Queensborough Flood Construction Level (FCL) Review PHASE 1 REPORT. Submitted By:

Transcription

1 Queensborough Flood Construction Level (FCL) Review PHASE 1 REPORT Submitted By: EB January 2013

2 1. SUMMARY INTRODUCTION STUDY AREA FLOOD PROBABILITY FLOOD CONSEQUENCE SCENARIOS AND RESULTS DISCUSSION CONCLUSIONS EB3774 January 2013 i

3 1. SUMMARY The Queensborough neighbourhood lies within the floodplain of the Fraser River and is subject to both river freshet and, increasingly, to storm surges from the ocean. With a typical ground elevation of 1 metre (Geodetic Survey of Canada - GSC), the natural grades frequently lie below the surrounding water levels during high tides. During extreme events, the water levels can rise over 2 metres above the natural grades. Developed properties are protected by a perimeter ring dike and by constructing the livable floor space above Flood Construction Levels (FCLs). The dikes protect to a level of approximately 3.5 metres (GSC) while the FCLs vary between homes that predate the requirement for FCLs and were built approximate at existing grades (as low as 1 metre GSC) to 4.2 metres for newer subdivisions near the east side of Queensborough. Currently, 80% of the value of property improvements sit above 3.5 metres. The elevation of 3.5 metres was set by the province to match the regulatory flood profile of the Fraser River. As a general strategy, Queensborough residents can be protected by the ring dikes, FCLs, or both. This study focused on FCL strategies in residential neighbourhoods only, and assumed that the ring dikes will at least be maintained in their current state. Although the ring dikes provide the primary defense, FCLs provide a secondary level of protection in case the ring dikes are breached or overtopped. In such instances, the level that habitable space is built to will impact the risk to property and life. Four FCL strategies were reviewed to assess their impact to economic and safety risks. The strategies were: 1. Current Practice (subdivisions build to current FCL (3.5m or 4.2m), reconstructions build to 1.5m or 0.15m above the adjacent road) 2. Intermediate Protection (all new homes to 3.5m or 4.2m) 3. Increased Level (all new homes to 4.7m) 4. Relaxed Uniform (subdivisions build to current land elevation or 1.5m or 0.15m above the adjacent road) Two significant factors that arose in the evaluation are the expected increase in property value and sea level rise. The increased property value means that even strategies that improve the protection offered by FCLs could result in more damage because of the additional development at risk. Current Provincial guidelines require municipalities to plan for 1.0m of sea level rise by the year This substantially impacts the results of the assessment of the various FCL strategies. EB3774 January

4 2. INTRODUCTION Delcan was retained by the City of New Westminster to help develop an approach to the Flood Construction Levels in the Queensborough area. Flood Construction Levels (FCL) are the minimum height that new development is required to build to in order to protect the new development from flooding damages. The City is currently preparing a Community Plan for Queensborough and a key aspect to the plan will be implementation of the City s approach to FCL. Two key questions that need to be understood are: 1. What level of FCL should the City use? 2. How can this be applied without conflicting with other community needs? The project will be completed in two phases, one to answer each of the above questions. 2.1 Scope In this report we provide an overview of the current FCL scenario in Queensborough, estimate the probability of a flood, both today and in the future, and quantify the consequences of a flood on the residential properties in Queensborough. The results are presented so that the decision makers in New Westminster can make an informed choice as to the FCL to apply and the level of flooding risk to accept. Floating homes are not part of this study. 2.2 Application of FCLs in Queensborough Flood Construction Levels (FCL) are regulations applied by local authorities to new development requiring structures to build habitable space at or above a specified elevation. They are sometimes referred to as minimum building elevations or dry flood proofing levels. In Queensborough, the FCL was originally set by the Province and has since been adopted by New Westminster. Reasons to apply FCLs are: a. To provide protection from flooding in areas not protected by perimeter defences; b. To provide a second tier of defence to perimeter defences; and c. To provide protection against internal flood events. Because the vast majority of Queensborough is protected by dikes, the value of FCL is b and c above. Currently, the FCL is 3.53 (or 4.2 metres east of Boyd Street) above mean sea level as per the Geodetic Survey of Canada (GSC). These levels are based on the 2008 update of the Fraser River model by the Province which modeled the regulatory flood event in the Fraser River. Presently, construction to the FCL is required for all new subdivisions. Typically this is applied during a planning process which involved lot creation such as a subdivision process. Reconstruction on previously existing lots is not required to be built to the previously established FCL but instead to the minimum EB3774 January

