Go onto any construction site and you will find silt fences, straw bales, fiber rolls, and/or geosynthetic barriers around homes and inlets or in drainage channels. Today, a multimillion dollar industry provides a variety of products created to meet the EPA’s mandate of installing construction-site best management practices (BMPs) to “minimize pollutants” in the discharge of runoff waters. Contractors install these temporary sediment-control BMPs with the understanding that designers have made a selection to meet the EPA’s mandate. However, many times they (and regulatory agencies) find the effectiveness of temporary BMPs to remove suspended particles out of discharge waters is minimal.
Even if the installation, inspection, and maintenance of BMPs occur in an optimal manner, sediments (a form of pollutant) still discharge from construction sites for most runoff events-and often in significant quantities. The only BMPs that truly “minimize pollutants” in the discharge of runoff waters are those that lessen their creation. It is for this reason that the generation of large quantities of sediment by rainfall (as well as wind) happens when erosion control practices are nearly nonexistent. Since excavating earthen material, developing roads, and building homes cannot occur on top of erosion control practices, contractors are forced to rely on temporary sediment-control measures, which usually are barriers.
Hydrographs and “Sedigraphs”
The process of soil disturbance by rainfall, flowing water, and wind is known as erosion. Once erosion occurs and dislodges soil particles, the transport of suspended particles (also known as sediment) happens. Increased flows result in increased sediment transport, with a maximum value occurring when peak flows occur. Finally, as the flow of runoff waters decreases, deposition of sediment, also known as sedimentation, occurs.
The “sedigraph” illustrates one possible scenario by which the transport of sediment and deposit of sediment may occur. Because sediment is associated with runoff waters that can be represented by a hydrograph, viewing such an illustration as a sedigraph can represent the transport of sediment particles within the flows.
Perhaps the most important item to note with the sedigraph is when deposition (i.e., sedimentation) occurs. Only when flows become small does sedimentation become large. Thus, unless runoff waters are contained behind a structure, the effectiveness of any barrier structure to remove sediment from runoff waters may be small for flood-flow events.
Effectiveness of Barriers to Remove Sediment from Runoff
While erosion control BMPs are continually tested for their effectiveness, few temporary sediment-control measures undergo similar scrutiny. Unfortunately, actual construction-site data on the effectiveness of temporary BMPs to remove sediment out of large volumes of runoff are limited. However, Figure 2 shows a conceptual evaluation of barrier effectiveness to remove sediment from runoff waters.
The effectiveness of total containment behind a barrier to reduce sediment becomes 100%. However, once runoff waters discharge through and over a barrier, the effectiveness of a structure to contain sediment becomes less than 100% and may approach nearly zero for large flood-flow events.
Barrier Spacing for Sheet Flow Conditions
Because barriers are commonly found on construction sites to intercept sheet flow runoff waters, one might ask, “What spacing might result in 100% effectiveness?” Flow lengths between two barriers can be approximated by the following equation (Equation 1) when sheet flow conditions exist:
L = (Î1 x Y2) ÷ (Z x RO)
Where
L = Flow length between barriers (meters or feet)
Î1= 50,000 (SI Units) or 600 (English Units)
X = Horizontal distance between barriers (meters or feet)
Y = Height of barrier (meters or feet)
Z = Percent slope between barriers
RO = Runoff (millimeters or inches per unit area)
It is important to note that the upper barrier, illustrated in Figure 3, can be replaced by any structure, including the construction of a dwelling.
Figure 4 illustrates how flow lengths vary for 10% land slopes before overflow conditions occur for different types of bare ground soils (Type A soils are sandy material and Type D soils are predominately clays) and various barrier heights exist. Notice that if 75 millimeters (3.0 inches) of precipitation falls on Type C soils, there are little differences in upstream flow lengths between 230- or 300-millimeter (9- or 12-inch)-high barriers before overflow conditions happen. However, notice that the flow length upstream of a tall silt fence barrier is more than 12 times longer than that for fiber logs. Thus, depending upon land slope, shallow barriers can be ineffective in capturing runoff (and sediment) when long flow lengths exist.
Figure 4 also implies that using barriers for perimeter protection (e.g., during the construction of homes) requires BMPs to have an adequate height for proper containment. Of course, this conclusion is invalid if proper maintenance of the BMPs does not occur. For example, the effectiveness of perimeter barriers having low heights (e.g., fiber logs compacted by vehicles entering the lot) to minimize the discharge of runoff and sediment is very small.
Curb Inlet Protection
It appears that throughout the United States, regulatory agencies have a passion for “protecting” curb inlets by requiring placement of barriers in front of the opening. Such barriers on a grade will divert runoff waters and cause downstream flooding and sedimentation. When sump conditions exist, creation of a small pond in front of the inlet opening occurs, which causes localized flooding. However, the pond also creates sedimentation.
