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Village of Zeballos December 21, 2018 Zeballos River Floodplain Modernization & Future Landslide Risk Assessment Project No.: 1849001

Zeballos Landslide and Flood Risk Assessment.docx Page 54

BGC ENGINEERING INC.

4.0 RISK ASSESSMENT




4.1. Introduction

Risk is a measure of the probability and severity of an adverse effect to health, property or the environment, and is estimated by the numerical product of hazard probability and consequences (Australian Geomechanics Society (AGS), 2007). The geohazard SQRA completed for this study involved identification of geohazards and estimation of the likelihood that a geohazard event will occur, impact an element at risk, and cause some magnitude and type of damage or loss. The principal steps in this semi-quantitative risk assessment are:

1. Identification of geohazard scenarios. 2. Estimation of the likelihood that a geohazard scenario will result in some undesirable

outcome in the categories of economics, environment, company reputation, and human safety.

3. Estimate the economic, environmental, and safety consequences of the unwanted outcome.

4. Combine the likelihood of unwanted outcome and its consequences to arrive at a risk classification ranging from Very Low to Very High.

Risk estimates considered in this report represent the present case (i.e., consider the existing mitigation measures), which is a necessary assumption to estimate geohazard risk for the purpose of prioritizing mitigation measures should those be contemplated by the Village in the future. The risk estimates do not consider every possible consequence resulting from a geohazard occurrence. Rather, the estimates consider a range of potential outcomes that guide the prioritization of eventual risk reduction measures.

Figure 4-1 shows the risk evaluation matrix used to combine likelihood of unwanted outcome and consequence assessment to determine a risk rating for flooding and slope hazards, respectively. The probability of the undesirable outcome and the severity of the consequence define an intersection point in the matrix that ranks the risk scenario from “Very Low” to “Very High”. The risk ranking of all sites can then be used to prioritize risks for potential further study or more detailed design of risk reduction measures.

The top five rows of Figure 4-1 guide possible responses by the Village to each risk level, but depend on the Village’s risk tolerance criteria. In BGC’s experience, most clients choose to plan for mitigation measures for “High” and “Very High” risks and address “Moderate” risks where practical and cost-efficient.

BGC evaluated flooding hazards at the present time and in the year 2050-2100, which considers increased sea level and greater precipitation. Likewise, BGC evaluated slope hazards both pre- and post-fire.

It should be noted that Figure 4-1 represent a semi-quantitative assessment of risk and differ from the QRA conducted by BGC for FLNRORD (Appendix I). That work addressed safety risk specifically for buildings in a defined area that were evacuated near A, B, and C creeks on the east side of the Village. The QRA addresses individual buildings and its results are significantly




impacted by both the assumed number of residents in each building and whether the house is occupied on a full or part-time basis. The SQRA on the other hand takes a broader approach to risk evaluation and does not consider factors such as the number of individuals in specific buildings. This reasoning is described in more detail in the following sections.

Figure 4-1. BGC’s semi-quantitative risk matrix for geohazard risk assessments.

4.2. Risk Scenarios

4.2.1. Introduction

The risk assessment is conducted on a series of hazard scenarios to characterize the risk of each hazard type. For example, a rock fall hazard may impact a single house with no one home, one person, or several people at home. It may also break into several fragments and impact more than one house, with various number of people occupying each home. BGC does not consider it necessary to gather detailed occupancy data for each home or the typical hours each person occupies their home. Rather, these risk scenarios are meant to present a range of occupancies representative of a typical dwelling in Zeballos. As such, risk estimates provided here are “partial” risk, rather than “total” risk. Partial risk is the level of risk imposed by one geohazard scenario. Total risk would be the sum of all the geohazard scenarios that could affect a particular location.




4.2.2. Flooding

The coastal and river hazards are interdependent as the sea level sets the base level for the river. By increasing the base level elevation of the river, high tides, storm surges, and sea level rise all produce a backwater effect in the Zeballos River. This raises the water level on the Zeballos River, increasing the spatial extent and water depth of riverine floods. Similarly, the Zeballos River provides the base level for Keno Creek; a given flood discharge on Keno Creek will produce higher water levels, and have more potential for overbank flooding, if it occurs when the Zeballos River is also in flood.

To account for this interdependence BGC considered several scenarios in the hazard assessment, and these same scenarios form the basis of the flood risk assessment. The scenarios include:

• A 20-year flood on the Zeballos River during a 20-year storm surge • A 200-year flood on the Zeballos River during a 20-year storm surge • A 200-year flood on Keno Creek during a 20-year flood on the Zeballos River • A 200-year storm surge.

Climate change is anticipated to produce an increase in sea level, raising the base level of the Zeballos River, as well as increased precipitation and runoff. To account for these impacts BGC conducted a separate SQRA for the current conditions and the projected conditions in the year 2050-2100.

