Erosion Control – Natural Channels & Culverts
Providing for the control of erosion is a necessary part of the complete design of any residential, commercial or highway construction project. These provisions must include measures for the control of both sheet and gully erosion during and after construction. This section is concerned with methods and criteria for the control of gully erosion in roadside channels and at culvert outlets. Control of sheet erosion is discussed in Embankment Protection and Pollution, Erosion and Sediment Control.
Control of ditch erosion is given careful consideration during design to reduce maintenance costs and to improve appearance. All ditches are analyzed to determine erosion control measures necessary to minimize maintenance. Ditch erosion is controlled by widening ditches, flattening ditch grades, or by application of proper ditch protection such as mulch, sodding, erosion control blanket, rock ditch liner and paved ditches. Economy is the prime factor for the final selection of the type of ditch protection. It is usually cheaper to widen ditches or flatten special ditch grades than to use sod, erosion control blankets, rock or pavement.
The selection of proper erosion protection consists of estimating the design flow rate at points along the channel length and determining the depth of flow and corresponding shear stress at these points, for various types of linings. The type of lining which will provide the necessary protection at the least cost is generally selected. The design flow rates at various points along the channel are computed by means of the rational formula as discussed in Hydrologic Analysis. The depth of flow in the channel is determined by means of open channel flow computations as discussed in Open Channels. The shear stress is determined by the method given below.
Ditches are designed at least 1 ft. (0.3 m) deeper than the depth of flow. Runoff and corresponding ditch flow depths and shear stresses are computed at random points as necessary to determine the limits of the various types of ditch protection required. After these limits have been determined, the design of all ditches is reviewed toward eliminating or minimizing required ditch protection by widening ditches or flattening ditch grades. Permanent conditions are analyzed in order to determine if a grassed waterway is adequate. If a grassed waterway is not adequate, then a rock ditch liner, erosion control blanket or paved ditch is specified and the design is complete. In medians it is sometimes more economical to provide for additional median drains to reduce velocity than to use ditch protection to prevent erosion. When ditches are located within the clear zone of a roadway, care must be taken to ensure the type of erosion control selected for the ditch does not negatively impact the traversability of the clear zone. Specifically, the erosion control should not snag, roll or otherwise prohibit an errant vehicle from regaining control within the clear zone. If rock ditch liner is used in the clear zone, only Type 1 is permissible. Ditches are designed without abrupt bends, where practicable. If abrupt bends are necessary, they are analyzed to avoid the possibility of overflow due to superelevation of flow and to determine the increased shear stress caused by the bend.
After the design flow rate has been computed and the channel slope has been established, the ditch section (usually a standard section) is analyzed as a grassed channel. This analysis is carried out by procedures outlined in Design of Channels with Flexible Linings. The permissible shear stress for various lining types is given in the Permissible Shear Stresses for Ditch Protection table. If the analysis shows that the permissible shear stress for a grassed channel is exceeded, then one of two things may be done. First, the ditch may be modified by widening the ditch section or flattening the ditch slope, if these actions are feasible. Second, the ditch may be lined with rock ditch liner or an appropriate erosion control blanket or the ditch may be paved. It should be noted that if the ditch is lined or paved, consideration should be given to reducing the channel dimensions. If the channel dimensions are reduced, a lined transition section should be used. The rate of channel width reduction should be on the order of 1 ft. (1 m) of channel width for each 3 ft. (3 m) of channel length. That is, each side of the channel should converge at a rate of 1 ft. (1 m) in each 6 lineal feet (6 lineal meters).