Lesson 5: Measuring Soil Erosion and Sediment Control: Best Management Practices

Learner Outcomes

Students will investigate and analyze the best management practices commonly used to control soil erosion, sedimentation and turbidity and their effectiveness to reduce pollution and improve water quality.  

Success Indicator

Describe best management practices for erosion and sediment control.

Skill Level

Advanced; ages 14-18

Time Needed

One 90-minute block schedule or two 45-minute classes

Life Skills

  • Decision making
  • Learning to learn
  • Record keeping
  • Wise use of resources

Materials

  • Lesson Five slides
  • Runoff samples from Rainbox Experiment
  • Turbidity tube (purchased from a science education company or homemade – see Appendix B)
  • Turbidimeter (optional, possible borrow from local water quality firm or soil science consultant)
  • Filter paper
  • Toaster oven
  • 5 Mason Jars or another clear container
  • Conical shape coffee filters
  • Polyacrylamide – PAM (Included as part of  the ordered curriculum kit or through a Soil Erosion, Sediment Control company, www.ntuinc.com)
  • PAM SDS (Safety Data Sheets – included with the purchase of PAM) 

Lesson Slides

Lesson Slides

Click on “File” and then select “Make a Copy ” of the lesson slide deck to use it with your youth.

Introduction

Background Information

BEST MANAGEMENT PRACTICES (BMP’s)

Erosion Control

The best way to manage soil erosion is to prevent it from occurring in the first place.  Strategies for soil conservation regardless of an agronomic or urban development context are based on similar principles: covering the soil to protect it from raindrop impact; increasing the infiltration capacity of the soil to reduce runoff; improving the aggregate stability of the soil; and increasing surface roughness to reduce the velocity of runoff and wind (Morgan, 2005).  Any land disturbance that removes the vegetative cover, increases the slope, decreases rainfall infiltration, or increases surface-water flow can increase the rate of erosion.  Measures that stabilize bare soil areas should be implemented quickly.  

Crop Rotation

image of corn and soybean fieldRotating crops that require intensive cultivation of the soil in turn with crops that increase soil structure and provide soil cover increases the amount of land that can be farmed.  Soil that is highly erodible can only support row crops such as corn once every three, five, or seven years depending on the soil and BMP’s.  For example, a silt loam soil with a slight slope can easily lose topsoil if planted in corn every year.  The best practice, in this case, would be to plant a corn-wheat-clover rotation, with corn being planted once every three years with reduced tillage.

Row crops include corn, soybeans, cotton, sorghum, peanuts, rice, small grains, and tobacco.   Regardless of soil type, most farmers rotate crops, both for soil conservation as well as insect, weed, and disease management. 

Cover Cropping

Cover cropping is an effective BMP in agriculture that promotes soil health.  Using a cover crop between times of seasonal production suppresses weeds, protects the soil from the impact of rain splash and runoff, improves soil aggregate stability, and reduces surface crusting.  It also allows the soil time to replenish some of the essential nutrients that are often depleted during the harvest season such as nitrogen, if the cover crop is turned under the soil at maturity or fixes nitrogen in symbiosis with Rhizobia.

Strip Cropping

Strip cropping is the practice of planting a row crop and densely seeded crop in alternating strips along a contour.  Detached soil is trapped within the buffer of the densely seeded crop.  Plants that provide effective strip buffers include densely seeded legumes (members of the bean family)  or grasses. Often strip cropping plans are developed between a landowner and the local Soil and Water Conservation District.

High-Density Planting

The idea of high-density planting is to increase the canopy cover provided by a crop to mitigate the impact of the rainfall and still maintain yields.

Conservation Tillage

Standard tillage operations are a necessary process in building seed beds, controlling weeds, and loosening compacted soil, but they leave bare soil exposed to a risk of erosion.  It is appropriate with many crops to practice conservation tillage methods.  Conservation tillage is a practice that leaves at least 30 percent cover on the soil surface after planting.  No-till is a type of conservation tillage where operations are restricted to only what is necessary for planting seed.  Crop residue is left in the field after harvest to protect the soil and then special equipment is used to drive through the residue to plant a seed in the soil.  Strip tillage is another method that prepares only a narrow strip of soil to be planted, with the remaining area left undisturbed.  Crops like soybean, wheat, and corn are well-suited to conservation tillage.

