Sediment Control Practices

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07/12/2024 Added

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[00:00:00.810]Now we'll discuss the various techniques
[00:00:02.670]and practices for sediment control on construction sites.
[00:00:05.940]Many times during the life of a project,
[00:00:08.070]the main BMPs that can be utilized
[00:00:09.990]will be sediment controls.
[00:00:11.700]They don't prevent the erosion process from happening.
[00:00:14.520]But they can lessen it in some instances.
[00:00:16.710]And they capture runoff waters.
[00:00:18.780]Allowing sediment to drop out of suspension.
[00:00:21.690]We'll talk about seven different groups
[00:00:23.250]of sediment control BMPs, including the following:
[00:00:26.790]silt checks, silt traps, earth and rock checks,
[00:00:30.560]mulch perimeter control or slash mulch,
[00:00:33.780]silt fence, turbidity barriers, and inlet protection.
[00:00:38.280]All sediment control devices will require maintenance
[00:00:40.740]once capacity has been reduced by 50%.
[00:00:44.070]Let's start with silt checks.
[00:00:45.840]No matter what type of silt check is called for,
[00:00:48.240]installation is key.
[00:00:49.770]Looking at a silt check across a ditch
[00:00:51.600]or swale bottom, make sure that the base
[00:00:53.970]of the ends on either side of the swale
[00:00:55.680]are higher than the top point of the check
[00:00:57.270]in the bottom of the swale.
[00:00:58.800]This ensures that flows do not compromise the check
[00:01:01.380]and erode soil around the ends, but flow over
[00:01:04.140]the top of the check structure as designed.
[00:01:07.200]For silt checks, as they're identified
[00:01:09.060]in standard specifications,
[00:01:10.770]there are four different types based on
[00:01:12.330]their material makeup and application or use.
[00:01:15.660]Type one, low and high straw wattles.
[00:01:18.660]Type two, low and high wood fiber wattles.
[00:01:21.840]Type three, low and high coconut wattles.
[00:01:24.840]And type four, synthetic checks.
[00:01:27.840]Wattles used as silt checks
[00:01:29.310]can be made with different materials.
[00:01:31.230]With type 2 being made with shaved wood fiber.
[00:01:33.900]The density of the wattle will dictate
[00:01:35.580]how much runoff filters through the device.
[00:01:38.160]For NDOT projects, typical sizes for wattles
[00:01:41.400]are nine-inch diameter for type 2 low,
[00:01:43.770]and a 12-inch diameter for type 2 high.
[00:01:46.500]Type 2 silt checks are common for sediment control
[00:01:49.110]and energy dissipation in ditches during construction
[00:01:51.390]where final grading hasn't been completed.
[00:01:53.700]Their proper spacing will be dictated
[00:01:55.440]by the ditch slope steepness and length.
[00:01:58.170]Let's look at a type 2 wattle for use
[00:02:00.090]as a temporary sediment control device
[00:02:02.010]on rough graded or bare soil areas.
[00:02:04.500]Following the installation principles
[00:02:06.060]we talked about earlier, where point A
[00:02:08.400]should always be higher than point B,
[00:02:10.530]install one-inch by two-inch wood stakes,
[00:02:13.020]two feet in length, with spacing every two feet
[00:02:15.435]across the entire length of the wattle.
[00:02:18.090]Angle the stakes at 45 degrees pointed
[00:02:20.310]down slope from the direction of flow
[00:02:21.960]to give the wattle check strength.
[00:02:24.300]Type 3 wattles are made of coir or coconut fiber material.
[00:02:27.990]They are also specified as type 3 low or high,
[00:02:30.960]and are specified in steep rural ditches.
[00:02:33.390]Along wetlands and stream banks.
[00:02:35.880]Silt check type 4, synthetic checks,
[00:02:38.280]are manufactured products that can also
[00:02:40.080]be selected and installed for projects.
[00:02:42.510]These products will need to follow
[00:02:43.800]manufacturer's instructions for proper installation.
[00:02:47.040]For any type 4 silt check installation,
[00:02:49.470]make sure the outside points of the check
[00:02:51.420]are higher than the center.
[00:02:52.920]Proper check spacing is critical to effectiveness.
[00:03:00.780]Earth checks are simple and effective devices
[00:03:02.970]to install when other products are not available.
[00:03:05.580]When used in conjunction with a depressed area
[00:03:07.680]on the upstream side of the earth check,
[00:03:09.690]known as a silt trap,
[00:03:11.160]it adds more sediment-holding capacity.
