Soil erosion and sedimentation is the gradual physical stripping away of the Earth’s surface over time. It is a natural process in the life cycle of soil, and a result of dynamic activity from erosive agents like wind, water, plants, animals, and humans. In Napa County, our watersheds produce high amounts of sediment due to unique geographic features. A healthy amount of sediment is only natural and actually good for ecosystems, however, excessive soil erosion can lead to negative impacts on ecosystems, contaminate water sources, and long-term health considerations.
According to the Napa County Watershed and Conservation Council, soil erosion poses a unique threat to the watersheds of Napa County by depositing sediment and contaminating water sources, removing the rich, fertile topsoil in our region, and threatening the Napa River system as well as vital native species.
Erosion can also cause the growth of more hardy but undesirable vegetation – such as weeds and shrub species – in the wake of the loss of fertile topsoil. Additionally, the sediment carried away by erosion can be transported and accumulate in hazardous areas, causing major infrastructure hazards by burying plants and roads and contaminating important bodies of water.
In order to prevent excessive erosion, we must first seek to understand the different types of soil erosion. There are five main types of soil erosion: Splash, Sheet, Rill, Gully Erosion, and Streambank Erosion.
Why Should We Be Concerned about Erosion?
Erosion removes the topsoil of the earth, which contains vital, fertile materials with high levels of biological organic materials that are most conducive to plant growth. It reduces soil fertility, and loss of topsoil from erosion will make it very difficult for plant communities to recover from damage.
Water erosion is the process of soil stripping brought on by the natural erosive agent water. The process can be natural, or accelerated by human activity. Water droplets from rainfall can hit soil strongly enough where they break soil aggregates, and thus prevent water from infiltrating and moisturizing the soil. Instead, water accumulates atop the soil and carries away valuable topsoil with runoff water. Soils only break up if they are not well structured, and plant or litter covers can prevent breakage from rain drops. There are four main types of erosion within water erosion – splash and sheet erosion, rill, gully and tunnel, or piping erosion.
Splash erosion, or rain drop impact, is the erosion of valuable topsoil through the process of heavy rainfall. This typically represents the first stage in soil erosion. It occurs when heavy raindrops make impact with bare soil. Particles become displaced by the sheer force of falling raindrops, and these individual particles ‘splash’ up onto the soil surface. The bombardment of the soil surface from raindrops leads to soil detachment and soil disintegration, the splashed up particles blocking spaces between soil aggregates, forming a crust that prohibits infiltration and increases runoff.
Sheet erosion occurs as a shallow ‘sheet’ of water flowing over the ground surface, which in turn results in the removal of a uniform layer of soil from the surface. The thin layer of topsoil can be removed through the process of sheet erosion as a result of heavy rainfall that cannot be absorbed by the soil, causing water to pool and collect in a shallow ‘sheet’ just above the surface. Sheet erosion is one of the least apparent forms of erosion and can occur undetected – in fact it is nearly imperceptible to the untrained eye. In this form of erosion, instead of soil particles being detached, they are transported, eroded soil deposited elsewhere.
The best preventative measure against sheet erosion is a robust vegetative cover. Maintaining vegetative cover and planting organic matter can act as a glue for rainfall, stabilizing pore spaces which can absorb water and reduce the proliferation of ponded water that carries away fertile topsoil. Cover crops can be made more robust through the inclusion of supportive materials such as straw mulch, erosion control blankets, and or/ irrigation to germinate seeds prior to the onset of heavy winter rains.
Rill erosion occurs when surface runoff water accumulates in small channels and appears mainly on hills. This form of erosion is more easy to spot than sheet erosion, creating well-defined cavities or channels called “rills.” They become apparent when the rate of soil loss exceeds 20 tons per acre per year. Rill erosion lines are less than 30 cm deep and are most common in bare agricultural lands.
Channels that reach more than four inches deep can be classified as gullies. These deeper rill channels form in exposed natural drainage-ways, removing soil along drainage lines through surface water runoff. When left unchecked, gullies will continue to move by headward erosion or by slumping off the side walls. The excessive runoff brought on by gully erosion can be prevented through vegetation, which moderates the amount of soil that cannot absorb runoff water through the strong roots of organic elements which soak up water and hold the soil together.
Gully erosion leads to the loss of agricultural land, generates sediment and increases flooding that can impact critical infrastructure, like fences, bridges, and culverts. The soil instability of gully banks can lead to the collapse of bank slopes, especially in the rainy season, when the soil is most prone to producing runoff. The sediment deposited from gullies can also cloud nutrients in water supplies, increasing the turbidity of rivers, dams, and streams.
Large gullies are very difficult to repair, because once the erosion has reached bedrock, it is nearly impossible to revegetate the area and plant trees. It can also be very costly to maintain a catchment-wide approach to repairing gully erosion. That’s why prevention is a more effective method to curb gullies from forming in the first place. A stalwart ground cover is the best method to prevent gully erosion – according to the New South Wales Environment Protection Authority, a minimum of 70% ground cover, a 30% stubble cover on cultivated areas, and establishing gully heads and stabilizing points where the channel slope changes sharply is the best way to prevent erosion.
Tunnel erosion is a form of erosion that develops from tunnels beneath the surface, due to the removal of subsurface soil layers by water. On a rolling or steep landscape, water penetrates through a vulnerable soil crack or hole, such as a rabbit burrow, or where an old root has decayed, and soil disperses and is carried away within the flow to form a wide underground channel or small tunnel. As the surface soil continues to be eroded, the tunnel enlarges during wet periods, growing larger and larger, and eventually the soil roof collapses and forms a gully. Sheet erosion is typically the precursor to tunnel erosion.
Tunnel erosion is extremely hazardous and can cause the loss of desirable land and siltation to be carried away downstream in the slurry. It can be highly costly to maintain tunnel erosion. The best measure to prevent tunnel erosion is through re-establishment of vegetation, which binds and strengthens soils, creates a preventative cover, and maximizes uniform water use. Resurfacing vulnerable lands with topsoil and organic agents like grass or trees will ensure that water uptake is evenly distributed and runoff does not accumulate in tunnel holes.
Streambank erosion is the most common problem plaguing many streams in today’s waterway systems, occurring when water forces overtake bank materials and vegetation. Changes in streamflow brought on by rainfall changes or large scale single-storm events can lead to streambank erosion as the force of water flow removes soil particles from the banks, making the flow of the water too strong for the banks of the creek or river to contain. Streambank erosion leads to increased proliferation of debris in waterway systems as coursing water carries soil particles into primary sources of water. In addition to natural events such as increased rainfall or storm events, human activity can exacerbate streambank erosion. Man-made structures such as dams or levees can cause acute erosion due to increased water flow. Additionally, human-made structures can alter soil composition located on streambanks and stifle plant life growth on river banks, making banks weaker and more susceptible to erosion.
Plant organisms can be replanted in bank soils, taking root in the dirt and strengthening it, making it more resistant to streambank erosion. Robust vegetation is the best method for counteracting streambank erosion, but if plant life proves to be difficult, more targeted manual methods can also be effective and necessary. Placing rocks and logs strategically to redirect water flow can lessen the velocity of streamflow and safeguard against erosion. Other devices, such as rock riprap, gabions, and other soil stabilization methods can also be effective.