The Myth of Barriers to Remove Sediment From Runoff Waters

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All figures courtesy of Jerald S. Fifield
Figure 1. Fiber log barriers on a bare ground hillside

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By Jerald S. Fifield

Today, a multi-million-dollar industry provides a variety of products to meet EPA’s mandate of installing construction site best management practices (BMPs) to minimize pollutants in the discharge of runoff waters. On any construction site, one will find numerous barriers to perform this task, including silt fence, straw bale, fiber roll, and geosynthetic structures around homes and inlets or in drainage channels.

What is unknown is the effectiveness of barriers to remove sediment from runoff waters. One would like to believe that the installation of temporary sediment control BMPs would meet EPA’s mandate. Unfortunately, what is overlooked is that even if the installation, inspection, and maintenance of barriers occur in an optimal manner, sediment (i.e., a pollutant) nearly always continues to discharge from construction sites during runoff events.

The more effective BMPs that truly minimize pollutants in runoff waters are those that lessen their creation, such as erosion control practices. Unfortunately, because excavating earthen material, developing roads, building homes, and so forth cannot occur by implementing only erosion control practices, contractors have to rely upon temporary sediment control measures such as barriers.

Figure 2. Illustration of a barrier capturing runoff

Figure 3. Spacing of barriers for total containment of sheet flow conditions

Effectiveness of Barriers to Remove Sediment From Sheet Flows
It is unfortunate that while some erosion control BMPs are tested for their effectiveness (e.g., rolled erosion control products like blankets and mats), few temporary sediment control measures undergo similar scrutiny. This article tries to overcome this shortcoming by providing a technical insight into their ability to remove sediment from runoff waters.

Consider the use of barriers to remove sediment from runoff waters if installed as illustrated in Figure 1. As long as runoff does not overtop the barrier and failure of the structure does not happen, sediment removal will occur by containment of water. Unfortunately, total containment of runoff rarely occurs during significant rainfall events, and failure is often evident as shown in Figure 1. Thus, one must be careful when specifying or requiring barriers on bare ground to remove sediment from runoff waters.

Two possible scenarios for containment of sheet flows by a barrier are illustrated in Figure 2. The “sediment control scenario” demonstrates how a barrier causes sedimentation by creating a containment pond for runoff, which is dependent upon the contributing upstream area as represented by Equation 1:

A_sc = L_sc x W
(Equation 1)

where

A_sc = Maximum contributing area before overflow conditions (m² or ft.²)

L_sc = Maximum contributing flow length before overflow conditions (m or ft.)

W = Width of barrier (m or ft.)

Fifield (2011) demonstrated that the maximum upstream flow length before overflow conditions occur is represented by Equation 2:

L_sc = a₁ x (50 x Y²) ÷ (Z x RO)
(Equation 2)

where

α₁ = 1,000 (SI units) or 12 (imperial units)

Y = Height of barrier (m or ft.)

Z = Percent slope of contributing land

RO = Runoff per unit area (mm or in.)

Figure 3 illustrates results of Equation 2 for different barrier heights and land slopes when 50 millimeters (2.0 inches) of rainfall falls on Type C soils. Notice that with a 300-millimeter- (12-inch-) high barrier, spacing on 10% slopes is about 70% longer than what is needed for 33% slopes before overflow conditions occur. Similar patterns will exist for other rainfall events.

Figure 3 also illustrates that spacing of barriers on hillsides for sediment control should include an evaluation of design storm event conditions. In some cases, anticipated runoff values may be more critical than topographic features and height of barriers.

It is important to note that Equation 2 and Figure 3 do not provide information on maximum spacing between barriers for erosion control. The bottom illustration of Figure 2 depicts how control of erosion on bare ground can be achieved with barriers. Essentially, when a pond is created between barriers that ensure upstream runoff will discharge into contained waters created by a downstream barrier, then erosion control conditions exist.

The maximum distance between the two barriers for erosion control (L_ec) before overflow conditions occur is represented by Equation 3:

L_ec = ^Y/_Z x (Z² + 10⁴)^½ ≈ 100 x ^Y/_Z
(Equation 3)

Figure 4. Diversion of runoff by a barrier in front of a street inlet on a grade

Figure 5. Contained runoff in front of a sump inlet

Figure 6. Hypothetical illustration of contained runoff in front of a barrier

Figure 7. Contributing runoff areas vary for different street slopes.

