Flowing water at the land surface - Fundamentals of the Physical Environment - page 437

contribute water. However, these parameters cannot be considered in isolation from
channel form, which interacts with river flow. We look first at the water behaviour in
channels before considering their geomorphic development.
RIVER CHANNELS
Links between stream flow and channel morphology, which conserve discharge from one
segment to another in the example given (see box on p. 295), must also embrace changes
in discharge over time. It might be argued that ideal channels should accommodate most,
but not all, peak discharges and that bankfull discharge leading to flooding should be
exceeded either annually or only after abnormally wet periods or high rainfall events.
Channel efficiency decreases and friction losses increase as discharge falls in fixed-
geometry channels. Similarly, wetted perimeter increases and efficiency decreases
dramatically in over-bank conditions when the flooded valley floor briefly becomes part
of the channel. Whilst regular flooding is indicative of channels poorly adjusted to
discharge, there are excessive 'costs' in maintaining channels which never flood. It is
easy to see this in the case of large, engineered channels with higher financial costs in
land and construction, but it is also evident in the energy costs of natural channels. In
bedrock, rivers often cut channel-in-channel forms and shrink into the smaller channel
during low flows. River flow cannot sustain large channels in soft sediments at low
flows, and bank caving effectively reduces channel dimensions (Plate 14.7).
Deep water, gliding smoothly down a channel as tranquil flow, may enter a steeper
segment over a visible fall or hydraulic drop in the water surface. Water depth falls as
rapid flow develops and the water surface becomes disturbed (Plate 14.8). If the river
then enters another less steep segment, the transition back to deeper tranquil flow is
marked by a hydraulic jump or standing wave. This also clarifies the links between the
shape of the water surface and bed forms in sandy channels, both of which can change
over short distances and time scales.
CHANNEL HYDRAULICS
key processes
Channel geometry is defined by width, depth, length and slope, best seen in a short
channel segment which emphasizes the role of water level in two further, derived
channel parameters. The wetted perimeter ( wp ) equals 2 d + w in a rectangular channel
and hydraulic radius R is the cross-sectional area A ( d × w ) divided by wetted perimeter
( A /(2 d + w )) (Figure 1). This allows us to measure discharge as:
where v mean is the mean velocity. Natural channels have irregular wetted perimeters,
and the magnitude of energy loss at the bed emphasizes the role of bed roughness. This is
assessed through the Manning equation , which defines the v mean in terms of hydraulic
radius, channel slope ( S ) and a roughness coefficient ( n ):
n ranges from 0·02 for smooth, straight channels to 0·1 for rocky channels.