Can we change stormwater pond design? An alternative approach (Nautilus Pond)

I have worked with stormwater ponds in the past, particularly with the integration of wetland features to stormwater pond design.  There are many challenges to managing stormwater in urban environments.  Some local Calgary folks, Cory Albers and Bernie Amell have come up with what might be a game changer.  The premise is managing the inflow and pond circulation to (1) better manage short-circuiting, (2) enhance density based separation, (3) displays similar functionality under low and high flows, and (4) provides for the incorporation of wetland features.  The materials pasted below can be found on the Nautilus Pond Inc. website.

THE NAUTILUS POND

A Nautilus Pond (patent and trademark pending) is a high performance surface water treatment environment in both form and function.  The core “pond” portion of the environment is specifically designed to allow water that is resident in the pond prior to a stormwater inflow event to be displaced toward a central drain rather than mixing with the inflowing stormwater.  Resident water is then preferentially flushed in favour of retaining newly inflowing stormwater to a greater degree than is typically possible using designs consistent with current stormwater best management practices.  The result is a longer average stormwater retention time and better opportunities for achieving sediment and nutrient removal objectives.

The conceptual sketches below illustrate how a representative treatment cell in a Nautilus Pond layout directs water to enter near and generally tangential to the pond perimeter.  Water generally follows an inward spiralling path and leaves the pond by a central drain which may or may not include a central island.  A semicircular island berm is shown that can beneficially provide some inflow confinement during the energetic rising limb of a significant stormwater inflow event when circulation patterns are developing in the initially quiescent resident water.  Once the peak of the hydrograph has passed, the desired circulation patterns should be established and the berm may be flooded.

General hydraulic control over the outflow is achieved within the control structure located outside the pond perimeter for easy access by  maintenance staff.  A single pipe can be used to draw water from the central island drain and deliver it to the control structure.  Water may be drawn from one or multiple elevations in the water column.

What is a Nautilus Pond?

In an effort to begin assessing how effective a Nautilus Pond design could be relative to traditional pond designs, Nautilus Ponds Inc. (NPI) commissioned a conceptual computer modeling study.   The study results have been reported in a paper the abstract for which is presented in the box to the right (click on the title to download a copy).

Two images selected from the paper are shown below that illustrate the flow patterns and relative concentration of stormwater vs. resident water in a Nautilus and in a traditional pond design as they respond to the same inflow event.  Each pond is almost identical in storage volume though they differ significantly in geometry when viewed in plan.  Blue coloured water represents resident water present in the pond prior to the storm inflow event whereas yellow water represents undiluted stormwater entering the pond at a constant concentration of 10 ppm.  Intermediate colours represent a mixture of the two source waters with, for example, 6 ppm representing a mixture of 60% new and 40% resident water.  Note that the resident water in a Nautilus Pond tends to be displaced toward the central drain so that it may be preferentially discharged.  This results in a better approximation to a “plug flow” water treatment system and a longer overall stormwater retention time.

Changing the Stormwater Pond Design Game 
(click on the title to download a PDF copy)

By Cory Albers and Bernie Amell

Submitted for publication to the NOVATECH 2010 Conference in Lyon, France, 10 pages.

ABSTRACT Current stormpond designs focus on volumetric attenuation of peak stormwater inflow rates to protect downstream pipes from surcharging. Though stormponds can perform this function effectively, water movement and mixing within stormponds is poorly understood as are the dominant mechanisms determining water quality. A conceptual study is presented comparing discharge characteristics of traditional stormpond designs against a new stormpond design called the Nautilus Pond. Results from computational fluid dynamics simulations show that a traditional stormpond design is prone to short- circuiting during a significant stormwater inflow event thus resulting in stormwater treatment inefficiencies and the potential for early environmental releases of contaminants. A Nautilus Pond is comparatively shown to mitigate against short-circuiting during the same inflow event by achieving a better approximation to plug flow. The effectiveness of a Nautilus Pond comes from the use of a perimeter inflow spiraling toward and flowing out through a central outlet. Key design features are discussed that enable the desired spiral flow pattern to develop passively and without human intervention even though stormwater inflows are highly transient events. The modular design of a Nautilus Pond is discussed as it relates to implementation in constrained or discontinuous sites where traditional stormpond designs are particularly susceptible to short-circuiting and dead zone formation.

How effectively can a Nautilus Pond perform?

Key Nautilus Pond Benefits

RESISTANT TO  Short-circuiting and the formation of dead zones.

DENSITY-BASED SEPARATION  A Nautilus Pond can be more effective than conventional designs at separating both sediments and floatables from stormwater inflows.

STORAGE EFFECTS  Local storage in a Nautilus Pond generally allows for the outflow infrastructure to be smaller than the inflow infrastructure.

OPERATION UNDER HIGH AND LOW FLOWS  Unlike large and expensive manhole vault oil-grit separators, which typically operate under low flow conditions with a high flow bypass, a Nautilus Pond can be expected to operate under both low and high flow conditions.

MODULAR DESIGN  Individual Nautilus Pond treatment cells can be placed in series and/or parallel to take advantage of challenging sites that are highly constrained or discontinuous in terms of the footprint area or in terms of changes in grade across or between sites.

WETLAND HABITATS Relative to conventional designs, a larger fraction of the stormwater pond area can be developed into wetland habitats when implementing a Nautilus Pond design.

STORMWATER HARVESTING By preferentially discharging resident water before newly inflowing stormwater, a Nautilus Pond may be used to support the beneficial reuse of stormwater to offset against withdrawals from fresh water sources.

Nautilus Pond

Traditional Pond