[00:03:13.560]These are temporary BMPs, and a plan should include
[00:03:16.620]when and how to stabilize the area with vegetation
[00:03:19.110]and erosion controls as soon as possible.
[00:03:21.900]During the life of earth checks, they will need maintenance.
[00:03:25.020]The contractor shall remove and dispose
[00:03:27.000]of accumulated sediment behind checks
[00:03:28.800]during and after construction completion.
[00:03:31.260]Remove sediment when the depth reaches
[00:03:32.910]one half the height of the check.
[00:03:34.620]After sediment removal,
[00:03:36.210]repair the check to working condition.
[00:03:38.760]Another check that gets used in temporary situations
[00:03:41.310]is high porosity silt fence.
[00:03:43.500]You can tell this particular silt fence
[00:03:45.240]allows more water to pass through it.
[00:03:47.310]As you can actually see through the fabric material.
[00:03:50.070]While allowing water to pass through,
[00:03:52.020]it slows down the flow, catching sediment in the process.
[00:03:55.350]Conversely, low porosity silt fence,
[00:03:57.990]typically what is installed for perimeter sediment control,
[00:04:01.140]isn't as effective as a silt check
[00:04:02.760]placed in concentrated flows.
[00:04:04.770]And is not designed or specified
[00:04:06.510]to be installed as a silt check.
[00:04:08.640]For high porosity silt fence, make sure
[00:04:10.800]post spacing is six feet or less on center.
[00:04:14.130]Ideally the more support posts,
[00:04:15.900]the better especially for steeper slopes.
[00:04:18.270]This extra support is critical
[00:04:19.740]to the effectiveness of this fence.
[00:04:22.140]High porosity silt fence should also be installed
[00:04:24.630]in conjunction with a silt trap,
[00:04:26.370]which we will learn about in a moment,
[00:04:28.110]providing more sediment retention capability.
[00:04:30.990]Always make sure the outside bottom
[00:04:32.610]of the check is at least six inches higher
[00:04:34.410]than the top height of the middle of the check.
[00:04:36.540]Checks allowing flow to go around the ends have failed.
[00:04:39.810]Silt traps are typically used
[00:04:41.460]in conjunction with an earth check.
[00:04:43.470]Earth checks are made from compacted soil.
[00:04:45.840]With the excavated area immediately behind the check
[00:04:48.450]leaving a depression for sediment to accumulate.
[00:04:50.970]It provides more energy dissipation
[00:04:52.710]behind the check and is easy to install.
[00:04:55.590]When used in a series, silt traps can be an effective
[00:04:58.170]short term BMP for capturing small drainage areas.
[00:05:01.590]Maintenance is critical, as silt traps can
[00:05:04.350]and will fill with sediment quickly in many instances.
[00:05:10.230]Silt traps should normally be excavated no deeper
[00:05:12.870]than one foot to allow for sufficient drainage
[00:05:15.570]and evaporation of captured runoff.
[00:05:17.970]As with all sediment control BMPs,
[00:05:20.310]the silt trap should be cleaned out
[00:05:21.900]when it reaches 50% capacity.
[00:05:24.570]Rock checks are temporary or permanent check structures
[00:05:27.300]constructed across a swale or channel.
[00:05:30.000]They're used to reduce velocity of concentrated flows.
[00:05:33.120]The use and availability of rock for checks
[00:05:35.190]may be limited depending on location.
[00:05:37.710]The same check principles apply.
[00:05:39.720]Make sure the side elevations
[00:05:41.190]are higher than the middle of the check.
[00:05:43.050]Giving stormwater flows no choice
[00:05:45.000]but to flow over the middle of the check.
[00:05:47.460]Slash mulch is a byproduct of the removal of trees
[00:05:50.160]where construction projects encroach
[00:05:51.870]into repairing the areas.
[00:05:53.550]The key to making the proper slash mulch
[00:05:55.500]is to use a tub grinder.
[00:05:57.240]Creating pieces of different sizes.
[00:05:59.310]Which makes for better cohesion and strength
[00:06:01.230]as a slash mulch barrier can slowly settle
[00:06:03.960]the organic material, binding together as it breaks down.
[00:06:07.200]It's a great alternative to burning tree piles.
[00:06:09.780]They're low maintenance once they're installed,
[00:06:11.940]and it's a great green solution utilizing
[00:06:14.460]biomass that was grown right on the site.
[00:06:17.220]For payment, both grinding and placement
[00:06:18.990]are paid for by the cubic yard.
