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A Landscape Architect faces many challenges in aquascape design. One of those challenges is balancing the ornamental aspects of the design while accommodating stormwater detention within the water feature.

When Alan Thompson of Ragan Smith set out to design an ornamental feature for the Dowell Springs project in Knoxville, Tennessee, he faced that challenge.

Originally, Thompson was alerted to the Dowell Springs project by his boss Randy Caldwell. Ragan Smith was doing the streetscape and landscape work at the entrance to Dowell Springs at the time.

Once a 120 acre farm, Dowell Springs is now a business park development in the Bearden/West Hills district of Knoxville. The development should attract business and retail because of its prime location and view of the nearby Smoky Mountains. The Millennium edition of "Places Rated Almanac" ranked metropolitan Knoxville the "best place to live in the United States and Canada" among cities. The study analyzed 354 metropolitan areas for living costs, transportation, job outlook, education, climate, crime, the arts, health care and recreation to determine the rankings.

The pump house was pre-built and equipped by Wesco Fountains. A 20 horsepower pump and a 5 horsepower pump trigger the flow of water throughout the system. The 20 hp pump takes the water from the bottom of the system to the top pond.

Depending on business requirements, one to 20 acre parcels are available. An architectural review board will offer guidance to all developers to ensure the integrity and appearance of the property.

The development adjoins the 1.6 acres where the Lonas-Dowell House sits. It is one of the oldest historical and continuously occupied archaeological sites in Knox County. The house was originally built in 1858 from bricks molded out of Tennessee clay. Dowell Springs gifted this historical landmark to Knox Heritage, Inc. for its restoration and preservation.

The walking trails and trailheads on the Dowell Springs Business Park property guide visitors to historical markers for the property's other archaeological sites, including a slave cemetery and an operational Spring House.

A large natural spring on the property near the Lonas-Dowell House feeds a nearby creek. This spring and creek provided Thompson the inspiration needed to create a water feature that runs through the center of the development.

The 5 hp pump takes water just above the main waterfall to the lowest pond to replish the water source quickly, since that is where the water is drawn from. The pumps deliver the water at a rate of 400-gallons-per-minute.

"We wanted a water feature with a passive feel that is pedestrian friendly, and that could be enjoyed at lunch," Thompson said. "The biggest trick of the design was working an ornamental stream system through a retention/detention pond system."

Retention/Detention System

Four retention ponds sit within the system, all north of a road that runs through the property. When the retention ponds fill to a certain point they flow into the detention system that bypasses the stream. Multiple waterfalls interlink these ponds and there is approximately 80 feet of elevation difference between the lowest pond and the upper most pond in order to assist the flow of the water.

Water flow is triggered through the stream with a 20 hp and a 5 hp pump which draws water from the lowest pond and delivers it to the uppermost pond at approximately 400-gallons-per-minute.

"We came up with a 20 hp pump that takes the water all the way to the top pond," Thompson said. "And then we have a 5 hp pump that takes it just above the big main waterfall at the lowest pond because we wanted to get a little extra water there quickly."

The intake pipes lead to the 5 horsepower and the 20 horsepower pumps. Water levels get low in bottom pond so a 450-500 foot deep well was built the brings subgrade water into the pond.

Help with the hydraulics came from Wesco Fountains. They provided a pre-fabricated pump house, figured out the flow calculations and came up with what they thought would work best in conjunction with Bob Morris of Rockscapes International who provided the rock for the project. The storm drainage system was pre-designed by Mike Thorne of Barge Waggoner Sumner & Cannon, Inc., the site engineers at the time.

"We all had to work together to get this system as close to the original concept that we were trying for in the first place which was to accommodate detention and also provide a nice ornamental setting at the same time," Thompson said. "It's come off really well."

According to Thompson, development of the Dowell Springs Business Park was started in 1998. Ragan Smith came aboard in the spring of 1999 to do streetscape work, and that summer made plans for the amenity. The stream was up-and-running in the spring of 2000.

A flow through pipe, located underneath the first bit of rock treatment is in front of the top retention pond. That water then runs through another set of rock treatment and fills two smaller ornamental ponds. This process continues on down throughout the system filling smaller ponds and retaining water in the larger ponds as well. The natural rock rip-rap is placed in front of the inflowing waterfall giving it the look of water exiting a natural spring.

Water needed to get to the lowest pond quickly, so a 5 horsepower pump was installed during construction of the main waterfall.

When water is taken from the lower pond, coupled with evaporation, the water level in that pond is going to drop in order to get the system charged. To replenish the water supply in the pond, a 450-500 feet deep well was built that brings sub-grade water into the pond.

For the detention part of the project, a stand pipe is located in the larger ponds, which are approximately 4 feet deep, where storm water overflows into these pipes and bypasses the stream and goes right into the bottom pond.