Comparing Equation 2 with Equation 3, one will observe that distances between barriers for erosion control is independent of runoff, but directly proportional to barrier height. Thus, to use barriers for erosion control on steep hillsides, one needs to be concerned only with the height of a downstream barrier and land slope. When comparing results of Equation 2 to those of Equation 3, it will become evident that barrier spacing for sediment control is about 80% longer than what is needed for erosion control.

In summary, it is a myth that any spacing of barriers on bare ground steep slopes will control erosion. Erosion control on bare ground slopes will occur only when barriers are spaced closely together.

Effectiveness of Barriers to Remove Sediment From Concentrated Flows
Within any development, it is common to find barriers in front of inlet openings as construction of homes or other buildings occurs. Sometimes referred as “inlet protection,” these mislabeled BMPs (why do inlets need protection?) are often perceived as providing effective sediment control. This perception is usually incorrect!

On-Grade Inlet Barriers. Because subdivision streets are rarely flat (except perhaps in Florida), installing barriers in front of on-grade inlets is a major cause of downstream flooding and sedimentation. Instead of capturing and treating runoff, on-grade barriers usually become major diversion structures.

Figure 4 illustrates a rock barrier diverting runoff around an on-grade inlet. Notice that while some debris is caught by the rock material, little (if any) removal of sediment from runoff waters is evident.

Ironically, when there is a regulatory requirement to install barriers in front of on-grade inlets, the mandate may be creating downstream noncompliant conditions. Thus, contractors should not be held responsible for downstream ramifications if they were installing barriers as required by an approved stormwater pollution prevention plan (SWPPP).

Sump Inlet Barriers and Sedimentation. When runoff encounters sump barriers, water will either flow through the material and/or create a pond as illustrated in Figure 5. Notice that contained runoff in front of a sump inlet barrier has a geometric surface area that approximates one half of an ellipse. Using what is illustrated in Figure 6, Fifield (2011) showed that a net contributing area of runoff to the structure before overflow conditions develop can be estimated by Equation 4:

A_RO = α₁ [5,236 ÷ (Z_a x Z_b) x Y³ – V_bar + V_SEEP] ÷ RO
(Equation 4)

where

A_RO = Maximum contributing area before over-flow conditions occur (m² or ft.²)

α₁ = 1,000 (SI units) or 12 (Imperial units)

RO = Runoff (mm or in.)

V_bar = Volume of runoff displaced by the barrier (m³ or ft.³)

V_SEEP = Volume of seepage through the barrier (m³ or ft.³)

Y = Height of barrier (m or ft.)

Z_a = Side (a.k.a. curb line) slope (%)

Z_b = Crown (a.k.a. street) slope (%)

Figure 7 illustrates the results of Equation 4 when impermeable barriers are installed in front of a sump inlet. Notice that when relatively flat streets exist, contributory runoff areas are larger than for steep street slopes, which may result in localized flooding problems.

Seepage Through Barriers. Because most sump inlet barriers are not impervious, seepage through porous material will occur. Unfortunately, scant data are available about long-term seepage rates for different barrier materials and their impact on the environment. However, work by Jiang et. al. (1998) made it possible to assess seepage rates of water through rock barriers with Equation 5:

Q = 10.24 x ρ x W x (D ÷ T)^0.5 x H^1.5
(Equation 5)

where

Q = Seepage rate (cm³/sec.)

ρ = Porosity of the rock barrier

W = Total width of the barrier (cm)

D = Mean diameter of the rock (cm)

T = Thickness of the barrier (cm)

H = Hydraulic head (cm)

Figure 8 illustrates how much seepage may occur for different rock diameters and depths of runoff in front of a sump inlet. For example, when runoff is about 140 millimeters (5.5 inches) deep, seepage through a 2.0-meter- (6.6-foot-) wide barrier will be about 23 liters per second. (0.83 cubic foot per second) for mean rock diameters of about 50 millimeters (2.0 inches). If the barrier rock has a mean diameter of only 13 millimeters (0.5 inch), then seepage rates for the 2.0-meter-wide barrier drops to about 10.6 liters per second (0.38 cubic foot per second).

