Increases in runoff volume and velocity lead to increased erosion (i.e., gullying), sediment transport, and offsite delivery (sedimentation). These increases must be addressed when implementing sediment and erosion control measures. Runoff-control measures are practices that mitigate the erosive and sediment transport forces of stormwater during and after construction activities. Some examples of runoff control include:
- Outlet protection
- Diversion dikes and swales
- Temporary slope drains
- Rock-lined channels
- Check dams
- Grass-lined channels
- Temporary stream crossings
Erosive kinetic energy associated with runoff waters can be expressed as:
KE = mV2 [Equation 1]
where: KE = Erosive kinetic energy of runoff
m = Mass of water
V = Runoff velocity
Notice when the flow velocity is reduced by 50%, the erosive energy is reduced by 75%. Thus, reducing flow velocity is one method to reduce erosion.
Assume V0 = the initial flow velocity and V1 = reduced velocity that is 50% of V0.
D KE = (KE0 – KE1) ¸ KE0
= (m V02 – mV12) ¸ mV02
= V02 – (0.50 x V0)2 ¸ V02
= 1 – 0.25
D KE = 75%
Rill and gully erosion is caused by concentrated runoff. To reduce runoff energy, erosion control practices that follow the “4 D’s” should be implemented:
- Decrease the amount of runoff.
- Detain water to decrease flow velocity.
- Divert runoff to less erodible areas.
- Dissipate runoff away from the site.
Methods that reduce runoff velocity (e.g., check dams, vegetated channels, or riprap) will also reduce the potential for sediment transport. Reducing the runoff energy of stormwater can also be realized by conveying flows through or along nonerodible surfaces such as culverts, slope drains, and open channels.
Diversion dikes intercept and divert runoff waters that are conveyed in drainage swales. Transporting runoff in open drainage channels can minimize erosion. This results in reduction of pollutant loading associated with runoff. Appropriate uses of diversion structures include the following:
- Diverting runoff from upslope drainage areas away from disturbed areas and toward a stabilized outlet.
- Diverting sediment-laden runoff from a disturbed area to a sediment containment structure.
- Using an upslope dike or swale to improve working conditions at the construction site and prevent erosion.
- Using a swale or dike to ensure that sediment-laden stormwater will not leave the site without treatment.
- Using methods that are sometimes preferable over a silt fence because they are more durable, less expensive, and require less maintenance when properly constructed.
The channel behind a dike must have a positive grade leading to a stabilized outlet. When the channel slope is less than or equal to 1%, stabilization might not be required on most soils. However, channel slopes greater than 1% must be stabilized to prevent degradation.
Sediment-laden waters must be diverted into a temporary sediment trap or sediment basin. In addition, runoff from undisturbed lands must discharge onto an outlet protection system. Once these two conditions are met, downstream impacts will be minimized.
When using diversion dikes and drainage swales, follow these guidelines:
- Install and utilize permanent dikes, swales, and ditches early in the construction process.
- Stabilize conveyance systems. High flow velocities will require a lined ditch.
- Use onsite material to develop temporary dikes or swales.
As with all structural measures to control sediment and erosion, frequent inspections and maintenance must occur with diversion dikes and swales. The following guidelines should be used when conducting inspections and maintenance:
- Inspect temporary facilities before and after significant storm events and at least once per week during the rainy season.
- Channel linings, embankments, and channel beds must be inspected for erosion and sedimentation.
- Removal of debris and prompt repairs should always occur.
Design and Installation of Slope Drains
Constructed slopes are often exposed to erosion between the time they are graded and the time they are permanently stabilized. During this period, the steep land is vulnerable to erosion. Fortunately, slope drains and diversions can provide valuable protection.
Slope drains consist of flexible tubing, pipe, or other conduit extending from the top to the bottom of a cut or fill slope. It is important that slope drains be sized, installed, and properly maintained since their failure could result in severe slope erosion. Entrance to the drain must be well entrenched and stable. Increased inflows can be realized by installing flared end sections on the pipe. Finally, the drainpipe should extend beyond the toe of the slope to a stable area or appropriately stabilized outlet. Other points of concern to consider during the design and maintenance of slope drains are:
- Failure from overtopping because of inadequate pipe openings.
