Small Dams in Canada
What is a small dam? Who is a small dam owner? Simple enough questions but they are continuously raised as an item for discussion, not only in Canada but internationally. Large dams are significant engineering achievements and their profile with the public is justifiably high. By contrast, small dams have a relatively low public profile. However, it is intuitively understood in the dam safety community across Canada that small dams outnumber large dams by a wide margin. Moreover, small dams frequently have Consequence Classifications or High or greater; however, their lower profile with the general public often results in dam safety programs for these structures being a lower priority.
The benefit of seeking clarity on the characteristics of small dams and small dam owners is of importance to the dam safety community because failure of a small dam can be just as fatal as failure of a large dam. Generally speaking, small dams fail more frequently than large dams. ICOLD Bulletin 109 "Dams less than thirty metres high (1997)" hinted that while a failure at large dams is catastrophic for the downstream population, the cumulative effect of more numerous small dam failures is greater and that the incident of dam failure is greater for small dams (i.e., >2% and opposed to 2% for large dams).
Qualitatively, there are several reasons why this may be the case.
- Small dams have different uses than large dams, which are typically used for hydroelectric generation and tailings retention. Small dams are typically used for irrigation, municipal water supplies, storm water management, etc. and as a consequence, tend to be, by default, closer to the service population.
- The effort required by engineers for small dam design and construction monitoring is proportionally as time consuming as for large dam activities. However, due to cost constraints, typically less time is approved for engineering and construction monitoring. This leads to reduced investigation, design and construction monitoring costs which may lead to substandard designs and poor construction quality.
- Dams frequently are engineered structures and viewed by the public to carry implied levels of reliability and safety that may not have been imparted to the structure during design or construction.
- Often, less experienced "local" contractors are used to construct, repair or maintain small dams. These contractors have limited resources and sometimes use "farm dam" practices during construction with resulting poor long term performance.
- There are some beliefs that imply that old dams tend to get better with age (or at least do no experience any deterioration) and therefore do not require regular maintenance or attention.
- The limited resources of a small owner may result in regulatory requirements being viewed as too onerous for higher consequence structures that are candidates for rehabilitation to meet current safety standards.
- The lower profile afforded to small dams could results in downstream consequences of dam failures not being fully considered in permitting for development downstream of the dam (e.g., residences, schools, roads, or pipelines).
There is no established definition of a small dam in Canada or internationally. The ICOLD definition of a large dam is presented in Table 1.
Table 1: ICOLD definition of a large dam
* Height of dam is measured from the foundation.
By ICOLD standards, any dam greater than fifteen (15) metres in height is a large dam. For dams that are between ten (10) metres and fifteen (15) metres in height, they are considered a large dam if they are greater than 500 metres in length, retain more than 1,000,000 m3 of water at maximum operating level, or have a flood discharge greater than 2,000 m3/s. By default, a small dam is typically defined as anything that doesn't qualify as a large dam.
One difference between the CDA Guidelines and ICOLD is the definition of height. ICOLD measures height from the foundation, including any cut-off, where as Canadian practice is to measure from the downstream toe of the dam. While this is of interest to ICOLD, the definition adopted for the current study defines height as measured from the downstream toe.
Another definition of interest is the definition of a dam in the Canadian Dam Association (CDA) Dam Safety Guidelines (CDA Guidelines), 2007, which defines a dam as:
A barrier which is constructed for the retention of water, water containing any other substance, fluid waste, or tailings, provided the barrier is capable of impounding at least 30,000 m3 of liquid and is at least 2.5 m high. Height is measures vertically to top of the barrier, as follows:
i. From the natural bed of the stream of watercourse at the downstream toe of the barrier, in the case of a barrier across a stream or watercourse; or ii. From the lowest elevation at the outside limit of the barrier, in the case of a barrier that is not across a stream or watercourse.
In these guidelines, the term dam includes appurtenances and systems incidental to, necessary for, or in connection with, the barrier. The definition may be expanded to include dams less than 2.5 m high or with an impoundment capacity less than 30,000 m3 if the consequence of dam operation or failure are likely to be unacceptable to the public, such as:
i. Dams with erodible foundations that, if breached, could lower the reservoir more than 2.5 m; or ii. Dams containing contaminated substances.
The CDA Guidelines definition captures a much broader definition of dams than ICOLD, and it was used in the current study to identify the lower bound definition of a small dam. Structures that are smaller than the CDA Guidelines definition that were considered in this study are classified as "other" structures.
It is the experience of the authors that dams approaching or greater than 15 m in height are more likely to have some, if not all, of the usual elements of a dam safety management program in place. Conversely, the reverse is generally true for dams under 6 m in height, especially those owned by an individual or corporate entity that owns a limited number of dams.
