Hundreds of thousands of people flock each year to the Florida Keys for world-class fishing, diving and the breathtaking scenery. The Keys, about 112 miles long from Key Largo to Key West, are home to about 80,000 people.
Wending their way through the waterfront homes are 481 canals, each of which is vital to ensure the region's water quality.
According to George Garrett, director of the Monroe County Marine Resources Department, there are currently about two dozen county canals with no access to open water – they remain plugged due to changes in environmental regulations imposed in the 1970s. Residents have also long reported a problem with flotsam entering open canals, with the accumulating seaweed decaying and fouling the waterway.
Monroe County officials were concerned about water degradation in the canals. In order to evaluate what type of remedial actions may be necessary for the canals in consideration of existing water quality data and other available information, MACTEC, Inc. undertook a survey of canals conditions throughout the Keys.
Major survey tasks included:
A base map of canal and water quality layers were developed using ESRI's ArcView 3.2. Environmental and geographic data compiled were incorporated into a base map. A GIS database of physical attributes was developed that could potentially impact water quality in the canals and a GIS-based water body layer was created by delineating the footprint of each water body observed as a separate polygon within the layer.
Existing GIS layers were reprojected to the canal projection and spatially analyzed through customized programming using ArcView to populate each individual canal. Various physical attributes like length, number of mouths, and orientation of a canal was derived through GIS; polygons were also spatially analyzed. These attributes were used to evaluate potential impacts to water quality in a canal. Some of the water quality data available in electronic format was summarized and linked to the associated canal system using ArcView. A query function was used to select the most applicable and cost effective canal specific treatment technology combination.
The residential canal project needed first to identify those canal systems that were developed mostly for private homes and residences. The database included information about single-family homes, trailer parks, condominiums, duplexes, apartment complexes and some businesses that reside along canals that are mostly residential.
Aerial photographs for the Florida Keys, derived from Florida Department of Transportation (FDOT) were utilized to digitize all the water bodies observed during the desktop aerial interpretation. The aerials were selected due to their relatively high resolution and their extensive coverage of the study area.
The aerial and field data were incorporated into a geographic information system (GIS). Each canal was subject to a comprehensive 48-category checklist (e.g., length, development near the canal, physical depth, turns, etc.) to develop a dataset. The database was then paired with water quality data that resulted in a canal classification system based on those characteristics.
Analyzing Available Treatment Options
The survey has provided Monroe County with a number of treatment options. Some of these include:
Advanced Wastewater Treatment/Storm Water Management Technology: Advanced wastewater treatment technology can include on-site units such as dry-composing toilets, wastewater hydroponic gardens or centralized sewage treatment facilities. Storm water management measures include porous pavement, retention basins, baffle boxes, dry wells with pretreatment, oil and grease separators, storm water wetlands, buffer strips, and vegetated swales.
Best Management Practices: Poor management practices are a major contributor to canal water quality degradation. Implementation of best management practices by landowners can be implemented in all canals to assist in the reduction of land-based nutrient and pollutant loadings. Best management practices (BMPs) relating to the canals include: no dumping into canals, no cleaning of fish in the canals, and the prevention of bank runoff. Landscaping and land use measures presented by the Florida Yards and Neighborhoods Program (a state program that addresses how to control runoff, reduce the use of pesticides and fertilizers, plant native plant buffers, etc.) should be included in the best management practices. Berms can be planted with native, low maintenance plants. There are a variety of vines, small trees, grasses and herbs that occur naturally in the coastal zone, and are attractive in landscaping for the Keys.
The effectiveness of this technology can be increased if homeowners take additional measures to reduce harmful runoff, such as reducing irrigation, using mulches around plantings, and reducing the use of pesticides and fertilizers. These measures can be implemented to reduce storm water runoff even on lots that are too small for the installation of a berm and swale.
Swinging Weed Gates: Swinging weed gates are mechanical devices that physically block seaweed from passing through the device. The weed gates are designed to prevent floating, wind-driven flotsam from entering and accumulating in man-made canals where it typically sinks and fouls with water. Weed gates consist of a floating arm that contains hanging flaps often made of rubber. The gates are placed at a canal mouth to prevent floating seaweed from entering the canal. The gate swings open when applied with slight pressure to allow access for boat traffic.
Air Curtains: These are aeration devices that consist of perforated piping connected to a blower. They are typically installed at the mouth of a canal. The curtains consist of perforated PVC pipe, a blower, control panel, electric, and protective pump housing. The PVC pipe is installed on the canal bottom across the entire canal mouth and anchored in place. The pipe is connected to a blower located on the adjacent shore. Blower selection strongly influences the capital and operation and maintenance costs of the system.
