Environmental Engineering Reference
In-Depth Information
• shape and size of material
• vegetation
• surface irregularities
(construction, maintenance type)
• shape and size of material
• surface irregularities
(bed forms)
roughness in the bed
roughness in the sides
• shape of canal
• bed width to water depth ratio
equivalent roughness
Figure 5.3.
Process to derive the equivalent roughness.
the development of bed forms on the bottom, different roughness along
the bottom and the sides, and vegetation on the side banks. Canals with
different roughness along the perimeter need an equivalent roughness that
should be based on the weighted roughness of the various components. In
the past, several methods have been developed to compute the equivalent
roughness from some basic assumptions concerning the flow in the canal.
5.2.2
Bed form development
In alluvial canals there exist two flow stages, one without any movement
of bed material and the other with movement of the bed material. Canals
without movement of bed material can be compared with a rigid boundary
canal having an equivalent roughness height (
k
s
) equal to the representative
bed material size (
d
). The resistance to flow in a movable bed consisting
of sediment is mainly due to the grain roughness and form roughness. The
grain roughness is generated by a skin friction force and form roughness
due to forces acting on the bed forms.
Bed forms will continuously change with the flow parameters
(velocity, depth) and therefore the bed roughness will also change.
There are two approaches to estimate the bed roughness:
•
methods based on hydraulic parameters such as mean depth, mean
velocity and bed material size;
•
methods based on bed form and grain-related parameters such as bed
form length, height, steepness and bed-material size.
The methods based on the hydraulic parameters by Einstein and
Barbarossa (1952), Engelund (1966), White et al. (1980), and Brownlie
(1981b) are the most widely used ones. These methods do not explicitly
Search WWH ::
Custom Search