Environmental Engineering Reference
In-Depth Information
(glacial) or temperate (interglacial) stages; (d)
stratigraphic complexity in the upper Thames
around Oxford, where cold-stage terraces overlie
or are incised by temperate-stage channels. Height
in metres above modern flood plain.
Source: Boulton (1992).
KEY POINTS
1
Earth's land surface is divided into a hierarchical series of drainage basins or
catchments which convert precipitation to river flow. Each catchment contributes
surface and subsurface water to a specific stream or a major trunk river, separated
from its neighbours by a watershed. At continental scales, the watershed probably
coincides with prominent morphotectonic features. Catchment topography, geology,
vegetation and land-use sy stems retain, store and transfer water. They introduce
delays and losses to onward water transfer.
2
The fate of precipitation falling on the catchment can be quantified in a water balance
equation, with the volume of precipitation minus evapotranspiration generating river
flow, known as the discharge. Discharge volume and pattern over time are plotted on a
hydrograph and reflect the contribution of the component catchment stores.
3
Gravity-induced overland and subsurface flow is inefficient when diffuse, encountering
high resistance and friction loss. Channel flow is more efficient and is initiated where
surface/subsurface flows converge, initially as intermittent rills and gullies. Efficiency
continues to develop downstream as river channels enlarge with discharge. Channel
forms compensate for falling gradients and potential energy. Channels connect to form
a catchment-wide network, with recognizable patterns and drainage densities
determined by catchment hydrometeorology and hydrogeology.
4
Flowing water plays an important role in continental denudation. Erosion occurs
through fluid stressing by water itself and/or by water movement of abrasive tools.
Flow competence and sediment entrainment, transport and deposition are a function of
velocity and particle size, summarized by Hjulström's curve. Stream flow creates
distinctive landforms composed of straight, meandering and braided channels, channel
networks and flood plains.
5
Channel segments respond to flow regimes, sediment delivery, slope, etc., and undergo
almost continuous change. At whole-landscape scale, upper catchments are dominated
by bedrock channels and deep, narrow valleys, etc., with sediment pockets. Lower
catchments tend to display extensive flood plains of primary and reworked alluvial
sediments. Quaternary sea-level change has driven repeated marine/landward
extensions of the flood plain and Holocene delta construction.
