Erosion Control Principles: Protecting Soil and Water Quality

Erosion control is essential for protecting water quality, preventing property damage, and meeting regulatory requirements. Whether for construction sites or permanent installations, understanding erosion mechanisms and control strategies is fundamental to effective design.

Understanding Erosion

The Erosion Process

Erosion occurs in three stages:

1. Detachment - Soil particles are dislodged by raindrop impact or flowing water
2. Transport - Particles are carried by water or wind
3. Deposition - Particles settle when energy decreases

Types of Water Erosion

Sheet Erosion:

• Uniform removal of thin soil layers
• Hard to detect until severe
• Caused by overland flow

Rill Erosion:

• Small channels (< 1 foot deep)
• Flow concentrates in low areas
• Can be repaired by tillage

Gully Erosion:

• Large channels (> 1 foot deep)
• Severe concentration of flow
• Requires structural repair

Channel Erosion:

• Scouring of stream/ditch banks and bottoms
• Caused by high velocities
• Can be natural or accelerated

Factors Affecting Erosion

Rainfall Erosivity (R):

• Intensity and duration
• Drop size and velocity
• Kinetic energy

Soil Erodibility (K):

• Soil texture
• Organic content
• Structure and permeability
• Sandy and silty soils most erodible

Slope Length and Steepness (LS):

• Longer slopes = more erosion
• Steeper slopes = faster flow, more erosion
• Combined effect is multiplicative

Cover and Management (C):

• Vegetation dramatically reduces erosion
• Mulch protects surface
• Residue and organic matter help

Support Practices (P):

• Contour farming
• Terracing
• Diversions

The RUSLE Equation

The Revised Universal Soil Loss Equation predicts annual soil loss:

Where:

• A = Annual soil loss (tons/acre/year)
• R = Rainfall erosivity factor
• K = Soil erodibility factor
• LS = Slope length and steepness factor
• C = Cover and management factor
• P = Support practice factor

Example: Bare construction site vs. established lawn:

Regulatory Framework

Federal Requirements

NPDES Construction General Permit:

• Required for sites ≥ 1 acre disturbed
• Part of Clean Water Act
• Requires SWPPP and inspections
• Penalty potential: $25,000+ per day

SWPPP Requirements:

• Stormwater Pollution Prevention Plan
• Site map and BMPs
• Inspection and maintenance schedule
• Documentation and records

State Requirements

Most states have additional requirements:

• State-specific permits
• Approved BMP lists
• Inspection certifications
• Design standards

Local Requirements

Municipalities often add:

• More stringent standards
• Specific BMP requirements
• Performance bonds
• Third-party inspection

Erosion Control Strategies

Strategy 1: Minimize Disturbance

Principles:

• Disturb only what’s necessary
• Clear in phases, stabilize as you go
• Preserve existing vegetation
• Limit disturbance duration

Implementation:

• Phase clearing to match construction
• Flag limits of disturbance
• Protect trees and natural areas
• Stabilize within 14 days

Strategy 2: Protect Steep Slopes

Principles:

• Slopes are highest erosion risk
• Reduce effective slope length
• Protect surface from direct impact
• Control concentrated flows

Implementation:

• Slope breaks at 50-foot intervals
• Slope drains for concentrated flow
• Temporary or permanent cover
• Erosion blankets for steep slopes

Strategy 3: Control Concentrated Flow

Principles:

• Concentrated flow causes severe erosion
• Convey clean water around site
• Line channels or reduce velocity
• Provide energy dissipation

Implementation:

• Temporary diversions
• Lined waterways
• Check dams
• Outlet protection

Strategy 4: Establish Perimeter Controls

Principles:

• Sediment must not leave site
• Last line of defense
• Size for expected loads
• Maintain regularly

Implementation:

• Silt fence at proper locations
• Inlet protection
• Sediment basins/traps
• Buffer zones

Erosion Control BMPs

Temporary Seeding

Purpose: Stabilize soil surface with vegetation.

Application:

• Areas idle > 14 days
• Slopes and disturbed areas
• Temporary cover until permanent planting

Design Considerations:

• Select appropriate species (annual for temp)
• Apply at proper rate
• Consider season and climate
• May need mulch protection

Permanent Seeding

Purpose: Establish long-term vegetative cover.

Application:

• Final grading complete
• Slopes, channels, basins
• All disturbed areas

Design Considerations:

• Native species preferred
• Proper seed mix for conditions
• Soil preparation essential
• Establishment care required

Mulching

Purpose: Protect soil surface from raindrop impact.

Types:

• Straw (most common)
• Wood chips
• Compost
• Hydromulch

Application Rates:

• Straw: 2-4 tons/acre
• Anchor if slope > 3:1
• Apply immediately after seeding

Effectiveness:

• Reduces erosion 70-90%
• Retains moisture
• Moderates temperature

Erosion Control Blankets (ECBs)

Purpose: Protect slopes and channels during vegetation establishment.

Types:

• Straw blanket (temporary)
• Coconut fiber (longer term)
• Synthetic (permanent)
• Turf reinforcement mat (TRM)

Application:

• Steep slopes (> 3:1)
• Concentrated flow areas
• Channels
• Critical areas

Installation:

• Anchor at top with trench
• Overlap rolls (shingle down)
• Staple pattern per manufacturer
• Seed before or after

Slope Drains

Purpose: Convey concentrated flow down slopes without erosion.

Types:

• Corrugated pipe
• Half-round pipe
• Concrete flume

Design:

• Size for design flow
• Energy dissipation at outlet
• Anchor to prevent movement
• Tie into diversion at top

Sediment Control BMPs

Silt Fence

Purpose: Filter sediment from sheet flow.

