Vegetation and Levee Maintenance

The detrimental effects of vegetation on flood control levees have been recognized for centuries. Printed below are excerpts from a rare Spanish book, as translated by Amalio Gomez.^[6] The book, "Hidrologia Historica del Rio Segura", deals with the struggles of the City of Murcia, Spain, from 1535 to 1879 against floods, droughts, and other calamities.

Murcia Levee Ordinance

1. It is hereby ordained that Murcia's levee shall not be swept by any person, with any type of broom, because the soil of which the levee is built will be blown away by the wind, and in the course of time the levee will lose height. Therefore, he who violates this ordinance shall be fined . . . The city's share [of the fine] shall be used to pay the wage of a levee patrolman.

2. No person shall plant any trees, including mulberry bushes and fruit trees, within ten paces of the toe of the levee, along the entire length of the levee, and on both sides of the levee . . .

3. All trees (see 2. above) located within the ten pace strips described, should be pulled out within thirty days of the publication of this ordinance . . .

4. No person, for any reason, shall farm within the land occupied by the levee with a plow, hoe, or other implement . . .

5. No person shall accumulate a pile of manure within 100 paces of the toe of the levee all along its length . . .

6. It is ordained that no cattle of any kind shall be permitted to walk along the levee crown, with or without an attendant; such cattle must walk the road built parallel to the levee for that purpose. Cattle may cross the

[6] Former Chief, Engineering Division, Sacramento District, US Army Corps of Engineers.

levee only at the paved cattle crossings. Owners of cattle found violating this act shall be fined . . .

7. Grass grown on the levee crown and side slopes shall not be cut or pulled by any person . . .

8. The levee shall not be used as a playground; no person shall damage it by making holes or otherwise. No damage shall be done to any improvements on the levee crown, such as statues, paintings, stone seats, housewalls, stone pavements and hermitages. Violators shall be punished . . . It is further ordained that if the patrolman hired by the city to patrol the levee and enforce these ordinances does not apprehend the violators, he shall be removed from office and appropriately punished. (Gomez 1976)

Each prohibition of the ordinance carried a penalty. The usual procedure was to divide the fine, giving one-third to the judge, one-third to the accuser, and one-third to the city. The ordinance was enacted by the City of Murcia on July 25th, 1737. It was approved by the Royal Council of Castilla, August 30th, 1737.

Corps of Engineers Regulations

The Code of Federal Regulations contains the CE regulations for the operation and maintenance of local flood protection works.^[7] These regulations, as written, apply to flood control projects throughout the nation and are of necessity general in character.

With regard to vegetation, the regulations require that measures be taken to promote the growth of sod, exterminate burrowing animals, and provide for routine mowing of the grass and weeds and removal of wild growth and drift deposits. The regulations also encourage planting willows and other suitable growth on the river side of levees to retard bank erosion; they discourage activities which retard or destroy the growth of sod, such as burning grass and weeds.

CE has several regulations which further address vegetation and the operation and maintenance of levees. Excerpts from "Project Operations, Levee Maintenance Standards and Procedures" (US Army Corps of Engineers 1968) are presented below.

Maintenance Standards

The levees will be maintained as necessary to insure serviceability against floods at all times. Standards for accomplishing the foregoing are as follows:

A good growth of sod will be maintained where feasible with grass height from 2 inches to 12 inches, substantially free of weeds.

All brush, trees, or other undesirable wild growth will be removed from the levee embankment. Vegetation specifically planted for aesthetics or recreation purposes may remain.

Maintenance Procedures

Maintaining sod growth—Maintenance of a sturdy sod growth on levee embankments is highly important as sod is one of the most effective means of protecting the levee against erosion from rain, current and wave wash. Periodic mowing is essential to maintaining a good sod growth, and should be done at such intervals as necesssary to keep down weeds and other noxious growth and to prevent the grass height from exceeding 12 inches. The grass should be mowed to a height of 2 inches or more. The number of mowings required each season will depend on local conditions, but experience has indicated that in most parts of the United States two or more mowings are necessary each season where pasturing is not used. The last mowing of the season should be accomplished under conditions which will allow the grass to obtain a height of approximately 8 to 10 inches going into the winter season. Mowing should be performed to a distance of at least 5 feet beyond the toe of the levee or berm.

Burning grass and weeds will not be permitted in the levee maintenance program, except during appropriate seasons when it will not be detrimental to the growth of sod.

"Project Operations, Inspection of Local Flood Protection Projects" (US Army Corps of Engineers 1973) addresses items to look for during inspections. Some excerpts from that regulation are presented below.

Are trees and other growth which might jeopardize the stability or watertightness of the structure removed at regular intervals?

Have trees and undesirable growth been cleared from the levees and along side floodwalls?

