Flood — Boulder, Colorado

2013-09-12 to 2013-09-15 · near Superior, Boulder, Colorado

Event narrative

Heavy rain, ranging from 6 to 18 inches through the storm event, caused widespread flooding along the entire drainage systems of Boulder and St. Vrain Creeks. Three vehicles were partially submerged when floodwaters undercut Dillon Road near CO287, between Lafayette and Broomfield. When the road first collapsed, several people were able to get out of the water quickly, but three were trapped in their vehicles and had to be rescued. The three drivers were taken to local hospitals for treatment. Extensive flooding was reported in Longmont and forced several road closures. Floodwaters tipped over two storage tanks containing sulfuric acid at the Longmont Brewery. Several shelters were set up for evacuees. Numerous bridges, culverts and roads were damaged in eastern Boulder County. The town of Lyons was completely isolated by floodwaters. The wastewater treatment plant was destroyed. There were numerous rock and mud slides in the area. Mandatory evacuations were ordered for the entire town of Lyons. Train tracks were washed out near the intersection of the Diagonal and Jay Road in Boulder. Boulder Canyon was closed from Nederland to Magnolia. South Boulder Road was closed due to flooding between Cherryvale and 55th. About 40 buildings on the University of Boulder campus were damaged and 400 students evacuated.

According to FEMA, 10,588 homes were impacted by flooding. Preliminary damage assessments for Boulder, Lyons, Longmont, Jamestown, Superior and unincorporated Boulder County included: 337 homes destroyed; 428 homes with major damage; 3,426 homes with minor damage; 33 commercial buildings with major damage; 54 commercial buildings with minor damage. The most extensive damage impacted lakes and rivers; in particular, along and near the St. Vrain River corridor.

The breakdown for the damage estimate in Boulder County included: $132 million for the city of Longmont; $91.2 million in damage to government facilities, roads and bridges in unincorporated Boulder County; $49 million for the city of Boulder's infrastructure, parks and open space; $25 million for the county's parks, trails and open spaces.

Wider weather episode

A deep southerly flow over Colorado, ahead of a near stationary low pressure system over the Great Basin, pumped copious amounts of monsoonal moisture into the area. In addition, a weak stationary front stretched along the Front Range Foothills and Palmer Divide. As a result, a prolonged period of moderate to heavy rain developed across the Front Range Foothills, Palmer Divide, Urban Corridor. By the 14th, storm totals ranged from 6 to 18 inches, highest in the foothills of Boulder County. The headwaters then moved down the South Platte River and caused widespread flooding with record flood stages at several locations as it made its way downstream into Nebraska. Record high flood stages along the South Platte River were recorded at Balzac, Weldona and Julesburg. As a result, widespread flooding occur along the South Platte for several days as the headwaters continued downstream, with much of the damage reported from Fort Morgan and Brush to around Sterling. The floodwaters resulted in widespread evacuations. Numerous water treatment facilities were contaminated and had to be shut down. The headwaters remained at or above flood stage east of Sterling to Julesburg, but the flooding was not as widespread and the damage not as extensive.

Initially, the showers that developed on the afternoon of the 11th were moving fast enough and were small enough to produce generally moderate rainfall amounts. However, rain along the Front Range began to intensify and become more widespread in the evening. A narrow band of very heavy rain began to organize just north of Denver and move westward toward the foothills. By 9 pm mdt on the 11th, heavy rain was becoming more widespread, with rainfall rates near 2 inches per hour in the heaviest showers. From 10 pm mdt to Midnight mdt, very heavy rain locked itself in against the Front Range. Flash flooding became more widespread, with several road closures from northwest Jefferson County northward through Boulder and Larimer Counties. The heaviest rain in the foothills of Boulder, Larimer and northern Jefferson Counties finally began to abate, but widespread flash flooding continued due to the extremely heavy rainfall during the evening and early morning hours of the 12th. Meanwhile, torrential rain redeveloped in southwest Weld, northeast Adams, eastern Denver and northwest Arapahoe Counties in the early morning hours of the 12th; which impacted northeast Denver and Aurora.

Atypical for September in Colorado, 24-hour rainfall totals ending 7 am mdt on September 12th included 6.5 to 8.6 inches in Boulder; with 3 to 6.5 inches across the Larimer and Boulder County foothills, as well an area stretching from northeastern Metro Denver area into southwest Weld County. By 9 am mdt, heavy rain and considerable flash flooding continued through the morning in the Aurora area. Numerous streams remained at or above bankfull levels across Boulder, Larimer, and southwest Weld counties. By the evening of the 12th, flooding continued in many areas and increased in severity. Heavy rain, while not as widespread as the previous day, had a greater impact on stream levels as streams were already elevated and the ground was completely saturated. Two peaks of flows occurred on Boulder Creek near downtown. The peak flow on the evening of September 12th surpassed the 1969 Boulder flood, but initial indications are that it fell short of the 1938 and 1894 floods. Boulder Stream Gauge 'peak flows occurred near 0100 mdt on the 12th, and 2100 mdt on the 12th. Similar massive stream flows occurred on many other creeks and rivers up and down the Front Range, including: Big Thompson River, Lefthand Creek, Little Thompson, Cache la Poudre, St Vrain Creek, Coal Creek, South Boulder Creek and Sand Creek. Similar massive stream flows occurred on many other creeks and rivers up and down the Front Range, including: Lefthand Creek, Little Thompson, Cache la Poudre, St Vrain Creek, Coal Creek, South Boulder Creek and Sand Creek. The headwaters moved down the South Platte with floodwaters impacted Weld, Morgan, Washington and Logan Counties. Extensive flooding occurred in and around Greeley, Fort Morgan and Sterling. Large sections of the counties' roads and highways were washed away, with property and crops completely inundated.

