FLH PDDM Chapter 9 - Eastern Federal Lands Highway Division

9.4.E. Drainage Design. (continued)6. Underdrains and Horizontal Drains. The geotechnical section should design underdrain systems andhorizontal drains based on field observations and exploration of subsurface conditions. The designer will haveto incorporate them in the plans and provide detailed drawings for their construction.7. Riprap. (See Section 7.4.C.3, Channel Stabilization). The Hydraulics engineer determines the class,thickness, and cross section of riprap for slope protection along streams and lakes and for ditch and channellining. The roadway designer incorporates the data in the plans. The class, thickness, and typical section mustshow in the plans and specifications.Place riprap around culvert inlets and outlets to prevent erosion and undercutting.8. Energy Dissipators and Outlet Basins. (See Section 7.4.D.10). In areas of erodible soils, consider energydissipators at the outlet of downdrains and culverts with high outlet velocities and in channels at points wherethe grade flattens. Energy dissipators may be in the form of riprap outlet basins, stilling wells, weirs, orconcrete structures.9. Erosion Control. (See Section 7.4.H). Determine the need for various items of erosion control and includethe items in the contract. The type and extent of erosion control measures will depend mostly on the soils andstreams on a project.9 - 92