Recommendations for Floodway Vegetation Managment

By using the velocity contours from the hydraulic model, the restoration planner can target where on the floodplain vegetation can be used or placed to affect desired outcomes. Areas of slow velocity, 1 – 2 feet per second, can usually be planted with trees and shrubs at fairly moderate densities (200 plants per acre) because the slow velocity is caused by the geomorphology of the channel and the characteristics of the flow at various depths, not the vegetation. Higher velocity areas (5-10 fps) can be planted with flexible-stem plants and grasses that lie down under the weight of the rapid flow.

The characteristics of flexible stem species offer management options that are not currently being utilized by floodway managers.

1. Flexible stems slow erosion while maintaining conveyance.

Example planting designs:

• Hedgerows of flexible stem woody species with wide strips of herbaceous species or grasses between them to allow for sediment transport.
• Higher density plantings of mostly flexible stemmed species that would be equal or less than Manning’s n without plants.

2. Levee surface erosion

Riparian trees and shrubs can be planted near the base of a levee to form a wall of vegetation that will break the force of wind waves and protect the levee from erosion. Surface erosion on the face of the levee could be slowed or halted by planting short-stature flexible-stem species on the levee such as blackberry or rose. Alternatively, a wide row of large trees and shrubs could be planted parallel to the base of the levee to intercept wind waves before they can break on the levee and cause erosion.

3. Reduction of maintenance cost

By using the velocity contours from the hydraulic model, the restoration (and maintenance) planner can target where on the floodplain vegetation can be placed to affect desired outcomes.