5 building elevation in the Building Bylaw (1.5m GSC or 150mm above the road centre line, whichever is greater). 3. STUDY AREA The study area includes the neighbourhood of Queensborough which is on the eastern end of Lulu Island. The main Fraser River Annacis Channel is to the south and the North Arm of the Fraser River is to the north. On the west, Boundary Road defines the border between the City of Richmond and the City of New Westminster. The focus of the study is the residential properties. No consideration is given to commercial, industrial and agricultural properties. The flood damages discussed do not include damage to city infrastructure and public buildings or the economic disruption consequences of a major flood event. 3.1 Existing Dike Queensborough is protected by a shared perimeter dike around Lulu Island. The City, as the Provinciallyregulated Dike Authority, is responsible for dike management and flood hazard land use decisions. The perimeter dikes were constructed or upgraded during the Fraser River Flood Control Program which ran between 1968 and These dikes typically have 3.5 to 4.0 metre crest widths, 2.5:1 to 3:1 side slopes and incorporate 0.6 metres of freeboard over the designated flood level (at the time of construction). In Queensborough, the dikes typically have crest levels between 3.8 and 4.2 metres (GSC). Figure 1 shows the dike crest levels for Queensborough from the 2007 dike survey, including areas where crest deficiencies are noted. Other deficiencies, such as poor soils that increase the risk of dike breaches, are not as readily observable and detailed construction records do not existing to determine subsurface conditions. EB3774 January

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7 3.2 Existing Housing Types Queensborough is a mix of industrial, commercial and residential development as well as some agricultural area. The focus of this study is to determine an approach to residential FCL so other land use and infrastructure was not included. The residential buildings in Queensborough were visually inspected in August and September Approximately 1350 buildings were classified. The purpose of the inspections was to classify each building based on its elevation to provide an inventory of the different building classifications to assist in flood protection planning. The inspections were done by visiting each residential street in Queensborough and visually inspecting each building. A map with GIS data for each of Queensborough s buildings was used to locate the residential buildings. One of the following classifications was assigned to each residential building: Classification A Living space at a minimum elevation of 3.53m (Figures 2, 3, and 4) Classification A 4.2 Living space at a minimum elevation of 4.2m (Figure 5). These homes are located in the east of Queensborough and are typically elevated by using fill. Classification B Living space elevated from road, typically 1.0m above road (Figure 6) Classification C Living space at approximately the same elevation as the road (Figure 7) Classification R Site currently under development Outside Dike Building outside of dike, typically along South Dike Road Most buildings in Queensborough are well described by the characteristics above and are easily classified. Those building which have been built to meet the FCL were often easy to recognize because of their unique building type. The numbers in each category are meant to provide a good estimate of the percentage of each building type and be sufficient to assess the risks of flooding and expected damage for a given return period flood. Some photos of example buildings in the categories are shown below. Figure 2 Example of house at 3.53 m GSC Figure 3 Example house at 3.53 m GSC EB3774 January

8 Figure 4 Example house at 3.53 m GSC Figure 5 Example house at 4.2 m GSC Figure 6 Example house at 1.0 m above road Figure 7 Typical house level with road Table 1 below summarizes the number of homes in each category. Figure 8 shows a colour coded inventory of the existing homes in Queensborough. Table 1: Summary of Building Classification Classification Description Number of Buildings A Living space at a minimum elevation of 3.53m 656 A4.2 Living space at a minimum elevation of 4.2m 187 B Living space 1.0m higher than road; 107 C Living space at the same elevation as the road 371 R Site Currently Under Development 4 Outside Dike Building outside of dike 19 EB3774 January

