Split-Flow Method: Introduction of a New Stormwater Strategy

July 1, 2002

It’s becoming accepted that comprehensive stormwater management systems need to focus on controlling peak rate, quality, frequency, duration, and volume of runoff. This focus is a vast improvement over our traditional systems that relied on detention to simply control peak flow rates. It has also become apparent that onsite infiltration and bioretention currently offer the greatest opportunities for solving our urban runoff and nonpoint-source pollution problems. In practice, these methods have not gained wide acceptance as practical stormwater management methods. This is due in part to the problems at previous infiltration facilities and an overriding perception that bioretention alone is not a reasonable method for controlling peak flow rate. Most of these problems have come about because we continue to address stormwater as a hazard that should be collected, treated, and removed rather than as a valuable natural resource that needs to be managed to restore aquatic ecosystems. It is imperative that stormwater management professionals gain access to new strategies that can demonstrate their efficacy in controlling peak flow rate, quality, frequency, and volume and their economic and design flexibility.

This article explains the justification for development and application of a new patent-pending split-flow method with the aim of offering revisions to established ecological and economic precedents for managing stormwater. Ideally, new principles of split-flow systems will encourage further investigation that can evolve into ecologically based stormwater management policies and practices. A stormwater management method for separating out runoff created by development is created by dividing runoff through a proportional stormwater flow-splitter based on the predevelopment runoff and the difference between predevelopment and postdevelopment runoff (United States patent pending filed 2002, West Virginia University Research Corporation).


The basic principle of our traditional systems is that excess runoff is simply detained and released to control peak flow rates. This has nothing to do with replicating the natural hydrological system but rather focuses on mitigating damage from elevated flows to downstream properties. The concept is illustrated as a simplified hydrograph in Figure 1. The simplified hydrograph geometry is based on the equivalent triangular hydrograph (Federal Highway Administration [1996] fig. 3.7 and US Soil Conservation Service [1972] fig. 16.2).

Onsite infiltration and bioretention have been used as supplements to detention systems intending to infiltrate the first flush of runoff from impervious surfaces. In this configuration, infiltration or bioretention acts as a simple water-quality add-on for a detention system while the peak flow rate is still controlled using the detention system as shown conceptually in Figure 2.

To calculate the first flush, a site is divided into drainage areas based on impervious surfaces. The area in acres of each impervious surface is multiplied by 0.04 to get the first half inch of runoff volume from impervious surfaces to be infiltrated in cubic feet. The infiltration system can then be designed as aboveground or belowground facilities based on the available land area and percolation rates. The amount infiltrated is not based on the runoff hydrograph and does not control peak flow rate. Therefore, a detention system is still needed to maintain the predevelopment peak flow rate, and the first-flush infiltration facilities are simply distributed throughout the site based on the location of impervious surfaces as a water-quality treatment system.

Using infiltration to control peak flow rate is commonly based on using one of two options. The first is to infiltrate the portion of runoff that exceeds the predevelopment peak flow rate, as shown in Figure 3 (Ferguson, 1995). This volume of flow can be diverted using an in-ground overflow splitter, as shown in Figure 4. The overflow splitter can also be designed as a surface system, as represented Figure 5.

Based on the hydrograph geometry shown in Figure 3, the volume to be infiltrated for a specific design storm can be calculated using a simplified hydrograph with the following equation:

Volume = (post Qp — pre Qp) x (ToC — [{pre Qp/post Qp} x ToC]) x 80.1

The overflow splitter is designed so that all runoff that exceeds the peak flow rate will overflow into an offline infiltration basin. The bypass pipe is sized for the maximum predevelopment peak flow rate. There is no need to run a reservoir routing because the basin accommodates all runoff that exceeds the peak flow rate for the design storm and the bypass pipe accommodates all other runoff. The infiltration basin is sized for the overflow volume plus runoff from the surface of the overflow basin area. This method uses infiltration to control the peak rate for a specific design storm.

There are two drawbacks to a system based on skimming off the volume that exceeds the predevelopment peak flow rate. The first drawback is that the system is designed for a specific design storm by sizing the bypass pipe capacity based on the predevelopment peak flow rate, and the infiltration basin is sized only for the excess runoff from the same storm. Uncontrolled excess runoff from larger storms is discharged over a spillway. The second drawback is that the method does not address nonpoint pollution issues because the first flush is allowed to pass downstream untreated, as well as all the flows from smaller storms.

