Following the European example, flood hazard maps can follow at best three different probability scenarios: low (20 year), medium (100 and 200 years) and high (500, 1,000 and 2500 years) which are reflected in Tables E-1, E-2 and E-3. These probabilities will, at least to some degree, hinge on the available data for the river or stream in question as well as the flood-producing process.
Table E-3: Proposed frequency probability scenarios for different watershed areas Large river systems Moderate and small
rivers and large streams or small streams with low gradients
Small steep streams subject to debris floods and debris flows
Typical length of gauged record
> 50 years 0 - 50 years rarely gauged record
Typical watershed area > 1000 km2 10-1000 km2 0.1- 10 km2 Flood- generating process rainfall snowmelt rain-on-snow ice-related floods rainfall snowmelt rain-on-snow landslide dam outbreak floods volcanic debris flows log jams
beaver dam failures ice related floods
landslide dam outbreak floods debris flows lahars extreme rainfall Proposed flood return periods** shown on hazard maps 20-year* 100-year 200-year 1,000-year 2,500-year*** 20-year* 100-year 200-year 500-year 1,000-year 20-year* 200-year 500-year 2,500-year***
* should only be considered for areas where there are no flood defence structures or where the existing ones are likely to fail or be overtopped for an event of this return period.
**The return periods serve as guides only and will need to be adjusted depending on the elements at risk on the floodplain to suit the objectives of the respective flood hazard or risk assessment. Also, the return period estimates beyond 200 years only make sense if a reasonably long gauged record is available from the river in question or from regional analysis.
*** Peak flows, stages or debris volumes (debris flows) for return period exceeding 1,000 years are exceedingly uncertain and are in many cases at the limits of the available Quaternary dating methods. Such extrapolations also
must contend with significant climate variability and thus variability in the geomorphic response. The 2,500-year return period will thus only apply to Class 3 and 4 (Table E-2) assessments.
Table E-3 provides guidance on the range of return periods to be used for different flood- generating process and associated typical watershed sizes. For example, for Lillooet River in the Pemberton Valley, work by Friele et al. (2008) has shown that lahars (i.e., volcanic debris flows) may reach the township of Pemberton, on average, every 2,000 years and that,
measured by risk tolerance standards developed elsewhere, risk to inhabitants in Pemberton is currently considered unacceptable. For this reason, a 200-year and 2,500-year floodplain map may be considered a reasonable compromise. Similarly, for the Squamish River (watershed area: 2330 km2), large landslide dams from the Quaternary volcano Mt. Cayley
have been dated using radiometric methods. For developments in the upper Squamish River valley, a 2,500-year return period landslide dam breach would form a reasonable basis for floodplain mapping.
For the Fraser River, given the very high potential consequences, flood hazard maps including a 1,000-year return period event and a 2,500-year event may be warranted as this river has been dammed by rock avalanches several times in the past in the Fraser Canyon. Outbreak floods from large landslide dams would likely result in greater flood depth than normal floods for some sections of the river. It is worthwhile comparing the 1,000-year and 2,500-year return period herein to return periods considered in the Canadian Dam Safety Guidelines (2007). For a High dam class with permanent population at risk and loss of life of <10, the suggested return period for deterministic assessments of dam safety is defined as 1/3 between the 1,000- year return period flood and the PMF (Table, E-4). The PMF has no associated annual
exceedance probability (AEP). In the case of a landslide dam break and imperfect evacuation given that there are currently no emergency management plans for such event, one could argue that the potential loss of life could be significantly higher (>100 people). In this case, the Canadian Dam Safety Guidelines proposed the PMF as the appropriate design flood level. Given these suggested design standards, the return period levels suggested above (1,000- year for snowmelt and rain-on-snow floods and 2,500-year for landslide dam outbreak floods) appear reasonable.
Table E-4: Dam classification and suggested design return flood return periods (Canadian Dam Safety Guidelines, combined tables 2-1 and 6-1, 2007)
Dam Class Population at risk [note 1] Incremental losses Loss of life [note 2] Environmental and
cultural values Infrastructure and economics
Design Flood
Return Period* Low None 0 Minimal short-term loss
No long-term loss
Low economic losses; area contains
limited infrastructure or services 100
Significant Temporary only Unspec.
No significant loss or deterioration of fish or wildlife habitat
Loss of marginal habitat only
Restoration or compensation in kind highly possible
Losses to recreational facilities, seasonal workplaces, and infrequently used transportation routes
100 to 1000
High Permanent <10
Significant loss or deterioration of important fish or wildlife habitat Restoration on compensation in kind highly possible
High economic losses affecting infrastructure, public transportation, and
commercial facilities 1/3 between 1,000 and PMF Very High Permanent <100 Significant loss or
Professional Practice Guidelines - Legislated Flood
APEGBC June 2012 Assessments in a Changing Climate in BC 95
fish or wildlife habitat Restoration or compensation in kind possible but impractical
highway, industrial facility, storage
facilities for dangerous substances) PMF
Extreme Permanent >100
Major loss of critical fish or wildlife habitat Restoration or compensation in kind impossible
Extreme losses affecting critical infrastructure or services (e.g., hospital major industrial complex, major storage
facilities for dangerous substances) PMF Note 1. Definitions at risk:
None – There is no identifiable population at risk, so there is no possibility of loss of life other than through unforeseeable misadventure.
Temporary – People are only temporarily in the dam-breach inundation zone (e.g., seasonal cottage use, passing through on transportation routes, participation in recreational activities).
Permanent – The population at risk is ordinarily located in the dam-breach inundation zone (e.g., as permanent residents); three consequence classes (high, very high, extreme) are proposed to allow for more detailed estimates of potential loss of life (to assist in decision-making if the appropriate analysis is carried out).
Note 2. Implication for loss of life:
Unspecified – The appropriate level of safety required at a dam where people are temporarily at risk depends on the number of people, the exposure time, the nature of their activity, and other conditions. A higher class could be appropriate, depending on the requirements. However, the design flood requirement, for example, might not be higher if the temporary population is not likely to be present during the flood season.
* PMF has no associated annual exceedence probability
On the lower spatial spectrum, consider a small (<10 ha) fan that is subject to infrequent debris floods as preliminarily determined through consideration of the watershed morphometry and fan gradient. The fan contains two homes and the owner of one of those wishes to double the square footage of his house with liveable space. An Approving Officer needs to determine if such development can be permitted and seeks the help of a consultant. In this case, the QP
would orient himself/herself on the last column in Table E-3. A site visit would likely include some machine-aided test pitting to at least 2 m depth and perhaps some dendrochronology of impact-scarred trees. If buried organic materials are found, a few samples should to be taken to obtain an idea as to the frequency of debris floods on the fan. The methods should allow an interpretation of debris flood magnitude for at least a 500-year return period (0.2% annual probability of occurrence). The APEGBC (2010) Guidelines for Legislated Landslide
Assessments for Proposed Residential Developments in BC provides additional guidance as
to requirements to conduct a debris flow or debris flood study.
For each of the above sample scenarios, the minimum requirement would be for the flood
hazard map to show flood extent, the water depth, and where appropriate, the maximum flow
velocities. This type of information is not provided in the floodplain maps that have previously been published by the MFLNRO.