FIELD METHODS IN ENVIRONMENTAL GEOLOGY

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FIELD METHODS IN ENVIRONMENTAL GEOLOGY GEOS 3110
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Content
- Watershed boundaries - Streamflow / stream
gauging
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Watershed Definition
A drainage basin or watershed is the area
throughout which surface water drains into a
particular body of surface water. NOTE A
surface-water drainage basin is not necessarily
the same as a ground water drainage basin.
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Watershed Boundaries
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Delineating Drainage Basins
REASONS FOR DELINEATION 1. Determine the
impact of some activities that might affect
quality or quantity of water in a basin. 2.
Determine the potential development of water
resources in a basin.
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Delineating Drainage Basins
Steps for Delineating a Drainage Basin 1. For
a stream drainage basin, find the point where the
stream enters another body of water (its base
level). 2. Put a pencil tip at the base level,
and then move it, intersecting contour lines at
right angles, to the topographically highest
nearby location. 3. Continue in this fashion.
The goal is to draw a line surrounding the area
on which rainfall would drain into the stream of
interest. Rain falling on the other side of the
line would flow into a different stream body of
water.
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Marble Test
Interpretation of the topographic contour
lines Set a pencil point down anywhere on the
map, and imagine what would happen if a marble
hit the ground at that point. Would it roll
ultimately into the stream of interest ? If so,
then that point belongs in the drainage basin of
that stream. If it would end up in a different
stream, then it does not belong to our stream of
interest. The point where it is unclear which
way the marble would roll, because it might roll
either way, is situated on the drainage divide or
boundary of the drainage basin.
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Marble Test
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Stream Gauging - Purpose
The goal of this field exercise is to determine
the discharge (volume of water per
cross-sectional area per time) of a stream at
several locations. Any variability in measured
discharge with distance downstream will be
analyzed for its hydrologic significance.
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Equipment
- a good coat, gloves and hat (if the weather is
cold or wet) - wading shoes or boots, shorts or
fast-drying pants. - a field notebook and pen -
measuring tapes and/or taglines - anchors for
taglines (stakes or nails) - surveyors tape or
marker for marking tag lines - yardsticks - timer
(stopwatch, watch with second hand, etc.) -
floats (for velocity determination)
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Field Observations
1. General topographic setting 2. Site-specific
topography and relief (a sketch or profile may
be helpful. 3. Character of the floodplain and
floodplain development. 4. Description of the
stream banks and bed. 5. Sediment and rock
exposed in cuts and in the stream bed. 6. Soils
on the bank and washover deposits. 7. Vegetation
plant species, density and condition 8. Evidence
of animal activity in the stream 9. Field
observation of moisture content of the floodplain
soils. 10. Depositional features 11. Erosional
features 12. Human development 13. Evidence of
flooding events 14. Bank stability
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Parameters
- Streams width
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Width
DEFINITION The distance between streams banks
in a perpendicular direction to
flow. METHODS Using a fiberglass or steel
tape measure. If the stream is wide, the tag may
sag, resulting in a measurement that is too high,
so pull it taut. The approximate widht of a
large stream may be measured on a
map. NOTE The width of a stream changes as
discharge changes.
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Parameters
- Streams width - Streams depth
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Depth
DEFINITION The distance between streams
surface and streams bottom. METHODS Using a
plastic or metal graduated rod. Because rivers
depth may vary significantly from bank to bank,
it is important to take many measurements to get
an accurate cross-sectional view of the
stream. NOTE Depth may change over time as
the stream moves, erodes and deposits sediments.
When discharge changes, depth changes.
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Depth
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Depth
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Parameters
- Streams width - Streams depth - The gradient
of the stream
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Gradient
DEFINITION The gradient of a stream is the
change in elevation of the water surface between
two points divided by the horizontal distance
traveled by the water. METHODS Using simple
surveying instruments (meter stick, measuring
tapes, theodolite). NOTE The gradient of a
stream changes along the course of the stream.
