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Topographic Reconstruction: Drainage Basin Reconstruction
When both hillslopes and channel reconstruction are necessary for
reclamation, the establishment of entire drainage basins is required.
These may be small projects involving the construction of one single
basin drained by one permanent stream channel. The projects become
more complicated when reconstruction involves a nested drainage
basin system. For example, at the highest elevations, three drainage
basins may be constructed with three permanent stream channels that
flow into two downstream drainage basins. The drainage from these
basins may then flow into one drainage basin and into one large
stream channel.
Basin characteristics that are important to topographic reconstruction
include: (i) drainage-basin area, (ii) basin shape, (iii) drainage
pattern, and (iv) drainage density. The design basin characteristics
should support the concept of an approximate dynamic equilibrium
in drainage basin development. As a result of drainage-basin area
changes, runoff, stream discharge, and total sediment discharge
also change. These changes, in turn, alter stream channel design.
The shape of the basin is important because it influences the time
distribution of runoff. In a long, narrow basin, flow in tributary
channels reaches the mainstream at different times, thus distributing
the total runoff over a long span of time. In wide, square, or fan-shaped
basins, tributaries feed into the main-trunk stream at the same
time and tend to produce a sudden high peak flow. Drainage pattern
refers to the spatial arrangement of stream channels within a drainage
basin. Drainage density, a drainage pattern parameter that describes
the total channel length divided by basin area, is regarded as the
most important spatial measure of the channel system in a drainage
basin. Drainage density increases as mean annual runoff, mean annual
flood, percent bare surface (unvegetated), and sediment yield increase.
Drainage density decreases as the length of overland flow, baseflow,
or evapotranspiration
increases.
Drainage basins are designed incrementally from the highest to
lowest elevations involved in the reconstruction. Upper basins are
designed first with peak flow values calculated at the mouth of
each basin. Basins at a lower elevation are designed next with peak
flow values being recalculated for the entire area above the mouth
of the basin under design. Large errors can occur if peak flows
from each individual basin are simply added together. Channels flowing
from basins at the lowest elevation of reconstruction will eventually
converge with undisturbed channels. Care should be taken to ensure
that excessive erosion or overtopping of the channel does not occur
at the point of convergence. Small detention structures or ponds
are commonly constructed to prevent the potentially destructive
consequences of two high flow streams converging.
Topographic reconstruction is fundamental to successful reclamation.
The resulting surface is the foundation upon which other practices
are implemented and subsequent land uses take place. Much of reconstruction
design is a combination of art and science. Our understanding of
rebuilding mountains and streams is limited in many ways and, therefore,
the theories and models we use to design landforms are also limited.
We use our scientific understanding to get started and then we learn
by trial and error what works and what doesn't. Success is often
a not a result of answers found from an elaborate scientific model
but from experience as to what has worked in the past for a given
situation. A factor of safety is built into topographic reconstruction
design which accounts for the uncertainties in our understanding
of geomorphic processes and the landforms they create and change.
Conservative assumptions and estimations are used to ensure public
and environmental welfare. As a result of our ability to combine
art and science and the inclusion of a factor of safety, topographic
reconstruction successes far outnumber failures, especially during
the past decade or two. It is not unreasonable to expect landforms
and landscapes to possess both mass- and surface-stability and approximate
dynamic equilibrium.
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
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