Contour Tillage

image of wheat fields planted along the contour of the slopePlanting along the contour of a sloped land can reduce soil loss compared to cultivated rows that run up and down the slope.  Local Soil and Water Conservation employees work with growers to develop planting plans using survey equipment.

 

Terracing

Terracing is a practice used in both urban development as well as agriculture to control erosion and sedimentation.  Terracing allows land managers to create breaks along the terrain which causes a reduction in the velocity of water moving downslope.  By controlling the amount of water and the rate at which it moves along the contours the amount of soil loss is controlled.  This type of BMP also gives the water time to infiltrate into the soil profile supplying moisture to plants.

Grassed Waterways

Grassed waterways are used as a point of discharge for other conveyances upslope, such as terraces.  As the water runoff enters these grassed waterways it loses energy and high loads of nutrients attached to eroded soil particles are filtered out.  The grassed waterways allow excess water (runoff) to leave the site (construction or agriculture) and enter the waterways cleaner.

Mulching

Mulches are effective agronomic and land development strategies that provide protection from raindrop impact and reduce runoff velocity.  In agronomic settings, the soil is mulched with crop residues such as straw, maize stalks, or standing stubble like wheat stalks.  In smaller spaces like residential landscapes or gardens, appropriate mulches include decomposed leaves, straw, cypress mulch, or pine bark.  In the context of a construction site, prior to establishing permanent vegetation, protective practices that provide temporary covers such as straw or mulch are used.  Products like erosion control blankets made from organic materials such as coconut fiber (coir), jute, or aspen shavings can also protect the soil.  Refer to the background information in Lesson Four: The Rainbox Experiment.

Sediment Control

Once the soil has been detached and is being transported across the landscape, slowing the velocity of water to enable sediment to drop out of suspension becomes important.  Providing barriers which the sediment-laden water will encounter can keep more soil onsite.  As a last line of defense, sediment basins capture runoff and provide a space for the sediment-laden water to slow or still, giving the soil particles one last opportunity to fall out of suspension and remain out of waterways.

Sediment Basins and Baffles

Sediment basins and traps at construction sites, agricultural operations, and other disturbed areas provide temporary pools for runoff that allow sediment to settle before the water is discharged into a river, stream, or landscape.  The basins trap sediment and other coarse material.  Unfortunately, these traps and basins are not efficient when the swift, turbulent water pours along with a straight-line flow that takes runoff quickly to the basin’s outlet, also known as short-circuiting.  Baffles are porous barriers that slow, calm, and distribute the water across the whole width of the basin to help solve this problem. Baffles can lengthen the flow path and distribute the flow more widely.  They significantly increase the amount of sediment captured by decreasing the velocity of the water entering the device compared to that of an open basin.  They do not capture the very small, or clay, particles, however, that are the main cause of turbidity.  

Skimmers and other surface outlets

A skimmer is a surface drain that floats on top of the water in a sediment basin. The skimmer inlet controls the rate of outflow and rises and falls as the basin fills and drains. It releases the cleanest water in the basin from near the surface. 

Flashboard risers and solid risers are commonly used in agriculture to control the level of water in area ponds.  These devices have been adopted in many other industries, including sediment and erosion control due to their ability to de-water structures from the top of the water column.  This practice allows time for the heaviest sediment particles to drop out of suspension.