The second and more common infiltration method used to control peak flow rate is to infiltrate all of the runoff that precedes the point where the postdevelopment hydrograph descends to the predevelopment peak flow rate, as shown in Figure 6 (Ferguson, 1995). The volume of runoff that precedes this point is referred to as the truncated hydrograph. Its volume can be calculated for the simplified hydrograph shown in Figure 6 using the following equation:

Volume = (post Qp x ToC x 80.1) — ([{ToC x 1.67} x {pre Qp/post Qp}] x pre Qp x 30)

A basic principle of the method is that all runoff up to this specific volume will be captured and infiltrated. When rainfall events occur in which the peak flow rate does not correlate with the specific volume, the predevelopment peak flow rate might or might not be controlled, depending on the size of the storm. Systems based on the truncated hydrograph are designed as online facilities and are based on a specific design storm. Unlike the overflow system mentioned above, however, smaller storms will be captured and held in the infiltration facilities. If the total flow volume exceeds the facility’s storage capacity, runoff will overflow the facility and be released downstream uncontrolled. This becomes a major concern when rainfall events are larger than the design storm. As a result, systems based on the truncated hydrograph can be sized for larger design storms to ensure they will effectively control peak flow rate.

One of the main drawbacks of the truncated hydrograph method, however, is that large runoff volumes must be infiltrated to control peak flow rates. This is an issue because the truncated hydrograph volume is always more than the increase in volume actually created by a given development. This can be demonstrated with simplified hydrographs by comparing the volumes between the truncated hydrograph using the equation

Volume = (post Qp x ToC x 80.1) — ([{ToC x 1.67} x {pre Qp/post Qp}] x pre Qp x 30)

and the difference in volume between predevelopment and postdevelopment using the equation

Volume = (post Qp — pre Qp) x ToC x 80.1.

It is therefore best to apply the truncated hydrograph method in developments with limited change in the overall runoff coefficients or where the potential for flood damage is low. This shortcoming greatly limits application of truncated hydrograph methods in a wide variety of land development intensities.

A New Strategy

The purpose of split-flow infiltration systems, as defined in this article, is to allow us to preserve the predevelopment stormwater flows in terms of rate, quality, frequency, duration, and volume. A method that infiltrates the total difference in volume created by development can be designed by using paired Vee-notch weirs as flow splitters connected to small infiltration facilities distributed throughout a site. Unlike other stormwater management systems, split-flow systems are designed to infiltrate the same volume of runoff proportioned to time at the same rates that existed prior to development. The paired weirs can be designed to split the runoff so that the portion of the postdevelopment hydrograph created by buildings and impervious surfaces is diverted into infiltration facilities, and the natural runoff that existed before development is routed downstream. The total difference in volume between predevelopment and postdevelopment design storms can be calculated with the following equation:

(post Qp — pre Qp) x ToC x 80.1

This method closely re-creates the predevelopment hydrograph by infiltrating the additional runoff volume created by development, as shown in Figure 7.

One weir is designed to capture the predevelopment runoff and bypass the infiltration facility, and the other weir is designed to capture the increase in runoff caused by development. The paired weir system is based on predevelopment and postdevelopment conditions. The bypass weir angle is sized for predevelopment flow rate, and the diversion weir angle is sized for the difference between the predevelopment and postdevelopment flow rate. These two weirs combine to act as a proportional flow splitter, as shown in Figure 8.

Using a common Vee-notch weir nomograph, such as that shown in Figure 9 (Ferguson, 1998), each weir angle can be sized with identical heads and different flow rates based on the predevelopment flow rate and the difference between the predevelopment and postdevelopment flow rate. For example, if the predevelopment peak runoff rate is 5.6 cubic feet per second (cfs) and the postdevelopment peak runoff rate is 8.5 cfs, the bypass weir angle would be sized for 5.6 cfs and the diversion weir angle would be sized for 2.9 cfs. Using the Vee-notch weir nomograph, the bypass weir angle could be 120º and the diversion weir angle 90º as long as the weirs were constructed at the same elevation. Another option would be to set the bypass weir angle at 75º and the diversion weir angle at 45º, as shown in Figure 10 (Ferguson, 1998).

