FIELD METHODS IN ENVIRONMENTAL GEOLOGY GEOS 3110

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 Observations

1. 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

Width

DEFINITION:The distance between stream’s 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.

Parameters

- Stream’s width- Stream’s depth

Depth

DEFINITION: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 Method

Two operators are needed to run the float test. One should bepositioned 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