Tame the Torrent: Hydro-Brake? Revolutionize Flood Management

Consider this, the rise of large cities coincided with advancements in water management. The ability to reliably control and distribute water allowed populations to concentrate, fostering trade, innovation, and cultural progress. It's a stark reminder of the fundamental role water plays in the formation and sustenance of human civilization.

Controlling flooding is an engineering task as old as civilization. Water is truly key to life. Large cities started to be formed about 7500 BCE and water, stormwater, and wastewater management was critical to allowing that to happen.

Just like our ancestors, we also have to adapt to changing water demands and climatic conditions. However the option to simply move, or vanish, is probably not on the top of the list of immediate actions!

Managing stormwater is not a new engineering science and a simple look at recent rainfall data, even a basic analysis of rainfall records, reveals a clear shift in rainfall patterns in recent decades that we need to understand in order to make better decisions about our current approaches to stormwater management.

Historic Hourly Rainfall Data

Figure 1a below, is derived from hourly rainfall data from NOAA for Minneapolis St. Paul Airport between 1948 and 2022 (74 years). Data reveals an upward trend in total annual rainfall depth over several decades. This trend underscores the critical need for resilient and adaptable stormwater management solutions. Relying solely on historical data for future planning risks leaving vital infrastructure, business and housing vulnerable.

Figure 1a shows a 15% increase over the past 74 years on trendline depth, however this does not reveal the significant impact of the change in rainfall patterns.

Figure 1b presents the same data as Figure 1a, but instead charts the greatest depth (intensity) recorded for a single hour within each year. This shows a clearer picture of how rainfall patterns are changing. The peak hour’s intensity has climbed from just under 1.1 inches/hour to around 1.5 inches/hour. While this may seem small it represents a significant 36% increase in the largest storm’s intensity. So both depth and intensity have increased!

Delving deeper into the data, we can break out the data into three categories:

• Light rainfall (≤ 0.1 in/hr)
• Moderate rainfall (> 0.1 ≤ 0.3 in/hr)
• Heavy rainfall (> 0.3 in/hr).

This breakout reveals further insights.

As Figure 2a illustrates the number of light rainfall events is decreasing, we see from Figure 1a that the total annual rainfall is on the rise, suggesting a shift in trend towards larger rainfall events. Figure 2b shows that the trend for the total annual volume in light rainfall is constant at about 12 inches per year. Given that the number of events has decreased this implies that the volume per event in this range is increasing.

Figure 3a paints a concerning picture, the number of moderate hourly rainfall events has surged from around 40 to 55 per year on trendline, representing a significant 37% increase. This trend is further amplified when we consider the total depth of rainfall within this category, which Figure 3b reveals. From a trendline of 6.3 inches per year, the moderate rainfall depth has skyrocketed to 9.5 inches, a staggering 52% increase.

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Figure 4a & 4b confirms this escalating trend in large rainfall events also. The number of large events has increased by 54% on trendline and the trendline depth of these large events has surged by a staggering 71%, Figure 4b.

Similar trends can be observed across the US and Canada. The risk of flood severity and frequency becomes readily apparent, as does the inadequacy of detention systems designed using outdated historical data. This raises several critical concerns:

• Existing detention systems, based on historical data, are ill-equipped to handle current rainfall trends.
• Release rates are becoming increasingly restrictive as regulatory bodies attempt to control flooding through policy measures. This, combined with increasing runoff volumes and decreasing allowable release rates, necessitates larger detention systems with longer cycle times.
• Longer cycle times render detention systems vulnerable to stress from frequent, sequential storms. Even minor consecutive events could fill a system, leaving it unable to effectively manage runoff during a critical event.

What are the problems you face as a designer of stormwater detention systems? Too much runoff? Restrictive discharge rates forcing excessive footprint size of detention systems? Minimum outlet sizes too large to control the flow? High cost of detention?

Regulators are adjusting policy and design requirements that puts greater responsibility for management of stormwater on the property developer.

This in turn passes cost onto the developer and yet we still default to the same technology and techniques developed 1000's of years ago; ponds, cisterns, orifice, and weirs. There are a better ways to control outflow from detention systems that can reduce the cost of detention and improve climate resilience.

