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Know Your Flood Hazard

Causes of flooding

Flooding is an overflowing of water onto land that is normally dry. Floods can happen during heavy rains, when ocean waves come on shore, when snow melts too fast, or when dams or levees break. Flooding may happen with only a few inches of water, or it may cover a house to the rooftop. They can occur quickly or over a long period and may last days, weeks, or longer. Floods are the most common and widespread of all weather-related natural disasters. 

Flash floods are the most dangerous kind of floods, because they combine the destructive power of a flood with incredible speed and unpredictability. Flash floods occur when excessive water fills normally dry creeks or river beds along with currently flowing creeks and rivers, causing rapid rises of water in a short amount of time. They can happen with little or no warning.

Floods in Aurora are generally caused by heavy rains, usually during summer storms. Sometimes snowmelt adds to the amount of water runoff during a storm. River channels, creeks, gulches and storm sewers can only carry so much water. Even in natural settings, streams overflow their banks every year or two. Flooding can be further aggravated when debris blocks the waterway, which is a common problem at road crossings. Hail can also make flooding worse by clogging storm sewer inlets and impeding surface drainage.

Urban development changes the natural environment. Pavements and rooftops mean that less rainwater can soak into the ground. Gutters and storm sewers speed the runoff to the channels. Our pattern of streets and buildings has interrupted some of the natural drainage ways and reduced the width of some channels. As a result, more water runs off more quickly, and the drainage system becomes overloaded more frequently. Goldsmith Gulch flooding near Iliff and Monaco. The combination of heavy precipitation and an overloaded drainage system can result in three principle types of flooding:

(1) overbank flooding, 
(2) irrigation ditches/canals, and
(3) streets and low-lying areas.

Each type of flooding is associated with somewhat different hazards.

(1) Overbank flooding. The most dangerous kind of flooding in Denver occurs when rivers, creeks, and smaller tributary streams overflow their banks. Overbank flooding occurs every year in Denver. Fortunately, most of these events cause little damage, usually flooding trails, parks, and open space areas. Serious overbank flooding can occur on every stream in Aurora.

(2) Irrigation ditches/canals. These facilities divert water from mountain streams and carry that water to reservoirs for domestic and agricultural use. Most of the water that you use in your home is transported by these ditches or canals. They are called irrigation ditches because most of the water that they carry gets used by farming operations. The ditches run along hillsides, following natural contours and crossing streams and other natural drainage patterns. Consequently, they occasionally intercept stormwater runoff. For the most part, ditches are not designed to handle excess stormwater; when they do, they can spill or collapse and cause considerable property damage. Experience shows that there is little or no warning for this type of flooding.

(3) Except at certain underpasses, street flooding and yard ponding usually do not get deeper than a foot or two. Street flooding and yard ponding are often viewed more as a nuisance than a major hazard. Traffic is disrupted, and some streets may have to be closed for a while. Parking lots may be designed to store runoff temporarily and release the stormwater at a slower rate. This helps reduce downstream flooding for a short distance, but it also means that flooding in those parking lots may be deeper than expected. If it is raining, be aware of this possibility when parking your car, and pay attention to signs identifying stormwater detention areas. Beware of the potential risk of entering any flooded area, especially one with moving water. There can be high-velocity flows in areas with only shallow flooding. People and vehicles can be swept away by shallow moving water. Drainage pipes or culverts may be submerged and impossible to see. The force of the water entering these pipes can overpower you quickly. On August 17, 2000, a Denver firefighter lost his life attempting to rescue a citizen trapped in a situation like this.

Understanding flood size and frequency

The term “100-year flood” is commonly misunderstood. The reason it is talked about so much is that it represents the magnitude or size of a flood used by engineers for designing drainage and flood control facilities. It is also used by the National Flood Insurance Program as a minimum standard for regulating new construction and as a basis for determining insurance premiums and mapping flood hazard areas. So, what does this term mean to you when considering flood protection for your home and family? Before trying to answer this, consider the following: There are many ways to define the 100-year flood, the most common being a flood that has a 1% chance of being equaled or exceeded each year. Statistically, this equates to a better than 1 in 4 chance of experiencing a flood of this size during a 30-year period—the length of a typical home mortgage. Regardless of what may be the best definition, the 100-year flood is simply a large event, not the worst possible. Colorado’s flood history attests to this fact.

If your property is near a mapped floodplain but not actually in a floodplain, these facts should help you make a more informed decision about your flood insurance needs. If you are located in a floodplain, it is likely that you already have flood insurance as a condition of your mortgage.

To determine your flood risk, call the Public Works Engineer on duty at 303.739.7335.

Determine Your Flood Risk

An interactive map  from the Mile High Flood District. This map provides hazard information by address. 

An interactive city flood risk map 

map of all bodies of water in the city of Aurora 




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