1 best management practices -structural measures – level ii: introduction to design education and...
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Best Management Practices
-Structural Measures –Level II: Introduction to DesignEducation and Certification for Persons Involved in Land Disturbing Activities
Issued May 2009
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The Manual for Erosion and Sediment Control In Georgia
• Referred to as the Manual or Green Book• Chapter 6 of the Manual focuses on the standards and
specifications for planning, design and installation of erosion and sediment control measures.
Updates to the Manual can be found on GSWCC’s website at www.gaswcc.georgia.gov under
Programs Urban Lands New Updates to the Manual for Erosion and Sediment Control
• Page includes newly approved practices, approved products list and other associated documents.
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Best Management Practices
Definition
• Vegetative measures & structural measures
• Properly designed, installed, & maintained in accordance with specification in the Manual for E&S Control
• Provide effective erosion prevention & sedimentation control
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Check Dam
Definition • A small temporary barrier constructed across
a swale, drainage ditch, or area of concentrated flow
Purpose• Reduce velocity• Filter sediment• Stabilize grade
Not to be used in a live stream
Cd
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Check Dam
Design Criteria
There is no formal design. The following standards shall be used:• Drainage area shall not exceed
• Two (2) acres for stone check dams• One (1) acre for haybales
• Height - the center of the check dam must be at least 9 inches lower than outer edge. Height should be 2 feet maximum measured to center of check dam.
Cd
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Check DamDesign
How to Compute L:
L = Height of Cd / Slope as decimal
Graded 2”-10”
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Check Dam
Example problem Independence School – School Road Slope From Topo = 5% – 13%
Find length between check dams for
1. 5%
2. 10%
3. 13%
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Check Dam
Solution1. L = Height of Cd / Slope as Decimal
L = 2 ft. / 0.05 = 40 ft.
2. L = Height of Cd / Slope as Decimal L = 2 ft. / 0.10 = 20 ft.
3. L = Height of Cd / Slope as Decimal L = 2 ft. / 0.13 = 15 ft.
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Check Dam
Example
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Channel Stabilization
Definition• Improving, constructing, or stabilizing an open
channel or waterway
Purpose• Prevent erosion and sediment deposition• Provide adequate capacity for flood water,
drainage, or other water management practices
Ch
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Channel Stabilization
Design Criteria• Typical linings include vegetation, riprap, and
concrete• Lining selection depends on the velocities within
the channel• Vegetative lining shall be established using
erosion control blankets or matting or sod
Ch
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Ch Channel Stabilization
For Design Velocity
0 - 5 Ft/Sec Vegetation (Ds3, Ds4, Mb)
5 - 10 Ft/Sec Rock Riprap (Appendix C)
> 10 Ft/Sec Concrete
• Grade stabilization structures may be needed to reduce velocities
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Channel StabilizationCh
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Channel StabilizationExample
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Construction Exit
Definition• Stone stabilized pad located at any point where
traffic will be leaving a construction site to enter a public right-of-way, street, alley, sidewalk or parking area.
Purpose• To reduce or eliminate the transport of mud from
construction area.
Co
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Construction Exit
Design CriteriaNo formal design. The following standards shall be used:• Aggregate size – stone in accordance with National Stone
Association R-2 (1.5 to 3.5 inch stone)
• Pad thickness – gravel pad minimum thickness of 6 inches
• Pad width – minimum width should equal full width of all points of vehicular egress, but not less than 20 feet wide
• Pad length – minimum of 50 feet
• Washing - Wash tires if action of vehicles over gravel does not remove sediment. Divert tire washing to proper area
Co
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Construction ExitCo
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Construction Road Stabilization
Definition• Travel way constructed as part of the construction
plan including access roads, subdivision roads, parking areas, and other on-site vehicle transportation routes
Cr
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Construction Road Stabilization
Design Criteria
• 6” coarse aggregate applied immediately after grading
• Geotextile applied for additional stability
• Grades should be < 10% for slope lengths less than 200 ft
• Road Widths:• 14 ft for one-way traffic
• 20 ft for two-way traffic
• 24 ft for trailer traffic
Cr
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Construction Road Stabilization
Geotextile underliner
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Stream Channel Diversion
Definition
• A temporary channel constructed to convey flow around a construction site while a permanent structure is being constructed within a streambed.