[00:06:21.180]Slash mulch perimeter controls
[00:06:22.800]are used to slow and filter water.
[00:06:24.960]This is a distinction that many
[00:06:26.340]sediment control BMPs can say they do,
[00:06:29.010]but rarely do they actually filter runoff water.
[00:06:31.800]Slash mulch has that ability
[00:06:33.330]in the early part of its lifecycle.
[00:06:35.430]They can be left in place, slowly biodegrading over time.
[00:06:38.850]And once the material is on site, and made,
[00:06:41.760]they're relatively quick to install.
[00:06:44.220]In wet boggy areas, slash mulch can be a much easier
[00:06:47.100]installation than other BMPs such as silt fence.
[00:06:50.430]Again, if left in place,
[00:06:52.020]it poses no adverse effects to the environment.
[00:06:54.630]Slowly degrading over time well after its needed use.
[00:06:58.710]Here's an example of slash mulch
[00:07:00.420]as a perimeter control at the Toba slope.
[00:07:02.940]Also installed up and around a culvert.
[00:07:05.370]As vegetation becomes established on the slope above.
[00:07:08.550]For removal, it is at the discretion of the project engineer
[00:07:11.700]as to whether or not the slash mulch berm should be removed.
[00:07:15.060]Or if possible, the material can simply
[00:07:17.460]be incorporated back into the existing landscape.
[00:07:20.220]With no hauling away of waste material.
[00:07:22.950]Slash mulch can also be installed as a check
[00:07:25.230]in a ditch application, similar to a silt check.
[00:07:28.410]The diagram Option B on the right
[00:07:30.120]shows how the leading edge,
[00:07:31.650]or upstream side of the barrier, should be added.
[00:07:34.380]A six to one slope, 18 to 24 inches high,
[00:07:37.920]with a backside at a three to one slope.
[00:07:40.290]Ensuring a broad, stable, and sturdy check or barrier.
[00:07:44.430]Obviously the most widely used
[00:07:46.290]and misused sediment control BMP
[00:07:48.360]in the entire country is silt fence.
[00:07:50.927]A silt fence should be used as a perimeter of control.
[00:07:54.290]Capturing sheet flows, posts,
[00:07:56.280]steel T posts, or wood stakes.
[00:07:59.110]Question: is this the only BMP on site?
[00:08:03.906]The answer is yes, and this is where
[00:08:05.846]a lot of the confusion and misuse of BMP stems from.
[00:08:08.850]In the example here, a vegetated buffer
[00:08:10.860]is set in place behind the silt fence barrier.
[00:08:13.636]Creating a ponding effect.
[00:08:16.125]In the same example, we're infiltrating
[00:08:18.330]the soil away from the toe of the slope.
[00:08:20.640]Allowing plenty of room for capacity of sediment
[00:08:22.800]to settle out and be retained behind the fence.
[00:08:25.740]For perimeter silt fence,
[00:08:27.240]install in effective locations prior to mass grading.
[00:08:30.570]Keep long runs of fence to a minimum,
[00:08:32.430]utilizing bays or J-hooks as necessary,
[00:08:35.370]and be sure to follow NDOT's
[00:08:37.050]installation specifications and standard plans.
[00:08:40.920]One method for installing silt fence
[00:08:42.780]is by using a specialized installation machine.
[00:08:45.810]Commonly known as the slicing method.
[00:08:47.940]These machines cause minimal soil disturbance.
[00:08:50.610]Forcing the silt fence fabric down
[00:08:52.320]into a narrow slice, with compaction achieved by simply
[00:08:55.380]driving back over the sliced area with the tractor tire.
[00:08:58.620]This makes for effective compaction.
[00:09:00.570]And minimizes the risk of the fence
[00:09:02.100]getting undercut with runoff water.
[00:09:04.230]Installing silt fence in extremely dry soils
[00:09:06.720]can also lead to compaction issues in time.
[00:09:09.810]Another method is by using a trenching machine
[00:09:12.030]which excavates a six inch trench.
[00:09:14.100]Silt fence fabric is pinned in the bottom of the trench.
[00:09:16.950]Backfilled and compacted.
[00:09:18.930]Here we see some examples of poorly installed silt fence.
[00:09:22.350]Water is always going to take the path of least resistance.
[00:09:25.440]And when we install silt fence to capture sediment,
[00:09:28.110]and temporarily hold back runoff water,
[00:09:30.480]it's going to find any weak point in the installation.
