What Is Water Erosion? Aren t they the same thing? What Is Sediment? What Is Sedimentation? How can Sediment Yields be Minimized?

Size: px

Start display at page:

Download "What Is Water Erosion? Aren t they the same thing? What Is Sediment? What Is Sedimentation? How can Sediment Yields be Minimized?"

Stewart Webster
5 years ago
Views:

Jerald S. Fifield, Ph.D. CISEC HydroDynamics Incorporated Parker, CO 303-841-0377 Aren t they the same thing? What Is Sediment?

particles displaced by the action of wind or water What Is Sedimentation?

Maintain and/or implement erosion control measures Vegetation, mulches, soil binders, etc.

basins Reduce flow velocity Check structures Continued use results in higher costs for contractors What Is Water Erosion?

1 Jerald S. Fifield, Ph.D. CISEC HydroDynamics Incorporated Parker, CO Aren t they the same thing? What Is Sediment? Soil particles deposited or suspended in water or air The process of depositing soil particles suspended in water or air Soil particles displaced by the action of wind or water What Is Sedimentation? Soil particles deposited or suspended in water or air The process of depositing soil particles suspended in water or air Soil particles displaced by the action of wind or water How can Sediment Yields be Minimized? Maintain and/or implement erosion control measures Vegetation, mulches, soil binders, etc. Timely use results in lower costs for contractors Use barriers to intercept runoff Silt fences or fiber rolls Contain runoff Sediment basins Reduce flow velocity Check structures Continued use results in higher costs for contractors What Is Water Erosion? Soil particles deposited or suspended in water or air The process of depositing soil particles suspended in water or air Soil particles displaced by the action of wind or water 1

2 True or False Good sediment control is good erosion control FALSE Nearly all sediment control BMPs are very ineffective in removing suspended particles from runoff waters Types of Erosion Illustration from SCS, 1978 Photo by Calvin Palmer Illustration of Creep Illustration of Saltation Wind Erosion < 10% Which transport method moves the most soil? 50% TO 80% Illustration of Suspension Sahara Desert 5% TO 25% South America Illustration from SCS, 1989 One little raindrop makes a large crater and dislodges many soil particles. 2

Dislodged particles are conveyed by sheet flow Sheet Erosion Rills and Gullies o Causes additional erosion o Can transport large amounts of soil

On steep slopes, rills and gullies begin to form Notice that rills are the beginning of gullies RILLS ARE 75 mm ( 3 in.

) rill, about 0.038 m 3 (1.35 ft. 3 ) of soil is lost? Soil from 184 rills can fill up a 7 m 3 (9.2 yd.

3 Dislodged particles are conveyed by sheet flow Sheet Erosion Rills and Gullies o Causes additional erosion o Can transport large amounts of soil material Illustration from SCS, 1978 Rills Are 75 mm ( 3 in.) Deep Gullies Are > 75 mm (> 3 in.) Deep There are 40 rills along this strip. On steep slopes, rills and gullies begin to form Notice that rills are the beginning of gullies RILLS ARE 75 mm ( 3 in.) DEEP GULLIES ARE > 75 mm (> 3 in.) DEEP There are 40 rills along this strip. Did you know that for a 75 mm x 75 mm x 6.8 m (3 in. x 3 in. x 22 ft.) rill, about m 3 (1.35 ft. 3 ) of soil is lost? Soil from 184 rills can fill up a 7 m 3 (9.2 yd. 3 ) truck More Gully Erosion Did you know that in a 1.0 m x 1.0 m x 7.0 m (3.3 ft. X 3.3 ft. X 23 ft.) gully, about The 7.0 BMPs mspecified 3 (9.2 yd 3 to ) of soil control is lost? erosion Stream Bank Erosion Illustration from SCS,

Stream Bank and Channel Erosion Other Forms of Erosion

for Minimizing Water Erosion on Construction Sites

effective Rolled Erosion Control Products (RECP)

Mats (TRMs) Diversions and check structures Hard

Impervious covering Plastic Homes Roads & sidewalks

Climate events Soil erodibility Low with gravels High

organic matter Decreases with low void ratios and high

4 Stream Bank and Channel Erosion Other Forms of Erosion Shoreline Landslides Debris Flows Geological Methods for Minimizing Water Erosion on Construction Sites Vegetation, mulches and soil binders Most cost effective Rolled Erosion Control Products (RECP) Erosion control blankets (ECBs) Turf Reinforcement Mats (TRMs) Diversions and check structures Hard armoring Riprap and gabions Articulated blocks Impervious covering Plastic Homes Roads & sidewalks USLE RUSLE MUSLE Factors That Impact Water Erosion Climate events Soil erodibility Low with gravels High with silts & fine sands Decreases with clays and organic matter Decreases with low void ratios and high moisture content Increases with higher sodium adsorption ratios and decreasing ionic strength of water 4

Factors That Impact Water Erosion Climate events Soil erodibility Length of flow Slope of the land Critical parameter Erosion

= R x K x LS x C x P E = Annual Erosion Rate (tons/ac./yr. or tonnes/ha/yr.

management factor P = Sediment Control Also known as the support practice factor Rainfall Factors Determining Erodibility 72% silt

50 Imperial Units Possible Hierarchy of Soil Erodibility (From RUSLE2) Silt Silt Loam Loam Sandy Loam Silty Clay Loam Clay Loam

