FLOOD10617 ‎(2)‎

• County: Boulder
• Community: Boulder County and Incorporated Areas
• Title: FIS - Boulder County and Incorporated Areas - Vol 1
• Date: 10/4/2002
• Prepared For: Boulder County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

On May 8 and 9, 1957, approximately 3 to 5 inches of rain
the entire basin of St. Vrain Creek. The rain
approximately 10 p.m. and stopped at approximately 6 a.m.
at approximately 1 a.m., St. Vrain Creek peaked at
3,060 cfs. The flood damaged and destroyed irrigation
works and bridges downstream fram Lyons.

fell over
began at
On May 9,
Lyons at
diversion

~n 1969, heavy rainfall combined with snowmelt caused prolonged high
flows OD St. Vrain Creek. The worst flooding occurred on May 7
and 8 and from JuDe 15 to JuDe 21. Roads and bridges along the
stream were extensively damaged, streambanks were eroded, and
faruaands were flooded. On May 7, the peak discharge at Lyons was
2,900 cfs. (photographs of the 1969 flood are shown on Figures 17,
18, 19, and 20.)

The steep stream slopes create swift currents during a flood, which
produces added damages. Debris carried ~ the fast-moving water not
only threatens bridges and culverts, but batters houses or other
structures on the floodplain. The bridge and culvert crossings
often result in channel restriction, raising the water-surface
profile. Erosion undercuts and destroys structures which would
otherwise receive little damage fram inundation. Large quantities
of rocks are often deposited in portions of the channel, leaving
little capacity for future floods. Vegetative growth in the channel
also reduces the channel capacity and causes increased overbank flow
during high floods. Major flood damages in the study area are to
roads, culverts, and residential buildings.

Early records of floods in the Town of Nederland study area are
fragmented and lacking in detail. A description of the more-recent
floods follows:

June 22, 1951 - Boulder Daily Camera:

Nederland reservoir at the head of Boulder Canyon is full
for the first time in two years.

with water pouring over the 132-foot-high spillway at the
dam and heavy rains falling in the mountains, a flood
threat developed in Boulder Creek Thursday night but there
was little damage. Contrary to broadcast reports, the
water did not reach the canyon highway or cause any
interference with road bridges . . . It is not unusual for
the reservoir to fill; it has happened more than half of
the years since the dam was completed in 1910. But with
the dam unable to hold any more water, it means a possible
flooding of Boulder Creek if there are heavy rains and fast
thaws of snow at the headwaters.

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