In preparation for our exercises, our exercise planner, Tony Reidell, always assembles a background paper on the scenario to help put a face and scale on real, related events in Colorado history to our exercise scenarios. I always find these interesting and since we are preparing for a State-level flood exercise today, I thought I would pass along Tony's write-up. If you have any questions about the background paper or exercise planning, you can contact Tony at email@example.com
Historical Colorado Flood Events
The average number of thunderstorm days per year in Colorado varies from less than 40 near the western boundary to over 70 in the mountains along the Front Range. The thunderstorm flood season in Colorado is from the middle of July through October.
Cherry Creek Basin Flood of July 14, 1912.
During Sunday afternoon and evening, July 14, 1912, torrential rains occurred in the greater portion of the Cherry Creek drainage basin, especially between Castle Rock and Denver. This stream is an unimportant tributary of the South Platte River, and forms a junction with that river about 200 yards above the Sixteenth Street viaduct in Denver. It has its source about 50 miles to the south of Denver, and its course to the South Platte lies through the heart of Denver. Its ordinary flow is insignificant, and this is true of the volume of water carried by it during most of Sunday.
Between 3.25 p.m. and 6.30 p. m., 2 inches of rain fell in Denver. Of that amount 1.72 inches fell in a period of 30 minutes. At Castle Rock the rain began at 5.15 p.m. For a period of 25 minutes it was very heavy, and by 7 p.m. 1 inch had fallen. It continued after 7 p. in. in the form of a drizzle, and by the morning of the 15th an additional 0.65 inch had occurred. in the southern portion of Denver and in that part of the drainage area lying to the north of Castle Rock the rainfall was apparently considerably greater than at the Weather Bureau station.
Aside from flooding streets, the rainfall in Denver proper did not contribute materially to the flood that later in the night inundated the low-lying districts of the city. However, it caused a rise of about 3 feet in the South Platte between 3.30 p.m. and 4.30 p.m. up to 8 p.m. there was but little increase in the flow in Cherry Creek, and no evidence of flood conditions were observed in that stream in the vicinity of Denver, but between 8.30 p.m. and 9.30 p.m. the water came down Cherry Creek with a rush. By 9:45 p.m. the stream was bankfull, and by 10 p. m. the water overflowed the banks and spread out two blocks on either side of the creek, north of Welton Street. In the vicinity of the railroad yards and Union Depot the area flooded was much greater.
At 10.45 p.m. the water was about 2 feet above the floors of the bridges that cross the stream in the downtown districts, and in the vicinity of the Union Depot it was from 2 to 3 feet deep. The maximum height attained was about 11 feet, and the maximum flow about 11,000 second-feet,. After 10.30 p.m. the flood subsided rapidly. At 11 p.m. the water was well within the banks of Cherry Creek. At midnight the stream had fallen decidedly, and by the early morning of the 15th only the ordinary flow remained.
Sand and silt were deposited to a depth of several inches in all localities that were flooded. Much damage resulted to crops, gardens, parks, streets, bridges, buildings, and to merchandise stored in the basements of buildings in the wholesale district. Traffic was interrupted on railroad , and the city tramway service was suspended for several hours. The Denver Gas & Electric Company, suffered material damage, and the Mountain States Telephone & Telephone Company, had many of its lines put out of commission. In aggregate, the damage to property was considerable, and is variously estimated at from $500,000 to $1,000,000. Two lives were lost and several persons were injured.
Golden Flash Flood of June 7, 1948.
"A cloudburst in the mountains northwest of Golden sent a 25 foot wall of water flashing down normally dry Tuckers Gulch through the northern section of Golden early yesterday causing damage estimated at between a quarter and a half million dollars."
"Although no one was reported killed or injured seriously, three small houses and two railway and three automobile bridges were washed away, basements' flooded and lawns and streets covered with tons of silt and debris."
"The stream in the canon (sic), normally only a trickle, soon was out of its banks and washed out state highway 58 in several places.
Another wall of water poured down Crawford's Gulch into Golden Gate Canon (sic) and then on to the outskirts of Golden where a third normally dry gulch, Christman (sic) Gulch, poured additional water into the stream. The water from the three streams poured into Tucker's Gulch and backed up behind a bridge carrying the railway spur to the Golden Fire Brick Co. plant, north of town.