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10 4. FLOOD PROBABILITY The risk of flooding is a combination of the probability that something will happen and the consequences (damage) when it does. Flooding Risk is defined as the product of probability times the consequence: Risk = Probability x Consequences In Section 3 the probability of flooding is discussed and Section 4 quantifies the consequences. There are two main mechanisms for flooding from the Fraser River: Breach and Overtopping. Breaches occur when a dike fails even though the exterior water level hasn t reached the top. The chance of a breach generally goes up as the exterior water level goes up. Overtopping occurs when the water levels exceed the crest elevations of the dike. Overtopping will often cause a breach once water begins to flow over a dike. Both breaches and overtopping have a potential to cause flooding damages. Risk of earthquake induce damage to dikes causing flooding has not been included in this report. 4.1 External Flood Levels Current Conditions The Fraser River around Queensborough is subject to high water level from Fraser River freshets and ocean storm surges. Historic flooding and water level data has been used to determine the water levels in various return period events. The flood elevations are higher to the east and lower to the west but for the purpose of this high level study a single value is used to represent flood levels in New Westminster. The regulatory flood is based on the 1894 flood of record and this event has an estimated return period of approximately once every 500 years Climate Change The Province has recently released new guidelines on potential sea level rise in British Columbia titled Climate Change Adaption Guidelines for Sea Dikes and Coastal Flood Hazard Land Use - Draft Policy Discussion Paper (Ausenco Sandwell 2011). For vertically static areas, the report recommends that affected areas allow for a 1.0 meter rise in sea level by the year They also recommend an increase of 0.5 metres be considered for Table 2: Flood Water Levels in the Fraser River Return Period Water Level (GSC) (years) EB3774 January

11 4.2 Chance of Overtopping The dikes in Queensborough have crest elevations between 3.8 and 4.2 which include 0.6 metres of freeboard. The freeboard is intended to account for waves during high water. There are a few isolated sections of dike that are lower than the regulatory level and therefore it was assumed that a 200 year freshet or storm surge flood would overtop the dikes. It is assumed the overtopping would be a significant flow rate and as a result the inside water level would match the outside water level. 4.3 Chance of Dike Breach A dike crest does not need to be overtopped for a dike to fail. A number of failure mechanisms could cause a breach. Figure 9 shows three typical failure types. To determine the susceptibility of a dike to a breach failure requires a geotechnical site investigation followed by a checking of each possible failure mechanism. The data does not exist to determine the potential failure mechanisms for the Queensborough dikes so the chance of breach failure was estimated by other means. Figure 9: Some types of dike failures Anecdotally, there have been two recent dike breaches in the Queensborough dikes in the last ten years. One was caused by a watermain break within the dike, and one was caused by poor quality backfill around a pump station. Neither of these breaches occurred during high water levels so flooding was limited. In The Netherlands, there is a detailed health check of the diking system every five years. The assessment program is now in its third cycle. From these health checks there is a better understanding on governing types of failure mechanisms. The failures from breaches exceed those from overtopping. In general, pipe and landside stability are the leading deficiencies in Dutch dikes. Two site specific studies on a large diked area indicated that breaches were about five times more likely than overtopping. Unless you collect geotechnical data and carries out a probabilistic assessment of the Queensborough dikes, one cannot quantify the precise probability. There are different soil conditions, construction techniques, inspection procedures and hydraulic loading. However, in order to make a relative comparison EB3774 January

12 of FCL scenarios some breach events need to be considered. Based on the limited information available, it has been assumed that a breach is approximately five times more likely than an overtopping event. The higher the event, the higher the hydraulic loading and the higher the chance of breach. For this project the 5 and 50 year breach events are used. We have assumed that a 5 year event has a 10% chance of causing a breach and a 50 year event has a 50% chance of causing a breach. Improving the accuracy of this assumption would require extensive investigation and study and even after that, the assumption would still have uncertainty. EB3774 January