Orifices, rectangular weirs, and V-notch weirs, struggle with inflexibility. Their operation depend solely on outlet size and head pressure, neglecting water velocity and other fluid dynamics for outflow control. This often leads to oversizing or under sizing detention systems or excessively long drain down times and extended duration of elevated flows downstream. Think of detention like a melting snow pack. We know the impact of a big winter snow, lots of downstream flooding in the spring that can go on for weeks.

With inflow predetermined by the design storm, the outlet control structure becomes a multifaceted challenge. With increasing restrictive discharge rates orifice outlets become smaller, meaning they restrict even small inflows that many not need to be detained. Sequential small storm events can overwhelm the system, triggering bypasses and compromising detention efficiency. As a result, traditional detention designs often err on the side of oversizing, leading to increased costs.

As we have seen from Figures 1a & 1b even these small storms are increasing in volume. This makes sequential storms more problematic.

The ability to more effectively manage the detention system is a vital part of cost control and climate adaption, and this is done using the outlet control structure.

Fully automated, predictive computer-controlled outlet gates, though ideal, pose challenges. They're expensive, require power, demand regular inspection, and involve significant operator involvement. Cyber security and reliability risks also merit significant consideration.

Can we agree that a better design of outlet control that reduces cost, potentially reduces detention size, and could also be used for climate resilience is something we all need?

Can we adapt existing infrastructure and design new systems to be more effective? Absolutely! Two promising options exist:

• Active predictive outlets:?These connect rainfall prediction to actively controlled gates, offering precise control but requiring power, computers, and complex management. While ideal for specific applications, their cost and complexity limit their widespread feasibility and the supporting infrastructure is not yet in place.
• Passive Hydro-Brake? vortex flow control:?This entirely passive device utilizes a unique vortex generating design to provide superior flow control compared to traditional orifices and weirs, without needing power or complex management. Figure 5.

Invented in the 1960s by Bernard Sisson, a pioneer in water management and co-founder of Hydro International, the Hydro-Brake? emerged as a solution to a critical challenge in combined sewers: controlling flow while preventing blockages. Since then, Hydro-Brakes have become a mainstay in wastewater flow control.

What is a Hydro-Brake? vortex flow control valve?

Imagine the swirling vortex formed by water draining down a sink. The Hydro-Brake works in a similar way, harnessing the energy of the water flow (hydrodynamic flow) to create an air core within the valve body. This air core acts as a dynamic barrier, limiting the amount of water that can pass through the outlet. Compared to traditional flow controls like orifice plates and weirs, this water-powered system offers several advantages:

• Customizable head discharge curves:?This enables precise control over the outflow hydrograph.
• Larger outlets:?These minimize blockages and maintenance needs.
• Optimized detention volume:?By optimizing detention volumes, the Hydro-Brake translates to cost savings and enhanced climate resilience.
• Fully passive operation:?No moving parts mean minimal maintenance.
• Energy Dissipation: Water is discharged in a low energy state due to the use of the hydro-dynamic energy to create the air vortex.

Before delving into the Hydro-Brake, let's take a closer look at traditional outlet controls: orifices and weirs. Detention system outflow typically aims to match pre-development conditions, using a single release rate or several rates corresponding to critical design storm return periods like the 10-, 25-, and 100-year events. Controlling multiple outflow rates typically involves a tiered system: a low-flow orifice for smaller storms, a weir for the next design storm level, and an overflow weir for the peak event.

Figure 6 illustrates the design of a typical detention outlet control structure.

The complexity of designing outlet control structures is underappreciated and has a significant impact on the effectiveness and cost of a detention system. We see a variety of outlet shapes and sizes in practice, driven by the need to balance inflow and outflow while optimizing the detention system's size within available space. Orifice and weir designs themselves have inherent limitations, necessitating creative solutions.

Let's revisit the discharge curve shapes of the most common control structures: orifices, rectangular weirs, and triangular weirs.

