RIVERBANK RESTORATION SOLVES CONTAMINATION PROBLEM WHILE PRESERVING CRITICAL HABITAT

A 275 linear foot section of contaminated bank and over 4,800 square foot area of adjacent wetlands in a major river in eastern Massachusetts required removal and reconstruction. Assessment of the extent of hazardous material impact from a disposal site indicated that a significant lateral and vertical section of river bank required removal. The impacted reach of bank was characterized by a steep, organic rich bank face overlying thick deposits of glaciofluvial outwash. The river stretch is characterized by shallow depths, a sand-cobble bottom and typical seasonal flow fluctuations. The river is lined by mature deciduous shade trees and directly adjacent to the impacted bank are a rich population of Massachusetts NHESP protected fresh water mussels including the triangle floater (Alasmidonta undulata) and eastern pondmussel (Ligumia nasuta). The river system includes fallen trees and a dense mat of undercut roots, primarily of red maples (Acer rubrum) along the bank, which provides valuable habitat for a variety of riverine species including perch and nesting for waterfowl, as well as basking and denning structure for small mammals, reptiles and amphibians.

Critical design elements included preserving an oxbow-like feature of wetland behind the stream bank, subject to seasonal and high flow event flooding from bank over topping as well as creating habitat features with similar characteristics to existing features. A bio-engineered restoration solution for the impacted area was designed and permitted. For construction, 300 feet of steel sheet piling was installed in a curvilinear path adjacent to the riverbank to protect the mussels in the main channel of the river from sediment and to dam water to allow for adequate dewatering during removal of contaminated materials and bank restoration. The bank and adjacent wetlands was excavated to depths below the river bed. A bio-engineered bank was installed that used a brush-layering technique to incorporate live native plants between lifts of soil wrapped in geotextile fabrics and river stone at the base. The restored bank and wetlands incorporate a wide variety of native trees, plant and herbaceous vegetation to provide a stable, natural setting with significant habitat features that successfully blends with adjacent sections of undisturbed riverbank.