Inlet Protection
Definition
A sediment filter or an excavated impounding area around a storm drain, drop inlet, or curb inlet. Used to prevent sediment from entering storm drainage systems prior to permanent stabilization of the disturbed area. This practice allows for early use of the drainage system.
Where Practice Applies
Inlet protection is a temporary measure used where the drainage area to the inlet or inlets of a storm drain system is disturbed and it is not possible to divert sediment-laden water away from the system. Storm sewers which are put into use before their drainage area is stabilized can convey large amounts of sediment to natural drainage ways. This practice should not be used to replace other sediment trapping devices, but it should be used in conjunction with these devices to help prevent sediment from being transported into the system and ultimately downstream or offsite.
- Runoff from disturbed areas larger than one acre should be routed through a temporary sediment trap or basin.
- Filter fabric is used for inlet protection when storm water flows are relatively small with low velocities.
- Block and gravel filters can be used where velocities are higher.
- Gravel and mesh filters can be used where flows are higher and subject to disturbance by site traffic.
- Sod inlet filters may be used if sediment load in the storm water runoff is low.
Advantages
- Prevents clogging of storm drainage systems and siltation of receiving waters.
- Reduces the amount of sediment leaving the site.
Disadvantages/Problems
- May be difficult to remove collected sediment, especially under high flow conditions.
- May cause erosion elsewhere if clogging occurs.
- Practical only for low sediment, low volume flows.
Planning Considerations
- Installation of this measure should take place before any soil disturbance in the drainage area. Inlet protection should be used in combination with other measures, such as small impoundments or sediment traps, to provide more effective sediment removal.
- The type of inlet protection device chosen depends on site conditions. Straw or hay bale barriers or sediment fences can be constructed around inlets. A small sediment basin can be excavated around the storm drain inlet. In other cases, gravel filters may be used around or directly over the storm sewer opening.
- The major considerations in deciding the type of protection to be used must be based on the type of inlet, the conditions around the inlet, and the area adjacent to the inlet that may be damaged or inconvenienced because of temporary ponding of water.
Design Recommendations
- Grates and spaces of all inlets should be secured to prevent seepage of sediment laden water.
- All inlet protection measures should include sediment sumps of 1 to 2 feet in depth, with 2:1 side slopes.
- The inlet protection device should be constructed so that any ponding resulting from the installation will not cause damage to adjacent areas or structures.
- The device must be constructed so that clean-out and disposal of trapped sediment and debris can be accomplished with minimal effort.
Drainage Area
The drainage area normally should be no more than one acre.
Capacity
Runoff from 10-year storm must enter storm drain without bypass flow.
Types of Inlet Protection
Straw or Hay Bale Barriers
If the area adjacent to the inlet is unpaved, straw or hay bale barriers can be constructed around the drain inlet. Permeability through bales is lower than for other types of inlet protection, such as sediment fences. Provide sufficient storage space for runoff or sufficient lineal footage of bales to allow storm flow to pass through the bales.
Excavated Drop Inlet Trap
If the area adjacent to the inlet is paved, an excavated trap can be constructed around the drain inlet.
- This method of inlet protection is applicable where relatively heavy flows are expected and overflow capability is needed. Applicable where the inlet drains a relatively small (less than one acre) flat area, on less than 5 percent slope.
- This practice works well for trapping coarse grained material. Do not place fabric under gate as the collected sediment may fall into the drain when the fabric is retrieved.
- This practice should not be used where the area is paved because of the need for driving stakes to hold the material.
- Excavated traps may be used to improve the effectiveness and reliability of other sediment traps and barriers such as fabric, or block and gravel inlet protection.
Installation
The trap should be excavated around the inlet to provide 67 cubic feet of storage per acre of drainage area to the inlet. The trap should be no less than 1 foot deep or more than 2 feet deep when measured from the top of the inlet. Side slopes should be 3:1 or flatter. Dimensions of the excavation should be based on the site conditions. Normally the traps are square. If there is concentrated flow being directed into the trap, however, then the trap should be rectangular with the long dimension oriented in the direction of the flow. When necessary, spoil may be placed to form a dike on the downslope side of the excavation to prevent bypass flow.
Common Trouble Points
- Sediment fills excavated basin and enters storm drain: Sediment-producing area too large for basin design or inlet not properly maintained.