When assessing the risk associated with any given dam, the "size" of the owner can be as important of a consideration as the height of the dam or volume of the reservoir. Dam owners with significant operating budgets (E.g., hydroelectric companies, watershed authorities, government departments) can more readily afford the costs associated with a dam safety management program and typically maintain knowledgeable technical and operations staff who are more likely to have dam safety as a priority in their job descriptions. "Small" owners of dams more often than not lack the staff with sufficient knowledge to maintain a dam safety management program and/or lack the financial resources required to establish such a program.
To date, the only available public information on the characteristics of dams in Canada is a CD and a short document "Dams in Canada" prepared by the CDA (CDA, 2003) and some limited data on the CDA Website. The CDA Website lists an approximate number of dams for each province in the Regulators section. Given the large number of small dams that are intuitively known to exist in Canada, the authors undertook this study to provide greater insight on the numbers of small dams in each database and the characteristics of small dam owners. This study is of particular importance to the CDA as it engages, involves and serves the Canadian dam community.
4. SOURCES OF DATA
Regulation of dams is a provincial jurisdiction in Canada and each province delegates responsibility for regulation through a departmental agency. The agencies that were requested to provide dam databases to the authors are as follows:
- Newfoundland & Labrador Department of Environment and Conservation, Water Resources Management Division;
- Nova Scotia Department of Environment and Labour;
- Quebec Centre d'Expertise Hydrique;
- Alberta Environment; and
- British Columbia Ministry of Environment - Dam Safety Branch.
Combined, the five regions have submitted databases that encompass nearly six thousand dams registered across a broad spectrum of sizes, classes, etc. The databases provided were assessed for the current study. A general description of the data is presented in Table 2.
Table 2: Summary of data
*About 1000 dams in British Columbia have no recorded height. There are 188 dams greater than 10 m in height. **The database provided for Quebec did not include about 2550 dams under 2.5 m in height or with reservoirs less than 30,000 m3
There are some discrepancies between the number of the dams included in each database and what is presented in the Regulators section of the CDA Website.
Overall, the five jurisdictions represented in the study represent fifty two (52) percent of the population of Canada, or just over seventeen (17) million Canadians and thirty seven (37) percent of the land mass, or 3.36 million square kilometers. Additionally, the database includes provinces from across Canada in various physiographical regions that include the Maritimes, the Canadian Shield, the Prairies and the Western Cordillera and west coast regions. It is the opinion of the authors that the sampled population of dams considered in this study is sufficiently representative of the complete dam population in Canada to support the discussions presented in this paper. The authors welcome and encourage additional studies to further investigate the statistics of small dams in provinces and territories not included in this study.
Another source of data used to compile the overall statistics was the ICOLD registry of dams. Where practical, data from the provincial databases provided to the authors was compared with the ICOLD registry to ensure all structures were included in the database. In some cases, the names were different and it is possible the database provided to the authors may have missed or double counted a structure. However, due to the high level nature of this review, we believe the instances are minimal and do not effect the overall assessment and results.
5. DATABASE REVIEW
Data which were common to most the databases include:
- Number, type, and height of dams in the province;
- Consequence classification;
- Uses of existing dams; and
- Reservoir volume.
The systems used for data entry were not completely consistent between the various databases and some data fields were not completely filled out for all dams within certain databases. Additionally, some of the databases were in the process of being revised and improved when the study discussed herein was conducted. In the case of the British Columbia database, a summary "central" database was provided to the authors for use in this study. Regional databases maintained by the various dam safety officers are understood to be complete. The "central" British Columbia database is currently being revised.
For all provinces considered, the type of owner and/or purpose of the dam were generalized into the following groups
- Municipality (e.g., water supply, sewage lagoons);
- Private ownership (includes agricultural);
- Government; and
- Non-Government Wetlands Conservation.
The type of ownership has ben approximated to a reasonable degree. In the interests of maintaining the privacy of the various owners (mandated by some of the regulators who provided databases to the authors), confirmation of the nature of ownership was not conducted. This is a potential source of error in the processing of the various databases.
The databases maintained by the various regulators do not use consistent consequence classification systems. The databases reviewed by the authors generally use either the consequence classification systems presented in 1999 or the 2007 CDA Guidelines or some similar system. The consequence classifications used in the various databases have not been reconciled to a single classification system, which is a potential limitation of this study.
Finally, all databases contained dam ownership and location details with some common data fields, e.g. height of dam, consequence classification. However, in several cases in some of the databases, some field were blank.
