climate continues to change and weather becomes increasingly unpredictable,
many concerns arise. Of those, extreme weather is increasing in risk. This
means more droughts, more floods, and larger storms. According to the Federal
Emergency Management Agency, floods are the most common and costly natural
disaster in the US. Millions of dollars in damage occurs due to flooding every
Thousands of miles of streams flow through developed
areas. As urban districts continue to grow, it is apparent that water is
essentially unavoidable. Figure 1 displays a map of the Denver Metro area
including all major streams located within those boundaries, and not only major
streams are at risk to flood. There are many more miles of drainage ways in
Denver that are not shown in the figure.
Figure 1: Drainage Ways in
the Denver Metro Area 2
This increase in development also increases flood
risks as humans create impermeable land surfaces. Vegetation removal,
structures, and pavement cause an increase in excess runoff as stormwater is no
longer allowed to seep into soils. This higher peak runoff has in turn
increased the floodplain adjacent to water ways.
There are many ways to prevent flooding. Options
include modifying buildings, relocating out of the floodplain, and planting
more vegetation. However the most efficient strategies also happen to be the
most controversial. These include stream restoration/modifications and an increase
in storage ponds.
Stream modification is done in the form of
construction and movement of soils, vegetation, and boulders. This allows
engineers to completely reform streams shape, size, and material. By altering
the cross sectional area of a stream, the carrying capacity increases to allow
the passage of more water and therefore reduce the risk of flood. Sometimes it
is also necessary to alter the channel material, for example from a natural
stream bottom to a concrete channel. This allows the water to flow at a higher
velocity without risk of erosion. Higher velocity flows will be less likely to
get backed up, and therefore reduce risk of flood. Another modification often
used is a drop structure. These are used to dissipate energy when velocities
are too high due to a steep grade. High velocities erode soils in channels at a
much 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 property
endangerment. These projects can also help the economy. An ongoing project
called Nissen Reservoir at Perry Street is located in Broomfield, CO is
currently in design. This is a stream that runs the perimeter of an open lot
currently inhabited by prairie dogs. The stream itself is in poor condition
with no real shape or capacity. This project is occurring primarily to
eliminate the flood plain, which currently includes a large mobile home
development to the north as well as the open land to the south. However, it has
been discussed by local government and business owners that a revamped stream
with a pathway will boost foot traffic in the area and therefore increase
business incentive to build on the open lot. A restaurant has already discussed
building a back patio open to the water way as an attraction. Additionally, the
land plots in the mobile home development will increase in value as they are
now ‘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 disapprove
construction on drainage ways as it disrupts the natural local habitats and
landscapes. Vegetation in and around the stream are completely removed.
Everything from shrubs to grasses to massive trees. An occasional tree may be
saved, 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 the
size of the project. Additionally, the argument of water quality has risen when
opposing construction on drainage ways. Earth moving and construction equipment
such as excavators, skid loaders, and side dumps may cause sediment to become
suspended solids in the water of the stream. Some oppose stream modification
purely for aesthetics. They prefer the natural streams to manmade manicured.
However, some also enjoy the look of manicured drainage ways. Weeds are
controlled, snakes and rodents are kept to minimum, and the area appears to be
more open and clean.
Although these concerns are legitimate, most are
addressed during design and construction of stream management projects. Various
permits are required for construction which require certain amounts of
remediation. For example, non-invasive indigenous tree species must be replaced
post construction. Mulch and seed blankets are often placed on the banks to
promote vegetation growth as well as additional erosion control. Certain Best
Management Practices (BMPs) must also be followed throughout construction as
well. For example, check dams must be placed in the stream during earth
movement to reduce sediment in the flow and grass swales must line the project
to prevent mud/sediment flows into the water. The Section 404 permit is
required if wetlands habitat is included in the project. There are three
different levels of the permit varying in restrictions dependent on how
invasive the project is. For example, the Nation Wide 404 permit disallows
changing stream length to ensure there is no loss of habitat to wildlife post
construction. This permit includes many other regulations filling 150 pages to
ensure disturbance to the environment is kept to a minimum 3. Depending on
the 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 the
aesthetic of the surrounding residents and business owners.
Detention ponds face similar controversy to stream
modification. Water storage is created through excavation of an open area to
create an indentation, or the construction of a dam. By withholding storm
water, the release rate of the water is controlled. This reduces peak flows and
therefore risk of flood.
to reasons stated prior, detention areas are often opposed by residents and
business owners as they require much open space to be effective. During a storm
event when a storage area fills, there are complaints of mosquitos and smells
from the standing water.
These concerns are also addressed in design and
construction of a detention pond. The large areas often double as parks or
sports fields, a perk to the local community. This eliminates the ‘waste’ of
open space. Of the detention areas that are not created into grassy public
land, they are usually natural wetlands, providing habitat for countless local
wildlife species. Additionally, it is required that all standing water be
drained within 72 hours of a storm event due to water rights issues. This
addresses mosquito and odor concerns. The controlled release also can improve
water quality as water is allowed time to filter through vegetation and for
suspended solids to settle. Increased water quality is beneficial to the
environment as well as society. Although storage ponds are costly and some may
oppose the construction, they are very efficient in flood control.
Water is one of the most dangerous and destructive
forces on the planet. Our fight is to control this force within our society.
Some may prioritize the environment, and promote avoiding building in areas
with flood risk. However many structures were constructed prior to the
knowledge of hydrology we have now, and with society continually expanding, it
will be increasingly difficult to avoid floodplains without modifying them. Stream
modification and storage projects are necessary and unavoidable as our
community continues to expand and climate continues to change.