sequence.Elevationsarebasedontheheightabovemeansealevel.Greydenoteselevationaboveͼ 2.2 GeologyoftheGreaterChristchurcharea Christchurch is located on the eastern edge of an advancing (aggrading) gravel outwash plain at the southern end of Pegasus Bay. The central city is underlain by low lying Holocene age coastal margins and abandoned overbank flood channels of the Waimakariri River. Figure 2.2 shows a diagrammatic geological cross section through Christchurch. The Port Hills lie to the southeast of the city. These represent part of the northern rim of the extinct Lyttleton volcano and range in age from 5.8 to 12 million years old (T&T 2011). The coastal margin of Christchurch, which is situated on the eastern side of the city, is made up of estuaries, lagoons and swamps. This forms part of the eastward advancing coastline which has been formed over approximately the past 6,500 years due to sediment input from Waimakariri River floods and coastal storm/current activity in Pegasus Bay. Increased Flood Vulnerability: Geological Processes Causing Increased Flood Vulnerability T&T Ref. 52010.150 Final Chapman Tripp on behalf of the Earthquake Commission (EQC) August 2014 3 The overbank flood channels of the Waimaikariri River, which mainly consist of sands and gravels, are predominately in the western side of the city and slope gently toward the east. These flood channels form the basis of the present day Heathcote, Avon and Styx rivers. The Avon, Heathcote and Styx Rivers meander through the city in a west to east trending direction. The Avon and Heathcote drain into the Estuary. The Styx River drains into the Brooklands Lagoon to the north. Figure2.2ȂGeologicalcrosssectionthroughChristchurchshowingQuaternarydeposits(GNS2008) 2.3 Catchmentareas The Greater Christchurch area has a history of flooding because of its proximity to both rivers and the coast (refer Section 5.1 for more details). There
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