The general approach for estimating n values consists of the selection of a base roughness value
for a straight, uniform, smooth channel in the materials involved, then additive values are
considered for the channel under consideration:
n = (no + n1 + n 2 + n3 + n 4 )m 5
Base value for straight uniform channels
Additive value due to cross-section irregularity
Additive value due to variations of the channel
Additive value due to obstructions
Additive value due to vegetation
Mulitiplication factor due to sinuosity
Detailed values of the coefficients are found in Cowan (1956), Chow (1959), Benson and
Dalrymple (1967) and Aldridge and Garrett (1973). Typical values are given in Table 2.2.
Arcement and Schneider (1984) proposed a guide for selecting Manning's roughness coefficients
for floodplains. For steeper streams, the reader is also referred to the work of Jarrett (1984,
The roughness characteristics on the floodplain are complicated by the presence of vegetation,
natural and artificial irregularities, buildings, undefined direction of flow, varying slopes and other
complexities. Resistance factors reflecting these effects must be selected largely on the basis of
past experience with similar conditions. In general, resistance to flow is large on the floodplains.
In some instances, conditions are further complicated by deposition of sediment and
development of dunes and bars which affect resistance to flow and direction of flow.
The presence of ice affects channel roughness and resistance to flow in various ways. When an
ice cover occurs, the open channel is more nearly comparable to a closed conduit. There is an
added shear stress developed between the flowing water and the ice cover. This surface shear
is much larger than the normal shear stresses developed at the air-water interface. The ice-water
interface is not always smooth. In many instances, the underside of the ice is deformed so that it
resembles ripples or dunes observed on the bed of sandbed channels. This may cause overall
resistance to flow in the channel to be further increased.
With total or partial ice cover, the drag of ice retards flow, decreasing the average velocity and
increasing the depth. Another serious effect is its influence on bank stability, in and near water
structures such as docks, loading ramps, and ships. For example, the ice layer may freeze into
bank stabilization materials, and when the ice breaks up, large quantities of rock and other
material embedded in the ice may be floated downstream and subsequently thawed loose and
dumped randomly leaving banks raw and unprotected.
2.4.4 Average Boundary Shear Stress
The average shear stress at the boundary τo for steady uniform flow is determined by applying
the conservation of mass and momentum principles to the control volume shown in Figure 2.11.
The conservation of mass Equation 2.16 is then: