From The Pueblo Chieftain (Chris Woodka):
The type of storm that would creating the worst flooding on Fountain Creek and the Arkansas River east of Pueblo might just seem like another rainy day for much of the region. But the lessons of floods in 1965 and last September’s close call for Pueblo show that Fountain Creek can froth up in a hurry when rains hit El Paso County to the north. Putting a small dam here and there would not be the most effective way to stop the water.
A recent U.S. Geological Survey study of dams on Fountain Creek shows that an 85-foot tall dam north of Pueblo would be the single-most effective way to mellow out flood waters and trap sediment. The drawbacks of the dam are that highways, railroad crossings and utilities might have to be relocated. There would also be the chore of removing sediment after large storms.
Smaller detention ponds, with dams no higher than 10 feet, are touted by many as a better alternative. But as Colorado Springs and Pueblo already are discovering, smaller ponds also require high maintenance. Similar dams failed to hold stormwater in the South Platte during last September’s record rains. And the cost of flooding to utilities and roads was a major side effect of the 1965 flood.
A different study of flooding was done by the USGS in 1974, nine years after the disastrous 1965 flood. Unlike the current study, it largely eluded the spotlight and has not been widely cited during the 40 years since it was written. It looked at floods in the Arkansas River basin in three states, Colorado, Kansas and New Mexico and assessed the causes, effects and damage caused by heavy rains from June 13-20, 1965. The study chronicled $60 million of damage overall, with $40 million in Colorado. In today’s dollars, that would be about $300 million. Of that, 55 percent of the damage was to agriculture; 20 percent to roads and utilities; and 25 percent to cities and businesses, with about 85 percent of that amount in Pueblo.
The study also looked at peak flows within the basin during the 1965 flood and compared them to other major floods, particularly the 1921 flood on the Arkansas River. The flows were considerably less in 1965 than in 1921, mainly because storms were centered over tributaries that fed into the Arkansas River below Pueblo, rather than in the watershed upstream from Pueblo.
The study found a huge benefit to Lamar from John Martin Reservoir, which cut two-thirds of the peak flows raging from upstream. The Lamar area did not escape the wrath of the storm, however, because of large storm cells centered above Two Buttes and Holly. The Arkansas River stayed swollen for days after the rains.
The heaviest rainfall in the 1965 storm came from Colorado Springs and the Holly-Two Buttes area, where 12-18 inches fell over a four-day period. Pueblo saw only a couple of inches during that time. The ground already was saturated from rains the previous two months throughout the region. Flows on Fountain Creek reached 47,000 cubic feet per second at their peak, while neighboring Chico Creek hit 52,000 cfs.
The 2014 study by the USGS modeled a 100-year storm that would send about 37,000 cfs from Colorado Springs to Pueblo and then looked at hypothetical dams along the way.
“A dam at any location could be modeled,” said David Mau, head of the Pueblo USGS office.
The intensity of that storm would not be as great as the 1965 flood. In addition, Colorado Springs today has five times as many people and many more square miles of parking lots, roof tops and streets that shed water quickly and would make flooding that much worse for Pueblo.
Levees were built on Fountain Creek to protect Pueblo, but sediment has reduced their effectiveness. Some structures meant to protect Pueblo were damaged by the relatively small flow last September.
The attention in Colorado Springs is focused on the accelerated runoff from the Waldo Canyon and Black Forest fires. Structures are being built. Town meetings are preparing neighborhoods for flooding. A vote to create a regional stormwater fee is heading for the ballot in November.
Colorado Springs also made a commitment to Pueblo County in its permit process that new development from the Southern Delivery System won’t worsen the condition of Fountain Creek.
While the rains may hit Colorado Springs first and make flooding more intense because of the fires, the 1974 USGS study shows the bigger wallop would come to Pueblo and the Lower Arkansas Valley.
More Fountain Creek watershed coverage here.