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The Science of Vegetation on Levees

Definition of Soil

Soil is a three part system/phenomenon composed of mineral and organic particles, air, and water-nutrient solution. The soil mineral particles are classified by average diameters – or texture-classes. The pore space between the soil particles is where the root hairs must grow. The water-nutrient solution is adhered to the surface of the soil particles, and will fill the pore space during rain or irrigation. After a rainstorm or irrigation, the water in the pore spaces slowly drains away by gravity, and pulls air (including oxygen) into the pore spaces. Plant roots must be able to metabolize all their functions within this environment. Soil texture influences all three components, and consequently the vigor of the plants growing on it. Soil texture composed of predominantly fine sand or silt-sized particles appears to result in the most vigorous growth by plants in the riparian zones in the Central Valley. Larger texture sizes – sand and gravel – do not have sufficient moisture in the pore spaces between the soil particles to support plant growth. On the other hand, the smaller particle sizes of clay may not have sufficient pore space to allow root hairs to grow between them.

Soil particle sizes

(Above) Illustration of the relative diameters of soil particles.

Root double diagram

(Above) (a) Close up of germinating root with root hairs. (b) Diagram of root hair growing in between soil particles.

Root morphology

(Above) Diagram of root morphology

How Plant Roots Grow through soil

Root Morphology. Plant roots are highly developed organs for collecting moisture and nutrients from the soil, and for providing physical support to the entire plant. The root tip is the location that most water and nutrients are absorbed by way of single cell root-hairs. There are thousands of root-hairs at the tip of each root. As the root grows through the soil, new root-hairs are formed. Individual root hairs on an actively growing plant function for about 1-2 days.

Roots are respiring organs and require for growth and development oxygen, moisture, nutrients, and space. Nutrients and soil moisture are required for internal physiological processes and for physical growth. Space within the soil matrix (between soil particles) is essential for the root to grow into. Where all four of these conditions are met, roots proliferate. When any one of the four is not available, root growth stops.

In perennial woody plants, root growth develops over the years into a characteristic architecture that must physically support the tree and meet its needs for moisture and nutrients. Most (90 percent) of the root biomass is located in the upper two feet of the soil profile (Shields and Gray 1994; UC extension pamphlet). It is in the upper two feet of the soil profile that the four requirements for root growth are met: the soil is mixed and loosened by invertebrates and rodents allowing oxygen and water to percolate into it and the roots to easily grow through it. A decades old tree can have horizontal surface roots many feet beyond its dripline. (Examples: Valley Oak with a three inch diameter horizontal root only six inches below the soil surface and over 100 feet from the base of the tree; Fremont cottonwood, two-years old from a cutting that was excavated by a flood had five individual roots that were over 25 feet long).

Oak shallow root

(Above) Plant roots can extend well beyond the drip line of the shrub laterally, but tend to remain in the upper two feet of the soil.

Cottonwood long roots

(Above) This two-year old cottonwood (planted as a cutting) was excavated by a flood and found to have five individual roots that were over 25 feet long.

From the above discussion, several conclusions fall out: Trees that grow on the levee will likely grow only into the surface two feet of the levee structure and not into the center where oxygen, moisture and nutrients are much less than at the surface. Thus, tree roots as a source for piping is suspect. However, with roots in the surface two feet of soil, the surface soil will be much more porous than compacted levee material. When flood water rises onto the side of the levee it will rapidly move through the porous soil and begin moistening the interior of the levee. On the other hand when flood water backs off, the levee will drain much faster because of the porous surface soil, and the plants will transpire moisture out of the levee as well.

Root systems reinforce levee structure (improved resistance to shear stress)

Definition of Shear. An action or stress caused by applied forces that cause two parts of a body to slide on each other.