13 5. FLOOD CONSEQUENCE The understanding of the full consequence of a flood event is an essential part of making decisions about flood protection measures. Typically, the consequences of flooding can be defined as: economic damage; number of casualties; and damage to natural and heritage features (ecosystems, archaeology). For this study, only economic damage and number of causalities have been considered for residential properties. 5.1 Flood Damage The methodology used for this flood risk assessment was developed by The Netherlands Ministry of Transport, Public Works, and Water Management. This method determines the consequences of a large flooding event in terms of economic damage, casualties, and social and environmental effects. Figure 10 below shows the information requirements to determine flood damage during an event. Figure 10: Flood Damage Methodology EB3774 January

14 Damage functions relate the flood inundation depth to a percentage of the maximum potential damage. Damage functions were based on actual flood damage values and research from projects within the US and The Netherlands. The damage function for low-rise dwellings is shown in Figure 11. The damage factor relates the depth of flooding to damage by providing the percentage of maximum damage that occurs at the flood depth. Figure 11: Damage Function for Low rise dwellings Delcan combined the GIS property information, contour data and the results of field work on estimated building levels to determine the level of the habitat space of each home. The value of the home was obtained from the BC Assessment data provided by New Westminster. Data gaps were few but where they existed, assumptions based on Queensborough averages were used to fill in the blanks. The City also provided Delcan with currently known development that is in the planning process. These areas are already in the process and deemed significant enough to the overall totals that they were manually put into the data as already developed. These properties could be impacted by a change in FCL policy made today and as a result they have been included in the future scenarios as development but not in the 2012 data. This provided all the information shown in Figure 10 that allows for the quantification of flood damage during a particular event. EB3774 January

15 5.2 Risk to Life There are a number of papers that have attempted to quantify the fatalities of flood events. There are a number of main factors that influence mortality of a flood event including: Water depth; High flow velocities; The combination of larger water depths and rapid rise of waters is especially hazardous; Timely warning and evacuation; The possibilities for shelter within the community; If the flood induces the collapse of buildings; and Characteristic of population. There are a few quantification methods based on above characteristics. For this project we have selected the method based on the statistical analysis of almost 2000 historical floods performed by Duiser and Waarts. The formula related height of flood (h) to the deaths caused (F D ). This method was used to calculate the number of deaths. The height of water was taken to be the level of water above the living space of a dwelling. Figure 12 below illustrates the mortality equation above. During depth less than 1.5 metres the mortality rate is low. As the depth increases the mortality rises at a faster and faster rate. Figure 12: Flood Depth vs. Mortality EB3774 January

16 6. SCENARIOS AND RESULTS Delcan, through discussion with New Westminster, developed a number of scenarios to compare different approaches to flood construction levels. The damage and casualties are calculated for each scenario for 2012, 2050 and A key component in the analysis is the development/redevelopment rate. The 2012 values are the same for each scenario but for the 2050 and 2100 timeframes the elevation of the properties changes depending on the implementation of FCLs. When redeveloping, properties build to the new FCL. A long term development/redevelopment rate of 1% was used for all scenarios and all development types. Although this is a lot lower than the current rate it is thought that the current high pace of development will soon slow as greenfield or large scale subdivision type development opportunities are used up. The 1% range represents average development rates to 2050 and With redevelopment, the value of properties within Queensborough grows. When larger lots develop they go from having one home to multiple. As this is applied over the whole neighbourhood, the total value of the homes in Queensborough increases about 40%. Some other assumptions include: Existing grades used for the year 2012 of each scenario; For properties that develop, the future FCL is the greater of the land elevation or the FCL. In the case of relaxing FCLs, building elevations can go down but not lower then land elevations. Population is 3.1 people per home/unit and for future scenarios the population is increased with development of new homes/units. 6.1 Scenarios Five scenarios were run for the study. One current practice, two higher levels of protection, one lower level of protection and one current practice without sea level rise Scenario 1 Current Practice Presently the implementation of FCLs is for all properties that undergo a subdivision process. The minimum size for a property is 4000 square feet (743 square metres). Therefore, any single family lot 8000 square feet or larger has the potential to subdivide into two or more lots. For the modeling in this scenario, it is assumed that when redevelopment occurs on large lots they will subdivide and therefore be required to build to the FCL. For lots smaller than 8000 square feet when the houses redevelop they build to either 0.15 metres above the road, 1.5 metres elevation or to existing level, whichever is greater. The table below shows how the application of this FCL policy changes the value of homes in each elevation category over time. EB3774 January

