When are flood waters unsafe for humans?Design guidelines used by flood plain managers, water resource engineers and planners consider both the depth and velocity of water to assess a person's stability in the water. These criteria are used to establish flood risk maps that may preclude development or, if not possible, require special design considerations to reduce risk to occupants (e.g., flood proofing).
The Calgary Bow River flood risk map shows the risk areas on a plan view for the 100 year flood (an event that has a 1% chance of occurring in any year).
Some provincial guidelines are presented below. General rules of thumb are:
“3 x 3 rule” - developed in the U.S. based on 3 ft depth and 3 ft/s velocity values. The rule suggests that people would be at risk if the product (multiple) of the velocity and the depth exceeded 0.8 sq.m/s (9 sq.ft./s). So slow moving water at 0.1 m/s and 0.8 m deep has a product of 0.8 sq.m/s and would the just stable for an adult, and so would 0.2 m/s and 0.4 m deep.
Some detailed discussion from provincial guidelines follow:
(1) Flooding as a Threat to Life
Hazard to life is linked to the frequency of flooding, and to depth of
flood waters and the velocity of flow in the floodplain. Depth increases buoyancy
and velocity increases instability, so that each of depth and velocity should
be studied independently or as a combined function.
(a) Depth
Any person in the midst of a flooded area will be acted upon by
a buoyant force equal to the weight of water displaced by that person. The
volume of displaced water and this force increases with depth until neutral equilibrium
is reached and the person begins to float.
Average adults and teenage children remain stable when standing
in flood depths up to about 1.37 m (4.5 ft.). The average school child 6 - 10
years old would float at about 1.1 m (3.5 ft.), although smaller, younger children
in this range would float at a depth of about 0.98 m (3.2 ft.).
Hence, in terms of depth and individuals who could be present
in the floodplain during a flood:
· depths in excess of about 0.98 m (3.2 ft.) would be sufficient
to float young school children;
· a depth of about 1.37 m (4.5 ft.) is the threshold of stability
for teenage children and most adults.
(b) Velocity
Moving water in the floodplain exerts a lateral force resulting
from momentum thrust of the flood flow. This force acts to displace objects in a
downstream direction. The shear force of friction of a person on the wet surface
of the floodplain resists this force. However, even relatively low velocities of flow
in the floodplain can pose possible flood hazards.
The force exerted by various flow velocities can be developed
for different age and size groups, but because its effect is tied to depth, a better
appreciation of velocity effects can be gained by looking at both depth and velocity
in combination.
(c) Combination of Depth and Velocity
As a guide for personnel involved in stream flow/depth monitoring,
the simple “3 x 3 rule” was developed in the U.S. based on 3 ft depth and 3
ft/s velocity values. The rule suggests that people would be at risk if the product
(multiple) of the velocity and the depth exceeded 0.8 m2/s (9 ft.2/s).
The Water Survey of Canada has the same rule of thumb and
its Hydrometric Field Manual (1981) states, “a general rule of thumb which has
been used in the past is arrived at through the product of the depth and velocity.
Generally speaking, if the bed is firm and provides good footing, the product of
these two factors should be slightly less than 1 m2/s, or roughly 9 ft.2/s”.
It should be noted that this rule of thumb applies to trained
professionals whose regular work accustoms them to the dynamic forces of
river flows, buoyant forces from partial submergence and recognition of potential
hazards, e.g. rocks, depressions, etc. They also enter the stream with
equipment which will assist them in maintaining stability, e.g. tag line, wading
rod, strap-on cleats for greater stability.
It is considered highly unlikely that such equipment would be
available to most occupants of floodproofed buildings in the flood plain. It
seems equally unlikely that these occupants would have the same level of
experience as water survey staff in dealing with high depths, current speeds,
unsteady footing, or cold weather/water conditions.
As a result, it is likely that the simple rule of 3 x 3 product (1 m2/
s or 9 ft. 2/s) represents an upper limit for adult male occupants in the flood
plain and that it would be reasonable to consider something lower as being
more representative of a safe upper limit for most flood plain occupants.
As noted earlier, any person on foot during a flood may be
subject to a number of forces in the floodplain. Excluding impact by ice and/or
other debris, these forces include:
· an upward buoyant force, equal to the weight of the
fluid displaced;
· a lateral force exerted by the moving water (linear
momentum);
· unbalanced hydrostatic forces.
Resisting these forces are:
· the shear force of friction acting through the weight of
the person standing on a wet surface in the floodplain.
Figure 6-1 provides a graphical representation of depth and
velocity hazards in the flood plain to show the limits of stability. Unit weights of
976, 1464 and 1952 kg/m2 (200,300 and 400 lb/ft.2) are used. Adults of average
size would fall into the range between 976 -1952 kg/m2 (200 - 400 lb/ft.2)
but young children would more appropriately fall into a range of 732 - 1464 kg/
m2 (150 - 300 lb/ft.2). Only 7% of Ontario’s population is within the 6 - 10 year
age range, i.e. young children (Statistics Canada, 1981).
