Blind Inlets to Reduce Sediment Loading from Farmed Depressional Areas
A blind inlet, also known as a French drain, is a structure that is placed in the lowest point of farmed depressions or pot-holes to minimize the amount of sediment, and potentially other contaminants, that would be transported to receiving ditches or streams.
The most common practice used to drain farmed depressions is a tile riser, which is essentially a pipe that conveys surface water from the field to the receiving ditch or stream without the benefit of filtration through subsoil. A traditional blind inlet or French drain is used to filter sediment and therefore any chemicals bound to that sediment, from the surface water prior to leaving the field. A modified blind inlet additionally removes certain dissolved contaminants, such as dissolved phosphorus (P) through the use of P sorption materials, effectively serving dual purpose as a P removal structure.
How Does This Practice Work?
Replacing tile riser drains with blind inlets allow for the filtration of sediment through gravel, prior to surface water entering the subsurface tile drainage system.
To drain a 10-20 acre depression in Northeast Indiana, with a 2.5 inch (1½ year return period storm) the following steps were followed: 1) a 15’ X 15’ hole was dug 30 inches deep; 2) approximately 2 inches of # 4 limestone gravel was added; 3) a perforated pipe was installed in a 10’ X 10’ section, with the holes located at 4 and 8 o’clock (4) # 4 limestone was added to within 8 inches of the surface; 5) geotextile placed over the top of the gravel layer; and 6) added 8 inches of high quality coarse sand or washed pea-gravel.
The size of the blind inlet should be determined by the area of the depression and local precipitation patterns. An engineer should be consulted in designing a blind inlet to ensure adequate ability to remove excess runoff water.
Where This Practice Applies and Its Limitations
This practice is applicable to any landscape where surface drainage patterns result in isolated depressional areas (aka pot-holes), and the climate is sufficiently humid to result in reduced trafficability or the loss of crops due to excessive amounts of water in the depressional area.
One of the primary benefits to the farmer is the ability to drive equipment over the blind inlet, as opposed to having to drive around a standard tile riser. From the perspective of water quality, there is only a benefit when a blind inlet is constructed in place of a currently existing surface tile riser i.e. the water quality benefit of a blind inlet is only relative to the negative consequences of a tile riser.
Take caution in choosing a media for the upper sediment filter “cap” layer. Several blind inlets have clogged due to use of poorly sorted sand containing excess clay/silt particles that re-orient and reduce infiltration. Such a risk can be avoided by using washed pea-gravel for the cap.
Sediment removal, which is nearly equivalent to total P removal in most cases, varies with the texture of the surface cap material. A 12-yr study showed 40% overall sediment and total P removal (Penn et al., 2020). Traditional blind inlets will not remove dissolved P. Replacing the gravel with sieved steel slag or mixing 8% (w/w) steel turnings results in dissolved P removal via creation of a “modified” blind inlet that additionally serves as a P removal structure (Gonzalez et al., 2020).
Cost of Implementing the Practice
Costs include gravel, pipe, geotextile, and earthwork/labor. Total cost varies from 2 to 3K, where 3K is typical for a modified blind inlet that contains steel turnings or is constructed with sieved steel slag.
Operation and Maintenance
The cost of operating and maintaining this practice is minimal. If infiltration reduces below acceptable levels, remove and replace sediment “cap” layer on top of the geotextile as the underlying gravel will still be clean.
Penn, C., Gonzalez, J., Williams, M., Smith, D., & Livingston, S. (2020). The past, present, and future of blind inlets as a surface water best management practice. Critical Reviews in Environmental Science and Technology, 50(7), 743-768.
Gonzalez, J. M., Penn, C. J., & Livingston, S. J. (2020). Utilization of steel slag in blind inlets for dissolved phosphorus removal. Water, 12(6), 1593.
For Further Information
Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit https://www.nrcs.usda.gov/contact/find-a-service-center. This practice may be eligible for cost share as USDA-NRCS Conservation Practice 620 (Underground Outlet), but may require the addition of a grass buffer around the blind inlet.
|Douglas R. Smith
Editing and Design
NC State University
University of Tennessee
Penn, C. 2023. Blind Inlets to Reduce Sediment Loading from Farmed Depressional Areas. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/blind-inlets-to-reduce-sediment-loading/