In summary, net containment of runoff flowing to a sump inlet barrier will be impacted by street slopes, barrier seepage rates, soil types, topographic conditions, and rainfall. Thus, one should expect that discharges of runoff and sediment into a storm sewer system will occur for nearly all significant runoff events when barriers exist. It is a myth to believe otherwise!

Sump Inlet Barriers and Sedimentation. Sedimentation in front of a sump inlet barrier might be perceived as the BMP having a high effectiveness in capturing sediment (Figure 9). However, only when a small contributing area and little runoff exist might this perception be valid.

Because runoff can be represented by a hydrograph, and sediments are suspended particles found in runoff, it is feasible to represent sediment yields by a similar illustration, which the author defines as a “sedigraph” (Figure 10).

Depending upon flow velocities, depth of overflows, and seepage, sedimentation will be minimal during flood flows over the top of and through a sump inlet barrier. Only when flow velocities and shear stress of moving waters become minimal can significant sedimentation occur. This is represented by the recession limb of a sedigraph.

One method for calculating the effectiveness of a sump inlet barrier is to compare the volume of sediment contained in front of the structure after a runoff event to the total amount of sediment generated by the storm event. Within Equation 4, containment volume in front of a sump inlet can be calculated by Equation 6:

V_con = 5,236 ÷ (Z_a x Z_b) x Y³ - V_bar (Equation 6)

The total amount of sediment associated with runoff can be calculated by applying results generated by the SCS Curve Number Method and MUSLE models. Of course, results will vary depending upon soil types, contributing area, topographic features, and precipitation.

Figure 11 provides an example of runoff and sediment volumes from 0.40 hectare (1.0 acre) of bare ground for different storm events. Notice the volume of sediment is only about 3% to 4% of total runoff volume when comparing 15 millimeters (0.6 inch) to 60 millimeters (2.4 inches) of rainfall, respectively. Similar results will occur when runoff and sediment yield calculations are completed on other type of soils, different contributing areas, and changing topographic conditions.

Figure 8. Possible seepage rates of runoff

Figure 9. Sedimentation in front of a sump inlet barrier

Figure 10. Representation of sediment transport by a sedigraph

Figure 11. Volume of runoff and sediment from Type C bare soils

Combining results from Equation 6 with information found in Figure 11 provides an insight into the possible effectiveness of a rock sump inlet barrier to remove sediment from runoff waters, which is illustrated by Figure 12 and the following.

Effectiveness values vary depending upon the depth of sediment in front of the barrier.
Maximum effectiveness exists for small precipitation and runoff events.
Effectiveness becomes smaller as larger precipitation events occur.

Figure 12 illustrates an idealized condition of being able to capture all upstream sediments. The results are not encouraging for advocates of “inlet protection” to reduce downstream sedimentation. Overflow and/or seepage conditions nearly always occur, which indicates the effectiveness of installing barriers in front of sump inlets to remove sediment from runoff waters is small.

It should be noted that Figure 12 illustrates a worst-case scenario, namely, what happens when 100% upstream bare ground conditions exist. As homes are built, driveways installed, landscaping occurs, and so forth, less sediment is generated during storm events and barrier effectiveness may increase because there are fewer incoming suspended particles. Eventually, upstream erosion control practices will render the role of sump inlet barriers to capturing trash and debris rather than sediment in runoff waters.

In summary, one should not get excited about observing sediment captured in front of any barrier. A barrier’s effectiveness is directly proportional to the amount of containment volume existing in front of the barrier, conditions of upstream disturbed lands, and contributing runoff. Hence, believing sump inlet barriers have a high effectiveness to remove sediments from runoff waters is definitely a myth!

Sump Area Drains and Sedimentation. Conclusions about the effectiveness of sump inlet barriers also apply to structures installed around an area drain (a.k.a. catch basin). Specifically, installing barriers around on-grade area drains (e.g., highway medians) will divert runoff and cause downstream flooding. Only when sump area drains exist might it be feasible to contain runoff for sedimentation (Figure 13).

