Riparian Vegetation on Flood Control Project Levees

Constraints and Opportunities^[1]

Lee W. Carter and Gene L. Anderson^[2]

Abstract.—Efforts are being made to find and evaluate alternative construction methods and maintenance practices that are responsive to environmental and esthetic considerations in conjunction with flood control needs. Although some constraints on vegetation on levees are necessary, progress is being made to reach a compromise between the environmentalists and the flood control project builders and operators to allow riparian vegetation on and adjacent to flood control project levees.

Introduction

Can riparian vegetation on flood control project levees be managed differently in the future than it has been in the past? What constraints on vegetation are necessary to maintain the integrity of flood control project levees?

The answers to these questions are complex and controversial. This paper is primarily based upon experience with levees in the Central Valley of California.

Background

Maintenance of flood control project channels and levees has come under considerable public criticism in the last several years. Some of this criticism can be attributed to the lack of environmental consideration on the part of the maintaining agencies. Some of the criticism is because the public fails to acknowledge the primary purpose of the levees. Public criticism is often triggered by removal of riparian vegetation and its replacement with rock bank protection.

In recent years, the California Reclamation Board and the Department of Water Resources (DWR) have advocated retention of a greater amount of riparian vegetation on and adjacent to flood control project levees. The US Army Corps of Engineers (CE) has resisted relaxation of present flood control project maintenance standards because of the high potential for loss of human life and property associated with levee failures. The CE has expressed particular concern about the increased wild growth developing in the rock revetment sites on the Sacramento River flood control project levees.

The differences between guidelines for allowable vegetation on levees proposed by the Reclamation Board and those in the operating manuals prepared by the CE have received considerable attention during the past three years. The staff of the CE and that of the DWR have jointly proposed a revised guide for allowable vegetation consistent with Title 33, Code of Federal Regulations. This revised draft of present guidelines was circulated for review within the DWR, CE, other interested State agencies, and local levee maintaining agencies. The local maintaining agencies and the South Pacific Division of the CE expressed opposition to relaxation of the CE's standards. The Department of Fish and Game expressed concern that the proposed guide does not adequately encourage the retention and protection of riparian vegetation.

The next step will be to submit the proposed guide to the Reclamation Board for its recommenmendations considering the comments that have been received.

Constraints on Vegetation on Levees

First, it is useful to discuss briefly the reasons for restricting vegetative growth on levees.^[3] Most levees were constructed for one purpose—to protect the adjacent land from flooding. Early levees were constructed by local landowners. Later State and Federal governments

[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].

[2] Lee W. Carter is Chief, Data and Operations Branch, Central District, Department of Water Resources. Gene L. Anderson is Senior Engineer, Department of Water Resources, Sacramento, California.

[3] For more information on this subject, see Nolan (1981).

assisted in providing funds and expertise to upgrade the integrity of flood control projects. For the most part, the levees were considered single-purpose flood control structures. They were designed as dams to withstand the hydrostatic pressure and relatively high velocities of flow exerted on the levees during high water.

The design, construction, and maintenance standards developed by the CE were designed to protect human life and property. The State of California gave assurances to the Federal government that the levees would be maintained to the standard outlined in the operation and maintenance manuals prepared by the CE for project levees. The most economical and efficient way to inspect the levees was to restrict the amount of vegetation so that the levee slopes could be inspected from the levee crown. Most inspections are made from an automobile moving along the levee at about 5 mi. per hr. or 1 mi. per 12 min. If the inspector were required to stop and spend time inspecting levee slopes on foot, the inspection time could easily be tripled. Under the present inspection procedures, the State is spending around $200,000 per year for inspections. Maintenance and flood fight activities also are easier to perform without wild growth on the slopes. Therefore, many maintaining agencies annually mow, burn, or spray vegetation. The maintaining agencies then fill eroded areas with soil or rock, exterminate burrowing rodents, and maintain levee heights to the design level.

In some cases, the local maintaining agencies have cleared more vegetation than is required by the CE standards; in other cases, they are doing less than is required by standards. For example, the minimum-size levee on a major stream is 20-ft. minimum crown width, a design freeboard of 3 ft. to 5 ft., and slopes of 3:1 horizontal to vertical on the waterward levee slope, and 2:1 horizontal to vertical on the landward side of the levee. On smaller tributary streams, the minimum crown width is 12 ft., freeboard 3 ft., and the same side slope requirements. According to CE engineering manuals, this basic levee structure must be root-free except for grasses and low-growing, shallow-rooted groundcover plants. In certain areas, brush and small trees may be retained on the waterward levee slope to prevent erosion, wave wash damage, and for environmental values.

Unrevetted Levees

Although the Federal regulations still require the basic structure to be root-free, many levees are larger than the minimum standard levee. Where adequately overbuilt levees exist, trees and shrubs can be allowed to remain on the levee slope. However, to facilitate inspection of the levees and flood fighting efforts, spacing requirements are considered necessary. Selective clearing to satisfy spacing requirements is expensive.

The magnitude of the expense of retaining vegetation is difficult to evaluate. The expense would vary depending on the extent and type of vegetation and the present maintenance practices. Many local maintaining agencies contend they cannot provide for the additional expense required for selective clearing. No sources of funds, except for the unfunded Z'Berg Bill (Water Code Section 8450, etseq .), have been identified to subsidize local maintaining agencies for the increased cost of maintaining riparian vegetation.

Critics of the CE standards argue that the standards are too stringent and that vegetation can be a deterrent to erosion of the levee slopes. In some instances, their argument may be valid; however, the standards were developed to protect levees under adverse soil-type and foundation conditions, erosional forces, and problems associated with trees and shrubs.

