Shoreline Characteristics
When planning a shoreline stabilization project, it is important to consider the shoreline characteristics. The erodibility of a shoreline is a function of the amount of erosive forces reaching the shoreline versus the resistance characteristics of the contents of the shoreline. Wind, waves, gravity and currents cause erosion on natural lakes. On reservoirs, erosion processes are similar, but with additional complicating factors such as water level fluctuations. Listed below are characteristics of the shoreline that need to be considered to determine which stabilization technique will be best suited for the project.
Shoreline Characteristics to Consider
- Classification of the shoreline based on the Duke Energy Shoreline Management Plan
- Existing vegetation
- Fluctuating water levels
- Wave energy (from boat traffic and/or wind) and depends on water depth, wind fetch and site location
- Slope configuration above and below the waterline
- Soil type and condition above and below the waterline
- Primary source of erosion
- Amount of sunlight exposure
- Shoreline maintenance requirements
Shoreline Classification
When planning a shoreline stabilization project, a Duke Energy lake representative should be contacted in order to determine the Duke Energy classification of the shoreline based on Duke Energy’s Shoreline Management Plan. The shoreline classification and bank height will help identify which stabilization techniques can be constructed. Shorelines are classified as environmental, natural, impact minimization zones, and other, by the Duke Energy Shoreline Management Plan, depending on their current use, vegetated cover, habitat value, water depth, substrate and location. Depending on the shoreline classification and bank height only certain stabilization techniques will be allowed by the Duke Energy Shoreline Management Plan, some of these classifications may have additional shoreline modification restrictions.
Existing Vegetation
Any existing vegetation should be saved and used as part of the erosion control plan, if possible. If the shoreline possesses existing vegetation, simply adding to the already established vegetation may solve the erosion problem. Obviously, the existing plant species have survived at the site, but the cause of any barren areas on the shoreline (e.g., vegetation and shallow topsoil may have been removed during a single storm event or the overhead canopy shades the barren area) should be determined. In either case, the surrounding area can be altered to re-establish lost vegetation. When establishing vegetation, only native vegetation should be used. Planting native vegetation is recommended because they can be less expensive and offer a higher degree of transplant survival -- that means more shoreline protection.
Planting native vegetation can be less expensive, offer a higher degree of transplant survival, and provide more shoreline protection than non-native plants. Non-native vegetation is not recommended because many of the species can out-compete native vegetation and negatively impact biodiversity. Aquatic invasive species such as hydrilla and water hyacinth can clog waterways, degrade water quality, restrict boat traffic and recreational water use, and disrupt groundwater flow. They can make lakes virtually unusable and require millions of dollars for continued aquatic plant management.
Water Level Fluctuations
Reservoirs usually have fluctuating water levels that cause various degrees of shoreline erosion. The fluctuations vary depending on several factors, including power generation, weather conditions, recreational use, regulatory agreements, municipal water needs and flood control.
In reservoirs with little water level fluctuation, the waves can usually build up a natural terrace, which can deflect waves and create a calmer protective area where plants may be colonized. In reservoirs with high water fluctuation, a natural terrace cannot be built up without human intervention.
While the annual lake level range will vary from lake to lake on our river systems, the annual fluctuation of each lake will be relatively consistent from year to year. Lake levels are measured in feet. A full lake (also known as “full pond”) is considered 100 feet, and is the designed maximum water level of the lake. It also corresponds to an elevation above sea level that varies between lakes. For example, a lake level of 97 feet means the water level at that time is three feet below full and three feet below the designed maximum sea level elevation.
The water fluctuation ranges establishes the normal full, low, and mean water levels for the lake which, in turn, determines the zones of the shoreline (see Figure 1.1). The size of the zones will vary according to the range of the water levels in a given lake and the slope of the shoreline, therefore will be site specific. This is extremely important when determining which vegetation should be used to help insure the highest degree of survival, and where along the shoreline the vegetation should be planted. While these zones may vary with weather conditions and power generation, the annual lake level ranges are an acceptable gauge to determine the zone locations. Occasionally, extreme lake levels occur and may cause problems. Different species of plants will grow and survive in different zones of the shoreline.
top
Figure 1.1: Shoreline Regions Defined by Zones
Zone 1: The Submersed Aquatic Zone. Defined as the annual low water level in the lake and is usually continuously submerged.
Zone 2: The Emergent Zone. Defined to be between the mean water level and low water level and is usually under water for most of the growing season.
Zone 3: The Emergent/Shrub Zone. Defined to be between the mean water level and high water level and is under water for short periods during the growing season.
