The hard, jagged surface of rock riprap used for erosion control in ditches and along roadsides gives the impression of solidity and strength. While riprap offers effective protection against high-flow water events, there are long-term safety issues to consider. The risk of damage and injury to vehicles, passengers, and pedestrians is greater in accidents on jagged rock than on a vegetated surface.
Turf reinforcement mats (TRMs) provide a safer alternative. Designed to effectively replace riprap in many applications, the mats provide the aestheticand environmental advantage of natural vegetation. Roadside projects can often be executed more efficiently, at a lower cost, and with safer, more satisfying permanent results through the use of TRMs.
Selecting a TRM
TRMs are a type of rolled erosion control product. These products come in many forms, such as temporary erosion control blankets that are predominatelymade of photodegradable synthetic mesh with or without fillers or woven biodegradable natural fiber netting, and the highly engineered TRMs designedfor permanent use.
TRMs are composed of non-degradable synthetic fibers, filaments, nets, wire mesh, or other elements that have been processed into a permanent, 3-D matrix of appropriate thickness for the application. They impart immediate erosion protection, enhance vegetation establishment, and permanently reinforce vegetation. The mats are commonly selected for use in hydraulic applications, such as high-flow ditches and channels, and on steep slopes.
A good place to begin researching TRMs is the Erosion Control Technology Council (ECTC) Web site, www.ectc.org. Resources include a specification, comparative tables for product class evaluation, an installation video, case studies, and information on material providers.
A useful design manual for roadside channels is the 2006 Federal Highway Administration (FHWA) Hydraulic Engineering Circular #15 (HEC 15). This document can be accessed at www.fhwa.dot.gov or via a link in the "Product Selection and Design Tools" section of the ECTC Web site.
In addition to the safety benefits, vegetated linings such as TRMs do not require as much excavation, the need for heavy installation equipment, or specialized labor to install. Their lighter weight generally reduces to-site transportation costs.
Riprap, on the other hand, may require a bed that is twice as deep as the mean diameter stone and a geotextile liner to improve performance.
In short, the mats offer significant savings even before the project has been completed. This is particularly important in the public sphere where everydollar spent must be justified.
Installation efficiency, long-term performance parity, and general cost savings make the vegetated mats an advantageous design choice for many applications. But the safety and aesthetic advantages may be the most important factors for the everyday citizen who benefits, often unknowingly, from that roadside work.
Andy Constantine is the TRM product manager with Profile Products LLC, Buffalo Grove, Ill. Tim Lancaster is president of North American Green, Evansville, Ind.