The July 28,1997 Fort Collins Flood:

Putting the Stormwater System to the Test

How Did it Serve to Minimize Societal Impact?

Matt KelschMatt KelschHydrometeorologistHydrometeorologistField Trip LeaderField Trip LeaderUCAR/COMETUCAR/COMET

August 12, 2008August 12, 2008

Material Contributed by:Material Contributed by:Marsha Hilmes-RobinsonMarsha Hilmes-RobinsonFloodplain AdministratorFloodplain AdministratorCity of Fort Collins, ColoradoCity of Fort Collins, Colorado

Stormwater Utility

• System Repair and Maintenance

• System Construction

• Development Review

• Floodplain Administration

• Water Quality

• Master Planning

Annual budget is approx. $6 million.

What is a 100-year flood?

• The 100-year flood is a statistical designation for a specific streamflow that has a 1% chance of being equaled or exceeded in any given year.

• The 500-year flood has a 0.2% chance of being equaled or exceeded in any given year.

• Short period-of-record and urbanization greatly affect the accuracy of these measures.

FloodplainsFEMA:

Poudre River, Spring Creek, Dry Creek, Boxelder Creek, Cooper Slough

Locally Designated:

Fossil Creek, Old Town, West Vine

The Meteorology

Low level flow into the terrain (upslope).

Dewpoints of 60+ ( a big deal for Colorado).

The Radar View

The Localized Nature

Localized area of extreme rain on SW side of town.

Spring Creek watershed (13 sq miles) takes the bulk.

Water surges eastward along west-east roads.

Rainfall 5:30-11:00 PM 28 July 1997

Spring Creek Discharge Estimates

Location 100-year Discharge

(cfs)

500-year Discharge

(cfs)

July 28, 1997 Discharge

(cfs) Taft Hill Rd. 1,492 2,347 3,500

Drake Rd. 1,635 2,575 4,200

Shields St. 2,135 3,325 8,250

Remington St. 1,528 1,846 5,000

Riverside St. 2,187 2,920 5,500

NWS Radar Estimatesvs. Actual Precipitation

Time NWS Actual 6-7 pm 0.6 1.02 7-8 pm 0.6 1.02 8-9 pm 1.2 2.03

9-10 pm 2.5 4.24 10-11 pm 0.6 1.02 11-12 pm 0.4 0.68

TOTAL 5.9 10.01

NOAA 100-year, 6-hour rainfall = 3.5 inches

Timetable of Flood Events5:30

8:00

8:30

9:00

9:40

10:30

10:55

1:20

Rain begins, Flash Flood Watch is in Effect

EOC Activated

Ponds Overflowing, rapid water rescues begin

Most intense rain commences

NWS Flash Flood Warning

Storm begins to dissipate and move northeast

Trailer Park Flooding, Fires, Train Derailment

Declared City Disaster

The Impact5 dead, dozens

injured, major destruction along Spring Creek

Over $100 million in damages at CSU.

Building Damages

Destroyed: 97 mobile homes, 5 houses

Damaged: 1,988 properties

Past Floods on Spring Creek• Prior to Horsetooth Reservoir 1902, 1904, 1938, 1949,1951• Recent Flooding 1975 and 1977

Spring Creek Improvements:$5 Million spent since 1989

• Acquisition and Relocation of Structures• Channelization• Storm Drainage Improvements• Reinforcement of Burlington-Northern Railroad

Embankment• Bridge Improvements

Reduced RiskBecause of Acquisition

• 30 Mobile HomesAssume 2 people per mobile home

• 9 Residential StructuresAssume 2 people per home

• 1 Retirement Home15 residents

• 1 Businessassume 3 workers, 2 customers

2 X 30 = 60

2 X 9 = 18

15

5

TOTAL = 98

Flash Flood Field Trip Route

18Boulder

Fort Collins

Big Thompson Canyon

Rocky Mountain National Park

Lawn Lake

Lunch in the Big Thompson Canyon

19

Rocky Mountain national Park

20