Use of Wetlands for Stormwater
Treatment
by Johnny Martin, PE & Tim Reid, PE
Moffatt & Nichol Engineers, Raleigh NC
Moffatt & Nichol Engineers was contracted by
the Town of Emerald Isle to model the drainage characteristics
of the southern tip of the coastal barrier island and find an
acceptable solution to treating floodwaters that occur after
major storm events. This was necessary due to changes in environmental
regulations governing how the Town could remove the floodwaters.
The study area is comprised of interior dunes and troughs that
direct all runoff toward a single outfall located on the southwest
end of the island. The entire area is heavily developed and
the natural trough system has been interrupted by the development
of roads across this natural trough. Fecal coliform is the major
contaminant of the stormwater due to residential wastewater
treatment systems that are mainly septic tanks with shallow
leach fields. The complexities involved with hydraulics and
environmental issues on a coastal barrier island along with
the physical infrastructure constraints provided the challenge
to design a viable solution.
Discussions with environmental agencies and residents gave
us the parameters required for an acceptable solution. These
parameters are: improve water quality, install with a minimum
of disruption to the community, permitted by environmental agencies,
little or no opposition from the community or other agencies
or groups, and no adverse affects on the beaches or sound.
Only the solutions that meet these requirements would be studied
in detail. The possible solutions included gravity feed through
pipe systems, ocean outfall, aquifer storage and recovery (groundwater
injection), packaged treatment station, land based infiltration
/ treatment, and the do nothing alternative. After evaluating
each of the alternatives using the criteria listed above, the
land/based infiltration/treatment system was chosen as the only
solution that met all of the stated parameters.
The use of constructed wetlands/infiltration basins is a recognized
method for treatment of stormwater in North Carolina, and therefore,
the use of existing wetlands is the option with the best chance
of being permitted in this case. This type of treatment system
allows for many of the contaminants in stormwater effectively
to be removed from the flow by natural means. The vegetation
helps to remove phosphorous and nitrogen while the vegetation
also slows the flow to allow sediments to settle. Downward percolation
through the soils also removes additional contaminants and enteric
bacteria. Fecal coliform removal efficiencies exceeding 90%
have been reported from constructed wetlands. The multiple filtering
of the runoff through sand filters and vegetated bays and then
through the marsh grasses located at the edge of the property
prior to entering the sound should yield similar results.
This
option had no opposition from the Division of Water Quality
or The Division of Shellfisheries. The Division of Water Quality
also expressed a desire to work closely with the Town if this
option was utilized to serve as a model solution for other coastal
communities. The North Carolina Coastal Federation also preferred
this alternative since it would require the acquisition of large
tracts of coastal property which would preserve open space and
wildlife habitat. Also, since large areas are required to effectively
treat the storm water, there was a good chance that the Town
could derive a secondary low impact use such as a park at the
site. The transfer mechanism to get the stormwater to the treatment
area consisted of a series of lift stations and forcemains.
As for design concerns, the proposed project was designed to
temporarily store runoff in existing shallow, vegetated pools
and depressions that will allow nutrient uptake and removal
of urban pollutants. The infiltration area also had to be large
enough to accept large quantities of water without adversely
affecting nearby property owners (through surface flooding or
amplification of groundwater levels). The treatment area would
also need to be high enough to eliminate the possibility of
a direct discharge during a storm surge. To answer these and
other questions, Moffatt & Nichol Engineers developed a
coupled surface-water, groundwater model. This model allowed
the engineers to view the affects of surface and sub-surface
flow on the proposed design. This also provided the guidance
required to adequately size the small dikes and other perimeter
elements to prevent impacting the nearby residents.
Another important factor to consider during design is that
both the current plant and wildlife species may be impacted
by the altered hydroperiod. These effects must be studied and
quantified to verify that impacts will be minimized. In our
case, the design allows a more complete wetland system to be
developed and maintained due to the ability to hydrate the system
throughout the year. The project is currently waiting funding
for construction.
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