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: 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 resources in a basin.

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.

“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.

“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 distance downstream 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 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

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 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.

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 direction perpendicular to flow.

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.

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 radius A = cross-sectional area WP = 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

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 of a stream varies: - from top to bottom of the flow profile - across the stream from bank to bank

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 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.

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=(R 2/3 * S 1/2 ) / n - for measurents in metric system V=(1.49*R 2/3 * S 1/2 ) / 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 = discharge A = cross-sectional area V = velocity