These flow-thru pipes, cast in a fabricated rock to blend in with the natural and other fabricated rock surrounding it. There are three weirs located at the front of the first three retaining ponds. All weirs are a minimum of 8 feet long and the heights are exactly eight inches above the flow-thru pipe inverts.

Ornamental Features

"This was a nice project, it was a different style," Morris said. "There was about 60 feet of falls. I counted about 20 or 21 falls that were at least 2 feet high."

The pedestrian trail and wood bridges are now lined with river birch crape myrtle. River birch is a common tree along streams in Tennessee and crape myrtle was added for what Thompson called "vertical interest."

Morris said he used cast-in-place, natural fitting artificial rock made to look like Tennessee limestone. There were some rock out croppings near the natural spring that Morris used to match with his cast-in-place rock. Approximately 6,000-square-feet of cast-in-place rock was used, as well as nearly 1,000 tons of natural boulders from a nearby mountain.

The stream bed is PVC lined with a concrete shell and textured for water proofing. Morris said the biggest challenge was regulating overflow outlets and mainining them. "The ponds are retaining ponds," he said. "We had to regulate the overflow outlets and on through the path. We had to retain water, but still have a retention area."

Thompson added that with water you never know what is going to happen. "Water does what it wants," he said.

Although Tennessee does not have the weather extremes of the northeastern or southern United States, there are definitely four seasons in the state. The average temperature in Knoxville is about 56 degrees with an average summer temperature of 77 and an average winter temperature of 36. Average snowfall in the region is about 11.4 inches.

A big test for the stream was how it performed in the winter months with the snow. According to Thompson, the small ponds and stream beds froze over but it did not affect the system.

"The system would come on and lift the ice, so there was no detrimental affect," he said. "I don't think it will see ice a majority of the winters, we have pretty mild winters.

"When it snows, it's nice looking, but it doesn't affect it much." Thompson said Morris, who has over 30 years of experience creating rock treatment for water features, reassured him that the fabricated rock and stream bed would make it through the test of the ice.

Along the stream bed and in the surrounding area, the plant material that Thompson chose for the project featured very colorful plants that blend in well with the nearby Smoky Mountains.

The upper amenity area looks over part of the stream and the Smokey Mountains can be seen in the background.

"I'm a big fall color fan, especially using those colors in east Tennessee with the Smoky Mountains all of 20 minutes away from Knoxville," he said. "This site is going up the south slope of a hill, and its view is of the whole run of the Smoky Mountains. It's pretty fantastic."

Although choosing plants for their beauty was obviously a factor in material selection, budget (as usual) played a factor in selecting plants. Thompson tried to stick with native plant material familiar to the region.

"What we are finding out these days is you do one landscape plan, you bid it out and find out what plants exist and then you retool it," he said. "The availability of plants in this economy is one year you'll have red maple and one year you won't have red maple, they just get used so much."

Thompson chose to use a lot service berry featuring a lot of fall color and redbud which is more of a spring tree and has a multi trunk feature and the branches that spread wide. Thompson also got nice multi trunk features out of river birch which was placed up close to the stream.

"River birch is a common tree along Tennessee-type streams, and we wanted to reflect that in our stream bed to give it a natural look also," he said.

River birch is unique because the bark begins to peel with age, as dark gray colored bark comes off in thin strips. Other plant material used includes red maple, sugar maple, blue atlas cedar, forest pansy redbud, sourwood, sycamore, yoshino cherry, bald cypress, otto laurel, kwanso daylillies and many others.

For "vertical interest," crape myrtles were added. Thompson said that because of the elevation difference throughout the park, and walking through the trails is like going through a hall in a natural landscape setting, these trees were added. But, color was still the biggest factor is plant material selection.

Thompson added that the landscaping around the facility was very improtant to Jerry Bodie, the developer of Dowell Springs Business Park.

"Jerry saw the value of landscaping and all the ornamentals," he said. "He made it priority one, and it only increases the value of the land."

Bridges cross the stream six times, including three rock slab bridges. The bridges designed by York Bridge Concepts, were made from Southern Yellow Pine.

More than 20 waterfalls, combined for nearly 60 feet of waterfalls in the Dowell Springs stream. Approximately 6,000-square-feet of cast-in-place rock was used, along with nearly 1,000 tons of natural boulders from a nearby mountain. Cast-in-place rock was used with some rock out-croppings near the natural spring at the lower end of the development.

The bridges and boardwalks are already getting put to good use according to Thompson.

"People are driving in from other job sites for lunch and walk it in the evenings," he said. "In about five or 10 years when everything is all mature, it is really going to be nice."


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November 13, 2019, 7:40 pm PDT

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