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

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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.

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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).

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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.

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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 are three major rivers in Christchurch: the Avon; Heathcote; and Styx. The majority of flooding occurs in areas adjacent to these rivers and their tributaries. The three catchment areas are shown in Figure 2.3. Further to the north of Christchurch City is an area including the Kaiapoi River catchment that is bounded by the Waimakariri and Ashley Rivers. Descriptions of the catchments are as follows (GHD, 23 August 2012).

2.3.1 AvonRiver The Avon River catchment is predominately located in the middle of the city. The Avon has its source in the suburb of Avonhead and runs through the suburbs of Ilam, Riccarton and Fendalton before reaching the CBD. It then passes through Avonside, Dallington, Avondale and Aranui before flowing into the Avon-Heathcote estuary.

2.3.2 HeathcoteRiver The Heathcote River catchment is located in the south of the city. The catchment starts in the west and drains to the Avon-Heathcote estuary. The catchment includes the suburbs of Yaldhurst, Wigram, Hillmorton, Hoon Hay, Spreydon, Cracroft, Cashmere, Beckenham, St Martins, Opawa, Woolston and Ferrymead. The northern slopes of the Port Hills are part of the catchment.

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2.3.3 StyxRiver The Styx River catchment is located to the north of the city. It has two main tributaries, the Smacks Creek and Kaputone Stream, along with several other small waterways. The river originates in Harewood and flows through the suburbs of Belfast, Marshland and Spencerville before flowing into Brooklands and entering the sea at the mouth of the Waimakariri River.

2.3.4 Coastalcatchments There are also minor catchments draining directly to the sea or Avon-Heathcote estuary, which include the coastal areas around Southshore, Ferrymead, Bromley and South New Brighton. In addition Sumner catchment drains directly to the coast and comprises of the suburb of Sumner and surrounding slopes of the Port Hills.

2.3.5 WaimakariritoAshleyRivercatchment The Waimakariri and Ashley Rivers (both of which flow from west to east) have permanent flood stopbanks. The catchments between these two major rivers is largely rural, with the two medium sized towns of Kaiapoi and Rangiora and the coastal settlements of The Pines Beach and Kairaki located at the mouth of the Wamakariri. Areas in Kaiapoi experienced land damage in the September 2010 and February 2011 earthquakes. The Kaiapoi River has stopbanks to prevent flood and tidal inundation of adjacent property.

Figure2.3ȂRiverCatchmentsinChristchurchCity.TheWaimakaririRiverisshownatthetopofthe figure

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3 GeologicalprocessesoftheCanterbury Earthquakesaffectinggroundelevations

3.1 Introduction Canterbury experienced four major earthquakes (Magnitude 5.9 to 7.1) during 2010 and 2011. The four major earthquakes were: x 4 September 2010; x 22 February 2011; x 13 June 2011; and x 23 December 2011. These major earthquake events and the associated aftershocks are known as the Canterbury Earthquake Sequence. The earthquakes caused significant changes to the land in some locations due to a combination of tectonic and liquefaction effects. The geological processes which are relevant when considering Increased Flooding Vulnerability caused by the Canterbury Earthquakes are those that have caused a change to the ground surface elevation. The majority of change in ground elevation caused by the Canterbury Earthquake Sequence is due to a combination of four geological processes: 1. Tectonic ground movements (plate movements that may result in vertical tilting or horizontal movement); 2. Ejection of liquefied soil; 3. Lateral spreading; and 4. Volumetric consolidation of liquefied soil. These geological processes can be grouped into two general categories, tectonic effects (Item 1 above) and liquefaction effects (Items 2 to 4). There are also ongoing physical processes that may cause changes to flooding vulnerability, such as long term ground level and sea level change, however these are not related to the Canterbury Earthquake Sequence.

3.2 Tectoniceffects During the main earthquakes in the Canterbury earthquake sequence, the bedrock on one side of the fault moved relative to the other side of the fault. In some locations within the bedrock, several metres of slip occurred in the rock across the fault. Much of this movement was in a horizontal direction, but a portion of the movement was in a vertical direction. This slip in the bedrock causes folding of the overlying soil deposits, resulting in subsidence and uplift of the ground surface near the fault trace, as well as horizontal movement. This process is summarised in the simplified cross section of the Port Hills fault shown in Figure 3.1. The resulting ground surface displacements typically ranged between about 150 mm subsidence and 450 mm uplift, and up to about 500 mm of horizontal (in the direction of the fault) displacement.