Zone 4: The Terrace Zone. This zone is rarely under water, but the soil is typically saturated
Wave Energy
A key factor in determining the appropriate stabilization technique to control erosion is the amount and strength of waves that reach the shoreline. Erosion caused by wave energy is related to wave height -- the higher the wave impacting the bank, the more erosion is likely to occur.
Wave height depends on three factors: fetch (the distance the wind blows across a body of water until it reaches the shore), water depth and wind speed. Generally, a greater fetch means a taller wave. In reservoirs, deeper water is typically located near the dam and shallow water is upstream from the dam. The deeper the water the wind blows across, the greater the height of the wave that will affect the shore.
Project sites located where the banks are exposed to the main lake body will typically have a long wind fetch and are exposed to a high amount of wave energy. Sites located in coves may be protected from the wind and have little wave energy due to the wind, but boat traffic must be considered. Locations that are continually exposed to high wave action due to wind and boat traffic may not be the best sites for planting native vegetation. The optimal solution may be a hard structural system, such as a seawall or riprap (see Figures 2.2, 2.3).
This information can also help determine if a temporary breakwater system, such as hay bales (see Figures 2.9, 2.10), will be needed to absorb some of the wave energy. The system will provide quiet water until the vegetation or erosion control systems are established.
The wave energy can be determined in several ways: observation and general knowledge of the shoreline, consideration of the distance the wind blows across the water until it reaches the shore, the water depth and the amount of boat traffic around the site.
Shoreline Slope
Shorelines with steeper slopes tend to experience a greater erosion effect on its banks from waves (see Figure 1.2A). Reducing the slope of a shoreline dissipates wave energy and lessens erosion. Generally, flatter slopes (see Figure 1.2 C) have less erosion and there are more options available to stabilize the shoreline. The slope of the shoreline will need to be determined in order to decide which erosion control system should be used and if any additional construction, such as backfilling, will be needed. Shoreline slopes steeper than 2:1 (see Figure 1.2B) may require slope modifications before shoreline stabilization with bioengineering techniques or vegetation can be attempted. The slope of the shoreline can usually be estimated with general observations. Figure 1.2 shows the general slope configurations for a 1:1, 2:1, and 3:1 slope.
Figure 1.2: Slope Configurations; Horizontal:Vertical
Soil Type and Condition
Shorelines generally consist of organic material, clay, silt, sand, gravel and rock. The extent of erosion depends on the specific make up of the shoreline. The soil type will be vital in determining the bank's ability to resist erosion and, along with the soil condition, will be important in selecting the plants to be used. Some plants may not grow in certain types of soils because the roots may not be able to penetrate to obtain an adequate foothold. The soil conditions, consisting of the pH, nutrient content, moisture content and mineral content, cause plants to survive and grow differently. The soil type and condition can be analyzed free of charge by sending a sample to the N.C. Department of Agriculture & Consumer Services Agronomic Division soil testing services. Local co-operative county extension offices should be able to provide materials and instruction on taking the sample.
Primary Erosion Source
Shoreline erosion is primarily caused by wave action driven by prevailing winds on the reservoir and, to a lesser extent, by upland runoff. Cuts, gullies or small ditches present on the bank or on the upland area, are good indicators of upland runoff. Lining the gullies with rock can help control upland runoff. Other methods to control shoreline erosion from upland runoff include drainage ditch modification or piping, limiting the amount of pedestrian traffic on the shoreline and enhancing the vegetation to increase the buffer. General observations of the site can usually identify the cause and location of erosion.
Sunlight Exposure
Sunlight is an integral ingredient in the growth and survival of plants. The amount of sunlight the shoreline and receding bank are exposed to determine what plants can be used as the erosion control method. Some plants tolerate partial shade while others require full sun.
The quantity of sunlight can also explain why certain vegetation does not grow on the shoreline. In some cases, pruning limbs of larger trees along the shoreline can expose the bank to more sunlight. The amount of sunlight can be estimated by identifying the direction the shoreline is facing.
Maintenance
Maintenance of the shoreline is an important consideration when selecting a stabilization technique. Maintenance includes weeding, pruning, removal and replanting vegetation. These maintenance requirements depend on the species of plants, weather conditions and soil conditions.
Vegetative plantings and soil bioengineering control systems may require more maintenance than hard structural systems, but the maintenance cost of a vegetative system will usually be less expensive than the cost of repair and maintenance on a hard structure system. The effectiveness of a vegetative or soil bioengineering system typically improves over time -- the vegetation becomes more established and begins to grow and thrive as the plants become older. Hardened structures, however, deteriorate over time and may require extensive and expensive maintenance as their structures age.
Before performing any shoreline stabilization work, contact Duke Energy Lake Services at 800-443-5193.