Round Orifice Controls

The round orifice discharge curve resembles a rising parabola, Figure 7. As the water level (head) above the orifice increases, the flow rate (discharge) out of the opening also increases, but at a steadily accelerating pace. This relationship is captured in the equation:

?Q = Cd A √(2gh)

?where:

• Q?is the discharge (volume per unit time)
• Cd?is the discharge coefficient,?accounting for real-world imperfections
• A?is the orifice area
• g?is the acceleration due to gravity
• h?is the head (water level above the orifice)

Rectangular Weir Controls

The rectangular weir discharge curve is a cubic function, Figure 8. As the head over the weir increases, the discharge initially rises slowly, then picks up pace, and finally levels off asymptotically. This behavior is described by the equation:

?Q = Cd B h √(2gh/3)

?where:

• ?B?is the width of the weir notch

V Notch Weir Discharge Curve

The V notch weir discharge curve is another cubic function, Figure 9, but steeper than the rectangular weir's. The flow rate over the triangular notch increases rapidly with increasing head, making it a good choice for situations where you need to handle a wide range of flow rates in a limited space. The equation for this curve is:

Q = Cd (5/8) tan(θ/2) h^(5/2)

where:

• θ?is the angle of the triangular notch

These are just the basic shapes of the discharge curves. Real-world factors like fluid viscosity, turbulence, and channel geometry can influence the actual behavior. However, understanding these fundamental curves is crucial for accurately predicting and managing flow rates in various hydraulic systems.

Designers often combine these control devices in a single outlet to achieve a compound discharge curve that satisfies their desired flow characteristics.

However, a key limitation of these individual controls is that their discharge curve shapes are inherently fixed. While designers can adjust the size of the control to manage the flow rate, they cannot alter the fundamental relationship between head and discharge. In essence, each control's curve represents a fixed response to the applied head pressure.

Hydro-Brake Discharge Curves

In contrast to the fixed discharge curves of traditional controls, the Hydro-Brake harnesses the power of hydrodynamic forces to create a highly customizable head-discharge response, offering greater adaptability and control than traditional methods. They are powered by the water itself, using hydrodynamic forces to control flow.

Once you have control over the energy in the water you can control the flow using that energy.

This unique approach allows for precise control of the shape of the head discharge curve of the Hydro-Brake. This enables designers to have more control over the head-discharge curve of the outlet control structure for optimal performance across a wide range of operating conditions. Figure 10 vividly illustrates this flexibility, showcasing six distinct head-discharge curves achievable with the same design point, with many more available.

Figures 11 through 13 describe the operation of the Hydro-Brake as inflow and head increase. The inlet on the left and outlet on the right. Remember the Hydro-Brake has larger inlet and outlet and uses hydrodynamic forces and air to control the flow.

Figure 11: Initial flow enters the Hydro-Brake and passes unobstructed in weir flow mode. During Stage 1 of the discharge curves, the flow rate remains below the allowable discharge limit, keeping the detention system empty.

Figure 12: As flow and head increase, the Hydro-Brake enters Stage 2, the transition stage. Approaching the allowable discharge the Hydro-Brake's design starts to restrict flow. The Flush-Flo point, controllable during design, marks the transition from weir flow to orifice flow. An air core begins to form, choking the outlet and causing the flow rate to decrease despite increasing head.

Figure 13: In full vortex flow control mode, the Hydro-Brake acts similarly to an orifice, restricting the flow to the allowable discharge at the design head.

Hydro-Brake to Orifice Comparison

Now let's compare an orifice discharge curve to a Hydro-Brake curve for the same design point of 1.36 cubic feet per second (cfs) at 4 feet of head, Figure 14.

At low head, the larger Hydro-Brake outlet allows more flow. This is permissible because the discharge limit has not yet been reached.

As the flow approaches the discharge limit of 1.36 cfs, with a larger outlet the Hydro-Brake can develop this flow with about 1.5 feet of head. The equivalent orifice requires almost 4 feet of head.

The Hydro-Brake starts restricting flow only as the vortex air core starts to form, slowing flow until it meets the orifice curve and the two have similar discharge.?

Eventually, both controls reach the design point. The area between the two curves represents the volume of water released by the Hydro-Brake and not stored, which would have been stored by the orifice control. However, at no point has the design discharge rate been exceeded.

Let's delve into a couple of design examples modelled in HydroCAD to demonstrate how this works in practice and the benefits it brings.

For demonstration we'll start with a hypothetical example to see how this works on a smaller scale site. We must match the post development discharge to the predevelopment rate using a detention pond.

