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.


How Do We Know That Corrugated Metal Pipe is Reliable?

Corrugated metal pipe has been used in drainage applications in the United States since the late 1800’s.

For most of the 20th century, the two most frequently installed stormdrain pipe materials were concrete pipe and galvanized corrugated steel pipe.

For instance, there are ~65,000 stormdrain pipes in Gwinnett County’s GIS database with installation dates between 1941 and 1995. About 17,000 of the pipes are concrete pipe (~27%) and about 45,000 of the pipes are galvanized or asphalt coated galvanized pipe (~70%).

How have these materials performed over time?

The Problem with Galvanized Corrugated Steel Pipe in Georgia

Every stormdrain pipe material has recommended applications and environmental condition restrictions.

Corrugated metal pipe is recommended for the following environmental conditions according to the Georgia Department of Transportation:

Georgia DOT Corrugated Metal Pipe Environmental Conditions

Out of all these corrugated metal pipe products, galvanized steel pipe (zinc coated) is the most sensitive to environmental conditions.

Compare the allowable pH range for galvanized steel pipe (minimum 6.0) vs aluminized type 2 steel pipe (minimum 4.50).

The pH system is a logarithmic scale, so a 1.5 lower pH value translates to an environment that is ~30 times more acidic.

Environmental Conditions in the Atlanta Area

The USDA Natural Resources Conservation Service provides soil survey information online, which allows you to select an area of interest and download the expected soil and water characteristics based on the recorded soil surveys.

Let’s look at the soil conditions in the Atlanta area, using Gwinnett County as an example again.

The USDA Natural Resources Conservation Service provides soil survey information online, which allows you to select an area of interest and download the expected soil and water characteristics based on the recorded soil surveys. Let’s look at the soil conditions in the Atlanta area, using Gwinnett County as an example again.

If you select Gwinnett County as an area of interest and pull up the “Soil Chemical Properties”, then you can select pH values and download a table of the expected soil types and pH values. You can download the information as a PDF report (USDA Expected Soil pH Values – Gwinnett County).

We have summarized the USDA information below.

USDA Gwinnett County pH Summary

If you compare the USDA information to the recommended environmental conditions for galvanized steel pipe, then you can see why the material is underperforming in the Atlanta area.

Galvanized CMP should be installed in environments with a pH of 6.0 or higher, but just 1.5% of Gwinnett County falls within that range.

Aluminized CMP is recommended for environments with a pH of 4.5 or higher, so none of the USDA’s expected environmental conditions fall outside of the recommended range for aluminized steel pipe.

Why Does Aluminized Type 2 CMP Perform Better in More Aggressive Environments Compared to Galvanized?

Aluminum and aluminum alloys are very stable when they are exposed to an environment with pH values between 4.0 and 9.0. Included below is a chart from Uhlig’s Corrosion Handbook.

Aluminum Stability pH 4.0 to 9.0 Uhligs Corrosion Handbook

Field measurements of pH may vary, so the industry leaves some safety cushion in the recommended environmental conditions for AT2 CMP, so the recommended lower pH limit is 4.5 instead of 4.0.

How Do We Know That Aluminized CMP will Outperform Galvanized CMP in Georgia?

Let’s start with some visual comparisons of aluminized CMP and galvanized CMP installed at the same time and in the same environments.

The image below is a pipe that was manufactured partially from galvanized steel and partially from aluminized type 2 steel. The pipe was left in service for 30 years and then examined.

The visual difference between the two materials is dramatic.

Let’s start with some visual comparisons of aluminized CMP and galvanized CMP installed at the same time and in the same environments. The image below is a pipe that was manufactured partially from galvanized steel and partially from aluminized type 2 steel. The pipe was left in service for 30 years and then examined. The visual difference between the two materials is dramatic.

One of the easiest places to compare the performance of galvanized and aluminized CMP is at a headwall connection. Most precast headwalls are manufactured with a galvanized CMP stubout, so when an aluminized type 2 CMP pipe is connected to the headwall then you can see a direct comparison of the products.

Galvanized vs Aluminized at Headwall

This pipe is located in Barrow County, Georgia. It was 17 years old at the time the picture was taken. The galvanized stubout is beginning to fail at the invert but the aluminized pipeline is performing well.

The red coloration you can see in the pipe invert was easily removed with a brush and the pipe surface still has a silver sheen because of the intact aluminum coating.

The aluminized pipe was measured as part of the NCSPA’s Atlanta area study of AT2 CMP. This pipe still had more than 90% of its aluminum coating remaining when this picture was taken.

