TheGeoRoomHydrology and Fluvial Geomorphology

Definition of the Day
Atmospheric Instability
A condition in the atmosphere where air parcels are heated and therefore are warmer than the surrounding atmosphere resulting in unstable weather conditions such as heavy rains and hailstorms. See Atmospheric Stability & Instability

River Velocity and Efficiency

River velocity is influenced by several factors such as:


Steep gradients increase the kinetic energy of a river and therefore speed


If the channel bed is colonized by large boulders velocity can significantly decrease as water flows on a rough jagged terrain.Boulders are mainly found in the upper course and they can significantly decrease river velocity especially during dry seasons.But after a heavy storm velocity is reduced slightly.


Obstacles such as vegetation can decrease a river's velocity.

Channel Shape

Wide shallow channels are less efficient and have low velocities than deep narrow channels. This can be illustrated by the hydraulic radius of a stream (the flow depth of water). High hydraulic radius means a stream is more efficient and fast flowing.Hydraulic radius is calculated by dividing the cross sectional area of the channel by the wetted perimeter of the channel. The wetted perimeter is the water that is in contact with the channel bed and sides.
measuring hydraulic radius diagram
Hydraulic radius of A : 54/26= 2.3 B : 48/20= 2.4
Stream B is more efficient than A

Different methods of measuring velocity has been put forth. The Chèzy equation measures velocity by relating gradient, hydraulic radius and coefficients of gravitational and frictional forces. The Manning equation calculates velocity by relating hydraulic radius, gradient and the Manning roughness coefficient (v = Rs/n R is hydraulic radius, s is gradient and n the Manning Coefficient). However, these equations measures mean velocities not the actual velocity.

Types of Flows

Turbulent FlowTurbulent flows are characterised by violent eddy flows. They usually occur in fast moving waters and boulder colonized streams. Water collides and deflect on these boulders creating hydraulic jumps and drops. Eddies with pebbles may drill the river's bed creating potholes.
Turbulent Flows
Turbulent Flows along a waterfall
Hellicoidal FlowA type of turbulent flow that sway sideways in a river.
LaminarLaminar flows are very rare and almost negligible. They involve straight and fluent flows. Usually laminar flows are induced by humans when straightening rivers to reduce floods.

Share Newsletter

Comment Below
*Email will not be published*


You May Like

Test Yourself Quiz


Geomorphology Cheatsheet
Subscribe To Newsletter or Follow on Facebook To Get New Content, Quizzes & more

The Geo Room Sites

The Discussion Room
The Career Room