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

Erosion

• Accelerated erosion occurs at construction sites when stabilizing vegetation is removed

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

A Generic BMP Gameplan

Examples of BMPs—straw mulch

Examples of BMPs—rock check dams

Examples of BMPs—erosion control fabric

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

NRCS TR-55 Storm Types

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

Soil Physics

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

Rapid & Robust

Sparse

Slow & Stunted

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

Wheat versus Western wheatgrass

Western Wheatgrass

(perennial plant)

Wheat (annual plant)

Progressive Sizing of Sediment Control BMPs

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

Single family residence homesite $34,800 fine

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

Monitoring in response to precipitation or snow melt

Changing Site Hydrology/Seasonal Changes in Climate

Some BMPs Require Removal

Importance of Soil Quality

Importance of Surface Roughening and Seedbed Preparation

Contour Tillage

Surface Protection

Streambank Stabilization

Note sediment in culvert

Nick Point without appropriate anchoring

2002

2003

2006

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