Does brush cover or other growth interfere with inspection?

Are there any obstructions to vehicular passage along the crown of the levee?

Is sod cover on levees being encouraged?

Is there any unauthorized burning of grass and weeds?

Is sod cover mowed regularly?

[7] Code of Federal Regulations Section 208.10, Title 33, Chapter II. Part 208—Flood Control Regulations. Corps of Engineers, Department of the Army.

Is sod or other desirable cover fertilized and reseeded as necessary?

Is there an effective program for control of burrowing animals?

"Project Operation, Maintenance Guide" (US Army Corps of Engineers 1967) also contains information on levee maintenance. Appendix I of the regulation, "Inspection Guide and Maintenance Standards," contains the following information:

Close inspection (should be made) for settlement, sloughing, slides, erosion, condition of ground drainage, the presence of burrowing animals, the presence of debris, need of mowing, encroachments that tend to weaken levees, rutting of crown, depressions or other defects.

Keep embankments sufficiently smooth to permit mowing by power equipment. Newly filled areas shall be sodded or seeded.

Levees should be mowed with tractor-operated equipment to a height of 2 inches or more when the grass reaches a height of 5 to 7 inches or when excessive uneven growth of grass or weeds becomes unsightly. Reseeding and fertilizing is limited to kind and weight necesssary to sustain vegetative cover for the control of erosion by wind and water.

Figure 2 is an example of a levee where vegetation interferes with inspection.

Figure 2.
Levee along Sutter Slough where dense
vegetation interferes with levee inspection.

Title 33 of the Code of Federal Regulations^[5] also requires the CE to furnish local interests with an operation and maintenance manual for each flood control project (e.g., US Army Corps of Engineers 1955, 1959). The manual assists the responsible authorities in carrying out their obligations by providing information and advice on the operation and maintenance requirements of each project. Specific maintenance requirements that are applicable to an area are spelled out in special instructions in the manual for each project. This has been done, for example, in the special instructions contained in the "Standard Operation and Maintenance Manual for the Sacramento River Flood Control Project" (US Army Corps of Engineers 1955) which allow burning. Burning was allowed after the California Department of Public Works (DPW) in 1949 contended that due to the nature of the levee vegetation, burning weeds and brush during appropriate seasons should be allowed in order to determine danger to the levee such as slips and cracks and to permit the detection of holes caused by burrowing animals. Figures 3 and 4 present a good comparison between a burned levee and a mowed levee.

Figure 3.
Tractor-mounted hydraulically operated and controlled rotary
mower as utilized by the American River Flood Control District
(California Department of Water Resources 1973).

Figure 4.
Burned levee along the Sacramento River near Knights Landing.

Another example of special instructions in the "Standard Operation and Maintenance Manual for the Sacramento River Flood Control Project" (US Army Corps of Engineers 1955) deals with the provision in Title 33^[5] which encourages growth

riverward of the levees to retard bank erosion. The special instructions in the manual expand on this provision and allow retention of brush and small trees on the waterward levee slope where desirable for preventing erosion and wave wash. The manual contains the specific requirements for maintenance of each project.

Others besides the CE recognize that vegetation can be detrimental to levees. In December 1954, a year before the disastrous flooding of Yuba City and Nicolaus due to a levee break (fig. 5), the RB and the DPW expressed concern about trees on levees. The following is extracted from an 8 December 1954 letter from the RB to the District Engineer, CE, Sacramento:

The Reclamation Board has through many years of experience found that the existence of trees on the river banks is a serious threat to the integrity of the levee system. In that respect the regulations of the Corps of Engineers require the removal of wild growth from levee banks and this agency is obligated to enforce these regulations. However, the Reclamation Board concurs in the view that trees constitute a menace to flood control works. Tree root systems penetrate levee sections, inducing seepage paths. Trees situated on the edge of the river banks that are uprooted by high winds, particularly when the banks are saturated, create craters in the sandy banks and levee sections rendering both of them susceptible to serious erosion during flood periods. Furthermore, the presence of trees on or adjacent to a levee often seriously interferes with routine maintenance operations such as slope dragging and control of weed cover.^[8]

Figure 5.
Break in Feather River levee in Reclamation District 1001.

The following are excerpts from a letter dated 10 December 1954 from the DPW, Division of Water Resources, to the District Engineer, CE, Sacramento:

Through the years, large trees became established on the levee slope and the heavy superimposed load produced slip-outs when the relatively unstable bank and levee section became saturated. Wind action on trees also tends to loosen the embankment and accelerates slippage when moisture is present. Trees overhanging the water's edge tend to pull out and drop into the stream when pressure is released from saturated banks. Trees which are of sufficient size to offer resistance to the flow of water are undesirable in a levee slope. Fallen trees and bared roots caused scour, silting and eventual diversion of the current with the result that banks became undercut, additional trees are dislodged and serious levee slippages ensue.