Check Dams

On most construction sites, channels are installed to convey runoff along the perimeter of the property into sediment control basins. To keep the channels from eroding, erosion control blankets are installed to protect the bare exposed soil from eroding.  Check dams are usually installed along the length of the ditch to reduce the flow rates and pool the water so it moves down the slope gradually. The most common practice is to place a large stone in the channel with a weir, or low spot, in the center. An alternative approach is to use wattles or logs made of natural fibers such as straw, coir, wood fiber (excelsior), or compost. Such fiber check dams (FCDs) have recently proved to be both more effective and more economical than rock check dams in construction site channels (SoilFacts, 2009). Most FCDs are relatively light and can be easily installed by one person in about 15 minutes. Unlike rock dams, FCDs are biodegradable and do not have to be removed after construction is complete. 

Silt Fences

Silt fences are a control measure that captures sediment in the runoff before the site is stabilized with vegetation.  They are among the last line of defense, installed along the perimeter of a site.  Made of slowly permeable fabric, runoff flows into the fence and pools allowing sediment to be deposited.  Silt fences and other collection devices should be maintained and accumulated sediment regularly removed.  Silt fence is not designed for concentrated flow and should never be placed in ditches or streams.  They are installed along the contours of the land.

Polyacrylamide (PAM) use in erosion, sediment, and turbidity control

Polyacrylamide (PAM) is a water-soluble, synthetic polymer that acts as a highly effective binding agent with fine silts and clays. These particles are bound together with PAM to form flocs, which can settle out of the water to significantly reduce turbidity. As an erosion control method, PAM does not directly protect soil from rain droplet impacts, but reduces soil detachment, maintains the soil’s structure, and increases infiltration rates early in the rain event when used along with all types of mulches (see a picture of hydraulically applied mulch on previous pages in this lesson). 

PAM is a term describing a wide variety of chemicals based on acrylamide and acrylate molecular units. When linked in long chains, these molecular units can be modified to result in a net positive, neutral, or negative charge on the PAM molecule. The negatively charged, or anionic PAMs, as well as the neutrally charged, or no charge PAMs are nontoxic in the environment. These types of PAM can be used for either erosion or turbidity control. All references to “PAM” are to the anionic forms in the following discussion. 

NOTE: The positively charged, or cationic, PAMs, are not used for turbidity control due to their potentially toxic nature.   Toxicology tests have shown a decrease in the reproduction rate of very small water fleas.  These cationic PAMs, however, can be used when applied to soil to control erosion, due to their tight binding to the soil which prevents their release into runoff.   

Flocculation is the process of causing small, suspended materials to stick to each other to form “flocs.”  These larger flocs, now weighing more than individual particles, settle more readily.  PAM can create flocs in many of the suspended sediment types found throughout the country, as well as worldwide.  Because there are so many different types of PAMs and soils it is important to do a simple “jar test” to determine which PAM will work for your specific soil.

One of the key factors in making a flocculant work is to ensure that it is dissolved and thoroughly mixed with the runoff water.  This can be accomplished in several ways. Introducing PAM to the runoff at a point of high flow velocity helps to provide the turbulence and mixing needed to maximize the suspended sediment exposure to the large PAM molecules (Soil Facts, 2006).  Examples include placement in storm drain junction boxes where a pipe is dropping water, in a slope drain, or in other areas of falling or fast-moving water that are upslope from a sediment trap or basin or on check dams placed in a ditch. 

Polyacrylamide-treated runoff should be directed into a sediment basin or similar device prior to discharge to effectively trap the flocculated material before the water is discharged off the site. PAMs are permitted in states like North Carolina if flocs are captured prior to discharge.  Not all states allow the use of flocculants so it is important to check with the proper state and local agencies before using them for turbidity and erosion control.

For more information on PAM and its use in erosion and sediment control, visit the Resources section at the end of the curriculum. 

Note: Anionic PAM has been proven to be relatively non-toxic to the environment when used correctly.

Daily Blog Breakdown

Give students a few moments to debrief on the most recent blog post. Encourage the students to point out pieces of information that were interesting or thought-provoking. Have them recap why it is important to know about soil properties in relation to erosion, sediment, and turbidity control. Ask the students about their ideas in regards to their peer’s postings. What were some common themes or thoughts? How were viewpoints the same? Different? Where can they go to find additional valid information? Why do they think it is important to have plenty of information before making decisions?

