The US Army Corps of Engineers have a published article regarding the vegetation management of Embankment Dams etc.
For brevity’s sake, I have extracted relevant passages. ETL1110-2-583
Examination of this document should give the reader an insight as to why the Canal Authority is carrying out the long overdue work to restore the embankments to the conditions laid out here.
Levees, floodwalls, embankment dams, and their appurtenant structures serve a common purpose in that they are designed to contain water and prevent flooding for varying lengths of time. They must also be readily accessible by equipment and personnel essential to reliable operation and maintenance. The possibility for long-term saturation of levee materials or levee and floodwall foundations, together with their specific operation and maintenance requirements, makes it necessary to exercise caution in the design of landscape planting and vegetation management at these structures. This ETL describes important characteristics of levees, floodwalls, embankment dams, and their appurtenant structures.
2-2. Vegetation-Free Zone
a. The vegetation-free zone is a three-dimensional corridor surrounding all levees, floodwalls, embankment dams, and critical appurtenant structures in all flood damage reduction systems. The vegetation-free zone applies to all vegetation except grass. Grass species are permitted, as described in Paragraph 4-8, for the purpose of erosion control.
b. The primary purpose of the vegetation-free zone is to provide a reliable corridor of access to, and along, levees, floodwalls, embankment dams, and appurtenant structures. This corridor must be free of obstructions to assure adequate access by personnel and equipment for surveillance, inspection, maintenance, monitoring, and flood-fighting. In the case of flood- fighting, this access corridor must also provide the unobstructed space needed for the construction of temporary flood-control structures. Access is typically by four-wheel-drive vehicle, but for some purposes, such as maintenance and flood-fighting, access is required for larger equipment, such as tractors, bulldozers, dump trucks, and helicopters. Accessibility is essential to the reliability of flood damage reduction systems.
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.
3-4.Embankment Dams and Appurtenant Structures
Purpose. “Tree and woody-vegetation penetrations of earthen dams and their appurtenances have been demonstrated to cause serious structural deterioration and distress that can result in failure of earthen dams” (Association of State Dam Safety Officials 2002). Proper establishment and control of vegetation is critical to dam safety. This Paragraph establishes minimum requirements for landscape planting and vegetation management at embankment dams (earth fill, rock fill, or earth and rock fill), including multipurpose projects with both concrete and embankment dam structures (wing dams) and perimeter saddle dams (dikes); abutments; and appurtenances, such as spillways, outlet works, and inlet and/or outlet channels.
b.Policy. The following five areas are vegetation-free zones:
(1) The dam or the dam-toe area.
(2) Areas in or around seepage monitoring systems, or critical downstream areas where seepage observation must be vigilant and continuous.
(3) Groin abutments and areas immediately adjacent to groin abutments.
(4) Spillways and spillway channels, including spillway slopes and approaches to spillways where vegetation could, in any way, impede the efficient operation of the spillway.
(5)The outlet-works discharge channel.
4-8. Appropriate Ground Cover in the Vegetation-Free Zone
a. The only acceptable vegetative ground cover in the vegetation-free zone shall be perennial grasses. Their primary function shall be to reliably protect against erosion. They shall be maintained as necessary to ensure the health and vigor of the primary species providing erosion protection. The species selected for each project shall be appropriate to the local climate, conditions, and surrounding or adjacent land uses. Preference should be given to the use of native species. Invasive or weed species shall not be acceptable. The species selected must be able to tolerate mowing to heights as low as 3 in. as follows: at least once each year for inspection, and in anticipation of flood conditions and associated monitoring and flood-fighting activities.
5-3. Removal of Non-Compliant Vegetation
a. All vegetation not in compliance with this ETL shall be removed. A detailed removal plan shall be submitted to the local USACE District Levee Safety Officer for review and comment before removal of vegetation. The removal plan shall expand on the following basic requirements.
(1) By excavation, remove the trunk (or stem), stump, rootball, and all roots with diameters greater than ½ in. All such roots in, or within 15 ft of, the flood damage reduction structure shall be completely removed.
(2) Assure that the resulting void is free of organic debris.
(3) Fill and compact the void according to the original soil and compaction specifications: or, if no specifications exist, match adjacent soil and compaction.