sediment and erosion control: field performance of construction site bmps and optimized designs for...
TRANSCRIPT
Sediment and Erosion Control: Field Performance of Construction Site BMPs and Optimized Designs for
Enhanced Stormwater Control
Stuart Jennings
Reclamation Research Group, LLC
Format of Presentation
• Review of BMPs and resources
• Estimating erosion severity
• BMP strategies
• Observations from construction sites
On-site Stormwater Control
• Which BMPs are appropriate to my site?– What are the unique soil and
vegetation characteristics of my site?
– What can I expect for precipitation?
– How long will it take for stabilization by vegetation?
Families of BMPs
• Surface Stabilization BMPs—hydromulch, straw mulch, erosion control blankets, temporary seeding, slope roughening, others
• Sediment Control BMPs—silt fence, check dams, sediment basins, lined channels, straw bales, others
• Non-storm water BMPs—Equipment decon areas, stockpile management, tracking control, stabilized site entrances, others
Revisiting the Construction BMP Palette Using EPA and Montana Department of
Transportation Guidance
• http://cfpub.epa.gov/npdes/stormwater/menuofbmps/
• http://www.mdt.mt.gov/research/projects/env/erosion.shtml
MDT Menu of BMPs– Section 3 - Best Management Practices
– Section 3a - Soil Stabilization BMPs
– Section 3b - Sediment Control BMPs
– Section 3c - Wind Erosion Control
– Section 3d - Snow Management
– Section 3e - Stabilized Construction Entrance/Exit
– Section 3f - Water Conservation Practices
– Section 3g - Material Delivery and Storage
50 page .pdf, 15 BMPs
41 page .pdf, 11 BMPs
3 page .pdf, 1 BMP
7 page .pdf, 3 BMPs
Developing a site specific strategy
• Soil type• Slope steepness• Potential rainfall• Adjacent water resources• Seasonality of work• Construction sequencing• BMP selection, installation, monitoring,
maintenance• Emphasis on surface stabilization or sediment
control BMPs, or both
Erosion Prediction using the Universal Soil Loss Equation
A=RKLSCP• A = Average Annual Soil Loss (tons)• R = Rainfall Amount• K = Soil Erodibility• L = Slope Length• S = Slope steepness• C = Cover Factor• P = Conservation Practices
RUSLE Computer Program: http://www.techtransfer.osmre.gov/NTTMainSite/Library/hbmanual/rusle.htm
Rainfall Factor (R)
• Rainfall intensity and duration
• Built in database for rainfall intensity
• As rainfall intensity increases, erosion increases
• As rainfall duration increases, erosion increases
A=RKLSCP
Soil Erodibility Factor (K)
• Relates to soil texture and rock content
• Silty soils are typically most erodible
• Sandy soils have better infiltration, larger particles
• Clay soils have better cohesion
A=RKLSCP
Slope Factors
• L = Length of Flow Path
• Longer flow paths have more erosive power
• S = Slope steepness
• Steep slopes have higher runoff velocities
A=RKLSCP
Cover Management Factor (C)
• Erosion rate directly proportional to the amount of vegetation cover protecting the soil surface
• Vegetation reduces rainfall impact energy• Vegetation promotes infiltration• Vegetation reduces runoff velocity• Vegetation traps sediment• Have control over this factor
A=RKLSCP
Conservation Practices Factor (P)
• Manipulation of the soil surface to discourage erosion
• Provides slope storage
• Surface roughening• Pitting• Implemented on the
contour
A=RKLSCP
Example 1, Consider a 1 acre area
• In an area that receives 12 in of annual precipitation, 1 acre-ft of water is applied to the soil surface annually as rain and snow
• 1 acre-ft = 325,000 gallons per acre per year• Therefore a 1 inch rainstorm = 27,000 gal water• 27,000 gal water or 112 tons of water• Question: How much erosion will occur?• Answer: It all depends
Example 2, consider 1 acre of bare soil
• Apply 1 in of rain over a 1-hour period
• Sandy loam texture 90% infiltration, 3,000 gal runoff
• Silt loam texture 40% infiltration, 16,000 gal runoff
• Clay loam texture 20% infiltration, 22,000 gal runoff
Model example
Sub-Watershed
Watershed
Area (acres)
Horizontal Slope Length (feet)
Slope Steepness
(%)
A 4.8 300 10
B 9 500 20
C 4.4 200 50
This example uses an 18.2 acre watershed and the Colstrip 16-18 climate record
Sub-Watershed
Watershed
Area (acres)
Horizontal Slope Length (feet)
Slope Steepness
(%)
A 4.8 300 10
B 9 500 20
C 4.4 200 50
Colstrip Sediment Yield-Watershed A
8.5 9.2
4741 44
226
0
100
200
300
Loamy Sand Silty Clay Silt
Soil Texture
Sed
imen
t Y
ield
(to
ns)
.
