FLOOD01888

• County: Statewide
• Title: River Hydraulics
• Date: 10/15/1993
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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EM 1110-2.1416
15 Oct 93

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Pipe flow

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Open-channel flow

Figure 2-1. Comparieon between pipa flow and open-channel flow

R = VL
e -
V

(2-1)

where

Re = Reynolds number (dimensionless)
V = characteristic flow velocity (ft/sec)
L = characteristic length (ft)
v = kinematic viscosity of water (f~/sec)

In open channels, L is usually taken as the hydraulic
radius; i.e., the cross-sectional area normal to the flow
divided by the wetted perimeter. Care must be taken 10
use a homogeneous system of units for these terms so'
that the Reynolds number is dimensionless. An open
channel flow is laminar if the Reynolds number is less
than 500. Flows in open channels are classified as turbu-
lent if the Reynolds number exceeds 2,000, and they are
transitional if Re is between 500 and 2,000 (Chow 1959).
Laminar flow is characterized by the dominant effects of
viscosity. In laminar flow, parcels of fluid appear 10
travel in smooth parallel paths. Laminar flow occurs
very rarely in natural open channels. When the surface
of a river appears smooth or glassy, it does not necessar-
ily mean that the flow is laminar; rather, it is most likely

tranquil, though turbulent flow. Laminar open channel
flow can occur, however, when a very thin sheet of water
flows over a smooth surface; otherwise, it is usually
restricted to specially controlled laboralory facilities.

(3) In turbulent flow, pulsatory cross-current velocity
fluctuations cause individual parcels of fluid 10 move in
irregular patterns, while the overall flow moves down-
stream. One effect of the microstructure of turbulent
flow is the formation of a more uniform velocity distri-
bution. Figure 2-2 shows the differences between typical
laminar and turbulent velocity prof1Ies in an open channel
and a pipe. Much greater energy losses occur in turbu-
lent flow. The energy required 10 generate the random
cross current velocities must come from the Iotal energy
of the river, but it is of no real help in transporting the
flow downstream. Therefore, open channel flow rela-
tions for turbulent flows describe energy and friction
losses differently than for laminar flows.

(4) Because flows in natural rivers are always turbu-
lent, methods of analyzing turbulent open channel flows
are presented exclusively in this document. Readers
interested in the analyses of laminar flow conditions

2-5