Introduction
This document provides an overview of consequence classification for dams in BritishColumbia. It outlines a rough method for assessing consequence and some key concepts
that require consideration in assessing consequence. If the method provides a clearly
defined consequence classification then a consequence classification can be assigned. If
the results are uncertain, use of the higher possible consequence classification is
appropriate or a more detailed assessment method should be used. For larger structures or
complicated downstream channel conditions more detailed procedures may be required
These guidelines are only intended for consequence of failure classification. They are not adequate for the preparation of inundation mapping for Emergency Preparedness Planning (EPP), or for the assessment of hazards and risk analysis.
Consequence Classification Guide
The BC Dam Safety
Regulation - Schedule 1 “Downstream Consequence Classification Guide”
outlines a classification guide for all dams in British Columbia. The
consequence classification (very high, high, low, or very low)
identifies the potential for damage and loss in the unlikely event of a
dam failure. The consequence classification is not a reflection on how
safe the dam is; thus age and condition of the dam are not reflected in
the Consequence classification.
The consequence
classification is used to determine the design requirements for a
particular dam, with dams of higher downstream consequence having higher
design standards. Suggested design requirements for dams falling under
the various consequence classifications are identified in the “Dam
Safety Guidelines” published by the Canadian Dam Association.
Dam Breach Flood Determination
The flood hydrograph
resulting from a dam breach is dependent on many factors. The primary
factors are the physical characteristics of the dam, the volume of the
reservoir, and the mode of failure. The dam characteristics such as dam
geometry, construction materials, and mode of failure; determine the
dimensions and timing of breach formation. Breach formation, volume of
reservoir storage, and reservoir inflow at the time of failure determine
the peak discharge and the shape of the flood hydrograph.
The
following sections provide a method for estimating dam breach
parameters and peak flow discharges for earthfill dams. Earthfill dams
are focused on because the great majority of small dams are earthfill.
When estimating concrete gravity dam breach parameters, a complete
failure of a discrete number of monoliths is considered. For concrete
arch dams a complete dam failure is considered. Breach times for
concrete gravity dams generally fall between 0.1 and 0.5 hours and for
concrete arch dams they generally fall between instantaneous and 0.1
hours.
Estimation of Dam Breach Parameters
Work by MacDonald and
Landridge-Monopolis (MacDonald, 1984) were successful in relating
breaching characteristics of earthfill dams to measurable
characteristics of the dam and reservoir. Specifically, a relationship
exists between the volume of material eroded in the breach and the
Breach Formation Factor (BFF):
BFF = Vw (H)
where:
Vw = Volume of water stored in the reservoir (acre-ft) at the water surface
elevation under consideration
H = Height of water (feet) over the base elevation of the breach
Interpretation of data (MacDonald, 1984) suggests that the estimates of material eroded from earthfill dams may be taken to be:
Vm = 3.75 (BFF)0.77 for Cohesionless Embankment Materials; and
Vm = 2.50 (BFF)0.77 for Erosion Resistant Embankment Materials
where:
Vm = Volume of material in breach (yds3) which is eroded
Using
the geometry of the dam and assuming a trapezoidal breach with
sideslopes of (Zb:1) the base width of the breach can be computed
(MacDonald, 1984) as a function of the eroded volume of material as:
Wb = [27Vm – H2 (CZb + HZbZ3/3)] / [H (C + HZ3/2)]
where:
Wb = Width of breach (feet) at base elevation of breach
C = Crest Width of dam (feet)
Z3 = Z1 + Z2
Z1 = Slope (Z1:1) of upstream face of dam
Z2 = Slope (Z2:1) of downstream face of dam
If
the calculated breach width is negative then the reservoir volume is
not large enough to fully breach the dam and a partial breach will
result. In this case the head of water (H) needs to be adjusted to
estimate the breach depth and peak discharge. Maximum breach
widths
have historically been limited to breach widths less than 3 times dam
height (Fread, 1981). In addition site geometry often limits breach
width.
The time of breach development (τ) in hours, has
been related to the volume of eroded material (MacDonald, 1984).
Interpretation of data suggests that the time for breach development can
be estimated by:
τ = 0.028 Vm0.36 for Cohesionless Embankment Materials; and
τ = 0.042 Vm0.36 for Erosion Resistant Embankment Materials
There
is a large uncertainty in the eyewitness accounts for many of these
failures; thus these equations may tend to overestimate breach times. In
addition, these equations appear to produce unrealistically short
breach development times in the case of small dams. A lower limit for
the breach development time of perhaps 10 minutes for dams constructed
of cohesionless materials and 15 minutes for dams constructed of erosion
resistant materials seems reasonable.