The next visual is a video that was published by Contech Engineered Solutions. They examined a 30-year old installation where a 78” diameter aluminized CMP culvert is outperforming the galvanized end section that is connected to it.

What Information is Available on the Real-World Performance of Aluminized Type 2 CMP?

Aluminized type 2 corrugated steel pipe has been around since the early 1950’s. The product has been thoroughly studied for decades.

NCHRP Report 801 “Proposed Practice for Alternative Bidding of Highway Drainage Systems” states that:

“Currently, there are more well defined methods to estimate the service life of steel culverts than any other type of material” (page 57).

Information Source 1: Minnesota DOT

Minnesota DOT published a report in 2013 titled “Improving the Selection and Testing of Culvert Pipe Materials”. They recommend using aluminized type 2 CMP because its service life is 3 to 8 times better than galvanized CMP.

A research team at the University of Minnesota presented to MDOT in 2013. The presentation was called “Culvert Pipe Materials and Durability: Making the Right Selection for Minnesota”.

The research team examined the DOT’s culvert inspection database to see how various pipe materials are performing. They recommended replacing galvanized CMP with aluminized CMP because of the better service life. Interestingly, they also state that 18% of the RCP in Minnesota DOT’s database has joint separation issues.

Information Source 2: Gwinnett County’s GIS Database

Gwinnett County has been assessing the condition of their existing stormdrain pipe infrastructure since 1999. Each pipeline was inspected and assigned a condition rating of “Good”, “Fair”, or “Poor”. This condition data was updated in their publicly available GIS database until 2006.

The database includes more than 145,000 pipelines and lists the pipe material, location, estimated date of installation, and pipe condition. The information in the database is summarized in the tables below.

The condition data was gathered internally by Gwinnett County for their own maintenance planning purposes, but the summary tables below were compiled by Southeast Culvert.

Material type acronyms:

  • RCP (reinforced concrete pipe)
  • BCCMP (asphalt coated galvanized corrugated steel pipe)
  • AT2 CSP (aluminized type 2 corrugated steel pipe)
  • HDPE (high density polyethylene pipe)
  • GALV (galvanized corrugated steel pipe)

GWINNETT All Pipes & All Time Periods

When the data is broken down by pipe material type, you can clearly see the performance differences among various material options.

  1. The worst performing pipe material in the database is galvanized CMP.
  2. Bituminous coated galvanized CMP is performing significantly better than galvanized.
  3. RCP is middle of the pack. It’s doing better than galvanized and BCCMP, but worse than HDPE and aluminized type 2 CMP.
  4. HDPE pipe is the second best performing material in the database.
  5. Aluminized type 2 CMP is the top-rated material by a significant margin.

The install dates for the database go back to 1941, so let’s look at the statistics for the oldest installations.

GWINNETT Stormdrain Pipe Conditions 1941-1995

Narrowing the time frame to the period between 1941 and 1995 reduces the number of pipelines to ~65,000. The performance differences between the various material types also becomes more pronounced.

Galvanized CMP is clearly performing worst with ~9% of the galvanized pipes in “poor condition”.

RCP, BCCMP, and HDPE are all in the same ballpark, around 3% in “poor condition.” HDPE is performing the best in that group.

Aluminized type 2 has the most pipes in “good condition” by a large margin (27% vs < 10% for the rest of the materials). It also has the least proportion of pipes in “poor condition” (~1%).

Information Source 3: Cobb County’s GIS Database

Cobb County has a GIS database that is comparable to Gwinnett County. The number of inspected facilities was smaller as of 2019, but the statistics are similar.

The condition data was gathered by Cobb County for their own maintenance purposes, but the summary tables below were compiled by Southeast Culvert.

Their database includes ~ 78,000 pipes categorized by pipe material, location, estimated date of installation, and a pipe condition assessment. They assessed each pipeline and assigned the pipe a condition of “Good”, “Poor”, and “Failing.”

Material Type Acronyms:

  • RCP (reinforced concrete pipe)
  • BCCMP (asphalt coated galvanized corrugated steel pipe)
  • AT2 CSP (aluminized type 2 corrugated steel pipe)
  • HDPE (high density polyethylene pipe)
  • Uncategorized CMP

Cobb’s statistics show a similar pattern as Gwinnett County.

  1. Aluminized Type 2 CMP has the smallest proportion of pipes in failed condition and the largest proportion in good condition.
  2. HDPE is the second best performing product.
  3. BCCMP / Uncatagorized CMP tie for second worse performing product with ~ 1 percent in failed condition.
  4. RCP is the worst performing product in Cobb’s statistics by a large margin.

