In the winter of 2002, the City of Indianapolis faced a critical situation. River bank erosion had exposed landfill waste and threatened to expose a petroleum pipeline, jeopardizing public safety and private property. EPA funding for design would not be available for 2-3 years; therefore, in order to protect the public and private property, as well as avoid a potential legal action, the city needed a quick, cost effective solution.
Naren Patel, PE, VS Engineering, Inc., was shown the site by the City in a canoe on the White River. When asked if he would help the City find a solution, Mr. Patel enthusiastically replied, “I will as long as you never make me ride in a canoe in winter on the White River again!” Mr. Patel discussed the possibilities of utilizing a hybrid soil biotechnical solution with local industry representative Jim Blazek. Mr. Patel explained that environmental compatibility, cost effectiveness, call for immediate action, quick permitting, and the difficulties associated with public “low bid” work were all challenges to be considered. Additionally, the system would need to be built year round and withstand potential flooding not only during, but immediately upon construction.
A hard armor solution was investigated and D50 = 30” was an appropriate design. The estimate for constructing this hard armor riprap solution along the affected area was $3,000,000. A second analysis was conducted to determine if soils colonized by synthetically reinforced indigenous vegetation soft armor would adequately protect the landfill and pipeline from exposure. A detailed design analysis was performed and a hybrid system was proposed. The installed estimate for the hybrid system was $2,200,000. Considering the project’s challenges and budgetary analysis, VS Engineering, Inc. proceeded with the hybrid design.
The hybrid design approach included a riprap wing deflector at the upstream end of the project as the affected river bank was approximately at a 30 degree skew to the river’s thalweg or primary deep channel. A riprap “aggregate raft” was constructed along the entire toe of slope composing of Tensar UX Geogrid and 1160N Mirafi non-woven geotextile. The aggregate raft design was utilized to minimize scour and differential settlement. It was constructed from three feet below the normal pool to a top elevation of two feet above normal pool. The top elevation coincides approximately with the two-thirds of bank full elevation, which is the typical transition point to the vegetated or “soft armor” solutions. A non-woven separation geotextile was laid on top of the aggregate raft to prevent the backfill from migrating through the stone’s void space. A 2:1 fill slope was constructed over the aggregate raft in front of the landfill waste and the pipeline. The backfill was reinforced with Tensar BX Geogrids in 18” vertical lifts. Indigenous seeds were sown on the constructed slope and then covered with North American Green P-550 permanent reinforcement mat. The bottom of slope included a 20” diameter, 9 lbs/ft3 density, prevegetated coir log, with site specific species. The coir log was placed on top of the P-550 at the transition to the vegetative system. Eight inch diameter, 9 lbs/ft3 density, vegetated coir logs were also placed to secure the overlap locations of the P-550 mat. The coir logs were used to ensure the successful introduction of over twenty indigenous plants species throughout the slope.
The project’s installation began in April 2003 and was completed by the end of October 2003. The first flood took place less than thirty days after the first four hundred feet of the project was installed (two thousand linear feet total). The last eight hundred feet was installed dormant waiting for Spring 2004 to germinate. The system withstood six flood events during the 2003 installation, with inundation periods up to seven days and a record setting nine inch “Labor Day 2003” rain event. The system has been monitored yearly since 2003 and continues to perform meeting all design expectations.