Lowering the Cost of a Reduced Runoff System

Since the Georgia Stormwater Management Manual revision in 2016, many of the municipalities in the Atlanta area have begun requiring stormwater runoff reduction for water quality and quantity control. The primary goal of the new standards is to retain and infiltrate the runoff volume from the first 1” of rainfall.

The typical RRV system is a modified detention system, either an above ground detention pond with an unrouted volume designed to infiltrate or an underground detention system with a stone aggregate base. The RRV seeps into the stone voids and then into the soil below the system.

Some common infiltration systems:

Stone Pit for RRV
Perf'd Pipe for RRV

These RRV configurations have one glaring problem – stone is expensive.

Value Engineering RRV Systems

The reason for the putting the reduced runoff volume into the stone voids is to allow the water to infiltrate into the soils below the stone. Storing the volume in the stone voids doesn’t provide any additional benefit to the system, but it does increase the amount of stone used in the system.

The simplest way to reduce the amount of stone needed for an infiltration system is to use perforated pipe or chambers with a minimal amount of stone below the systems. The volume is retained by using a weir wall or a raised outlet pipe.

Perforated Pipe with Elevated Outlet Pipe
Chambers with Weir Wall

Cost Examples of the Two Design Approaches

Let’s look at an example to compare the two design approaches.

Project requirements:

Required RRV: 9,000 cf
Required detention volume: 27,000 cf
Total system volume: 36,000 cf

Configuration 1: 96” Diameter Perforated Metal Pipe System with Deep Stone Base

96 Inch Pipe with Deep Stone Base

Stone envelope dimensions: 436’ long x 10’ wide x 13.2’ deep (2,132 CY)
4,360 sq-ft footprint
434’ of 96” pipe
1,320 CY of stone

Configuration 2: 96” Diameter Perforated Metal Pipe System with Shallow Stone Base

96 Inch Pipe with Shallow Stone Base

Stone envelope dimensions: 563’ long x 10’ wide x 8.5’ deep (1,773 CY)
5,630 sqft footprint
561’ of 96” pipe
730 CY of stone

Reduction in stone quantity: 590 CY
Reduction in excavation: 359 CY

The stone configuration for your RRV system will dramatically affect the installed cost of the system.


If you are working on a runoff reduction system and have some questions, feel free to contact our engineering team for support.

#1 Mistake Design Engineers Make that Increases Stormdrain Costs - 30%

#1 Mistake Engineers Make that Increases Stormdrain Costs

We get a lot of calls from design engineers with the same question:

“I am designing a project that will have 18”, 24”, and 36” stormdrain pipe. What pipe material option is the least expensive for these sizes?”

Our answer: none of them

The Surest Way to Lower Costs is to Allow Competition

The problem with this question is that it assumes that you have to specify a single product for each pipe line in your project. Instead of specifying a product, you should be designing a system that meets the project needs.

The specific material used in any system is only important in that it meets the design criteria of the project. You want the pipe to function hydraulically, perform structurally, and provide long term service life to the owner.

Aluminized corrugated steel pipe, HDPE dual wall pipe, and reinforced concrete pipe all meet these criteria.

Download a Sample Stormdrain Pipe Specification with Material Options

Hydraulic Performance

The spiral rib corrugation profile of steel pipe has the same Manning’s coefficient as HDPE and RCP (0.012). These products can be used interchangeable without impacting your hydraulics.

Structural Capability

All three products can support H-20 highway loads and are allowed per GDOT for cross drain applications.

Long Term Durability

All three products provide a 75-100 year minimum service life.

How Much Does it Cost to Specify a Single Product?

Basic economics tells us that competition lowers prices.

The effect of competition specifically on storm drain pipe costs has been well documented through independent research by the FHWA and various state DOTs as well as through industry research studies.

  1. The National Cooperative Highway Research Program (NCHRP) published a report in 2015 titled “Proposed Practice for Alternative Bidding of Highway Drainage Systems.” The report states that “The inclusion of multiple equivalent options during the bid phase of projects has been shown to reduce costs through increased competition.” Read the excerpt here
  2. South Dakota DOT published a report in 1998 titled “Evaluation of High Density Polyethylene (HDPE) Pipe.” The report states that “when polyethylene pipe is included in the construction plans as an alternate to reinforced concrete pipe or corrugated metal pipe, bid prices for each type of pipe are generally lower” and “…when projects are bid as reinforced concrete only, installed costs are an average of 29.2 percent higher on 305 mm to 915 mm (12 to 36 in) drain pipe than when polyethylene is allowed as an alternative.” Read the excerpt here
  3. BCC Research published a report in 2016 titled “Comparison of Stormwater Pipe Installation Lengths and Costs in Texas” that compared the installed storm drain pipe system costs for cities in Texas that a allow a single product choice for storm drainage vs cities in Texas that allowed more than one product choice for storm drain pipe. The report states that “key project findings indicate that communities with open competition enjoy lower pipe cost, on average, for stormwater projects, reaching savings of up to 57% in comparison to municipalities employing closed competition practices.” Read the report here

