Environmental Engineering Reference
In-Depth Information
Chapter
14
Flowing water at the land surface
Water is Earth's 'proud setter up and puller down of kings', to borrow Shakespeare's
description of Warwick the 'Kingmaker' during the Wars of the Roses. Water is the
catalyst for low-temperature melts and explosive volcanic activity, essential to
subduction orogeny, and then the principal agent in their denudation. Nowhere is its
ability to destroy, as well as create, mountain systems and continents more obvious than
in running water at Earth's surface. Precipitation is widely distributed over the land
surface but markedly concentrated at points of
discharge
through
trunk
rivers. The
catchment
, or land surface unit generating river flow, is a fundamental geomorphic and
accounting unit. Catchment slope processes are intimately linked with their water and
sediment transfers. Their collective yields then drive fluvial processes in river channels,
although channel-slope links are rarely in equilibrium. Water-sediment stores and fluxes
are measured or estimated, leading to calculation of
water
and
sediment balances
. This
is not done solely in the interests of geomorphology, for the catchment also determines
vital options for human occupation and land use. Water is required simultaneously for the
essential but conflicting purposes of water supply and waste stream disposal. Data
gathered for both geomorphological investigation and hydrological management
considerably aid our appreciation of water flow through the landscape.
We follow the cascade of water and sediment, from precipitation to the generation of
channel flow, the behaviour of water and sediment in river channels and their creation of
fluvial
landforms. At first sight the
terrestrial
component of the
global hydrological
cycle
appears to be negligible. Surface freshwater rivers, lakes and swamps account for
only 110,000 km
3
or less than 0·01 per cent of the global water balance of approximately
1·35 × 10
9
km
3
(Figure 14.1). Almost 175 times this amount is stored in terrestrial
glaciers, ice sheets and permafrost (see Chapter 15). However, high-energy, fast river
transfer over the land surface compensates for its diminutive mass (Figure 14.2). The
turnover time of surface waters is less than twenty days or 0·05 yr via rivers, compared
with 10
4
yr via continental ice sheets. Rivers are the principal route for water, sediment
and energy transfers from the continents.
GENERATION OF CHANNEL FLOW
THE CATCHMENT AND WATER CASCADE
The catchment or
drainage basin
converts water, snow and ice input (precipitation +
in
fluent ground water) to