What is the effectiveness of a barrier placed in front of a curb inlet opening to capture runoff and sediment? The answer to this question lies with how much runoff can be contained in front of “sump” barrier from contributing areas. Using Figure 5, Equation 2 can approximate this contributing area:
A = (Î2÷ RO) x [(104 x ∠x Y3) ÷ (6 x Za x Zb)- Vbar]
Where
A = Contributing area (m2 or ft2)
Î2 = 1,000 (SI Units) or 12 (English Units)
Y = Barrier height (meters or feet)
Za = Side street slope (%)
Zb = Crown street slope (%)
RO = Runoff/unit area (millimeters or inches)
Vbar = Volume displacement by barrier (m3 or ft3)
Figure 6 illustrates Equation 2 and demonstrates how contributing areas can become smaller before overflow conditions occur for increasing street slopes and 75 millimeters (3.0 inches) of runoff. Notice that when building of homes on Type B soils occurs, before overflow conditions exist, the amount of contributing lands for 1% street slopes will be nearly four times larger than for developments with 4% street slopes. This is due to the increased containment volume that can occur when shallow-slope streets exist.
Once overflow conditions occur, sedimentation in front of a sump barrier will consist of predominately large-diameter particles. The reason for this is that high flow velocities associated with overflow conditions will convey smaller (e.g., colloidal) particles into the storm-sewer inlet opening. It is only after overflows cease that sedimentation of large and small suspended particles found in runoff waters occurs, as is illustrated on the recession arm of a sedigraph.
Figure 7 illustrates zones of sediment discharge and sedimentation, respectively. Depending upon the storm intensity, type of soils, and the contributing area, it is feasible that significant amounts of sediment will enter a storm sewer system as major overflow conditions occur. Thus, even though sedimentation is evident on the street after runoff events cease, “inlet barrier protection” in front of a storm sewer sump inlet may be providing only minimal environmental benefits.
Using Barriers for Area Drain Protection
As with placing barriers in front of curb inlets when the streets are on a grade, installing a barrier around an area drain or catch basin located on a grade will also cause downstream flooding due to diversion of runoff. Instead, barriers around area drains should only occur when they are located within a sump.
Assuming uniform slope conditions exist, and using information shown in Figure 8, the following equation (Equation 3) can approximate the amount of contributing area to an area drain barrier before effectiveness is compromised for overflow conditions:
A = (Î2 ÷ RO) x [(10,000 x ∠x Y3 ÷ Z2) – Vbar]
Where
A = Contributing area (m2 or ft2)
Î2 = 1,000 (SI Units) or 12 (English Units)
Y = Barrier height (m or ft.)
Z = Average contributing land slope (%)
RO = Runoff/unit area (millimeters or inches)
Vbar = Volume displacement by barrier (m3 or ft3)
Figure 9 clearly illustrates that barriers placed around area drains must have sufficient height to allow for maximum containment of runoff waters. Notice that a 300-millimeter (12-inch)-high barrier placed around an area drain is essentially ineffective in containing runoff waters from Type C soils when 75 millimeters (3.0 inches) of precipitation occurs. Even a 500-millimeter (20-inch)-high barrier is not very effective when compared to a 750-millimeter (30-inch)-high barrier. Designers and reviewers need to be aware that barriers around area drains with low heights have minimal effectiveness in containing runoff waters and minimal sediment removal when overflow conditions occur.
Summary and Conclusions
A mathematical assessment of the effectiveness of temporary barriers to remove sediment from runoff waters has been presented. Based upon an assumption that maximum effectiveness happens when containment of all runoff waters occur, it has been concluded that once flood-flow conditions exist:
- Barriers must have sufficient height to capture sheet flows if they are to be effective in minimizing sediment discharges.
- Placing barriers in front of curb and area drain inlet openings that are not in a sump will result in downstream flooding and sedimentation.
- When overflow conditions occur with barriers placed in front of sump curb inlet, a majority of sediment suspended in the runoff will discharge into the storm-sewer system.
- Barriers placed around an area drain in a sump must have sufficient height to be effective in containing runoff and sediment when overflow conditions occur.
Designers, inspectors, and regulatory personnel must realize the limitation of placing barriers to intercept sheet flows from lands undergoing construction activities or concentrated flows in front of storm-sewer inlet openings. Proper selection, installation, and timely maintenance are essential.
Once design flood-flow events occur (e.g., a two-year, 24-hour storm event) that result in overflow conditions, the benefit of barriers to remove sediment from runoff waters and to protect the environment will become negligible.
