Permanent (Retention) Stormwater Management Systems

Many stormwater control structures are permanent and always contain water - maintaining a similar appearance to artificial ponds, lakes or canals. Retention basins, also known as wet ponds or retention ponds are the most common systems in this category. These stormwater storage systems are designed to collect surface runoff and improve its water quality.

As open-air closed water storage systems, wet ponds and retention basins effectively capture and store surface runoff and usually have an outlet in case of high water volumes during storm surges. As an erosion control and flood prevention system, runoff from impermeable surfaces like roadways, parking lots and structures is largely directed to retention basins via drainpipes and drainage ditches.

Permanent (Retention) Stormwater Management Solutions

These containment structures treat the runoff and overflow from roads and parking lots, filtering out sediments, nutrients and debris. The retention basins are often the destination of underground pipes and storm drain networks. Retention basins can support large amounts of incoming water and have outlets where small quantities of water can escape once the system has reached desired water levels.

Through this process, stormwater levels are well-controlled to avoid flooding. Another critical feature is that filtered, high-quality water can naturally be released while sediments and contaminants settle to the bottom of the pond.

Retention basins can be built with gentle side slopes or retaining walls along the sides. MagnumStone blocks are excellent for these applications as it allows engineers and developers to maximize water capacity by getting closer to property lines than other retaining wall systems. Saving space can be invaluable for urban areas, where space for parking or other infrastructure is a priority. If retaining walls are used, it is imperative that they can handle permanent water containment, rapid fluctuations in water level, and are durable with low-absorption properties.

Design Considerations for Wet Pond (Retention) Retaining Walls

High Water Mark - The average maximum height that a body of water is expected to reach during a period of flooding.

Low Water Mark - The average low water elevation that stays consistent and is typical throughout the year.

Rip Rap - Rip rap should be used along the toe of the walls to slow water flow, protect the wall facing, and to prevent scour. Additional scour analysis and rip rap sizing calculations should be completed separate of the wall design analysis.

Soils - A full soils analysis report should be prepared using boring logs to determine the suitability of the site soils. A Geotechnical Engineer should be consulted to perform the analysis and report findings.

Drain Gravel - Drain gravel is used in the retained infill zone or reinforced zone when using geogrids. A well draining coarse gravel (often referred to as AASHTO #57 stone) with little to no fines should be used 1 ft (304mm) above the high water mark. This material can be used inside the hollow cores of the retaining wall units and can also be used to backfill behind the wall.

Perforated Drainpipe - A perforated drainpipe, also known as a weeping tile, should be used for proper drainage behind the wall face. These pipes typically have a 4" (101mm) diameter and may be sock wrapped. All drainpipes may outlet (daylight) through the wall at a minimum of every 30 ft (9.0 m) on center. The elevation of the first drainpipe should be above the high water mark by a minimum of 6" (152mm). A second drainpipe should be added above the low water mark. Additional back drain(s) may be added where necessary at the discretion of the Design Engineer.

Filter Separation Fabric - Filter fabrics, also known as geotextile, should be used to separate the fine soils from the drainage layers. They may also be used between the backfill material a. retained material along the excavation cut. Wrapping the leveling pad will assist with scour and erosion of the base leveling pad.

TOW - Top of retaining wall elevation. Typically, the top of the cap; shown in plan views and wall profiles and are essential for construction.

BOW - Bottom of retaining wall elevation. The base of the wall where the ground level meets the retaining wall; shown in plan views and wall profiles and are essential for construction.

Why Use Wet Ponds and Stormwater Retention Systems?

Residential and urban developments have a shared shrinking commodity that is valuable to their expansion, that being available land. Since erosion, flooding and scouring have wreaked havoc on infrastructure with the dramatic increase of impermeable surfaces, wet ponds and other stormwater solutions have become a necessity for development. With erosion control top of mind, stormwater regulations and stormwater compliance has become a safety requirement for new developments.

While stormwater compliance does vary by city, municipality and county, regulations are becoming stricter as to how stormwater management is addressed. Wet ponds and detention basins provide a strategic way to capture surface runoff while also having unlimited landscaping potential. From retaining walls to shrubbery and flowerbeds, stormwater retention systems have the flexibility to have basic features or elegant aesthetics.

Features & Attributes of Permanent Stormwater Storage Systems

Permanent stormwater storage systems have key defining characteristics that indicate they are wet ponds and retention basins. These are typically large, shallow closed pool systems that accumulate and treat contaminated water runoff. They maintain water at all times, allowing for sediments, contaminants and pollutants of all sizes to settle along the bottom.

Wet ponds and retention systems often appear in the form of artificial lakes or water containment structures where retaining walls are embedded below the surface of the water. Vegetation and plantings surrounding the basin is popular with permanent stormwater storage systems and can make an area more aesthetic, much like retaining walls can enhance the containment area's perimeter.

Receiving waters downstream welcome the treated water into rivers, streams and lakes which can be recycled for other uses. Of the stormwater management systems, detention systems are one of the most effective ways to treat stormwater runoff.

Common Applications for Wet Ponds

Wet ponds and retention areas have become closely aligned and incorporated into outdoor recreational facility landscapes by community associations. Often explored to gentrify stormwater storage systems, amenities can be offered at or surrounding the wet ponds and marketed as nature parks or artificial lakes for newly developed communities.

In these cases, they can serve a dual-purpose for stormwater management while also becoming a great spot for walking paths. Since retention ponds perform natural water treatment processes, water activities including stand-up paddle boarding, canoeing and kayaking are generally safe. Wet Ponds can easily be beautified for pedestrians and cyclists, aesthetically enhanced by retaining walls, walking bridges and landscaping.

Wet ponds and retention basins are easy to maintain, are an excellent collection device for rainfall events and often self-regulate with an overflow outlet in case of extreme volumes. Water levels do tend to fluctuate at these locations depending on precipitation and runoff from contributing stormwater sources but are relatively minor. Wildlife habitats regularly develop in retention areas.

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MagnumStone's Advantages

An eco-friendly product, the hollow core design uses 40% less concrete than solid concrete systems
MagnumStone units are backfilled with clear crush gravel for exceptional drainage and permeability
Free-flowing drainage system built into and behind the blocks, to avoid sediment buildup and blockages
Gravel-filled MagnumStone blocks provide weighted strength for gravity walls, or with the help of geogrid for reinforced walls
Natural facing rejuvenates and enhances the look of temporary stormwater storage systems
MagnumStone's terraced and planter retaining walls provide environmentally friendly elements and more stylish aesthetics thanks to the block's hollow core design
Helps contain water to a designated area for long-lasting, durable water storage solutions
MagnumStone can either be used for a geogrid reinforced wall or a gravity wall for these simple or complex water applications
High-quality precast wet-cast concrete is weather-proof and minimizes water absorption
MagnumStone's strength can withstand high pressures and loads of close-proximity infrastructure (parking lots, roads, railways, buildings, etc.)
Perforated drainpipes are strategically placed along the retaining wall to guide and evenly disperse water runoff along the retaining wall
Lightweight blocks are quick and easy to install, excellent for tight excavation spaces, making better use of land and properties

Design Resources for Wet Pond Retaining Walls

The MagnumStone system has a powerful and easy to use design engineering software that is free to all users. The software not only checks for wall overturning, bearing capacity, and sliding, it also provides global stability, seismic loading, in water conditions and many more analysis that are critical to a comprehensive final design.

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