Economic Optimization of Sustainable Subsurface
Infiltration Systems

By William G. Young, P.E.; Derek Berg; and Heather McCall, E.I.

Learning Objectives

After reading this article you should understand:

  • The benefits of infiltrating runoff and the general types of infiltration systems available.
  • The basic principles for designing an underground infiltration system.
  • The importance of pretreatment and maintenance for infiltration systems.
  • The primary factors to consider when choosing the appropriate underground infiltration system.

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Overview

Infiltration is quickly becoming the centerpiece of stormwater management strategies across the United States. Rainfall infiltrating through the soil and recharging streams, lakes, rivers, and underground aquifers is a fundamental component of the water cycle. The amount of rainfall that infiltrates into the ground is largely dependant on the soil type, land use, level of soil saturation, and a number of other variables. Sandy soils tend to have higher infiltration rates than finer silt and clay soils because there are larger voids between coarse sand particles for water to pass through. Saturated soils typically have lower infiltration rates than relatively dry soils.

Where viable, infiltration is an effective means of managing stormwater runoff since it often allows practitioners to address both water quality and water quantity concerns. Stormwater is commonly infiltrated by directing runoff to a variety of engineered best management practices (BMPs) designed to optimize infiltration rates and water quality while minimizing ongoing maintenance needs. Common infiltration BMPs include trenches, basins, bioretention cells, rain gardens, and a growing list of underground BMPs such as perforated pipe, bottomless chambers, and vaults. Two major challenges to using infiltration to manage stormwater runoff are maintaining infiltration rates as the soil accumulates solids and other pollutants over time, and preventing the contamination of groundwater. As such, most regulatory bodies have developed strict design criteria geared toward overcoming these challenges.

Infiltration standards vary somewhat from one agency to the next, but a number of common variables exist. First, most agencies establish an allowable range of infiltration rates. If water infiltrates too slowly, the facility may never fully drain, but if water infiltrates too quickly, it may not be properly treated and create a contamination risk. Infiltration rates are assessed by conducting tests at the proposed BMP location. Most agencies discourage infiltrating when rates fall outside a specified range (0.5 to 5.0 inches per hour is common). Agencies generally specify a minimum separation between the infiltration facility and the groundwater and/or bedrock. Most agencies call for 2 to 3 feet of separation to allow adequate time for pollutants to be filtered by the soil.

To reduce maintenance frequency, it is also common to require that other BMPs be placed upstream of infiltration facilities, as pretreatment removes coarse solids and debris prior to discharge into the infiltration facility. Practitioners are also sensitive to the potential for groundwater mounding to occur. Mounding analysis evaluates the expected raising of the water table below the infiltration facility, which, if not properly accounted for, can create localized drainage and flooding issues.

Figure 1: A CDS hydrodynamic separator provides pretreatment
prior to discharge into a ChamberMaxx infiltration system.
A CDS hydrodynamic separator

Maintenance and pretreatment

Hands down, the most important factor in the sustainability of any infiltration system is proper maintenance, and the best way to minimize the maintenance burden is to capture as much of the sediment load as possible before the runoff reaches the infiltration area. As site conditions can prove to be unpredictable, it is in the design engineer's best interest to provide an aggressive pretreatment practice prior to infiltrating the water quality flow or greater flow events. Unforeseen upstream construction, erosion, annual tree debris, and winter maintenance treatments can quickly occlude infiltration facilities, putting them at risk for hydrolic failure and reducing water quality benefits.

There are numerous pretreatment design options available to accommodate the sediment loading, ranging from land-based options such as forebays and grass channels to manufactured treatment systems such as hydrodynamic separators (Figure 1). Incorporating pretreatment serves to extend the lifecycle of the infiltration system and eliminate the need for frequent maintenance. Another important safety factor to consider to prolong the life of the facility is sizing the infiltration system with a conservative infiltration rate; the design infiltration rate should be based on the assumption that the system has some level of occlusion. This will ensure that the system will function as intended even after the accumulation of sediment has started to reduce the original percolation rate.

Best maintenance practices include inspection of the pretreatment and infiltration systems on a routine basis to determine when cleaning is needed. In many cases, this includes inspection after heavy storms to evaluate sediment accumulation in the pretreatment system and drain down time of the infiltration system. Cleaning is needed when sediment accumulation or drain down time falls outside of acceptable levels.

Underground infiltration facilities

Understanding the basics When planning an underground infiltration system, it is important to have a thorough understanding of the mathematical relationships between composite volumes of pipes, chambers, and vaults surrounded by stone or other media. The basic stone and pipe leach field for home septic systems has been a staple for generations now; distribute the hydraulic and pollutant loading over as large an infiltration area as possible for the lowest installed cost. To achieve this, a designer would simply start with a box of rocks and run an array of perforated distribution pipes through them (Figure 2).

