3.2D – Construction – Grading and Excavation

Management Measure

1. Limit destruction of natural conveyance systems.
2. Reduce erosion and, to the extent practicable, retain sediment on site during and after grading and excavation activities; and
3. Implement approved erosion and sediment control plan.

Management Practices

Where possible, modification of natural drainage patterns should be avoided

Increasing impervious areas by paving and curbing contributes to water quality degradation by increasing peak flows and preventing the natural storm water treatment functions performed by vegetated areas. It is beneficial to route storm water over vegetated buffers, infiltration devices, or other pervious areas. Converting channelized storm water to sheet flow thus increasing its flow path allows these natural infiltration techniques to function properly and remove pollutants. Another option is using open vegetated swales in place of conventional conveyance devices.

Consider using landform grading techniques to restore natural drainage features on the landscape

Mass grading is defined as the movement of earth by mechanical means to alter the topographic features of a site, including elevation and slope to prepare for the construction of facilities. Mass grading results in a flattened, featureless landscape or a terraced hill with uniform engineered 2:1 slopes (see schematic A below). The soils are heavily disturbed and compacted in this process with any natural drainage features put into an underground culvert. Landform grading preserves and/or restores natural drainage features (see schematic B below).

                A. Mass graded slope

                B. Landform graded slope

Mass grading alters the pre-existing hydrograph, impedes soil infiltration and soil moisture thus limiting soil biota. Landform grading techniques create radial drainage patterns and concentrate flow in valleys, creating microclimates for the establishment of vegetation, often without the need for constant irrigation (as is the case for vegetated areas on 2:1 slopes). As a result of any grading, however, natural soil forming processes are disturbed and therefore, biologically active soil biota is unable to perform their pollutant transformation functions via aerobic nutrient cycling. During mass grading soils are compacted; eliminating soils texture and micropores thus decreasing the ability of the soil to absorb water and pollutants.

Erosion Control Practices are the first step in reducing sediment pollution from construction and demolition sites

There are several opportunities for erosion control, beginning at the planning stages of construction. Clearing and grading should be scheduled during the dry season when storm water runoff is expected to be minimal. Construction should be undertaken in a phased schedule, in contrast to the traditional practice of grading a site or excavating it all at once. In phased construction, clearing, grading, and building take place at only one part of a site at a time, and new parts of the site are cleared only after the last part is stabilized with permanent erosion controls and revegetated.

The use of chemical additives to stabilize the soil is sometimes recommended to reduce erosion of exposed, unvegetated areas. Polyacrylamide (PAM) is a common polymer for controlling erosion and promoting infiltration on irrigated agricultural lands, and it has been recommended to reduce erosion on urban construction sites and disturbed areas. It decreases soil bulk density, absorbs water, and binds fine-grained soil particles. Caution should be used when applying PAM in ecologically sensitive areas because its toxicity to aquatic life is unknown. Roa-Espinosa et al., (2000) provide a literature review and experimental results of PAM use at construction sites.

After clearing (including demolition), grading, and building are complete, temporary and permanent erosion controls should be implemented, including seeding, mulching, sodding, and installing erosion control blankets. Soil bioengineering techniques can be used in lieu of retaining walls for slope stabilization practices where sloped roadsides are deteriorating. Practices such as live stakes, fascines, brush layers, branchpacking, live gully repair, live cribwalls, vegetated rock gabions, vegetated rock walls, and joint planting are recommended for relatively moderate slopes where vegetation can be established (see Management Measure 5.2A, 6B and 6C for more information).