If uniform terrain conditions exist (which rarely occurs), then the containment surface area can be approximated by an ellipse. An idealized illustration of how the containment volume might occur before overflow conditions are evident is illustrated in Figure 14. Fifield (2011) showed that a net contributing area of runoff to an area drain before overflow conditions develop can be estimated by Equation 7:

A_RO= α₁ [10,472 ÷ (Z_a x Z_b) x Y³ - V_bar + V_SEEP] ÷ RO (Equation 7)

Figure 15 provides an illustration of what possibly might happen if a uniform containment volume exists around a sump area drain. Notice that the contributing area for a shallow barrier is relatively small when compared to taller structures. For example, the contributing area before overflow conditions occur for a 300-millimeter- (12-inch-) tall barrier is about 20% of what exists for a 500-millimeter- (20-inch-) high structure and only 6% of the potential containment area for a 760-millimeter (30-inch) barrier.

The above suggests that effectiveness of shallow barriers around a sump area drain to remove sediment from runoff waters is a myth! Not only will seepage and overflow conditions occur, but also shallow barriers do not create sufficient containment volumes for runoff. Only when tall (and adequately supported) barriers exist around sump area drains to create sufficient containment volumes will the structures begin to display some effectiveness to remove sediment from runoff waters—assuming seepage is minimal.

Sediment Containment Systems
As the above discussions demonstrate, inlet barriers have a limited effectiveness for removing sediment from runoff waters. However, when sediment-laden runoff discharges into a storm sewer system, designers can still provide an effective BMP to capture design-size suspended particles by using an effective sediment containment system (SCS, a.k.a. sediment pond/basin and trap).

In February 2012, EPA recognized that an SCS is more effective in removing sediment from runoff waters than “inlet protection” by stating in its Construction General Permit (Section 2.1.2.9):

If you discharge to any storm drain inlet that carries stormwater flow from your site directly to a surface water (and it is not first directed to a sediment basin, sediment trap, or similarly effective control), and you have authority to access the storm drain inlet, you must . . .

In addition, EPA went on to state (Section 2.1.2.1):

EPA does not consider stormwater control features (e.g., stormwater conveyance channels, storm drain inlets, sediment basins) to constitute “surface waters” for the purposes of triggering the requirement to comply . . .

In other words, EPA is advocating direct discharge of runoff into a storm sewer system when the sediment-laden waters enter into an effective SCS. Design and review professionals should adopt and implement this practical practice for effective sediment control on construction sites. Only in this manner can one expect effective capture of sediments in runoff waters.

Detailed information on developing an effective SCS can be found in Fifield (2011). The topic is complex (e.g., containment volume is not the most important parameter) and requires more time and space to discuss than what is available in this article. However, the following provides a summary of guidelines necessary for an effective storm sewer system and SCS:

Avoid storm sewer pipes having less than 0.5% slopes.
Ensure the design will minimize SCS backwater problems.
Ensure that adequate retention time within the SCS exists before discharges occur.
Always provide adequate horizontal flow path lengths and fall depths.
Provide properly designed discharge structures for the SCS using skimmers.
Ensure that sufficient SCS volume exists for design inflow and outflow conditions.
Evaluate the effectiveness of an SCS to capture design-size particles during overflow conditions.

Only when runoff waters discharge into a professionally designed SCS will effective treatment of runoff waters to remove sediment occur. Once this happens, it is not unreasonable to achieve effectiveness values of sediment removal from runoff waters that are 90% or better.

Summary
It has been demonstrated that structure height and width, hillside slope, flow length, and runoff are determining factors for barriers as to whether they provide sediment or erosion control of sheet flows. When maintenance and seepage issues are not a problem, then hillside barrier effectiveness requires assessing the maximum flow length necessary for total containment of runoff. Unfortunately, because maintenance and seepages issues are often major problems, hillside barriers are not effective in removing sediment from runoff waters—unless used in conjunction with mulch.

When runoff encounters on-grade inlet barriers, downstream flooding and sedimentation are often problems. While sedimentation in front of sump inlets barriers gives an appearance they are effective, they are actually ineffective when large contributing areas of bare ground exist. However, the effectiveness of sump inlet barriers does increase as upstream stabilization practices occur.

Lastly, the most effective sediment control BMP to implement on a construction site is a professionally designed SCS that accepts discharge waters from storm sewer systems. Once an effective SCS is implemented, the capture of runoff discharging unimpeded into on-grade inlets is feasible, which will result in a reduction of downstream flooding and sedimentation. Also, the use of an effective SCS negates the need for requiring the installation of most sump inlet barriers that cause localized flooding and sedimentation.