4.2.3. Geomorphic Processes

In addition to flooding, geomorphic processes (i.e., bank erosion and avulsion) may also pose a hazard to infrastructure. According to the geomorphic assessment (Section 3.5), bank erosion could occur along the dike on the east bank of the river during a 200-year flood on the Zeballos River under the future (year 2050-2100) climatic conditions. The additional risk associated with bank erosion has therefore been considered in the 200-year Zeballos River flood scenario for the future timeframe.

4.2.4. Slope Hazards

This risk analysis is based on debris flow, rock fall, and rock slide scenarios, which are defined as events with particular volumes and likelihoods of occurrence. Geohazard scenarios were chosen to represent the spectrum of possible event magnitudes. Consequence scenarios include the possibility: a) no one is present, b) one person is present, and c) more than one but less than ten people are present. The slope hazard scenarios considered in the risk assessment include:

• Debris Flow – 300 to 1000-year pre-fire and 30 to 100-year post-fire return periods, corresponding to approximate event magnitude of 4000 m3 for A and B Creeks and 230 m3 for C Creek.

• Debris Flow – 1000 to 3000-year return period, corresponding to approximate event magnitude of 6000 m3 for A and B Creeks and 300 m3 for C Creek.


Zeballos Landslide and Flood Risk Assessment.docx 8


• Rock Fall – Median, 66th, 90th and 99th percent event magnitudes observed during site visit.

• Rock Slide – 90th and 99th event magnitudes, corresponding to approximate 6000 m3 and 30,000 m3.

4.3. Methodology

4.3.1. Introduction

Geohazard scenarios range from the most frequent that could cause non-negligible damage to the largest credible events and are based on the results of the hazard assessment described in Section 3.0. These likelihoods populate the vertical axes of Figure 4-1. A separate assessment of the severity of the consequence (e.g., the range of economic recovery associated with the undesirable outcome) is conducted for areas of Zeballos affected by each geohazard scenario, and used to populate the horizontal axes of Figure 4-1.

4.3.2. Likelihood of an Undesirable Outcome

The likelihood of an undesirable outcome is a product of the: 1. Frequency or likelihood of the hazard occurring, PH. 2. Spatial probability PS:H that the hazard, should it occur, impacts the element at risk (a value

of 1 was assumed for all flood hazards). 3. Temporal probability PT:H that the element at risk is present in the hazard zone when the

hazard occurs (considered certain for buildings). 4. Vulnerability of the element at risk to damage or loss (the likelihood of the undesirable

outcome should the event impact the building).

Figure 4-1 defines categories used for likelihoods of an undesirable outcome, based on the product of the probabilities listed above.

4.3.2.1. Hazard Probability

Hazard probability, PH, corresponds to the annual probability of occurrence of each hazard scenario, which are defined as annual frequency ranges. The bounds of a given range are exceedance probabilities. For example, the 10 to 100-year scenario represents the probability that a slope hazard event or a flood will be larger than the 10-year event but not larger than the 100-year event.

Flood likelihood does not vary between the pre-fire and post-fire conditions. Similarly, BGC did not change the likelihood of each flood scenario between the current and future (year 2050-2100) timeframes. In other words, climate change does not change the hazard probability (0.05 or 0.005, or an “Unlikely” to “Moderate” likelihood rating) but instead influences the magnitude of the flooding for each event; a 20-year flood in the time period from 2050 to 2100 has a greater discharge, and results in more flooding than a 20-year flood would in the current timeframe.




Hazard probability, PH, corresponds to the annual probability of occurrence of each hazard scenario, which are defined as annual frequency ranges. The bounds of a given range are exceedance probabilities. For example, the 10 to 100-year scenario represents the probability that a slope hazard event will be larger than the 10-year event but not larger than the 100-year event. BGC estimated a tenfold increase in debris flow and rock fall PH based on the burn severity observed on the forest floor and the quantity of rock falls that reached Zeballos Main Road during the fire. A fivefold increase was applied to PH as well to test the ultimate risk estimate’s sensitivity to this estimation. The results present the combined annual risk from all debris flow, rock fall and rock slide scenarios, given that some parcels may be impacted by more than one scenario. Minimum and maximum frequency estimates were used in the risk analysis as approximate upper and lower uncertainty bounds for each frequency range.

4.3.2.2. Spatial Probability of Impact

Spatial probability, PS:H, is defined as the chance that the hazard, should it occur, reaches the element at risk. A wide range of inundation areas are possible for a given event magnitude. Specifically, more watery debris flows are expected to run out further than those with higher sediment concentration. Moreover, flow avulsions near the fan apex can result in flow trajectories primarily towards a certain sector of the fan. For a given hazard scenario, these factors influence the spatial probability of geohazard impact.