Physical Removal: Consists of removing debris or trash from canals in order to improve water circulation or water quality from decaying organic material. Canals without weed gates or air curtains can have accumulated floating seaweed removed by backhoe or specialized weed removal boats.
Pumping: Pumps can be installed to promote water circulation within a canal. Water can be pumped from a ‘dead end' canal to another adjacent canal or mangrove creek to increase turnover of water at the end of a canal system. Pump installation must be designed to prevent adverse secondary effects such as bottom scouring or impact to adjacent waters. Circulation studies by a qualified coastal engineer would be needed to provide an effective design.
Construct Flushing Channels: Flushing within canals can be improved by constructing channels between them. This alternative consists of trenching the ground so that adjacent canals are joined and then placing a precast concrete cover over the trench. This would only be applicable where there was sufficient energy to promote flushing such as a channel at the mouth outfall. These channels are prone to clogging, and residents often request gates or bars over the mouth of the channel for safety reasons. Circulation studies by a qualified coastal engineer would be needed to provide an effective design and ensure that no adverse impacts occur to adjoining waterways.
Culvert Installation: Culverts could be installed between canals or between canals and thin land strips to improve flushing within them in a similar fashion to flushing channels. Based on canal-specific hydrology, larger or smaller diameter culverts may be more applicable. Culverts would still need an energy source to induce flushing such as a channel at the outfall mouth. Like channels, culverts are even more prone to clogging, and require maintenance. And residents in Key Largo have complained that culverts serve as ideal habitats for the American crocodile so this technology may have some wildlife management issues. Circulation studies by a qualified coastal engineer would be needed here as well.
Canal Backfilling: Canal backfilling could be performed in order to decrease the depth of a canal to promote flushing and reduce/eliminate stratification. Filling in of abrupt changes in depth or sink areas would assist in increasing circulation. Backfilling attempts to mimic the hydrology of a mangrove creek, with the shallowest depth at the landward end of the canal, and greatest depth at the mouth of the canal.
Flushing is typically hindered by the fact that both the canal sill and the waters surrounding the Keys are typically shallower than the canal bottom. This difference in depth hinders mixing in the lower depths of the canals. This option would work best in canals where there was sufficient energy, either from tidal fluctuations or wind force, to promote flushing. Naturally there may be some restrictions to the size of boats that can utilize filled canals. Canal backfilling can be performed in order to decrease the depth of a canal to promote flushing and reduce/eliminate stratification. This option works best in canals where there is sufficient energy, either from tidal fluctuations or wind force, to promote flushing.
Circulation Devices: Aeration can be used in smaller canals to facilitate circulation. The design of aeration systems must be aimed at moving water out of the canal, not merely vertical mixing of the water column. Circulation improvement needs to be coupled with reducing the pollution loading within the canal. Circulation devices are specialized aeration devices that could be installed to increase both the transfer of oxygen to the water as well as the horizontal movement of water out of the canal. Circulation devices can help prevent hypoxic/anoxic conditions to a limited extent and promote aerobic processing of nutrients.
Nutrient Removal: Nutrient removal from canal waters is a developing technology. In brief, it consists of pumping the water to a structure that supports the growth of algae that utilize nutrients from the water. Special macroalgae species have been used effectively in small-scale nutrient stripping experiments in the Keys. Water is circulated through shallow troughs exposed to sunlight with small starter colonies of algae. Often airstones are used to vigorously circulate the water and stimulate nutrient uptake by the algae. The algae must be periodically harvested (and possibly sold). The water is recycled back to the canal after treatment with lowered nutrient content. In situ systems such as Aquamats are also being employed. This technology would likely work best on small canals with high inorganic nutrient loading.
Management of residential canal water quality will become increasingly important to Monroe County as build-out of the Florida Keys continues. A comprehensive and systematic approach to canal mitigation will be necessary to prevent long-term impact on adjacent marine resources.
"This report will give us the necessary tools to fully understand the situation and come up with cost-effective solutions," Garrett said. "We anticipate implementing many of these technologies in the near future because different solutions will be required for different areas throughout the canal system. Now that we have a full understanding of the situation, we can also launch a public outreach program so canal residents can be educated as well."
And the bottom line? Monroe County now has in place a methodology for groupings of canals by design features most likely to impact water quality, thus providing a method for design-based assessment of treatment technologies for individual canal systems.
Wendy Leonard, P.G., is senior hydrogeologist/project manager for MACTEC, Inc. and is based in the firm's Miami Lakes, FL office. MACTEC, based in Alpharetta, GA, is a $524 million environmental, engineering, construction management, and infrastructure services firm with 4,000 employees in over 100 offices nationwide. Contact Ms. Leonard at firstname.lastname@example.org, 305/826-5588.