Chapter 4: Fluvial Processes in Riparian Areas
Flow and Sediment Transport
- Water and sediment discharge vary in time and space.
- Discharge can be computed as the average flow velocity multiplied
by the cross-sectional area of flow.
Figure 4.9. Stream flow discharge is estimated by multiplying the mean flow velocity by the stream cross sectional area at a specific point.
- A plot of discharge versus time is called a hydrograph.
The shape of the hydrograph provides information on the character
of the flow event. Short durations and high intensity thunderstorms
result in flash floods that yield rapidly rising runoff hydrographs.
In contrast, a flat hydrograph is indicative of constant discharge.
Figure 4.10. Typical hydrograph.
- The magnitude and frequency of flows have important implications
for sediment transport. Although large flood flows erode and
evacuate large quantities of sediment, and are responsible
for channel formation, they are relatively infrequent. The
relative amount of work done by smaller flows in transporting
sediment may add up to a considerable amount.
- Total sediment load carried by water flowing through the channel
network is made up of several components.
- Suspended load includes the material that is suspended when
the drag and lift forces exerted on the particle by the flow
exceed the submerged weight of the particle.
- Bedload includes material that typically travels in short
bursts or rolls along the channel bed. The bedload is typically
made up of larger, coarser particles .
- Washload and dissolved load (salts, chemicals) consist of
material that are chemically dissolved in the runoff. The dissolved
load moves the smallest material.
- Sources of sediment that make up the load include hillslopes,
channel bed and banks and tributary channels.
- Erosion
occurs when the erosive forces (shear imparted by flowing
water) overcomes the resistive forces (soils cohesiveness,
vegetation cover).
- The two main features of the channel are the bed and the banks.
- Alluvial channel beds are comprised of cohensionless sediment
that is readily picked up, with increasingly large particles
picked up as discharge increases. In general, as the depth
of flow increases, the effects of grain roughness become less
important.
- Channel banks often contain a higher proportion of clay than the sediment on the channel bed. Bank steepness is related to the proportion of clay and throughout the southwest, vertical channel banks are a sign that these banks have relatively cohesive substrates. However, these banks are also subject to erosion during flows.
- Transport capacity is the maximum quantity of solid material
that a stream can carry.
- It is directly related to discharge (velocity).
- It is highest during storm generated runoff (more sediment
in water).
- The amount of transported sediment can be limited by the
available supply or by the capacity of the flow to transport
available material.
- Sediment transport capacity in dryland channels is often
limited by transport rather than by sediment supply.
- Sediment is naturally sorted during deposition along the stream
bed.
Figure 4.11. Deposition of sediments along a stream.
- In general, channel bed sediment exhibits a pattern of longitudinal
fining in the downstream direction.
- Steep upper reaches channels can become armored, or covered
with a layer of larger, less transportable rocks, as the supply
of finer material is depleted from the channel bed and transported
downstream.
- As channel slope lessens on the lower reaches of the channel,
increasingly smaller sediment particles are deposited.
Figure 4.12. The hydrologic and geomorphic changes among the three zones of the streams.
- The size of deposited sediment will also vary with discharge.
Larger discharge event will deposit coarser material. Coarse
deposits may be overlain with finer deposits during subsequent
flows and vice versa.
- Deposits that remain in place can provide
a record of past runoff events.