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Figure3.1ȂSimplifieddiagramoftectonicgroundmovementacrossthePortHillsfaultintheFebruary 2011earthquake,showingtheverticalportionoffaultslipdisplacement

3.3 Liquefactioneffects The ground shaking from the major events in the Canterbury Earthquake Sequence triggered localized-to-widespread minor-to-severe soil liquefaction in many flat-land areas. Liquefaction is the process that occurs during an earthquake, where ground shaking causes the soil particles to be rearranged and the soil mass compacted and decreased in volume (Figure 3.2). During liquefaction the soil behaves more like a liquid than a soil. Sand boils, water fountains and ground cracking are surficial evidence that liquefaction has occurred (Figure 3.3). Liquefaction typically occurs in loose saturated fine grained soils such as silts and sands. These types of soils are often found adjacent to rivers and streams as floodplain deposits or estuarine deposits. Poorly compacted man made fills can also liquefy (ECan “The solid facts on Christchurch liquefaction”).

Figures3.2(left)LiquefactionprocessofsoildensificationandFigure3.3(right)Ejectionofwaterand liquefiedsoilcausingsandboilsandlandsettlement/subsidence(ECan“ThesolidfactsonChristchurch liquefaction”)

The damage caused by liquefaction was severe in a number of suburbs as illustrated in Figure 3.4. Ground subsidence resulting from ejection of liquefied soil, liquefaction-induced differential settlement, and lateral spreading were the principal ground deformation modes that damaged residential land on the flat (plains) in the Canterbury region (van Ballegooy et al 2014).

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Figure3.4ȂObservedliquefaction-inducedlanddamageanddwellingfoundationdamagedueto Christchurchearthquakes(fromvanBallegooyetal2014)

Figure 3.5 illustrates a summary of the typical patterns of liquefaction and lateral spreading observed as a result of the Canterbury Earthquake Sequence. In general the observed damage is more severe the closer the land is to a river or stream. This is due to the process of the soil types from geological deposition associated with these watercourses, and also because the unrestrained land is able to move sideways towards the channel. Both liquefaction and lateral spreading can result in subsidence of the land. Lateral spreading can also result in narrowing of waterways and bed heave and other changes to the shape and hydraulic capacity of the channel. Figures 3.6 and Figure 3.7 shows mapping of the liquefaction and lateral spreading observations following the September 2010 and February 2011 earthquakes respectively. Initially in the September 2010 earthquake the observed liquefaction was concentrated along the Avon River and small areas around local streams. In the February 2011 earthquake the observed liquefaction became much more widespread.

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Figure3.5ȂSummaryofthetypicalpatternofliquefactionandlateralspreadingobservationsduringtheCanterburyEarthquakeSequence,andcategoriesusedfor mappingvisiblelanddamageafterthemainearthquakeevents

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Figure3.6ȂMappingofliquefactionandlateralspreadingobservationsfollowingtheSeptember2010earthquake

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Figure3.7ȂMappingofliquefactionandlateralspreadingobservationsfollowingtheFebruary2011earthquake.Note:Thismapdoesnotshowobservedlanddamage inKaiapoiandNorthernsuburbs.LiquefactionandlateralspreadingdidinfactoccurinthisareaduringtheFebruary2011earthquake,howeveritwasobservedthat itwasnomoresevereinrespectofitsextentthanduringtheSeptember2010earthquake.Thereforenodetailedmappingwascarriedout.