- Excessive ponding: Gravel over weep holes may be plugged with sediment. Remove debris, clear sediment, and replace gravel.
- Flooding and erosion due to blockage of storm drain: Install trash guard.
Gravel and Wire Mesh Filter
Applicable where flows greater than 0.5 cfs are expected and construction traffic may occur
over the inlet.
Installation
A wire mesh should be placed over the drop inlet or curb opening so that the entire opening and a minimum of 12 inches around the opening are covered by the mesh. The mesh may be ordinary hardware cloth or wire mesh with openings up to 1.2 inch. If more than one strip of mesh is necessary, overlap the strips. Place filter fabric over wire mesh. Extend the filter fence/wire mesh beyond the inlet opening at least 18 inches on all sides. Place 3.4 to 3-inch gravel over the filter fabric/wire mesh. The depth of the gravel should be at least 12 inches over the entire inlet opening.
Block and Gravel Inlet Protection
This method is best for paved areas adjacent to inlets. This method uses standard concrete block and gravel to provide a small, sturdy barrier to trap sediment at the entrance to a storm drain. It applies to both drop inlets and curb inlets where heavy flows are expected and an overflow capacity is necessary to prevent excessive ponding around the structure. Concrete blocks are laid without mortar closely around the perimeter of the drain. Gravel is then placed around the outside of the blocks to restrict the flow and form a sediment pool. For slower drainage and therefore more settlement time, the concrete blocks could be eliminated and the device made entirely of gravel. Pool depth should be limited to a maximum of 2 feet. Frequent maintenance is a must for this practice.
Installation
Place wire mesh over the drop inlet so that the wire extends a minimum of 1 foot beyond each side of the inlet structure. Use hardware cloth or comparable wire mesh with one-half inch openings. If more than one strip is necessary, overlap the strips. Place filter fabric over the wire mesh. Place concrete blocks lengthwise on their sides in a single row around the perimeter of the inlet, so that the open ends face outward, not upward. The ends of adjacent blocks should abut. The height of the barrier can be varied, depending on design needs, by stacking combinations of blocks that are 4 inches, 8 inches, and 12 inches wide. The row of blocks should be at least 12 inches but no greater than 24 inches high. Place wire mesh over the outside vertical face (open end) of the concrete blocks. Extend at least 12 inches around the opening to prevent aggregate from being transported through the openings in the block. Use hardware cloth or comparable wire mesh with 1.2 inch openings. Pile gravel, 1-inch diameter or smaller, against the wire mesh to the top of the outside face of the blocks to control drainage rate.
Common Trouble Points
- Top of structure too high: A Bypass storm flow causes severe erosion.
- Blocks not placed firmly against storm drain inlet: Scour holes develop.
- Drainage area too large: Poor trap efficiency and/or sediment overload.
- Approach to drain too steep: Causes high flow velocity and poor trap efficiency. Install excavated basin in the approach.
- Sediment not removed following a storm: Sediment enters storm drain.
- Stone in gravel donut not large enough or inside slope too steep: Stone washes into inlet.
Maintenance
Remove and replace gravel over weep holes when drainage stops.
Fabric Drop Inlet Protection
If the area adjacent to the inlet is unpaved, a fabric drop inlet protection is acceptable. This method consists of a temporary device consisting of porous fabric supported by posts and placed around a drop inlet. When properly braced and sealed at the bottom, the fabric restricts flow rate, forming a sedimentation pool at the approach to the inlet. The fabric allows the pool to drain slowly, protecting the storm drain from sediment. This method of inlet protection is effective where the inlet drains a small, nearly level area with slopes generally less than 5 percent and where shallow sheet flows are expected. The immediate land area around the inlet should be relatively flat (less than 1%) and located so that accumulated sediment can be easily removed. This method cannot easily be used where the area is paved because of the need for driving stakes to hold the material.
Height of fabric
1.5 ft maximum, 1 foot minimum; measured from top of inlet.
Stability
Structure must withstand 1.5-foot head of water and sediment without collapsing or undercutting.
Support posts
Steel fence posts or 2 x 4-inch wood, length 3 foot minimum, spacing 3 foot maximum; top frame support recommended.
Fabric material
Synthetic, extra-strength fabric. Burlap is acceptable for short-term use only (60 days or less).