Spatial probability estimates for a given lot were based on “lateral impact” probability. This factor addresses the question, “what is the chance that a flow will follow a particular trajectory that results in impact to a building (as opposed to travelling past but missing a dwelling)?” Values used in the analysis are based on the results of modelling and judgement. Flooding, debris flow and rock slides are sufficiently wide it is assumed an impact will impact the entire width of the building, which was conservatively assumed for the purposes of this analysis. For these hazards the lateral component of the spatial probability was assumed to be 1.0. Spatial probabilities for rock falls are the product of the runout exceedance and the proportion of the building impacted by the boulder. The median boulder size (2 m diameter) and an average building width of 10 m are assigned to each rock fall PS:H calculation.

4.3.2.3. Temporal Probability

Temporal probability considers the proportion of time residents spend within their dwelling. All else being equal, safety risk is directly proportional to the time residents spend at home (e.g., a resident who is rarely home has less chance of being impacted by a slope hazard). The proportion of time residents spend in dwellings varies annually, seasonally and from occasional to full time occupants. Unlike the QRA which is only applicable to the current occupancy, the semi-quantitative approach considers future occupancy and land use and assumes a person is present at the time of an undesirable outcome, PT:H=1.

As there are typically hours of advanced warning of flood events, it is assumed that most residents will evacuate affected areas prior to the modelled flood scenarios. Low temporal probability values




(PT:H of 0.01 to 0.1) were therefore used for the flood scenarios, reflecting occupancy of only 1% to 10% at the time of a flood event.

4.3.2.4. Vulnerability

Section 3.7.2 shows the criteria used to estimate the vulnerability of persons within buildings to flooding and slope hazard impact, where vulnerability is primarily an indirect outcome of building damage or collapse. Building impact by flow IDF <1 were assumed to pose negligible risk to life for persons within buildings and were excluded from the analysis.

4.3.3. Estimating Consequences

Consequences considered in this risk assessment are categorized as follows: • Human Safety • Economic Impact • Cultural/Historical Impact.

Safety consequences consider the potential for injury or fatality to one or more persons occupying buildings. Economic consequences consider the cost of repairs to a building, based on the median home price (including the building only) of $52,000 obtained from the 2018 BC property assessment data. Cultural/historical consequences consider intangible impacts of a geohazard event, such as loss of a museum, inundation of a park, or damage to a cemetery. The consequences are listed in order of importance (i.e., if there is a credible safety consequence, safety consequences govern the analysis). As such the areas affected by slope hazards (Drawing 16) are judged to be at risk of a safety consequence and economic and cultural/historical impacts are not assigned. Similarly, areas with low intensity flooding (Drawings 12 through 15) are not judged to pose a serious safety risk, and economic damages are considered the primary driver of risk.

4.4. Results

4.4.1. Flooding Risk Assessment

Given the presence of dikes along the Zeballos River and Keno Crescent, river flooding and coastal flooding are expected to produce low flow velocities within the Village. Furthermore, weather forecasts as well as rising river levels typically provide hours of warning time for such floods, largely eliminating fatalities. As a result, the consequence of flooding is primarily a result of the economic damage and social/cultural impacts caused by the inundation of infrastructure. The spatial extent of flooding, as well as the modelled water depths (rather than flow intensity, 𝐼𝐼𝐷𝐷𝐷𝐷), form the basis for the risk assessment of the flood hazard.

In assessing the potential damages associated with flooding BGC considered the overall economic losses within the Village. Flood damage is typically negligible unless the water depth exceeds the typical elevation of the first floor of buildings, which is assumed to be 0.3 m for houses




without basements. Beyond this water depth significant damage is sustained (e.g., 20% of the value of a house), and economic damage rises steadily as depth increases. Figure 4-2 shows a depth-damage function obtained from the U.S. Federal Emergency Management Agency (FEMA) software program Hazus-MH (v. 4.2). The function was compiled from FEMA using data for 457 different classified building types.

Figure 4-2 Example of a flood depth-damage function for residential homes from FEMA’s

Hazus-MH software. The "zero" flood depth corresponds to the first-floor elevation and is assumed to be 0.3 m in the current assessment.

BGC used the methods described in Section 4.2 to assess the risk associated with four risk scenarios for both timeframes considered. The spatial extent of flooding is shown in the hazard maps (Drawing 12 to 15). Figure 4-3 shows the results of the risk assessment for both the current and future (year 2050-2100) conditions. The associated input values are provided in Appendix J.




Figure 4-3. Populated risk matrix for river and coastal flooding scenarios.

4.4.1.1. Current Conditions

Coastal flooding (i.e., storm surges) affect the area to the south of the intersection of Pandora Avenue and Maquinna Avenue under the current climate conditions (Drawing 12). Flooding of the Zeballos River is predicted to overtop the dike for floods with a 20-year or 200-year return period, inundating the area between the dike and the Pandora Avenue intersection and increasing the water depths associated with coastal flooding to the south of the intersection (Drawing 14). Water depths within this area (directly south of the dike) increase by up to 0.5 m as the return period of the flood increases under the current climatic conditions. Minor inundation of the schoolgrounds in West Zeballos is anticipated during a 200-year flood in the Zeballos River, but inundation is not predicted to occur during a 200-year flood on Keno Creek (Drawing 16).