17 Table 3 Property Value % Total Value of Homes Year > 3.52m Elev 3.52 > x > 1.49 < 1.49m % 7% 10% $362,246, % 6% 7% $414,233, % 4% 7% $517,535, Scenario 2 Intermediate Protection The second scenario uses the same FCL level as the first scenario but applies it to every property as they rebuild or redevelop. This includes single lot rebuilds and would require building to the FCL to become a future requirement. Table 4 Property Value % Total Value of Homes Year > 3.52m Elev 3.52 > x > 1.49 < 1.49m % 7% 10% $362,246, % 3% 5% $414,233, % 0% 3% $517,535, Scenario 3 - Increased FCL This scenario presents the case to raise the FCLs to account for future sea level rise. Like Scenario 2, the FCL, in this case 4.7 metres, would apply to all redevelopment and rebuilds. Table 5 Property Value % Total Value of Homes Year > 3.52m Elev 3.52 > x > 1.49 < 1.49m % 7% 10% $362,246, % 3% 5% $414,233, % 0% 3% $517,535, Scenario 4 - Relaxed FCL Scenario 4 presents the results if the FCL was relaxed to only require houses to be 1.5 metres, or 0.15 metre above the road. This would result in some existing properties being built lower at time of rebuild. Table 6 Property Value % Total Value of Homes Year > 3.52m Elev 3.52 > x > 1.49 < 1.49m % 7% 10% $362,246, % 37% 5% $414,233, % 67% 3% $517,535,200 EB3774 January

18 6.1.5 Scenario 5 Current Practice and No Sea Level Rise This scenario was included to illustrate the impact of sea level rise on the increasing damage. The debate on global sea level rise is not if it is happening but only what the rate of the increase will be. This example was included to demonstrate the significance of sea level rise on FCL policy. The results of this scenario are discussed more in the subsequent sections of this report. Table 7 Property Value % Total Value of Homes Year > 3.52m Elev 3.52 > x > 1.49 < 1.49m % 7% 10% $362,246, % 6% 7% $414,233, % 4% 7% $517,535, Results The scenarios were each run through the flood damage and mortality methods discussed above. Each scenario was run for a 5 year, 50 year and 500 year event. They were also run for current conditions, conditions at 2050 and conditions at Below is a summary of the results. FCL Scenario 1 Name: Current Practice (FCL only applied to subdivided properties) FCL Level: year 50 year 500 Year Damage Mortality Damage Mortality Damage Mortality Year 2012 $13,000,000 6 $ 16,000, $25,000, $15,000, $ 31,000, $61,000, $33,000, $ 71,000, $104,000, FCL Scenario 2 Name: Intermediate Protection FCL Level: year 50 year 500 Year Damage Mortality Damage Mortality Damage Mortality Year 2012 $13,000,000 6 $16,000, $25,000, $10,000,000 7 $26,000, $56,000, $23,000,000 6 $61,000, $90,000, EB3774 January

19 FCL Scenario 3 Name: Increased FCL FCL Level: year 50 year 500 Year Damage Mortality Damage Mortality Damage Mortality Year 2012 $13,000,000 6 $16,000, $25,000, $10,000,000 7 $20,000, $38,000, $8,000,000 3 $15,000,000 5 $25,000, FCL Scenario 4 Name: Relaxed FCL to 1.5 metres or 0.15 above road FCL Level: year 50 year 500 Year Damage Mortality Damage Mortality Damage Mortality Year 2012 $ 13,000,000 6 $ 16,000, $ 25,000, $ 34,000, $ 52,000, $ 84,000, $ 88,000, $128,000, $ 174,000, FCL Scenario 5 Name: Current Practice and No Sea Level Rise FCL Level: year 50 year 500 Year Damage Mortality Damage Mortality Damage Mortality Year 2012 $13,000,000 6 $16,000, $25,000, $12,000,000 5 $14,000,000 9 $24,000, $11,000,000 4 $13,000,000 7 $24,000, EB3774 January

