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Drainage Design

Few things can cause more damage on construction site than water intrusion. It is critical for contractors to have a thorough understanding of designing and implementing effective drainage systems in various applications. For more information on the basics, you can refer to our article “Why is Drainage Necessary?” 

In this article we’ll dig a little deeper into the specific components of drainage systems and how and when to use them.


Creating a Checklist for Drainage Design

A good place to start is with a detailed checklist to define the type of drainage system, and various components you’ll need for your project. There are three primary types of designs: 

1. Fully analyze the jobsite's topography for the following elements. 

A. Record patterns of the drainage.
  • Determine if there is a source for water coming onto the site.
  • Determine the points where water is draining off of the jobsite.
  • See if the water is flowing perpendicular to the jobsite’s contour lines.

B. Assess the runoff and percolation.
  • Look for patterns starting with topographical high point, ridges, streams, swales, and valleys, and determine the flow of the water.
  • Determine the watershed locations based on the grading plan, then determine the square footage, concentration points, and low points.

C. Check your options for how you can dispose of the water, usually covered in local codes.

  • On-site disposal includes retention basins, ponds, etc.
  • Off-site disposal includes storm drains and streets
  • Natural drainage
  • Existing artificial systems
  • Planned or proposed artificial systems

2. Begin assessing the jobsite for conditions that could affect the flow.

A. Look for physical barriers and obstructions like concrete (patios, sidewalks, and drives), edging, landscaping, fences, and plants.
B. Determine the types of soils on the jobsite to understand the absorption.
C. Determine the vegetation and how it will likely affect erosion on slopes.

3. Determine where drainage structures are likely to be or where they could be established if needed.

4. Use the information from the previous steps to determine the sizes and types of drains you will need. The following sections will help you determine how to design a system that includes both surface and subsurface drainage and pipes. The pipe design should move water from the drains to the disposal point using the most direct and simplest possible route.


The following sections detail the different systems. Keep in mind that you will almost never have just one of them. By understanding the components of each of them, you will be better able to determine how to connect them for the most effective drainage system.


Surface Drainage Design

Surface drainage is needed wherever standing water occurs, or will occur, such as parking lots, garages, sloped surfaces, etc. Pooling water is a safety hazard and can cause damage to a property. By following these 10 best practices, you can ensure that your jobsite doesn’t have problems with standing water or other liquids. 

  1. Surface water will follow the path of least resistance, with gravity being your biggest asset in removing it. To make the most of gravity, you need a slope between 1 and 5% over grass swales, and a 1% slope for the inside of pipes to the place where it will discharge. Critical applications will need various slopes, in which case you should consult a professional to effectively remove the water.

  2. There are usually two types of water flow: slow and fast. Slow flows tend to create bogs, while fast flows erode the jobsite, including weakening foundations and creating gullies. The system needs to address these potential problems.

  3. Avoid draining over sidewalks or places where people will walk. Pedestrians should not have to step off of dedicated walkways to avoid large volumes of water rushing over their pathway. This is one instance when you will probably want to connect surface and subsurface designs to ensure the water can reach storm drains without adversely affecting or compromising pedestrian safety.

  4. Always account for erosion, and work to minimize it.

  5. Divide large drains into smaller drains to reduce erosion and improve the jobsite's aesthetics.

  6. You don’t have to divert the natural flow, but don’t increase the discharge when you design the drainage system.

  7. Always follow code requirements and any specified applications.

  8. Start at the location where you want to discharge the water, then work backwards to the highest point on the jobsite.

  9. Have a secondary route to handle overflow.

  10. Plan to have a fitting between every 50 and 100 feet or when the system has at least a 45-degree angle.

There are two primary types of surface drains. 

  • Direction drains are often used along with landscaping. By using the landscape, the water is moved around the property, and direction drains are often installed in lower locations and at swales.

  • Channel drains, also known as trench drains, are often used in hardscape applications. Hardscapes are locations where very little to no water will be absorbed, so it creates a greater amount of water. Trench drains provide modest slopes on these areas in a single direction instead of the water running off in multiple directions and potentially creating problems around the hardscape.

Modular Trench Drains

Hardscape surfaces should be sloped in one direction, similar to the way a roof is installed on a house to cause the water to fall to the gutters on the edges. There are three types of installation.
Drainage at Arizona Sun Devils Stadium
1.  Standard installation requires the excavation of wide trenches that are deep so that the channel and bedding can be fully accommodated. A string line is added to both ends of the drain to guide how the piping is laid. Installation of the channel begins at the discharge location, then goes backward up the trench. PVC should be cemented prior to the installation, and once the channel is installed the piping can be inserted into it. The rest of the installation will vary based on the type of traffic (vehicular, pedestrian, or none), so a professional can help ensure that your standard installation meets different needs.

2.  Suspended installations are done to retrofit current slabs and construction, suspending the channels so that they do not cause problems. After preparing the trench, use 2x4 boards to cover the full trench width, then bolt them to the grates. Add concrete around the channel.

