Stormwater | Aquascape Solutions

Aquascape Solutions for Stormwater Management

Beyond ornamental: Traditional landscaping features provide water-quality benefits.

By Bruce Phillips

Aquascape facilities have traditionally been considered ornamental landscape features, primarily serving aesthetic purposes in golf courses, parks, and residential developments. However, aquascapes—such as artificial lakes—can be applied with innovative design elements to function as primary infrastructure facilities in urban developments, replacing typical stormwater facilities and adding value to communities. These specialized types of aquascape systems integrate a living ecosystem into an urban environment, maintaining water quality through natural biological processes.

Planned aquascape features, particularly in semiarid areas, offer a combination of advantages for stormwater management in addition to other benefits that are not available in conventional engineered systems, including:

year-round natural treatment processes,
stormwater conveyance and storage,
enhanced water-quality treatment,
flood protection,
combined land-use elements,
significantly reduced infrastructure costs,
dry-weather flow treatment,
landscape and aesthetic treatment with natural water systems,
increased surrounding-land values,
natural ecosystem enhancement,
recreational design features,
urban design elements for communities.

The necessity for stormwater pollution control has received increased public attention, especially with the March 2003 deadline for the National Pollutant Discharge Elimination System Phase II stormwater regulations. Developers and municipalities are required to address stormwater quality by implementing standard structural control measures or best management practices. These methods generally have limited pollutant-removal effectiveness, perform single functions, often require considerable land space, have associated construction costs and maintenance issues, have difficulty integrating with the land plan, and have limited aesthetic appeal to the community. Integrating large-scale specialty aquascape systems through constructed lakes, ponds, small creeks, or other water features can replace traditional underground drainage infrastructure and provide highly effective stormwater treatment.

Standard Water-Quality Design Requirements
Specific design issues and requirements are commonly associated with an aquascape feature to maintain a minimum operational quality of a water body. For example, the ability to maintain long-term water quality, which generally focuses on algae control, nutrients, alkalinity, and temperature, is critical. All water bodies experience the natural eutrophication process related to the depletion of dissolved oxygen from increased nutrients and minerals, and this process can negatively result in such changes as algae blooms, scum formation, transparency, and odors.

Water features also experience daily evaporation, which increases the long-term alkalinity or presence of dissolved salts in the water. Conventional aquascape system designs attempt to prevent or eliminate stormwater runoff inflow in an effort to minimize nutrient input for algae control. One technique is to allow water exchange or flushing of the lake to limit nutrient concentrations. A maximum 30-day turnover can be tolerated in general and will allow for some nutrient buildup, but a seven-day turnover will result in clean water. Closed-system aquascapes with clean source water that do not integrate flushing generally require an annual replacement of the entire water supply to maintain its quality. Manmade lake systems for parks and golf courses commonly serve as irrigation storage reservoirs from which the stored water will be withdrawn for irrigation use, thus indirectly flushing the system. Irrigation lakes allow for a high turnover or replacement of the water volume, and multiple lakes can be interconnected to achieve the maximum benefits.

It is also desirable to achieve "plug-flow" circulation to improve the benefits of flushing. Plug flow eliminates stratification from the incoming flow and ensures complete turnover. Correctly locating the inlet and outlet at opposite ends of a water body and with the direction of the wind will assist in achieving plug flow to maximize horizontal-circulation benefits.

Improving low dissolved oxygen in manmade water features typically is achieved through aeration, either through a bubbler or a spray system and also through the natural wind action on the surface. Spray systems generally require more horsepower and increasing long-term costs without any additional advantages, except for the visual benefits of a fountainlike system. Bubblers or a fine-bubble diffusion system placed along the bottom of the water feature induces vertical recirculation that results in destratification and reduction of the surface-water temperature, avoiding conditions leading to undesirable algae blooms.

Another critical aspect of a water feature that affects operating characteristics and water quality is the horizontal layout and geometry. Constructed lakes within residential developments commonly maximize the amount of lake-waterfront homes through a layout incorporating numerous "fingers." A finger layout develops circulation dead-zones and poor water quality, however, and generally requires a more extensive plumbing infrastructure and pumping to offset circulation issues. A "snake" layout is preferred because it provides the same maximum-edge frontage as a finger system but also improves circulation due to plug flow. Additionally influencing lake quality is the normal operating water depth because this determines the effects of temperature, and biological reaction time increases with temperature. A minimum operating depth of 8-12 ft. eliminates light penetration, maintains a lower average temperature, allows for temperature stratification, and minimizes evaporation.

