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Construction Stormwater Runoff – Sediment Controls

John Poullain, P.E.


Course Outline

This four-hour online course provides general guidelines and practices for managing stormwater runoff and sediment control at construction sites. The purpose of sediment control is to slow the flow and pond the stormwater allowing suspended sediment to settle out and not leave the construction site. The methods discussed include earth berms, erosion control blankets, diversion and drainage ditches, sedimentation basins traps and combinations of these systems. Remedial actions and activities performed at construction sites must comply with federal, state and local regulations to protect water quality.

Among the topics covered in the course:

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will have covered these topics:

Intended Audience

This course is intended for engineers, planners, local governments, site developers, and

others involved in sediment control measures at construction sites. 

Benefit to Attendees

The student will become familiar with methods used to manage stormwater drainage at construction sites. The purpose is to reduce erosion, water ponding and runoff of sediments and pollutants onto downslope land and downstream water or streams. Ground water and surface water can contain contaminants such as volatiles, soluble organics, corrosive acids and alkalis. The student will better understand the application of interception or diversion methods such as ditches, berms, down pipes, and sediment basins. Basic guidance in the selection of the most appropriate practice to use are discussed.  

Course Introduction

This course covers the practices commonly used to manage the movement of storm water and sediments at construction sites. Early soil conservation efforts were driven by the wind erosion occurring during the “dirty thirties”. These first measures were aimed at soil conservation but as land development increased, runoff erosion and sedimentation became issues also. Remedial actions consist of site control to prevent off-site migration of surface water, sediments and contaminants. Land disturbed by construction activities requires precautionary measures to reduce soil erosion and sedimentation. 

Sediments contain not only soil particles but pollutants like petroleum products, metals, chemicals, pesticides and organics. Measures for erosion and sediment control  are necessary to protect property, waterways and wildlife habitat. Based on volume, sediment may be a larger pollutant of streams, rivers and lakes than toxic waste or chemicals. The amount of sediment could also be more from construction activities than would normally occur. In cases where the National Pollutant Discharge Elimination System (NPDES) for stormwater permits applies, a Stormwater Pollution Prevent Plan (SPPP) is required for construction sites. A SPPP will specify the control measures and deadlines to meet at a construction site. State and local governments also have ESC regulations that require special precautions for land-disturbing activities.    

Soil Erosion Causes

Land Development and Land Use Changes. Increased urbanization, more roofs, paving and clearing of woodland and grassland without proper conservation management create erodible land areas, speed up runoff and remove areas available for rainfall infiltration. Vegetation removal from land area further accelerates erosion and siltation.

Overbank Drainage and Groundwater Seepage. Overland stormwater drainage or runoff can severely erode unprotected slopes during runoff. Rainfall that does not runoff may seep downward through the soil resulting in a loss in shear strength. This condition may appear as wetness or piping flow down the slope. Clay soils swell and lose shear strength after absorbing surface and groundwater, resulting in creep or soil movement.

Management and Protection Practices

Management practices include overland flow diversion and perimeter protection of a site with earth berms and ditches, vegetation and by controlling development along fills. Structural methods include riprap, rock check dams, earth berms, sediment fences, compost blankets, straw bale traps and sediment basins.

Diversion Ditches and Berms - Runoff erosion of slopes can be reduced by intercepting and diverting water away from the top of slope with diversion ditching, diversion berms or a combination of both. They can be used to protect newly graded slopes until vegetation is established. Diversion ditches can be constructed across slopes to reduce the length of slope exposed to erosion, intercept flow at the top of slope or to convey stormwater at the slope base. The fill for berms is usually provided from the ditch excavation. The ditches may be seeded and mulched or sodded or for long-term erosion protection, stabilized with riprap or concrete or by any combination of these. The diverted runoff should be discharged into a stable area through pipes or ditches lined with riprap, concrete etc. for erosion protection.
intro

Diversion Ditches and Berms

Grading and Bench Terraces - Slopes that are too steep can be reshaped by grading the slope. It is more feasible to terrace slopes for small reaches. Terracing slows rainfall and runoff by breaking the gradient of long steep slopes in steps. Ditching on a bench terrace may not be necessary since the benches are constructed to typically slope inward toward the top of slope in order to divert runoff water along the bench rather than down the slope. Vegetation is then more easily established and maintained.  

Swales - Swales are used in a similar fashion, as ditches but do not block traffic as ditches due to their wider and shallower channel cross section. A wider and shallower cross section reduces the stormwater velocity and potential for erosion. A wider but shallower trapezoidal channel compared with a v-ditch of equal area for instance will be shown to have a lower velocity of flow. Grassed swales also promote infiltration and help to remove sediments.

