Asclimate continues to change and weather becomes increasingly unpredictable,many concerns arise.
Of those, extreme weather is increasing in risk. Thismeans more droughts, more floods, and larger storms. According to the FederalEmergency Management Agency, floods are the most common and costly naturaldisaster in the US. Millions of dollars in damage occurs due to flooding everyyear 1.
Thousands of miles of streams flow through developedareas. As urban districts continue to grow, it is apparent that water isessentially unavoidable. Figure 1 displays a map of the Denver Metro areaincluding all major streams located within those boundaries, and not only majorstreams are at risk to flood. There are many more miles of drainage ways inDenver that are not shown in the figure.Figure 1: Drainage Ways inthe Denver Metro Area 2This increase in development also increases floodrisks as humans create impermeable land surfaces. Vegetation removal,structures, and pavement cause an increase in excess runoff as stormwater is nolonger allowed to seep into soils. This higher peak runoff has in turnincreased the floodplain adjacent to water ways.There are many ways to prevent flooding.
Optionsinclude modifying buildings, relocating out of the floodplain, and plantingmore vegetation. However the most efficient strategies also happen to be themost controversial. These include stream restoration/modifications and an increasein storage ponds. Stream modification is done in the form ofconstruction and movement of soils, vegetation, and boulders. This allowsengineers to completely reform streams shape, size, and material. By alteringthe cross sectional area of a stream, the carrying capacity increases to allowthe passage of more water and therefore reduce the risk of flood. Sometimes itis also necessary to alter the channel material, for example from a naturalstream bottom to a concrete channel.
This allows the water to flow at a highervelocity without risk of erosion. Higher velocity flows will be less likely toget backed up, and therefore reduce risk of flood. Another modification oftenused is a drop structure. These are used to dissipate energy when velocitiesare too high due to a steep grade.
High velocities erode soils in channels at amuch quicker pace than low velocity flows. Erosion of streams cause steep,often dangerous, banks that may also begin to cut into structure foundations.Erosion may also deteriorate natural berms, in turn increasing flood risk.Stream modification is beneficial to society, as it removes public and propertyendangerment. These projects can also help the economy.
An ongoing projectcalled Nissen Reservoir at Perry Street is located in Broomfield, CO iscurrently in design. This is a stream that runs the perimeter of an open lotcurrently inhabited by prairie dogs. The stream itself is in poor conditionwith no real shape or capacity.
This project is occurring primarily toeliminate the flood plain, which currently includes a large mobile homedevelopment to the north as well as the open land to the south. However, it hasbeen discussed by local government and business owners that a revamped streamwith a pathway will boost foot traffic in the area and therefore increasebusiness incentive to build on the open lot. A restaurant has already discussedbuilding a back patio open to the water way as an attraction. Additionally, theland plots in the mobile home development will increase in value as they arenow ‘stream front’ property and no longer located in the flood plain.This practice seems necessary, and it often is,however many people oppose this strategy. Environmentalists often disapproveconstruction on drainage ways as it disrupts the natural local habitats andlandscapes. Vegetation in and around the stream are completely removed.
Everything from shrubs to grasses to massive trees. An occasional tree may besaved, but the area will no longer have the natural ‘forest’ environment.During construction, the riparian or wetland area is inaccessible to wildlife.This time frame may be anything from one week to two years depending on thesize of the project. Additionally, the argument of water quality has risen whenopposing construction on drainage ways. Earth moving and construction equipmentsuch as excavators, skid loaders, and side dumps may cause sediment to becomesuspended solids in the water of the stream. Some oppose stream modificationpurely for aesthetics. They prefer the natural streams to manmade manicured.
However, some also enjoy the look of manicured drainage ways. Weeds arecontrolled, snakes and rodents are kept to minimum, and the area appears to bemore open and clean.Although these concerns are legitimate, most areaddressed during design and construction of stream management projects.
Variouspermits are required for construction which require certain amounts ofremediation. For example, non-invasive indigenous tree species must be replacedpost construction. Mulch and seed blankets are often placed on the banks topromote vegetation growth as well as additional erosion control.
Certain BestManagement Practices (BMPs) must also be followed throughout construction aswell. For example, check dams must be placed in the stream during earthmovement to reduce sediment in the flow and grass swales must line the projectto prevent mud/sediment flows into the water. The Section 404 permit isrequired if wetlands habitat is included in the project. There are threedifferent levels of the permit varying in restrictions dependent on howinvasive the project is.
For example, the Nation Wide 404 permit disallowschanging stream length to ensure there is no loss of habitat to wildlife postconstruction. This permit includes many other regulations filling 150 pages toensure disturbance to the environment is kept to a minimum 3. Depending onthe location of the project, local plant species may be encouraged to regrow,or grasses will be planted for the ‘cleaner’ look. This is meant to appease theaesthetic of the surrounding residents and business owners.Detention ponds face similar controversy to streammodification. Water storage is created through excavation of an open area tocreate an indentation, or the construction of a dam.
By withholding stormwater, the release rate of the water is controlled. This reduces peak flows andtherefore risk of flood. In additionto reasons stated prior, detention areas are often opposed by residents andbusiness owners as they require much open space to be effective. During a stormevent when a storage area fills, there are complaints of mosquitos and smellsfrom the standing water.These concerns are also addressed in design andconstruction of a detention pond. The large areas often double as parks orsports fields, a perk to the local community. This eliminates the ‘waste’ ofopen space. Of the detention areas that are not created into grassy publicland, they are usually natural wetlands, providing habitat for countless localwildlife species.
Additionally, it is required that all standing water bedrained within 72 hours of a storm event due to water rights issues. Thisaddresses mosquito and odor concerns. The controlled release also can improvewater quality as water is allowed time to filter through vegetation and forsuspended solids to settle. Increased water quality is beneficial to theenvironment as well as society. Although storage ponds are costly and some mayoppose the construction, they are very efficient in flood control.
Water is one of the most dangerous and destructiveforces on the planet. Our fight is to control this force within our society.Some may prioritize the environment, and promote avoiding building in areaswith flood risk. However many structures were constructed prior to theknowledge of hydrology we have now, and with society continually expanding, itwill be increasingly difficult to avoid floodplains without modifying them. Streammodification and storage projects are necessary and unavoidable as ourcommunity continues to expand and climate continues to change.