Stormwater
runoff, consisting of moving water both during and in the aftermath of a
downpour, may be managed (or mismanaged) in a number of ways. As described in
the Alabama
Handbook for Erosion Control, Sediment Control and Stormwater
Management,
“In an undeveloped area, stormwater runoff is managed by nature through the
hydrologic cycle. The cycle begins with rainfall. Rain either stands where it
falls and evaporates or it is absorbed into the ground near the surface, to feed
trees and vegetation, ultimately to be returned to the atmosphere by
transpiration; or it percolates deeply into the ground replenishing the
groundwater supply. The remainder of the rainfall collects into rivulets. This
collected runoff increases in quantity as it moves down the watershed, through
drainageways, streams, reservoirs and to its ultimate destination, the river and
then the sea.”
But,
the document continues, “This simple explanation of the hydrologic cycle belies
its complexity. Nature’s inability to accommodate severe rainfalls without
significant damage, even in undeveloped areas, is very apparent. Nature’s
stormwater management systems are not static but are constantly changing.
Streams meander, banks erode, vegetation changes with the seasons, lakes fill in
with sediment and eventually disappear.”
Of
course, in developed areas, additional problems may present themselves. Some of
the potential hazards, according to the Alabama handbook,
include:
- An increase in developed areas exposed to
stormwater runoff and soil erosion
- Increased volumes of storm runoff,
accelerated soil erosion and sediment yield, and higher peak flows caused
by:
a) Removal of
existing vegetative cover
b) Exposure of underlying soil or geologic
formations potentially more erodible than the original soil
surface
c) Reduced capacity of exposed soils to
absorb rainfall due to compaction caused by heavy
equipment
d) Enlarged drainage areas caused by grading
operations, diversions, and street construction
e) Prolonged exposure of unprotected
disturbed areas due to scheduling problems or delayed
construction
f) Shortened times of concentration of
surface runoff caused by altering slope steepness, slope length, and surface
roughness and through installation of “improved” storm drainage
facilities
g) Increased impervious surfaces associated
with the construction of streets, buildings, sidewalks, and paved driveways and
parking lots - Exposure of subsurface materials that are
rocky, acidic, droughty, or otherwise unfavorable to the establishment of
vegetation
Along
with these problems come the various associated oil, grease, metals, pesticides,
fertilizers, and other waste products that are washed off with storm events,
adding to the pollutant load entering drainage channels.
Thus,
as stated in the North
Carolina Erosion and Sedimentation Pollution Control Program,
“Upstream development accelerates channel erosion by increasing the velocity,
frequency, and duration of flow. As a result, many natural channels that were
stable become unstable following urbanization.”
But
solutions abound. In stormwater pathways natural or manmade, a variety of soft
armor and hard armor resources are available to repair deteriorating drainage
channels.
 |
Photo: Bethlehem Precast |
| CC 45 Cable Concrete articulated concrete block mattress allows infiltration
between the blocks and can be overseeded if desired. |
When
channel flow velocities are slow enough to warrant it, natural vegetation of the
channel bed is often the ideal method of preventing soil from washing away.
According to the North Carolina publication noted above, “Dense, vigorous
vegetation protects the soil surface from raindrop impact, a major force in
dislodging soil particles and moving them downslope. It also shields the soil
surface from the scouring effect of overland flow and decreases the erosive
capacity of the flowing water by reducing its velocity. Suitable vegetative
cover affords excellent erosion protection and sedimentation control and is
essential to the design and stabilization
of many structural erosion control devices. Vegetative cover is relatively
inexpensive to achieve and tends to be self-healing.”
Another
benefit of channel vegetation is the enhanced infiltration of the water runoff
into the soil, filtering out both pollutants and sediment from the
flow.
For
higher flows, vegetation may be combined with geosynthetic fabrics, such as turf
reinforcement mats, in which vegetation grows through the fibers. Such mats
serve to add additional support to the existing
vegetation.
However,
when flow velocities exceed the capacity for vegetation-based solutions, various
forms of hard armor may be called for to protect the channel bed and to provide
bank stabilization. Some of these methods include poured concrete, articulated
concrete blocks, riprap, gabions, and cellular confinement
systems.
Poured
concrete can minimize channel and bank instability but has a number of
drawbacks. There is no infiltration into the soil, so any pollutants in the
runoff are simply carried downstream, instead of being removed from the flow. In
addition, flow volume and velocity increase, creating greater problems at the
point where the concrete lining ends.
Riprap,
essentially broken chunks of concrete or stone, is another solution to channel
and bank stabilization, but many communities view this as a measure of last
resort due to the often unattractive appearance it offers. In addition, the
riprap components may dislodge with heavy water flows, reducing or negating
their effectiveness.
The
other hard armor options noted above will be discussed in the case studies that
follow.