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4 ChangesingroundlevelcausedbytheCanterbury EarthquakeSequence

4.1 Observation,measurementandassessmentofchangesto groundelevation EQC undertook an extensive programme of geotechnical observation, measurement and assessment to evaluate damage to insured residential land. Mapping of observed liquefaction and lateral spreading affecting residential properties was carried out immediately after the September 2010, February 2011, and June 2011 earthquakes to assess the extent and severity of the surface effects of liquefaction. This work is summarised in Land Damage Index maps, which are further documented in T&T (February 2013). Additionally, a detailed land damage inspection program was undertaken at 65,000 insured residential properties for insurance claim damage assessment purposes. EQC also undertook or obtained post-earthquake LiDAR ground elevation surveys and aerial photography, a comprehensive geotechnical site investigation programme and flood modelling. This work provides a high-quality dataset to assess how changes to the land have changed the vulnerability to flooding in Canterbury.

4.2 Totalchangeingroundelevation By comparing the ground elevation measured in the LiDAR surveys undertaken before the Canterbury Earthquake Sequence to the surveys undertaken after each of the main earthquakes, the difference between the ground level surveys can be calculated. Figure 4.1 presents this map of “LiDAR difference” from before the September 2010 earthquake to after the June or December 2011 earthquakes (survey coverage depends on location across the region). This shows ground subsidence of more than 0.5 m in some areas around waterways (mostly red zone areas). Not all changes in ground elevation shown in the LiDAR survey difference maps are due to the earthquakes. For example, many of the areas of blue shading in Figure 4.1 are locations where fill has been placed at some stage between the initial surveys in 2003 to 2008 and the final surveys in 2011 to 2012. In addition, some of the difference between the LiDAR surveys is due to the limits of accuracy of the measurement, rather than an actual change in ground elevation. For example, in the western part of Figure 4.1 a series of “error bands” can be seen as yellow bands running in a south-west to north-east direction, which correspond to the flight paths of the survey aircraft and the different error magnitudes for each “strip” of the survey. Some of the error bands are indicated by the arrows in Figure 4.1.

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Figure4.1ȂMapshowingelevationdifferencebetweeninitialLiDARsurveysundertakenin2003to2008 andfinalsurveysin2011to2012

4.3 Contributionsfromtectonicandliquefactioneffects As discussed in Section 3, the majority of the total change in ground elevation caused by the Canterbury Earthquake Sequence is due to the combination of tectonic and liquefaction effects. When assessing the patterns of ground subsidence across the region, it is useful to understand how each of these effects contribute to the total settlement. Tectonic ground surface movements (tilting) caused by the Canterbury Earthquake Sequence have been estimated by GNS (2012). These models are indicative only in their nature and were incomplete at the time of the primary author’s death. Therefore the limitations and accuracy of these models have not been fully documented. The estimated total tectonic ground movement from the four main earthquakes combined is shown in Figure 4.2. The maximum total tectonic ground subsidence is about 150 mm to the east of the CBD, and the maximum total tectonic uplift is about 450 mm at the Avon-Heathcote estuary. As shown in Figure 4.2, the ground displacement from tectonic effects is relatively uniform over large areas, with smooth and gradual transitions between areas of greater and lesser change in elevation.

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Figure4.2ȂIllustrationshowingestimatedtotalcombinedtectonicgroundmovementfromthefourmain earthquakesisshown(CGD2014).Approximateearthquakeepicentresareshownwithstarsymbolsfor theFebruary2011(red),June2011(blue)andDecember2011(pink)earthquakes.Theapproximate locationsofthesurfaceprojectionofthefaultruptureareshownwithpinklinesfortheFebruaryand June2011earthquakes.

Subtracting the tectonic changes in ground elevation (Figure 4.2) from the LiDAR difference map (Figure 4.1) generates the map shown in Figure 4.3. This map is indicative of the contribution of liquefaction effects towards the total change in ground elevation, although as discussed in Section 4.1 it also includes “error bands” and is limited by the accuracy of the LiDAR survey and the tectonic displacement model. Figure 4.3 shows that the ground displacement from liquefaction effects can vary significantly across short distances. This non-uniform ground displacement is related to the geological variability of the near-surface soils across the area and the proximity to rivers (affecting lateral spreading). Comparing Figure 4.3 with Figures 3.6 and 3.7 shows that the greatest inferred liquefaction settlements (from differencing between the LiDAR surveys) occurred in the areas where the most significant liquefaction was observed, generally around the Avon River, with the greatest settlement in Red Zone areas. Comparing Figure 4.3 with Figures 3.6 and 3.7 also highlights the effects of the limits of LiDAR accuracy. For example, settlement of about 100 - 200 mm is indicated in some areas to the west and northwest of the city even though there was no liquefaction observed in those areas. Much of the apparent difference between the LiDAR surveys is likely to be related to the limits of accuracy in the surveys rather than a real change in ground elevation. This emphasises the need for careful engineering judgement when using the LiDAR data, to recognise the accuracy limitations and ensure that it is considered in context with the variety of other information that is available such as observed land damage to confirm the occurrence of subsidence.

