Arkansas study suggests a methodology and provides general guidelines.
Drainage ditches and canals play a major role in stormwater management and flood control, particularly in urban areas, where copious stormwater runoff can quickly overwhelm sewer systems. Too often, however, these ditches and canals are unsightly and polluted, due to excess sediments, nutrients, woody debris, bacteria, and trace metals in the water.
There are scientific ways to help improve the water quality in drainage canals: Nonpoint source pollution levels can be reduced, and canal flow velocity profiles altered to promote sedimentation. These steps can reduce the amount of suspended solids to improve the look and function of canals. Carefully implemented vegetation management plans can also help make drainage canals more useful and attractive.
Distributed along canal banks, vegetation pleases the eye, protects against erosion, shelters wildlife, and provides pollination sources. To do the most good, though, it has to be vegetation of the right kind and characteristics.
A 2010 research project (PDF) surveyed the existing vegetation along drainage canals in Jonesboro, Ark., that have FEMA flood designations. The project was funded by the City of Jonesboro, with additional support from the Department of Agriculture’s Agricultural Research Service and the Mississippi State University Department of Wildlife, Fisheries, and Aquaculture. Researchers analyzed the beneficial and detrimental effects of various types of vegetation, and proposed a vegetation management plan the City could follow going forward. While the specific recommendations address conditions in Jonesboro, several ideas are transferable to other regions and ecosystems.
The researchers identified plant species that were prevalent along each of the nine drainage basins they surveyed. They measured what percentage of the canal banks was bare of plants (typically covered with riprap for bank stabilization and erosion control) and what percentages held trees (taller than 5 meters), brush (less than 5 meters tall), forbs, and grasses. They also measured the percentage of canopy cover, i.e. branches overhanging the water surface, which could break and fall into the canal.
From a functional point of view, the biggest vegetation problems come from trees and brush that die, break, and deposit woody debris in the channels. This debris can block bridge crossings, pilings, and culverts, which in turn reduces the flow of stormwater and causes flooding. Areas with high percentages of canopy cover and tall, woody brush are most prone to this flooding, which may justify tree removal.
Vegetation considered most conducive to use along drainage canal banks share a number of characteristics and meet rigorous ecological criteria. Among these requirements are:
Ability to withstand upland conditions but, if necessary, survive flooding and soil saturation for limited time periods
- Native and non-invasive species
- Ability to reproduce vegetatively with rhizomes
- Ability to withstand the region’s weather conditions
- Sufficient robustness to survive periodic mowing
- Low-growing stature
- Broad basal and aerial coverage for bank stabilization
The Jonesboro researchers identified several species common in their survey sites that meet these criteria and recommended they be maintained. These include Chinese Privet (Ligustrum sinense), Possumhaw (Ilex deciduas), Wild Rose (Rosa multiflora), and Muscadine (Vitis rotundifolia).
Because they can be quite extensive and also serve a critical function, drainage canals pose some special vegetation management challenges. The goal is to keep canals flowing as needed, but without devoting too much time and manpower to maintaining the plants along their banks. The key is encouraging the growth of desirable plants, while removing or mowing overgrowth only when necessary. The use of broadleaf herbicides can suppress the growth of woody seedlings. Controlled burning can also be an effective way to suppress woody species and encourage desirable native herbaceous plants and grasses to grow, but may be too hazardous to use in urban settings.
Kenneth A. Hooker is a freelance writer in Oak Park, Ill.