Temporary Slope Drains, Flumes and Down Pipes - Flumes and downpipes are temporary structures used to carry concentrated flows down the face of a slope without erosive damage. Flumes are open channels lines with an erosive resistant material such as concrete, grouted riprap etc. Downpipes are made of corrugated plastic or metal. They have standard entrance sections and like flumes discharge to outlets stabilized with riprap aprons or sediment traps. The advantage of both practices includes prevention of erosion on steep slopes and they do not require special equipment or materials. They are intended for temporary erosion control until a slope is stabilized with vegetation and/or practices.

Straw Bale Traps or Check Dams - Use of straw bales like sediment fences is limited to small drainage areas subject to sheet flow and intended to intercept and detain small amounts of sediments. Straw bale traps however can be located across small swales. For large drainage areas and swales, which have concentrated flows, rock dams or sediment basins are more suitable. Erosion will occur around or under the bales at adverse sites with steep slopes. Installation of straw bales is similar to the requirements of sediment fences. A stabilized bypass should be considered for sites with large stormwater flows to protect the integrity of the straw bale trap.

Vegetation - Site specific conditions must be considered to use vegetation. Success of vegetation depends on the climate characteristics, slope grades, site preparation and compatibility of vegetation with these conditions. Vegetation is one of the most commonly used methods for stabilization. It is relatively easy to maintain and establish and properly selected plants and grasses are self-maintaining. Erosion control matting may be necessary to hold the seed and soil in place until the vegetation is established. Slopes flatter than 1V to 1 ½ H can accommodate vegetation but slopes flatter than 1V on 2H or 3H are preferable especially for ease and safety in maintenance.

Vegetation protects a slope with the roots and exposed branches, stems. Surface flow velocity is reduced and the capacity for infiltration and water withdrawal from the soils is increased. Seedbed preparation, fertilizers, planting dates, rates of application and type of grasses will depend on the region, specific area for planting, time of year and as specified in the design plans. Also there are temporary and permanent plantings. Permanent seeding is typically for periods longer than 12 months with perennial grasses. Nurse crops are sometimes uses in the seeding mix. Annual grasses provide winter protection for the permanent grasses to become established and help in controlling weeds.

Riprap - There are several ways to place riprap. It can be mechanically placed along the slope or in wire baskets as a blanket over the slope.

Riprap mattresses are relatively flexible and can adjust as changes from settlement or erosion occur. Minor damage can be easily repaired with additional stone to fill settlement or voids from erosion. A rule of thumb for mattress thickness is 1.5 times the thickness of the largest stone being used. Filter fabric or a drainage material is usually placed as an underlayment to protect from loss of fine soils and to allow for water seepage under the riprap.

Course Content

This course is based primarily on Chapter 6 (Section 4) from the Missouri Department of Natural Resources Manual, “Protecting Water Quality”, a guide to erosion, sediment and stormwater best management practices (BMP) for development sites, (2011 Edition, 108 pages), PDF file. 

MoDNR Manual “Protecting Water Quality”, Chapter 6 (Section 4)

Terms and Definitions

Please note the following errors in references to figures:
PAGE                  WRONG FIG REF           CORRECT FIG REF
Page 6-124          Reference to Fig 5.44 --> Fig 6.41 (pg. 6-123)
Page 6-138          Reference to Fig 3.83 --> Fig 6.52 (pg. 6-139)
Page 6-140          Reference to Fig 5.84 --> Fig 6.53 (on same page)
Page 6-141          Reference to Fig 5.85 --> Fig 6.54 (on same page)
Page 6-188          Reference to Fig 5.89 --> Fig 6.80 (on same page)
Page 6-198          Reference to Fig 5.87 --> Fig 6.84 (on same page)
Page 6-223          Reference to Fig 5.77 --> Fig 6.85 (on same page)

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Course Summary

State and federal regulations have to be complied with at construction sites in order to remove any threat to public health or the environment. This course considers the techniques and methods used to manage stormwater, the control of run-on and run-off and the control of soil erosion and sediment at construction sites to protect water quality. Among the methods considered include ditches, berms, flumes, and sediment basins. The design, installation, type of materials, advantage and disadvantages and the effects of the physical site condition are also considered. 

Related Links

For additional technical information related to this subject, please refer to:

http://www.errosioncontrol.com/ec.html
“Erosion control at Construction Sites” has information about erosion and sediment control and stormwater management from the journal “Erosion Control” published by the International Erosion Control Assc. 

http://www.cwp.com/esc_practices.htm
The Center for Watershed Protection provides technical tools to local governments and watershed organization watershed protection, restoration and research and stormwater management.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.