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Figure4.3ȂMapoftheindicativecontributionofliquefactioneffectstowardsthetotalchangeinground elevationbetweeninitialLiDARsurveysundertakenin2003to2008andfinalsurveysin2011to2012.As discussedabove,thismapalsoincludesnoisebandsassociatedwiththelimitsofaccuracyinthesurveys.

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5 Effectsofchangestogroundlevelsonflooding vulnerability

5.1 FloodinginChristchurch Flooding in Christchurch was a problem prior to the Canterbury Earthquake Sequence, refer to Figure 5.1 (Christchurch Drainage Board, 1989). As part of the assessment of potential IFV the pre-earthquake flooding has been assessed and found to be extensive, refer to Figures 5.2 and 5.3. The mechanisms that cause flooding in Christchurch are pluvial, fluvial and tidal flooding, refer to Figure 5.4 (which is further explained in Section 5.2). The management of flooding in Christchurch is summarised in T&T Volume 1 (2014). CCC uses Operative Variation 48 to the CCC City Plan to control activities and floor levels in flood prone areas, refer to Figure 5.5. CCC is currently reviewing the City Plan, including the provisions relating to natural hazards. CCC’s draft maps show a much greater areas for flood management.

Figure5.1PhotosoffloodinginAylesfordStreet(Flocktonarea)in1975(ChristchurchDrainageBoard 1989)

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Figure5.2(top)and5.3(bottom)showpreearthquakemodelledriverfloodandoverlandflooding respectively

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Figure5.4ȂWaysinwhichlandsubsidencecanchangevulnerabilitytoflooding: 1.Tidalflooding(top);2.Pluvialflooding(middle);and3.FluvialFlooding(bottom)

Figure5.5ȂFloodManagementAreasasidentifiedinCCCOperativeVariation48(CCC,2011)

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5.2 HowFloodinghaschangedasresultoftheCanterbury EarthquakeSequence 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 widespread prior to the Canterbury Earthquake Sequence, it has been increased in some areas by the earthquakes. A simple example of an increase in flood vulnerability as a result of settlement is shown in Figure 5.6. Note that Figure 5.6 is generally only representative in tidally affected areas. In upper catchments the flood depths are less influenced by land settlement as the flood level also decreases. In these locations there can be little or even no increase in flood depth on a property as a result of land subsidence. In some instances, there is actually a decrease in flood depths. This is further explained at the end of this sub-section.

Figure5.6ǦDiagramshowinghowgroundsubsidencefromtheCanterburyearthquakeshasmadesome propertiesmorevulnerabletoflooding(EQCFactsheetMay/June2014).

The three flooding mechanisms (refer to Figure 5.4) that cause flooding are listed below with explanations of how the earthquake has modified these mechanisms: x Pluvial flooding is caused by runoff that is in excess of the capacity of the stormwater systems and causes overland flow. It can be exacerbated in situations where settlement has occurred, as this settlement can change overland flow paths or reduce hydraulic gradients to stream/rivers. x Fluvial flooding is caused by flow in streams/rivers that exceeds the capacity of the channel and causes flooding of the adjacent land. The earthquakes have reduced the capacity of some stream/river due to lateral spreading, which has reduced widths and increased bed levels. Ground subsidence (particularly along stream banks) can increase the overflow from streams/rivers onto flood prone land, and can also result in inundation of previously flood- free land. x Tidal flooding is caused by extreme sea levels in coastal areas and lower rivers that cause flooding of adjacent land. Land settlement can make areas more prone to tidal flooding where the land settles to a level below tide levels if not protected. What this means at a property level is that some individual residential properties that previously were only exposed to infrequent flooding now have the potential to flood more regularly, whereas properties which had some existing flood vulnerability may have an increased area with potential to flood, or an increased flood depth due to this subsidence. Where the increase in flood vulnerability is due to on-site changes in ground levels this is covered by IFV. An increase in flood vulnerability due to off-site changes to streams/rivers and floodplains affecting the predicted flood levels is not covered by IFV, refer T&T Volume 1 (2014).