• Bioengineering uses live plant materials to provide erosion control, slope and stream bank stabilization, landscape restoration, and wildlife habitat. These techniques are used alone or in conjunction with conventional engineering techniques. Soil bioengineering refers to the use of amendments such as compost, mycorrhizae fungi, bacteria inoculations and other soil treatments that promote living soil and functional soil ecology.
• Seeding with native grasses can be used to establish permanent erosion control. There are several seeding techniques that can be used, including broadcast seeding, hydroseeding, and drill seeding. Broadcast seeding is the simplest method and involves scattering seeds by hand or mechanically. Hydroseeding involves spraying a slurry of seeds, fertilizer, tackifier, and water onto exposed soils. This method is more expensive but can be more effective at erosion control because the water and fertilizer additives promote fast growth and the tackifier provides immediate stabilization.
• Mulching of disturbed soils can be effective at reducing erosion. Materials used include tacked straw and wood chips and are often covered by erosion control blankets or netting. The mulch typically has a short useful life and is only a temporary measure. Mulching alone should be used when permanent seeding is not feasible, such as in arid or winter conditions when vegetative growth is slow or absent.
• Using sod permanently and immediately stabilizes an area with a thick vegetative cover and should be used in sensitive areas or where establishing permanent vegetation by seeding would be difficult.
• Erosion control blankets or turf reinforcement mats (TRMs) protect the soil from scouring due to runoff and can enhance vegetative growth. TRMs can raise the threshold of natural vegetation to withstand higher hydraulic forces on slopes and streambanks and in channels. They should be used in combination with seeding to achieve permanent results.
• Wind erosion controls, which include bales of hay, solid board fences, and snow fences, can be used to keep soil in place. Sprinkling with water can moisten the soil surface, but watering should be done in moderation to prevent a non-storm water discharge.
• Runoff can be intercepted above disturbed sites and conveyed to a permanent channel or storm drain. Conveyance systems may be earth dikes, perimeter dikes/swales, or diversions. A pipe slope drain or pipe drop structure is a temporary pipe placed from the top of a slope to the bottom of the slope to convey concentrated runoff down the slope without causing erosion.
• On long or steep, disturbed, man-made slopes, benches, terraces, or ditches can be constructed at regular intervals, or retaining walls can be erected, to intercept runoff, slow it down, and prevent it from becoming concentrated flow.
• Linings for urban runoff conveyance channels can be installed to prevent scouring. The first choice of lining should be grass or sod because it reduces runoff velocities and provides water quality benefits through filtration/infiltration. Also effective are turf reinforcement mats, riprap, concrete, and gabions.
• Flow control practices can be installed in channels to reduce runoff velocity. For example, check dams are small temporary dams constructed across a swale or channel and can be used to reduce the velocity of concentrated flow and, therefore, to reduce erosion in a swale or channel.

Sediment Control Practices

The second step in preventing sediment pollution from construction sites is to install devices that trap or filter sediment from runoff. These can include sediment basins and traps, filter fabric fences, inlet protection devices, and stabilized construction entrances.

• Sediment traps: These are typically installed in a drainage way or other point of discharge from a disturbed area. They are small impoundments that allow some sediment to settle out of runoff water before it is drained through a rock dam. Temporary diversions in the form of berms or channels can be used to direct runoff to the sediment trap.
• Sediment basins: The use of sediment basins is a popular way to retain sediment generated at the site during construction and can be adapted to serve as runoff control after the site is stabilized. A perforated pipe riser (sometimes wrapped in filter fabric) connected to a drain pipe constricts flow and slowly releases impounded water from the bottom of the basin. A recent modification to the standard sediment basin design is the use of a floating skimmer, which slowly drains relatively clear water from the top of the basin. These structures require regular inspection and maintenance to ensure that they are not clogged with debris or sediment.
• Filter fabric fence: Filter fabric or silt fences can be used along the perimeter of the disturbed area to filter out sediment as runoff flows through the fabric. Such fences should be used only where there is sheet flow (no concentrated flow), and the maximum drainage area should be one-half acre or less per 100 feet of fence.
• Fiber rolls: (also known as straw wattles) consisting of straw, mulch, or composted material rolled and bound (sometimes in filter fabric) can be staked on a hillside or other erosion-prone area. Fiber rolls are installed perpendicular to a slope to act as check dams. When properly installed, fiber rolls reduce the velocity of runoff and can provide sediment removal through filtration. Another advantage is that they can be left in place because they consist of natural materials. On construction sites with relatively flat slopes, fiber rolls are often used at the perimeter of the site between the disturbed area and a sidewalk as a perimeter control in lieu of, or in combination with, silt fence.
• Compost filter berms: are mounds of material, typically recycled wood chips and bark, that are installed at site perimeters or along slopes to act as a check dam and filter pollutant-laden runoff. The berms are designed to filter runoff by absorbing flows into the compost mixture’s void space, gradually releasing them into the ground or offsite. They are usually installed at the bottom of a slope, but they also can be installed at the top of the denuded area to prevent clean runoff from entering exposed areas. Berms are typically installed in lieu of silt fence and are sized at 1 foot high and 2 feet wide (Tyler, 2001). Caine (2001) found that flows through a compost filter berm mobilized humic and tannic acids from the organic material present. In areas that are susceptible to BOD or pH changes, compost material should be carefully chosen to ensure that the practice does not contribute to water quality impairments.
• Storm drain inlet protection: Sediment should be excluded from storm drains using inlet protection measures to trap sediment before it enters the storm sewer system. Common inlet protection measures include riprap wrapped in chicken wire, cinder blocks filled with gravel, straw wattles wrapped in filter fabric, drop-inlet bags, and other combinations of materials that filter runoff. Inspection and maintenance of these measures is essential to their effectiveness; failure to remove sediments and debris can result in reduced treatment of runoff and flooding due to clogging.
• Stabilized construction entrances: Construction entrances should be clearly designated and reinforced with gravel, corrugated metal sheets, or devices specially designed to clear tires of sediment and hold it for later cleanout. This practice of protecting construction entrances can minimize the loss of sediment associated with the equipment and traffic leaving the site.