Due to the
uncertainties in breach development parameters, a range of values should
be used to assess the computed dam break flood peak discharges. There
is a range of alternative procedures for estimating dam break
parameters. An example is the computer program BREACH, developed by
Fread (1987) which is used for larger complex dams.
Estimation of Dam Breach Peak Discharge
A number of computer
programs, such as DAMBRK (Fread, 1988), have been developed for
estimating dam break peak discharge. This computer model, and others,
utilises unsteady flow conditions in combination with user selected
breach parameters to compute the breach flood hydrograph.
Fread
(1981) gives an alternative method suitable for many planning purposes.
He developed an empirical equation based on numerous simulations with
the DAMBRK model. Estimation of the peak discharge from a dam breach is
computed as:
Qp = 3.1 W H1.5 [ A / (A + τ H0.5]3
where:
Qp = Dam breach discharge (cfs)
W = Average breach width (feet) W = Wb + ZbH
H = Initial height of water (feet) over the base elevation of the breach
τ = Elapsed time for breach development (hours)
A = 23.4 Sa / W
Sa = Surface area of reservoir (acres) at level corresponding to depth H
The
following Tables 1 & 2 contain estimates of dam breach peak flows
for overtopping induced failures of earthfill dams based on Fread’s
equation. The values used in developing these estimates are presented
after the Tables.
Selection of Reservoir Conditions for Breach Analysis
The
selected reservoir storage is an important consideration in dam breach
analysis. Normally a couple of reservoir conditions, normal pool and
maximum storage elevation during floods are considered. For smaller
unattended structures usually only the case of dam failure during
overtopping needs to be considered. Overtopping could result from a
debris blockage, or a beaver dam constructed, in overflow spillway
channel.
In evaluating the overtopping dam breach it
needs to be remembered that the reservoir storage and head on the dam
are greater than for normal pool levels.
Downstream Routing of Dam Breach Flood
As the dam breach
flood wave travels downstream there is a reduction in the peak flow.
This effect is governed by factors such as:
the channel bedslope,
the cross-sectional area and geometry of the channel and overbank areas,
the roughness of the main channel and overbank,
the existage of storage for floodwaters in off-channel areas, and
the shape of the flood hydrograph.
Small attenuation is associated with:
large reservoir volume,
small confining channel,
steep channel slopes, and
little frictional resistance in channel and overbank areas.
Large attenuation is associated with:
small reservoir volume,
broad floodplain and/or off-channel storage areas,
mild channel slopes, and
large frictional resistance in channel and overbank areas.
There
are a number of methods for modelling the attenuation of peak flow as
the breach flood wave travels downstream. For consequence classification
a simplified procedure based on generalised flood attenuation curves
developed by the USBR (1982) is often adequate. The curves presented in
Figure 1 should be used conservatively as they utilize generalised
solutions to approximate the reduction of flood peak discharge with
distance downstream of the dam. For example the attenuation would be
much smaller for a dam breach flow travelling down a steep narrow
valley.
Downstream Hazard Classification
Once the dam breach flood
inundation path has been determined, the resulting consequence of
failure classification can be determined. For BC, the classification
system is outlined in Schedule 1 “Downstream Consequence Classification
Guide” of the British Columbia Dam Safety Regulation. Refer to the
Regulation for Schedule 1. The highest consequence rating in one of the
three categories; loss of life, economic and social loss, and
environmental and cultural losses is the consequence rating for the dam.
In estimating loss of life in a dam breach one needs to consider:
Time of day of failure
Number of homes in inundation area
Flood depth and velocity
3 people per home (USBR, 1988)
Highways
Recreation
Warning time
Sources of uncertainty
For
further information on this topic the “Downstream Hazard Classification
Guidelines” produced by the US Bureau of Reclamation (USBR, 1988) are a
good starting point.
Other Considerations
There are many other factors that can influence the consequence of failure classification. They include:
Debris build-up and sediment transport can increase floodwave size and its destructive power,
Channel avulsions especially on alluvial fans,
Multiple dams on a river system, and
Current and potential future downstream development,
Warning
systems can be effective in reducing loss of life in the event of a dam
failure. Thus they are effective risk management tools, however they do
not change the consequence of failure classification.
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