9. Toll Creek Nursery
2023, University of Tennessee
Professor Chad Manley
Individual Work
01
Toll Creek is a small watershed in south Knoxville that terminates into the Tennessee river. While most of the creek is on private property, there is a small high order leg further downstream. This is the area of focus for these series of investigations. There is not a lot of life in the creek currently and it suffers from being over channelized. Like most urban streams, it suffers greatly from distributed source pollution from the adjacent roadway. This area is zoned as residential and features scattered patches of manicured lawn juxtaposed with young disturbed forest ecologies. Island Homes Way is the street that runs adjacent to Toll Creek for almost its entirety. Something special about this watershed is that it has Ijams nature center in it which mean that the areas not zoned for residential are relatively healthy functioning ecosystems. There is an abundance of growth and life all around Toll Creek, but not very much inside it. A few of the issues that could be improved upon within this watershed is reducing flow rate, reintroducing the historic flood plane, and allowing the stream to wiggle more than it is currently allowed to. Being that this is largely a residential area, this can be a bit of a challenge. The two biggest protagonists in this case become the road and the houses. A healthy stream wants to move and to wiggle and flood occasionally or have vernal pools, but these are all scary things to a homeowner who’s house may get in the way of this water. This is largely the case against streamy-ness. The Anthropocene is centered on acceleration. How can we get rid of water faster? How can we keep the water in one place away from the house? These are important questions to question. The reality is that humans have evolved into this Anthropogenic mindset that not only do we decide the fate of the world, but the world should function around our needs. Instead try asking yourself, what does the water want?
02
The toxicology of Toll Creek is similar to what you might expect from most urban streams in the twentieth century. The majority of the watershed is distributed source polluted by runoff from neighboring lawns and roadways. Included in this runoff is sediment, rubber from tires, various metals, fertilizers, herbicides, pesticides, engine oil, antifreeze, and any litter. While these are of great concern for the quality and health of the creek, there is one more serious manner to address. Located on the junction of Toll Creek and the Tennessee River is Ergon Terminaling. This terminal in particular houses Phillips 66 asphalt. This is a distributed source within a contemporary site. The site itself is located directly adjacent to Toll creek. The frequency of pollution on the site is considered constant but very slow. Pollutants such as Iron and Sulfur are flushed into the soil and creek as a by-product of the asphalt storing and distributing process. This effects plants and their root structures, as well stream ecologies. In the rare but not impossible case of a spill at this site, the contaminants would flow downstream to the City of Knoxville potable drinking water intake. While there is a variety of pollution concerns within the Toll Creek Watershed, they are largely centered on the same cause, stormwater runoff. The best way to design for runoff is to create various buffers around the stream that focus on slowing water down and catching/holding as much water and pollutants as possible before they reach the stream. Bio-swales are a useful strategy for this issue as well as wetland retention ponds which allow heavier pollutants to sink and catch at the bottom where they can be extracted. The goal is to use plants and soil as much as possible as a natural filter of these pollutants to stop as much as possible before they enter the stream.
03
The ecological breakdown of this watershed is comprised of a series of home/turf grass patches within the matrix of a mature hardwood forest. As mentioned previously these residential patches are the source for a significant amount of pollution in Toll Creek. Near the extents of these patches there is a lot of sunlight which allows for a unique edge condition at the ground level because of how harsh the transition is from mature forest to manicured lawn. Forests normally have much more of a transition at their edge because more sunlight penetrates through to the ground. This patch is ongoing and it will likely grow over time due to the human nature of expansion and urban sprawl. The main ecological corridor is the creek itself. This is an area with a high density of rolling hills which makes using the valley as a corridor more ideal for both humans and non-humans. Toll creek cuts right through the forest matrix in this area in South Knoxville and it has even shaped the traffic patterns of humans by means of the road. The stream outdates the road, however, the road shapes the creek. Cars and concrete don’t wiggle like streams, and over time the stream will fall victim to the rigidity of the pavement and become channelized. At times this corridor is surrounded entirely by preserved forest landscape and at other times is it a pattern for human development and it sits completely within private property. People use this corridor as a means to stay connected to the city of Knoxville but animals typically are trying to cross this corridor in order to eat and drink. The Toll Creek ecological corridor has very little non-human occupants however but there is hope in nature. With some encouragement, wildlife may find its way back home in this watershed.