Let’s look at some of the older pipe in Cobb’s database. When the data is filtered for install dates between 1980 and 2000, the number of pipelines drops to ~55,000.

The older pipes show a similar pattern. Aluminized CMP is the best performing product. BCCMP, Uncategorized CMP, and HDPE are middle of the pack, and RCP is the worst performing pipe material.

Information Source 4: Georgia DOT and Oregon DOT

Within the recommended environmental conditions, the expected service life of aluminized type 2 CMP is 75 years minimum for 16 gauge pipe and 100 years minimum for 14 gauge pipe.

Galvanized CMP has an average service life of 40 years in the recommended environmental conditions (which are not commonly found in Georgia).

Because of the performance differences between the products, most DOTs in areas with lower pH environments use aluminized CMP instead of galvanized.

The Georgia Department of Transportation updated their stormdrain standards in 2016 and revised their stormdrain pipe selection chart.

They upgraded the use of AT2 CMP to be allowed underneath roads with an expected traffic of 5,000 cars per day. GDOT uses a 75-year min service life guideline for pipe materials underneath a road.

Notice that galvanized (zinc coated) CMP is not allowed inside the roadbed on GDOT projects.

Georgia DOT Storm Drain Pipe Selection Chart 2016

The Oregon Department of Transportation explicitly lists the service life expectations for their stormdrain products in the ODOT hydraulics manual.

Galvanized CMP is listed with a service life between 15 and 40 years depending on the environment.

Aluminized CMP, RCP, and plastic pipe are all listed with a 75-year service life.

Information Source 5: “Durability of Aluminized Type 2 Corrugated Steel Pipe Exposed Throughout the United States” – Report by Elzly Technology Corporation

Aluminized type 2 has been studied on an ongoing basis since the early 1950’s. More than 125 AT2 CMP pipes were installed in coordination with 20 state and county DOT’s and revisited every 10 to 15 years for an updated performance report.

The most recent durability study was published by Elzly Technology Corporation in 2014 and included pipes that have been in service for 60 years.

The conclusions of the study support the 75-year minimum service life for 16 gauge Aluminized Type 2 CMP in culvert applications.

Culvert applications are significantly more aggressive than storm sewer applications because of bedload conditions. Storm sewer applications are considered to have little or no bedload according the FHWA guidelines.

In stormdrain applications, the 75-year minimum service life guideline for AT2 CMP is very conservative, which is why the next study’s results are so dramatic.

Information Source 6: “The Measured Performance of Aluminized Type 2 Steel Pipe in Storm Sewer Applications in the North Georgia Region” – Report by the National Corrugated Steel Pipe Association

In 2015 the NCSPA began building a database of aluminized type 2 CMP installations in the Atlanta area. In total, 140 pipes were inspected and measured for aluminum coating loss and the results were charted against the expected wear rates for the material to provide a 75-year service life.

The study locations were widely scattered across north Atlanta. Most of the pipes have been in service for more than 20 years at the time of inspection.

The results of the study show that AT2 CMP installed in stormdrain applications will provide a service life far in excess of 75 years.

The average wear rate of the aluminum coating was 0.84% loss per year. At that rate, there will be substantial aluminum coating on these pipes after 100 years of service.

The average coating wear rate for the pipes that have been installed for more than 30 years was 0.78% loss per year, which shows that the aluminum coating provides consistent protection even after decades of service.

You can find the full report on the NCSPA’s Studies and Reports webpage.


For most people, the distinction between the many types of corrugated metal pipe is blurry. Contractors, engineers, and municipal reviewers often have an opinion on the product as a general category.

A reviewer might say “we allow metal pipe” or “we don’t allow metal pipe” for a project, but the specific type of CMP material makes a big difference on the expected service life of the pipe.

In Georgia conditions, galvanized CMP does not perform very well. The soil and water pH values are generally outside of what is recommended for the product. In other regions of the country with different environmental conditions, the product performs very well and can exceed the 40-year service life expectation

Aluminized type 2 CMP has proven to perform very well in Georgia conditions, which is why it is allowed underneath major roads on Georgia DOT projects and in many Atlanta area municipalities.

If you are specifying a CMP product on a project in Georgia, then mostly likely you should use aluminized CMP.

Let us know if you have a question about whether the pipe is appropriate for a specific application on a project, or if you have any questions about CMP or HDPE products.

#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.