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Unformatted text preview: iment-transport rate, or total sediment load is the sum of the suspended-sediment
and bed-load transport rates; it is the total quantity of sediment, as measured by dry weight or
volume, that moves past a site during a given time. Often the suspended-sediment transport
rate is measured, but the bed-load transport rate must be estimated.
Total suspended solids (TSS), when used as a regulatory concept in the hydrologic subdiscipline of sedimentology, is a measure of the suspended-solids, or solid-phase, content of a
water sample. According to Standard Methods for the Analysis of Water and Wastewater
(1995), TSS is determined from a sub-sample of an original water sample, whereas the
American Society for Testing and Materials (ASTM) advocates that the determination be made
from an entire sample of the water-sediment mixture; the two approaches are not compatible.
Transect is a line or path along which measurements or observations are taken, generally at
equal intervals, as a means of compiling data for an investigation. As applied to
geomorphology or sedimentology, for example, it could be the measurement of the diameter
of each sediment particle or rock fragment occurring at 1-m intervals along a 100-m length of
channel bed or hillslope; as applied to plant ecology, a transect is a line along which
observations of vegetation are made, often within quadrats, that might include presence or
absence of species, height or truck diameter of trees, or surface area covered by individual
plants, as a means to study characteristics of a particular assemblage of species. The use of
transects in geomorphic or ecological investigations is a means to minimize the potential for
introducing bias into the study. 45 Transmission loss is the abstraction, or reduction, of flow in ephemeral or intermittent stream
channels as discharge migrates downstream; the loss occurs by infiltration of streamflow into
normally dry (unsaturated) sediment forming the channel bed and banks and therefore much of
the flow abstraction becomes recharge to the ground-water reservoir.
Transpiration is the process by which water in living organisms, primarily plants, passes into the
Transport is the movement, shifting, or carrying away by natural agents of sediment (and
dissolved load) from one place to another on or near the earth’s surface.
Transport-limited is a term to describe the short-term ability of a specified flow rate to
transport a specified type of fluvial sediment or a specified range of particle sizes of sediment;
when a transport-limited condition occurs, the sediment load, or flux, is limited by the flow rate
of the stream, not by the availability of sediment. Less commonly, transport-limited is applied
to hillslope-erosion studies, for which it recognizes a condition of insufficient overland flow to
move all eroded sediment down the slope.
Tree is a woody plant having a single main stem or trunk at least to some measurable height
above the surface; exceptions are caespitose trees, generally willows and cottonwoods, with
trunks growing in clusters or tufts because the main trunk was sheared at the surface by a
Turbidity is the optical property (state, condition, or quality) of opaqueness or reduced clarity
of a fluid, due to suspended, colloidal, and organic matter and dissolved solids that provide
color, that causes light to be scattered, absorbed, and diffracted rather than being transmitted
directly through the water.
Turbulent flow, as a hydrologic term, is water movement (flow) in which the lines of flow are
erratic and mixed and in which flow direction at all sites changes frequently and nearly
instantaneously; turbulent flow is typical of stream and other surface-water bodies whereas
laminar flow is typical of slowly moving ground water. U
Underflow is the down-valley movement of water in a near-surface alluvial aquifer that is
hydraulically connected and directly related to the stream channel; underflow is most
descriptive of near-surface ground-water movement beneath stream channels of arid and
semiarid areas where it typically provides water adequate to sustain phreatophytic trees such
as cottonwoods despite unreliable amounts of streamflow.
Understory is the undergrowth, or underlying layer of plants (shrubs, herbs, etc.) of forest
vegetation; the understory plants of a forest generally are adopted to conditions of limited light
owing to shade caused by trees that form the canopy. The understory affects erosion by
intercepting that rainfall penetrating the canopy, by causing evaporation of a portion of it and
preventing that portion from reaching the surface to become runoff, and by protecting soil
particles at the surface from erosion by rainsplash impact.. 46 Unmeasured sediment-transport rate, or unmeasured sediment load, is the difference
between the total-sediment rate and measured suspended-sediment discharge. V
Variable source area is a geomorphic concept acknowledging that within a drainage basin the
amounts of water, sediment, and organic matter entering the drainage network from subbasins (or source areas) vary depending on local characteristics such as climate, geology and
soils, topography, and history (especially floods, fires, and land use).
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This document was uploaded on 02/26/2014 for the course ES 322 at Western Oregon University.
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