The Cost of Specifying a Single Product Choice for a Project in the Atlanta Area

We pulled publically available stormdrain construction contracts to compare the installed cost of various stormdrain materials in the Atlanta area.

Stormdrain Pipe Materials Cost Comparison

Depending on the diameter, the installed cost of reinforced concrete pipe is 50% to 100% higher than the installed cost of corrugated steel pipe or corrugated HDPE pipe in these contracts.

You can see that the least costly pipe material choice varies based on the diameter of the pipe. This choice is also affected by municipal regulations and fluctuations in material prices over time.

How to Reduce Stormdrain Costs

The easiest way for you to ensure that your client gets the best storm drain system pricing is to allow multiple pipe material options. With this one simple step you:

  1. Increase the number of manufacturers competing for the project, which lowers bid prices.
  2. Protect your project from fluctuations in the material pricing for these products.
  3. Reduce your work on the back-end of the project because you do not have to review alternate pipe material proposals.

We have put together a sample storm drain pipe material specification you can insert on your drainage plan or pipe profile to allow contractors multiple pipe materials options. The specification includes the industry recommendations for pipe installation and the material requirements to meet GDOT standards.

Download a Sample Stormdrain Pipe Specification with Material Options

If you have more questions about this topic, please feel free to contact our engineering department.

Contact our engineering department

Large Diameter Corrugated Metal Pipe Underground Detention

How to Design an Underground Detention System: 3 Cost Saving Steps

The engineering team at Southeast Culvert has designed hundreds of underground detention systems. We partner with civil engineering design firms every day to provide them with resources and experienced guidance according to industry best practices. Our goal is to ensure that your storm drain systems are extremely cost effective, functional, and meet the standards of the municipality reviewing the project.

We regularly receive questions from design engineers about how best to design an underground detention system. A few of the more common questions are listed below.

• Should I use pipe, chambers, or another product?
• If I use metal pipe, what diameter and gauge do you recommend?
• How much cover will the underground detention need?

In this guide, we will answer these questions as well as addressing other important criteria in underground detention design.

As always, if you need some design guidance for an underground detention, water quality treatment device, or retention/infiltration system, let us know.

We usually return proposal requests within a day.

Step 1: How much depth do you have to work with?

The allowable vertical space for your underground detention system is one of the most important considerations for what product will work in the application. You have to ensure that the system will have an appropriate cover height to function structurally.

If you have a rough grading plan and your detention outlet pipe elevation, you can calculate the maximum depth (finished elevation to invert out) at the outlet location. Then you will check the edges of the detention’s allowable footprint to see if the finished grade increases or decreases significantly.

Use the shallowest depth you find as your initial allowable vertical space. You can probably manipulate the detention system’s shape and footprint later to increase that vertical space, but this depth is a good starting point for your design.

Step 2: What underground detention product will work in this space?

Every detention product has a minimum allowable cover to allow vehicular traffic loading on the finished system as well as construction loading guidelines. The construction loading minimum cover will always be more than the minimum allowable cover for traffic loading.

Start off your design assuming a minimum cover of 3’ over the detention system. That minimum cover will allow some medium construction loads on both pipe and chambers systems. This guideline does not cover all construction loads and you will need to double check the requirements of the product you specify against the equipment required during construction, but it is a good place to start.

Subtract 3’ from the vertical depth you came up with in step 1. That depth is the allowable vertical dimension of the detention system.

*Note: If the allowable detention footprint and depth based on these guidelines does not allow you to provide the required volume, then you can reduce the cover over the system. That will allow you to expand the system footprint and increase the size of the pipe or chamber system you are using. The minimum cover for either product is in the range of 12-24” to the bottom of flexible pavement or to the top of rigid pavement.

Designing the system closer to the minimum cover requirements will necessitate more coordination with the contractor to make sure that the construction loading requirements are maintained through construction.

Step 3: Choose the most cost effective detention product

Now that you have the maximum vertical dimension of your detention product, you can look at the different available options and do some value engineering.
Generally, the least expensive detention system product on the market is corrugated steel pipe. Designing with the product is fairly easy because the minimum and maximum covers on the product allow you a wider range of design than almost any product.

