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WEB-BASED HANDBOOK (Legacy Edition). CLICK HERE to submit comments.
Because of staff changes and Randolph Langenbach's retirement from FEMA, these handbooks were never published on the FEMA website.  These are the only copies available.


 

Chapter 1: ROADS

B. EMBANKMENT EROSION

Introduction

Problem:

Damage caused by erosion of inadequately protected slopes.

Mitigation Objective:

Harden the damaged embankment slope or redirect flows to avoid higher velocities.

B.1 Bio-Engineered Embankment Slope Protection

Provide bio-engineered embankment slope protection by covering the slope with deep rooting vegetation and where it is adjacent to and in contact with a live stream, strategically anchor large woody debris (i.e. root wads) that will hold the soil in place and protect it from erosion.

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Effectiveness:

  • Very effective in areas of low flow velocity.
  • Environmentally friendly.
  • Contributes to settling of particulate matter.

Limitations:

Vegetation must have sufficient time to become established prior to being exposed to floodwaters.
Generally not suitable for areas of sustained high velocity flows.

Considerations:


B.2 Install Half-Round or Spillway Pipes, or Rock Channels

Concentrate flows in a collection structure and install half-round or spillway pipes, or rock channels down steep slopes to eliminate erosion.

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Effectiveness:

  • Most effective for intermittent streams or surface water collection.
  • Pipes very effective when properly connected to drainage collection structures or roadway ditches.

Limitations:

  • Requires good anchors to eliminate failure by slippage of the pipes on the steep slope.
  • Consider constructing an energy dissipater at bottom of slope.

B.3 Change Geometry of Roadway Section

Alter the angle of an embankment slope through shoulder rounding and slope reduction to reduce erosion. Reducing the angle of slope generally reduces the velocity of the water running across it.

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Effectiveness:

Very effective in areas of roadway overtopping with low velocity flows.

Particularly effective if combined with armoring of the downstream embankment slope and road shoulder.

Limitations:

May reduce carrying capacity of ditch or stream.

Considerations: z-image026.png (6659 bytes)


B.4 Construct a Wall

Construct a wall to protect the slope from erosion and consequent sloughing and slumping. Walls can be constructed of various materials including rock, gabions, sheet pile, concrete, etc.

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Effectiveness:

Very effective.

Suitable for high velocity, high volume flood events.

May be designed to accommodate replanting of a damaged riparian area.

Limitations:

May preclude regrowth of riparian areas.

Considerations:


B.5 Place Riprap Along Eroded Slop

Place appropriately sized riprap along the eroded embankment slope in sufficient quantities to resist scouring effects of higher velocity flows and protect the embankment slope from future flood. damage.

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Effectiveness:

Very effective in areas of moderate flow velocity and volume when appropriately graded material is placed in sufficient quantity and layered so fines are not eroded.

Water velocity will determine size and volume of riprap.

Limitations:

Extensive use of riprap may preclude re-growth of riparian areas.

Access may limit where riprap can be placed.

 

Considerations:

 

NOTE:  None of the mitigation measures in these Handbooks should be considered ‘pre-approved’ or otherwise automatically eligible for FEMA funding. Only FEMA staff can determine eligibility, once they have determined that an applicant is eligible and they have reviewed a project proposal.

FEMA HAZARD MITIGATION HANDBOOKS                                                                        Updated: June 13, 2002