The Lower Neches Valley Authority (LNVA) is one of the 23 River Authorities created by the state of Texas to develop and manage the waters of the state. The LNVA is responsible for fresh water management in far southeastern Texas in the entirety of Jefferson, Hardin and Tyler Counties, and portions of Liberty and Chambers Counties. The watersheds of the Neches River and its tributaries occupy an area of approximately 10,000 square miles. The area receives generous amounts of rainfall, producing stream flows in the Neches and its tributaries of around 4,323,000 acre-feet of water a year at Port Arthur, TX, where it empties into Sabine Lake and, ultimately, into the Gulf of Mexico.
Although water in the Devers Canal System is pulled from the Trinity River, the LNVA acquired the Devers Canal System in 2008.
Known as the Jones Crossing, the site of a potential breach was approximately 21 canal miles downstream from LVNA’s Devers first pump station at the Trinity River. At this particular location about 20 miles southwest of Beaumont, an aged drainage system was piped under the Devers Canal via three parallel 48-inch reinforced concrete pipes. The canal, in turn, was piped directly under a farm access road also via three parallel 48-inch reinforced concrete pipes. All three features converged at various angles at the Jones Crossing point.
“We were experiencing problems with the existing reinforced concrete pipe joints separating and allowing canal water to leak into the ditch,” said Ryan Ard, P.E., LNVA engineering manager. “LNVA made several repairs over time to patch any leaks that appeared. Over the years, erosion and weathering had deteriorated the original timber headwalls in the drainage ditch. This ongoing erosion, exacerbated by the heavy rains in early 2015 and again in early 2016, eventually infiltrated and undermined the headwalls, allowing the banks of the drainage ditch to creep toward the canal. Had we allowed the problem to persist, the canal banks would have ultimately been breached by erosion, thereby allowing canal water to spill into the drainage ditch below.”
As originally planned, this project would be designed by LNVA staff and put out for public bid by a general contractor in mid to late 2017. Given the circumstances and the need for urgency, and because LNVA’s canal system is not in operation over the winter months (Oct. 31 through March 15) due to the agricultural cycle, LNVA decided to take on the construction project internally.
The scope of the repair would include excavating and removing the existing drainage ditch and canal crossings which both included three runs of 48-inch reinforced concrete pipe. The pipe in the ditch measured approximately 160 feet and the pipe in the canal was approximately 40 feet long. The total depth of the drainage ditch was approximately 12 feet.
LNVA engineers analyzed several alternative repair designs, but ultimately decided the best option was to rebuild the crossing in its original configuration. The layout was somewhat complicated by the fact that a private road crosses both the drainage ditch and the canal at this location.
Because the hydraulics of both the canal and the drainage culverts had been performing satisfactorily for many years, and also because of vertical space limitations, LNVA decided to use the same size pipe for the reconstruction. The driving consideration was trying to maximize the depth of the canal while maintaining the existing flow line of the drainage ditch.
The original headwalls in the drainage ditch were constructed of vertical timbers. “We wanted to go back with a hard armored headwall, such as concrete or articulated block,” explained Ard. “We determined a vertical headwall in the drainage ditch would be more costly to construct and maintain over the years compared to a sloped concrete liner. Based on this decision, we requested the pipe ends be mitered at a 2:1 slope from the factory. Once installed, the ditch slope would be graded to match and a concrete liner poured for erosion protection.”
LNVA considered different types of pipe material for this application. Although corrugated metal and concrete are most commonly used materials for drainage culverts, engineers reviewed the life-cycle cost benefit of fiberglass pipe. Leakage associated with corrugated and concrete drainage culverts are not generally of concern, but over time they will cause erosion and corrosion. For this particular site where a canal, a drainage ditch and a road all converged, the type of pipe material raised concern for LNVA engineers.
“At this diameter, the pricing was competitive with reinforced concrete pipe,” said Ard. “However, the benefits of fiberglass pipe outweighed that of a reinforced concrete pipe in terms of handling the material and placement during construction.”
While the needed pipe would measure only 600 linear feet of 48-inch diameter, generally an amount falling below the dollar value required for a formal bid process, LNVA nonetheless, publicly advertised for bids to supply fiberglass reinforced pipe (FRP).
The LNVA ultimately chose Hobas centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipe because, according to Ard, “We have a good service history with Hobas and it is installed in various other locations on our system. We liked the fact that the joints came in 20-foot lengths and would be watertight, the pipe is lightweight when compared to its concrete counterparts, and Hobas would factory miter the pipe ends to our specifications and install a headwall ring to be cast into a concrete headwall.”
At Hobas, the pipe is easily customizable for specialty jobs via a custom-built water jet cutter. Drawings are created in 3D mechanical CAD software and transposed into the required code. The water jet machine is then run by a computer-controlled program that produces perfectly cut pipes, quickly and in a more environmentally friendly manner.
All of these benefits facilitated the LNVA crews being able to install the pipe relatively easily with no previous experience installing this type of pipe. LNVA has several crews and operates a fleet of its own heavy equipment to perform day-to-day business. This allowed LNVA to help control costs by utilizing internal equipment and manpower to construct the project.
The installation of the three 160-foot runs in the drainage ditch took four days, while installation of the three 40-foot runs in the canal under the roadway was completed in only one day. Once the new pipe was installed, LNVA crews re-graded the drainage ditch slopes, and reconstructed the canal banks and rock road back to their pre-existing grades. Because repairs to the roadway and canal system were performed in the agricultural off-season, the road was closed for duration of construction, and no residences or local traffic were affected.