Best maintenance practice dictates that the levee should be kept clear of trees and brush. However, in actual practice this condition is not fully obtained. In the sand levees below Sacramento, the presence of sod and light growth is not particularly objectionable and, in many instances, considered desirable. However, trees which attain large growth should never be permitted to become established on the levees. At locations where a wide berm exists between the bank of the river and the toe of the levee slope there is no objection to large trees if located in clear of the levee and riverbank.

The assumption that the trees along the banks of the Sacramento River have provided a stabilizing effect is contrary to the experience of the engineers in the Division of Water Resources and others familiar with the conditions along the Sacramento River. Most severe slip-outs in levees of the Sacramento River can be directly attributed to the presence of large trees which place a superimposed load on the relatively unstable embankment, in excess of the ability of the levee material to resist when saturated.^[9]

Figures 6 and 7 are examples of some of the problems cited by the RB and the DWR.

[8] California Reclamation Board. 8 December 1954. Letter to the District Engineer, CE, Sacramento, concerning tree removal along both banks of the Sacramento River between Sacramento and Rio Vista.

[9] California Department of Public Works, Division of Water Resources. 10 December 1954. Letter to District Engineer, US Army Corps of Engineers, Sacramento, concerning tree removal along Sacramento River channels downstream from Sacramento.

Figure 6.
Exposed roots and erosion around roots of tree
along Sacramento River near Knights Landing.

Figure 7.
Exposed tree roots along Sacramento River near Cranmore.

The State maintains some portions of the Sacramento River Flood Control Project under the supervision of the DWR, with costs being defrayed by the State. The DWR also has exercised supervisory powers over maintenance performed by local districts on Sacramento River Flood Control Project levees. Such supervisory controls were accomplished generally in conformance with DWR "Standard Maintenance Procedures" (California Department of Water Resources n.d.) and "Guide for Levee Maintenance" (California Department of Water Resources n.d.). The following is taken from these manuals:

Clearing of the levee slopes of all growth, other than grass, should be accomplished at the earliest opportunity and once cleared the area should be maintained in that condition.

Burning of vegetation on levee slopes and shoulders should be performed annually.

Cut and pile brush, trees, and other obnoxious growths.

Clear brush, trees and wild growth, other than sod, from the levee crown and slopes. Herbicides applied with suitable equipment, under proper control and conditions, have been successfully employed in eradicating pernicious growth of vegetation.

Contrary to the often expressed belief that growth of trees and brush is beneficial for protection of the levee slopes, long experience has demonstrated that this is in error for the following reasons:

Under wind and wave action the larger growths tend to pull at their root systems, causing them to uproot themselves, disturb the soil or rock revetment and permit accelerated erosion to take place. Fallen trees may also cause harmful current deflection and accumulate drift, which can compound the erosive action. Roots of large trees also attract burrowing animals to the protective shelter afforded.

Removal of such growth promotes a growth of sod or grass, the pliable roots of which tend to provide a soil binding net.

Some detrimental effects of vegetation were pointed out again in "Findings and Recommendations Based on the Inspection of the Delta Levees During October 1980" (Department of Water Resources 1980a). Excessive wild growth was considered detrimental because it interfered with visual inspection of levee sections suspected of being inadequate. The DWR recommended some selective clearing to increase the capability for levee inspection and floodfighting.

The importance of levee maintenance has long been recognized. In 1925, in his report to the RB concerning revised plans for the Sacramento River Flood Control Project (California Department of Public Works 1925), the State Engineer stated: "This project will be incomplete and the expenditures largely wasted without adequate provisions for maintenance."

The importance of adequate levee maintenance is also recognized in "California Flood Management: An Evaluation of Flood Damage Prevention Programs" (California Department of Water Resources 1980b), which includes the statements:

Levees are . . . the method [of flood control] with the greatest potential risk of failure . . . Such failure can be caused by high waters overtopping a levee or eroding a portion of a levee, or by excessive seepage through a levee. Also, very large trees can be uprooted, leaving excessive voids in a levee, which can accelerate levee destruction . . . The effectiveness of

any method of structural flood protection depends on the maintenance of the completed projects . . . Levees are the weakest part of a structural flood control program . . . Maintenance is a continuing responsibility with the objective of retaining the functional capability of the structures to store or carry storm flows according to their design . . . Levees are the most critical area in the maintenance of structural flood protection works.

The overriding concern of the CE, Sacramento District, has been ensuring the integrity of the flood control levees. Hence, adequate maintenance must provide a condition which will ensure the integrity of the systems when needed during flood periods. Encroachments which compromise the levee integrity or interfere with inspection, maintenance, and operation activities are detrimental to levee integrity.