Status and Monitoring, Longmont Flood Preparedness and Spring Runoff Information
Understanding the Variables
Whether or not these factors cause flooding often depends on daily weather conditions over the region such as temperature and precipitation.
Deep snow can melt into a lot of water. Deep snow very rarely causes flooding by itself. Often, heavy rain and rapid warm ups combine with rapidly melting snow to cause major flooding problems. For reference and perspective as we head into 2014 spring runoff, download a chart detailing the top 5 runoff events on St. Vrain Creek since 1990. This chart helps demonstrate that - based on historical data - the estimated 2500 cubic feet per second (CFS) post-flood capacity of St. Vrain Creek should be sufficient to carry 2014 runoff if other influencing factors such as rain are not abnormal.
Frozen soil can not absorb as much water as unfrozen soil. Rain or rapid snow melt atop frozen soil can cause flooding that wouldn't have occurred if the soil were not frozen.
Saturated soil can't absorb rain and water from melting snow. The excess water becomes runoff that rapidly flows into rivers and streams. Unsaturated soil acts like a sponge, absorbing some of the water from rain or snow melt. Saturated soil by itself does not cause flooding. Usually, heavy rain or rapid snow melt combined with saturated soil causes the flooding.
Reservoirs are large, mostly man-made basins that hold water for irrigation and drinking. Reservoirs can alleviate river flooding by absorbing and spreading out flood crests flowing down the river. This would reduce the height in which the water rises downstream of the reservoir. If the reservoir is already full, then it can not absorb any water from swollen rivers.
High River and Stream Levels
Streams or rivers that are already at bankfull can be a precursor to major flooding. Heavy rain or rapid snow melt that flows into an already full river will cause the river to overflow its banks and flood nearby locations. A prolonged dry spell, however, can alleviate flooding concerns.
Widespread, Heavy Rain
This is perhaps the most important and influential factor of them all. Long periods of heavy rain can cause flooding even if all other factors are unfavorable for flooding. Often, heavy rain is a cause of some of the factors listed above such as wet soils, high stream levels and full reservoirs.
In addition to regular monitoring via technical tools and forecasts, City staff will also perform on-site, visual creek monitoring if predetermined flood triggers are met. In such cases, City staff will monitor conditions in water ways which have the potential to create flooding in Longmont. Localized flood locations and the major irrigation/storm water ditches are also monitored. An additional part of monitoring is the inspection of utility crossings of the river, bridges, problematic drainage grates and trash racks throughout the City.
Monitoring locations include vehicular and pedestrian bridges along:
- St. Vrain Creek
- Lefthand Creek
- Spring Gulch #1
- Dry Creek
- Spring Gulch #2
- Lykins Gulch
- S. Pratt Parkway
- Oligarchy Ditch
You Can Help, Too
Residents can also be on the lookout for potential incidents along the river corridor. If you see any of the following three conditions occurring, please report it to the appropriate number listed below.
- Debris (i.e. trees, granular, etc.) – in or near a streambed or ditch that is impeding flow or causing some other hazard. Community should call the PWNR Call Center at 303-651-8416.
- Very large debris (i.e. a shipping crate into a bridge)- causing damage or a life safety issue of some kind. Community should call 911.
- Overflow (i.e. water overflowing streambed, water overtopping the streambed and flooding adjacent property) - Community should call 911.