20 7. DISCUSSION 7.1 Probability of Events The results above can be hard to put into context simply by viewing the numbers. It can help to express the events in terms of the chance of occurring during a 70 year span, representing the timeframe of a longtime resident. Sections of the dikes are currently constructed to a height equivalent of a once in 500 year freshet event from the river, and a 200 year probability storm surge event from the ocean. Assuming the dikes are improved at the same pace as sea level rise, then this equates to a combined probability of an overtopping event once every 142 years. Similarly, a 50 year freshet or storm surge has a combined probability of occurring once every 25 years. However, a breach is estimated to occur only during half of such events and no overtopping would occur. The resulting probability of occurrence is once every 50 years. A smaller 5 year freshet or storm surge has a combined probability of occurring once every 2.5 years. However, a breach is estimated to occur during 5% of such events. The resulting probability of occurrence is once every 50 years. Therefore, an individual has an approximately 40% chance of experiencing a major overtopping flood event over a 70 year period. The chance of experience a dike breach with a more minor flood is higher. 7.2 Visualization of Events Another important tool for understanding flood risk is to be able to visualize what that event might look like. Flood events are rare occurrences and often residents who have not experienced flooding have trouble imagining it as a possibility in their neighbourhood. The next six pages show the flood levels for the 5 year, 50 year, and 500 year events rendered onto a series of typical Queensborough type homes. A no flooding image is also shown for reference. The homes represent homes constructed to the current FCL, both on fill and raised on structure, and homes below the FCL. EB3774 January

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27 7.1 Trend in Damage Related to FCL To better understand the trends in the flood damage results it is important to understand the following points: Damage function does not increase sharply with depth As shown in Figure 11 earlier, the damage function does rise quickly with increasing depth. For example a 0.5 metre flood causes 12% of the maximum damage while a 1.0 metre flood causes 18% damage. The damage rate increases as flood depths exceed 2.0 metres as after that point upper floors are being damaged, as well the overall structure of the house is at greater and greater risk. For most scenarios modeled the flood depth is occurring in the lower portion of the damage curve where damage does not increase as significantly with depth. So that means that low flood depths covering large areas cause more damage than flooding smaller areas to higher depths. This is evidenced by the relatively same difference between 5 and 50 year events compared with that of the 500 year event. For example, in the 2012 time frame (common for all FCL scenarios) the damage in 5 and 50 years events is $13 and $16 million respectively while the 500 year scenario is $25 million. The trend is generally seen in the data through many scenarios Property Values Increasing Over Time Over time undeveloped properties develop. Lower density properties become higher density properties. This results in an overall increase in the value of structures susceptible to flood damage from 2012 to This and sea level rise, discussed later, are the two main reasons that the damage usually increases over time, even for progressive FCL scenarios. The FCL scenario may be raising properties and reducing flood depth but the increased values means that damages increase. For example, a house on a lot larger than 8000 square feet may flood to a greater depth but the house value may be only $300,000. When the property subdivides and develops into two houses with suites, those houses experience a lower depth of flooding (because the homes will be built to the FCL) but the total value of those houses is more than four times the original property. This, combined with sea level rise and the shape of the flood damage curve leads to non-linear damage values over time (2012, 2050 and 2100) and over return periods (5 year, 50 year, and 500 year) Sea Level Rise One of the main findings of the study is that sea level rise is a major factor in flood risk for Queensborough. The increasing level of the sea means that a 500 year flood level today will be the same as a 5 year flood in That is why FCL scenarios based on 3.5 metres have little effect in reducing damage in 2050 and EB3774 January

