You are not on the FEMA website.  TOUCH FEMA home to go to the FEMA official website.

[CONSERVATIONTECH.COM-Home]     [ FEMA home (www.FEMA.gov) | FEMA Library (www.fema.gov/library/index.jsp)]


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

A. DITCH EROSION

Introduction

Problem:

Severe roadway and ditch erosion due to water velocities eroding the ditch and overflow areas.

Mitigation Objective:

To strengthen eroded areas and/or redirect floodwaters away from ditch areas vulnerable to erosion.  The following have been identified as common mitigation measures used to protect road facilities:

A.1  Increase Ditch Capacity

Increase the capacity of the roadway ditch by increasing its depth and/or width.

Ditch Capacity enlargement  CLICK ON IMAGES TO ENLARGE

Effectiveness:

  • Very effective in areas of low to moderate flow velocity where overtopping of the roadway ditch causes the damage. 

  • Effectiveness in high flows can be increased by lining the ditch, embankment slope protection, or installing check dams.

Limitations:

  • In areas of high velocity flows, or if ditch is constructed from fine, easily eroded, materials, additional measures may be needed. 

  • May require increased slope angles on either side of ditch. 

Considerations:

A.2  Install Lining in the Ditch

Line the ditch with rock, concrete, asphalt, or vegetation to prevent ditch erosion.  Use larger and coarse-grained materials to protect against high velocity flows.

image008.png (56121 bytes)  

Effectiveness:

  • Very effective.

  • Suitable for high-flow velocity, and high-flow condition. 

  • Grass lined ditches provide bio-filtration and sediment reduction.

Limitations:

  • Grass lining requires time for the grass to become established prior to use. 

  • Concrete lined ditches increase run-off and decrease time of floodwaters concentration.

Considerations:

A.3  Install Additional Cross Culverts

Install additional culverts across roadway to cut off the flow so as not to exceed the capacity of the roadway ditch.  This measure will address the problem of ditch erosion when excessive flow in the ditch is either overtopping the road or is causing erosion of the road prism.  Cross culverts are usually small-sized, ranging from 18 to 24 inches in diameter.

image012.png (39324 bytes)

Effectiveness:

·  Very effective in reducing the volume of runoff in the ditch line.

A.4  Install Check Dams

Install low-height barriers (check dams), usually made of loose rock, to slow the velocity of the storm water and reduce the scouring action of the flow.

image014.png (26669 bytes)

Effectiveness:

  • Very effective in reducing flow velocity and erosion.

Limitations:

  • Difficult to maintain.

  • May be more appropriate for temporary erosion and silt control.

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