Rivers and Streams, Characteristics - Hydro-Environmental Analysis: Freshwater Environments - page 26

Sediment size Stream slope

Coarse Fine Flat Steep

Q s • D 50 α Q w • S

FIGURE 2.14 Factors affecting channel degradation and aggradation. (Reproduced from the American
Society of Civil Engineers from Lane, E.W., Proceedings from the American Society of Civil Engineers , 81,
1-17, 1955. With permission.)

Streams will increase in width when the sediment supply exceeds the transport capacity, and often,
as a result, they lose their deined riparian zone.

2.2.1.4.3 Roughness
The low velocity in a channel is inversely proportional to the channel roughness and directly
proportional to the stream slope (usually to the 1.2 power). These concepts are incorporated into
all hydraulic models using roughness coeficients such as Manning's number (Manning's n ). For
example, a higher velocity would be expected in a stream with a smooth bottom than a stream with
boulders, cobbles, or vegetation.

2.2.1.4.4 Dominant Discharge
The dominant discharge is that discharge that carries the majority of the sediments and is respon-
sible for creating or maintaining the size and shape of the channel (Leopold et al. 1964; Knighton
1984). The dominant discharge is also known as the “channel-forming” discharge and occurs fairly
frequently (it is not a rare event). For rivers and streams that are at equilibrium with the sediment
loads (not degrading or aggrading), the dominant discharge is about equal to or less (for incised
streams) than the bank-full discharge, or the discharge that ills the banks before spilling onto the
loodplain. When a streambed ills in (supply exceeds transport out) it is said to be aggraded, while
conversely, a stream with a reduced supply may have a capacity greater than its load and will down-
cut in order to increase its bed load, which is called degrading.
Determining the bank-full discharge (Figure 2.15) is often subjective so a common alternative
is to specify the recurrence interval of the discharge, which may vary between once every year to
once every 5 years, with a common recurrence interval being once every 1.5 years (Leopold et al.
1964). Another metric commonly used is the effective discharge, which is deined as the increment
of discharge that transports the largest fraction of the annual sediment load over a period of years
(Andrews 1980).

2.2.1.4.5 Sediment Transport Mechanisms
The term sediment covers a wide range of particle types and particle-size classes, ranging from very
ine clays to boulders. The factors impacting transport will also vary between some of the sands that