Purpose• To protect the stream channel from erosion and
allows work “in the dry”.
Dc
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Stream Channel Diversion
Design Criteria
• Drainage areas < 1 square mile (640 acres)
• Bottom width shall be a minimum of 6 feet or equal to the bottom width of the existing streambed, whichever is greater
Dc
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Stream Channel Diversion
Design Criteria
• Side slopes shall be no steeper than 2:1
• Table 6-12.1 gives channel linings and acceptable velocities. Note differences from channel stabilization.
Dc
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Stream Diversion Channel
Stream Diversion Channel Linings
Lining Material Acceptable Velocity
• Geotextile/ 0 - 2.5 fpspolyethylene
film or sod
• Geotextile alone 2.5 - 9.0 fps
• Type 1 Riprap & 9.0 - 13.0 fps Geotextile
Dc
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Stream Channel Diversion
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Diversion
Definition• A ridge of compacted soil, constructed above,
across, or below a slope to safely convey runoff to a stable outlet
Purpose• To reduce the erosion of steep or otherwise
highly erodible areas by reducing slope length, intercepting storm runoff and diverting it to a stable outlet at a non-erosive velocity.
Di
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Diversion
Design Criteria • Location determined by outlet conditions, topography,
land use, soil type, length of slope, seep planes, and the development layout
• A diversion consists of two components:• Ridge Design – have stable side slopes, no steeper
than 2:1 and minimum width of 4 ft at the design water elevation after settlement. Design shall allow 10% for settlement
• Channel Design – Land slope must be taken into consideration
Di
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Diversion
Selection of design storm based on type of diversion
to be used Temporary 10-yr, 24-hr storm Permanent 25 or 50-yr, 24-hr storm
Di
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Diversion
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Temporary Downdrain Structure
Definition• A temporary structure used to convey
concentrated storm water down the face of cut or fill slopes.
Purpose
• To safely conduct storm runoff from one elevation to another without causing slope erosion and allowing the establishment of vegetation on the slope.
Dn1
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Temporary Downdrain Structure
Design CriteriaThere is no formal design. The following standards shall be
used:• Placement – located on undisturbed soil or well
compacted fill• Diameter – provide sufficient capacity required to convey
the max runoff expected during the life of the drain• Sized according to its contributing drainage area
* 0.3 Ac = 10 in. * 0.5 Ac. = 12 in.
* 1.0 Ac. = 18 in.
Dn1
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Temporary Downdrain Structure
Commonly used in conjunction with Diversions (Di)
• Removed once the permanent storm water disposal system is installed and functioning
• Storm drain outlet protection (St), shall be placed at the downdrain outlet
Dn1
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Dn1
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Temporary Downdrain Structure
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Temporary Downdrain Structure
Placement
Diversion
Outlet protection and lined channel
Anchored every 10 ft
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Permanent Downdrain Structure
Definition • A permanent structure used to safely convey
surface runoff from the top of the slope to the bottom of the slope.
Purpose• Minimize erosion due to concentrated storm
runoff on cut of fill slopes
Dn2
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Permanent Downdrain Structure
Design Criteria
• May be constructed of concrete, pipe, pre-fabricated sectional conduit or other adequate materials
• Should be designed by professionals familiar with these structures
• All structures shall satisfy GDOT Standards and Specs
• Shall safely convey the 25-yr, 24-hr storm
• Outlets must be stabilized
Dn2
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Permanent Downdrain Structure
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Filter Ring
Definition • A temporary stone barrier constructed at storm drain
inlets and pond outlets.
Purpose• Reduces flow velocities, preventing the failure of other
sediment control devices. Prevents sediment from leaving the site or entering drainage systems, prior to permanent stabilization.
Fr
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Filter Ring
Design CriteriaThere is no formal design. The following standards shall be used:
• Location- Shall surround all sides of the structure receiving runoff and should be placed no less than 4 ft. from the structure. When placed in front of a retrofit it should be placed no less than 8-10 ft. from the retrofit
• Stone Size – Constructed of stone no smaller than • 3-5 inches in diameter for inlets with diameters less than 12 inches• 10-15 inches in diameter for pipes with diameters greater than 12
inches
• Height – no less than two feet from grade
Fr
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Filter Ring
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Filter RingExample
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Definition• Gabions are large, multi-celled, welded wire or
rectangular wire mesh boxes, used in channel
revetments, retaining walls, abutments, check
dams, etc.
