Deteriorating canal infrastructure uses LID design to manage erosion & drainage problems
Today’s engineers are not just taught construction theory, shear strength and soil type. Environmental influences have become a focus of education and best management practices.Better understanding downstream impacts has introduced us to new requirements in storm water management, Low Impact Development (LID) design and evolutions in construction materials selection.
In largely urban areas, such as Duval County, home of Jacksonville, Fla., LID is becoming an increasingly important engineering principle. LID is intended to “preserve or conserve site features and assets that facilitate natural hydrologic function.” Canal design alternatives, such as rip-rap and concrete, may control flow and channel storm water to off-site storm water collection drains, but the impact does not facilitate “natural hydrologic function.” As a result, government entities across the country are increasingly adopting standards for LID.
In addition, Turf Reinforcement Mats (TRMs) and High-Performance Turf Reinforcement Mats (HPTRMs) are becoming commonplace. Polypropylene blankets are allowing vegetation to hold back the forces of nature, while achieving the beneficial flow necessary for the job. These blankets are constructed to hold soil stability as its concrete and stone predecessors previously had, while also allowing for natural absorption and infiltration at the site of precipitation.
Effectiveness of TRMs and HPTRMs is analyzed and quantified by several organizations across the U.S., including the Erosion Control Technology Council, Colorado State University’s Hydraulics Laboratory, TRI Environmental and the Texas Transportation Institute.
An off-site cross-cut displays root establishment through the polypropylene blanketing material.
Jacksonville offers an example of an application of LID. As the city upgrades storm water infrastructure, the process, benefit and result of advancing engineering knowledge and construction materials is visible. Jacksonville experiences more than 50 in. of annual precipitation on average, making canal- and vegetation-devastating flows a certainty.
At the Woodruff Avenue pedestrian bridge site, this rang true. Engineers and contractors were asked to not only remove the pedestrian bridge, but also to reestablish the canal and install off-site storm water collection drainage.To reestablish the canal, engineers developed a plan to redevelop the canal utilizing polypropylene blanketing to reinforce the soil and manage flows into the offsite storm water drainage and associated retention. For this task, East Coast Erosion Control’s T-RECS product was selected due to its ability to stand up to brutal conditions.
Third-party testing documents a 15-lb-per-sq-ft vegetation shear stress rating, and 25-fps vegetation velocity ratings made the dome-shaped polypropylene blanketing a suitable stabilization matrix for Jacksonville environmental conditions. The construction materials used in this project underwent rigorous testing through a GAI Accredited Independent Laboratory, consistent with American Standards for Testing Materials. These tests evaluate such metrics as Apparent Opening Size, compression strength, light penetration, mass-per-unit area, tensile properties, water absorption, hydraulic shear and UV-resistance, among others.
Following removal of the pedestrian bridge, the canal side slopes were fine-graded to a smooth profile relatively free of vegetation, clods, stones and roots. Once graded, trenching was dug at the top and bottom of the slope. This trenching was used to anchor the erosion control blanketing. HPTRM blanketing was installed with 12-in. steel pins body diameter and a 1.5-in. steel washer at the head of the pin. Bohia sod was installed on top of the blanket. HPTRMs allow root growth down through the stabilization matrix to form a strong composite mat of established vegetation.
The dome-shaped and open-weave netting provides greater surface area and structure for native or otherwise appropriate vegetation, thus allowing permeability and soil stabilization on 0.5:1 slopes in high-flow channels. In this case, traditional concrete alternatives would have resulted in decreased permeability, and rip-rap can collect urban debris as it channels flow.
Reaping the Benefits
Installation of the LID-engineered solution was completed during spring 2013. By the conclusion of the growing season, vegetation was well established and ready to face the forces of Mother Nature. Moreover, the new polypropylene construction materials allowed municipal departments to mow the grass and keep urban areas LID-compliant and attractive in the eyes of the public.