For instance, the minimum allowable cover on 72” diameter steel pipe for H-20 highway loading is 12” to the bottom of flexible pavement and the maximum cover is 52’. No other product has that range of cover heights.

Corrugated steel pipe is available in a wide range of diameters from 12” to 144”, so you can fine tune your design to maximize your storage volume in the smallest footprint.

Cost of Corrugated Steel Pipe Storage Volume Reduces with Larger Diameter Pipe Detention

For corrugated steel pipe detention systems, choosing the largest diameter allowable leads to a cheaper price for the installed system. The reason for this characteristic of the product is that the amount of steel in the pipe increases according to the perimeter of the pipe (pi)*Diameter while the volume stored in the pipe increases according to the cross sectional area of the pipe (pi)* (Diameter/2)2.

For example, the weight of 48” diameter 16 gauge corrugated steel pipe is 38.6 lbs/ft and the cross sectional area is about 12.5 sqft. The weight of 84” 14 gauge corrugated steel pipe is 82.2 lbs and the cross sectional area is about 38.5 sqft. The amount of steel in 48” diameter pipe is about 3 lbs/sqft while the amount of steel in 84” pipe is about 2.15 lbs/sqft.

There would be approximately 30% less steel required per cubic foot of storage for an 84” diameter steel pipe detention system compared to a 48” diameter system. The system footprint would also be smaller.

If the allowable depth does not allow large diameter steel pipe, then you could look at getting budget pricing on a CSP system compared to a chamber product style system.

The general rule for when the cost of a chamber system is competitive with a metal pipe underground system is when the allowable pipe diameter is 54” (4.5’) or less. If your project only has room for a system around this size, then you should get a budget proposal for a metal pipe detention system and a chamber system and compare the costs and system footprints. It is important to consider the amount of stone required for both systems in addition to the material cost.

Want help with designing an underground detention system?

Ask our engineering team for help

Contact the Engineering Department

Should the corrugated steel pipe detention be perforated or solid?

Designing corrugated steel pipe detentions fully perforated and surrounded in a stone envelope has become very common since municipalities have begun requiring some level of retention and infiltration of storm water on new projects.

Perforating the detention system has several design benefits:

1. The system size is significantly reduced
2. The system is backfilled in stone, which speeds up the installation of the detention
3. The new Georgia Stormwater manual gives full water quality treatment credit if you can show the first 1” of rain being retained and infiltrated.
4. A perforated steel pipe detention typically requires significantly less stone than a comparable chamber system.

What is the least expensive diameter of corrugated steel pipe for a detention system?

As a general rule, the larger diameter you use for an underground detention system, the cheaper it becomes per cubic foot of storage. There are some diameters that are cheaper than others because they are the largest diameter before the pipe has to be made with a heavier gauge of steel.

The least expensive per cubic foot pipe diameters are: 60”, 84”, 108”, and 144”.

Those sizes are based on the industry standard gauging guidelines, but some municipalities have heavy gauge requirements. If you have a question about a system you are designing, you can contact our engineering team. We can tell you the best practices for the specific municipality reviewing your project.

As always, if you want to send your project to our engineering team then we will put together a detention proposal that includes stage storage tables, a layout detail, and budget pricing for the system.

We typically return a proposal within a day of getting a request.

Additional Tips

Use bulkheads in lieu of manifolds

Using Bulkheads in a Detention System Reduces Cost

Metal pipe detention systems consist of individual runs of pipe joined together with a manifold on the end. The typical configuration for a detention has both ends of the system joined into a manifold, but we recommend leaving the upstream end of the system un-joined and putting bulkheads on the pipe.

The fabricated tees on a manifold a significantly more expensive than bulkhead caps, so using bulkheads reduces the system cost. It also reduces the number of band connections and speeds up the installation of the system.

Use a smaller outlet pipe

One mistake we see a lot on underground detention designs is to tie the detention pipe itself directly into the outlet control structure on a system. For larger diameter pipe detentions, that configuration leads to large precast OCS structures. Reducing the outlet pipe to 36” pipe or something similar can reduce the cost of the OCS structure by thousands of dollars.

Design the OCS weir plate inside the detention pipe

Outlet Control Structure Weir Plate Inside Detention System

A custom weir plate can be fabricated inside the pipe to eliminate the OCS structure. Designing the system with an internal weir plate can reduce the overall cost of detention, especially for large diameter systems where the required OCS box would be expensive.