The economic consequences of flooding are similar for all four scenarios. As a result, the differences in risk are primarily attributable to event likelihood. As the 20-year Zeballos River flood has the highest likelihood (roughly a 5% likelihood of occurring in any given year relative to a 0.5% likelihood for the 200-year flood) the 20-year scenario is associated with the greatest risk; the maximum risk is categorized as “High” for the 20-year Zeballos River flood, and “Moderate” for all other flood scenarios (Figure 4-3 and Appendix J).




4.4.1.2. Climate Change (Year 2050-2100)

The area inundated by flooding does not change significantly between the current and future conditions (Drawings 12 through 15), with the exception of increased inundation in West Zeballos. Climate change increases the predicted water depths associated with flooding for all scenarios by over half a metre in 2050-2100 as a result of the increase in sea level elevation and peak flow discharge. As economic damage depends on water depth (Figure 4-2), flooding in 2050-2100 will result in greater economic damages for the same return period event. In other words, an event with the same likelihood will produce greater consequences, and therefore be associated with higher risk, in the future.

Bank erosion is predicted to occur during a 200-year flood on the Zeballos River in the future (year 2050-2100) and has therefore been considered in the 200-year Zeballos River flood risk scenario. Bank erosion may increase the economic consequences of flooding by damaging the dike along the left (east) bank of the Zeballos River, and may also increase the economic damages to properties adjacent to the river. However, the increase in damage is not sufficient to change the consequence rating, or the risk category, associated with the 200-year flood under the future climate conditions, as the economic damage will remain less than $10,000,000. Bank erosion is also not expected to increase the risk to safety as there will likely be days of warning in advance of the flood event, allowing for evacuation of the properties adjacent to the dike.

Once again, the economic consequences of flooding are similar for the scenarios; economic damage is considered “Severe” for all four scenarios, while the Cultural/Historical consequence is rated as either “Moderate” or “Major” depending on whether the school is inundated (Appendix J). As a result, the highest risk is again associated with the 20-year flood on the Zeballos River, which has a risk rating of “High”. The maximum risk for all other flood scenarios in the year 2050-2100 is categorized as “Moderate” (Figure 4-3 and Appendix J).

4.4.2. Slope Risk Assessment

The 300 to 1000-year return period debris flow governs slope risks (Figure 4-4 and Appendix J) near Pandora Crescent and Ferris Road, and rock fall governs risk at the north extent of the Village north of Pandora Crescent. Depending on occupancy, pre-fire debris flow posed a “Moderate” to “High” risk that increases to “High” to “Very High” risk post-fire. The northern extent of Maquinna Avenue outside of the debris flow runout extent is exposed to a “Moderate” to “High” rock fall risk, as evidenced by the accumulation of historical rock fall deposits in this area (Drawing 16).




Figure 4-4. Populated risk matrix for slope hazards.

4.5. Risk Assessment Limitations

This assessment is based on a combination of quantitative likelihood estimates paired with expert judgement and field observations. An effort was made to allow for variations in the numerical values entered in the analysis to reflect the inherent uncertainty underlying various assumptions. The biggest uncertainty lies in the estimate of return period (frequency) of the geohazards considered, as few direct observations are available.

As additional information on geohazards (timing, location and size) becomes available in the coming months, aspects of the SQRA can (and should) be refined which could change the outcome of the present risk assessment.




4.6. Risk Tolerance

There are currently no nationally or provincially adopted risk criteria in Canada. The use of risk of loss of life tolerance criteria originated in the United Kingdom and the Netherlands during the 1970’s and 80’s in response to the need to manage risks from major industrial accidents (Ale, 2005). Hong Kong adapted the United Kingdom criteria for the management of landslide hazards, and similar approaches have been applied in Australia, Switzerland and Austria. While risk tolerance levels vary amongst jurisdictions and the evaluation criteria for individual and societal risk are different, some common general principles apply (Leroi, Bonnard, Fell, & McInnes, 2005):

• The incremental risk from a hazard to an individual should not be significant compared to other risks to which a person is exposed in everyday life.

• The incremental risk from a hazard should be reduced wherever reasonably practicable. • If the possible number of lives lost from an incident is high, the likelihood that the incident

might occur should be low. This accounts for society’s particular intolerance to many simultaneous casualties and is embodied in societal tolerable risk criteria.

• Higher risks are likely to be tolerated for existing developments than for new proposed developments.

Appendix K provides a summary of Canadian and international risk tolerance criteria as a guide for decision makers within the Village of Zeballos.