It is important to note that the Quebec database provided for review only included "High Capacity" dams. The definition of a "High Capacity" dam used by Centre d'expertise hydrique is illustrated in Figure 1. The percentage of High Consequence Dams in the "Low Capacity" and "Really Small" categories is believed by Centre d'expertise hydrique to be minimal.
Figure 1: Centre d'expertise hydrique definition of dam categories in Quebec.
6. RESULTS OF ASSESSMENT
The combination of inconsistent consequence classification systems, inconsistent data fields and instances where various data fields were blank for dams in some of the databases has confounded a complete detailed statistical review of the data. However, some general observations can be readily made regarding dam ownership, height and consequence classification.
Of the nearly six thousand dams in the combined database provided to the authors, only 444 are greater than fifteen (15) metres and 314 less than 2.5 metres in height ("other" dams). In other words, given the definition based on height descried earlier, approximately eight (80) percent of the dams registered in Canada are small dams, ten (10) percent are large dams and ten (10) percent are not classified as dams based on the CDA Guidelines.
It is interesting to note that municipalities, private owners and non-government organization (NGO) wetlands conservation authorities are responsible for anywhere form one quarter to three quarters of dams in the assessed jurisdictions. While not a clear demarcation, these owner classifications are generally described as "small" owners of dams. Their share of dam ownership is presented in Table 3. In some cases the totals do not add up to 100% due to ownership of some dams not being stated in the databases.
Table 3: Dam ownership
The majority of dams owned by municipal, private and non-government wetlands conservation authorities are small dams as shown in Table 4. The available data indicates that in general, the majority of these dams are under ten (10) metres in height.
Table 4: Heights of municipal, private and NGO wetlands conservation dams
*The BC database did not include dam heights for between 40 and 57% of the dams in each ownership group.
A review of the consequence classification compared to dam height indicates a significant portion of dams with high consequence classifications qualify as small dams as shown in Table 5. The available data indicates the majority of these high consequence dams are under ten (10) metres in height.
Table 5: Percentage of small dams with high consequence classification
Both the ICOLD and CDA definitions of a dam use height as a focal point for defining dams, but both organizations also recognize the importance of reservoir retained volume in their definition. Other countries have brought in legislation that defines dams based on combining dam height and reservoir capacity, recognizing the fact that a small dam impounding a lot of water may be as much of a danger as a high dam impounding little water. Recent changes in French regulation changed the dam classification system, introducing a graded system with four classes of dams (Crouchon 2009).
The system in France introduces a new parameter, H2V½ which is expressed in hm3, million cubic metres, where H is the maximum operating head in the reservoir and V is the volume at the maximum operating level. The category system used by Centre d'expertise hydrique presented in Figure 1 illustrates his concept in practice in Quebec. Helwig and Smith (1993) have also discussed the importance of reservoir volume in addition to dam height during dam safety assessments.
Reservoir volume was the least consistently recorded parameter in the various databases. In many cases, more so in some databases than others, reservoir volume was not included and in others there was only a "Yes/No" indication if the reservoir volume was 30,000 m3 or greater. A plot of normalized reservoir volume versus height for dams that had recorded reservoir volume is presented in Figure 2.
Figure 2: Relationship between normalized reservoir volume versus height
While there are a few anomalies which are likely related to record errors, there is a clear definable limit for definition of dams. The labels are plotted differently for structures with height of greater than fifteen (15) metres, between 2.5 metres and fifteen (15) metres and below 2.5 metres. The dashed lines represent the definition of a large dam as defined by the ICOLD and any dam as defined by the CDA. A normalized reservoir volume parameter (H2V½) of 225 provides a reasonable fit for the lower definition of an ICOLD large dam while (H2V½) of 1 provides a reasonable fit for the CDA definition of a dam (i.e., a 2.5 m dam with 30, 000 m3 of retained volume has a normalized volume of unity (1)).
What is informative is that, by and large, approximately half or better of any dam over ten (10) metres in Canada should be considered a large dam by the definition of the ICOLD. Equally important, a significant number of small dams that have normalized volume values greater than what is traditionally thought of for large dams. The normalized volume measurement provides a qualitative means of assessing the potential consequences of a dam failure and can be used in the preliminary assessment of dams.
The financial challenges of "small" owners of dams are well known to consultants and regulators who work in the field of dam engineering and dam safety in Canada. The constraints run throughout the life cycle of a dam; from engineering cost for design, construction, operations and capital maintenance through to surveillance and eventual decommissioning. As discussed, small dams and small dam owners collectively, represent as great a risk to the public as large dams and large dam owners, if not more so. There are no easy answers to this quandary. One topic of recent debate of interest to small owners of dams is the selection of flood return periods for High Consequence dams.