Integrating Stormwater Treatment
Incorporating stormwater treatment into an aquascape relies on re-creating a natural ecosystem that can use biologic processes for treatment of urban pollutants in runoff and on maintaining the normal health of the aquascape system. The primary elements integrated into this type of aquascape treatment system include (1) wetland planters, (2) lake biofilter beds, (3) wetland pretreatment filters, (4) aeration, and (5) stormwater retention volume/capacity. When successfully applied, these can achieve exceptional water-quality results. Pretreatment of the stormwater is a critical element in enhancing its overall performance and should be applied to all aquascape inflows to trap larger sediments. Wetland filters or vegetated "first flush" basins can be provided as pretreatment devices and should be installed at the outfall of all storm drains to treat water before it enters the lake.

Wetland treatment system

Lake biofilters, through which lake water is circulated and distributed by a slotted-pipe system, consist of separate, self-contained, submerged gravel beds adjacent to the perimeter of the lake. A naturally occurring biological mass of microorganisms coats the gravel and strips the water passing through the filter of nutrients, such as nitrogen and phosphorous, that would otherwise promote algae growth. In addition, the recirculation pumping reintroduces oxygen into the lake system and increases the overall dissolved-oxygen content. The combination of limited food supply and aerobic conditions reduces the potential for lake eutrophication.

A critical feature that needs to be included in the biofilter design to ensure long-term performance is the ability to effectively perform periodic backwashing to remove material that accumulates within filter voids and prevents adequate filtration. Simply reversing the direction of flow through the biofilter piping system is not sufficient to distribute flow; a separate backwash system should be used, which involves a portable pump lowered into the screened standpipe within the biofilter. Lake water is drawn through the biofilter in a reverse direction to the screened standpipe, discharging to the sanitary sewer. It is recommended that maintenance personnel use rodding or a mechanical means to break up the gravel bed "packing."

Other important design aspects of the biofilters involve layout and location of these features. To promote the maximum water-quality benefit and enhance the overall lake-water circulation, the biofilters should be placed at terminal ends of the lake geometry. They are usually designed so the velocity does not exceed 0.5 gpm/ft.2, assuming minimum 24-in.-thick gravel filters. The number of biofilters needed to treat the lake system depends on the amount of turnover or lake recirculation time for treatment. Industry averages for lake turnover rates without biofiltration range from five to 15 days.

A stabilized biological lake system requires maintenance of the dissolved-oxygen levels, which eliminates the potential for odor problems and other lake-operating issues. A fine-bubble diffusion system can maintain the necessary dissolved-oxygen levels. Additional benefits of aeration include destratification of a lake’s water column to reduce surface-water temperature and enhancement of the natural vertical movement or circulation patterns. Aeration uses low pressure and is sized to provide turnover every three to four hours. Extremely fine bubbles can be achieved through the use of aeration disk pods with a flexible rubber skin that precisely controls the size of the bubbles. Fine bubbles, compared to the large bubbles from a simple perforated-pipe system, increase contact area and enhance oxygen transfer. The size of the bubble system is typically based on the shape or geometry—rather than the size—of the constructed lake to eliminate any dead zones. The movement of lake surface water from wind and other water-feature elements, such as waterfalls or fountains, can provide additional aeration.

Individual wetland planters incrementally located along the perimeter of the lake edge assist in promoting the overall water-quality objective for the lake system. The wetland planters can be constructed along shelves in the lake shoreline with walls separating the lake, except for the crest, to allow for submergence from the lake-water level. The wetlands filter out waste from the lake water through various natural chemical and biological processes. Methods to determine the amount of wetland area required for treatment involve correlations with the concentration or amount of nutrients in the lake water.

Providing sufficient additional storage volume to allow retention of a portion of the design stormwater runoff volume enhances the stormwater treatment capabilities of the aquascape system. The majority of stormwater pollutants are captured in the initial 0.5 in. of runoff; retention of this initial storm volume dramatically reduces pollutant levels through the settling process before discharging to other downstream receiving waters. In addition, once the water has been retained in a constructed aquascape, additional treatment can occur by recirculation through lake biofilters and wetland planters. Linear constructed-lake systems also have the advantage of providing stormwater conveyance and can potentially significantly reduce earthwork requirements on development projects.