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Gradient
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Parameters
- Streams width - Streams depth - The gradient
of the stream - Cross-sectional area
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Cross-Sectional Area
DEFINITION The cross-sectional area of a stream
is the area of the stream perpendicular to
flow. NOTE It varies along the course of the
stream, and it varies as a function of current
velocity and discharge. METHODOLOGY It should
be estimated by first constructing an accurate
cross section of the stream, based on width and
depth measurements.
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Cross-Sectional Area
METHODS 1. Trace the cross section onto graph
paper and count the numbers of grid blocks
enclosed in the area. 2. Consider the area as
being composed of many trapezoids. Each
trapezoid is defined by the water surface, the
stream bed and two measurements of depth made
along the transect across the stream.
Calculate the area of each trapezoid by summing
the two depth measurements and dividing by 2
and multiplying by the width of the section.
Sum the areas of the several trapezoids to get
the total cross-sectional area of the stream.
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Cross-Sectional Area
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Parameters
- Streams width - Streams depth - The gradient
of the stream - Cross-sectional area - Wetted
perimeter
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Wetted Perimeter
DEFINITION The wetted perimeter (WP) is the
length of a line along the part of the stream bed
that is under water, in a direction perpendicular
to flow.
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Wetted Perimeter
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Wetted Perimeter
METHODS Constucting an accurate, to-scale cross
section of the stream based on width and depth
measurements, then measuring the WP on the cross
section. NOTE The wetted perimeter of a
stream is always greater than its width.
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Wetted Perimeter
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Parameters
- Streams width - Streams depth - The gradient
of the stream - Cross-sectional area - Wetted
perimeter - Hydraulic Radius
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Hydraulic Radius
DEFINITION The hydraulic radius is the
cross-sectional area of a stream divided by the
wetted perimeter. R A / WP R
hydraulic radius A cross-sectional area WP
wetted perimeter
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Parameters
- Streams width - Streams depth - The gradient
of the stream - Cross-sectional area - Wetted
perimeter - Hydraulic Radius - The stage
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The Stage of a Stream
DEFINITION The stage of a stream is the
elevation of the water surface above a datum.
The most commonly used datum is mean sea
level. METHODOLOGY Gages are used to measure
the stage of streams. Types of gages -
recording - non-recording
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The Stage of a Stream
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The Stage of a Stream
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Parameters
- Streams width - Streams depth - The gradient
of the stream - Cross-sectional area - Wetted
perimeter - Hydraulic Radius - The stage -
Streams velocity
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Velocity
Velocity of a stream varies - from top to
bottom of the flow profile
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Velocity
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Velocity
Velocity of a stream varies - from top to
bottom of the flow profile - across the stream
from bank to bank
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Velocity
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Velocity
Velocity of a stream varies - from top to
bottom of the flow profile - across the stream
from bank to bank - along the flow course of the
stream
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Velocity
METHODOLOGY 1. Direct methods - float method
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Float Method
Two operators are needed to run the float test.
One should be positioned upstream and the other
downstream. Distance between them should be
measured. The upstream operator releases the
float and stats the clock and the downstream
operator catches the float and signals to stop
the clock. Velocity is the distance traveled
divided by the time it takes to travel that
distance. NOTE Because the float is at the
surface of the water, this method does not give a
representative measurement of the average
stream velocity. Depending on the width of the
stream, it may be necessaty to divide the stream
into sections of channels and run the test in
each.
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Float Method
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Velocity
METHODOLOGY 1. Direct methods - float
method - using a current velocity
meter - using tracers 2. Indirect
methods - Manning Equation
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Manning Equation
METHODOLOGY This equation uses four factors
to estimate velocity - streams slope (S) -
wetted perimeter (WP) - cross-sectional area
(A) - a roughness factor (the Manning n
value) V(R2/3 S1/2) / n - for measurents
in metric system V(1.49R2/3 S1/2) / n -
for measurents in english system
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Parameters
- Streams width - Streams depth - The gradient
of the stream - Cross-sectional area - Wetted
perimeter - Hydraulic Radius - The stage -
Streams velocity - Discharge
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Discharge
DEFINITION Discharge (Q) of a stream is its
volumetric flow rate, in units of volume per
time. METHODOLOGY Discharges may be estimated
by calculations based on velocity and area
measurements. Q A V Q
discharge A cross-sectional area V
velocity