Village of Zeballos, Seton Portage December 21, 2018 Integrated Hydrogeomorphic Assessment – DRAFT Project No.: 1849001



5.0 MITIGATION OPTIONS




5.1. Introduction

Mitigation can be used to reduce the risk to a level considered tolerable by the Village. Mitigation involves reducing either the magnitude, intensity, or probability of the hazard, or the severity of the consequences. This section describes the techniques that can be used for river flooding, coastal flooding, and slope hazard mitigation. There are two categories of mitigation techniques:

• Structural measures involving the construction of barriers, dikes, or slope stabilization • Non-structural measures involving temporary or permanent removal of elements at risk

from hazardous areas or changing people’s behavior to reduce vulnerability.

An overview of the proposed mitigation measures is provided in Drawing 17.

5.1.1.1. Structural Mitigation Conceptual Design

The objective of the conceptual design phase is to identify and develop feasible mitigation design options. Identification of feasible structural mitigation measures primarily depends on the accessibility of the watershed, channel and fan apex, and the level and type of development on the fan. It is also important to consider the types of processes that contribute to the hazard, such as side-slope landslides or channel erosion.

Once feasible mitigation options have been identified, the options are developed and sometimes combined into conceptual systems or “functional chains” that meet the design basis established during project scoping. A mitigation system can be a set of structural and non-structural measures that interact to provide redundancy and meet hazard or risk reduction targets. The mitigation systems consider factors including:

• Measures requiring minimal maintenance or intervention are likely to perform better and cost less in the long run, but sometimes can cost more to construct

• Measures that avoid disrupting the normal streamflow are generally preferable from an environmental and regulatory perspective

• If the sediment load is removed from a high discharge flow by a regulation or retention structure, the flow will tend to scour and entrain debris downstream of the structure, unless the channel is sufficiently protected (Piton & Recking, 2015).

The conceptual design process typically identifies several feasible system options for comparison.

Figure 5-1 shows selected examples of structural mitigation measures. These measures are often combined to create a “functional chain” of mitigation (Hübl & Fiebiger, 2005). The most effective mitigation systems include a range of different techniques, to provide redundancy and optimize risk reduction.




Figure 5-1. Examples of debris-flow mitigation structures: (a) an earth-fill retention berm on

Glyssibach, Brienz, Switzerland; (b) a stone diversion berm, Trachtbach, Brienz, Switzerland; (c) a conveyance channel with earthfill berms, Rennebach, Austria; (d) log crib check dams, Gesäuse, Austria; and (e) a flexible debris net for debris flood mitigation, Cougar Creek, Canmore, Alberta. Photograph (d) by M. Jakob, other photographs by E. Moase.

5.1.1.2. Non-Structural Measures

Non-structural measures for flooding and slope risk management typically include the following options:

a)

b) c)

d) e)




• Education – Provide training for residents and workers who are commonly exposed to hazards. Training topics include: how to interpret hazard maps and identify areas exposed to hazards; causes and triggers of events; measures that individual property owners can take to protect themselves; emergency preparedness; and actions to take during an event. This can reduce the vulnerability of individuals to hazardous events.

• Emergency Management Planning – Develop plans to respond during or immediately after an event. This would typically involve plans for evacuation, checking in with neighbors, and staging of equipment and materials. This can reduce the consequences of a hazard event and improve resilience of the community.

• Relocation – Remove buildings from hazard zones. This can eliminate safety and economic risk from hazard sources, but the costs and tradeoffs can be prohibitive.

• Temporary Evacuation – This can include precautionary evacuation from hazard zones during periods of heavy rainfall, or alarm systems that signal an event has started. An alarm would provide only seconds or minutes of advance notice for the processes affecting Zeballos. This method can reduce safety risk but does not reduce property damage. This method can be difficult to implement effectively because of large uncertainties in predicting events, the possibility of frequent false alarms, and the requirement for occupants to evacuate quickly and without assistance.

• Development Restrictions – This involves creation of zones where future development is not allowed. This should be based on hazard maps that are updated as conditions and topography change. Particularly, construction of structural mitigation measures can change the debris flow and debris flood impact location and extents.

Selection of appropriate mitigation depends on several factors, including the process type, current and future land-use, and budget.

5.2. Conceptual Flooding Mitigation Options

According to the SQRA there is a “High” risk from flooding under the current conditions. The risk is governed by the 20-year Zeballos River flood scenario; all flood scenarios produce similar consequences (i.e., “Major” economic damage), so the likelihood of the flood event dictates the overall risk category. In the future climate conditions (year 2050-2100) the risk associated with the 20-year Zeballos River flood is also rated as “High.”

In BGC’s experience, clients typically develop mitigation plans to address risks rates as “High” or “Very High,” while “Moderate” risks are only addressed where practical and cost-efficient. This suggests that mitigation measures should be developed to address the flood risk, given the risk rating of “High”. Table 5-1 summarizes the potential mitigation measures.




Table 5-1. Potential mitigation measures for addressing the coastal and river flood hazard.

