THE CORPORATION OF THE CITY OF PORT COQUITLAM C ¢ U N C

MEMORANDUM «JAN 5 1981

Mayor and Council December 31, 1980

F.E. Peters, P. Eng.,
City Engineer

Report on Flooding during Boxing Day

As a result of certain areas of the City flooding twice
in two years I am sure that Council as well as the public will
have questions. This report is to explain the operation of our
drainage system and in particular the problems in the Cedar Drive
and Dominion area. The following are a few facts about the

system.

1. The pump at the Cedar Drive Pump Station has been
designed to remove all runoff from a one in twenty-five year
Summer storm over a period of 24 hours. This assumes that the
river will be in freshet so that the floodgates will remain
closed. The capacity of the pump-is 40 cubic feet per second
which is approximately 15,000 imperial gallons per minute.

-2. The actual storm in question deposited approximately
67 millimeters of rainfall over a period of 15 hours. It was
therefore not as large as last year's storm which devosited that
amount over a period of 8 hours. The estimated frequency of the
actual storm is approximately one in five years. However, a one
in five year winter storm is greater than a one in twenty-five
year summer storm and thus exceeded the capacity of the pump.

3. The calculated runoff from the above storm is about
147 cubic feet per second which is three and one-half times
greater than the capacity of the pump.

4. The design criteria for the pump station was that
winter storms are to be handled by the floodgates and that even
during the worst winter tides, the floodgates would open enough
to allow the discharge of a one in twenty-five winter storm over
a period of 24 hours. .

5. The actual Capacity of the floodgates depends on how
high the water builds up behind the floodboxes. ‘The two flcod-—
boxes have a capacity of approximately 400 cubic feet per second
when the water level is one foot higher on the upstream side than
on the downstream side. Thus, with a water difference in
elevation of one foot the floodgates have ten times the capacity

of the pump.