There are loads of stormwater management options available for use in a range of construction projects, each offering its own mix of functionality and aesthetic beauty. Stormwater management is an important part of the job, and it goes without saying that there are two necessary steps to take before implementing a solution: First, do your homework and choose the option that best fits your project (and location)‘s needs. Second, make sure it doesn’t fail.
No one plans on failure—yet sometimes it happens. The vast majority of stormwater management failures come down to one of three reasons:
- Inadequate maintenance
- Functional failure
- Structural failure
Let’s see how these failures occur in four of the most popular stormwater management options, and how to prevent them.
Swale/Bioswale Failures
A swale or bioswale is simply a stretch of vegetation that borders a paved area. You’ll often see swales separating dual-direction roads; or in-between a sidewalk and road. As an unpaved area, swales and bioswales provide permeable space in highly paved regions. Properly functioning swales feature highly permeable soil and vegetation, allowing stormwater to collect, filter and drain into the earth.
Why they fail: Improper maintenance
Swales and bioswales with dead or dying vegetation, or non-maintained soils, won’t be as permeable, causing poor water conveyance and failed treatment.
To prevent a maintenance failure in swales and bioswales, you should design and follow a weekly or monthly maintenance plan for swales and bioswales to support soil and vegetation quality.
Why they fail: Functional failure
Improper planning in the design of a swale or bioswale can result in an insufficient stormwater management capacity, which may result in flooding or other failures.
To prevent a functional failure in swales and bioswales, be sure to design and execute your planned swales/bioswales carefully. Research stormwater estimates for the area in conjunction with all stormwater management features, and design each swale or bioswale to retain more stormwater than necessary.
Why they fail: Structural failure
As time passes, swales and bioswales will slowly degrade naturally. As water comes into the swale/bioswale and drains out, soil or vegetation can be washed away. Extreme rain can reduce ground permeability and cause flooding, which can further degrade swale/bioswale effectiveness.
To prevent a structural failure in swales and bioswales, you should design and follow a quarterly or yearly maintenance plan to replace lost soil and vegetation with fresh, highly permeable soils and naturally filtering vegetation.
Pond System Failures
Pond systems are commonly used in most geographic climates, and work by naturally filtering polluted stormwater as it enters the system. As large particles settle to the bottom of the pond, biological processes are activated to combat and neutralize pollutants.
Why they fail: Maintenance failure
Poor ongoing maintenance is the primary cause of pond failure. Retention/detention ponds that are poorly maintained can result in unwanted algae blooms, foul odors and a build-up of trash or debris resulting in the clogging of inlet or outflow pipes as well as clogged permeable base material.
To prevent a maintenance failure in pond systems, be sure to design and follow a weekly or monthly maintenance plan to check algae levels and remove debrisas necessary.
Why they fail: Functional failure
Ponds that are overloaded with pollutants due to insufficient routing of incoming stormwaters or insufficient sizing of the pond will be unable to keep up with filtration needs as designed, and may require aeration to aid natural biological processes.
To prevent a functional failure in pond systems, always design pond systems to be able to meet expected stormwater needs, and follow a monthly maintenance plan to review water quality and overall pollution levels. Intervene if/when monthly pollution levels begin to rise month over month.
Why they fail: Structural failure
Heavy rains or other stormwater influxes naturally erode at concentrated inflows to ponds, and unexpected flooding can also erode surrounding areas.
To prevent a structural failure in pond systems, make it a priority to locate probable areas for stormwater entry points into the pond and pay specific attention to that point on a monthly basis. Alternatively, you might consider installing a small concrete or other more permanent solution in the point of entry area to prevent future erosion.
Please note: Stormwater ponds should always be marked as non-recreational areas and fenced off to restrict human entry. Ponds that are poorly maintained may pose serious health and safety risks, including bacterial and drowning hazards.
Metal and Plastic Pipe System Failures
Metal and plastic pipe systems are straightforward options for transporting stormwater from one place to another. They may run from street or sidewalk drains to retention areas/ponds, or between one retention area/pond and another.
Why they fail: Maintenance failure
Metal and plastic pipe systems that do not feature an access point to clean and maintain the device may become clogged with debris or other buildup.
To prevent a maintenance failure in metal or plastic pipes, it’s best to use drainage pipes that feature maintenance access points for maintenance as necessary. When this isn’t an option, aim to use wide pipes for short distances, routed in straight configurations at a decline.
Why they fail: Functional failure
As with all other stormwater management systems, corrugated metal and plastic pipe systems depend on proper design at the beginning of a project to accurately calculate the required storage volume and ensure optimal function of the system before, during and after any storm event.
To prevent a functional failure in metal or plastic pipes, ensure during the design phase that the right types of pipes are being chosen specifically for each job in regards to material, length and diameter. Also be sure pipe route, configuration and decline are properly designed to allow for natural water movement.
Why they fail: Structural failure
Metal and plastic pipes that aren’t laid on a solid foundation, or do not have the minimum cover above, may collapse. Pipes that are laid with insufficient connecting bands and/or envelope closures are also apt to fail, causing stormwater leakage or backfill infiltration into the pipe. In addition, plastic pipes can fail from exposure to extreme temperatures and thermal expansion.
To prevent a structural failure in metal and plastic pipes, first carefully choose which material would best be applied on your job site. Use a compacted fill to support a traffic load that exceeds the minimum required. Ensure that all points of connection are sealed within a tight envelope and are incapable of disconnecting.
Concrete System Failures
Concrete systems are known for being strong, highly dependable, long-lasting and eco-friendly options.
Why they fail: Maintenance failure
Like other stormwater management systems, concrete solutions that don’t have debris cleaned out on a consistent basis will not be able to collect, treat and/or transport accumulated stormwater as designed.
To prevent a maintenance failure in concrete systems, use designs with large vault systems, wide diameter pipes and manhole access to allow for easy ongoing maintenance.
Why they fail: Functional failure
Improper hydraulic design in concrete stormwater solutions can lead to inadequate water flow, insufficient treatment or the inability to handle runoff volume. Improper site preparation, meanwhile, leads to differential settlement of components.
To prevent functional failure in concrete systems, it’s necessary to create a site-specific hydraulic design, taking into account geographic features, current/natural stormwater drainage and rerouted design.
Why they fail: Structural failure
Although factory-produced precast is typically superior to poured-in-place concrete, improper structural design, such as inadequate reinforcing within concrete, and designs that do not consider site-specific conditions can lead to reduced service life of any concrete stormwater system. Poured-in-place concrete, which takes 28 days to cure, can also be degraded by moisture in the air and other weather patterns during that time frame.
To prevent structural failure in concrete systems, first weigh the benefits of factory-produced precast and poured-in-place concrete. Be sure the structure design meets site-specific needs and includes adequate reinforcing within concrete.
With the right planning and execution, long-term stormwater management is possible. The key is finding the right solution for your construction job. Our stormwater management guide is the perfect place to start planning your strategy.