When the bridge was washed out, the wall of water swept on down, carrying out the second railway bridge, the three automobile bridges and the main gas line of the Colorado Wyoming Gas Co...."
"Old timers in Golden were in disagreement as to whether yesterday's flood was worse than the one in 1896."
"...Although two houses were swept from their foundations and dissolved in the roaring torrent and many thousands .of dollars damage was done no lives were lost in the spectacular flash flood which descended on north Golden from Tucker's Gulch early Monday morning. It was estimated to have been the most serious in the city's history.
Advance warning of the oncoming wall of water, telephoned to Golden and relayed by sheriff's radio and Golden's police department, is credited with having prevented any loss of life." Damage Near $500,000
The 1965 Flood of the South Platte River
On the evening of June 16, 1965, a wall of water described by some as fifteen feet high came roaring down the South Platte River, the result of extremely severe thunderstorms many miles south of Littleton. By midnight, the torrent crested at twenty-five feet above normal and was carrying forty times the normal flow. In its wake, the course of the South Platte River from Littleton to the Colorado-Nebraska border was a mud-encased, wreckage-strewn landscape of desolation. The great South Platte River flood of 1965 was not Littleton's first flood, nor only disaster -- it was simply the biggest and costliest, by far.
In 1864, just two years after the homesteading of Richard Little and his neighbors, two weeks of constant, heavy rainfall sent the river out of its banks around Littleton and nearly destroyed Denver downstream. Another flood in 1914 lasted for six weeks, following record snowfalls the previous winter which at one time measured four feet deep on Main Street and even prevented trains from moving. Yet another flood occurred in 1900 when the Goose Creek Dam up Platte Canyon broke. In 1946, the snow again fell -- for seventy-one straight hours; in 1932 there was a severe drought; and in 1865, pioneer Mollie Sanford wrote of yet another plague: "In three days time an army of grasshoppers had destroyed the work of weeks." Life hasn't always been easy in Littleton.
There had been talk for some time about construction of a dam on the South Platte River above Littleton, but it inspired little interest. In the valley, Cherry Creek seemed to be the real nemesis, and that had been dammed in the 1930s. The wide, shallow, slow moving South Platte, even with the few examples above, didn't seem to warrant the same precaution. What the South Platte had become, instead, was a waste dump. All along its length through the Denver area it was an eyesore littered by abandoned cars, refrigerators, construction debris and everything else that people looked to discard. In 1965, there was an accounting for that lack of respect for the South Platte River.
Residents of Littleton and metropolitan Denver had little reason to anticipate a flood on Monday afternoon, June 16. Although a rare tornado and severe thunderstorms had hit Loveland a couple of days before, the forecast was for scattered thundershowers typical for a summer afternoon. In fact, it was not even local precipitation which fueled the flood, but a violent cloudburst many miles south near Castle Rock. The ground was saturated from previous days' rains, so the normally dry east and west branches of Plum Creek became raging torrents heading north to meet the South Platte, which was swollen itself by rains to the southwest.
Police were able to give people in Littleton several hours warning, so they could be evacuated. The first local casualty was the Columbine Country Club southwest of town, whose golf course and luxury homes were devastated. Overland Park golf course north of town suffered a similar fate. In between, Centennial Race Track, which was within days of opening its racing season, had most of its track and stable areas inundated. A massive rescue operation by owners, trainers and jockeys saved some 140 horses. The City's water supply, which consisted mainly of a series of wells along the river, was nearly destroyed. A network of fire hoses run from the nearest Denver outlets provided emergency water for months.
As the flood continued north, it was more than just water bashing the countryside -- it now included all the old cars and refrigerators and both old and new debris. This battering ram carried away or destroyed 26 bridges, including every one from Littleton north to the Colfax viaduct. Both Public Service Company power plants along the river were shut down, and emergency circuits became waterlogged and shorted out. As the flood continued north, other tributaries added their weight, Sand Creek and Clear Creek, and further north the Bijou and Little Beaver and the Poudre River. The communities of Sterling, Fort Morgan and Brush became isolated as the waters spread out over a quarter-million acres of farmland.