5 Factors That Impact Water Erosion Climate events Soil erodibility Length of flow Slope of the land Critical parameter Erosion control Construction site Sediment control Construction site Ɵ Revised Universal Universal Soil Soil Loss Loss Equation Equation E = R x K x LS x C x P E = Annual Erosion Rate (tons/ac./yr. or tonnes/ha/yr.) R = Rainfall Factor K = Soil Erodibility Factor LS = Topographic Factor C = Erosion Control Also known as the cover and management factor P = Sediment Control Also known as the support practice factor Rainfall Factors Determining Erodibility 72% silt & very fine sand 20% sand 1.0% Organic matter Fine granular material Moderate permeability K = 0.50 Imperial Units Possible Hierarchy of Soil Erodibility (From RUSLE2) Silt Silt Loam Loam Sandy Loam Silty Clay Loam Clay Loam Loamy Sand Silty Clay Sandy Clay Loam Sand Sandy Clay Clay Most Erodible Least Erodible K IMP 0.69 (K SI 0.091) K = 0.50 K IMP 0.24 (K SI 0.032) K IMP 0.02 (K SI 0.026) Topographic Factors Which is more critical: Slope or flow path length? 5

Topographic Factors Topographic Factors What happens when land slopes are doubled?

Doubling the flow path length results in about 6 Topographic Factors Old C-Factor & P-Factor Values Erosion potential is impacted

An indicator of how much erosion may be reduced What is meant by a C-FACTOR = 0.30?

rainfall at the time of assessment TRM An indicator of how much sediment will pass by (or through) a treatment system What is

6 Topographic Factors Topographic Factors What happens when land slopes are doubled? Doubling the slope results in about 26 times more erosion What happens when flow times more erosion path lengths are doubled? Doubling the flow path length results in about 6 Topographic Factors Old C-Factor & P-Factor Values Erosion potential is impacted more by changes in slope than changes in flow path length What Are C-Factors? What are P-Factors? An indicator of how much erosion may be reduced What is meant by a C-FACTOR = 0.30? Erosion will be about 30% of what will occur for bare ground conditions o 100% - 30% = 70% effective in reducing erosion due to rainfall at the time of assessment TRM An indicator of how much sediment will pass by (or through) a treatment system What is meant by a P-FACTOR = 0.75? About 75% of the sediment in runoff will bypass the BMP and not be captured o 100% - 75% = 25% effective in removing sediment from runoff waters at the time of the assessment Where Into the did storm all the sediment sewer system go? 6

Bare ground erosion rates may be over 100 times larger Potential 93% reduction in erosion rates Methods for Calculating Sediment Yields Sediment Delivery Ratio Uses results from USLE/RUSLE2 Modified

0 (Imperial) V = Volume of runoff Q p = Peak flood flow K, LS, C, P same as USLE Might be able to use RUSLE2 results MUSLE and Hydrologic Assessments Calculating Runoff Volumes V RO = α x A RO x RO

7 Bare ground erosion rates may be over 100 times larger Potential 93% reduction in erosion rates Methods for Calculating Sediment Yields Sediment Delivery Ratio Uses results from USLE/RUSLE2 Modified Universal Soil Loss Equation T = 2 x (V x Q p ) 0.56 x (K x LS x C x P) Where T = Sediment yield for the storm event (tonnes or tons) 2 = 89.6 (SI units) or 95.0 (Imperial) V = Volume of runoff Q p = Peak flood flow K, LS, C, P same as USLE Might be able to use RUSLE2 results MUSLE and Hydrologic Assessments Calculating Runoff Volumes V RO = α x A RO x RO Where V RO = Volume of runoff (cubic metres or acre-feet) α = 10 for SI Units and for Imperial Units A RO = Contributing runoff area (ha or acres) RO = Runoff (mm or inches per unit area) Calculating Peak Flows Q p = Q u x A x RO x F p Where Q p = Peak discharge in m 3 /sec. (ft. 3 /sec.) Q u = Unit peak discharge A = Drainage area in km 2 (mi. 2 ) F p = Pond and swamp adjustment factor Bare ground sediment yield rates may be over 40 times larger Potential 91% reduction in sediment yield Estimated average annual erosion rate based upon 10-year, 24-hour rainfall events E = R x K x LS x C x P Calculated by statistical assessments of 10-year, 24-hour storm events Why isn t there a better agreement as to shape? Estimated sediment yield based upon a single storm event of 76 mm (3.0 inches) of rainfall T = 2 x (V x Q p ) 0.56 x (K x LS x C x P) Calculated by hydrologic assessments of runoff due to specific storm event 7

Results from both models should only be used to compare how BMPs might reduce the generation and/or discharge of pollutants Conclusion Modeling erosion rates and sediment yields is not an exact

8 Results from both models should only be used to compare how BMPs might reduce the generation and/or discharge of pollutants Conclusion Modeling erosion rates and sediment yields is not an exact science Modeling results should only be used to compare how reductions of erosion rates and sediment yields might occur when implementing BMPs Never assume modeling results are a true representation of erosion rates or sediment yields Jerald S. Fifield, Ph.D. CISEC HydroDynamics Incorporated Parker, CO

Continuing Education Associated with Maintaining CPESC and CESSWI Certification

Continuing Education Associated with Maintaining CPESC and CESSWI Certification Module 2: Stormwater Management Principles for Earth Disturbing Activities Sponsors: ODOTs Local Technical Assistance Program