The 2007 CDA Guidelines present a relatively new challenge to many small owners of small dams with High Consequence Classifications. The 2007 CDA Guidelines require that the Inflow Design Flood (IDF) for a dam with a High Consequence classification should be 1/3 of the way between the 1:1,000 year flood and the Probable Maximum Flood (PMF). Calculation of the PMF is not a trivial matter and can be quite costly on a proportional basis, especially when the financial resources of a small owner of dams are considered. There are some semi-empirical methods to calculate PMF which rely on Probable Maximum Precipitation (PMP) such as Hirshfield's method presented in Environment Canada (1985). Other more rigorous watershed or regional specific relationship methods exist but their availability is generally the exception rather than the rule. Additionally, the 1:1000 year flood is not as trivial to estimate accurately due to limited meteorological data available.
The BCMoE Dam Safety Branch recognized this challenge to "small" owners of dams in July of 2008 and issued a draft "Interim Consequence Classification Policy for Dams in British Columbia" (BCMoE, 2008). This policy states that "...the 1999 Guidelines may be used for dams constructed before 2008. The main reason for this policy is the change of the IDF and MDE recommended for High Consequence Dams in the 2007 Guidelines. For dams with less than 10 people at risk, this results in a more conservative recommendation of IDF and MDE than presented in the 1999 Guidelines. Some owners of dams previously classified as High had been previously informed that a minimum IDF and MDE of 1:1,000 would be acceptable" (BCMoE, 2008). Immediately requiring re-assessment with the more conservative IDF and MDE was judged by the BCMoE Dam Safety branch to be inappropriate at the time. The target ownership group for this policy has ben confirmed by BCMoE-Dam Safety to be the "small" owners of dams. This study has shown that there is a high incidence of small owners of dams owning High Consequence structures.
With regards to the CDA Guidelines recommendation to use the PMF in the determination of the IDF for High Consequence dams, it is hoped that the need for economical "short cut" methods to estimate a PMF will be identified, publicized and accepted by the various regulators in Canada. These methods used with a screening tool could conceivably be used to address the majority of instances, with detailed and costly engineering analysis being required to refine designs where the costs of implementing a conservative solution outweigh the cost of a detailed engineering study to optimize the design. In such instances, engineering judgement and practicality would be the final arbiter of whether or not a "short cut" solution could be used economically. Such approaches were discussed and recommended for further research by Helwig and Smith (1993) and could be of great use to the community of small dams and "small" owners of dams in Canada, a community that has ben shown in this study to be the majority in Canada.
The study discussed herein supports the following conclusions:
- Over three quarters of dams registered in the sampled provinces are small dams;
- The largest population of dam owners appears to be "small" owners of dams;
- A large percentage of small dams have High Consequence ratings;
- The largest population of dam owners in the sampled provinces generally have limited resources available for adequate engineering design, construction supervision or dam safety management programs when compared to owners of hydroelectric and government/watershed authority dams; and
- The financial resources available to "small" owners of dams generally results in economic difficulties in estimating IDF events that require estimation of the PMF.
There is a clear need for methods for estimating the PMF that are conservative yet economical for use in assignments where small owners of dams with High Consequences of failure are required to meet the requirements of the 2007 CDA Guidelines.
The authors thank the various regulators for providing their databases for review during this study. The authors also wish to acknowledge the efforts of Matthew Weatherby, EIT and Tobin Stetson, EIT for data processing and Mrs. Jennifer Then for preparing the manuscript.
British Columbia Ministry of Environment, 2008. Draft Interim Consequence Classification Policy for Dams in British Columbia
Canadian Dam Association, 2007. Dam Safety Guidelines
Canadian Dam Association, 2003. Dams in Canada
CRUCHON P., DEGOUTEET G., LE DELLIOU P., MONIE N., ROYET P. (2009) - "New regulations on the safety of dams and levees in France", ICOLD-CIGB XXXII Congress, Brasilia, May 2009
Environment Canada, 1985. Rainfall Frequency Atlas for Canada.
Helwig, P.C. and Smith, W.L., 1993. Some Thoughts on Dam Safety Assessment: Small Dams with Not-So-Small Reservoirs. 1993 CDSA/CANCOLD Conference, St. John's, Newfoundland, pp 93-103.
ICOLD (1997) - "Dams Less Than 30 Metres High", International Commission on Large Dams (ICOLD), France 1997
USCOLD (1970) - "Model Law for State Supervision of Safety of Dams and Reservoirs", United States committee on Large Dams, USA, 1970
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