Successful Aquascape Treatment Applications
A new 70-ac. planned residential community known as Bridgeport was developed by Newhall Land in the city of Santa Clarita, located in northern Los Angeles County, CA. Bridgeport incorporates a manmade lake system for stormwater management and treatment. Although a residential development plan that used a conventional storm drain system but did not offer stormwater treatment had been originally adopted, this site plan was modified by Pacific Advanced Civil Engineering Inc. (PACE) during the initial planning process to eliminate all underground storm drainpipe infrastructure within the project by integrating a manmade lake system that extends throughout the interior of the entire development. The lake system had three primary functions:

It included an aesthetic focal landscape feature for the project.
It served as the primary drainage conveyance within the development.
It provided runoff water-quality treatment from the development.

Newhall Land realized additional benefits from the constructed lake system through the cost savings from eliminating the storm drainpipe and increased residential-lot premiums for lake views.

Runoff from the 70-ac. residential area is tributary to the 15-ac. lake, which has an operating water volume of 105 ac.-ft. and depths averaging approximately 7 ft., with a maximum depth of 12 ft. The lake is lined with 30-mil PVC, and a leak detection system is located below the liner. Water-quality treatment features incorporated into the lake system include aeration, lake biofilters, wetland planters, and vegetated pretreatment basins. These features manage the urban storm runoff quality or the health of the lake system to ensure that any discharges to the adjacent Santa Clara River have an improved water quality.

The Bridgeport lake system places 15 biofilters at the end of each lake finger to promote overall circulation. Each biofilter is approximately 1,000 ft.2 and treats lake water at 500 gpm, providing a total biofilter flow rate for the lake of 7,500 gpm. The biofilters are typically 3-4 ft. deep, filled with gravel, and submerged 18-24 in. below the lake’s surface. Water pumped from the lake is distributed through the biofilter via a herringbone slotted-pipe system underneath the gravel bed. Recirculation occurs 24 hours a day through the biofilters, and the time needed for complete filtration of the lake is approximately three days. Because the Bridgeport lake system has a higher-than-industry-average turnover rate and incorporates biofilters, it provides a significantly higher level of water quality and aids in stabilizing it.

Aeration for the Bridgeport lake is provided via a fine-bubble diffusion system placed at the bottom of the lake. This system uses 12 5-cfm compressors that deliver a total of 60 cfm to 30 aeration disks in six groupings. The aeration system is sized to turn over the lake every three to four hours if the aeration is operating 24 hours per day.

The Bridgeport lake water quality is also enhanced through the pretreatment of all stormwater runoff entering the lake and submerged wetland planters along the perimeter that function as a natural kidney for the lake system. Approximately 18 water-quality filters, with an average surface area of 250 ft.2, are used to intercept runoff before it enters the lake.

Click here for a larger view

Filters are located at all the inflow points. The typical configuration consists of the storm drainpipe discharging into a vertical standpipe, the bottom end of which allows nuisance flows to discharge into the gravel bed underneath the entire water-quality filter. High flows overtop the standpipe into the vegetated filter area. Approximately 40 wetland-planter areas result in a combined total surface area of 12,000 ft.2 The planters are located intermittently along the lake edge based on project aesthetics and overall lake water-quality operation.

Click here for a larger view

Click here for a larger view

The constructed Bridgeport lake system has been operating since 1999, and an active water-quality monitoring program evaluates the overall health and operation of the lake (see Figures 1 and 2). The trends observed from the data indicate that (1) the makeup water required to offset evaporation is only 70% of the anticipated amount because of nuisance-water reuse and (2) application of algaecides is 50% less than the normal requirement for a lake of this size. An example of the successful application of constructed aquascape systems to provide stormwater treatment, the Bridgeport system demonstrates that exceptional water quality can be achieved far beyond the traditional methods and still benefit the surrounding community. The success of a project of this nature is highly dependent upon the the developer, the engineering team, and the landscape architect, whose collaboration in the case of Bridgeport resulted in an innovative lake system that became its "crown jewel" centerpiece. Bridgeport can credit its accomplishment to Newhall Land, PACE, Collaborative West (landscape architect), and Alliance Land Planning and Engineering (civil engineer).

Bruce Phillips, P.E., is vice president of stormwater management with Pacific Advanced Civil Engineering Inc., a water resources consulting engineering firm in Fountain Valley, CA.

SW January/February 2004

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