|Bays Mountain Lake Spillway Failure|
|Nestled among 3,000 acres of pristine parkland in Kingsport Tennessee, Bays Mountain Lake makes for a wonderful tourist attraction and is widely considered to be one of northeast Tennessee's most valued natural treasures. Thousands of school children, hiking enthusiasts, bird watchers and mountain bikers visit the park each year to enjoy its abundant beauty.|
However, on a rainy day in March 2002 a failure in the emergency spillway for the lake provided an unforgettable lesson in hydraulic force with Mother Nature supplying the newest park attraction - the Bays Mountain waterfall.
With thousands of gallons of water cascading over the failing spillway, Public Works crews received a calm but concerned call from the Park Director asking for assistance. As Public Works crews arrived on the scene they witnessed the uncontrolled water sheet flow over the spillway, across the only road into the Park, and propel off a vertical drop of 75 to 100 feet from the edge of pavement - the force of which washed out the guardrail,
|trees, large boulders and ultimately undermined a 35' section of the road base. Although no rainfall measures were available on the mountain, area recordings indicated that the preceding rain event approached 50-year storm levels which meant that the lake was likely to remain in a flood stage for at least 2 weeks and would impede any efforts to fix the failing spillway pipe that was under 4-6 feet of rushing water.|
The force and volume of water precluded any immediate spillway repair options so under the direction of Ronnie Hammonds, Public Works crews focused on devising methods to contain and direct the overflows in a manner that was least likely to cause any further damage in order to preserve the integrity of the remaining portions of the road until repairs could be affected. In consultation with city engineers Bob Winstead and Jill Trent, Ronnie had the Public Works crews use jersey barricades to channelize the water flow and establish a safety zone around the weakened roadway section that also served as a mobilization area and allowed safe ingress and egress to the Park in a single travel lane.
With a safety zone established the imminent hazards were under control and Public Works crews shifted their focus to road restoration. Weather forecasts called for additional storm activity approaching the area in the next couple of days, so Public Works crews had a narrow window of opportunity to make road repairs and attempt to restore the spillway before heavy flows were likely to return. In the absence of any as-built plans on the original roadway or data on the prior condition of the roadway sub-grade, Public Works crews had to engineer road stabilization efforts in the field based upon conditions observed as they open-cut the weakened road sections.
|After excavating a 35' linear section of the road 12'wide by 12' deep, Public Works crews filled the void using successive lifts of large rip-rap stone, 2" stone, and crusher run backfill with 3" of asphalt binder mix and 2" of asphalt topcoat which was rolled using a steel drum vibratory roller to achieve maximum densities. The same rip-rap and stone backfill was also used to re-establish the side-slope grades necessary to support the roadway.|
|Despite the significant site constraints, including extremely limited access and steep elevations, Public Works crews succeeded in making the road restorations in the course of two days. However, before re-opening the street to the public the City retained the services of a Professor from Ohio State University that specialized in roadway stabilization in mountainous terrain to verify the structural integrity of the road. After an extensive field investigation, the Professor validated the integrity of the repaired roadway sections and it was re-opened to the general public.|
Public Works crews were then able to shift their attention to the spillway that appeared to be the source of the problem. The spillway consisted of a 48" corrugated metal pipe with an inlet at the base of the dam. The pipe proceeded underneath the existing roadway for approximately 90' with an outfall into an existing creek downgradient of the road. Despite the heavy flows, it was evident that the inlet was damaged which severely restricted pipe capacity and caused the water to sheet flow across the road.
As water levels gradually receded in the days that followed, further spillway inspections revealed acollapsed inlet and internal pipe deformation that was most likely a result of substandard installation techniques used in 1971 when the pipe was constructed. In addition, significant portions of the metal pipe suffered from severe corrosion which allowed water to exfiltrate from the pipe into the surrounding road base and exacerbated adjacent roadway stability problems.
Public Works crews were able to re-open the collapsed portions of the pipe as an interim fix but it was clear that a long-term solution was needed. Working in cooperation with the University Professor, Bob Winstead and Jill Trent designed a long term strategy that included:
1. Repair the existing 48" corrugated metal pipe with a cement coating applied to the interior to restore some structural strength and eliminate exfiltration;
2. Install a concrete headwall on the existing pipe for additional inlet reinforcement;
3. Install a secondary 48" spillway structure (reinforced concrete pipe) as a back-up in the event of primary failure;
4. Reinforce rip-rap on side-slope with concrete slurry for additional stabilization;
5. Use natural vegetation (trees with strong tap roots, e.g., Ash, Oak) along the side slope for additional protection against slope failure;
6. Re-establish the roadway drainage structures using asphalt ditch and shoulder with surface rip-rap for energy dispersion.
|The total cost of the Bays Mountain Road restoration is estimated at $30,000. In light of the importance of the Park to the Kingsport community, the value of the work far exceeds these costs. And thanks to the professional response of Public Works, school bus loads of children are again able to enjoy the Lake every day and learn the lessons that nature offers around every curve in the trail, including - water runs downhill.|
History of Bays Mountain Lake
|In 1915, trees and buildings were removed from the area to be covered by the lake. On April 1916 work began on the dam and stone was quarried from about a hundred and fifty feet below the dam itself. The stone was hauled to the dam site by teams of mules and a crane, powered by a mule, was used to hoist the stones up on the dam.|
|When the dam was completed, all the property owners were moved out except one woman, who for several years continued to cultivate a field near the lake. Water began flowing to Kingsport in November of 1916, three months before the city was incorporated. In 1944 the lake was abandoned for use as a reservoir but it remains an integral part of the park today.|