Above-Grate vs. Below-Grate Inlet Protection: What Does My Site Need?

Sediment control measures at storm drain inlets essentially work one of two ways. One approach is to intercept sediment at the surface, before it enters the inlet at all. The other is to place the filter underneath the storm grate and catch the sediment as it drains with the runoff.

Both product types are meant to keep sediment out of the stormwater system…so why two styles? How do you choose between above-grate and below-grate inlet protection? Basically, it comes down to the shape of the inlet and the way runoff reaches it.

Take a closer look at how each product works and so you can select the right one for each inlet at your site.

Above-Grate vs. Below-Grate Inlet Protection

Above-Grate Inlet Protection

Above-grate inlet protection sits at the surface around or over the storm drain opening. Runoff encounters the filtration material on its way toward the inlet. Sediment is captured outside the drainage structure while filtered water continues toward the storm drain.

Surface placement makes accumulated sediment easy to see during routine inspections. Crews have clear access for cleaning because the captured material remains above the grate.

Still, the inlet configuration still has to match the above-grate product. Dandy has options available for flat storm grates as well as curb openings (which receive water laterally). Keep flow volume in mind, too — the protection product will need to preserve adequate drainage while it filters the runoff.

➨ Choose Above-Grate Protection When You Have Room Around the Inlet

The U.S. Environmental Protection Agency classifies above-grate products as “external inlet protection.” These kinds of sediment controls require more space for stormwater ponding and are, generally speaking, more effective. If you slow runoff outside the inlet, it gives suspended material an opportunity to settle as the filtering material captures the sediment.

It also gives crews immediate access to accumulated sediment. Inspectors can see buildup from the surface and maintenance crews can remove it without opening the storm drain. Convenient!

However, you do have to be aware that above-grate inlet protection is exposed to traffic around the site. Any disturbances from vehicles or foot traffic could damage or move the device and create gaps that let sediment through.

Choose an above-grate option when you’re confident it won’t be disturbed, and the surrounding surface accommodates the filter and any temporary ponding that could come along with it.

Below-Grate Inlet Protection

Below-grate inlet protection puts the filtration system beneath the storm grate. Runoff enters through the grate, then encounters a suspended filter that captures sediment inside the inlet structure.

The tradeoff is where sediment collects. Crews will need access beneath the grate for inspections and sediment removal, though, so plan ahead for that. Filtration performance decreases when sediment controls become full. While below-grate inlet protection is less visible, routine maintenance is no less important, so keep it top of mind.

➨ Choose Below-Grate Protection When Surface Space Is Limited

The biggest benefit of this design is that it keeps the filtration device out of the surrounding surface area. If your storm drain is located within a roadway or another heavily traveled surface, an external filter might occupy too much space and act as an obstacle around the drain. Ponded runoff on a roadway also interferes with traffic.

Choose below-grate inlet protection and move your sediment filtration inside the drainage structure when the surface needs to remain clear — for example, if there’s a lot of site traffic or ponding would be a problem. The EPA agrees: their guidance says that internal inlet controls are best for areas with high construction traffic or where roadway flooding is a concern.

Match the Filter to the Inlet

Okay, after you’ve compared above-grate vs. below-grate, you’ve narrowed the field. However, the category alone does not a product selection make.

Look at the inlet type and dimensions next. Then account for expected runoff volume and sediment load. Maintenance requirements should match the access crews will have throughout construction.

Dandy Products has options for both above and below-grate configurations along with products for curb inlets and other storm drain applications. Browse our stormwater management FAQs for additional guidance on sizing and product selection, or contact Dandy with the inlet dimensions for help identifying an appropriate solution.