Site Parameters:

In HydroCAD, the post-development condition was modeled with a detention pond having a total storage volume of 19,733 cubic feet and a peak water elevation of 5.9 feet. A 4.6-inch orifice was used for outlet control. Figure 15 shows the hydrograph for this configuration. The green is the inflow and the blue the outflow from the pond.

Utilizing Hydro's online Hydro-Brake design software, a 9.9-inch Hydro-Brake was sized to discharge 1.3 cfs at 6 feet of head. The resulting Hydro-Brade head discharge table was imported as a custom user defined outlet. This replaced the orifice control while maintaining the existing pond configuration. The updated model, as shown in Figure 16, achieved a smaller required storage volume of 18,680 cubic feet and a corresponding peak elevation of 5.58 feet.

Without further modifications, this change reduces detained water by 1,053 cubic feet, lowers the peak water elevation by 0.32 feet, and shortens the drain-down time by approximately 4 hours.

The designer now has options of reducing the pond size by 1,053 cf or leave the pond as is and consider it better adapted to larger storms. The outlet of the Hydro-Brake is also considerably larger, likely eliminating any blocking concern.

Now think about this as an existing pond and adapting existing infrastructure for future rainfall projections. How would fitting existing detention systems with better outlets impact climate resilience?

Our second example dives into a project in Chicago, Illinois, showcasing its multifaceted benefits. This redevelopment site, with a target release rate of 3.37 cfs for the 100-year event, exemplifies how this approach can be applied in practice.?

The original design relied on a large underground concrete stormwater vault, with the engineer employing a 7.3-inch orifice for system control. Figure 17 showcases the inflow-to-outflow hydrograph, revealing a storage volume of 64,929 cubic feet, a peak elevation of 11.91 feet, and a drain-down time of approximately 31 hours.

Leveraging Hydro's online sizing tool once again, a 15-inch Hydro-Brake was chosen to optimize the detention system. Figure 18 illustrates the transformed hydrograph, revealing that the Hydro-Brake benefits.

Note that the Hydro-Brake does not begin to store water until around 10 hours into the event, at a flow rate of approximately 1.8 cfs. This change significantly reduced the detention tank size required by 23,823 cf, translating to a cost saving of $233,465.00 based on the estimated unit cost of $9.80 per cf.

Furthermore, the Hydro-Brake delivered additional benefits, including a reduced drain-down time of approximately 5 hours and 1.3 feet of extra freeboard within the tank. This enhanced capacity equips the system to better handle larger events.

This cost saving is not an isolated case, but rather a common outcome in real-world applications. Hydro-Brakes typically deliver significant value.

If you could go to your client and tell them you can reduce the detention cost significantly, how would that impact your business?

Stormwater Detention is Critical Infrastructure

A key challenge highlighted in the first part of this article is that rainfall patterns are changing and there is more stormwater to deal with now. Existing stormwater infrastructure was designed for historic rainfall patterns, not the current and future extremes predicted by climate models. This leaves many existing detention systems undersized and potentially contributing to local flooding due to their inability to handle the increase in frequency and depth of rainfall.

Critical infrastructure refers to the interconnected systems and assets that are essential for the functioning of modern society and whose disruption or destruction would have a debilitating impact on national security, economic security, public health, or safety.

Retrofitting these systems with improved outlet control structures, both passive and active, is crucial for managing flooding. Simply restricting discharge limits and mandating larger detention ponds will not adapt the cityscape to be more climate robust, as resulting longer drain-down times hinder detention effectiveness when faced with frequent sequential storm events.

Hydro-Brakes as part of a suite of solutions, offer a compelling solution for both retrofitting and designing new outlet control structures. Their controlled release mechanism can more effectively manage larger runoff volumes while minimizing peak flow duration, reducing the risk of downstream flooding. Additionally, their faster drain-down times compared to traditional orifices allow them to handle frequent storms more efficiently.

What is your experience with detention design? Please start a conversation, post your thoughts and share this post.

More Information

To arrange training on Hydro-Brakes please contact me:

Phillip Taylor CPSWQ,

Technical Manager, Hydro International [email protected]

Hydro-Brake Page: https://hydro-int.com/en/hydro-brake-flow-control-series

Hydro-Brake YouTube Video: https://youtube.com/watch?v=Byk2VcHcqKI

Hydro International

Oldcastle Infrastructure