Purpose• Used to stabilize steep or highly erosive slopes
GabionGa
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Design Criteria
•Construction plans and drawings should be prepared by
professionals familiar with the use of gabions
•Should be securely “keyed” into the foundations and
abutment surfaces
GabionGa
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Gabion
Examples
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Grade Stabilization Structure
Definition • Structure used to stabilize the grade in natural or
artificial channels
Purpose• Prevent the formation or advancement of gullies
and reduce erosion and sediment pollution
Gr
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Grade Stabilization Structure
Design Criteria• Structures – designed in accordance with sound engineering
practices – can be constructed of concrete, rock, masonry, steel, aluminum, treated wood
• Types - straight drop, drop inlet, box inlet, chute spillway• Capacity – Conditions of adjacent areas is considered when
determining the storm frequency• Residences/commercial 100-yr, 24-hr
& recreation buildings
• Recreation & 25-yr, 24-hrlandscape areas
• Agricultural Land 25-yr, 24-hr
Gr
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Grade Stabilization
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Grade StabilizationExample
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Level Spreader
Definition• A storm flow outlet device constructed at zero
grade across the slope whereby concentrated runoff may be discharged onto stabilized ground and converted to sheet flow.
Purpose• To dissipate storm flow energy at the outlet by
converting storm runoff into sheet flow and to discharge it onto areas stabilized by existing vegetation without causing erosion.
Lv
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Level SpreaderDesign Criteria• Length – Determined by 10-yr, 24-hr storm
• Width – Minimum 6 feet
Lv
Peak Q (cfs) Minimum Length (ft)
Up to 10 10
11 – 20 20
21 – 30 30
31 – 40 40
41 – 50 50
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Rock Filter Dam
Definition • A temporary stone filter dam installed across small streams
or drainageways
Purpose• Capture and filter sediment for removal when working in a
stream or water body. • Reduce velocity of water.
*The use of a rock filter dam in a stream is considered fill by
the U.S. Army Corps of Engineers and is not allowed in their
permit.
Rd
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Rock Filter Dam
Design CriteriaThere is no formal design. The following standards shall be used:
Drainage area - shall not exceed 50 acres
Height –should not be higher than the channel banks -center should be at least 6 inches lower than
outer edge
Slide slopes – shall be 2:1 or flatter
Location – as close to the source of sediment as possible
Stone size - determined by design criteria for Riprap
Rd
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Rd Rock Filter Dam
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Rock Filter Dam
Figure 6-18.1
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Retaining Wall
Definition • A wall constructed of concrete masonry, reinforced
concrete cribbing, treated timbers, steel pilings, gabions, stone drywall, rock riprap, etc
Purpose• Used to stabilize cut or fill slopes where stable
slopes are not attainable without the use of wall.
Design Criteria• Requires design specific to the site
Re
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Retaining WallExample
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Retrofitting
Definition • A device or structure placed in front of a
permanent storm water detention pond outlet structure to serve as a temporary sediment filter
Purpose• Allows permanent storm water detention basins to
function as temporary sediment basins for LDAs
• Shall not be used in detention basins on live streams or DA >30 acres for Rt-P
Rt
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Perforated Half-Round Pipe with Stone Filter
• Should be used only in detention ponds with less than 30 acre total drainage area.
• Never to be used on exposed pipe end or winged headwall.
• Diameter of half-round pipe should be 1.5 times the diameter of the principal pipe outlet or wider than the greatest width of the concrete weir.
• Perforations and stone sizes are shown in Figure 6-19.1.
• Shall be fixed by specified means (bolts, etc) to concrete outlet structure.
Rt-P
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Slotted Board Dam with Stone
• Can be used in detention ponds with drainage areas up to 100 acres.
• Can be used with open end pipe outlets, winged headwalls, or concrete weir outlets.
• Should be installed with minimum size 4x4 inch posts.
• Boards should have 0.5-1.0 inch space between them.
Rt-B
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Rt Retrofitting
Design Criteria• Height should be approx. ½ the height of outlet
structure.• Pond must be capable of storing the required
volume of storage in addition to required stormwater volume.