Best Management Practices

Best Management Practices (BMP) 
  1. Pass out the student handout, “Measuring Turbidity, TSS and PAM,” and have students think about the opening questions found on their handout and write down their ideas. 
    • How does turbidity impact water quality?
    • Why is it important to know about Total Suspended Solids?
    • What are the best ways to manage soil erosion and control sediment and turbidity?
  2. Show students the lesson’s slides on best management practices (BMP’s) for controlling erosion and sedimentation. 
  3. The slides have notes and questions for educators to guide the discussion with the class.  Teachers should make connections between the BMP’s illustrated in the slides to any similar practices that the students may have used in their rainbox experiment. 
  4. Teachers should also relate the materials to observations students made during their site assessments or even more broadly to similar situations in the community. 
  5. Have the students compare their initial answers to the opening questions to what they know now after the slides presentation.
Porous Baffles Dye Demonstration

Watch this video demonstrate the effectiveness of using baffles in a sediment basin to drop sediment out of suspension.

Observing and Measuring Turbidity

Use this student handout for instructions need to do the turbidity activities.

Measuring Turbidity

Measuring Turbidity 

Hold up a sample of the rainbox runoff water collected by the student rainbox experiments.  Show the students a particularly cloudy sample and ask, “Why is this water so muddy?” Students should be able to share their soil erosion knowledge and explain that rain detaches the soil and surface water runoff transports it down a sloping landscape.  

Ask the students to describe where they think all the muddy water ends up.  Replies should include; streams, rivers, lakes, reservoirs, ponds, harbors, and estuaries, or any body of water.  Reiterate the question, “Why do we care if sediment reaches our waterways?” Be sure to cover the ideas below:

  • Clogs drainage ditches and stream channels, increases flood damage potential
  • Deposits silt in reservoirs, lowering the storage capacity
  • Transports pesticides and fertilizers that can contaminate or pollute downstream water sources and recreational areas.
  • Contaminates drinking water
  • Acts directly on fish, killing them or reducing their growth rate by affecting their gills, reducing their resistance to disease
  • Covers fish spawning grounds, preventing successful development of fish eggs and larvae
  • Reduces recreational use of waters for fishing and swimming
  • Increases the cost of water treatment for drinking and food processing. 

Display the muddy sample in one hand and hold up a cleaner runoff sample in the other hand. 

  1. Ask the students which sample would have a greater detrimental effect on the environment.  
  2. Explain that the muddier sample has higher turbidity levels and therefore would cause more environmental damage. 
  3. Tell the students that turbidity is the cloudiness of the water caused by soil particles suspended in the sample.  Turbidity is measured by the amount of light that passes through the sample using a turbidimeter or a secchi meter.  Turbidity is measured in Nephelometric Turbidity Units or NTUs.  The higher the NTU, the more sediment is in suspension.  
  4. Bring up the Lesson Five slides and share the slide of different turbidity levels. 
  5. Ask students if they have observed water with varying degrees of turbidity before and to describe the experience.

Measuring Turbidity with a Turbidimeter

The most accurate assessment of turbidity is using a turbidimeter.  Turbidimeters are expensive tools, but could possibly be borrowed through a local soil and water conservation district (SWCD) or through local soil erosion and sediment control program.  To find a soil conservation program, visit the National Association of Soil Conservation Agencies: nascanet.org

There are two different types of turbidimeters that are most commonly used; a probe-style turbidimeter and a meter-style turbidimeter.  

  1. If using the probe turbidimeter, demonstrate to students how to insert a cylindrical probe into the sampling water.  
  2. The probe sensor is located at the tip of the probe and should be carefully wiped clean of any sediment and debris between sampling to ensure accurate measurement of the water. 
  3. Students should write down the turbidity level in their Rainbox Throwdown Lab Sheet.  Teachers might also put a version of the lab sheet on the chalkboard, so students can collectively view and compare their results. 