Either of these configurations would emulate the difference in the predevelopment and postdevelopment hydrographs. One of the advantages of this method is that the design for the paired Vee-notch weirs is based on the notch angle and not the overall size or capacity of the weir. The runoff is split into the same proportions regardless of the Vee-notch height or storm size. Therefore, as long as the flow splitter’s capacity is larger than peak rate, the stormwater would be divided at the same proportions regardless of the total flow. In other words, the system is not designed for a specific design storm but, rather, designed to split the flow into a specific ratio for all storms large and small. This fact provides an immense advantage over the two preceding methods.

The key to success with this management strategy is to install proportional flow splitters for each impervious surface and distribute the flow from the diversion weir into individual infiltration facilities. This requires that the stormwater flow splitters be designed to divide the runoff from each of these surfaces into portions that emulate the predevelopment runoff flow and the difference in predevelopment and postdevelopment flow for each individual surface, which will not be the same as the ratios for the entire drainage area. This is done by sizing each individual pair of Vee-notch weir angles based on the predevelopment runoff and the increase in runoff caused by the impervious surface (US patent pending filed 2002, West Virginia University Research Corporation). For example, a parking lot built on land with a predevelopment runoff coefficient of 60 would require that the Vee-notch angles be based on 6 cfs and 4 cfs. This would result in the bypass weir requiring a 45º Vee-notch angle and the diversion weir requiring a 30º Vee-notch angle. To control the peak flow rate for the entire development, this ratio would be used in all flow splitters used for impervious surfaces built on land with a runoff coefficient of 60, including all the buildings. Similar ratios could be calculated for other predevelopment runoff coefficients, as well as other runoff methods, such as the TR-55. This allows the stormwater management system for each impervious area to be designed independently based on unique site conditions. These weirs can easily be built into inexpensive plastic drop inlets, as shown in Figure 11. These drop inlets can be distributed throughout a site much more cost-effectively.

The importance of dividing the site into as many drainage areas as possible and distributing the development hydrograph over the entire site in numerous small infiltration facilities cannot be overemphasized. An advantage of this strategy is that the volume to be infiltrated is precisely the excess runoff caused by the development, as shown in Figure 7, and not any larger, as is the case with the truncated method shown in Figure 6. This is an especially important benefit when utilizing infiltration on sites with clay soils where very little water is infiltrated into the ground before development. The proportional flow splitter would ensure that the same volume and no more would need to be infiltrated into the ground after development to control the peak flow rate. Without adequate distribution, however, the system will not work because sufficient soil area must be provided for the diversion volume to able to infiltrate in a reasonable time. This basic design concept is illustrated in Figure 12.

This concept will succeed in controlling peak flow rates where other infiltration strategies have not because the amount of stormwater to be infiltrated in each facility is carefully controlled and it is never concentrated in large quantities. The system still controls the peak flow rate by distributing and infiltrating the difference in volume proportional to time over the entire site. The volume of runoff that needs to be infiltrated for each individual impervious surface can be calculated with the following equation:

Volume = individual impervious surface area x ([post Qp x ToC x 80.1] — [pre Qp x ToC x 80.1]) / total onsite impervious surface area

This total volume should be based on the largest design storm chosen according to the acceptable level of flood risk for the site design.


A further objective is to lengthen the time of concentration and control the first flush by emulating the reduction in runoff that was adsorbed in the predevelopment initial abstraction. This reduction in runoff is most easily emulated by using existing bioretention techniques. These facilities can be located in-line as bioretention areas before the proportional flow splitter or off-line as a separate facility to ensure that the first-flush pollutants are not resuspended and released downstream. The facilities can be sized based on the predevelopment initial abstraction runoff depth in inches for each impervious surface area, as shown in Figure 13 (US Department of Agriculture, Soil Conservation Service, 1986). Alternatively, a first-flush depth can be selected based on local nonpoint-source guidelines or regulations.