28 7.2 Trends in Mortality Related to FCL Mortality is an exponential function. At low depths the risk of death is relatively low but rises quickly once the depth increases. For example, the mortality rate for a 0.5 metre depth flood is about one out of every 850 people while a 2.5 metre flood has a mortality rate of one out of every 80 people. This relationship means that the higher mortality figures are those associated with greater depths, not necessarily the greater number of people. This is why in some scenarios although the damage is increasing the mortality is going down. For example, in Scenario 2 (current FCL but applied to all redevelopments) the 50 year flood in 2100 is more than double that of However the mortality over the same period of time drops from 16 to Adaption to Sea Level Rise As the results indicate, the single most significant factor in changing potential for flood damage and fatalities is sea level rise. Sea level rise is an accepted phenomenon but also surrounded by many uncertainties. The current rate of rise can be estimated based on recent trends but the longer into the future you look the less consensus there is on what will occur. This change is expected to occur in a relatively linear manner, at least in the short term. Annual rates for sea level rise estimates range from 4 mm/year to 13 mm/year. Using FCLs to adapt to sea level rise has many more challenges because FCLs only get implemented at time of (re)development. This means that if, in the future, additional information on sea level rise becomes available, it is not practical to make adjustments until properties redevelop. Conventional perimeter defences such as dikes or floodwalls are much simpler to raise to keep up with sea level rise. 7.4 Applying FCL to home Re-builds One common element to Scenarios 2 and 3 was the application of FCLs to all homes, not just those involved in a subdivision or rezoning. This was applied to these scenarios because of the trends seen on the overall mortality from flood events. The existing low properties are the source of the greatest risk to life. By requiring them to apply the FCL when they redevelop the overall risk in the community is improved. This can be seen by comparing Scenario 1 (Current Practice) and Scenario 3 (High Uniform). For Scenario 1 by the year 2100 the mortality is 53 for the 500 year event. Using the same FCL (3.5 metres) but simply requiring all properties to meet the level when they redevelop reduces the mortality to 19 in Scenario Reliance on Perimeter Dikes The current FCL approach in many other communities is that the FCL forms the primary flood protection measure. In this scenario, the goal is to remove all properties from the flood risk as they redevelop by raising them above the flood level. Figure 13 shows that those properties that are constructed above the EB3774 January

29 FCL are protected from flooding even if the perimeter dike is non-existent or ineffective during the design event. The existing properties that are not constructed above the FCL are left at risk until they redevelop. Figure 13: FCL as primary protection At the other end of the spectrum is a system in which the publicly maintained perimeter defences are expected to provide all required flood protection. This is the situation in The Netherlands, where external barriers are designed and maintained to a high standard. Figure 14 illustrates this configuration. Property elevations are constructed to be protected only from internal drainage events such as a large rainfall event or pump failure. In this system, because the dike is maintained by the municipality, part of the tax base and/or development fees are used to protect all properties. If there is a failure of the perimeter dike system, the properties behind the dike are at risk. Figure 14: Low FCL and relying on perimeter defences The backbone of this approach is a strong reliable perimeter defence system. Perfect perimeter defences that never fail are impossible and there is always some risk of failure. However, an improved reliability could be obtained by conducting regular health checks of the existing dikes and structures. Currently, New Westminster preforms annual visual inspections and notes deficiencies. However, this program could be expanded to include geotechnical testing, field survey, and engineering analysis. All potential failure mechanisms could be checked and where deficiencies were noted, those sections of dikes could be repaired. There are also the challenges associated with raising the existing dike in limited horizontal space to expand. EB3774 January

