Snow Pack and Ice Thaw

Flood Zones and Spring Thaw

2021’s decent snowpack is not enough to prevent an earlier-than-average thaw

In Colorado, the beginning of April generally marks the time when the mountain snowpack reaches its peak. At this point, the rate of melting is usually greater than the rate of snow accumulation, and the spring thaw begins. For Coloradans, this means water, and lots of it, bringing the potential for flooding, landslides, debris flows, and even dam failure. For property owners, knowledge of the warning signs, adequate preparation, and emergency planning can make the difference between a total loss and keeping your head above water.

Snowpack

While the snowpack in Colorado struggled at the onset of 2021, it made up ground thanks to a stormy February and March. By March 17, the snowpack statewide reached 91% of its median snowpack for mid-March, and areas in all but the west and southwest parts of the state rested at or close to average.

The characteristic of snowpack most important is the amount of water held within the snowpack, known as the snow water equivalent. Snowpack is tracked as an indicator of how much melt to expect in the springtime, which directly contributes to the filling of reservoirs and replenishing water supply. In fact, as much as 75% of water supplies in the West come from snowmelt.

Snowpack also keeps the ground and soil moist while it melts into spring and summer, which can have an effect on delaying the fire season as well as the severity of wildfires. Although observations over the last 50 years has indicated a decrease in total snowfall and snowpack in the West, the risks associated with spring thaw remain.

Snowmelt and Flash Flooding

While major floods in Colorado generally happen due to rainfall events, localized and isolated flooding in the spring can result from snowmelt. In fact, eight of the most significant floods of the 20th century in the United States, classified by area affected, property damage, and fatalities, fell into the category of snowmelt flooding.

Snowmelt flooding is common in the spring, and can be exacerbated by what is known as “rain-on-snow” events. When heavy precipitation occurs, especially as the snowpack is melting, water cannot be absorbed into the ground and runs along the surface, causing mass-wasting of hillsides, damage to areas along streams, and downstream flooding. Coupled with frozen ground, high soil moisture, or warmer temperatures, which make for a rapid thaw, this can lead to prime flooding and flash flooding conditions.

Flash flooding occurs in Colorado from May through September and is caused by severe storms which produce runoff from the mountains. According to the Mile High Flood District, four of the five most destructive floods to affect the Denver region occurred in May or June.

In 1969, the Boulder Creek Flood was caused by severe river flooding after 60 hours of continuous rain walloped the area. In 1864, the Cherry Creek Flood caused an estimated $1 million in property damage, and resulted from the culmination of a snowy winter, heavy spring rain, and a strong thunderstorm event. Snow and rain were also the cause of the 1973 South Platte River Flood, which killed 10 people.

Finally, the most destructive flood in the state’s history was the 1965 Flood, which resulted in the formation of the Flood Control District. This event, which pummeled the Denver region with rainfall totals as high as 15.5 inches, also prompted the construction of the Chatfield and Bear Creek Reservoirs.

Mud and Debris Flows

Aside from flooding, rapid snowmelt can bring on landslides and debris flows. Especially in areas where wildfires have occurred, vegetation may no longer be present and soil properties may be altered, leaving bare ground behind which tends to repels water. These areas are known as burn scars and can also produce mud and debris flows.

These flows occur when normal, dry soil becomes over saturated. The saturated soil then enters a liquid state and flows downhill just like water. These hazardous events can destroy property, wash out roads and bridges, and knock down trees.

If you work in an area near the site of the 2020 wildfires, pay close attention to soil conditions this year.

Dam Failure

2013 marked the 7th historic flood in Colorado since 1902, indicating a frequency of historic floods of every 16 years. During the 2013 floods, nine dams of low-hazard failed. At this rate of flooding, every person involved with dam safety can expect to see an event like this during their careers.

The causes of these particular dam failures range from natural—rainfall, landslides, and erosion—to human—involving improper maintenance and negligent operations. Dam failure is categorized into three causal groups that include overtopping, excessive seepage, and structural failure.

According to FEMA and the National Inventory of Dams, approximately one-third of the more than 80,000 dams in the United States pose a “high” or “significant” hazard to life and property if failure occurs. These failures, or even levee breaches, can occur without warning, but they take a lot longer to develop as opposed to a flash flood. Where a flash flood can occur within a few minutes or hours after a rainfall event, dam breaks and levee failures can take days or weeks to occur.

Flood Preparation and Safety

Members and districts should visit fema.gov/flood-maps to utilize their map service center tool. This tool shares flood maps for all regions to help determine if an organization is within a regulatory floodway or within a region that is susceptible to flooding. This site also shows where flood risk is elevated due to close proximity to a dam or levee. This can be a valuable tool when determining what type of emergency preparations an organization needs to undertake.

And make sure that staff understands what to do both before and during a flood event.

Before a flood strikes:

  • Create an evacuation plan for staff.
  • Have an emergency communication plan in place to notify employees on and off site.
  • Have a contingency plan in place for continuing operations if you cannot return to work. Talk with the CSD Pool about Business Income and Extra Expense Coverage to verify your limit is sufficient to cover operating costs in the event of an emergency and for the retention of employees for up to one year.
  • Verify the values you have chosen for buildings and equipment to ensure that you can clean up premises, remove debris, replace facilities, and pay for these extra expenses at catastrophe rates.
  • Understand our flood coverage. The CSD Pool offers a $2 million limit for locations in Flood Zone A.  FEMA only offers $500,000 for buildings and the same for contents, with no Business Income or Extra Expense coverage.
  • Understand how to document damage and post-flood response options if damage occurs.

During a flood:

  • Move to higher ground.
  • Do not attempt to walk across flowing streams or drive through flooded roadways.
  • If you come in contact with floodwater, wash hands with soap and disinfected water.

After a flood:

  • If you operate or maintain a dam or levee, assess its structural integrity and review operations.
  • If your property is damaged, email cxcsd@sedgwick.com to file a claim.
  • For clean-up efforts, contact SERVPRO. SERVPRO uses advanced equipment and techniques to remove water quickly. They closely monitor and document the drying process to verify your customer’s property is dried properly and thoroughly. Pool members and their constituents are eligible to receive pre-disaster pricing and priority service.
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