Mitigation Description River Flooding

Coastal Flooding

Keno Crescent Training Berm extension

Raise and extend Keno Crescent Training Berm -

East Zeballos River and Sea Dike

Raise the dike along the Zeballos River and create a sea dike along the shoreline of Zeballos Inlet

Home Raising Raise existing and future houses to FLC

Relocation Permanent or temporary relocation of people and infrastructure

Emergency preparedness

Protocols to manage and limit risks if a disaster occurs

Notes: Mitigations marked with - are expected to be ineffective or do not apply to the hazard. Mitigations marked with are considered potentially effective but challenging. Mitigations marked with are considered potentially effective and practical.

The potential mitigation options include: • Keno Crescent Training Berm Extension – the Keno Crescent training berm effectively

prevents overtopping due to flooding on Keno Creek under the current conditions. However, flooding of West Zeballos (including the schoolgrounds) is possible during a flood on the Zeballos River under the current conditions, and additional flooding could also occur as a result of overtopping during the future (year 2050-2100) conditions. A possible mitigation option is to raise the existing Keno Crescent training berm to an elevation of approximately 10 m (GSC), which translate into an increase between 0 and 1 m. The dike was not surveyed, and those values are for information purposes. The berm should also be extended southward approximately 300 m along the left (west) bank of the spawning channel to Parkway Road (Drawing 17).

• East Zeballos Dike – the hazard mapping shows that the existing dike could be overtopped during a 20-year flood on the Zeballos River, if combined with a 20-year storm surge. However, the majority of the inundation (and economic damage) in East Zeballos is attributable to the 20-year storm surge rather than the overtopping of the dike by the river. In other words, raising the Zeballos River dike alone would have little effect on the economic damage and would not lower the risk rating associated with flooding. To address the river and coastal flooding it would instead be necessary to both raise the existing dike and to create a sea dike along the shoreline of Zeballos Inlet. As shown in Drawing 17 the dike would be over 1.5 km long. Given the required dike elevation (approximately 7 m GSC) this option is likely to be cost-prohibitive and is considered impractical.

• Home Raising – an alternative option is to raise houses to the FCL. Those levels are obtained by adding wave allowance and freeboard to the designated flood level representing the elevation of still water. Wave allowance is specific to the structure interaction effects and different protections against wave impacts could lead to distinct




allowances. This approach would not be effective for houses with basements, if any are present, and would require a cost-benefit assessment during the mitigation planning.

• Relocation – The area to the south of the intersection of Pandora Avenue and Maquinna Avenue is susceptible to inundation from both coastal and river flooding. If structural mitigation, either building a dike or raising homes, is deemed impractical in East Zeballos, relocation of the southern part of East Zeballos (with the exception of the southernmost part of Pandora Avenue) could also be considered. There is higher terrain available in West Zeballos and structural mitigation in that area (i.e., extending the Keno Crescent Training Berm) is less costly than structural mitigation in East Zeballos. Higher terrain is also present outside of the presently-developed areas (Drawing 17) and BGC could provide guidance on the suitability of these higher elevation areas.

• Emergency Preparedness – evacuation and emergency preparedness (e.g., promoting awareness of flood hazards and alerting residents in high hazard intensity areas) should be used as a means to minimize the safety risk associated with both coastal and river flooding. However, while it is necessary for public safety, emergency preparedness alone is unlikely to reduce the economic damage associated with flooding and will therefore not reduce the overall risk ratings for the current and future timeframes.

Table 5-2 summarizes the practicality and effectiveness of each mitigation strategy.




Table 5-2. Comparison of flood mitigation options.

Criteria Keno Crescent Training Berm

Zeballos River and Sea Dikes Home Raising Relocation Emergency

Preparedness

Cost* High – would require detailed engineering design and construction quality assurance

Very High – would require detailed engineering design and construction quality assurance

Moderate to High – would be Moderate for individual homes but High if all affected homes are considered

High – would require Provincial/Federal assistance

Low

Risk reduction

Would significantly reduce risk in West Zeballos by preventing inundation.

Would eliminate risk in East Zeballos by preventing inundation.

Would raise elements at risk above the water level, reducing the risk to near-zero.

Would remove elements at risk from areas with potential for flood inundation.

As there is typically warning prior to flooding areas can be evacuated, keeping the safety risk low. Additional education and emergency preparedness are unlikely to decrease economic damage, so overall risk would be unlikely to change.

Impact to residents

Moderate – land use changes to vacant land may impact residents


Low to Moderate – would require structural changes to many residents, requiring temporary relocation

High – depends on land availability

Low – success of the measure depends on resident’s willingness to evacuate if a hazard is identified

Note: * Cost comparison categories are approximate, as follows: Low means <$10,000; Moderate means $10,000 - $100,000; High means $100,000 - $1,000,000, and Very High means >$1,000,000.