The meter version of the turbidimeter uses a clear glass vial into which you pour your sample.  Be sure to wipe the vial with a paper towel or cloth to remove any water or smudges before inserting it into the meter.  

  1. Demonstrate to the students how to insert the vial into the turbidimeter and push a “read” button which sends a beam of light through the sample.  The speed at which the light travels through the suspended particles and is reflected back is then recorded and displayed in NTUs.

Measuring Turbidity using a Turbidity Tube

Turbidity tubes can be purchased through a science education supply store (see Resources at the end of the curriculum) or constructed using simple supplies.  For a step-by-step guide on making a turbidity tube (Myers and Shaw, 2006) see Appendix B, page 83, PDF in this tab section:  Distribute the student handout for measuring turbidity, Total Suspended Solids, and PAM lab. Let the students use their student handout “Measuring Turbidity, TSS and PAM” to complete the instructions on measuring turbidity using the turbidity tube.  Groups can take turns watching each other make measurements. 

TSS Measurement Lab

Total Suspended Solids (TSS) is a measure of the total amount of suspended particulates in a given water volume (typically measured in mg/L).  TSS is measured by analyzing a known volume of sediment-laden water and pouring it through a pre-weighed filter.  The filter is then dried at 105 degrees Celsius and then weighed again.  By subtracting the weight of the filter with the sediment from the weight of the filter alone you will obtain the weight of the sediment alone.  You then have the amount of sediment (mg) in a known volume of water (L).  Students should use their handout, “Measuring Turbidity, TSS and PAM,” to begin the process to measure for TSS.  Measurements will need to be finished after the soil has dried on the filter.

Observing Turbidity

Take a hike down the rivers of Davie County and observe water samples pulled by Danny Lough, Davie County 4-H agent. Is there a difference between the stream and river samples? Why or why not?

Talking it Over

Turbidity and PAM

Share… what you did

What were the measurement results of the turbidity and TSS labs?  Which group had the lowest results?  The highest? What do these results mean?  Did you expect these results?  What did you do to get the results that you did?  In what ways would you improve your rainbox?  Which group is the winner?  Why would PAM be useful as a turbidity control practice? 

Reflect… on the results

Why is it important to be able to make accurate measurements?  Why does gathering data for an experiment matter?  Did your PAM work?  What if it didn’t work?  What does this mean to the scientific process? 

Generalize… to your community

What can you do with your turbidity and TSS measurements that might matter to your community?  Have you observed turbidity in your local creeks?  In what ways might you make a difference?

Apply … to your community

Can you think of ways you might be able to take the information you learned and apply it to a problem in your neighborhood?  Why is this important?  What resources will you draw on?

Turbidity Control: PAM

Turbidity Control: PAM Lab 

Just as there are best practices to manage erosion and sediment, there are best ways to control turbidity.  Refer to the turbidity slides in the Lesson 5 PowerPoint.  Tell students that construction sites are the most prevalent situations where turbidity levels in runoff are regulated and therefore need to be managed.  Agricultural environments focus on managing erosion and sediment and not turbidity.  Sediment basins on construction sites work well to manage sand and coarse silt-sized particles, but finer silt and clay particles can remain in suspension for a long time.  The only way to control these fine particles is through a chemical treatment. These chemical treatments are flocculants that bind soil particles together until they fall out of suspension. They range from mineral-based-flocculants like alum, which is a metallic hydroxide with a polymeric structure, to polyacrylamides (PAM). In order for PAM to be effective, it needs to be introduced to the runoff at a point of high flow velocity (SoilFacts, 2007).  The turbulence and mixing of the water, sediment, and PAM is needed to maximize the suspended sediment exposure to the large PAM molecules. Examples include placement in storm drain junction boxes where a pipe is dropping water, in a slope drain, or in other areas of falling or fast-moving water that is upslope from a sediment trap or basin check dams placed in a ditch. 