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Retrofitting
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Retrofitting
Example
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RetrofittingExample
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Sediment Barrier
Definition • Temporary structures typically constructed of silt
fence supported by steel or wood posts. Other types may include sandbags, straw bales, brush piles or other filtering material.
Purpose• Prevent sediment carried away by sheet flow from
leaving the site and entering natural drainage way.
Sd1
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Silt Fence
Design Criteria• Shall not be installed across streams, ditches, waterways
or other concentrated flow areas• Structure and all accumulated sediment will be removed
as soon as project is permanently stabilized• Types
• Type A - 36” wide• Type B - 22” wide • Type C - 36” wide
• Wire reinforced, high flows and velocities
Sd1
Alternatives for Type – B and Type – C are available at www.gaswcc.georgia.gov
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Criteria for Silt Fence Placement
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Silt Fence• For stream buffers and other sensitive areas,
two rows of Type C silt fence or one row of Type C Silt Fence backed by hay bales shall be used
Sd1-C
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Type A silt fence
Wooden
Posts
6’ Post Spacing
Sd1-A
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Silt Fence
Example
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Mulch Berms
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Straw Bale Detail
Sd1-Hb
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Detail of Type A
Silt Fence
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Silt Fence
Detail of Type B
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Silt FenceDetail of Type C
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Inlet Sediment Trap
Definition
• Temporary protective device formed around a storm drain drop inlet to trap sediment.
Purpose• To prevent sediment from leaving site or from
entering drainage systems prior to permanent stabilization of disturbed area
• Should be installed around all storm drain drop inlets that receive runoff from disturbed areas.
Sd2
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Inlet Sediment Trap
Design Criteria• The drainage area shall be no greater than
one acre
• An excavation may be created around the inlet sediment trap to provide additional sediment storage at the rate of 67 cubic yards per acre of drainage area
Sd2
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Excavated Inlet Sediment Trap
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Inlet Sediment Trap (Sd2-F)
• For inlets that drains a relatively flat area (slope
no greater than 5%) and shall not apply to inlets
receiving concentrated flows
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• For inlets receiving runoff with a higher volume or velocity
Inlet Sediment Trap (Sd2-B)
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Inlet Sediment Trap (Sd2-Bg)
• For inlets where heavy flows are expected and where an overflow capacity is necessary to prevent excessive ponding
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Inlet Sediment Trap (Sd2-G)
• For inlets where heavy concentrated flows are expected
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Inlet Sediment Trap (Sd2-P)• For inlets once pavement has been installed
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Temporary Sediment Basin
Definition • A basin created by excavation or the construction
of a barrier or dam across a concentrated flow area
• Consists of a dam, pipe outlet, and an emergency spillway
Purpose• To detain runoff waters and trap sediment from
erodible areas to protect properties and drainage ways
Sd3
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Temporary Sediment Basin
Design CriteriaSize according to location, size of drainage area, soil type, and rainfall pattern
• Location – shall never be placed in live streams
• Volume – shall be 67 cubic yards per acre drained
• Surface Area
• Shape – Length to Width Ratio greater than 2:1
• Spillways – Principal and Auxiliary. Even if the principal spillway is designed to convey the peak rate of runoff from a 25-yr, 24-hr storm, an emergency spillway shall be present
Sd3
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SpillwaysPrincipal
Typically CMP Conduit through dam (8 inch minimum) Perforated Riser (1/2 inch holes, 3 inches apart) or
Skimmer outlet Anti-seep Collars St – Outlet Protection
Emergency Ch – Channel Stabilization
Temporary Sediment BasinSd3
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Temporary Sediment BasinSd3
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Temporary Sediment BasinSd3
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Baffles
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Baffles
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Baffles
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Baffles
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Baffle Detail
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• Examples
Temporary Sediment BasinSd3
Incorrect filter rock size
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Temporary Stream Crossing
Definition• Temporary structure installed across a flowing
stream or watercourse for use by construction equipment.
Purpose• Provides a means for construction vehicles to
cross streams or watercourses without moving sediment into streams, damaging the streambed or channel, or causing flooding.
