Page 85 - Fundamentals of Geomorphology

68 INTRODUCING LANDFORMS AND LANDSCAPES

Table 3.3 Springs
Type Occurrence Example

Waste cover Dells and hollows where lower layers of soil or Common on hillslopes in humid
bedrock is impervious environments
Contact Flat or gently dipping beds of differing Junction of Totternhoe Sands and
perviousness or permeability at the contact of underlying Chalk Marl,
an aquifer and an aquiclude. Often occur as Cambridgeshire, England
a spring line
Fault Fault boundaries between pervious and Delphi, Greece
impervious, or permeable and impermeable,
rocks
Artesian Synclinal basin with an aquifer sandwiched Artois region of northern France
between two aquicludes
Karst Karst landscapes Orbe spring near Vallorbe, Switzerland
Vauclusian U-shaped pipe in karst where water is under Vaucluse, France; Blautopf near
pressure and one end opens on to the land Blaubeuren, Germany
surface
Thermal Hot springs Many in Yellowstone National Park,
Wyoming, USA
Geyser A thermal spring that spurts water into the air at Old Faithful, Yellowstone National Park
regular intervals

Streamflow
Water flow may be turbulent or laminar. In lami-
Rivers are natural streams of water that flow from nar flow, thin layers of water ‘slide’ over each other,
higher to lower elevations across the land surface. with resistance to flow arising from molecular viscosity
Their continued existence relies upon a supply of (Figure 3.8a). In turbulent flow, which is the pre-
water from overland flow, throughflow, interflow, base- dominant type of flow in stream channels, the chaotic
flow, and precipitation falling directly into the river. flow-velocity fluctuations are superimposed on the main
Channelized rivers are streams structurally engineered forward flow, and resistance is contributed by molec-
to control floods, improve drainage, maintain naviga- ular viscosity and eddy viscosity. In most channels, a
tion, and so on. In some lowland catchments of Europe, thin layer or laminar flow near the stream bed is sur-
more than 95 per cent of river channels have been mounted by a much thicker zone of turbulent flow
altered by channelization. (Figure 3.8b). Mean flow velocity, molecular viscosity,
Water flowing in an open channel (open channel fluid density, and the size of the flow section deter-
flow) is subject to gravitational and frictional forces. mine the type of flow. The size of the flow section
Gravity impels the water downslope, while friction from may be measured as either the depth of flow or as
within the water body (viscosity) and between the flow- the hydraulic radius. The hydraulic radius, R, is the
ing water and the channel surface resists movement. cross-sectional area of flow divided by the wetted perime-
Viscosity arises through cohesion and collisions between ter, P, which is the length of the boundary along
molecules (molecular or dynamic viscosity) and the which water is in contact with the channel (Figure 3.9).
interchange of water adjacent to zones of flow within In broad, shallow channels, the flow depth can approx-
eddies (eddy viscosity). imate the hydraulic radius. The Reynolds number, R e ,