Stormwater on the Plateau

The portion of the campus of the University of the South that sits atop the plateau is comprised of more than 50 first-order catchments, ranging in size from 4 to over 61 ha. The vast majority of the streams are intermittent, flowing only during the winter and spring months, and rarely during particularly wet summers and autumns. The few perennial streams, such as Abbo’s Alley, are typically fed year-round by springs. During storms (usually defined by hydrologists as at least 1 inch of rain per hour) soils are quickly saturated and much water flows directly to streams as overland flow, often resulting in a dramatic rise in stream level. In non-urbanized catchments on the plateau, about 20%– 25% of the rain that falls during such a storm enters the stream as overland flow (hydrologists refer to this as the “hydrologic response” of the watershed). This is due primarily to thin (0–5.2 m, 1.2 m average), sandy loams that overlie the relatively impermeable sandstones and conglomerates of the plateau top. As expected, partly urbanized catchments on campus have significantly higher hydrologic responses. Here, impermeable surfaces such as roads, sidewalks, roof tops, and parking lots create increased amounts of overland flow. The upper Abbo’s Alley area is such a catchment, exhibiting a hydrologic response to storm events of ~40%. This is primarily due to the catchment consisting of 20% impermeable surfaces (rooftops, roads, sidewalks, etc.). Additionally, extensive grass surfaces in yards and parks have infiltration rates (10–100 mL/min) that are lower than those of the undisturbed, nearby forest fl oor (250–300 mL/min). Stormwater in urbanized catchments is not directed into the sewer system. Rather, the storm drains that one sees along roadsides and parking lots direct water into the nearest drainage.

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Storm drain with marker on Sewanee’s campus.

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The diagram above shows storm hydrographs for Abbo’s Alley and Split Creek, a 100% forested, undeveloped catchment of similar size on the campus. These hydrographs were generated during the same rain event (5.9 cm in one hour) in November 2006 and demonstrate the increased hydrologic response of partly urbanized watersheds in the area.   Notice that the Abbo’s Alley stream rises much more dramatically in response to the storm compared to the Split Creek stream.

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Abbo’s Alley stream during normal flow (left) and during a storm event (right).

Below is a description of a storm event at Abbo’s Alley from a Sewanee student:

“… At 5:53 the Abbo’s Alley watershed was experiencing a downpour. It began to thunder and lightning, and the stream rose enormously. A fast spike in the water level occurred, then the level fell again before rising to the peak. Between 6:00 and 6:30 a wave of insects washed by clinging to floating brown leaves (last fall’s leaves) and a few dog toys floated down the stream, along with a beer can and several salamanders. Around 6:10 water began to flow over the road about 30 meters closer to the meadow where Hodgson Pond used to be instead of through the culvert. At 6:19 peak flow was attained and the culvert was completely full of water. At 6:38 the rain stopped and the sky began to clear. It was partly cloudy until after sunset when another storm system began to move in and cloud over the sky.”

The increased stormwater flow in urbanized watersheds means they are also characterized by higher degrees of erosion and soil loss compared to their undeveloped counterparts, exhibiting steeper, higher stream banks, undercut stream banks, and exposed roots (see photo below). Plateau catchments often show macropore development along the axis of the stream channel. Here, erosion has created karst-like, sinuous passages in the soil. These macropore systems are discontinuous along the length of the stream channel, and therefore alternately move water above and below ground. In developed watersheds, these macropore systems often become overwhelmed with water during storm events, leading to accelerated lateral and upward erosion. Occasionally, this upward-enhanced erosion breaches the surface of the ground, leaving tree roots suspended across the resulting open channel (see photo below). Such “macropore blowout” structures are the hallmark of increased erosion on developed watersheds on the plateau.

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“Macropore blowout” structure behind range pole in stream on an urbanized watershed on the campus of the University of the South. Notice lateral erosion, bank undercutting, and sandstone bedrock stream floor. Range pole divided into 1 foot sections.

Some significant differences in water quality may be detected during storm events between developed and non- developed watersheds. The former are characterized by increased levels of Na, Ca, and Mg. Increased Na is attributed to winter road salting, while Ca and Mg are likely derived from weathering of carbonate road gravels. First- flush sampling on developed watersheds has also indicated the presence of oil and grease (<5 ppm), while non-developed watersheds have remained free of these contaminants. Finally, pH values on developed watersheds range from 7.2 to 7.6, compared to 4.6–5.5 on non-developed counterparts during all flow conditions. This elevation of pH is likely due to the presence of carbonate road gravel and concrete in the developed catchments.

The University of the South has recently taken more aggressive steps in order to minimize the negative effects of increased stormwater flow in urbanized catchments. Prior to 2010, the only evidence of stormwater flow control on campus was limestone rip-rap that was strategically placed to reduce erosion. With the renovation of Snowden Forestry Building in 2010, a large bioswale was constructed to filter roof water from the building. The bioswale is sufficiently large to accept roof water from the new Ayers Hall dormitory, which is currently under construction. A bioswale is a depression that is filled with permeable materials designed to slow and filter water as it passes through.

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Bioswale that will accept roof water from Ayers Hall (foreground) and Snowden Hall (near background).

Additionally, several detention basins have been constructed adjacent to parking lots, dorms, and on the University’s golf course. These basins temporarily hold stormwater, permitting sediment to settle out and slowing the water down.

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Tiered detention basins adjacent to student parking lot.

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