Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 62

Dragarm assembly

Draghead

FIGURE 3.20 Self-propelled hopper (hydraulic) dredge. (Courtesy of the U.S. Army Engineer Research and
Development Center, Vicksburg, MS.)

Alternatives include hopper dredges, as illustrated in Figure 3.20, cutter heads, and other dredge
types. Disposal can occur in open water, conined disposal facilities (CDFs), or contained aquatic
disposal (CAD), or increasingly the dredged material is used for beneicial use applications. The
U.S. EPA and the U.S. Army Corps of Engineers have a cooperative program for the beneicial use
of dredged materials, such as for beach nourishment, berm creation, capping, land creation, land
improvement, replacement ill, and shore protection. Additional information is available through the
Corps Dredging Operations Technical Support (DOTS) program, accessible online at: http://el.erdc.
usace.army.mil/dots/.

3.3.2 w Hat a re tHe I MpactS ?
Channelization obviously has a number of positive economic and societal beneits. However, chan-
nelization can have a number of adverse environmental impacts as well. Many streams in agri-
cultural landscapes have been straightened or otherwise modiied to the point that they no longer
provide ecological services such as habitat diversity, lood mitigation, or nutrient retention (Shields
et al. 2008). Extreme channel incision often follows these modiications (Darby and Simon 1999).
One common consequence of channelization is the destabilization of the channel sediments.
The impact of destabilization and the expected changes following destabilization may be illustrated
using the channel evolution model of Simon and Hupp 1986 (Figure 3.21):

• Stage I: The waterway is a stable, undisturbed natural channel.

• Stage II: The channel is disturbed by some drastic change such as forest clearing, urbaniza-

tion, dam construction, or channel dredging.

• Stage III: Instability sets in with scouring of the bed.

• Stage IV: Destructive bank erosion and channel widening occur due to the collapse of bank

sections.

• Stage V: The banks continue to cave into the stream, widening the channel. The stream

also begins to aggrade, or ill in, with sediment from eroding channel sections upstream.

• Stage VI: Aggradation continues to ill the channel, reequilibrium occurs, and bank ero-

sion ceases. Riparian vegetation once again becomes established.

The channel evolution model is widely used to determine the successional stages of the evo-
lution of rivers and streams, in order to identify impairments or to plan for river restoration or
rehabilitation.