Sediment yield per acre Total sediment yield
Bare soil, no BMPs
Sub-Watershed
Watershed
Area (acres)
Horizontal Slope Length (feet)
Slope Steepness
(%)
A 4.8 300 10
B 9 500 20
C 4.4 200 50
Colstrip Sediment Yield-Watershed B
9.9 106189 90
549
0
200
400
600
Loamy Sand Silty Clay Silt
Soil Texture
Sed
imen
t Y
ield
(to
ns)
.
Sediment yield per acre Total sediment yield
Bare soil, no BMPs
Colstrip Sediment Yield-Watershed C
19 24
10084 106
440
0
200
400
600
Loamy Sand Silty Clay Silt
Soil Texture
Sed
imen
t Y
ield
(to
ns)
.
Sediment yield per acre Total sediment yield
Sub-Watershed
Watershed
Area (acres)
Horizontal Slope Length (feet)
Slope Steepness
(%)
A 4.8 300 10
B 9 500 20
C 4.4 200 50
Bare soil, no BMPs
Effect of BMPs, Loamy Sand Texture
0
50
100
150
200
250
300
Bare BMP DenseGrass
Bare BMP DenseGrass
Bare BMP DenseGrass
Soil Condition
To
tal
Sed
imen
t Y
ield
(to
ns)
.
Watershed A
Watershed BWatershed C
Sub-Watershed
Watershed
Area (acres)
Horizontal Slope Length (feet)
Slope Steepness
(%)
A 4.8 300 10
B 9 500 20
C 4.4 200 50
BMP= soil rough, broadcast seeded, woodfiber mulch
Sediment Yield by Soil Texture
0
20
40
60
80
LoamySand
LoamySand
LoamySand
Silt Silt Silt SiltyClay
SiltyClay
SiltyClay
Soil Type
Sed
imen
t Y
ield
per
acr
e (t
on
s)
.
A
A
AB
B
BC
C
C
Sub-Watershed
Watershed
Area (acres)
Horizontal Slope Length (feet)
Slope Steepness
(%)
A 4.8 300 10
B 9 500 20
C 4.4 200 50
BMP= soil rough, broadcast seeded, woodfiber mulch
Erosion Control by Dense Grass
SubwatershedArea
(acres)Soil Management
Sediment Yield (tons per acre)
Total Sediment Yield (tons)
A 4.8 Loamy Sand Dense Grass 0.003 0.0
B 9 Loamy Sand Dense Grass 0 0.0
C 4.4 Loamy Sand Dense Grass 0.01 0.0
A 4.8 Silt Dense Grass 0.008 0.0
B 9 Silt Dense Grass 0.019 0.2
C 4.4 Silt Dense Grass 0.04 0.2
A 4.8 Silty Clay Dense Grass 0.005 0.0
B 9 Silty Clay Dense Grass 0.01 0.1
C 4.4 Silty Clay Dense Grass 0.02 0.1
Sediment Yield ExampleSite Subwatershed Area Soil Management
Sediment Yield,
tons per acre
Total Sediment
Yield (tons)
Colstrip A 4.8 Loamy Sand Rough, Bare 8.5 41
Colstrip A 4.8 Silty Clay Rough, Bare 9.2 44
Colstrip A 4.8 Silt Rough, Bare 47 226
Colstrip A 4.8 Loamy SandRough, Bare, broadcast seeded,
woodfiber mulch2.9 13.9
Colstrip A 4.8 Silty ClayRough, Bare, broadcast seeded,
woodfiber mulch3.1 14.9
Colstrip A 4.8 SiltRough, Bare, broadcast seeded,
woodfiber mulch17 81.6
Colstrip A 4.8 Loamy Sand Dense Grass 0.003 0.0
Colstrip A 4.8 Silty Clay Dense Grass 0.005 0.0
Colstrip A 4.8 Silt Dense Grass 0.008 0.0
Colstrip B 9 Loamy Sand Rough, Bare 31 279
Colstrip B 9 Silty Clay Rough, Bare 33 297
Colstrip B 9 Silt Rough, Bare 180 1620
Colstrip B 9 Loamy SandRough, Bare, broadcast seeded,
woodfiber mulch9.9 89
Colstrip B 9 Silty ClayRough, Bare, broadcast seeded,
woodfiber mulch10 90
Colstrip B 9 SiltRough, Bare, broadcast seeded,
woodfiber mulch61 549
Runoff Curve Numbers
• NRCS National Engineering Handbook
• Runoff Curve Number– Dependent on soil characteristics and cover
• Simpler planning tool for runoff outcome expressing the percentage of rainfall that is likely to runoff
• Four soil types with increasing runoff potential compared to typical land uses
Runoff Coefficients
Cover TypeSoil
Type A(sand)
Soil Type B
Soil Type C
Soil Type D (clay)
Open space—good grass 39 61 74 80
Paved parking lot 98 98 98 98
Gravel road 76 85 89 91
Fallow agricultural land—bare soil
77 86 91 94
Good rangeland/pasture 39 61 86 89
Hay meadow, no grazing 30 58 71 78
Sagebrush with good grass understory
- 35 47 55
Newly graded areas 77 86 91 94
Example
• How much runoff would result from a 0.4 inch rainstorm falling on a 2.5 acre construction site with the following characteristics:– Type B soil– 25% undisturbed pasture– 17% paved parking– 10% gravel roads– 48% newly graded areas
Calculation
• 0.41 inch rainstorm across 2.5 acres=– 0.41”/12 x 2.5 acres x 43,560 ft2 x 7.48 gal/ft3= 27,831 gallons of
rainfall on site– 22, 734 gallons runoff– 5,096 gallons infiltration
Cover Type % CN Runoff (gallons)
Undisturbed pasture 25 61 4244
Paved parking 17 98 4636
Gravel roads 10 85 2366
Newly graded areas—bare soil
48 86 11,489
Review
• Erosion is a naturally occurring process that is greatly accelerated on construction sites when bare soil is exposed
• There are many types of BMPs that may be used to control erosion (Internet guides)
• Erosion can be predicted by computer models and vast amounts of sediment can be eroded from small areas
How do we control erosion?