Installation
Space support posts evenly against the perimeter of the inlet a maximum distance of 3 ft apart and drive them at least 8 inches into the ground. The stakes must be at least 3 feet long. Overflow must fall directly into the inlet and not on unprotected soil. Build a supporting frame of 2 x 4-inch lumber, maximum height 1.5 ft above the drop inlet crest. The frame adds stability and serves as a weir to control storm overflow into the drop inlet. Alternatively, use wire fence (14 gauge minimum, with a maximum mesh spacing of 6 inches) to support fabric. Stretch fence with top level to provide uniform overflow. Extend wire 6 inches below ground. Excavate a trench approximately 8 inches wide and 12 inches deep around the outside perimeter of the stakes. Cut fabric from a single roll to eliminate joints. Place bottom 12 inches of fabric in trench adjacent to the drop inlet. Fasten fabric securely to the posts and frame or support fence, if used. Overlap joints to the next post. Backfill the trench with 3.4 inch or less washed gravel all the way around. Do not place fabric under grate as the collected sediment may fall into the drain when the fabric is retrieved. Stabilize disturbed areas immediately after construction.
Common Trouble Points
- Posts and fabric not supported at the top: Results in collapse of the structure.
- Fabric not properly buried at bottom: Results in undercutting.
- Top of fabric barrier set too high: Results in flow bypassing the storm inlet or collapsing structure.
- Temporary dike below the drop inlet not maintained: Results in flow bypassing storm inlet
- Sediment not removed from pool: Results in inadequate storage volume for next storm.
- Fence not erected against drop inlet: Results in erosion and undercutting.
- Land slope at storm drain too steep: Results in high flow velocity, poor trapping efficiency, and inadequate storage volume. Excavation of sediment storage area may be necessary.
Sod Drop Inlet Protection
A permanent grass sod filter area around a storm drain drop inlet in a stabilized, well vegetated area.
Where Practice Applies
- Where the drainage area of the drop inlet has been permanently seeded and mulched and the immediate surrounding area is to remain in dense vegetation.
- This practice is well suited for lawns adjacent to large buildings.
- The drainage area should not exceed 2 acres,
- The entrance flow velocity must be low, and
- The general area around the inlet should be planned for vegetation.
Other Inlet Protection Practices
There are several types of manufactured inlet filters and traps which have different applications dependent upon site conditions and type of inlet. One is a catch basin filter that prevents sediments and other contaminants from entering storm drainage systems. The catch basin filter is inserted in the catch basin just below the grating. The catch basin filter is equipped with a sediment trap and up to three layers of a fiberglass filter material.
This is a changing field. New products are being developed and brought to the market. For the most recent information see a trade journal such as Erosion Control or Land and Water. For example, Eco-Blok (www.eco-blok.com) provides an inlet protection product from recycled plastic. Another source for inlet protection is www.thebmpstore.com.
Maintenance
All trapping devices and the structures they protect should be inspected after every rain storm and repairs made as necessary. Sediment should be removed from the trapping devices after the sediment has reached a maximum of one half the depth of the trap. Sediment should be disposed of in a suitable area and protected from erosion by either structural or vegetative means. Temporary traps should be removed and the area repaired as soon as the contributing drainage area to the inlet has been completely stabilized.
Systems using filter fabric
Inspections should be made on a regular basis, especially after large storm events. If the fabric becomes clogged, it should be replaced.
Systems using stone filters
If the stone filter becomes clogged with sediment, the stones must be pulled away from the inlet and cleaned or replaced. Since cleaning of gravel at a construction site may be difficult, an alternative approach would be to use the clogged stone as fill and put fresh stone around the inlet.
References
Massachusetts Department of Environmental Protection, Office of Watershed Management, Nonpoint Source Program, Massachusetts Nonpoint Source Management Manual, Boston, Massachusetts, June, 1993.
Minnick, E. L., and H. T. Marshall, Stormwater Management and Erosion Control for Urban and Developing Areas in New Hampshire, Rockingham County Conservation District, August 1992.
North Carolina Department of Environment, Health, and Natural Resources, Erosion and Sediment Control Field Manual, Raleigh, NC, February 1991.
U.S. Environmental Protection Agency, Storm Water Management For Construction Activities, EPA-832-R-92-005, Washington, DC, September, 1992.
Washington State Department of Ecology, Stormwater Management Manual for the Puget Sound Basin, Olympia, WA, February, 1992.