Opa-locka Canal
Project
Located
in Miami-Dade County, FL, the city of Opa-locka has experienced significant
flooding throughout much of its 4.2 square miles for many years. Erosion and
sediment control firm R. H. Moore & Associates explains the problem: “This
is due, in part, to the geographic location of the city, which was developed on
a flood plain. Several factors have contributed to the flooding of the city’s
low-lying areas, including a limited plan of action to manage surface water
runoff conditions and no regularly scheduled canal cleanup plan to prevent
obstructions that impede flow. The canal system that surrounds the entire city
had been experiencing severe erosion resulting in sediment laden canals, which
reduced the volume for stormwater runoff, as well as harming the water quality.
It was clearly determined by city managers that the canals would have to be
dredged and the side slopes protected from future
erosion.”
 |
Photo: Contech Construction Products |
| The Antelope Creek project looks natural, yet has the ability to withstand the events that occur there. |
Larry
Larson Jr. of R. H. Moore & Associates describes a problem common to
Florida’s coastal communities. “Florida canal embankments are being eroded
during storm events with undermining occurring at the waterline [that is] caused
by wind and boat-generated wave chop,” he says. “When undermining occurs at the
toe, the slopes begin to slough off into the water. Slopes steeper than 2H:1V
normally start sliding under saturated conditions until the soil reaches a point
of equilibrium. Florida is especially susceptible to this type of erosion due to
our sandy soils.”
Larson
explains that in 2002, a hurricane that struck south Florida wiped out most of
the Opa-locka Canal at the water line, resulting in steep 2-foot dropoffs in
places. This presented a serious danger to the many children who enjoy fishing
along the canal’s banks. Because the sand in this part of Florida is very fine,
it “turns into soup and washes away,” Larson says. Layers of soil keep falling
into the canal, causing these hazardous dropoffs. Worse still, tall weeds began
sprouting, presenting ideal hiding places for the area’s
alligators.
Because
of the city’s proximity to the Atlantic Ocean, the Opa-locka Canal experiences
wave chops, for which a 4:1 (horizontal to vertical) bank slope is generally
required. In addition, the water level within the canal fluctuates quite a
bit—lower in the winter and higher in the summer.
A
decision had to be made regarding which of many solutions to incorporate into
the stabilization of the canal. Larson explains some of the factors considered.
“We looked at many types of products for this canal, from soft-armor
bioengineering solutions to hard armor such as articulating block and gabions.
The Geoweb system [manufactured by Presto Products Company] was chosen based on
the velocities of the canal and the overall look that the city was trying to
achieve. They were trying to give these canals a sort of linear park along both
sides. The city was looking for a place that the residents of the area could use
as a park and where they could have access to the water without the fear of
hidden alligators. The Geoweb system solved that problem.”
The
Geoweb cellular confinement system is designed for either intermittent or
continuous levels of low-to-high water flow, according to Bill Handlos, P.E.,
CPESC, of Presto’s Geosystems group. “The Geoweb system acts like a flexible
form,” he explains, “confining a variety of infill materials in typical
applications such as swales and drainage ditches, stormwater diversion or
containment channels, process water channels, culvert outfalls, and
spillways.”
Larson
found that the various options for infill material were particularly useful for
this project. “Geoweb offers a unique option for erosion protection for canal
side slopes; you can fill the cells with rock or concrete to about one foot
above the mean high water line. The choice between rock and concrete is based on
the velocities and wave attack encountered. Above the mean high water line, you
can fill the cells with soil and sod or seed directly over the filled cells. A
rock infill below and sod above was chosen for the Opa-locka
project.”
 |
Photo: R. H. Moore & Associates |
| The city of Opa-locka has experienced significant flooding throughout much of its 4.2 square miles for many years. |
More
than 5 miles of the Opa-locka canal system was restored in this manner, although
severe storms pose a constant threat. “The canal was designed to meet certain
criteria based around a 25-year storm event,” Larson says, “but designing to
meet hurricane-force conditions is almost impossible. This area may never be hit
again by the type of storm that did all the damage, but, on the other hand, we
may get hit four times in one year like we did back in
2004.”
Frost Hollow
Swale
The
Frost Hollow swale is a channel running through a residential development in
Forks Township, in Easton, PA. Historically, during heavy storms, this channel
has been unable to move the stormwater runoff quickly enough, causing localized
flooding that entered the basements of nearby homeowners. In addition, the
channel banks were eroding away, and Frost Hollow had formed several 90-degree
turns. Deep erosion was occurring, and nearby properties were eroding as
well.
Then,
in 2005, the area suffered two or three “100-year floods” according to David
Faust of Bethlehem Precast Inc. He notes that the current concept of a 100-year
flood may be somewhat antiquated. “Since there is so much building and
construction going on,” he says, “we’ve got a concrete jungle and a rising water
table, especially in highly populated areas.”
