An Integrated Pest Management Approach to Levee Vegetation

An IPM approach to vegetation management offers a way out of the traditional dilemma of levee maintenance practices which exacerbate weed and rodent pests. Furthermore, this method shows promise as a means of achieving the objectives of maximum levee safety while simultaneously improving environmental quality of riparian areas.

Traditional approaches to levee maintenance have a single objective—that of flood safety. In contrast, the IPM approach recognizes flood safety as the primary maintenance objective but places high value on other goals including improvement of wildlife habitat and recreational opportunities, enhanced aesthetics, and reduced pesticide use. At the level of implementation, the main points at which the approaches differ are in the way in which vegetation is viewed and the process by which maintenance decisions are made.

Despite the wide variation in biotic and abiotic conditions of various levee reaches, traditional maintenance practices have tended to give all levee reaches equal attention and treatment. Thus, a uniform policy of slope clearing to facilitate inspection is implemented irrespective of characteristics of the levee vegetation. By contrast, the IPM approach gives more recognition to the uniqueness of each levee stretch in

the belief that improved practices can be developed which both maximize flood safety and improve environmental quality.

When reduced to their basic structural forms, levees can be defined as "fill slopes." When viewed in this manner, both engineering (US Army Corps of Engineers 1978) and biological (Lines etal . 1978) expertise would agree that vegetation can and does play a key role in stabilizing these slopes against the erosive forces of wind, water, temperature fluctuation, and damage by animals, humans, or vehicles.

Damage to levees due to erosion, cracking, slumping, seeps, etc., may originate in soil type, construction techniques, seismic action, burrowing rodents, water pressure, maintenance practices, or other forces. Whatever the origin of the problem, the presence of vegetation holding the soil mass together can help reduce (and in some cases, prevent) the onset or impact of such problems.

Thus, protection and encouragement of certain vegetation-types on levee slopes can be seen as an important tool in maximizing the structural integrity—and therefore the safety—of these structures. Should a conflict appear to arise between the presence of vegetation and the need to inspect the levees, a site-intensive levee monitoring program offers a solution (fig. 4).

Figure 4.
Diagram showing a levee slope monitoring
process. Key variable is slope visibility.

Under an IPM vegetation management system, information on current site or pest conditions is integrated with historical data on the construction and maintenance history of a levee reach. Site conditions are evaluated through use of a variety of monitoring techniques and record-keeping systems which vary in intensity, depending on a priority assigned to a given site. Utilizing the monitoring data, injury and action levels are established for the vegetation, and selective treatments are chosen. Spot treatments, selected from mechanical, cultural, biological, or chemical controls, are timed to minimize side effects on non-target organisms. Strategies and tactics are evaluated for long-term effectiveness and cost.

Utilizing this system, the maintenance manager has greater flexibility when it comes to the apparent conflict between vegetation and inspection of levees. By prioritizing sites to be monitored, those with no significant history of maintenance or other problems can receive a less intense level of monitoring, freeing maintenance personnel to focus major monitoring attention on areas with chronic flood history or maintenance problems. (This situation may occur defacto under the traditional approach but is not recorded, planned, or approached in a systematic manner.)

Ideally, levee inspections occur just before, during, and just after flood season (i.e., November–March each year). Under the IPM system, spring inspections (which detect damage from winter flows) should be timed to occur before spring grasses have grown more than one foot tall. By shifting existing maintenance personnel to inspection/monitoring roles in early spring, it

should be possible to thoroughly monitor most sections of levee prior to dense growth of vegetation. If vegetative growth becomes too dense to detect potential levee damage, selective removal of that vegetation can occur. If damage is found, repairs can be made. If warranted, the vegetation at that site could be removed periodically, or vegetation more suited to the inspection and maintenance needs at that site could be encouraged.

Encouraging Appropriate Vegetation on Levees

A major strategy in an IPM program for levees is the development of practices which select for and encourage certain existing grass species and low-growing broad-leafed plants which reinforce the structural integrity of levees by reducing erosion and ground squirrel habitat. Examples of candidate species include salt grass (Distichlisspicata ); creeping ryegrass (Elymustriticoides ); perla grass (Phalaristuberosa 'Hurtiglumis') and saltbush (Atriplex spp.).

Under some circumstances it may be appropriate to introduce new species to the levees (Daar etal . 1979), particularly in areas adjacent to residential subdivisions, where concerns about the relation of levee management to fire danger, recreation, and aesthetics are the focus of attention. Five candidate species for replacement vegetation in urban areas are showing promise as relatively low-growing, dense, low-maintenance slope covers in test plots at two levee locations near Sacramento. These species include: sageleaf rockrose (Cistus salvifolius ), Cleveland sage (Salviaclevelandii ), Australian saltbush (Atriplexsemibaccata ), dwarf coyote brush (Baccharispilularis 'Twin Peaks No. 2'), and Noel grevillia (Grevillianoellii ).

Benefits of IPM Vegetation Management

The benefits of a selective vegetation-management system will be most evident during the flood season. The condition of levees at any given site will be known in intimate detail due to the data recorded by the monitoring program. This results in greater predictability of a given levee reach when under flood stress. At sites with chronic maintenance problems or flood histories, vegetation on the land side of the levee (the side visible during high water) will have been selectively managed to retain its rootmass while restricting its height, thus maximizing the stability of the slope as well as its visibility in case of a flood fight. Decisions regarding setting of priories and scheduling work, deployment of labor and materials, and evaluation of efficacy of maintenance efforts will be aided by the data collected by the monitoring process.

Other benefits of particular relevance to the riparian system would include reduction in the use of pesticides, more extensive wildlife habitat, enhanced recreational opportunities, and an increase in the aesthetic quality of the levee environment.

In conclusion, an IPM approach to managing levee vegetation shows promise as a method for achieving the dual objectives of maximum levee safety and improved environmental quality.

In recognition of this potential, the DWR is moving to implement new IPM practices including more intensive levee-monitoring and record-keeping systems, development of injury- and action-level concepts, and use of selective vegetation-management techniques to encourage the presence of certain grasses and other vegetation compatible with multiple-use management objectives. It is hoped that these and other IPM practices will become adopted Department-wide and serve as a model for the state's 7,000 local water districts, whose activities have a profound impact on California's dwindling riparian resources.