A blind inlet, similar to a French drain, is a structure that replaces a tile riser. The blind inlet is placed in the lowest point of a farmed depression or pothole so as to reduce the amount of sediment, nutrients, and other contaminants that would otherwise be transported to receiving ditches or streams. The blind inlet is an in-field practice.
Figure 44. Layers used for a blind inlet installation. Credit: Justin McBride, ODA-DSWC.
A blind inlet should be located at a field’s lowest elevation point, where drainage patterns result in reduced trafficability or crop losses occur due to frequent saturated soils. A blind inlet can be installed any place where a tile riser is recommended, or where drainage in depressional areas is causing a problem.
Water that flows through a blind inlet filters first through soil and rock before entering the tile system, compared to a tile riser where water goes through the tile system without being filtered. Compared to tile risers, blind inlets reduce plugging from debris and reduce the export of nutrients, pesticides, and sediment.
Installations of blind inlets remove field obstructions (risers) that need to be farmed around. Installations improve drainage in depressions where no tile riser exists.
- Studies in the pothole region of northeastern Indiana show that, compared to tile risers, blind inlets reduce sediment loads by 79 percent and total phosphorous by 78 percent in water moving through them (Smith et al., 2013; Smith et al., 2015).
- Accumulated sediment throughout a 12-year lifetime did not appreciably decrease flow rates or cause maintenance issues. Monitoring throughout 12 years indicated reduced soluble-reactive phosphorus and sediment loading. Soluble-reactive phosphorus removal is partly a function of the choice of gravel media. For example, steel slag will significantly improve removal compared to traditional limestone gravel (Feyereisen et al., 2015).
- As a sediment-control structure, a blind inlet has an estimated lifetime of 10 years. One hundred percent no-till management is encouraged for this practice to minimize soil disturbance and maximize its lifespan.
- Once constructed, a blind inlet requires no routine maintenance; however, care is required to avoid tearing the geotextile during deeptillage operations.
- If significant disturbance to the field surface is required, protect the blind inlet using silt fences or other soil-erosion protection.
- Try to install the inlet during dry or frozen conditions to minimize soil compaction of the zone around the inlet.
- Unlike tile risers, blind inlets do not impede farm machinery. Blind inlets can support heavy machinery traffic.
- A blind inlet will not “fix” a poorly functioning tile outlet. Properly functioning downstream drainage is required.
Figure 46. A completed blind inlet. Credit: Justin McBride, ODA-DSWC.
Figure 47. A blind inlet after a rainfall. Credit: Justin McBride, ODA-DSWC.
- Estimated cost of the blind inlet depends on the size of the inlet and raw materials availability
- NE Indiana and NW Ohio contractors are estimating costs between $1500 and $3000 for complete installation.
A blind inlet consists of coarse material (gravel) backfilled around perforated pipes, which are installed 2 inches above the bottom of the excavation. The coarse material acts as a storage zone for water infiltrating laterally through the soil and vertically through a 12-inch layer of sandy material (pit run). Pit run filter media and soil sidewalls of the excavation are separated from the coarse gravel material by a geotextile to limit clogging of the storage media that envelope the perforated drain pipes. Runoff water from the closed depression infiltrates through the filter material, slowing its journey to the tile and depositing sediment, nutrients, and pesticides in the filter material and on the soil surface surrounding the blind inlet.
Figure 45. A blind inlet during construction, showing coarse material and a perforated pipe. Credit: Justin McBride, ODA-DSWC.
Extensive design details are provided in Ohio NRCS Standard 620: Underground Outlet