5.3. Conceptual Slope Hazard Mitigation Options

Locations exposed to “High” or “Very High” slope risks can be mitigated by a combination of measures. Table 5-3 is a list of conceptual mitigations with BGC’s assessment of the effectiveness and practicality.

Table 5-3. Conceptual slope hazard mitigations.

Mitigation Description Debris Flow Rock Fall Rock Slide

Stabilize source zone Remove or stabilize unstable rocks/debris - - -

Stabilize creek bed Reduce erodible material in creeks - -

Consolidate creek Elevate creek bed to stabilize side slopes - -

Debris retention Permanent or temporary debris storage -

Debris regulation Temporarily storing debris - -

Energy dissipation Decrease size or velocity of flow -

Diversion Redirect flow away from Village - - -

Improve conveyance Improve a defined travel path to guide debris to a desirable location

- -

Protect buildings/infrastructure

Use of damage-resistant construction methods -

Relocation Permanent or temporary relocation of people and infrastructure

Emergency preparedness

Protocols to manage and limit risks if a disaster occurs

Notes: Mitigations marked with - are expected to be ineffective or not relevant to the hazard type. Mitigations marked with are considered potentially effective but impractical. Mitigations marked with are considered potentially effective and practical.

Potential mitigations could include:

• Education and Rain Fall Warning System – Slope hazards are most commonly triggered by periods of intense rainfall and earthquakes. The Village could implement a system of alerting and evacuating residents when a pre-determined rainfall intensity is reached, or an earthquake of a pre-determined magnitude is reported. Much like a tsunami warning system, the alerts would require automation and enforcement by local officials.




Hazard awareness education could involve information flyers that are mailed to homeowners, as well as public meetings.

• Debris Basins to Contain Avulsion Potential – The majority of the debris flow risk comes from avulsion scenarios in which flows spill out of the existing channel and impact a portion of the fan away from the existing channel. Improving the channel would require extensive earthworks and, by comparison, installing basins on the fan could be a cost-effective option for managing risk. Potential locations are vacant lot 116 in the former school location and vacant lots between lots 208 and 214 (Drawing 17). Basins would require an outlet structure and ditching/culverts to direct flow toward the river.

• Relocation – The area delineated on Drawing 17 is subject to rock fall, rock slides and debris flows. The August 2018 wildfire has worsened the hazards and thus risks. Should structural mitigation not be feasible (i.e., too expensive), and should the Village decide that risk to certain properties remains intolerably high, relocation of some properties may be an appropriate response. This has already occurred with the old high school which is now located in West Zeballos. High-lying terrain exists within the Village and adjacent areas and BGC could provide guidance on the geotechnical suitability of such sites.

• Monitoring – Monitoring slope deformations can provide advanced warning of impending rock falls, rock slides, and debris flows. Thick vegetation limits the usefulness of most remote sensing techniques and the best tool may be periodic visual inspections by a qualified geohazard professional. With repeated inspections the forest regeneration can be tracked, and the hazard likelihood updated. When paired with communication and evacuation protocols such an early warning system would provide an alert to the community when the rock slope movement is accelerating and likely to detach from the slope and become a slope hazard. BGC recommends that detailed records be kept by the Village on any rock fall that enters the Village (time, date, location, boulder size), or any changes in the behaviour of A, B, or C creeks (changes in flow direction, flow magnitude, coloration, debris movements). Furthermore, weather records from the Village’s weather station should be digitized and made available so that the geomorphic response to major rainstorms and wind storms can be analyzed. A monitoring program involving measurement of debris in A, B and C creek at particular locations over time could also help to adjust especially the frequency of the post-fire geohazards and further adjust the risk assessment.

• Seasonal Land Use – All else being equal, safety risk is directly proportional to the time residents spend at home. Safety risk within houses and hotels that are intermittently occupied (e.g., vacation homes) is therefore less than the category shown on Figure 4-4. Reducing occupancy during the wettest months of the year where debris flow events tend to occur will reduce the associated safety risk to occupants.

Table 5-4 provides a comparison of the identified slope hazard mitigation options.




Table 5-4. Comparison of slope hazard mitigation options.

Criteria Warning System Debris Basin Relocation Monitoring Seasonal Land Use

Cost* Low to moderate - depends on the scale of the work

High – would require detailed engineering design and construction quality assurance

High – would require Provincial/Federal assistance

Moderate – depends on frequency of inspections

Low to Moderate – depends on resident’s intended occupancy and tourism

Risk reduction

Depends on the resident, including degree of advance preparation and long-term memory about the recommendations

Would reduce flow intensity, even if broken or damaged in the event; design should consider potential risk transfer caused by flow diversion

Would remove elements at risk from high intensity hazard zones

Depends on inspection frequency. Inspections could provide advanced warning on large scale events but would likely not for rapidly initiating slope hazards.

Would reduce resident’s exposure to slope risks.

