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Flow Free Grates: Products

 can be applied in a variety of situations to capture debris. Here are examples of the most common applications:

Prevent Blockage of Culvert Inlets:

Flow-Free-Grates for culverts 1

By far the most common application of FLOW-FREE GRATES is to prevent blockage of culvert inlets.

The economic and environmental cost of a plugged culvert can easily be a hundred times more than the cost of a single grate. Each situation is unique and should be carefully evaluated to determine which size and model is most appropriate for your needs and where to install it.

FLOW-FREE GRATES are designed to trap large material and accumulations of debris that will obstruct your culvert inlets and other stormwater drainage components. Research has shown that most debris blockages formed during an average storm begin with just a few pieces of woody debris not much longer than the culvert’s diameter. FLOW-FREE GRATES are not intended to trap small size material, sediment and other pollutants that are normally carried by average flows. FLOW-FREE GRATES will however, capture the pulses of small material transported by high flows before it reaches the culvert.

Some organizations discourage the use of trash racks for fear that accumulated debris in the rack will result in a clogged culvert faster than one without a rack. This is because most trash racks are placed against the culvert inlet. But a properly installed FLOW-FREE GRATE placed upstream of the culvert inlet eliminates this problem. The recommended minimum distance between the culvert inlet and the FLOW-FREE GRATE is 1.5 times the culvert diameter. So for example with a 24-inch diameter culvert, the FLOW-FREE GRATE should be installed at least 36 inches upstream of the inlet.

flow-free grates for culvertsflow-free grates for culverts

flow-free grates for culvertsWhen a FLOW-FREE GRATE becomes obstructed with debris, the backwater effect causes water to flow over and around the grate and back into the channel before reaching the culvert.

A FLOW-FREE GRATE must be installed where it will not cause water levels to overflow the channel banks or cause bank erosion

A FLOW-FREE GRATE can also be installed in an alternate manner that will encourage debris to pass through a culvert instead of trapping it. This method can work well under some situations such as where channels leading to the culvert are much wider than the inlet. In this application, the FLOW-FREE GRATE is installed in the channel parallel to the direction of flow. Floating debris carried downstream will then hit the upstream edge of the grate and be forced to pivot so that it will enter the culvert longitudinally.

flow-free-grates for culverts

Prevent Accelerated Erosion in a Ditch:

flow-free grates for culvertsDrainage ditches usually have a minimum grade to

1) force water to flow deliberately to its intended destination (such as a culvert inlet) and 2) to have enough velocity to be “self cleaning.”

Sometimes, however, it is necessary for ditches to have much steeper gradients. Heavy runoff on steep grades can create flow velocities that are capable of severely eroding an un-lined channel.

flow-free grates for culvertsFLOW-FREE GRATES can be used to incrementally capture debris that would otherwise end up at a culvert inlet. This debris will act as a baffle to slow the velocity of water and reduce its erosive effects. Where debris is lacking such as construction sites and post-wildfire settings, straw bales or flakes of straw bales may be used. This application can also be made to serve as a non-permanent grade control structure or a check dam.

flow-free grates for culvertsRemoving debris from a FLOW-FREE GRATE is simple compared to a clogged culvert:-

The FLOW-FREE GRATE should be much easier and safer to access than the culvert inlet.flow-free grates for culverts

The FLOW-FREE GRATE can be cleared quickly and easily with a pitchfork or rake.

There is no need to stand in the channel to reach into the culvert opening. Cleaning a clogged culvert can be a very difficult, expensive and messy project which can be avoided by using our grates.

Prevent Animal Access into a Culvert:

flow-free grates for culvertsCulverts can be very attractive hiding places for pets, and wildlife such as skunks, raccoons, mountain lions, alligators and other creatures that you may not wish using this space. FLOW-FREE GRATES can be highly effective at preventing animal access into a culvert by placing a grate against each end of the culvert. Note that this application is contrary to the recommended method for debris protection. If the upstream channel is likely to carry debris capable of obstructing the culvert, we recommend you either install additional FLOW-FREE GRATES upstream to capture this debris higher up or move them prior to the rainy season to accomplish both objectives!

flow-free grates for culverts


FLOW-FREE GRATES are built in two sizes and in various configurations to suit your specific situation. Choosing the right size and configuration is critical to the success of your application. General recommendations are provided below but we recommend you consult with an expert in stormwater management to insure you select the proper model and install it correctly.



Overall Height

Above Ground Height

Horizontal Opening

Culvert Diameter/ Channel Width*

























* Culvert size recommendations are general.

** These models have smaller openings for their size to capture smaller material. Although these models capture more debris, they also require more frequent inspections and cleaning.

flow-free grates for culvertsFLOW-FREE GRATES are solid steel and built with quality workmanship. The powder coat finish is more resistant to cracking, peeling, abrasion, corrosion and chemical deterioration than paint and is environmentally friendly because it uses no solvents and has no VOC emissions.

If using a sledgehammer to install your FLOW-FREE GRATE into hard material, we recommend placing a piece of plywood or lumber across the top rail for a striking surface. This will protect the finish during installation. Powder coated finishes will be damaged by striking it with a steel face hammer.

A quality automobile finish paste wax can be applied annually to extend the life and richness of the powder coat finish if you wish.

Eventually the finish will succumb to abrasion by debris and deterioration from the elements. Oxidation (rust) will then develop where steel becomes exposed. Despite this, no loss of strength should occur for many years.

flow-free grates for culvertsYour local soil, water and weather characteristics will have a strong influence on the rate of oxidation of both your FLOW-FREE GRATE and culvert. Some soils have corrosive properties that can reduce the service life of steel and concrete culverts. High-density polypropylene (HDPE) culvert can be much less effected by this type of deterioration but are vulnerable to other impacts such as fire and chemicals.

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