Sediment clogs our nation’s waterways, costing us billions of dollars in restoration and cleanup each year. Runoff from construction sites is one source of sediment that can be easily addressed. Following the basic design steps outlined here, a system can be developed for controlling this runoff and preventing the sediments from travelling off-site and into downstream water bodies.

By Jerald S. Fifield

The role of any structural sediment control measure is to create conditions for sedimentation; that is, to allow for the deposit of soil particles that are in suspension. When transport mechanisms that carry soil particles, such as water or wind, move at a slow rate, particles might settle out of suspension.

The Theory of Sediment Containment Systems

Sediment containment systems are hydraulic controls that function by modifying the storm-runoff hydrograph and slowing water velocities. Some of the more common names for these structures are sediment basins, sediment ponds, and sediment traps. The function of any sediment containment system is to contain suspended particles found in runoff waters.
      When designed correctly, all sediment containment systems complete the following:

1. provide containment storage volume for incoming runoff waters,

2. create uniform flow zones within the containment storage volume,

3. discharge water at a controlled rate.

Retention Versus Detention. When all runoff waters are captured, efficiency of the containment system is 100%. However, the feasibility of retaining all runoff waters from a construction site is nearly impossible. Instead of trying to retain all runoff waters, a containment system should provide sufficient volume for capturing suspended particles and still be able to drain.

Calculating Minimum-Containment Surface Areas. If a sediment containment system is to be effective for capturing a design-size particle, it must allow for the flow of water through the system and provide time for deposition of suspended particles.

Defining the Three Types of Sediment Containment Systems

Traditionally, when sediment containment structures captured runoff from 1.2 to 2 ha (3 to 5 ac.) or less, a sediment trap was used. When the contributing area to the structure exceeded these values, then a sediment basin or pond was used.
      The problem with using a single value for the total contributing area is that it does not take into account the type of soils from which runoff is occurring. A suggested method that uses suspended design-size particle criteria to distinguish which type of containment system to use appears in Table 1.

Type-1 Sediment Containment System. A Type-1 sediment containment system will require development of a structure to capture the maximum possible number of medium silt and smaller suspended particles. Since these-size particles have low settling velocities, large storage volumes, long flow-path lengths, and controlled discharges will be required. Type-1 systems are designed to have the highest possible net efficiency and are best represented by the traditional sediment basin and trap.

Type-2 Sediment Containment Systems. The Type-2 sediment containment system will capture suspended particles having faster settling velocities than particles requiring Type-1 structures. Consequently, smaller storage volumes and shorter flow-path lengths can be used. As with a Type-1 structure, these sediment control systems will also have controlled discharges. Whereas their net effectiveness for the inflow of all suspended particles may be low, Type-2 systems will still have a high apparent effectiveness.

Type-3 Sediment Containment Systems. The least effective methods to control suspended particles in runoff waters are represented by Type-3 sediment containment systems. These are not necessarily design structures, as found with Type-1 and Type-2 systems, but are often best management practices (BMPs) found on construction sites. Examples include bale and silt-fence barriers, inlet control structures, and drainage ditch-check structures. Whenever significant runoff occurs, all Type-3 systems have very low net and apparent effectiveness to control suspended particles. When runoff is very low, however, the Type-3 sediment control systems can be effective in reducing suspended particles as long as they are continuously maintained.

Minimum Sediment Containment System Volumes

In order to calculate a minimum containment system volume, it is necessary to decide upon a minimum settling depth. Theory suggests that any depth will be sufficient. Practicality dictates sufficient depth must exist to ensure sufficient deposition zones.

Type-1 Sediment Containment System Minimum Storage Volume

To compensate for actual conditions, such as short-circuiting and dead areas, it has been recommended (Goldman et al., 1986) that systems have a minimum settling depth of 0.61 m (2 ft.). Field experience has demonstrated that Goldman’s suggestion for depth is functional for construction sites. Therefore, the following criteria are suggested for providing minimum runoff volumes (VOL_RO) for sediment containment systems (SA_m = minimum water-surface area):

In order to reduce maintenance on any sediment containment structure, additional depth should be provided near inflow areas. This allows heavier particles to be immediately trapped to allow more deposition space for design-size particles within uniform flow zones.