  1. Finish the lesson slides with PAM and explain to students that they will be trying to control turbidity for their own samples. 
  2. Have the students measure a teaspoon of soil and mix with 250ml of water in a small clear container with a lid (mason jars and lids make great containers). Students can also use leftover runoff samples from the Rainbox Throw down. 
  3. Shake the jar for 10 seconds to ensure thorough mixing.  
  4. Add a pinch (8 to 10 granules) of the granular PAM to the sediment mixture and shake again, ensuring that the PAM has adequate mixing time. 
  5. Be careful not to add too much of the PAM as it will quickly form a gel inside your container and waste some of the product.  
  6. If the PAM is suitable for the soil you will begin to instantly see flocs forming, sediment settling out of the suspension, and the water clearing.  
  7. If the PAM is not appropriate for the particular soil nothing will happen.  In this case, try another type of PAM if it is available or a different soil.  This experiment can be expanded by using more than one soil and PAM.  
  8. Have the students rank the effectiveness of the PAMs on these individual soils by charting their results on the board.  
  9. Ask the students what happened to their sample after the addition of PAM. Most should notice the sediment flocculating and sinking to the bottom of their jar, with a definite delineation between the sediment and the clear water.  (NOTE: different PAMs work best with specific soil types, so everyone might not always have this result, but you should still see some sort of flocculation). 

Why would PAM be useful as a turbidity control practice? Students should recall the sediment basin and the use of the skimmer to pull the cleanest water off the top of the water column.  PAM makes the water at the top of the basin cleaner and therefore creates a cleaner discharge. 

PAM Skimmer Demonstration:

After the PAM Lab, the teacher can demonstrate to students the basics of how a skimmer functions.  Ask the students to envision (or show them) the BMP slides illustrating the sediment basin with the skimmer.  Simply pour out the clean water while containing the flocculated sample.  Students should see the clean water leaving the jar and the soil aggregates remaining at the bottom. Ask the students to relate this idea back to what is happening in a sediment basin. 

Soil Flocculation

PAM works as a flocculant, gathering soil particles into clumps so they are heavy enough to fall to the bottom of the water. What do you notice in this video? Is one water sample clearer than another? Why?

Turbidity Control with Wattles and Polyacrylamide

What happens to the water after it flows over the wattles treated with PAM?

Assessment

Save Our Waters Video

Have each group create a 30 second to 1-minute video to convince the public (their peers/or the class) “Why clean runoff matters!”  Encourage the students to be creative and work together to develop a message about the importance of clean water and the role we play in helping manage soil erosion, sedimentation, and turbidity.

The videos can be used to assess students’ conceptual understanding as well as the life skills they have developed around teamwork. 

After the project is completed, ask the students about their experience.

  • “How did your group work together?” 
  • “What did you learn as a group that you might not have learned alone?”
  • “What was easy or difficult about working in a group?”
  • “Describe five ways in which new ideas are communicated to you.”
  • “In what other ways could you apply the skills gained in this activity?”
Assessment

The Daily Blog

Assign one or two students to blog about the day’s labs.  Encourage them to describe what they did and discuss what they learned when conducting the activities.  Ask them to describe why it is important for people to know about turbidity, total suspended solids, and best management practices in erosion, sediment, and turbidity control. What advice might students give to others who are concerned about these problems? Have them describe a time where they might need to use the skills they learned today.  Always include any photographs, videos, and links.  Have the students that are not blogging comment on previous postings as well as today’s writings.

Learn More

Leran More

Students who would like to continue their interest in erosion and sediment control can connect with local organizations that monitor and work to improve their watershed.  This could mean doing river clean-ups, monitoring construction activities to reduce sediment pollution, or helping frame local policy.  In many states across the country, there are Riverkeeping groups – they may be associated with parks and recreation, state parks, or environmental non-profits.  For example in North Carolina, there is the NC Riverkeeper’s Muddy Water Watch.

Note:  It is very important to remind the students never to go on to a construction site uninvited.  It is extremely dangerous and considered trespassing.

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