As
an aside, a number of engineers appear to agree with Faust’s assessment. The
Alabama
Handbook for Erosion Control, Sediment Control and Stormwater
Management
comments, “Studies have shown that most natural stream channels are formed with
a bank-full capacity to pass runoff from a storm with a 1.5 to 2-year recurrence
interval. As upstream development occurs, the volume and velocity of flow from
these relatively frequent storms increase. Even smaller storms with less than
1-year recurrence intervals begin to cause streams to flow full or flood. Stream
channels are often subject to a 3- to 5-fold increase in the frequency of
bank-full flows in a typical urbanizing watershed. This increase in the flooding
frequency places a stress on the channel to adjust its shape and alignment to
accommodate the increased flow.
“Unfortunately,
this adjustment takes place in a very short time period (in geologic terms) and,
the transition is usually not a smooth one. Meandering stream channels which
were once parabolic in shape and covered with vegetation typically become
straight, wide rectangular channels with barren vertical banks. This process of
channel erosion often causes significant property damage, and the resulting
sediment which is generated is transported downstream, further contributing to
channel degradation.”
This
was the process occurring with the Frost Hollow swale. “There is a huge
development upstream,” notes Faust. “There is water feeding in from area roads;
there is the stormwater runoff. There are some runoff basins for water
retention, but these are not sufficient for major storms.”
To
solve the problem, the Frost Hollow channel was widened. Initially, soft
protection was used, in which a screening mesh was applied and overseeded with
grass. However, these measures were put in too late and the grass couldn’t get
established before the next heavy storm event. It washed out, and then the
decision was made to replace the screening mesh with CC 45 Cable Concrete
articulated concrete block mattress manufactured by International Erosion
Control Systems. Design engineer Ryan Christman of Keystone Consulting Engineers
was free to select what he thought was best for the project—there were no
specific municipal requirements. But Faust notes that the Pennsylvania
Department of Environmental Protection has approved Cable Concrete as an
acceptable best management practice for stormwater management and adds that the
CC 45 is highly effective in dealing with the high water velocity and the
regular strong storm events in the area.
Cable
Concrete consists of pyramidal-shaped concrete blocks interwoven with stainless
steel cable and underlaid with an erosion-minimizing geotextile fabric. It
allows infiltration between the blocks and can be overseeded if
desired.
Faust
explains that the purpose of the Frost Hollow channel is both to move stormwater
runoff and to allow infiltration into the ground. “The benefit of the
articulated blocks is that grass grows over and between the blocks, causing a
filtering benefit as well as a better aesthetic appearance,” he says. “Within a
relatively short period of time the area looks very natural. In this specific
project, we did not overseed with grass but let nature take its course. Over
time, the area silts up and vegetates naturally. Now, a year and a half after
the project’s completion, it looks quite natural, and before long, you won’t be
able to see the articulated blocks at all. This vegetation, of course, slows
down the velocity of stormwater, and you get infiltration, which is what you
want.”
The
Frost Hollow project was completed in September 2007, and Faust notes that
engineers involved with the effort are quite pleased. A total of 362 eight- by
16-foot Cable Concrete mats were used, along with 181 four- by 16-foot mats,
totaling nearly 58,000 square feet. The channel has been successfully widened,
erosion appears to have been arrested, and the channel treatments have performed
exactly as desired. There have been several major storm events since the
installation, with no reports of any new basement
flooding.
Antelope
Creek
Antelope
Creek is a small stream running through Lincoln, NE. As noted in the Antelope
Valley Study issued by the city of Lincoln, the creek meanders in an open
channel underneath a number of street bridges and through many residential and
business neighborhoods until it moves underground into an enclosed conduit. From
there, it travels underneath several buildings, re-emerging aboveground a couple
of blocks later. The final leg of the creek again meanders in an open channel
underneath more street bridges, passes through the campus of the University of
Nebraska, travels underneath railroad tracks, and finally empties into Salt
Creek.
“Because
of increased runoff caused by urban development in the lower reaches of the
Antelope Creek basin,” states the Antelope Valley Study, “only a four-year or
smaller storm is calculated by the engineers to fit into the conduit, and any
larger storm would exceed the conduit and cause the excess water to travel
overland, flooding many … neighborhood streets and
properties.”
Furthermore,
a number of street bridges over the creek are so small and low that as the creek
rises, these bridges act as small dams, contributing to the flooding of nearby
properties. The study estimates that in a 100-year flood event, some areas would
be under up to 6 feet of floodwater. In an analysis by the US Army Corps of
Engineers, it was determined that even an 8-year flood would result in
widespread damage.
Over
the past century, Antelope Creek has flooded numerous times, most severely in
1908, when 10 lives were lost.