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Of note is that a change in land level does not necessarily mean an increase in flood depth. Figure 5.7 shows that near the sea the increased flood depths are roughly equal to subsidence as the extreme sea level is unchanged. Whereas in the upper catchments the flood depths may not increase by much as the land and the water levels drop by the same amount.

Figure5.7ȂSimplificationoftheeffectofsettlementoflandnearandfarfromtidalboundary.Notethe hydraulicgradientisapproximatedbytheslopeoftheground.

5.3 Floodmodellingusinggroundlevelsbeforeandafterthe CanterburyEarthquakeSequence Flood modelling was commissioned by EQC to assess flood levels for the catchment areas detailed in Section 2.2, refer T&T Volumes 2 and 3 (2014). This modelling was based on a 1% Annual Exceedance Probability (AEP) event. Flood modelling was undertaken for the ground levels before the Canterbury Earthquake Sequence (Figure 2.1) and then repeated for the ground levels measured after the December 2011 earthquake (Figure 5.8). By comparing the predicted flooding depths before and after the earthquakes, the change in flooding vulnerability can be assessed. The engineering assessment to determine whether an increase in flooding vulnerability has occurred is described in T&T Volume 1 (2014).

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Figure5.8ȂLiDARgroundelevationmodelofChristchurchandKaiapoifollowingtheDecember2011 earthquake.Elevationsarebasedontheheightabovemeansealevel.Greydenoteselevationaboveͼ

5.4 Floodeventsandrainfallfrequency

5.4.1 Introduction Rainfall data for five rainfall stations in Christchurch, with long observed records, were analysed to determine the temporal distribution of large rainfall events in the region and to add context for the recent flood events during 2013 and 2014.

5.4.2 Rainfallstationanalysis The six rainfall stations selected for analysis are listed in Table 5.1.

Table5.1.Rainfallstationsusedinanalyses

Station number Name Period of record Number of years 324610 Firestone Factory, Papanui 1981 to 2014 34 325507 College of Education 1964 to 2014 51 325616 Botanical Gardens 1962 to 2014 53 325617 Horseshoe Lake 1987 to 2014 28 325618 PS24 Sparks Road 1967 to 2014 48

Rainfall data at hourly time step were obtained for the stations. These data were accumulated to determine 1, 2, 6, 12, 24 and 48 hour running totals. The annual maxima were input to frequency

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 21 analyses to determine storm rainfall depths for return periods of 2, 5, 10, 20, 50 and 100-years based on an Extreme Value Type 1 distribution.

5.4.3 Temporalandspatialdistributionofstormrainfall The observed storm events that equalled or exceeded a 2-year 48-hour storms were plotted to show the temporal distribution of rainfall in Christchurch, refer Figure 5.9. The 5, 10, 20, 50 and 100-year storm rainfall depths are also shown on the plots to add perspective to the severity of the rainfall events. In addition to this predicted ‘flood events’ have been added to the Botanical Gardens gauge. This data is from the CCC (2014) presentation “Dudley Creek - Trial by Water”. CCC found that the 40- hour rainfall depths in excess of 75mm caused flooding in the Flockton area, which is why the analysis in Figure 5.9 also shows the similar 48 hour rainfall duration. This shows that the spatial variation of rainfall varies across the city i.e. high rainfall may occur in one part of the catchment, but this is not consistent across the whole city, which results in variability of flooding across the city for any given rainfall event. The predicted flood events by CCC have occurred on four occasions since the earthquakes. Prior to this the last event was in 1999 and prior to that in the early 1990’s. 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 22

