Lake Tomahawk forebay rehabilitation complete

Sediment control and bridge stability addressed by project

Fred McCormick
The Valley Echo
January 27, 2021

A boulder weir in the forebay of Lake Tomahawk is one of several upgrades recently completed at the Town of Black Mountain park. Work included stabilization of the pedestrian bridge abutment and stone walls to allow access for dredging and sediment management. Photo by Fred McCormick

A project designed to reduce the amount of sediment entering Lake Tomahawk and stabilize a pedestrian bridge is complete. 

The work, which concentrated on the inlet and forebay in the northwest corner of the park, should improve the water quality in the popular public attraction.  

Town of Black Mountain officials in October of 2019 approved a plan to repair a previous sediment management system where Tomahawk Branch feeds the lake, after it was damaged by severe flooding in 2018. The failure led to an increase in sediment accumulation in the nine-acre body of water. The flood water also undermined the substructure of the pedestrian bridge that crosses the inlet. 

Aldermen approved $130,000 for the rehabilitation work, which Town Manager Josh Harrold presented as a more natural and holistic method of addressing sediment issues in the lake than the previous system. 

The project, which began in early 2020 with a topographic survey of the area and an evaluation of the abutments supporting the bridge, was constructed by South Core Environmental and designed by Asheville-based Wildlands Engineering, Inc. Work was completed around the end of the year.

Crews removed a large cement structure from the lake, south of the pedestrian bridge, and repositioned the channel underneath, according to a report from Wildlands Engineering Senior Water Resources Engineer Jake McLean. A boulder weir was constructed in the bed of the inlet, north of the bridge, allowing sediment to drop into a pool as water flows into the lake.

Stones and rocks of various sizes were added to support the bridge abutment. 

“In addition, the dredge capacity was significantly increased over its prior capacity and a reinforced access for Town digging and hauling equipment was established along the length of the new forebay,” McLean stated in the report. “The boulder walls running along the banks of the forebay were also rehabilitated, and buttressed on the water side with additional rock below the water surface to allow for an increased depth of dredging and to support the weight of Town equipment.”

The access will allow public works crews to dredge the forebay regularly. 

“Sediment is the primary pollutant of concern in the Upper Swannanoa River basin, and in most river systems throughout N.C. The Upper Swannanoa is on the State’s 303(d) list of impaired water bodies making it a target for improvements to protect aquatic life and public uses,” McLean wrote in the report. “Recent watershed studies, stream restoration and stormwater treatment projects have started the process of making improvements to Tomahawk Branch, and reducing sediment sources in the watershed.”

Minimizing sediment in Lake Tomahawk, which feeds into the Swannanoa River, is one of many steps taken by the Town in an effort to have the 2.6-mile Black Mountain segment of the river removed from the impaired waterways list. 

“The rehabilitation of the forebay, a measure that acknowledges that we still have a ways to go to reduce sedimentation of our waterways, expands the capacity so that with each maintenance dredging effort as much as 150 cubic yards of sediment can be removed before entering the lake, roughly equivalent to 250 tons,” McLean’s report stated.   

The new system will allow the Town to monitor sediment collection in the forebay and adjust maintenance accordingly, likely reducing the need to contract larger dredging projects within the lake bed.