FLOOD09853

• County: Denver
• Community: Denver
• Stream Name: South Platte River
• Basin: South Platte
• Title: Effects of the May 5-6, 1973 Storm in the Greater Denver Area
• Date: 12/31/1973
• Prepared For: State of Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

(Varnes, 1958, pI. 1). The well-done sketches
of rockfall, slump, debris slide, and earthflow
were drawn by Natalie J. Miller of Menlo Park,
Calif" and were first published by Tor H. Nil-
sen (1972). A companion report, dealing with
the rainfall-runoff frequencies of the storm
(Ducret and Hansen, 1973), has been published
by the Urban Drainage and Flood Control Dis-
trict (Denver).
INUNDATION

A large part of the total storm damage was
caused simply by inundation, with all its costly
side effects. In the private. sector, inundation
was the chief cause of loss. Many homes, com-
mercial buildings, warehouses, outbuildings,
utilities, and railroad rolling stock and yards
received damage. Side effects included the re-
lated spoilage of myriad household goods and
other personal properties, waterlogging of crop-
lands and pastures, the drowning of livestock,
and the indirect loss of income of the flood vic-
tims. The list is endless.
Inundation resulted not only from overbank
flooding of streams but also from the backup
of water behind obstructions at bridges and cul-
verts along tributaries that otherwise would
not have gone out of their banks, or along arti-
ficial drainages that had not been intended to
accommodate more than moderate volumes of
storm runoff, Much flooding was aggravated
by intensified drainage in urban areas where
roofs, driveways, walks, streets, parking lots,
and other impermeable surfaces contributed to
high peak runoff.
The main stem of the South Platte River was
out of its banks from Douglas County to the
Nebraska State line, having crested in Denver
late Sunday night, May 6, and early Monday
morning, May 7. At peak discharge, its flood
attained an approximate recurrence interval of
50 years at Littleton, 1.1 X 50 years at Denver
below the mouth of Cherry Creek, and 1.4 X 50
years at Henderson (Ducret and Hansen, 1973,
table 3), Flooding along the South Platte in the
metropolitan area was heaviest in the vicinity
of Chatfield Dam (under construction in 1973)
and Centennial Racetrack in Littleton; in the
vicinity of West Alameda Avenue and South
Santa Fe Drive in the Westwood section of Den-
ver, where Interstate 25 Highway was closed to
traffic for several days; in the lower downtown

area in the vicinity of the railroad yards; in
Commerce City; in Henderson; and in Brighton.
Elsewhere in Greater Denver, urban flooding
was heavy locally where tributaries went out of
their banks or backed up behind obstructions
(fig, 2), In particular, extensive areas in Engle-
wood were inundated by Little Dry Creek, the
peak discharge of which had an approximate
recurrence interval of 20 years (Ducret and
Hansen, 1973, table 4). Many places throughout
Greater Denver were flooded by overflow from
canals and ditches.

OVERWHELMING OF ARTIFICIAL DRAINAGE SYSTEMS
In many built-up areas artificial drainage
systems were inadequate to accommodate the
runoff, and water quickly rose to flood levels
(fig. 3). Proportionately, the amount of runoff
that reached trunk drainages in built-up areas
was much larger than in non built-up areas of
equal size, simply because runoff, which other-
wise would have infiltrated the ground drained
off totally from impervious roofs, sidewalks,
streets, gutters, and pavements. Drains designed
to accommodate modest rainstorms were over-
whelmed or blocked with debris, and as tribu-
tary drain systems merged, the problem was
compounded, Many areas, such as much of
Lakewood, simply lack adequate facilities to
handle heavy storm runoff (The Lakewood Sen-
tinel, May 10, 1973). Facilities that may have
been adequate in the past, moreover, were sud-
denly unable to accommodate the surge of water
from new subdivisions higher in the watershed.
As growth continues, smaller and smaller
storms are able to generate larger and larger
discharge.

PROBLEMS WITH CANAlS AND DITCHES
Many irrigation canals and ditches were
quickly filled to the point of spillage and wash-
out despite efforts to shut them off at the source,
But the flow of a canal cannot be turned off
like a water faucet just by closing a headgate
that is, perhaps, miles away. Canals flow nearly
along the contour of the ground, and they inter-
cept storm runoff from many minor drainage-
ways that normally carry no water. Highline
Canal is a good example. It heads at the mouth
of Platte Canyon and flows northeastward
across the Greater Denver area to a point in
Adams County east of the Rocky Mountain Ar-

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