Illinois Native Plant Guide - Native Plant Applications for Streams and Stormwater Facilities

Illinois Native Plant Guide

Native Plant Applications for Streams and Stormwater Facilities

Stormwater Detention Basins--Shoreline Stabilization
Streambank Stabilization
Stormwater Detention Basins & Streambanks-Upland Slope Buffer
Vegetated Swales

Stormwater Detention Basins--Shoreline Stabilization

This application includes wet-bottom detention basins and ponds that have a permanent pool of open water as well as wetland detention basins, which only detain water for a limited period during and after rainfall events. Wet-bottom detention basins are those that have a permanent pool of open water, while wetland detention basins do not have permanent open water but maintain saturated conditions such that wetland vegetation can grow on the bottom. Dry detention is not a recommended Best Management Practice (BMP). Shoreline erosion is caused by wave action and water level fluctuations that can erode the shoreline and contribute to sedimentation of the pond. This in-filling of sediment from erosion of the shoreline can decrease the storage capacity of ponds, reduce water quality and clarity, and cause the shoreline to recede. The receding shoreline frequently presents additional problems with the integrity of adjacent facilities and structures, such as buildings and trails. In many cases traditional solutions to this problem, such as rip-rap, do not provide adequate long-term stabilization. Traditional engineered solutions have the added problem of providing little or no wildlife habitat, or in the case of seawalls can have a negative affect on wildlife. In addition, these traditional engineering approaches are often visually unappealing.

Anumber of native wetland plants can be used to effectively correct shoreline erosion. The deep roots of the plants will stabilize the soil. Stems and foliage dissipate the wave energy before it hits the shoreline. An added benefit is the wildlife habitat provided by such vegetation far exceeds that offered by rip-rap, mowed turf grass, or steel seawalls. Many native species are more tolerant of periodic inundation from wave action or fluctuating water levels than turf grasses. A continuous band of emergent plants will reduce nuisance geese problems. These shoreline plantings also provide water quality benefits through removal, uptake, and transformation of pollutants in runoff.

Plantings for shoreline stabilization in ponds can consist of two components. The first is the lower shoreline zone, which occupies the shallow water around the basin’s perimeter. Generally, it is from the water’s edge to a water depth of 6 inches, with a seasonal maximum water depth of 18 inches. If such a shallow shelf is not present around the pond, this zone cannot be established and limited regrading to flatten slopes and establish a shelf should be considered. For shoreline stabilization, a wider shelf provides more benefit, but a minimum width depends on storage and safety needs. The second zone, the upper shoreline zone, is from the water’s edge to the point where the soil is no longer saturated or wet most of the year. Plant deep-rooted plants naturally adapted to shoreline settings in these areas to hold the soil together and reduce shoreline erosion.  [back to top]

Streambank Stabilization

exceed the ability of the soil and vegetation to hold the banks in place. Natural rates of streambank erosion vary with stream size, velocity, amount of vegetative cover, and the type of soil. Under well-vegetated conditions, lower order (smaller) streams show little erosion over decades. Larger streams often show erosion on the outside bank of bends, but under natural vegetated conditions, this becomes a part of the naturally meandering stream morphology. Under natural conditions, stream channels are continuously changing as the outside banks of the meanders are carved out while deposition continues on the point bars. With the urbanization of the region, society has sought to keep streams in one place so as to not disrupt the man-made environment. Furthermore, many streams have been straightened to increase conveyance and provide surface and subsurface drainage to urban and agricultural landscapes. This has caused increased velocities and in turn increased erosion. To exacerbate this further, increasing amounts of impervious surface increase the flow rates and volumes in urban streams after each storm event. In many places, these high velocity, straightened channels are lined by shallow-rooted turf grass areas and other features of the urban landscape. In this type of setting nearly all of the factors that once held streambanks in place have been removed. By reestablishing deep-rooted native vegetation along streams, streambanks can be stabilized in a more natural and longer lasting way.

Bioengineering or bio-erosion control in some cases involves the use of structural elements such as coconut fiber rolls, concrete A-jacks, lunkers, and others. In most cases it also involves the establishment of native vegetation. This native vegetation can be herbaceous or woody, depending on the situation. In most cases the banks have become nearly vertical, due to the continued severe erosion in both urban and agricultural environments. In order for the native herbaceous vegetation to be successful, these slopes must be reduced to approximate those that existed prior to the accelerated erosion caused by human activities. Gradual slopes (no steeper than 5:1 horizontal: vertical) are recommended. Flatter slopes should be considered whenever possible. Woody vegetation can also be used, especially in more severe erosion situations. Trees and shrubs provide root reinforcement of the soil. Roots mechanically reinforce soil by transfer of the shear stresses in the soil to tensile resistance in the roots. Woody vegetation also provides soil reinforcement by the buttressing and arching support provided by embedded roots and stems. Native woody vegetation can also provide similar benefits to those described for the herbaceous vegetation for upland slope buffers.  [back to top]

Stormwater Detention Basins & Streambanks-Upland Slope Buffer

Stormwater runoff on slopes can cause significant soil erosion problems and non-point source pollution if the runoff is not slowed and the soil is not protected. Native herbaceous vegetation can provide a solution to these problems through a number of mechanisms. The above-ground portions of the plant, the leaves and stems, absorb rainfall energy and thus reduce the impact on the soil surface. Root systems physically bind the soil together so that it remains stabilized during runoff events. Root channels also help facilitate infiltration of the rain. Through these mechanisms the erosion potential on the slopes is greatly reduced. In addition, once runoff reaches the slope, the vegetation serves to slow it down and filter out sediment. Deep-rooted native species have a much greater capacity for overall erosion prevention than shallow-rooted, mowed turf grass. As with shoreline vegetation, a native plant buffer on slopes around basins and along streams offers considerably more habitat value for wildlife than mowed turf grass or other more traditional urban treatments, yet is unattractive to nuisance Canada geese. Buffers should be as wide as possible for maximum benefit. Their size is dependent on drainage area and slope, but as a rule of thumb, a 25-100 foot minimum is recommended.  [back to top]

Vegetated Swales

Erosion can be a problem in drainageways where the gradient is steep or water flow velocities are high. Deep-rooted native species can help to bind and stabilize the soil. Dense native vegetation can also slow runoff, thereby reducing erosive forces while filtering out some sediments and contaminants. Most native species however, are not tolerant of high salt concentrations or other contaminants which often accumulate along roads and in ditches. Caution should be exercised by using the tolerance information provided with each species in this Guide. Ditches should be configured as swales with gentle side-slopes rather than traditional square or V-shaped ditches in order to maximize water quality benefits and infiltration. See also the Illinois Urban Manual for design information. Drainage swales are often recommended as an alternative to pipes or ditches for water quality benefits in stormwater management systems. Swales have gentle side slopes and should be low gradient. If vegetated with deep-rooted native species, swales can provide wildlife habitat, reduce erosive forces, slow runoff rates, promote infiltration, and filter out sediments and nutrients. The species used for drainage swales should be selected based on how wet the swale will be between rain events, the water quality expected, and anticipated flow conditions. The selection of any particular species or mix of species must also include its effect on the capacity or size of a vegetated swale.  [back to top]