Management alternatives to enhance water quality and ecological function of channelized streams and drainage canals

Abstract:

Drainage practices in humid regions historically focused on straightening and deepening natural channels to increase their hydraulic capacity and minimize upstream flooding. In most cases, traditional channel improvements disassociated the channel from the natural floodplain, especially for smaller storms, degrading water quality and ecological functions associated with the riparian floodplain. Pilot studies were implemented to demonstrate and evaluate alternative channel management strategies and design geometries to identify alternatives that might enhance water quality functions while maintaining the necessary drainage function. Channel alternatives included: establishment of in-stream and riparian wetlands, lowering of the floodplain to reconnect the channel with the floodplain, redesign of channels using natural channel design principles, and establishment of conservation easements to encourage establishment of perennial riparian buffer vegetation. This paper summarizes seven projects that were implemented between 1995 and 2006 to provide better management of drainage water in order to enhance water quality and other ecological functions in large drainage canals. Hydrology and water quality were monitored from two to four years at five sites. In addition, plant communities and macro-invertebrates were monitored at three sites. Nitrogen concentrations and transport were reduced by 20% to 70% with in-stream and constructed storm water wetlands. Reconnecting the channel with the floodplain dampened the hydrograph peak and reduced the “out-of-floodplain” risks outside the project area. The alternative practices were more expensive and resulted in two to three times more land area being taken out of production compared to conventional drainage practices. However, the benefits were improved water quality, lower peak outflow stage, and enhanced habitat for wildlife. Practices described here are beginning to demonstrate that there are multipurpose, environmentally friendly alternatives to traditional practices of routinely cleaning and mowing trapezoidal ditch channels for the sole purpose of satisfying drainage requirements.