Suspended load

Composition

The boundary between bed load and suspended load is not straightforward because whether a particle is in suspension or not depends on the flow velocity – it is easy to imagine a particle moving between bed load, part-suspension and full suspension in a fluid with variable flow. Suspended load generally consists of fine sand, silt and clay size particles although larger particles (coarser sands) may be carried in the lower water column in more intense flows.

Suspended load vs suspended sediment

Suspended load and suspended sediment are very similar, but are not the same. Suspended Sediment contains sediment uplifted in Fluvial zones, but unlike suspended load no turbulence is required to keep it uplifted. Suspended loads required the Velocity to keep the sediment transporting above the bed. With low velocity the sediment will deposit.

Velocity

The suspended load is carried within the lower to middle part of the water column and moves at a large fraction of the mean flow velocity of the stream, with a Rouse number between 0.8 and 1.2. The rates within the Rouse number reveal how at which the sediment will transport at the current velocity. It is the ratio of the fall velocity and uplift velocity on a grain.

Diagrams

Suspended load is often visualised using two diagrams. The Hjulström curve uses velocity and sediment size to compare the rate of erosion, transportation, and deposition. While the diagram shows the rate, one flaw about the Hjulström Diagram is that it doesn't show the depth of the creek giving an estimated rate.

The second diagram used is the Shields Diagram. The Shields Diagram (based on the Shields formula) uses the critical shear stress and Reynolds number to estimate transportation rate. The Shields Diagram is considered a more precise chart to estimate suspended load.

Measuring suspended load

Shear stress

To find the stream power for sediment transportation, shear stress helps determine the force required to allow sediment transportation.

${\displaystyle \tau =Pw.g.d.s}$

Critical shear stress

The point at which the sediment is transported within a stream

${\displaystyle \tau \text{c}=\tau \text{c}.g.(p\text{s}-p\text{w})d50}$

Suspended load transport rate

${\displaystyle q\text{s}=w.h.c\text{a}.[((a/h{)}^z-(a/h))/((1-a/h)Z.(1.2-Z))]}$