Sr
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Temporary Stream Crossing
Design Criteria• Not to be used on streams with a drainage area
greater than one square mile (640 Acres)• In place for less than one year• Design based on drainage area and slope of
watershed• Must convey 2-yr, 24-hr storm• Bridge or culvert crossings may be used• Installed perpendicular to stream
Sr
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TO BE SHOWN ON THE ES&PC PLAN
1. Drainage area (ac), average slope of watershed (%), and stream flow rate at bankfull flow (cfs).
2. Detailed dimensions of components for the type of crossing to be used.
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Temporary Stream Crossing
• Example
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Storm Drain Outlet Protection
Definition • Paved and/or riprapped channel sections, placed
below storm drain outlets
Purpose• Reduce the velocity of flow from storm drain outlets• Reduce erosion of receiving channels• Stabilize grades
St
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Storm Drain Outlet Protection
Design Criteria• Capacity • Tailwater Depth • Apron Length and Thickness• Apron Width• Bottom Grade• Side Slope• Alignment• Geotextile• Materials
St
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Outlet Protection
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Outlet Protection
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16
0.6
Outlet ProtectionDesign
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Outlet Protection
Example
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Surface RougheningDefinition • Providing a rough soil surface on the contour
Purpose• Aid in the establishment of vegetative cover with seed • Reduce runoff velocity and increase infiltration• Reduce erosion and provide for sediment trapping
Soil surface should not be roughened if slope is to be
stabilized with matting or blankets
Su
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Surface RougheningDesign Criteria
Selection of an appropriate method of surface
roughening depends on the type of slope. Slope
steepness, mowing requirements and whether the
slope is formed by cutting or filling should be
considered when choosing one of the three methods
of achieving a roughened slope surface.• Stair-Step Grading• Grooving• Tracking
Su
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Surface Roughening
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Tracking with bulldozer treads on clay soils is not recommended unless no alternatives are available.The soil surface is severely compacted and runoff is increased.
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Topsoiling
DEFINITION• Stripping off the more fertile top soil, storing it, then spreading it over the disturbed area after completion of construction activities.
PURPOSE• To provide a suitable soil medium for vegetative growth on areas where other measures will not produce or maintain a desirable stand.
Tp
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Conditions• This practice is recommended for sites of 2:1 or flatter slopes where:
1.The texture of the exposed subsoil or parent material is not suitable to produce adequate vegetative growth. 2. The soil material is so shallow that the rooting zone is not deep enough to support plants with continuing supplies of moisture and food. 3. The soil to be vegetated contains material toxic to plant growth.
TopsoilingTp
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Vegetated Waterway
Definition• A waterway that is shaped or graded to required
dimensions and stabilized with vegetation.
Purpose• Dispose of stormwater runoff• Prevent erosion• Reduce sedimentation
Wt
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Vegetated Waterway
Design Criteria
• Capacity – At a minimum convey peak runoff expected from a 25-yr, 24-hr storm
• Velocity –Is well within the limits of permissible velocity
• Drainage - Subsurface drainage measures shall be provided for sites with high water tables or seepage problems. When base flow is present a stone center or lined channel is required.
Wt
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Vegetative Waterway
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Vegetative Waterway
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Vegetative Waterway
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Sediment Storage Requirements
67 cubic yards per acre drained of sediment
storage is required for each project
Temporary Sediment Basins Excavated Inlet Sediment Traps Retrofitted storm water detention ponds
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Design Storms for Structures
2-year, 24-hour Storm Temporary sediment basin
(Principal spillway)
Temporary stream crossing 10-year, 24-hour Storm
Level spreader Diversion (Temporary) Grade stabilization structure
(Agricultural)
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Design Storms for Structures
25-year, 24-hour Storm Temporary sediment basin
(Emergency spillway)
Diversion (Permanent) Grade stabilization structure
(Landscaping & recreation)
Storm drain outlet protection Vegetated waterway
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Clean-out Elevations
One-Half (1/2) Full Silt fence Check dams Rock filter dams Inlet sediment
traps
One-Third (1/3) Full Temporary
sediment basins Retrofitted
detention ponds
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Review• BMPs
• Required on LDAs by Erosion and Sedimentation Act of 1975
• Proper design, installation, and maintenance
• Vegetative Measures• Control erosion• Treat at source
• Structural Measures• Control sedimentation• Treat after erosion has begun