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FIELD METHODS IN ENVIRONMENTAL GEOLOGY GEOS 3110. Content. - Watershed boundaries - Streamflow / stream gauging. Watershed Definition. A drainage basin or watershed is the area throughout which surface water drains into a particular body of surface water. NOTE: - PowerPoint PPT Presentation
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FIELD METHODS IN ENVIRONMENTAL GEOLOGYFIELD METHODS IN ENVIRONMENTAL GEOLOGYGEOS 3110GEOS 3110
Content
- Watershed boundaries
- Streamflow / stream gauging
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.
Watershed Boundaries
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 resourcesin a basin.
Delineating Drainage Basins
Steps for Delineating a Drainage Basin:
1. For a stream drainage basin, find the point where the streamenters another body of water (its “base level”).
2. Put a pencil tip at the base level, and then move it, intersectingcontour lines at right angles, to the topographically highest nearby location.
3. Continue in this fashion. The goal is to draw a line surroundingthe area on which rainfall would drain into the stream of interest. Rainfalling on the other side of the line would flow into a different stream body of water.
“Marble Test”
Interpretation of the topographic contour lines:
Set a pencil point down anywhere on the map, and imaginewhat 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.
“Marble Test”
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 distancedownstream will be analyzed for its hydrologic significance.
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)- surveyor’s tape or marker for marking tag lines- yardsticks- timer (stopwatch, watch with second hand, etc.)- floats (for velocity determination)
Field Observations1. General topographic setting2. Site-specific topography and relief (a sketch or profile may behelpful.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 condition8. Evidence of animal activity in the stream9. Field observation of moisture content of the floodplain soils.10. Depositional features11. Erosional features12. Human development13. Evidence of flooding events14. Bank stability
Parameters
- Stream’s width
WidthDEFINITION:
The distance between stream’s banks in a perpendiculardirection 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.
Parameters
- Stream’s width- Stream’s depth
DepthDEFINITION:
The distance between stream’s surface and stream’s 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 anaccurate 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.
Depth
Depth
Parameters
- Stream’s width- Stream’s depth- The gradient of the stream
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.
Gradient
Parameters
- Stream’s width- Stream’s depth- The gradient of the stream- Cross-sectional area
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.
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.
Cross-Sectional Area
Parameters
- Stream’s width- Stream’s depth- The gradient of the stream- Cross-sectional area- Wetted perimeter
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 directionperpendicular to flow.
Wetted Perimeter
Wetted Perimeter
METHODS:Constucting an accurate, to-scale cross section of the
stream based on width and depth measurements, then measuringthe WP on the cross section.
NOTE:The wetted perimeter of a stream is always greater than
its width.
Wetted Perimeter
Parameters
- Stream’s width- Stream’s depth- The gradient of the stream- Cross-sectional area- Wetted perimeter- Hydraulic Radius
Hydraulic Radius
DEFINITION:The hydraulic radius is the cross-sectional area of a stream
divided by the wetted perimeter.
R = A / WP
R = hydraulic radiusA = cross-sectional areaWP = wetted perimeter
Parameters
- Stream’s width- Stream’s depth- The gradient of the stream- Cross-sectional area- Wetted perimeter- Hydraulic Radius- The stage
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
The Stage of a Stream
The Stage of a Stream
Parameters
- Stream’s width- Stream’s depth- The gradient of the stream- Cross-sectional area- Wetted perimeter- Hydraulic Radius- The stage- Stream’s velocity
Velocity
Velocity of a stream varies:- from top to bottom of the flow profile
Velocity
Velocity
Velocity of a stream varies:- from top to bottom of the flow profile- across the stream from bank to bank
Velocity
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
Velocity
METHODOLOGY:
1. Direct methods : - float method
Float MethodTwo operators are needed to run the float test. One should be
positioned upstream and the other downstream. Distance betweenthem should be measured.
The upstream operator releases the float and stats the clockand the downstream operator catches the float and signals to stop theclock.
Velocity is the distance traveled divided by the time it takesto 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 streamvelocity. Depending on the width of the stream, it may be necessatyto divide the stream into sections of “channels” and run the test in each.
Float Method
Velocity
METHODOLOGY:
1. Direct methods : - float method - using a current velocity meter - using tracers
2. Indirect methods: - Manning Equation
Manning Equation
METHODOLOGY:
This equation uses four factors to estimate velocity:- stream’s 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=(1.49*R2/3 * S1/2) / n - for measurents in english system
Parameters
- Stream’s width- Stream’s depth- The gradient of the stream- Cross-sectional area- Wetted perimeter- Hydraulic Radius- The stage- Stream’s velocity- Discharge
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 = dischargeA = cross-sectional areaV = velocity
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