In order to understand how levees are able to hold back flood waters that may be several feet deep, one must begin with examining the individual soil particle sizes that make up the levee structure. Historically, levees along Central Valley rivers were constructed of whatever soil or sediment material that was readily available, most usually sand from the bed of the river. However, sand (individual sand grain size ranges from 0.2 – 2 mm diameter) is not the most effective soil particle size with which to build a levee. Sand cannot be compacted sufficiently to prevent water from moving through it, especially when the water is under head-pressure, as during a flood. Smaller particles, namely clay (0.002 mm diameter) and silt (0.02mm diameter)- are more effective at enduring shear stresses compared to sand when they are compacted. The reason is that silt and clay particles are small enough and are of light enough mass such that the natural electrochemical bonding forces that attract the silt or clay particles to each other are stronger than the force of gravity that is trying to pull them apart. This bonding attraction is termed cohesion, and explains the stickiness of clay soils and mud that we feel.

Today, construction engineers seek out the “ideal” texture for levee construction material, which is composed of silt and clay sized particles that are then physically compacted during construction, resulting in a relatively strong, shear resistant levee. Plant roots will grow into the upper layers of the levee structure and bind together the soil particles, thereby increasing the shear strength (resistance to shear) of the levee. Tree roots form a network that functions as a web or net – similar to how we use rebar when pouring concrete - that holds together when under shear stresses. The presence of roots in the soil profile greatly adds to the cohesiveness of the soil particles that make up the levee. High cohesiveness means that much larger external forces (e.g. the weight of flood water on the side of the levee, the force of high velocity flows, or the force of the wind-generated waves) are required before the cohesiveness is broken, initiating shearing of parts of the levee. This is most commonly seen as slabs of levee sliding down the side of the levee during or immediately after high water due to the added weight of the saturated soil that is caused by flood water entering the levee. In levees composed of sandier material, wetting of the interior of the levee during prolonged high flows with water high on the levee causes the loss of soil cohesiveness. The levee becomes saturated as flood water moves by capillary action horizontally (phreatic water movement) through the levee and the individual soil particles lose cohesiveness. (This situation may be the cause of individual trees that windthrow. That is, the soil that supports the tree loses cohesion and the weight of the tree is more than the weakened soil can support.)

Sycamore roots on Kern River

(Above) California Sycamore roots on a Kern River bank.

Roots like rebar

(Above) Roots in the soil can act like rebar in concrete to add reinforcement and stability.

Shields and Gray (1992) excavated trees and shrubs growing on a sandy levee on the Sacramento River, near the Sacramento International Airport, in order to determine the rooting patterns of valley oak, elderberry, box-elder, and willow trees rooted into the levee. They excavated trenches around the trees and shrubs and carefully mapped and measured the lengths of all roots in the wall of the trench. Results revealed that most roots occupied the upper 20 cm (8 inches) of the levee. Old root channels in the levee were full of sand from the surrounding matrix. Although ground squirrels occupied the levee, there was no evidence of roots nor rodent burrows that extended through the levee. Roots present in the surface layers increased The Factor of Safety for these layers well above that for levee material without roots. (Factor of Safety = increase in force required to shear soil due to presence of roots)

Vegetation composed of several species and many individuals growing on a levee will saturate the upper layers of levee with a network of inter-connected roots. The importance of this is that the individuals will physically support each other from the shear stresses from high winds or high flows. Wind-toppling of trees usually happens to isolated individuals.

Recently, researchers in Asia (Normaniza et al 2008) are studying the rooting characteristics and shear strength additions to slopes of four small tree species.

Vegetation Protecting the Surface of the Levee from Erosion

Waves breaking onto the side of a levee can erode the surface and eat into its’ structure if given sufficient time. Waves can be generated by three causes: 1. Wind during a high flow event when the floodway is full of water can generate one to two feet waves that can cause significant erosion on the face of the levee. If the waves continue for a long enough time severe damage and weakening of the levee can occur. 2. Stream flows over certain bed geomorphologies can generate waves on the surface of the water that can move to a levee and break onto it. 3. The wave-wake of boats can be a major problem on some of the frequently used water ways, as in the Delta.

Woody vegetation on or near the base of a levee can function to break the integrity of the wave energy, causing it to dissipate before reaching the levee. Recently, belts of vegetation are being designed into levee construction with the intention of protecting the levee from wind waves. When the floodway is full, it is a large open expanse of water that provides a long fetch for wind to generate waves many feet tall. Waves of this size will erode the sides of the levee quickly.