[00:09:33.390]Here an inspector sees the fence
[00:09:34.830]was backfilled with a little dirt.
[00:09:36.630]Not trenched or sliced into the ground.
[00:09:39.180]Another example, this fence being pinned down
[00:09:41.160]to the ground with rocks.
[00:09:42.630]Not even sliced into the ground.
[00:09:44.580]And fence being attached to steel rebar.
[00:09:46.860]Not able to be attached or cut into the ground correctly.
[00:09:50.040]Also, the fence wasn't placed correctly near this culvert.
[00:09:53.130]Allowing any runoff water to simply pass right on by,
[00:09:56.078]rendering the entire installation useless.
[00:09:59.430]Another common mistake with installing silt fence
[00:10:01.770]is that the designer or the installer
[00:10:03.420]doesn't think like water.
[00:10:05.040]Water runs downhill.
[00:10:06.630]And if the job of silt fence is to capture
[00:10:08.610]and hold runoff water for a period of time,
[00:10:11.040]it makes sense that we allow the fence
[00:10:12.660]to capture areas by installing silt fence base,
[00:10:15.600]or J-hooks as some refer to it.
[00:10:18.000]This technique allows for silt fence
[00:10:19.740]to do its job correctly.
[00:10:21.870]You see in the example on the right
[00:10:23.460]where fence is installed that may or may not
[00:10:25.590]keep sediment out of part of the roadway,
[00:10:27.720]but it's essentially pushing the problem somewhere else.
[00:10:30.600]In the example on the left, runoff is captured
[00:10:33.360]and held behind the fence effectively.
[00:10:35.790]This is a classic example of a silt fence bay installation.
[00:10:39.240]A manageable area is effectively captured
[00:10:41.340]behind the fence, with the next run of fence
[00:10:43.710]ready to capture another small area.
[00:10:46.020]Captured runoff is allowed to weed through the fence.
[00:10:48.930]And also slowly infiltrate behind it.
[00:10:51.750]When installing silt fence, ensure that
[00:10:53.760]proper post spacing is being used.
[00:10:56.160]NDOT specifications require six foot post spacings.
[00:11:00.000]Steel T posts with a minimum weight
[00:11:01.680]of one and a quarter points per foot is the specification.
[00:11:05.040]Another important installation detail
[00:11:06.840]is the attachment itself.
[00:11:08.550]Use UV stabilized black zip ties,
[00:11:11.040]50 pounds minimum tensile strength,
[00:11:13.260]and place three zip ties within
[00:11:14.820]the top eight inches of the installed fabric.
[00:11:17.610]As with all sediment controlled BMPs,
[00:11:19.830]be sure to remove sediment
[00:11:21.150]once storage capacity has reached 50%.
[00:11:24.570]Turbidity curtains are sediment barriers
[00:11:26.490]used in live watercourses such as lakes and ponds.
[00:11:29.730]They are normally used in coastal areas,
[00:11:32.070]but are occasionally used on NDOT projects.
[00:11:35.040]They have also been used as a limit to construction.
[00:11:37.800]If right of way for a project
[00:11:39.120]is encroaching on a pond or lake,
[00:11:41.130]the barrier will hang into the water,
[00:11:43.080]attached to supports on shore either side,
[00:11:45.540]with a weighted chain ballast
[00:11:46.860]extending the curtain down into the water.
[00:11:49.050]The barrier limits sedimentation to the contained area only.
[00:11:52.440]Preventing a larger sediment plume
[00:11:54.120]from impacting the entire water body.
[00:11:56.790]Now let's discuss inlet protection.
[00:11:59.040]There are two types of storm drain inlets.
[00:12:01.560]Area inlets and curb inlets.
[00:12:03.630]We'll first review area inlets.
[00:12:05.760]And in this case we're specifically discussing
[00:12:08.160]the use of silt fence for protecting area drains.
[00:12:11.370]An area inlet is designed as a permanent
[00:12:13.440]stormwater drainage conveyance feature.
[00:12:15.750]And that's an important thing to consider
[00:12:17.250]when installing sediment controls around inlets.
[00:12:19.980]When we essentially block an inlet
[00:12:21.510]from receiving stormwater flows,
[00:12:23.400]we have to consider how much water
[00:12:24.930]we are intending to treat or hold back.
[00:12:27.660]Also, if we block flows, where is that water going?
[00:12:30.930]Hopefully if it's an area inlet, it is in a sump condition.
[00:12:34.200]Meaning we're at the bottom of any hill or drainage area.
