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Stormwater Control Practices

John Poullain, P.E.


Course Outline

This two-hour online course provides general guidelines and practices to control stormwater runoff and sediment at construction sites and urban areas. The methods and techniques discussed include sedimentation basins, detention basins and constructed wetlands and combinations of these systems. Other stormwater management and protection practices are briefly described. Streambank protection is not discussed here but some of the practices discussed may also be suitable for streambank protection. Remedial actions and activities performed at construction sites must comply with federal, state and local regulations to protect water quality.

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:


Intended Audience

This course is intended for civil engineers and planners.


Benefit to Attendees

The student will become familiar with methods and techniques used to manage stormwater drainage at construction sites and developed areas. The purpose is to reduce erosion, water ponding and runoff of sediments and pollutants onto down gradient land and downstream water or streams. Ground water and surface water may contain volatile, soluble organics, corrosive acids and alkalis. The student will better understand the application of detention methods such as sediment and detention basins. Basic guidance in the selection of a practice to use is discussed. A matrix is shown as an aid in selecting erosion and sediment control practices.


Course Introduction

This course covers the practices commonly used to manage the movement of storm water and sediments at construction sites. The wind erosion occurring during the "dirty thirties" fostered early soil conservation efforts. 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 pollutants which can contaminate the soil, groundwater, wells and nearby bodies of water unless properly managed. Land disturbed by construction activities requires precautionary measures to reduce soil erosion and sedimentation. A construction site must be investigated for a wide range of conditions, including ground water level, surface drainage and subsurface ground conditions.

Sediment contains soil particles along with petroleum products, metals, chemicals, corrosive acids, pesticides, organics and other pollutants. Plans for erosion and sediment control (ESC) are necessary to protect property, waterways and wildlife habitat. An ESC plan establishes the specific control practices intended to prevent erosion and off site sedimentation. The plan clearly defines the location, installation and necessary maintenance of the practices to control the anticipated erosion and to contain sediment. Based on volume, sediment from erosion may be a larger pollutant of waterways and lakes than are toxic waste or chemicals. In cases where the National Pollutant Discharge Elimination System (NPDES) for stormwater permits applies, a Stormwater Pollution Prevention 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.
Erosion occurs naturally but is accelerated by land development and land use changes. Increased urbanization, with increased roof areas and paving and the 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 will further accelerate erosion and siltation. Using detention or sedimentation basins attempts to keep the stormwater runoff rate from exceeding the predevelopment rate. In order to develop an area and properly manage the stormwater, the predevelopment and post development runoff must be estimated and compared for the area.

The stormwater control practices described in this course are keyed to the detention of runoff and the deposition or trapping of sediment. If the containment could retain all runoff the efficiency would be 100%. Retention of runoff is impractical and would not be feasible and prohibitively expensive considering the required acquisition of land and the loss of future development. Detention practices serve to reduce the frequency of erosive downstream flooding and to trap the sediment and contaminants or nutrients carried by the runoff. Water is discharged at a controlled non-erosive rate and suspended particles are allowed to settle.

Detention Basins and Ponds

Detention basins and sediment basins both reduce stormwater runoff velocities, trap sediment in water pools and then discharged the water at a controlled rate of flow. The differences between these practices however, are the detention times, drainage areas served and height of dam embankments. Most detention basins are designed to store the first ½ inch of runoff from a design storm and release the remainder at a rate equal to that rate of runoff existing before development of the site. Acceleration of downstream erosion from the site is then prevented. Larger detention basins are designed to store the first 1-inch of runoff. Inspection and preventative maintenance becomes a very important consideration for detention basins because of their longer detention times, higher embankments and drainage areas of 50 acres or 100 acres for larger or extended basins.

Generally stormwater runoff from an area will be more polluted than subsequent runoff. It is important to capture this most polluted runoff. Proper management will in cases require on-line flow-through pollution control. Later runoff can then be directly diverted to the stormwater system. However the first flush flush will depend on the nature, mobility, supply and source of pollutants and may not always be observed for these reasons:

a. Drainage area Size - Runoff from farthest sources of large areas may not reach the stormwater control devices for some time after the storm starts. Small areas would not have that time lag.

b. Some pollutants are less mobile, oil and grease, and are not moved as quickly as more soluble materials.

c. First flush may only occur when the supply of pollutants is limited unlike the supply of soil particles eroded.