Figure 2: Composite pipe and stone systems within a "box of rocks."
Composite pipe and stone systems within a box of rocks.

Driven by constrained lots, many innovative infiltration systems have been introduced, but the basic concept still remains the same.

The following main principles can be learned from leach field design:
  • Pre-treatment is essential, and make sure that it is cleaned regularly as solids in the leach field will lead to rapid system failure.
  • Include a composite leaching bed of distribution pipes, chambers, or vaults surrounded by stone to distribute hydraulic and pollutant loading evenly and utilize the entire filter area.
  • Maintain minimum required separation to seasonal high groundwater.
  • Be keenly aware of the installed cost of all specified materials and incidentals during the design process.

Combining quality and quantity requirements

It can be extremely advantageous to size infiltration systems large enough to address peak flows in addition to managing the water quality event. Doing so allows practitioners to match pre- and post-development peak runoff rates, which is required by many jurisdictions. It also allows the total volume of runoff (i.e., the area under the runoff hydrograph) to match the pre-development condition, which has become an increasingly important concept known as matching pre- and post-development hydrology. However, infiltrating a large volume of water heightens the risk of mounding and groundwater contamination, so these issues should be carefully evaluated. Addressing water quality and quantity concerns also provides an opportunity to reduce the amount of infrastructure required to meet stormwater management objectives. To maximize the economical benefit of incorporating water quality and water quantity requirements into a combined infiltration system designers must remember two simple rules:

  1. Make the system as deep or as tall as possible within a given footprint while still maintaining minimum separation to groundwater and a large enough overall footprint to drain the system completely within the required maximum time after a storm event (usually less than 48 hours).

  2. Maximize the open area inside the "box of rocks" using the most costeffective materials possible.

Maximizing vertical space: Every inch counts

Increasing the depth of an infiltration system allows for more water storage in the same footprint. For example, doubling the diameter of perforated pipe yields four times as much storage volume, which translates to a significant cost savings per cubic foot of storage. The savings in cost per cubic foot of storage increases as the pipe diameter increases (Table 1).

Fully perforated, aluminized Type II steel corrugated metal pipe (CMP) can be readily made in diameters as large as 144 inches, but use of pipes larger than 96 inches is not always advantageous from a cost perspective. Once the pipe exceeds 96 inches in diameter, pipe material gage requirements start to increase quickly and complications in shipping to the jobsite (such as escorted and permitted loads) may arise. However, these larger diameters can be implemented for a site that is heavily constrained in regard to subsurface space, but may have very favorable soil conditions and a very low groundwater table.

Table 1: Comparison of decreasing cost per cubic foot of storage as pipe diameter increases for 16-gage, aluminized Type II steel, fully perforated corrugated metal pipe.
Pipe diameter Pipe material cost
($/ linear foot)
Storage (cubic feet)
per linear foot
Storage cost
(per cubic foot)
inside pipe*
12 inches $9.00 0.78 $11.54
30 inches $21.00 4.9 $4.29
48 inches $33.00 12.5 $2.64
72 inches $57.00 28.8 $1.98
96 inches $75.00 50.2 $1.49
*Pricing is estimate only. For exact pricing, please contact manufacturer.

Thinking outside the box: Infiltration system layouts

Historically, underground infiltration beds have been laid out in either square or rectangular shapes, sometimes far away from where the actual rain drops fall. These large systems are typically fed by a series of upstream catch basins and conveyance pipes. Several factors contribute to site layout, but mostly it simplifies the hydrologic and hydraulic modeling process, and keeps infiltration beds away from more sensitive underground infrastructure.

The emergence of low impact development (LID) principles, however, has encouraged designers to focus on managing stormwater onsite and close to the source using uniformly distributed, decentralized, mirco-scale controls. A site that had one or two large infiltration systems a couple of years ago might be designed with numerous smaller infiltration systems spread throughout the site today. Additionally, rectangular or square systems will not always fit the requirements of an LID site design, so designers may be forced to think "outside the box" in respect to the shapes of their systems. Underground infiltration systems can be implemented on a site in just about any shape or configuration to allow designers to develop decentralized systems and provide infiltration on portions of the property where other LID practices may not be applicable.

Historically, many designers incorporated flow-control and flow-splitting structures into subsurface infiltration systems using a separate structure to perform each function. With advances in precast concrete manufacturing capabilities, increased plastic customization capabilities, and custom welding of metal systems, many flow-control and flow-splitting structures (Figure 3) can be placed integrally inside the main chambers or pipe to reduce cost and the overall impact to a site's footprint.

