|
1. Definition
| Name |
FLOODPLAIN
AND CHANNEL MORPHOLOGY |
| Brief
definition |
Channel
and flood plain morphology can be described through a number
of related measurements of channel dimensions and the ratios
between them. The most significant are derived from:
- Channel width
- Flood plain width
- Channel width:depth
ratio
- Plan sinuosity
- Braiding index
Indicator values may be
standardised for position in the network to facilitate downstream
comparisons.
|
| Unit
of measure |
Lengths
(m) and/or dimensionless ratios |
| Spatial scale |
|
| Temporal scale |
|
2. Position
within the logical framework DPSIR
3. Target and
political pertinence
| Objective |
To
provide an integrated indicator of the physical impact of erosion
within a catchment. |
| Importance
with respect to desertification |
Changes
in channel morphology, and to a less extend flood plain form,
respond sensitively to changes in the rates of hillslope erosion.
Repeat measurements therefore provide a sensitive and integrated
indicator, relating impacts to the source areas responsible. |
| International
Conventions and agreements |
The
UNCCD emphasizes that combating desertification must be tackled
within the general framework of actions to promote sustainable
development. |
| Secondary
objectives of the indicator |
If
repeat surveys of channel cross-sections are made throughout
a catchment, then they can be combined with erosion estimates
to create a sediment budget for the catchment, which is relevant
to reservoir sedimentation and other impacts farther downstream. |
4. Methodological
description and basic definitions
| Definitions
and basic concepts |
The following measurements
can be made either in the field or, in some cases (starred),
from air photographs. Most can be defined for dry as well
as flowing channels. They are given in order of importance.
- * Channel bank-full
width (‘bank-full’ is the level above which the
channel spills on to the flood plain) in m.
- Channel bank-full depth
in m.
- Ratio of bank-full
width to bank-full depth: dimensionless ratio
- * Channel sinuosity
(ratio of channel mid-line length, following bends etc,
to reach length): dimensionless ratio.
- * For single-thread
rivers (at bank-full), meander wavelength in m (following
centre-line around bends between alternate points of inflexion
in plan form). For multi-thread rivers (at bank-full), the
mean number of channels in a flood plain cross-section.
This can be defined for a single cross-section, or calculated
as for sinuosity, summing over every active channel.
- * Floodplain width
in m.
|
| Benchmarks
Indication of the values/ranges of value |
Increased
catchment erosion is commonly accompanied by the following changes,
which demonstrate valley bottom deposition. These can, however,
be modified by reservoirs, water diversions etc.
- Increases in channel
width, width: depth ratio, flood plain width
- Increased width-depth
ratio
- Shifts from single
to multi-thread channels.
In comparing between different
sites in a catchment, widths tend to increase approximately
as (Catchment Area)**0.5, and depths slightly less, so that
width: depth ratios typically increase more as (Catchment
Area)**0.1.
|
| Methods
of measurement |
From
field, air photo or RS images. |
| Limits
of the indicator |
Channel
form is also modified by other factors, for example current
tectonics or, near the sea, coastal erosion/ progradation. It
is also modified by direct human modification of the channel
(e.g. impoundments diversions, channel straightening, cross-sectional
engineering), which can also influence reaches up- and down-stream. |
| Linkages
with other indicators |
Drainage
density, Flooding
frequency, Rainfall-runoff relationship |
5. Evaluation
of data needs and availability
| Data
required to calculate the indicator |
Air
photographs, high resolution RS images `(<5m pixels) or DEMs
(<5m grid resolution). If these are not available, and for
confirmation, field survey. |
| Data
sources |
Necessary
data are usually available and accessible to researchers. |
| Availability
of data from national and international sources |
Data
can be obtained from various regional, national or international
institutions involved in the collection and the analysis of
survey data. Older air photographs are widely available, though
not always of adequate quality. |
6. Institutions
that have participated in developing the indicator
| Main
institutions responsible |
University
of Leeds |
| Other contributing
organizations |
|
7. Additional
information
| Bibliography
|
Leopold,
L.B., Wolman, M.G. and Miller, J.P., 1964. Fluvial Processes
in Geomorphology. Freeman, SF. |
| Other
references |
Beven,
K.J. and Kirkby, M.J. (Editors), (1993). Channel Network Hydrology,
John Wiley, Chichester.
Leopold, L.B. and Maddock,
T. (1953). The hydraulic geometry of stream channels and some
physiographic implications. USGS Professional Paper 252.
|
| Contacts
Name and address |
Mike
Kirkby, School of Geography, University of Leeds, LS2 9JT, UK
mike@geog.leeds.ac.uk |
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