30 7.6 Risk to Life Making decisions about the level of risk to life is a decision that must be made by a community (and their elected representatives). The information presented in this report is intended to provide Queensborough with an indication of the level of risk to life. With this information, current investments and decisions within the context of flood defence and mitigation measures can be better developed and evaluated. Based on information regarding future flood risks it can be decided what risk levels are acceptable or whether additional risk reducing strategies are needed. 7.7 FCLs in other Jurisdictions Flood construction levels are regulations applied by many local authorities to new development requiring structures to be built at or above a specified elevation. In some locations they are sometimes referred to as minimum building elevations or dry flood proofing levels Corporation of Delta In areas throughout Delta, FCLs are currently specified for new developments. Table 8 summarizes the minimum building elevation for some areas of Delta. Table 8: Flood Construction Level Area Elevation Ladner Tilbury Island Agriculture areas Beach Grove Boundary Bay Airport Boundary Bay Village 1.6 m 2.74 m to 3.04 m 2.74 m to 2.9 m 1.6 m 2.9 m 1.6 m to 2.9 m The regulatory flood level for most of Delta is 2.9m with dikes constructed at 3.5m to include freeboard. The application of lower FCLs in some of the existing urban communities represents an approach that implies an acceptance that FCLs in those areas do not represent the primary level of protection. Two important differences should be noted between Delta and Queensborough: Delta has large flat agricultural areas; and Delta s flood risks are for the most part tidal and the peak would only last for a few hours compared to days or weeks in a Fraser River Freshet. EB3774 January

31 Because of the large flat agricultural area that surrounds the urban areas in Delta, a flood caused by a breach has the potential to spread over underdeveloped land and therefore not reach as high of a depth (unlike Queensborough). The tidal nature of the flood would mean that after a breach, there would be a period of low water where repairs could be made and pumping could lower the internal water level City of Richmond Similar to the Corporation of Delta, the City of Richmond has, within their floodplain bylaw, the requirement to develop to the FCL which is set based on the regulatory level of the Fraser River. It is 3.5 metres where Richmond borders Queensborough. However, in the urban areas in the west of Richmond they allow for exceptions and in those areas the properties are only required to build to the 0.3 metres above the crown of the road. As with Delta, one of the differences in Richmond is that large agricultural areas to the east could serve as flood storage and lessen the chance that a flood event would equal the outside flood level in the river. In Queensborough there are a number of barriers both existing and proposed in the form of road infrastructure and raised development that would not allow the water to spread if there was a breach. 7.8 Limitations This study has been prepared for the City of New Westminster to aid in making decisions about how to apply FCLs in Queensborough. The damage and mortality numbers were determined using the methodology outlined in this report. Although they are suited for the relative comparisons of options, they are not designed for making absolute decision on investment cost-benefits. The damage values do not include damage to road, infrastructure and non-residential structures. They also do not include economic disruption to the neighbourhood and adjacent communities. The mortality figures do not include the potential deaths in illegal suites constructed below the FCL. These suites likely represent a high risk to life as they are some of the lowest properties in Queensborough and may not meet other requirements for means of escape. 8. CONCLUSIONS The value of property improvements and the number of people residing in Queensborough is rising steadily. The majority of the living space is already above current flood design levels of 3.5m. The remainder could be raised above current flood levels through the use of FCL strategies. The probability of a major flood that would overtop the dikes is approximately 1 in 142, or 0.7% every year. Over 70 years, this equates to a 40% chance of experiencing a major flood that would see approximately 20% of the houses inundated to depths up to 1.7m. The expected damage in such an event to residential properties only is $25 million, with a potential loss of life 20. EB3774 January

32 Similarly, it is more likely that a smaller return period event would result in a dike breach with lower levels of flooding. For example, a 1 in 50 year event or a 1 in 5 year event is around four times more likely to occur, and would result in damages of around $10 to 15 million and a potential loss of life of Over 70 years, this equates to a 76% chance of occurrence. With an expected sea level rise of 1.0m over the next 100 years, as estimated by a Provincial study, the depths of inundation and the risk of overtopping the existing dikes rises significantly. For example, a current 500 year event would be equivalent to a 5 year event by the end of the century. Such a rise can be addressed with higher dikes, higher FCLs, or both. Under all FCL strategies except Strategy 4 (relaxed FCL), the level of risk to the community would decrease if it weren t for sea level rise. Factoring in sea level rise, the risk to the community increases for all strategies except Strategy 3 (Increased Level). However, the risks could be offset by dike improvements instead of increased FCLs but with sea level rise the level would need to be continually raised. The risks to life could also be reduced by emergency warning practices. EB3774 January