Impact to residents

Success of the measure depends on resident involvement


High – depends on land availability

Low – success of the measure depends on resident’s willingness to evacuate if a hazard is identified

Moderate to High – depends on resident’s intended occupancy

Note: * Cost comparison categories are approximate, as follows: Low means <$10,000; Moderate means $10,000 - $100,000 and High means >$100,000.

Village of Zeballos, Seton Portage December 21, 2018 Integrated Hydrogeomorphic Assessment – DRAFT Project No.: 1849001



6.0 CONCLUSIONS AND RECOMMENDATIONS

Seton Portage from the top of Bear Mountain




6.1. Introduction

This integrated hydrogeomorphic and slope hazard assessment focused on the Zeballos River, coastal process, and slope processes east of the Village. Hazards were quantified and mapped, risk to loss of life and economic losses were qualitatively estimated, and conceptual level mitigation measures were presented and compared to reduce risk.

6.2. Summary of Results

The principal risks identified were: • River floods inundating large portions of East Zeballos, and a smaller portion of West

Zeballos, and leading to major economic loss as well as social costs due to school closure • Coastal floods inundating East Zeballos • Rock fall impacting buildings and leading to loss of life or economic losses north of

Pandora Crescent • Debris flows impacting buildings and leading to loss of life or economic losses in the

Pandora Crescent and Ferris Road areas.

Comparing all geohazard risks analyzed it becomes apparent that there are few places within the Village boundaries that are free of hazards.

The 20-year return period Zeballos River flood is the largest contributor to flood risk and presents a “High” economic risk to the Village under current and year 2100 conditions. The economic damages are expected to be concentrated in East Zeballos south of Pandora Avenue and Maquinna Avenue, as well as in the schoolgrounds and surrounding areas within the West Zeballos.

The August 2018 wildfire increased the risk posed by slope geohazards. Debris flow and rock fall life loss risks are “High” to “Very High” in the Pandora Crescent, Ferris Road and northern-most extent of Maquinna Avenue. Such risk exposure requires a long-term risk reduction plan must be developed and implemented in a reasonable time frame.

6.3. Recommendations

Hazards and risks are presented in terms of both return period and severity to allow the Village to consider the timing and rigor of installing mitigations. The Village should weigh the hazards and risks through the lens of the current and future conditions.

6.3.1.1. Flooding

Given the “High” risk associated with coastal and river flooding, BGC recommends that the Village consider the mitigation options presented in Section 5.0. These options include both structural (e.g., dikes, raising homes or floating homes) and non-structural (e.g., relocation to safe locations) approaches.




As coastal flooding inundates a large portion of East Zeballos, raising the Zeballos River dike alone is unlikely to reduce risk sufficiently, and a sea dike would also be required. Given the required scale of the river and sea dike, this option is unlikely to be practical and home raising is likely the preferred structural mitigation option in East Zeballos. Risk reduction can be realized in West Zeballos, however – and likely at a much lower cost – by raising and extending the Keno Crescent Training Berm. BGC recommends that the Village also consider non-structural options including relocation and increased emergency preparedness. Relocation could be accomplished for individual properties, and, with a long-term view of the entire area of East Zeballos, apart from businesses that require deep-water access.

6.3.1.2. Slope Hazards

The August 2018 wildfire has increased rock fall and debris flow hazards and associated risks. The level of risk increase is uncertain. Observations on rock fall and debris flow activity including channel changes and water discoloration will be key to refine the original estimate of a factor of 10 increase in the frequency of rock fall and debris flows. Particularly during and immediately after intense rainfalls and wind storms, rock fall occurrences and any stream channel changes should be carefully recorded. This will, over the course of the fall and winter of 2018/2019 lead to a refinement of slope hazard likelihood.

Designated village members could be provided a form to record rock fall, streamflow in A, B and C creeks, and rainfall that would inform the short-term and cumulative rainfall associated with initiating a slope hazard. Collaboration with FLNRORD in a systematic monitoring system is highly desirable to understand the landscape’s response to the forest fire. An added benefit to monitoring would be that, with time, these monitoring data could be used to calibrate a rainfall-based debris flow warning system.

Should the Village be interested in pursuing structural mitigation measures, BGC recommends that a pre-design study for such measures be undertaken. Such study would include the conceptual design of various structural options including approximate dimensions and an option analysis to optimize layout. It would also include an initial cost estimation and a cost-benefit analysis of such measures.

Village of Zeballos

Zeballos River Floodplain Modernization & Future Landslide Risk Assessment

December 21, 2018

Project No.: 1849001

7.0 CLOSURE

We trust the above satisfies your requirements at this time. Should you have any questions or

comments, please do not hesitate to contact us.

Yours sincerely,

BGC ENGINEERING INC.per:

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Geological Engineer

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Matthias Jakob, Ph.D., P.Geo.

Principal Geoscientist

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Zeballos Landslide and Flood Risk Assessment.docx


Page 80




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