325616 Botanical Gardens: Observed 48-hour rainfall

CCC 75mm 5-year 10-year 20-year 50-year 100-year Observed floods 180 160 )

m 140 m (

t 120 p e

d 100

l l a

f 80 n i a r

60 m r

o 40 t S 20 0 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

324610 Firestone Factory Papanui: Observed 48-hour rainfall CCC 75mm 5-year 10-year 20-year 50-year 100-year 180 160 )

m 140 m (

t 120 p e

d 100

l l a

f 80 n i a r

60 m r

325507 College of Education: Observed 48-hour rainfall

CCC 75mm 5-year 10-year 20-year 50-year 100-year 180 160 )

m 140 m (

t 120 p e

d 100

l l a

f 80 n i a r

60 m r

325617 Horseshoe Lake: Observed 48-hour rainfall

CCC 75mm 5-year 10-year 20-year 50-year 100-year 180 160 )

m 140 m (

t 120 p e

d 100

l l a

f 80 n i a r

60 m r

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 23

325618 PS24 Sparks Road: Observed 48-hour rainfall CCC 75mm 5-year 10-year 20-year 50-year 100-year 180 160 )

m 140 m (

t 120 p e

d 100

l l a

f 80 n i a r

60 m r

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 24

6 References

Christchurch City Council (2014) Dudley Creek - Trial by Water, WaterNZ Stormwater Conference

Canterbury Geotechnical Database (2014) https://canterburygeotechnicaldatabase.projectorbit.com/ Christchurch Drainage Board Wilson J (1989) Christchurch Swamp to City Christchurch City Council (2001) http://www.ccc.govt.nz/thecouncil/policiesreportsstrategies/districtplanning/cityplan/proposedv ariations/operativevariation48.aspx Environment Canterbury (undated) The solid facts on Christchurch liquefaction. http://ecan.govt.nz/publications/General/solid-facts-christchurch-liquefaction.pdf EQC Increased Flooding Vulnerability Damage Peer Review JOINT REPORT OF THE EXPERT PANEL, Final Report, April 2014. EQC Increased Flooding Vulnerability Damage Peer Review JOINT REPORT OF THE EXPERT PANEL, Final Report - Part 2 (Flood Modelling), 13 August 2014. EQC (2014) Increased Flooding Vulnerability Factsheet May/June GNS (2008) 1:250000 Geological map Geology of the Christchurch Area GNS (May 2012) Ground displacements and dilational strains caused by the 2010-2011 Canterbury earthquakes. GNS Science Consultancy Report 2012/67. S. van Ballegooy, P. Malan, V. Lacrosse, M.E. Jacka, M. Cubrinovski, J.D. Bray, M. EERI, T. D. O’Rourke, M.EERI, S.A. Crawford and H. Cowan (2014) Assessment of Liquefaction-Induced Land Damage for Residential Christchurch. Earthquake Spectra, EERI, 30(1). Tonkin & Taylor (2011) Christchurch Central City Geological Interpretative Report Tonkin & Taylor (February 2013), ref 52020.0300/v1.0 Canterbury Earthquake Series Flat Land Damage Apportionment Report Tonkin & Taylor (April 2014) Volume 1 Canterbury Earthquake Sequence: Increased Flooding Vulnerability Assessment Methodology. Tonkin & Taylor (August 2014) Volume 2: Increased Flood Vulnerability: River Modelling and Coastal Extensions Report Tonkin & Taylor (August 2014) Volume 3: Increased Flood Vulnerability: Overland Flow Model Build Report The Earthquake Commission Act (1993) Published under the authority of the New Zealand Government, Wellington, New Zealand.

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 25

7 Applicability

This report has been prepared for the benefit of Chapman Tripp on behalf of the Earthquake Commission (EQC) with respect to the particular brief given to us and it may not be relied upon in other contexts or for any other purpose without our prior review and agreement.

Tonkin & Taylor Ltd Environmental and Engineering Consultants Report prepared by: Authorised for Tonkin & Taylor Ltd by:

Mike Jacka Tim Fisher Senior Geotechnical Engineer Project Director Mark Taylor Senior Civil Engineer

MCNT t:\christchurch\tt projects\52010\workingmaterial\cat 9\reports & memos\geotechnical processes report\2014 08 14 geotechnical processes report_final.docx

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