All told, it was estimated that the damage came to some $540 million, plus 28 persons lost their lives. The state could count itself fortunate that so few citizens were killed in one of Colorado's worst natural disasters because it began in broad daylight and few people were caught without some notice. On the positive side, much of the eastern plains received relief from a three-year drought and farmers made the most of the situation. Plans were quickly finalized and construction began on the Chatfield Dam, being completed in 1972. And with a massive cleanup required all along the South Platte, municipalities began to turn the valley into a beautiful greenbelt which today belies its garbage dump past. The river finally got its respect.
Tucker Gulch Flash Flooding of Golden, July 23-24, 1965
Similar to past events, heavy rainfall resulted in severe flash flooding of Tucker Gulch through Golden. It was reported that 4.5 inches fell within a one-hour period in the Tucker Gulch basin, exceeding the estimated one-hour 100-year precipitation by a factor of 1.8. In the lower reaches of Tucker Gulch the floodwaters were reported to have spread over about 17 blocks, causing an estimated $112,000 damage to 69 residences, three commercial enterprises, three railroad bridges, four street bridges, and utility lines. Local sources recalled that major damage to streets, bridges, and utilities resulted from the high channel velocities carrying large debris and silt, blocking bridges and forcing floodwaters down streets.
Photographs of the 1965 flood provide an illustrative account of this major flood event. The following descriptive information was provided by Vie Seiferth, City Engineer for Golden:
Photographs show the structural damage sustained by the 10th Street bridge, formerly State Highway 58. Major damage resulted from the force of a railroad trestle striking the bridge. The same trestle passed beneath the 9th Street bridge without causing damage.
The debris which collected south of Clear Creek resulted from Tucker Gulch overtopping its banks between 9th and 10th Streets, forcing floodwaters down Ford Street and across the bridge. The Mitchell School property is located left of the photo and the old Safeway property is located to the right. The Coors Wellness Center currently exists where the Safeway Store was located in 1965.
Photographs taken from a location beneath the existing Highway 58 overpass. The 1965 flood having occurred prior to the construction of the new highway. The Church Ditch siphon can be seen near the center of the photo, the Coors Porcelain Company can be seen in the background. The exposed sanitary sewer line has been replaced by a new line running along the west side of Ford Street.
The worker cleaned debris by hand from the culverts under the old road embankment near Boyd Street. The stream channel has been located to the west, since the 1965 flood. Today, Garden Street is located where this channel once existed.
Big Thompson Flood of July 31, 1976
The Big Thompson Canyon Flood occurred on 31 July, 1976, after a stationary thunderstorm dropped torrential rain in the area. The storm produced between 12 and 14 inches of rainfall in less than five hours. It was one of the deadliest freshwater floods in U.S. history. This resulted in a disastrous flash flood in which a 19 foot wall of water and debris washed down the canyon. In under two hours there where 145 fatalities, 418 homes and 152 businesses destroyed, and another 138 home damaged. More that $40 million dollars in damages were reported.
Late in July the front range of Colorado is generally characterized by warm sunny days with intermixed thunderstorms. On the 27 of July, 1997, wet tropical air stream in northward from Mexico supplied the moisture and an approaching cold front from the west provide the trigger for setting off heavy thunderstorms as the two moist air masses met over the state.
Intense rainfalls from the 28th to the 30th of July,1997, produced more than 10 inches of over a three day period in the areas of Fort Collins in Larimer County, Atwood and Waldona in Morgan County as well as Logan County. Trapped against the foothills, this monsoonal flow was responsible for more than half of the annual average rainfall for this area in less than 10 hours.
This slow moving storm resulted in five fatalities and 100 injured when unexpected surge waters swamped a mobile home parks and the City of Fort Collins. 120 mobile homes were destroyed and 2000 of other homes were damaged with 90 business closed and an additional 600 damaged. 400 people required rescued from windows, rooftops and stranded vehicles in the area. This storm was estimated to exceed the 500-year flood levels in some drainages. City services were overwhelmed as streets floored; power was lost in some areas.
Down stream (Atwood and Sterling) approximately 13,700 acres of agricultural lands were inundated with 1,400 residences and more that 200 additional businesses being affected.
The greatest single financial loss was experienced by Colorado State University, where floodwaters caused more that $120 to $160 million dollars in damages to campus buildings, books, utilities and other resources. Roads and bridges were damaged by the floor in 10 additional eastern counties, and over 100 projects were later added to the federal declaration to help cover costs of public infrastructure repairs.