A plan for controlling erosion and sediment during construction will be prepared by the Roadside Environmental Unit and incorporated into the final project plans. The engineer developing the drainage plans will be responsible for the following items relating to sediment and erosion control:

A-BASINS

These are large sediment trapping facilities composed of a dam, storage/trapping area and an outlet spillway structure. They are generally limited in use to disturbed areas of 5 acres or more and require thorough analysis and design to insure;

• adequate storage volume for expected sediment
• adequate retention to allow settlement
• a dam and spillway capable of handling expected flow

The hydraulics engineer will include design details and recommendations for A-Basins at sites identified as potential locations during the initial design process. These will be included in the preliminary right-of-way plans and available for review during the preliminary field inspection. If the recommended basins are not felt to be required by Roadside Environmental or construction personnel responsible for erosion and sediment control during construction of the project, they are to be deleted from the plans. If additional or alternate sites are identified and requested by others for addition to the plans, detailing and right-of-way requirements will be developed for inclusion in the plans.

Detailing of the basins must be site specific to fit the local topography. Appendix K (sheet 1 of 2 , sheet 2 of 2) provides special detail sheets for documentation of the basin design. The following criteria will provide some minimum limits, special details and general guidance in the basin design.

Storage/Trapping Area

• Minimum storage below top of riser:

  2700 cubic feet per disturbed acre

• Minimum surface area:

  Q₂X 350 ft²

• Plan dimension:

  minimum length = 2 X width at dam

• Excavation:

  If the design requires excavation to attain minimum storage, slopes are to be 2:1.

Spillway (Riser)

• Minimum riser diameter: 15 inches

• Minimum riser height above barrel invert:

  2 X riser diameter, not to exceed 9.0 feet

• Diameter of riser is equal to barrel

• Riser hydraulic requirement:

  The riser must convey the Q₂ discharge with a head no greater than 0.5 times the diameter or 1.0 foot, whichever is less. A weir coefficient of 3.0 is to be used for the analysis of diameters of 15-36 inch, 3.5 for 36 inch and greater.

Spillway (Anti-flotation)

• Minimum depth of riser below barrel invert: 1.0 foot

• Weight of filter stone and trash rack are not considered in computing ballast force.

• Weight of the riser, steel base plate (if used),a portion of the barrel (2X diameter), and ballast (concrete or stone) are considered in computing ballast force.

• Weights to be used in computation:

  Concrete = 86#/cubic foot

  Stone = 62#/cubic foot

  Steel Plate = 9#/square foot(0.25 in. thick)

  CS Pipe:

  • 15"- 10 #/LF
  • 18"- 13 #/LF
  • 24"- 17 #/LF
  • 30"- 26 #/LF
  • 36"- 31 #/LF
  • 42"- 51 #/LF
  • 48"- 58 #/LF
  • 54"- 65 #/LF
  • 60"- 90 #/LF

• Volume of the entire riser above the invert and a portion of the barrel equal to twice the diameter are to be used to compute the buoyancy force.

• A minimum safety factor of 1.2 is required.

Spillway (Overflow)

• Must be founded entirely in natural ground including side slopes.
• Elevation must be 1.0 feet above top of riser.
• Must be adequate in size to convey the entire Q₅₀ discharge with a maximum weir head of 1.5 feet. A weir coefficient of 3.0 is to be used for spillway performance analyses.

Embankment

• Slopes 2:1 or flatter

• Minimum top width: 6 feet for 15"-36" / 8 feet for >36"

• Minimum top elev. = Q₅₀ WS + 0.5

• Maximum Height: Not to exceed 12 feet above lowest toe or barrel invert

CONSTRUCTION PHASING FOR BOX CULVERTS

This is a recommended step by step plan for the construction of a structure including requirements for; temporary handling of flow, required temporary erosion control items, and structure staging. This is an identified acceptable method. There may be others that are more appropriate and acceptable. This should be discussed and an agreed to plan developed during the field inspection. The final phasing plan must include:

• A means of handling flow through the site(ex. diversion pipes or channels)

• A sequence of construction and appropriate sediment controls

• Placement and sizing of a stilling basin for storage of pumped effluent for de-watering

• Detailing of any temporary easements required

PERMANENT CONTROLS

Permanent control measures such as ditch lining and pipe outlet protection are included in the drainage plan recommendations and report.