The
goal of the Antelope Creek flood control project is to reduce the estimated
annual flood damage by 80%, as well as to confine the 100-year storm event
within the channel banks.
Wayne
Wendel of Contech Construction Products in Lincoln notes that erosion concerns
existed as well, but because of increased surface runoff caused by the
impervious surfaces from city development, continual flooding was the primary
issue with Antelope Creek.
Wendel
explains that the Joint Antelope Valley Authority (JAVA) “pushed for a ‘green’
solution,” desiring “a channel that would look natural, yet have the ability to
withstand the events that occur here.”
JAVA
has produced a 20-year development plan for the creek basin, incorporating work
on three channel reaches. “I have been involved with all of them from the very
beginning,” Wendel says. “Presentations on ArmorFlex”—an articulated concrete
block revetment system—“and erosion control TRMs [turf reinforcement mats] were
given to JAVA prior to the design being completed. We then worked with the corps
and the engineer [Olsson Associates of Lincoln] to size the ArmorFlex and TRMs.
The project is ongoing. Hawkins Construction Company of Omaha has been the
contractor that has performed the work up until the most recent bid, which was
just awarded to Park Construction out of Minnesota. We have approximately
100,000 square feet of ArmorFlex to supply for the phase that just
bid.”
Continuing
downtown development increases the problems faced from channel flooding.
“Antelope Creek is the main drainage channel of the city, and with the
development of recent years, it had become undersized,” stresses
Wendel.
He
explains that curative options were somewhat limited. “There was no soft armor
available—and still isn’t—that can withstand the shear forces and velocities
present in the channel. I would estimate that any hard-armor solution is going
to cost approximately 2.5 to 4 times what a soft-armor solution would, the
caveat being that most times ArmorFlex is being considered as a design solution,
there simply is no soft-armor solution.”
The
Antelope Creek flood control project is still ongoing, but Wendel says that the
creek has been treated to allow silt to settle out, and the ArmorFlex used is
designed to allow the establishment of vegetation, producing some level of water
treatment. However, “the main goal was to move water.”
To
date, Wendel says, “The reaction has been very positive from the business owners
and public officials. There was some resistance to the project from some
homeowners and business owners that had to give up property. However, that
stemmed from the road and bridge projects that went along with the channel
improvements. It was the new road and bridges that caused most of the negative
reaction because of the eminent domain issues and establishment of easements.
Like many things, the public doesn’t like the delays and inconvenience during
construction, but will be very pleased upon completion, and are already pleased
with the performance of the channel.”
Neabsco Creek
Channel
Neabsco
Creek is a tributary of the lower tidal segment of the Potomac River located in
eastern Prince William County, VA. Alan Dinges, assistant area manager for
Maccaferri Inc., says, “The creek has served as a vital waterway for trade and
commerce in northern Virginia since the eighteenth century. The EPA Office of
Water has identified the Neabsco Creek watershed as an ‘area of significant
habitat degradation’ due to a loss of natural land cover and stormwater
management facilities designed without consideration for environmental
conditions.”
Because
of its sandy soil and a steep slope, a hillside drainage channel emptying into
Neabsco Creek was suffering severe erosion. The county hired engineering firm R.
A. Smith National Inc. and specified that gabions (essentially rock-filled
baskets) should be used to stabilize the drainage channel. This firm contracted
with Maccaferri in Williamsport, MD, for design work.
According
to Maccaferri’s Larry Bakner, “Maccaferri provided an initial design for
consideration. The project went through numerous revisions, and a final design
was decided on in the spring of 2008. Upstream development was the reason for
concern on this project. A large commercial development, 650,000 square feet,
was to have three outfall areas with 48-inch and 54-inch concrete pipes. The
design flows were 120 and 160 cubic feet per second for a 100-year event, with
velocities around 10 feet per second.”
Bakner
explains the primary goal of the work: “The channel was designed to move the
water from the commercial site into an existing stream, and also to dissipate
velocities. An inspector was continually onsite during construction to monitor
the water quality in the stream.”
Dinges
adds, “This project has very high visibility, and this is the reason it was
monitored so closely.”
As
with the Antelope Creek project, options were limited, especially because the
county had specified the use of gabions to repair the channel. “A soft-armor
solution was not considered,” Bakner says. “Gabions and riprap were both
considered as solutions, but due to the steep slope and water velocities,
gabions were chosen as the best solution.”
The
gabion installation was completed shortly after the final design approval. It
consists of approximately 1,400 cubic yards of galvanized gabions, placed on top
of Maccaferri’s MacTex MX-275 nonwoven geotextile material. The resulting gabion
structure is quite striking and appealing to the eye, in stark contrast to the
initial appearance of the eroded channel. Bakner notes that, not surprisingly,
the engineers were quite pleased with the results.