Pond Shoreline Assessment Tool (PSAT)
Abstract
Stormwater ponds in older developments are facing challenges related to shoreline erosion and sediment accumulation. Dredging to maintain pond functionality is a conventional engineering solution, but it is a reactive approach compared to more natural restoration alternatives. Given the prohibitive cost of dredging, a range of comparably low and moderate cost measures could be used to stabilize and naturalize a shoreline depending on the problem severity. To address these issues systematically, we developed the Pond Shoreline Assessment Tool (PSAT). PSAT offers an objective framework for evaluating and cataloging shoreline erosion and prioritizing restoration efforts. Here we present the Seaside Farms development with 13 stormwater ponds and approximately 6 miles of shoreline constructed approximately 20 years ago. Our initial observations, along with input from the community, highlighted a preference for nature-based solutions. This tool enables the HOA Board of Directors to allocate resources effectively and fairly, establish a timeline for addressing erosion, and thereby preserve the community’s environmental and aesthetic values. PSAT can be applied to stormwater ponds in other settings, and the supporting materials assist with landowner education.
Key words: stormwater, erosion, ecosystem restoration
Download and print your own PSAT field sheet here!
In developing the PSAT, we sought a balance between technical rigor and user friendliness, with concepts and language that are ideally understandable to a person without a background in science or engineering. Rapid visual assessment should be sufficient to assign a score for each of the risk factors, supporting multiple site assessments by a single individual.
Explanation of Risk Factors
Slope – Erosion is related to the bank angle, with slopes steeper than 2:1 (2 horizontal : 1 vertical) being more susceptible, contrasted with a preferred slope of 3:1 (3 horizontal : 1 vertical) or flatter. Exposed bank material is also an indicator of active erosion. The most extreme examples exceeded 1 foot of eroded bank height.
Sediment – Very tightly packed, cohesive banks (i.e., primarily clay) are much less susceptible to erosion, whereas loose, sandy bank material contributes to shoreline instability. Mixtures with silt and clay (e.g., loamy soil) fall in between these two extremes. Rapid assessment relies on visual observations rather than soil testing.
Vegetation – The extent and type of bank vegetation influences shoreline susceptibility to erosion. Ideally, there should be at least a 10 ft vegetated buffer that includes emergent wetland plants. In contrast, turf grass lacks the root structure to stabilize the banks. When located on top of a pond or embankment (or dam), large trees have roots that create pore spaces and flow pathways in the soil, contributing to potential erosion and instability.
Proximity – Higher levels of risk and priority correspond to the potential erosion impacts to buildings, pond structures, and other human infrastructure. A setback distance of at least 50’ is preferred. Areas of greatest concern are those that lie directly on top of a pond embankment (i.e., a dam).
Community – The relative degrees of visibility, public accessibility, and social value contribute to project priorities when considering multiple sites. For an HOA project with potentially conflicting property owner preferences, it is helpful to include a more objective metric that captures overall community risks and benefits.
Seaside Farms Basemap & Shoreline Stabilization Measures
Figure 1: An illustrative basemap of the Seaside Farms neighborhood in Mount Pleasant, South Carolina. This maps areas of existing erosion, categorized by the stabilization measure RDE recommends using for that area. These various stabilization measures correspond to the various degrees of erosion.
Budget and schedule allowed a single day in the field to assess 13 ponds and the existing shoreline conditions. Therefore, shorelines with visible or active erosion were assessed and scored using the Pond Shoreline Assessment Tool (PSAT). The total length of shoreline in the entire pond system is roughly 5.8 miles and the total length of eroded shoreline that we recommended be stabilized was 1.75 miles, so roughly 30% of the entire pond system. While assessing the shorelines, we had a good idea of the possible treatments that could be implemented at each reach based on the physical conditions of the site. Scoring on the PSAT was a secondary step in this process, to incorporate concerns and notes from residents in the more subjective criteria (Proximity and Community risk factors).
For the risk factor Proximity, the majority of shorelines were not near residents' homes or major infrastructure. The highest score (3) represented closer proximity to a resident’s fence. Regarding the Community risk factor, most pond shorelines were in residents’ backyards, thus the majority of shorelines scored (2). A score of (2.5) resulted from shorelines from the same pond that were touching private parcels, while at another reach, there was a public area possibly fronting a street or accessible by residents for fishing. Ponds with a high score of (4) were noted by residents as high-value areas that were used regularly. Areas around Ponds 2 and 6 were highly valued as they had walking trails, fishing areas, and in close proximity to the community pool.
PSAT Scores & Recommended Shoreline Treatments
Table 1: This table is organized by ponds’ total condition score with the lowest score at 9 and ranged to the highest score of 15. Possible shoreline treatments were discussed during site visits alongside scoring for PSAT. Possible shoreline treatments are divided into a low, moderate, high cost categories. Shorelines of the same pond were divided based on the expected treatment, e.g., P02-1 (Pond 2-Reach 1), P02-2 (Pond 2- Reach 2).
Recommended Stabilization Measures & Costs
Table 2: This table logs the total length of each recommended stabilization measure and total costs from 2023. These cost numbers can and will change to some degree, but this info will be invaluable to the process of planning and scheduling out projects over the next several years.
Field Assessment Examples
Minor Erosion
Bank slope of 2:1 and/or eroded foreslope 6” high or shorter
Primarily clay sediments
May (but not always) exhibit some existing aquatic, emergent, and/or buffer vegetation
Condition Score
Slope = 1
Sediment = 2
Vegetation = 3
Moderate Erosion
Eroded foreslope 6”-12” high
Some undercutting of banks
Sandy or clay sediments
Generally mowed turf grass to edge of water or top of foreslope
Condition Score
Slope = 3
Sediment = 3
Vegetation = 3
Severe Erosion
Eroded foreslope 18” high or taller
Significant undercutting of banks
Generally sandy sediments
Mowed turf grass to top of foreslope and/or exposed bare soil
Condition Score
Slope = 4
Sediment = 4
Vegetation = 3
Case Study: Seaside Farms
Since 2022, RDE has worked with the Seaside Farms community in Mt. Pleasant, SC, a neighborhood with 13 stormwater ponds and approximately 6 miles of shoreline. Originally constructed around 25 years ago, the stormwater ponds have increasingly been impacted by shoreline erosion, algae blooms and fish kills, and sedimentation.
The HOA Board retained RDE to assess the condition of the ponds and to propose projects and practices to maintain and improve the ponds for the next 25 years. RDE installed water level monitoring equipment, measured water quality, and reviewed the orginal design drawings for the pond system. RDE also prepared a GIS basemap of the neighborhood and developed a Pond Shoreline Assessment Tool (PSAT) to offer an objective comparative analysis of shoreline erosion issues in each pond.
RDE conducted a web-based resident survey and conducted several workshops with the HOA board and residents. Based upon the clear feedback from the neighborhood, RDE developed a framework for pond management and a pond masterplan—including a long-term budget and project implementation schedule for the entire neighborhood. Over the last two years, Seaside Farms has completed the first few phases of the masterplan as they’ve stabilized and planted several thousand feet of shoreline, restructured the governance and management of their pond system, and began working with the Town of Mt. Pleasant and neighboring communities.
Resources
Pond Shoreline Assessment Tool (PSAT) Field Shield for Multiple Shorelines
Have a site with multiple shorelines that you need to catalog conditions?
Native Plants for Pond Shorelines
A resource to visualize a typical stormwater shoreline that has been naturalized with plants native to South Carolina!
Seaside Farms Project
Read more about our project at Seaside Farms here!