The first flush water should be re-used or properly disposed of and re-use is preferable. Re-use includes alkaline first flush water for concrete plants. Other re-use includes hot-mix bitumen emulsion plants and nursery irrigation.

Porous Pavements

These pavements consist of a permeable asphalt 2-4 inches thick or concrete 4-6 inches thick placed on a crushed stone base course. This pavement structure is constructed over a stone reservoir of 1.5 to 3.0 inch diameter stone with a min thickness of 6 inches, which varies according to the design storage capacity. The base course also acts as a stone reservoir for the porous pavement and provide groundwater recharge and reduces the amount of stormwater runoff. Normally runoff from surfaced pavements is 90 to 95% with little loss to infiltration into the road subgrade. Porous pavements may reduce the runoff by 30% or more depending on the volume of the stone reservoir used under the pavement. In addition to the additional construction costs, porous pavements are limited to low traffic areas and require of maintenance to prevent clogging. Also sand or ash should not be used for snow removal to prevent clogging.

Other Management and Protection Practices Commonly Used

Management practices also include overland flow diversion and perimeter protection of a site with earth berms and ditches, terraces, temporary swales, vegetation and controlled development along fills. Structural methods include riprap, cellular mattresses, earth berms and sediment fences.

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 and across slopes to reduce the length of slope exposed to erosion. 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, asphalt concrete, concrete etc. or by any combination of these. The diverted runoff should be discharged into a stable area through pipes or ditches lined with riprap, Portland cement concrete etc. for erosion protection.

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.

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. The construction and maintenance of swales and ditches are similar.

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 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 and sediment fences is limited to small drainage areas of about two acres and is 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, 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. The bales should be entrenched at least four inches or backfilled with compacted soil and anchored with stakes. 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 on 2H or 3H is 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. Annuals such as wheat or rye provide winter protection for the permanent grasses to become established and help in controlling weeds.

Soil Bioengineering - Woody vegetation is used for slope protection and slide repairs and usually provides greater slope protection than grasses alone. The woody vegetation can also be combined with a structural system, concrete block or 3-dimensional synthetic grids on steep slopes. The structure allows immediate slope protection and time for the vegetation to become established. Woody vegetation practices are well suited for areas with limited access for equipment and for short reaches with steep slopes.

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.
Temporary Gravel Entrance/Exit - A gravel entrance/exit pad provides a stable entrance and exit for the construction site and helps to keep mud from being tracked on to public roads. The gravel pad is constructed of 2-3 inch stone at least six inches thick and usually 50 feet long. If wetness or soft soil conditions are problems, geotextile fabric may be placed to stabilize the foundation of the entrance/exit pad.

Course Content

This course is based primarily on Chapters 2 and 6 (selected sections) 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, 42 pages), PDF file and the Course Introduction paragraph.

The link to the Chapters 2 and sections of Chapter 6 are

Chapter 2
Chapter 6

You need to open or download above documents to study this course.


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 runoff, to control soil erosion and sediment at construction sites and to protect water quality. The methods considered include sediment and detention basins and constructed wetlands. The design, installation, type of materials, advantage and the effects of the physical site condition is also considered. Methods for collecting and detaining stormwater water from construction sites are presented.


Related Links

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

https://www.epa.gov/npdes/npdes-stormwater-program.htm
NPDES homepage-info about stormwater maintenance, erosion control and discharges at construction, industrial, city sites and additional info. 
                          
http://www.epa.gov/owow/lid
EPA Office of Water responsible for ensuring that watersheds, drinking water are maintained for ecosystems, human health, safety..

The course text is based on the State of Mo which uses the DIGRITE phone.  A national directory of phone numbers to call for safe digging in all states can be found at the link http://call811.com/811-your-state 


Quiz

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

Take a Quiz

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.