[00:12:37.560]For area inlets, installation is critical
[00:12:39.750]to the success of an area inlet protection device.
[00:12:42.810]When using silt fence, there should be
[00:12:44.580]an inner ring of low porosity silt fence.
[00:12:47.280]Typically installed two feet out from the inlet,
[00:12:49.950]then an outer ring of high porosity silt fence,
[00:12:52.980]ideally installed three feet out from the inner ring.
[00:12:55.860]Extra posts will likely be necessary
[00:12:57.630]due to increased pressure from runoff water
[00:12:59.640]ponding around the inlet protection for a period of time.
[00:13:02.850]For silt fence installed for protecting culverts,
[00:13:05.640]proper staking and support is critical.
[00:13:07.950]The fence will strain behind the tremendous
[00:13:09.720]amount of weight and pressure from water.
[00:13:11.820]The most important consideration for protecting culverts
[00:13:14.460]is to prevent sediment from getting
[00:13:15.900]to the culvert in the first place.
[00:13:17.790]By installing effective erosion
[00:13:19.680]and sediment controls upstream wherever possible.
[00:13:22.680]If a contributing area has been properly stabilized
[00:13:25.200]with seed, mulch, blankets, or other erosion controls,
[00:13:28.770]remove the inlet protection.
[00:13:30.390]And allow the inlet to do its job.
[00:13:33.030]In this example, low porosity silt fence
[00:13:35.550]would not be the correct material.
[00:13:37.890]Culvert inlet protection should use
[00:13:39.600]high porosity silt fence instead.
[00:13:41.940]Also, with the insulation of erosion controls
[00:13:44.130]such as blankets and straw mulch,
[00:13:46.170]installing a barrier around the culvert
[00:13:48.000]at this point may not be needed.
[00:13:50.340]For inlet protection to stand up to the weight
[00:13:52.350]and force of runoff water pushing in on it,
[00:13:54.840]and providing time for the control to slowly
[00:13:56.910]pass water through, it needs top reinforcement.
[00:14:00.300]In this example, the extra support around the top
[00:14:02.940]of the device keeps the steel T posts
[00:14:04.770]from bending inward as runoff water pools around it.
[00:14:07.650]Allowing it to do its job.
[00:14:09.480]There are several proprietary devices on the market as well.
[00:14:13.110]For curb inlet protection, the important key
[00:14:15.390]is to not fully block the inlet
[00:14:16.950]with whatever device you are using.
[00:14:18.840]These curb inlet protection devices are common.
[00:14:21.510]And it's important that they are placed
[00:14:22.920]and installed such that they can
[00:14:24.240]do what they're intended to do.
[00:14:26.040]Make sure the ends extend beyond the width of the inlet.
[00:14:28.950]And secure on both ends, if possible, with sandbags.
[00:14:32.670]Also, prevent the device from completely covering the inlet
[00:14:35.460]by placing a block between the device and inlet
[00:14:38.100]to allow for overflow drainage into the inlet.
[00:14:41.040]One important note on curb inlet protection.
[00:14:43.650]If these are the only BMPs being used on a job site,
[00:14:46.980]your chances of effectiveness and compliance are low.
[00:14:50.040]Do everything you can to prevent sediment
[00:14:51.780]from entering the street or right of way right.
[00:14:54.330]Inlet protection should be the last line
[00:14:56.130]of defense on any project.
[00:14:58.380]In these examples, you can see the issues
[00:15:00.510]that arise from poor installations.
[00:15:02.670]When an inlet is blocked, especially on an open street,
[00:15:05.820]there will be maintenance and cleanup issues.
[00:15:08.190]Also, you can actually lose these devices
[00:15:10.260]down into an inlet if not installed correctly.
[00:15:12.900]Which can cause blockage and street flaying.
[00:15:15.480]Again, always follow NDOT standard plans
[00:15:18.030]and specifications for installation.
[00:15:20.550]So wrapping up section three, sediment controls,
[00:15:23.700]we covered seven different BMP types:
[00:15:26.280]Silt checks, silt traps, earth and rock checks,
[00:15:29.834]mulch perimeter control or slash mulch,
[00:15:33.210]silt fence, turbidity barriers, and inlet protection.
[00:15:37.800]Remember, all sediment control devices will require
[00:15:40.920]maintenance once capacity has been reduced by 50%.
[00:15:44.400]Now close the slideshow and return to the course menu.
[00:15:47.400]And take the quick five question quiz
[00:15:48.960]to move on to section four.

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