Just north of Indianapolis lies Carmel, Ind., a city experiencing continual expansion from the Indianapolis metropolis. Meandering through the city of Carmel is Cool Creek, a freshwater tributary of the White River. Known for its native riparian habitat, the Cool Creek corridor is inhabited by many types of wildlife, and under storm conditions carries excess runoff from the surrounding residential areas. Over time, portions of the creek had experienced severe erosion damage as the amount of runoff had increased with the city’s expanding residential development.
In order to receive a new housing permit for a development adjacent to Cool Creek, a portion of the creek required mitigation to lessen the severity of its bank slopes. The city of Carmel was concerned about the impact of new residential development on the creek’s banks, as the Cool Creek corridor had seen many erosion problems associated with the growing development within its watershed.
The need for mitigation along the creek included reduction of the potential for property and soil loss under flood conditions along the outer bank of a sharp creek bend. Fluctuating water levels along with the increased scour potential along the outer bank had eroded the soils and native vegetation over time, leaving the stream bank vulnerable to even greater erosion. This portion of Cool Creek was in need of a redesign to satisfy the permitting requirements and to permanently stabilize the creek shoreline from erosion.
In 2005, the design engineer, Rich Kelly of Indianapolis, along with the erosion control specialists at D2 Land and Water Resource, developed an integrated erosion and sediment control plan designed to offer Cool Creek long-term protection. "The creek banks required armoring, and we analyzed many different systems," said Kelly.
The integrated erosion control design first required the site to be regraded with the eroded 1:1 (H:V) slopes pushed back to a more gentle 4:1 gradient. The bulk of the slope protection was provided by Tensar’s North American Green Vmax3 P550 turf reinforcement mat. The permanent P550 offered initial erosion protection while the site revegetated, and also provided permanent reinforcement of both the roots and stems of the native vegetation.
Along the upper slopes where the potential for high water shears is lower, a temporary biodegradable erosion control blanket SC150BN was recommended. The all-natural double-net straw/coconut erosion control blanket with natural jute netting provided good soil coverage for up to 18 months while the native species developed. For protection along the toe and to act as a transition between the mats and the normal water line, a 20-inch-diameter coir log was specified. The long-lasting, heavy-duty coir log helped anchor the bottom edge of the erosion control matting and acted as a wave break along the stream bank. To expedite the establishment of native vegetation at the toe, the coir logs were pre-vegetated with native wetland species. By pre-vegetating the coir logs, the native plants would quickly take root since the coir log would provide the necessary nutrient reserves and moisture for their initial growth. To round out the design, all of the erosion control products were installed over a specialized Midwestern native seed mix, designed for wet and riparian areas.
"The coir log, Vmax3 P550, and native plant system provided a permit-friendly option that was very effective and easy to install," Kelly said. "The use of the system proposed and supported by D2 Land and Water Resource afforded the owner a way to perform the necessary mitigation and get their property developed in an efficient, effective, and economic manner."
The mitigation project began in March 2007 with the grading of the sandy loam soils. Once the eroded slope had been regraded to a more gentle 4:1 gradient, the INDOT Seed Mixture (Type D) was applied. The native vegetation seed mix including graminoids, forbs, and other cover grasses (see "Native vegetation species") was applied along with approximately one-half inch of top soil.
In the wet to mesic areas, extending from the water line up the slope 12 to 18 feet, the P550 was installed over the seeded soil. Beyond providing temporary erosion protection of soil and seed during the vegetation reestablishment phase, the P550 also offered permanent reinforcement of that vegetation in flows with velocities as great as 25 feet-per-second and shear stresses as great as 14 pounds per square foot. Above the P550 in the riparian area, a temporary erosion control blanket, SC150BN, was installed. The SC150BN is a double-net straw and coconut fiber blanket with natural woven jute netting designed to protect the soil for as long as 18 months during the establishment phase.
As a means to protect the slope toe at the water line, and to aid in anchoring the bottom edge of the erosion control mat, a 20-inch, 9-pound density coir log was installed. The North American Green coir log was supplied with a polypropylene netting encasement for added durability and longevity. The coir log provides water dissipation and breaks to the turbulent water, and allows for soil capture and filling behind the log at the slope toe. The coir log was anchored with hardwood stakes and nylon rope lashing. Jeff Yeary, president of Start to Finish Landscaping and the installer on the Cool Creek Development Project, said: "The potential for erosion in this area of Cool Creek is extreme. We had not installed a system like this before but with upfront planning and field support from Jim Blazek at D2 Land and Water Resource, we were able to properly install this easy-to-use erosion control system."
"The installation took extreme flows immediately after installation," said Yeary; the creek saw high water runoff that first spring. Now more than four years after installation, the native plant community can be seen having established successfully through the turf reinforcement mats. The coir logs can still be seen in the low bank position and, because the design called for the use of 20-inch-diameter coir logs, sediment deposition behind the coir logs and on top of the turf reinforcement mats is substantial.
Yeary was happy to report that the city of Carmel and the general contractor were pleased with the appearance and performance of this integrated, native plant erosion control system – a system that provided stream bank stabilization armor equivalent to 24-inch rip rap. This natural system also provided the consultant and developer with an approach acceptable to the permitting authorities of Indiana Department of Natural Resources, Indiana Department of Environmental Management, and the U.S. Army Corps of Engineers. This section of Cool Creek now has it "made in the shade" with a dense stand of reinforced native vegetation protecting its previously problematic banks.
Jill Pack, CPESC, is manager of Technical Services for Tensar North American Green. She can be contacted at firstname.lastname@example.org.