New York State Canal Corporation
EMBANKMENT INSPECTION & MAINTENANCE GUIDE BOOK


1.3 DISTINCTION BETWEEN LEVEES AND CANAL EMBANKMENTS

Water-impounding embankments are typically divided into two main categories: earth dams and levees.

The canal embankments do not neatly fit into either category. Although they are similar to levees in some respects, (mainly that they are both linear features that retain water on one side protecting adjacent low ground on the other), there are important differences between levees and canal embankments including:

• Levees are temporary barriers to protect for infrequent, short-term duration rises in rivers and only function with full water loading in high recurrence interval storm events.

 In contrast to this, the canal embankments perform for the entire navigation season (6 months of the year) year after year.

This difference has the following major impacts:

1. the frequency that the levee embankments function is far less than for the canal embankments, so the risk of failure is correspondingly less (canal embankments function every year, levees may function only once in a period of decades,

2. the period during which the levees must function is only when the flood waters rise, the canal embankments perform for the entire period when there is water in the canal (roughly 6 months per year from mid-April to the mid-October),

3. because the levees are needed for known storm conditions and for shorter duration, a heightened awareness and increased monitoring of the levees is possible and cost- effective since the period of monitoring is infrequent and short-term (maintaining this heightened awareness and enhanced monitoring for the entire time the canal is watered every year is unrealistic), and

4. because the period that the levees hold back water is significantly less than for the canal embankments, there is less time for the phreatic surface to develop into a steady-state condition and less time for any flaws within the embankment to be compromised.

The important distinction between relatively infrequently loaded levees is recognized within US Army Corps of Engineers (USACE) guidance (Engineering Monograph [EM] 1110-2-1913 “Design and Construction of Levees”) which states the following:

Embankments that are subject to water loading for prolonged periods (longer than normal flood protection requirements) or permanently should be designed in accordance with earth dam criteria rather than the levee criteria given herein.

This distinction is critical. The USACE guidance indicates that the canal embankments should be treated as earth dams.

Vegetation management practices for earth dams are universally accepted within the engineering community with the rule that woody vegetation is not permitted on earth dams.

1.4 VEGETATION MANAGEMENT AND THE EXPERIENCE OF OTHER AGENCIES

The risk reduction strategy for the embankments that likely has the most impact on the community is the management of the vegetation that currently exists on the embankment. The dam safety engineering community recommends clearing of all woody vegetation from the embankments to eliminate concerns of seepage paths created by tree roots, the possibility of tree blowdowns creating large depressions that could weaken the embankment or cause a breach and the difficulty the vegetation causes to embankment inspection, among other factors. Adjacent landowners and canal users view the trees and vegetated slopes as  beneficial to the canal setting for such things as shade, visual site barrier to adjacent properties, wildlife refuge, and more. Finding a solution that addresses the needs and desires of both sides is critical.

Reducing the risk of a catastrophic embankment failure is a clear need. Developing a cost-effective option that allows some woody vegetation to remain is a goal. This same dilemma exists in many other locations where water impounding embankments exist in close proximity to recreational and residential areas. The guidance developed by the governing agencies in those locations can be useful for the canal embankments.

The experience and resulting design criteria from two agencies are summarized in the sections below. The two agencies were chosen because of their specific technical reputation and the similarity of the issues covered.

The NYS regulating agency requirements for dam embankments is covered in Section 1.5.

1. The USACE, whose guidance may be considered the best practice relating to vegetation management on embankment dams and levees.

2. The California Department of Water Resources which has dealt with the issue of balancing the risk and benefits of having woody vegetation on their levee embankments, particularly those within urban environments.

1.4.1 US ARMY CORPS OF ENGINEERS
The current guidance for vegetation management on water impounding embankments is contained in EP 1110-2-18 Guidelines for Landscape Planting and Vegetation Management at Levee, Floodwalls, Embankment Dams and Appurtenant Structures.

The Corps’ guidance makes it clear that vegetation may remain on and around embankment and related infrastructure, provided that “the safety of the structure is not compromised, and effective surveillance, monitoring, inspection, maintenance, and flood-fighting of the facility are not adversely impacted.”

A key aspect of the Corps’ guidance is the establishment of a vegetation-free zone which is a “three- dimensional corridor surrounding all levees, floodwalls, embankment dams, and critical appurtenant structures in all flood damage reduction systems.” No vegetation, other than approved grasses may penetrate the vegetation-free zone. 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.”