Fannin County, Texas -- Over the last two months, Texas Mitigation Solutions (TMS) started work restoring a key portion of the North Sulphur River as part of the Lake Ralph Hall project.
In the 1920s, the natural path of the North Sulphur River was straightened and channeled to help protect farmland along the river from periodic flooding. The original channel was 16-20 ft. wide and 10 ft. deep. Since that time, massive erosion has expanded it to 350 ft. wide and over 60 ft. deep–nearly ten times its original size. Wetlands, quality habitat and multiple state highway bridges have been destroyed in the process. The mitigation project is returning a section of the river downstream of the dam to its natural, meandering path.
The first phase of construction involves digging out the pathway for each tributary, making sure each has sufficient bends to slow down the flow of water.
“We are working on five main tributaries and have started on two of those,” shared Dustin Fitzgerald – Mitigate Texas Project Engineer and Texas Mitigation Solutions lead on stream mitigation for the Lake Ralph Hall project. Downstream, a subcontractor is working on the area where the new tributaries will connect into the existing North Sulphur River, removing soil so water will flow naturally into the existing riverbed.
“We’ve finished 1000 ft. on one tributary and 4000 ft. on another,” Fitzgerald explained. “Our team uses bulldozers and trackhoes to cut the channel.” Then they remove earth to create “a space for each tributary’s floodplain,” or the land area that floods with water during heavy rainfall. The tributaries range from around 8-10 ft. wide with floodplains from 100-180 ft. wide.
To keep the new riverbends from eroding, crew members are also installing “woody riffles,” or logs laid across the stream bottom in a pattern that naturally slows the water.
“In natural channel design, there are a lot of different structures that go in the channel for many reasons, including stability and habitat. To date, we’ve put in 5-6 riffles for river stability,” Fitzgerald explained. “Ultimately we will incorporate riffles every 250 ft. along each new stream. These slow the water and help it stay deeper and slower in curves to protect the stream banks. They also create more habitat by holding water longer, raising the water table and supporting bugs, plants and ecology in general. This is naturally supposed to be a wet area in the floodplain, so the stream work is helping it get back to that again.”
After they’ve created the streams’ structure, TMS team members are planting native grass and placing biodegradable matting along the banks to keep them from eroding. Next winter, they will plant trees around the new streams and native grass across the entire 400 acres planned for mitigation.
All told, the TMS team will be creating 5.7 miles of new stream, and currently have around 15 crew and subcontractor team members onsite. When finished, the mitigation will provide flood control for the North Sulphur River downstream, less erosion and new water and food resources for local wildlife. The site will be monitored for seven years after completion to confirm that the transformation’s working.
|
|