Programs

• SWRCB, Division of Water Quality, Storm Water Program Discharges of pollutants from construction activities are for the most part regulated under the NPDES permitting program. Regulated entities include all construction sites with one or more acres of disturbed area. Discharges of pollutants from construction sites smaller than 1 acre typically are considered nonpoint sources but might also be regulated at the local level. Construction site operators should contact the municipal department for more information about local requirements, including air quality requirements for dust control.

Information Resources

• City of Watsonville, Erosion Control Standards specifies temporary erosion control measures for the City of Watsonville, California. These control measures, effective during the rainy season (October 15 to April 15), are required to be installed and maintained in accordance with the San Francisco Bay Regional Water Quality Control Board’s Erosion and Sediment Control Field Manual.
• Kentucky Erosion Prevention and Sediment Control Field Guide, this field guide offers succinct, basic information on construction site preparation, project planning, and erosion and sediment control techniques. The guide offers an assessment of various construction site management practices along with photos and graphics discussing the practices.
• Association of Bay Area Governments (ABAG), Best Management Practices (BMPs) for construction site planning and management, erosion and sediment control, pollution prevention, and sampling guidelines. Descriptions of BMPs are concise and include full-color graphics and installation information including guidelines, timing and limitations. Also includes the new Phase II regulations, the State Board sampling and monitoring guidelines, and long-term maintenance information. Recommended by the San Francisco Bay Regional Water Quality Control Board, the Home Builders Association of Northern California and the Engineering Contractors Association. (Revised 2002)
• California Stormwater Quality Association (CASQA), Storm Water Best Management Practice Handbook: Construction outlines waste management practices in a set of fact sheets that include erosion controls (scheduling, velocity dissipation devices, slope drains, streambank stabilization, polyacrylamide, preservation of existing vegetation, hydraulic mulch, hydroseeding, soil binders, straw mulch, geotextiles and mats, wood mulching, earth dikes, and drainage swales), sediment controls (silt fence, storm drain inlet protection, chemical treatment, sediment basins, sediment traps, check dams, fiber rolls, gravel bag berms, street sweeping and vacuuming, sandbag barriers, straw bale barriers, stabilized construction entrances and exits, stabilized construction roadways, entrance/outlet tire washing), and wind erosion control.
• Caltrans Storm Water Quality Handbooks, Construction Site Best Management Practices Manual, Fiber Rolls fact sheet describes appropriate applications, limitations, standards and specifications, and maintenance and inspection considerations for fiber rolls. The manual also includes guidance on fiber roll assembly, installation, and removal.
• Horst, Stephen and D. Gray. 2007. Landforming: An Environmental Approach to Hillside Development, Mine Reclamation and Watershed Restoration. Wiley ed. ISBN: 978-0-471-72179-6.
• Horst, Stephen per City of Pasadena Arroyo Seco Master Plans Design Guidelines, Appendix C: Landform Grading Principles provides some design guidelines for landform grading techniques.
• Horst, Stephen. 1991. Comparative Definitions of Grading Techniques. Landscape Architect and Specifier News 9(11): 21-25.
• Metropolitan Washington Council of Governments, Smart Growth Begins at the Local Level offers a multimedia video and Web site to inform and educate elected officials, civic groups, the development community, and citizens about the land use and transportation challenges currently facing the region. In addition to the video, the site also offers an overview of smart growth principles, frequently asked questions, links, resources, and news.
• Washington State Dept. of Transportation, Roadside and Site Development, Soil Bioengineering information.
• USEPA, National Menu of Best Management Practices for Storm Water Phase II, guidance for small NPDES-regulated municipalities details numerous erosion and sediment control techniques to reduce the generation and offsite transport of dust, sediment, and construction site chemicals and materials.

References

Association of Bay Area Governments. 2003. Construction Site Planning. Downloaded from http://store.abag.ca.gov/construction.asp on April 22, 2003.

Caine, E. 2001. Quilceda-Allen Watershed Erosion Control Program Water Quality Monitoring Report. Snohomish County Department of Planning and Development Services, Everett, WA.

Roa-Espinosa, A., G.D. Bubenzer, and E.S. Miyashita. 2000. Sediment and runoff control on construction sites using four application methods of polyacrylamide mix. In National Conference on Tools for Urban Water Resource Management and Protection: Proceedings, Chicago, IL, February 7-10, 2000.

SWRCB. 2003. Storm Water Program. Downloaded from http://www.swrcb.ca.gov/stormwtr/index.html on April 22, 2003.

Tyler, R. 2001. Compost filter berms and blankets take on the silt fence. BioCycle (January 2001).

USEPA. 2005. National Management Measures to Control Nonpoint Source Pollution from Urban Areas. EPA 841-B-05-004. U.S. Environmental Protection Agency, Washington, DC. Downloaded from http://www.epa.gov/owow/nps/urbanmm on March 3, 2008.

(Updated 9/4/08)