• Limit the extent of disturbance
• Rapidly reestablish stabilizing plant cover
• Promote infiltration and prevent runoff
• Provide for capture of sediment if runoff occurs
Mass Balance
• Bozeman receives ~19 inches of annual precipitation, or 516,000 gallons of water per acre per year
• What happens to all that water?
• Mass balance is required:
P-ET+D±RO=∆SP=PrecipitationET=EvapotranspirationD=DrainageRO=Run-off/Run-on∆S=Change is soil storage
BMP Strategy
• We can’t change precipitation
• We want to maximize evapotranspiration (plants)
• We want to maximize infiltration (drainage)
• We want to minimize runoff
P-ET+D±RO=∆S
Infiltration and Evapotranspiration
• Infiltration of precipitation is greatly increased by soil structure and presence of plants. In Montana most all precipitation can infiltrate if the precipitation intensity is modest and the ground isn’t frozen. ET>>R
• Plants can evapotranspire hundreds of thousands of gallons per acre per year
Facilitating Rapid Vegetation Establishment
• Avoid compaction
• Maximize surface roughening
• Beware of fine textured soils
• Consider temporary seeding
The Revegetation Dilemma
• Vegetation is the glue that holds the soil in place
• Most construction sites use seeded vegetation and not sod
• Most construction sites are not irrigated
• Vegetation grows slowly
• Stabilization commonly takes 2 years in Montana
Rapid Revegetation (cont.)
• What we want for stormwater control is rapid vegetation growth to reduce stormwater discharge
• What we commonly get is slow vegetation growth and accentuated erosion from typical time of seeding in late fall until mid summer when a small plant may grow from the seed planted
Rapid Revegetation (cont.)
• We typically seed perennial native grasses• The seeding window for perennial native
grasses occurs twice a year—early spring and late fall.
• An opportunity exists to use a temporary seeding with an annual grass
• Annual plants grow quickly, can be planted anytime the soil is warm and damp.
• Annual plants are a good soil stabilizer and the seed is inexpensive
Progressive Sizing—Sediment Control BMPs
Discharge
Watershed
Boundary
Ditch/Drainage Bottom
76
543
2
1
In this 1 acre hypothetical watershed 100% of the area has been disturbed. A 1 inch rainfall occurs and 50% of the rainfall infiltrates into the soil. How much stormwater runs through each BMP? Assume that the spacing between BMPs is constant.
Progressive Sizing—sediment control BMPs
Discharge
Watershed
Boundary
Ditch/Drainage Bottom
BMP Number Subwatershed Size (acres)
Volume of Water Generated (Gallons)
1 0.05 (pink line) 679
2 0.1 (maroon line) 1358
3 0.2 (olive line) 2716
4 0.3 4073
5 0.5 6789
6 0.8 10862
7 1.0 13578
76
543
2
1
Why is erosion control important?
• Sediments are a leading cause of water pollution in Montana and in the U.S.– 3723 miles of Montana streams impaired by
sediments (MDEQ TMDL program 2002)
• Sediments in streams are a leading cause of fishery degradation
• Significant fines are possible– $436,000 in MT Water Quality Act fines since 2002– 40 active enforcement cases in FY 08
Observations from construction sites in MT, WY, ID
• http://stormwater.montana.edu/• CD (Stormwater Control: Implementing
Construction Site BMPs in the Northern Rocky Mountains)
Compost Application using Blower Truck
Several hundred feet of hose can be attached to the blower truck to allow for distant installation of compost blankets on steep slopes
Compost Blanket
Revegetation Results using Compost on steep highway cut slopes
Glacial silt parent material
Alluvial rock parent material
BMP Strategy Summary
• Understand unique site attributes: soil, slope steepness, climate, soil quality
• Promote infiltration of precipitation into soil• Need to achieve rapid and robust
vegetation establishment• Need Sediment Control BMPs until
vegetation established• Need maintenance and monitoring