- Low-diversion channel capacity from lack of maintenance.
- Small diversion berms height.
There are two types of slope drains. The first, basin slope drains, are temporary structures used to convey runoff from either onsite or offsite watersheds. Determining the diameter of pipe to select is based on the equation for orifice flow.
Q = 0.60 x A x (2 x g x H)0.5 [Equation 2]
where: Q = Orifice flow (ft.3/sec.)
A = Cross-sectional area (ft.2)
g = 32.2 ft./sec.2
H = Hydraulic head (ft.)
By assuming the hydraulic head does not exceed the pipe diameter (i.e., H = 2r = D), the following steps illustrate how pipe sizes can be selected to collect basin flows for specific design storm events.
Q= 0.60p x (D¸ 2)2 x (2 x 32.2 x D)0.5 [Equation 3]
When flows into the pipe are assumed to be equal to runoff from a small basin, then:
Q=3.78 x D2.5=Qu x Am i x RO Equation 4
where: Qu = Unit peak discharge (ft.3/mi.2 [csm] per in.)
Ami = Basin area (mi.2)
RO = Runoff (in.)
csm = cubic feet per second of square mile of area.
Finally, knowing that 1 mi.2 is 640 ac., Equation 3 can be solved for the pipe diameter (in inches):
D = 0.531 (Qu x Aac x RO)0.4 [Equation 5]
Equation 5 identifies a slope-drain-pipe diameter when the runoff from a small basin is known. It also provides a method for calculating the diameter of slope drainpipes to intercept runoff for a design storm event. It also makes it possible to develop general guidelines for selecting a slope drainpipe when certain assumptions are made.
Consider offsite basins that are 2 ha (4.9 ac.) or smaller and assume times of concentration do not exceed 12 minutes. Corresponding SCS Curve Number Qu values, when the time of concentration is 12 minutes, are as follows:
Type I = 405 csm/in. Type IA = 153 csm/in.
Type II = 800 csm/in. Type III = 565 csm/in.
Since a Type-II SCS rainfall distribution is dominant throughout the US, Equation 4 can be rewritten using the Type-II value:
DII = 7.7 (Aac x RO)0.4 (in.) [Equation 6]
By using the SCS Curve Number methods, runoff values can be calculated for different soil types and vegetative covers. This has been done in Table 1, which gives slope-drain-pipe diameters for various precipitation events when sandy and loam/silt/clay soils exist. Figure 2 illustrates a typical slope drain for runoff from small basins.
Once values are determined for a Type-II storm, conversion to other SCS rainfall distributions can be developed by the following:
Dnew ¸ DII = (Qnew ¸ QII)0.4 [Equation 7]
- Determine the pipe diameter using Table 1. If located where an SCS Type-II rainfall distribution does not exist, apply a correction factor. When an offsite area exceeds 2 ha (4.9 ac.), more detailed evaluations are recommended.
- Install the pipe through the diversion berm. After backfilling, the soil must be at least 45 cm (18 in.) above the top of the pipe.
- Install a containment dam in the diversion channel that is about 15 cm (6 in.) lower than the top of the berm to allow for spilling of excess waters.
- Position the pipe in a manner that discharges water into a rock-lined drainage ditch, stream, or bale energy dissipater. Avoid discharging directly into an embankment.
- Secure the pipe, either with stakes or bales, to keep it from moving when water flows in the conduit.
- Protect the dike and dam against erosion.
The following conversions shall be used for other SCS rainfall distributions:
Type-I Value = Type-II Value x 0.76
Type-IA Value = Type-II Value x 0.52
Type-III Value = Type-II Value x 0.87
Slope drains must be continually inspected to ensure that they function in a proper manner. Any damage to the containment basins above the pipe must be immediately repaired. If evidence exists of pipe movement down a hill, additional means of securing the conduit must occur.
The pipes must be inspected after each precipitation event to decide whether runoff waters are continuing to be adequately controlled. If evidence exists of erosion, it might be necessary to install additional mitigation measures.
