ETL 1110-2-583
30 Apr 14
b. A large tree, outside the vegetation-free zone, becomes a hazard tree when its root
system is severely damaged by construction activity, thereby increasing its susceptibility to
windthrow and the associated risk of damage to a floodwall. This tree should be removed.
2-4. Root Impacts.
As stated in Paragraph 2-2, the primary purpose of the vegetation-free zone is access. However,
it also serves a secondary purpose: it provides distance between root systems and levees,
floodwalls, embankment dams, and appurtenant structures, thereby moderating reliability risks
associated with the following two situations: potential piping and seepage due to root
penetration; and structural damage (a hole in the ground, surrounded by an area of disturbed
earth) resulting from a wind-driven tree overturning. Though not adequate for all situations, this
15-ft zone does provide a measure of risk reduction, as follows:
a. Root size and numbers diminish with distance from the tree trunk.
b. The hole and its surrounding area of disturbed earth, created by a tree overturning,
typically has a radius ranging from 6 to 12 ft. This secondary effect of the vegetation-free zone is
important to the reliability of flood damage reduction systems; it is not a root-free zone, but it is
a zone of reduced root impact.
2-5. Root-Free Zone. Planting design must consider the possible implications to foundation
strength and performance. The integrity of the foundation could be compromised if potential
seepage paths were created by root penetration and/or root decay. The root-free zone provides a
margin of safety between the greatest expected extent of plant roots and the beginning face of
any structure that is critical to the performance and reliability of the flood damage reduction
system. The list of such structures includes levees, floodwalls, embankment dams, seepage
berms, seepage drains, toe drains, pressure relief wells, and cut-off trenches. These critical
structures must be root-free, as shown in Figures A-13, A-14, A-15, A-17, and A-19. The rooting
respect to its potential to invade the root-free zone and compromise the reliability of the flood
damage reduction system. Landscape planting plans will reflect full recognition of the
importance of selecting appropriate plant species and varieties. Root barriers may be used to
provide an added measure of assurance, but they should not be a substitute for adequate distance
between plantings and root-free zones. Root barriers shall not retard groundwater or seepage
flow. Some root barriers include herbicides to enhance effectiveness; in every case, these shall be
evaluated before use to prevent negative environmental impacts.
2-6. Water-Current and Wave-Action Barrier. The use of suitable vegetation, such as shrub
forms of Salix (willow), riverward of the vegetation-free zone, is encouraged as an
environmentally beneficial means to moderate the erosive potential of water currents and wave
action.
2-3