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Types of Lake, Pond, and River Erosion

The three primary types of lake, pond, and river erosion are Sheet Flow Erosion, Rill Erosion, and Splash Erosion. But erosion can also occur on horizontal areas such as landscaping, ditches, swales, berms, walls, and structures. The difference is whether the eroded land is part of a diagonal slope or a horizontal plane. Erosion in the landscape generally consists of berms and ditches losing their depth and consequently their efficacy. This can be remedied by cleaning out the eroded material, re-grading the area while ensuring a proper pitch for correct water flow and drainage, and securing the top of the land with sod or an aggregate.

Above Picture: Part of a 300 foot swale that was re-graded and filed with #57 stone to prevent further erosion.

Sheet Flow Erosion: Sheet erosion is a major process in the denudation of land surfaces. It involves the falling of raindrops and their merging to form a near-continuous sheet which moves down hillslopes, gathering momentum and representing an erosive force of high potential.1 In Florida, this type of erosion begins in the back yards of properties, builds up, and cascades down the shoreline bringing native soil along with it. The longer the back yard, the more water volume. This type of erosion is half of the reason escarpments develop on shorelines- the other half being the varying water levels of ponds and lakes due to rainy season versus dry season or the current that occurs on streams, rivers, and canals.

Rill Erosion: Rill erosion is the type of erosion that occurs as water flows over a hillslope and cuts shallow, curvy channels into the top soil. If the rills are not mended, with more water flow they can form gullies. Gully erosion is the erosion that causes deeper cuts into the soil and is more detrimental to the environment.2 Rill erosion in Florida is especially detrimental due to the fact that the topsoil that is used in construction has a high silica (sand) content. Additionally, our high annual rainfall collects between structures (such as homes) and becomes focused in one area, usually on a lot-line, as it reaches the shoreline. This concentrated area receives a high volume of water, creating a cavity in the shoreline. If left unchecked, this cavity becomes larger and larger with time and can eventually cause settling in structures, cracks in pool decks, or injuries to lawn care personnel!

Above Picture: Rill erosion eventually leads to large cavities, damage to irrigation piping, and failing sod.

Splash Erosion: Splash erosion or rain drop impact represents the first stage in the erosion process. Splash erosion results from the bombardment of the soil surface by rain drops. Rain drops behave as little bombs when falling on exposed or bare soil, displacing soil particles and destroying soil structure. Studies in the US have shown that splashed particles may rise as high as 1 foot above the ground and move up to 5 feet horizontally. Splash erosion results in the formation of surface crusts which reduce infiltration resulting in the start of runoff.3

There is also erosion that occurs on bodies of water here in south Florida that is caused by the rising and falling of water levels each year due to our very wet rainy season and fairly dry winters. This loosens the soil around the lake or pond bank and makes it susceptible to the three types of erosion above. If you would like to learn more about how erosion is affecting your property or community, please contact us.

References

1 Encyclopedia of Earth Science

2 Study.com

3 https://nre.tas.gov.au/

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Stormwater Management System

June 7, 2024

Each planned community in Florida is required to have its own stormwater management system. It is a structured approach to manage and control the runoff water from rainstorms. These systems are designed to mitigate the negative impacts of stormwater, which can include flooding, erosion, and water pollution. Here are the key components and purposes of such a system: Retention and Detention Ponds: These ponds are designed to collect and store stormwater. Retention ponds hold water indefinitely, while detention ponds temporarily store water and slowly release it to prevent flooding. Swales and Ditches: These are shallow, vegetated channels that help move rainwater away from developed areas and towards larger bodies of water or stormwater management structures. Storm Drains and Inlets: These structures collect runoff from streets and other impervious surfaces, directing it into the stormwater management system. Permeable Pavements: These surfaces allow water to infiltrate through them, reducing runoff and promoting groundwater recharge. Landscaped areas: are designed to absorb and filter stormwater, using plants and soil to remove pollutants. Underground Systems: These include pipes and vaults that collect and convey stormwater to other parts of the system. It also includes outfall structures, weirs, and wells.

Maintenance Plans & Reporting

June 7, 2024

In South Florida, a long-term maintenance plans for lakes in private communities are essential to ensure that these stormwater management systems function effectively over time. These maintenance plans are typically required as part of the permitting process and are designed to ensure compliance with environmental regulations and to protect public health and safety. Here are the key components of these maintenance plans: Regular inspections are required, often on a quarterly or annual basis, depending on the specific requirements of the permit and the characteristics of the stormwater system. Inspections should cover the entire lake system, including inlets, outlets, embankments, control structures, and the lake itself. Inspection have reporting requirements set forth by each county in South Florida. Reports should be documented and kept on file, with copies submitted to the relevant regulatory authorities if required. Floating debris and trash should be removed regularly to prevent blockages and maintain aesthetic and environmental quality. Additionally, beneficial littoral (seasonally-flooded) plants should be managed to maintain their health and effectiveness in filtering pollutants. Invasive species must be controlled or removed. One of the most important factors (and most overlooked) is that the littoral plant around the shoreline must be maintained to prevent erosion and provide habitat for wildlife. This includes spraying herbicides, hand-pulling, and planting new littoral plants as needed to stay in compliance.

Lakes with Specific Volumes

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Engineers are the ones that determine the volume that lakes (or retention/detention ponds) need to be in planned communities in Florida through a comprehensive analysis of several factors. This process involves hydrologic and hydraulic calculations, regulatory requirements, and specific site conditions. The first factor is Hydrologic Analysis in which Engineers use historical rainfall data to determine the amount and intensity of rainfall events that the system needs to handle. This data is often provided by government agencies such as the National Oceanic and Atmospheric Administration (NOAA). Secondly, watershed characteristics help determine the size, shape, land use, and soil type of the watershed (the area that drains into the lake) are analyzed. This helps determine how much runoff will be generated during a storm (using a period of years as an average). Rain runoff coefficients are different on each land surface (e.g., asphalt, grass, rooftops) and generate different amounts of runoff. Engineers assign runoff coefficients to various land uses within the community to estimate the total runoff. A hydraulic analysis looks at peak flow calculations. They calculate the peak flow rate which is the maximum rate at which stormwater will enter the lake. The volume of the lake must be sufficient to store the runoff from design storms (e.g., a 25-year or 100-year storm event). This involves calculating the required storage volume to ensure that the lake can hold the water without causing flooding. Regulatory Requirements Engineers must comply with local, state, and federal stormwater management regulations. In Florida, the Department of Environmental Protection (DEP) and various Water Management Districts (e.g., South Florida Water Management District) provide guidelines and requirements for stormwater systems. Regulations typically specify design storm criteria, such as the volume of runoff to be managed based on certain storm events. Engineers design the lakes to meet or exceed these criteria. Soil permeability and the presence of high groundwater tables influence the design of stormwater lakes. Engineers conduct soil tests to determine infiltration rates and groundwater levels. Also, the presence of existing lakes, rivers, or wetlands can impact the design and volume requirements of new stormwater lakes.