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Is Your Stormwater System a Washout?

A guide to ensuring your pollutant removal system doesn't unintentionally release contaminants

By Richard E. Ayres, JD September 1, 2007

Throughout the world, thousands of stormwater pollutant removal systems are being installed in an effort to prevent watercourses from being polluted. As supply has risen to meet demand, a variety of proprietary, chamber-based systems have emerged, including hydrodynamic separators, which are designed to settle out and store sediments and associated pollutants, preventing them from being discharged to the natural environment.

However, recent research suggests that some systems may not be as effective as claimed. Some systems are subject to “washout” or “scour,” whereby captured and stored pollutants are flushed from a system during extreme wet weather.

Conventional Sizing Strategies
A common measure of stormwater separator performance is “pollutant removal efficiency” – the ability to remove pollutants from a contaminated flow. Net performance will depend on actual flow conditions faced, and the nature and characteristics of the pollutant load.

For a typical hydrodynamic treatment device, instantaneous efficiencies will tend to be highest at low flows and lowest at high flows. This means that over the duration of a storm, the net efficiency will fall somewhere in between.

This conceptual approach is useful, allowing designs to be assessed in line with defined efficiency targets. However, it assumes that pollutants, once captured, will remain captured, even during the most extreme storm conditions.

Retention Efficiency and Washout
In practice, poorly configured stormwater separators can be prone to “washout,” which occurs when captured and stored pollutants get resuspended and flushed out at high flows. This can be included in the performance calculation through “retention efficiency” – the ability to retain previously captured and stored pollutants.

Most real stormwater treatment systems are protected with high-flow bypasses. This prevents hydraulic overloading and also helps to prevent washout. However, it must be recognized that any bypassed flow will not be treated. A preferred approach is to select stormwater separators that have good retention performance (i.e., that do not washout) and then to maximize the treatment threshold.

Chamber Assessment
The “retention” efficiency of a stormwater chamber is, in practice, difficult to measure and express quantitatively, which may explain why it has not often been considered in practical assessments. However, using the outputs of visual experimentation and flow simulation, combined with a little intuition, it is actually relatively straightforward to predict which devices might washout and which might not.

Extensive research has been carried out at the Centre for Environmental Technology at Liverpool John Moores University in the UK (LCET), examining the interrelation between chamber design and pollutants washout. This work finds that the least-efficient chambers have exposed storage zones. The better performers have storage zones that are sheltered from the main flows. LCET has looked at a range of “generic” designs of chamber, configured to mimic the primary features of proprietary systems. These have included symmetrical flow chambers and vortex flow chambers, both with and without internal components.

Symmetrical Flow Chamber
Symmetrical flow chambers typically have perpendicular inlet and outlet points. They may contain internal baffles, though many do not. Catch basins and some proprietary systems fall into this category.

LCET looked at the retention of unexpanded polystyrene beads (equivalent in settling velocity to 75-micron sediment particles) during a 5-minute period of moderate-high flow to the chamber. Video clips were obtained, and washout quantities measured.

This system was found to exhibit high levels of washout, in fact approaching 100 percent -- i.e., none of the material stored in the base of the chamber remained at the end of the test.

Vortex Flow Chamber
Three different configurations of vortex chamber were looked at by LCET, including one with no internal components, one with a ring positioned part way up from the base, and a further one with a ring and base shield/outlet assembly.

The vortex flow chambers generally exhibited less washout than the symmetrical chamber, though this was found to depend on the actual configuration. With no internals, around half of the stored sediment was lost. Adding the ring component, while sheltering parts of the base, the vortex tended to penetrate and accelerate into this region, and again, around half of the stored sediment was lost. The chamber with the ring and base shield/outlet assembly, however, retained 99.7 percent of the stored material, along with dye added to the water at the start of the test.

Combined results from the work are presented on page 20, confirming the inter-relation between chamber configuration and performance.

Computer Simulations
The work carried out at LCET has been backed up by fluid flow simulations, carried out in collaboration with Fluent, a world leading provider of computational fluid dynamics software. The predictions confirm what is observed in practice. In the symmetrical flow chamber, the inlet flow creates significant mixing, pulling particles up from the base, directly into the path of the inlet flow. This corresponds to substantial washout in practice.

The vortex chamber was found to be more effective, having a more structured flow, and fewer particles passing to the outlet, though particles are nevertheless accelerated and stirred up into the main treatment area.

Adding a ring above the sediment storage region has some impact on the flow structure, but the vortex is still able to penetrate into the base, pulling material up.

The vortex chamber with ring and base shield/outlet assembly shows very little susceptibility to washout. While particles stored in the sump are lifted and rotated, their velocities are low, and they are contained in this area. This cor-

responds with the experimental outputs, demonstrating this type of configuration, with a sheltered sump, as being best at retaining stored material.

The Liverpool study revealed the following:

• The phenomenon of poor retention leading to the washout of previously captured solids from stormwater treatment chambers must be taken into account in system selection for practical application.

• In the worst cases, washout begins almost immediately at flows likely to be encountered even in the early stages of a storm event.

• The rate of stored pollutants washout is extremely sensitive to chamber design. In the current studies, reentrainment rates ranged from zero to 100 percent.

• The best-performing chambers were found to be those that had induced rotary flows and internal arrangements and flow modifying components that resulted in the sediment storage region being hydraulically separated from the main treatment region.

Does Your Stormwater System Have Washout Woes?
Washout is an important factor to consider in the selection of stormwater separators, yet it is clearly not taken into account in the design or verification of many proprietary systems. Those responsible for selecting the best solutions to meet particular site needs are faced with an array of technological options and an array of performance claims. However, the process of identifying the “performers” from the “failers” is not as complicated is it might appear. The key is in asking the right questions, and also applying a little intuition.

• Are stored sediments located in a region that is suitably sheltered from the main treatment region? Simple test – if you can draw a vertical line / vertical lines from the treatment region to the storage region without hitting a physical boundary then the answer is probably “NO” – the system may be liable to washout.

• Does the system have a “high-flows” bypass? If the answer is “YES,” washout may be avoided. However, beware that any bypassed flow will not be treated. Also, beware that systems that bypass directly through the main treatment region may still be prone to washout – again, the vertical line test can be applied.

Washout is a particularly important phenomenon, as stormwater treatment devices are typically maintained on an infrequent basis. In addition to removing pollutants, these systems must also retain and store them for later removal during maintenance visits.

A device that is effective at removing pollutants under low-flow conditions but is prone to resuspending and washing out previously captured materials when flows increase does not serve the functions it claims. It provides very little overall environmental benefit. It also provides a false sense of security to those relying on it to keep their water clean.

About the author

Richard E. Ayres, JD
Richard E. Ayres, JD is also a partner in the law firm.