Limitations:

• Sheet flow only (not concentrated)
• Maximum 0.25 acre per 100 feet
• Cannot filter large volumes
• Requires regular maintenance

Installation:

• Trench installation (6” minimum)
• Posts at 6-foot spacing
• Along contour, not down slope
• Ends turn uphill

Common Mistakes:

• Across channels or swales
• Wrong locations (not filtering area)
• No trench embedment
• Inadequate maintenance

Sediment Basins

Purpose: Settle sediment from larger drainage areas.

When Required:

• Typically > 5-10 acres disturbed
• Required by most permits
• Primary sediment control for large sites

Design Parameters:

• Volume: 1,800 cf per acre (typical)
• Minimum 2:1 length to width
• Sediment storage zone
• Dewatering device (skimmer)
• Emergency spillway

Sediment Traps

Purpose: Smaller-scale sediment capture.

Application:

• Smaller drainage areas (< 5 acres)
• Outlets of diversions
• Interim measure before basin

Design:

• Volume: 1,800 cf per acre (typical)
• Stone outlet or riser
• Maintenance access

Inlet Protection

Purpose: Prevent sediment from entering storm drains.

Types:

• Fabric barrier
• Stone (gravel bag) barrier
• Block and gravel
• Excavated sump
• Manufactured devices

Considerations:

• Must handle bypass flows
• Maintenance intensive
• Size for expected flows
• Avoid flooding streets

Check Dams

Purpose: Reduce velocity in channels, trap sediment.

Materials:

• Stone (most common)
• Straw bales (temporary only)
• Sand/gravel bags
• Manufactured devices

Design:

• Spacing based on slope
• Center lower than edges
• Stone size for stability
• Wrap ends into banks

Filter Berms/Socks

Purpose: Filter sediment from sheet or shallow flow.

Types:

• Compost filter berm
• Filter sock (compost-filled)
• Straw wattle

Advantages:

• Easy installation
• No trenching required
• Can be repositioned
• Biodegradable options

Construction Sequencing

Before Clearing

1. Install perimeter controls
2. Stabilize construction entrance
3. Protect existing features
4. Mark clearing limits

During Construction

1. Phase disturbance
2. Stabilize idle areas
3. Maintain all BMPs
4. Inspect after rain events

At Project End

1. Complete final grading
2. Establish permanent vegetation
3. Remove temporary controls
4. Conduct final inspection

Inspection and Maintenance

Inspection Requirements

NPDES Requirements:

• Inspect every 7 days AND after 0.25” rain
• Document all inspections
• Note deficiencies and corrective actions
• Keep records for 3 years

What to Inspect:

• All BMPs for damage or failure
• Disturbed areas for erosion
• Discharge points for sediment
• Sediment accumulation

Maintenance Requirements

Documentation

Maintain records of:

• Inspection date and inspector
• Weather conditions
• Areas inspected
• Deficiencies found
• Corrective actions taken
• Dates corrections completed

Permanent Erosion Control

Vegetated Channels

Design for:

• Maximum velocity < 4-6 fps
• Adequate capacity
• Proper side slopes (3:1 or flatter)
• Establishment period protection

Riprap Protection

Applications:

• Channel linings
• Pipe outlets
• Slope protection
• Shorelines

Sizing: Calculate stone size based on velocity:

Calculate outlet protection →

Drop Structures

Purpose: Safely drop flow elevation.

Types:

• Rock chute
• Concrete drop
• Pipe drop
• Gabion drop

Design Considerations:

• Energy dissipation
• Scour protection
• Stability
• Maintenance access

Retaining Structures

For stabilizing steep slopes:

• Retaining walls
• Gabion walls
• MSE walls
• Vegetated slopes with reinforcement

Common Erosion Problems and Solutions

Problem: Sheet Erosion on Slopes

Indicators: Loss of topsoil, rills forming

Solutions:

• Temporary cover (mulch, blanket)
• Permanent vegetation
• Slope breaks
• Surface roughening

Problem: Gullies

Indicators: Channels > 1 foot deep

Solutions:

• Lined channels
• Grade control structures
• Fill and stabilize
• Divert flow elsewhere

Problem: Channel Erosion

Indicators: Bank cutting, bed lowering

Solutions:

• Reduce velocity (check dams)
• Armor channel
• Reshape and stabilize
• Grade control

Problem: Pipe Outlet Erosion

Indicators: Scour hole at outlet

Solutions:

• Riprap apron
• Energy dissipator
• Plunge pool
• Level spreader

Summary

Effective erosion and sediment control requires:

1. Understanding erosion mechanisms - Address root causes
2. Erosion control first - Prevent detachment
3. Sediment control backup - Capture what escapes
4. Proper BMP selection - Right tool for the job
5. Correct installation - Details matter
6. Regular inspection - Catch problems early
7. Timely maintenance - Keep BMPs functioning
8. Complete documentation - Regulatory compliance

Related Calculators

• Outlet Protection Calculator →
• Rational Method Calculator →
• Manning’s Channel Calculator →

References

1. Environmental Protection Agency. (2012). Construction general permit (CGP). EPA.

2. Fifield, J. S. (2011). Designing for effective sediment and erosion control on construction sites (2nd ed.). Forester Press.

3. Natural Resources Conservation Service. (2006). National Engineering Handbook, Part 650: Engineering Field Handbook. USDA.

4. Federal Highway Administration. (1995). Best management practices for erosion and sediment control (FHWA-FLH-95-003). U.S. DOT.

5. International Erosion Control Association. (2020). Best management practices and design considerations. IECA.

6. Renard, K. G., et al. (1997). Predicting soil erosion by water: A guide to conservation planning with the Revised Universal Soil Loss Equation (RUSLE) (Agriculture Handbook 703). USDA.