04
There is an interesting spot lower in the watershed where a pedestrian bridge has simulated a small beaver dam analog which has allowed the water to rise and slow down a bit. This is where the hydrophilic design project will take place. The issue with the current status of the stream is that the top of bank is quite high at this location so the existing beaver dam analog is too small and not as effective as it could be. The water quality at this part of the riparian zone is slightly better than other parts of the stream because it is downstream from a healthier section of the creek where there is an abundance of trees and plants allowing the stream to bend and ripple. This portion of the stream, however, is quite eroded and during a heavy rain event you can anticipate very high velocity flow rates and an unhealthy amount of sedimentation in the stream as the banks are mostly eroded sediment. Because of the unintentional beaver dam analog, this area has only one speed of flow and it is quite slow. This allows for nutrient buildups which can be harmful to aquatic species trying to move through this area. There is also not a lot of room to spread out if you are a small fish trying to seek cover from predators. Outside of the water, the banks are relatively steep and they do not provide any ideal habitats for turtles, snakes, or other species to sit on the bank. These steep banks also prevent the water from being able to enter the historic flood plain as easily. If the stream was raised up, it would be able to overflow its banks occasionally and hold some of its water just a bit longer as well as diversify the aquatic habitats in this riparian area. The project proposes that some of the banks are dug out and the dirt is used to fill in the stream bed in order to raise it a bit. This process is then followed by the implementation of beaver dam analogs in a series with the intent of raising the current flood plain to the point where it will begin to consistently flood the surrounding wooded area. The intent here is to allow for the stream to redirect itself around and connect back, allowing the stream to decide its own path as well as providing a variety of depth and flows. This will also maximize the amount of room that aquatic species have to spread out within the stream. This would also be topped with a new pedestrian walkway that uses some of the BDA posts as supports to reduce impact and save on materials. This is simply a way to keep people connected to these areas and see the importance of healthy streams.
05
When designing with living systems it is important to consider the impacts that a design may have over time. An educated speculation of a possible future for this project begins with the installation of the series of beaver dam analogs which raises the flood plane and diversifies the ecology of the water. These new flood levels are prone to causing vernal pools. For nearby plants such as the Liriodendron tulipifera, it can no longer survive here due change in the saturation of the soil. As the tulip poplar continues to decay, bark beetles begin to make a new home here where they can feast, lay eggs, and complete their life cycle. Melanerpes aurifons, the golden fronted woodpecker will also call this tree home and feast on endless bark beetles. Over time the decaying core of the tulip poplar will structurally fail encouraging the growth of mosses and the introduction of snails. This creates the ideal habitat for Lampyridae, fireflies because young fireflies are carnivorous and target snails and slugs among others. The fireflies nest in the wet soil, lay eggs, and begin the next generation. Hyla gratiosa, the barking tree frog, has found his new home here as well and thrives on bark beetles, spiders, and fireflies. With a growing population of reptiles, insects, and invertebrates, Ardea herodias, the great blue heron, also moves in. The great blue heron perches on the downed log and excretes the seeds of a bald cypress. The bald cypress is able to thrive in this environment and sprouts up rapidly to compete with its mature habitat. This plant becomes of the host of the Isoparce cupressi, the baldcypress sphinx moth. With the introduction of this specialist moth, the designer strategically plants Verbena bonariensis to help attract these rare moths. The designer also installs binoculars on the pier to connect people to this special habitat.
06
The Bald Cypress Sphinx Moth is a specialist, meaning, it only loves on certain plants and it requires very specific flowers to find food. By designing a propagative strategy centered on providing habitat for the Sphinx Moth, we can provide simultaneously for generalist species. An in situ nursery will be constructed using similar processes as the beaver dam analogs. This will create microhabitat in the water as well as reduce the footprint of the structure as it leverages its simple structural column grid to lift the nursery above the water. These columns will be constructed of standard wooden telephone pole material. This same material will be used as playful stepping stones to encourage a risk taking approach to circulation. Also supported by these posts is an ADA ramp for accessibility. This in situ nursery will germinate bald cypress both from seed and from trimmings, and will propagate tall verbena, cardinal flower, and northern spicebush. Because this nursery is located over the water, it will be passively ex situ as well as seeds inevitably are deposited and carried downstream. The walls of the nursery are designed to be operable doors so that it may be opened and closed as desired as well as protecting from the harsh cold in the winter months. The design is intended to be simple but not simplistic. It is complex, but not complicated. It is low cost, and low impact. One nursery is not enough, however. To maximize impact, at least two more installation should occur, targeting areas of point source pollution and channelization. By creating a network of small in situ/ex situ nurseries along the stream there is much greater opportunity for nature to self design the spaces between.