An approximation of additional sediment-storage volume (VOL_SSV) can be found by overexcavating the bottom of the containment system by 10%, or:

Combining the results of Equation 1 and Equation 2 will provide the following minimum sediment containment volume (VOL_m) equation:

If a sediment containment system is a rectangular solid shape having sides of 0.67 m (2.2 ft.), then Equation 3 will be fulfilled. Construction of a containment system, however, will not necessarily create vertical sides. Therefore, designers should strive to create a sediment containment system volume having an average depth (D_avg) that meets the following criteria:

Environmental Protection Agency Requirements. When 4 ha (10 ac.) or more are disturbed, EPA (1992) requires that a storage volume of 252 m³/ha (3,600 ft.³/ac.) be developed to capture runoff. This is equivalent to the first 25 mm (1 in.) of runoff per unit area from a storm event of 76 mm (3 in.).

EPA selected 76 mm as being representative of an average two-year, 24-hour storm event for the nation. However, 76 mm of precipitation is not necessarily representative as an average two-year, 24-hour storm event in the Eastern United States. Review of similar maps for the Western United States demonstrates the same conclusion.

Finally, Figure 1 illustrates that only Type A (i.e., sandy) soils will produce 25 mm of runoff in response to 76 mm of precipitation. All other soils will have greater runoff values.

Click here for larger view of graph

In 1998 EPA revised its criteria. These criteria suggest that sufficient volume exists for a sediment containment system to capture the first 25 mm of runoff or to capture the runoff resulting from a two-year, 24-hour storm event.

The 1998 ruling leaves unanswered whether 25 mm of runoff is a maximum value to consider. When runoff from more impervious soils (i.e., clays) is considered, there may be large volumes generated that cannot be handled on a construction site in a practical manner.

In order to address EPA’s requirements, it is recommended that Type-1 sediment containment systems have a minimum volume by selecting the largest value as calculated by either of the following methods:

Type-2 Sediment Containment System Minimum Storage Volume

Development of a Type-2 sediment containment system has the advantage of selecting a design-size particle that has a relatively fast settling velocity. Usually these structures are smaller than a Type-1 system since they do not require large storage volumes for runoff waters. Therefore, the volume calculated by Equation 3 is sufficient for this containment system.

Type-3 Sediment Containment System Minimum Storage Volume

Since these systems represent the more "temporary" BMPs found on construction sites, calculating design volumes is usually not necessary for most structures. Thus, their net efficiency to control suspended design-size particles is very low. The only time these structures will be effective is when a small amount of runoff occurs from disturbed lands.

Summary of Type-1 and Type-2 Sediment Containment System Minimum Parameters

Table 2 provides a summary of the equations used to calculate minimum structural parameters for Type-1 and Type-2 sediment containment systems. These structures provide the greatest potential for reducing the number of suspended particles in runoff waters from construction sites.

Click here for a larger version of this table

Effective sediment containment systems require the following:

1. understanding soil-particle distribution of contributing upstream, disturbed lands;

2. identifying the design-size particle;

3. providing sufficient containment surface area, flow length, and volume for maximum net effectiveness;

4. identifying and providing a design discharge value;

5. developing a maintenance plan to ensure that an average depth of at least 0.67 m (2.2 ft.) is available at all times for the containment volume.

Designers must always be cognizant of hydrologic constraints of sediment containment and detention systems. What is provided in Table 2 is only a guideline for determining minimum parameters needed to capture design-size particles in runoff waters. Professional judgment on proper drainage designs for developing areas must never be compromised. For example, large contributory areas often require development of large, centralized detention facilities. When this occurs, designers must verify that the minimum parameters for a sediment containment system are part of the centralized detention facility. In this manner, efficient control of sediment in runoff waters will be realized.

References

Goldman, S.J., K. Jackson, and T.A. Bursztynsky. Erosion and Sediment Control Handbook. McGraw-Hill, pp. 5.1-5.31. 1986.

US Environmental Protection Agency. "Final NPDES General Permits for Storm Water Discharges from Construction Activities." Notice, Federal Register, Part II. US Government Printing Office, Washington, DC. September 9, 1992.

US Environmental Protection Agency. "Reissuance of NPDES General Permits for Storm Water Discharges from Construction Activities." Notice, Federal Register, Part II. US Government Printing Office, Washington, DC. February 17, 1998.

Jerald S. Fifield, Ph.D., CPESC, is president of HydroDynamics Inc. in Parker, CO. This article is an excerpt from Designing for Effective Sediment and Erosion Control on Construction Sites, which will be published this spring by Forester Media .