If the volume needed in cubic feet is set equal to the initial abstraction runoff, the depth in inches is divided by 12 and multiplied by square feet of impervious area. For example, a typical 65-ft. parking bay 10 ft. across would park two cars with the driving aisle. If the 650-ft.2 parking area was built on land with a predevelopment curve number of 60, the volume would be calculated as Volume = (1.3/12) x 650. So a 650-ft.2 parking area would need 72 ft.3 of bioretention area. If the bioretention facility were built 1 ft. deep, then it would fit in an 8.5-ft.2 space, as shown in Figure 14.

The facility could be designed to use less land area if the depth was increased. This simple ratio of parking to bioretention area can be applied easily to small parking lots and still allow area for tree planting in the medians, as shown in Figure 15.

If the facility is sized for 1.3 in. of runoff, any storm with less than 1.3 in. of rainfall would be held completely on-site. Storms with more than 1.3 in. of rainfall would overflow into the flow splitter. After entering the flow splitter, this overflow would be separated into the natural and development hydrographs. The flow from the natural hydrograph would be released downstream, and the flow from the development hydrograph would be diverted to an underground infiltration chamber and allowed to infiltrate back into the ground, just as it could have before development. The infiltration chamber can be located in a landscape area between the parking areas directly under the bioretention facility. The basic design concept is shown in Figure 16.

Locating the entire facility within the landscape median and not under a parking lot provides many other benefits besides a convenient location for the bioretention facility. By locating the system in the landscape medians, the soil would not be compacted by the parking. These areas could either be protected from compaction during the construction process or dug out with a backhoe after the parking is in place to restore the infiltration capacity.

Likewise, the system could be maintained and repaired if needed or dug up and replaced with a new system much less expensively than removing a parking lot. Placing the entire system in the landscape medians would also eliminate the need for expensive concrete catch basins by allowing lower-cost, lightweight materials to be used in manufacturing most of the typical system components, including the pipes connecting downstream, as shown in Figure 17. The filter screen on top is sized to prevent mosquitoes from reproducing inside the system and keep sediment, organic debris, and trash out of the infiltration chambers.

Another advantage is that the infiltration chambers would not be susceptible to crushing from the weight of large, overloaded vehicles in the parking lot. A final advantage of locating the facility within the landscape median is that it would be much easier to design a terraced, infiltration-based stormwater system for sloped sites, as shown in Figure 18. Locating a system in landscape medians would require them to be built about 65 ft. apart. Some concern exists about not holding to a central principle in infiltration design that stormwater should be infiltrated as close to source as possible. The addition of stone-filled infiltration beds under the infiltration chamber, as shown in Figure 18, should help mitigate some of these concerns. This, however, is not an insurmountable concern given the amount of lateral flow commonly seen in systems built under similar conditions, as long as the infiltration facilities are built level to the contour. Additional measures might need to be considered under adverse conditions to ensure that the systems will perform as intended.


In theory, a distributed split-flow infiltration system should duplicate the predevelopment hydrological peak rate, frequency, duration, and volume of runoff. Furthermore, if the first flush containing the highest pollution amount can be diverted and isolated in effective bioretention facilities, the reduction in downstream degradation should be quite substantial. This would not preclude the use of existing infiltration-based practices, such as porous pavement, dry wells, infiltration trenches, and basins, that are an integral part of low-impact development practices. In practice, however, the use of existing infiltration methods to achieve the goal of peak rate control has worked best in areas of low-intensity development or highly permeable soils or with the use of very large infiltration facilities. The split-flow method adds another strategy that can be applied to developments of much greater intensities or on less permeable soils while resolving many of the problems that current stormwater management methods need to address. A site could be developed so that the flow from each building, parking lot, plaza, or other impervious surface can be directed into an individually designed split-flow facility, and the cumulative volume infiltrated in these facilities would be equal to the runoff volume created by the development. The end result is that the first flush would be captured on-site in bioretention facilities and the overflow would drop into the flow splitters, where it would be divided into the natural flows and the flows caused by development. The natural flows would then be released downstream while the flows caused by development would be infiltrated back into the ground as it was in the predevelopment condition. Thus, the predevelopment hydrology would be preserved.
About the Author

Stuart Echols

Stuart Echols focuses on developing innovative stormwater systems to reduce the negative impact of development. He teaches full-time at West Virginia University while completing his Ph.D. from Virginia Tech in environmental design and planning. He is also an adjunct assistant professor at Penn State, working with graduate students in the Center for Watershed Stewardship.

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