From the Guidebook, explains why GRASS is the best compatible vegetation for Earthen Dams
March 2021
NYSCC Embankment Inspection & Maintenance Guide Book
7.3 VEGETATIVE MAINTENANCE
Of particular importance for safety is the proper maintenance of vegetation on embankments. For embankment maintenance, vegetative cover is divided into two general categories:
• Compatible vegetation – this includes grasses and other similar plant cover. This vegetation is low growing, is easy to mow, and develops shallow root systems.
• Non-compatible vegetation – this category includes most brush, bushes and trees. This vegetation can develop deeper root systems and is typically prevented by regular mowing, after initial removal by clearing and grubbing.
While there are some positive benefits of woody vegetation and tree cover, these benefits do not outweigh the substantial risks associated with embankment failure that could be initiated by the presence of the vegetation such as flooding, property damage, environmental damage and loss of life.
In contrast, grass or “soft” vegetation is beneficial to the embankment. The grass and its root system prevent erosion damage from rain events, foot traffic, and even from minor overtopping events that are small and of short duration.
7.3.1 WHY IT’S NECESSARY
Proper maintenance of embankments and their ancillary features is of utmost importance in protecting lives and reducing the flood risk of adjacent communities. This includes the proper maintenance of vegetation on embankments and ancillary structures.
Non-compatible vegetation can harm the structural integrity of these impoundment structures, obscure visibility of the ground surface (necessary for inspections for other types of failures), impede access for maintenance and inspection, and encourage burrowing by rodents by providing habitat.
Woody vegetation with robust root systems can disturb the soil structure in the embankment. Roots that penetrate the phreatic surface in the embankment increase the risk of internal erosion known as piping, the early stages of which can go undetected for decades resulting in a sudden failure of an earthen embankment.
Animal burrows pose a similar piping potential – the animal burrow shortens the seepage path potentially leading to piping at the burrow location.
Additionally, shade caused by woody vegetation can impede growth of more compatible grassy vegetation.
Furthermore, large trees can be uprooted by winds/erosion and leave large holes in the embankment, root systems can decay and rot creating passageways for water through the embankment.
Once a significant seepage pathway is initiated, catastrophic embankment failure could be expected to occur within one to two hours. The presence of brush and trees can also hinder critical emergency responses to flooding or repair operations.
The proper maintenance of vegetation for water impounding structures is well understood and accepted by the dam safety community and the various regulatory and advisory agencies tasked with dam safety including the USACE, FERC, USBR and FEMA. In addition, all federal flood protection projects including embankment dams and levees in New York State must maintain a woody vegetative free zone on the embankment.