Introduction to Stream Restoration - Hydro-Environmental Analysis: Freshwater Environments - page 255

Geogrid Geonet

Geocell

FIGURE 8.37 Illustration of geotextile types. (From NRCS, Geosynthetics in stream restoration, Technical
supplement 14D, National Engineering Handbook , Natural Resources Conservation Service, 2007c.)

8.4.25 r Iprap
Riprap (graded stone or crushed rock) is one of the most commonly used streambank stabiliza-
tion techniques. Riprap is a permanent, erosion-resistant ground cover of large, loose, angular
stone, as illustrated in Figure 8.40. The purpose of this technique is to protect the soil surface
from the erosive forces of concentrated runoff, slowing the velocity of the runoff while enhanc-
ing the potential for iltration (RI DEM 2007). Riprap is generally used on streambanks at the
toe (bottom) of the slope, with other structures placed upslope to prevent soil movement (NRCS
1996a) (Figures 8.40 and 8.41). There are a variety of riprap types, including rock riprap, rubble
riprap, gabions, preformed blocks, grouted rock, and paved linings. Riprap design is included in
engineering manuals from a variety of federal agencies, including the Corps (USACE 1994), the
Federal Highway Administration (FHWA 1989), and the NRCS (1996a), as well as state agencies
(e.g., RI DEM 2007).
Riprap also has some moderate beneit for stream habitats and for some organisms. The
primary beneit is the stabilization of sediments, reducing the sediment loads and allowing
the reestablishment of riparian vegetation. Hehnke and Stone (1978) showed that bird species
diversity and density were signiicantly lower on the riprapped banks than on the unaltered
sites. Fischenich (2002) indicated that systems with excessive erosion due to anthropogenic
causes are most likely to beneit ecologically from riprap while those with healthy riparian
vegetation communities would most likely be harmed ecologically by the addition of riprap
structures.