Figure 3: Various integrated flow-control structures for CMP retention and detention systems
flow-control structures for CMP retention and detention systems

Choosing the right underground infiltration system for your site

As previously mentioned, infiltration can be achieved in a variety of forms, ranging from landscape-based systems to underground systems. Determining the right form of infiltration is usually based on factors such as site constraints, climate, and land value economics. Once the desired form of infiltration has been determined, the actual practice to implement can be evaluated. For example, if the desired form of infiltration is an underground system, then an evaluation of the following factors will help lead to the selection of the most appropriate type of underground system for the site:

  • Available footprint and vertical depth — Which system is a best fit for the site constraints?
  • Agency approval — Is there a preference for a particular system?
  • Soil type — Which system is best suited for the environmental conditions where it will be installed?
  • Service life — Will the system adhere to service life expectations?
  • Structural loading requirements — Does the system material adhere to structural requirements?
  • Maintenance — How easy is it to maintain the system? What are the annualized costs associated with maintaining the system?
  • LEED considerations (if applicable for the project) — Which system will provide the most LEED credits for recycled materials and regional materials?

Assuming that more than one underground infiltration system adheres to the above criteria, the biggest factor to consider in choosing the right system may come down to cost. When evaluating the cost of underground infiltration systems, consider not only the capital cost of the system itself, but also costs of all auxiliary equipment, as well as costs of construction and ongoing maintenance.

Total cost includes the following:
  • capital cost of equipment (including flow controls, pretreatment, and other components);
  • cost of construction (excavation, stone, installation, etc.);
  • cost of ongoing maintenance.
Figure 4A (left) Figure 4B (right): Comparison of two viable underground infiltration systems Comparison of two viable underground infiltration systems

Figure 4a is an example of a given site designed with a composite underground chamber system for infiltration. In this example, site constraints require that the underground retention system fit into a 100-foot by 300-foot rectangular footprint while being able to temporarily detain as much as 88,660 cubic feet of runoff. Additionally, the site has only 5 feet of available vertical depth. Cost estimates are based on best available information in today's market.

An alternative design to the above uses fully perforated 42-inchdiameter, 16-gage CMP (Figure 4b). In the alternative design, a cost savings of 16.5 percent is achieved while requirements around site constraints are kept intact. Value engineering in this example was achieved by maintaining the same volume of storage in the CMP system as the chamber system while keeping the footprint and burial depth the same. If site redesign is allowed, the storage volume of the CMP system could easily be increased by using minimum spacing requirements, therefore allowing you to fit more pipe in at 100 percent void space, instead of utilizing the stone at 40 percent void space. This redesign would maximize the storage efficiency in the available space at the lowest possible cost. This example highlights the need to evaluate multiple options and configurations to find the most appropriate infiltration system for your site.

Summary

With the change in philosophy that LID has brought about, it is imperative that a designer has an effective toolbox of systems and practices at his or her fingertips that can be strategically placed and not interfere with other underground infrastructure while still minimizing the amount of total land excavated during installation. The sheer number of proprietary underground infiltration systems available on the market today can be overwhelming. More often than not, the system that the designer is most familiar with gets specified without performing an in-depth cost analysis. Recently, hydrologic modeling software packages, such as HydroCad Version 9, have begun to incorporate cost fields right inside the pond nodes, allowing designers to make cost analysis part of their design in real time. This can very quickly allow one to hone in on the most cost-effective system to meet the stated hydrologic and hydraulic requirements. Also, in addition to capital costs, consider longterm maintenance and operating costs.

Thousands of combinations of layouts, materials, shapes, sizes, and hydraulic controls can be incorporated into the design of a subsurface infiltration system. While this article has only compared two possible configurations, the basic methodology outlined for making rapid comparisons can be applied across the entire spectrum of composite infiltration systems. When designing your next subsurface infiltration system, remember the basics. First and foremost, design for long-term sustainability, and then squeeze every bit of value possible out of the design while minimizing both short- and long-term site disturbance, as well as impacts to surrounding hydrologic and hydraulic features. This will ensure that your next design exhibits great value to stakeholders while truly striving to be a low impact development.


William G. Young, P.E., is a sales engineer with CONTECH Construction Products Inc.

 

Derek Berg is a regional regulatory manager for CONTECH

Heather McCall is a stormwater design engineer for CONTECH.


REFERENCES

  • American Iron and Steel Institute, 1994, Handbook of Steel Drainage & Highway Construction Products, Fifth Edition, Washington, D.C.
  • Smart, Peter, 2008, HydroCAD Version 9, HydroCAD Inc., Chocorua, N.H.