After little rainfall on Friday, September 13th, the flash flood threat returned for Saturday. This time, storms with heavy rainfall were concentrated in Douglas County, but also extended northeast into the Aurora area once again. Up to 3 inches of rain fell in one hour. Plum Creek and other small creeks and streams flooded, along with significant street flooding. The last day of this prolonged period of very heavy rainfall was on Sunday, September 15th. Another weak front had pushed through the area Saturday night, leaving the atmosphere unseasonably moist and unstable. Heavy rain developed by mid morning and then became more widespread and peaked in intensity by the noon hour. This time, flash flooding was most significant in the northern portions of the Denver metro area from Northglenn to around Broomfield. Rain finally tapered off during the late afternoon and evening.

The devastating flood damage encompassed 4,500 square miles of the Colorado Front Range, left seven dead, forced thousands to evacuate, and destroyed thousands of homes and farms. Record amounts of rainfall generated flash floods that tore up roads and lines of communication, leaving many stranded. Nearly 19,000 homes were damaged, and over 1,500 destroyed. The Colorado Department of Transportation estimated that at least 30 state highway bridges were destroyed and an additional 20 seriously damaged. A preliminary assessment of the state's infrastructure showed damage of $40 million to roads and $112 million to bridges. Repair costs for state and and county roads were likely to run into the hundreds of millions of dollars. Miles of freight and passenger rail lines were washed out or submerged, including a section servicing Amtrak's iconic California Zephyr. The town of Lyons in Boulder County was isolated by the flooding of St. Vrain Creek, and several earth dams along the Front Range burst or were over-topped. Floodwaters also swept through Estes Park, the gateway to Rocky Mountain National Park and a major tourist attraction, and damaged hundreds of buildings and destroyed large sections of the two main roads that led into town; U.S. 34 from Loveland and U.S. 36 from Lyons/Boulder. U.S. 34 suffered the most damage, with 85 percent of its roadway and bridges destroyed. In Weld County, approximately 1,900 gas wells were damaged and had to be closed off as the floodwaters inundated entire communities.

Sewage treatment plants and other utilities were knocked out in a number of towns, and standing water left by floods on prairie farmlands posed the threat of significant damage to crops already planted in the region. Northeast Colorado Health Department has advised with the local CSU Extension office in Logan County regarding local crops and gardens that had been exposed to flood waters. As the flood waters in our area should be considered contaminated, local crops and gardens may not be considered suitable for human consumption or animal feed. In the flooded areas, an estimated 23,000 acres of cropland was impacted and stretched from Boulder to Logan Counties and included: alfalfa, corn, hay, silage and sugar beets. The estimated loss of production ranged from $3.4 million to $5.5 million.

Governor Hickenlooper declared a disaster emergency on September 13th, in 11 counties across northeast Colorado including: Adams, Arapahoe, Broomfield, Boulder, Denver, Jefferson, Larimer, Logan, Morgan, Washington and Weld. By September 15th, federal emergency declarations covered those counties as well as Clear Creek County. Projected losses from the flooding statewide was nearly $3 billion in property damage. Half of the damage estimate was from housing and half from the commercial and government sectors, according to Eqecat, a catastrophe modeling firm; this was the first comprehensive estimate of the disaster's economic toll. The projected losses for residential property alone were about $900 million. Another $1 billion was attributed to commercial and goverment property, including roads and bridges. In addition, more than 11 thousand people were evacuated, reportedly the largest since Hurricane Katrina. President Obama declared a state of emergency for Boulder and Larimer Counties, with an additional 10 counties added September 16th including: Adams, Arapahoe, Broomfield, Clear Creek, Denver, Jefferson, Morgan, Logan, Washington and Weld Counties. The president also declared a major disaster specifically for Boulder County, which provided for federal recovery assistance.

There were six fatalities directly attributed to flash flooding. By county, those fatalities included 4 in Boulder County and 2 in Larimer County. Two 19-yr old teenagers died on the evening of the 11th, after they were swept away by floodwaters after abandoning their vehicle on Lindon Drive in Boulder. In Jamestown, a 72-yr old man was killed when the building he was in collapsed. An 80-yr old Lyons resident was killed in the early morning hours of the 12th, when his truck was swept into the St. Vrain River near his home. On the evening of the 12th, a 79-yr old Larimer County resident was killed when she was swept away while trying to climb to safety from her home in Cedar Point. A 61-yr old Cedar Point resident died when her home was swept down the Big Thompson River by the floodwaters. An indirect fatality occurred on the 16th. An 80-yr old Idaho Springs resident drowned in Clear Creek when the embankment he was standing on collapsed.

Along the South Platte River, the crest of the flood at Kersey was near 18.8 feet on September 14th, which was significantly higher than the record crest of 11.73 feet set on May 8, 1973. On September 15th, the flood crest was near 13.7 feet at Balzac which exceeded the record of 13.3 feet set on June 18, 1965. On September 18th, the flood crest was at Julesburg was 10.6 feet, which broke the previous record of 10.44 feet set on June 20, 1965.

In Boulder, some of the monthly records broken included: one-day all-time record: 9.08 inches which shattered the previous wettest day of 4.8 inches set on July 31, 1919; one-month record of 18.16 inches, which broke the previous all-time monthly record of 9.59 inches set in May of 1995; wettest September on record which broke the previous record of 5.5 inches set in September of 1940; one-year record of 34.15 inches broke the previous wettest year of 29.93 inches set in 1995. In Denver, the total precipitation for the month of September was 5.61 inches, which was 4.65 inches above the normal of 0.96 inches. This is the most precipitation ever recorded in Denver for the month of September.

View location on OpenStreetMap → (39.9200, -105.1500)

Source: NOAA Storm Events Database, event_id 474687. Narrative written by the NWS forecast office that issued the report.