The causes of levee failures are difficult to document. Failures have been attributed to levee subsidence because of foundation conditions, rodent activities, and caving of the levee from erosion. No levee failure has been attributed directly to the existence of riparian vegetation on the levee slopes. However, vegetation that hinders the local maintaining agency in performance of adequate maintenance increases the risk of levee failure. Consequently, frequent and adequate inspection of levee slopes is considered imperative to a good maintenance program. Restrictions on vegetation species, spacing, and pruning are, therefore, considered necessary.

In general, unacceptable vegetation species either: 1) provide a food supply for rodents; 2) are detrimental or destructive and difficult to control or eradicate; or 3) are characteristically thorny or intrusive, such as roses, blackberries, bamboo, and vines.

The proposed revised guidelines provide that spacing between trees or clumps of trees, shrubs, or clumps of trees and shrubs (up to 6 ft. in diameter), shall be no closer than 25 ft. The 25 ft. were considered an average inter-tree or inter-clump distance between trees on levees. Also, spacing encourages the tree to branch out rather than grow tall. Space requirements are for three general reasons: 1) to facilitate inspection of the levee; 2) to make flood fights and repairs less difficult; and 3) to encourage trees to develop a good root system and to branch out rather than grow tall. Tall trees have a greater tendency to be blown over.

Revetted Levees and Berms

The present CE guidelines prohibit growth of trees and shrubs in rock revetment on a standard levee section. Under the proposed revised guide, trees and shrubs are allowed in the revetment on either levees or berms when the distance from the landward shoulder measured at design freeboard level to the top of the revetment is 150 ft. or greater. When the stream velocity is less than 5 ft. per sec. and the channel is relatively straight, this distance can be reduced to 75 ft.

The rationale for this conservative guideline is: 1) revetment has been placed in areas where erosion has been a problem; and 2) flow pattern irregularities caused by trees and shrubs can cause displacement of the rock and failure of the revetment.

Opportunities for Vegetation on Levees and Berms

As mentioned earlier, many levees along the major streams are large enough to accommodate vegetation under the proposed revised vegetation guidelines.

Unrevetted Levees

Most unrevetted levees that have two-lane surfaced roads on the crown exceed the 30-ft. width requirement at freeboard that is proposed in the revised vegetation guide for retention of vegetation. Also, the waterward slope of the basic levee under the proposed revision has been changed from 3:1 horizontal to vertical, to 2:1 horizonal to vertical. This change increases the area at the waterward base of the levee where roots are allowed. This would provide for considerably more trees and shrubs near the waterward toe of the levee.

Unrevetted Berms

The proposed vegetation guide provides that where a berm exceeds 10 ft. in width, vegetation is allowed with no requirement for species, spacing, height, or pruning unless special conditions require some restriction.

Low Rock Revetment

In the case of the Steamboat Slough bank protection work, the CE constructed a 10-ft. berm just above the normal low water level and placed rock on the face of the berm. This is an example of innovative design of a levee to provide a condition where vegetation can be allowed. This design is consistent with the criteria for vegetation on berms established in the proposed revised vegetation guide. It should be recognized that this special construction feature increased the cost of providing flood control. In this case the CE and the State provided the funds to pay the extra capital costs. The local maintaining agency will be responsible for maintenance.

Low rock was also placed by the CE in several reaches of the Sacramento River downstream from Sacramento as early as 1940. In many locations vegetation has grown in and above the rock. Normal deterioration of the rock has resulted in some areas and repairs are needed, but very few problems have been attributed to the vegetation. The rock has reduced erosion and has not been significantly displaced in 40 yrs., indicating that low rock has been successful under these flow and soil conditions.

Funding to Protect Riparian Vegetation

Erosion of berms and levees is continuing at a high level, resulting in the loss of riparian vegetation at an alarming rate. Federal and State funds to protect levees and berms are decreasing. We will discuss briefly the status of two programs used in the past to help retain vegetation on levees and berms, the Sacramento River Bank Protection Program, and the 1973 Z'Berg Bill, codified as Section 8450, etseq ., of the Water Code.

The Sacramento River Bank Protection Program began in June, 1963. Under this program, the CE repaired levees along the Sacramento River flood control project. Costs were shared, with the Federal Government paying two-thirds and the State paying one-third. Under Phase I of this program, a large amount of riparian vegetation was removed from the project levees. Due to environmental concerns, the CE (under Phase II of the program) received authorization to spend up to 10% of the project costs to mitigate the loss of streamside vegetation resulting from the bank protection work. The presently authorized work under Phase II is nearing completion. Work on Steamboat Slough, in Unit No. 36, is the last work authorized for construction under Phase II.

The continuation of Phase II bank protection is conditioned on Congressional authorization of mitigation for Phase I. Even though Phase II provides for environmental consideration, the Secretary for Resources has not authorized State participation with the CE on additional units of bank protection work under this phase until the replacement of wildlife habitat lost during construction of Phase I has been authorized.

Conclusion

We believe more vegetation can be retained on and adjacent to flood control levees if: 1) the levee section is designed as a multi-purpose structure, that is, enlarged to provide a zone for roots outside the basic structure required for flood control; and 2) if the levee and vegetation are adequately maintained. Both of these conditions require additional funding. The question arises: who is willing to pay? In this time of austere budgets, support will be required from many sectors to obtain the high priority necessary for funds to protect riparian vegetation on and adjacent to flood control levees.

Literature Cited

Nolan, Michael F. 1981. Vegetation on Corps of Engineers' project levees in the Sacramento-San Joaquin Valleys, California. In : R. E. Warner and K.M. Hendrix (ed.). Proceedings of the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981]. California Water Resources Center Report No. 55. University of California, Davis.