3.  Slurry installations can be used instead of standard installations, and they act as a barrier to horizontal water flow. Complete the trench excavation, then ensure the correct elevation using a stringline. Place the concrete slurry to a depth of at least 4” in the trench. Install the channels in the slurry, then check the alignment with the grade. When the slurry finishes setting, you can add more concrete around the channel.

ACO Installation Tool

ACO makes an innovative tool to assist in the installation of catch basins.

They've created a template to automatically space and place rebar. Then the installation devices mount to the rebar and stay in place to perfectly align the channels as you install them. Once the channels are locked in place, you can install the grates and place the concrete. This tool improves placement accuracy, and also greatly increases productivity.


Installation Approaches

Contractors traditionally have either used one of two alignment methods to position modular trench drains. For shallow modular trench installation and on some types of replacement projects, contractors often use the “patty” method. Contractors place channels on small clumps of stiff/low slump fresh concrete while establishing the trench path and grade. 

For new installations that require the channels to be positioned a foot or more above the sub-grade, contractors use the hanging method. Workers attach wooden plywood supports or a cross-fit bar to cut to each channel. These supports are positioned on the trench drain’s centerline while board suspended using threaded bar supports driven into the sub grade. Final grade is established by adjusting the nut that holds the support on the threaded rod.


Tooling up for efficiency

One manufacturer, ACO Inc, has created an installation device that refines and speeds up modular trench drain positioning as compared to the hanging method. These sacrificial chairs enable workers to quickly align the channels in all three dimensions assuring alignment and assembly stability. 

Workers first layout the trench’s path, identifying where to drive in supports cut from rebar. They then slide the installation device onto the embedded rebar. Then they clamp the installation devices at each end of the channel. The workers lift the channel to the designed grade to control the elevation. Workers slip the supports onto the vertical support stake embedded into the sub grade. Once the channel is positioned to the desired elevation, workers secure the chair in its position on the rebar using a set screw. 

The special installation devices secure the entire drain system restricting any movement or float when filling the area surrounding the trench drain during concrete placement. Along with better grade control, the installation tool’s clamping feature aligns the channel and helps maintain levelness between the grate seats.


Ensuring a good placement

While these modular trench drains are easy to install, there are some practical tips that can make the installation easier and create better placement. 

Before beginning installation, determine that adjacent structural reinforcements are properly placed. In some jurisdictions, special inspections are required prior to placement.

Start the layout from the outlet. This will help maintain the drain grade’s design.

Choose sturdy support stakes. If so, be sure the threaded bars are straight and free from defects. Use either a 1/2” or 5/8” threaded rod with bolts instead of stakes to help establish grade. This will allow you finely raise and lower the channels to your string line.

During set-up, be sure to drive the support stack deep into the subgrade to establish a secure attachment. This may require longer stake lengths in areas that appear to be soft. Nearby activity from skid steers and ready-mixed concrete delivery can cause the subgrade to pump and disrupt the grade and line of placement.

When laying out the trench devices, be sure all assemblies match. Also position all fixed ends of the channels on the same side of the excavation. Some manufacturers cast directional flow arrows on the outside of the channel to help ensure proper flow.

When filling the cavity outside the modular drain’s channel with concrete, be sure to properly consolidate the concrete leaving no voids. The best way to accomplish this by filling is by filling the cavity in two passes. The first placement should fill in the cavity so that the top of the concrete is above the frost keys or very bottom of the channel to reduce the chance of floating. The second placement should be atop of the first placement filling the sides up to the top of grade. Use a low frequency internal vibrator to insure proper consolidation.

Catch Basin & Inlet Installation

Typically, installations for catch basins and inlets are fairly straightforward. However, there are over a dozen steps you need to take to ensure optimal drainage. 

  1. Understand the typical rainfall in and around the area, then select a basin or inlet that is large enough to accommodate that amount. You need to account for surface area and soil type as these elements both affect how much water will flow on the property and how much might be absorbed. You may need more than one for regions with frequent storms. Inlets are usually placed on the drain line, while catch basins are installed where runoff debris tends to collect. The primary purpose of the catch basin is to prevent the debris from clogging or obstructing the pipes.

  2. Determine the low areas where water is most likely to pool.

  3. Dig in the location, making the hole deep enough to accommodate the height of the basin and grate.

  4. Place the basin in the hole, making sure there is a firm base under it, then work backwards from the discharge location to the place where the grate is. Make sure there is a 1 to 2% slope from the trench base so that water drains in the right direction.

  5. Connect the basin and the pipe, then backfill the area.

  6. Verify that the incoming part of the drain line is elevated to the same level or higher than the outlet.
  7. Equip the catch basin outlet with an elbow that faces down so that debris can move out of the pipe.

  8. Before placing concrete, install the grate and tape it so debris is kept out.

  9. Recess the grates based on traffic. For places with low to no traffic, use a minimum grade of 1.8”, and 1/4" in areas of greater traffic.

  10. Encase basins and inlets in concrete if you anticipate a heavier load or if there are asphalt/hot mastic applications.

  11. Use PVC primer and a PVC solvent to solidify the connections between different components of the system. Use tape in places with corrugated pipe.

  12. Install basins over compacted gravel, concrete, or sand, and add holes to the basin bottom to prevent water from pooling.
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