Field Studies: Geomorphology and Water Quality

Watershed Term Definitions

  • Discharge (Q): The volume of water passing a point in a stream per unit time. Also known as Flow.
  • Velocity (V): The rate of water movement, measured as distance per time.
  • Cross-sectional Area (A): The area of a vertical plane cutting across the stream.
  • Stage (d): The elevation of the river surface above its bed, equivalent to water depth.

Stream Gauging

  • Stream gauging is used to accurately measure the volume of water moving past a specific point over time.
  • The U.S. Geological Survey (USGS) provides real-time stream gauging data online.

Discharge

  • Discharge is a fundamental property of a fluvial system, representing the amount of water flowing at a given time.
  • Definition: the volume of water passing a channel cross-section during a specific time interval.

Discharge Equation

  • The discharge (Q) is calculated by multiplying the velocity (V) of the water by the cross-sectional area (A) of the stream:
    Q=V×AQ = V \times A
  • Units:
    • Cubic meters per second: m3s\frac{m^3}{s}
    • Cubic feet per second: ft3s\frac{ft^3}{s}

Calculating Discharge: Measuring Velocity

  1. Measure a stream section's length and mark the start/finish with tapes across the stream.
  2. Drop a float in the stream's middle, upstream of the start line. Begin the stopwatch when the float passes the start line.
  3. Stop the stopwatch when the float crosses the finish line, then recover the float.
  4. Record the float's travel time in seconds.
  5. Repeat steps 2-4 nine more times at different channel locations.
  6. Calculate average float time by dividing the sum of all time values by the number of trials (10).
  7. Compute average velocity by dividing the distance by the average float time.
  8. Multiply the result by a 0.8 velocity correction factor to account for surface water moving faster than water near the stream bottom.
    • This correction factor is generally recognized in hydrology for float velocity tests.

Measuring Velocity (V)

  • Float Method: Time the movement of a float in the water.
  • Flowmeter: A flowmeter (current meter) measures water velocity directly.

Using the Flowmeter

  • A flowmeter is used with a wading rod to measure water velocity at different depths.
  • Depicts a current-meter measurement setup in rivers, including the wire, rope, suspension cable, winch, and boat, with the current meter positioned at 60% depth.

Discharge Examples

  • Mississippi River: Q=15,000m3sQ = 15,000 \frac{m^3}{s}
  • San Marcos River: Q=5m3sQ = 5 \frac{m^3}{s}
  • Amazon River: Q=150,000m3sQ = 150,000 \frac{m^3}{s}
  • Red River: Q=100m3sQ = 100 \frac{m^3}{s}

Calculating Discharge: Measuring Cross-Sectional Area

  1. Hold the 0 marker of the tape even with one stream edge.
  2. Measure taught tape distance to the other edge to determine width of the stream section.
  3. Every foot measure the stream depth using a stadia rod.
  4. Calculate the area for each 1-foot section as width (1 ft) x depth, then sum all the areas.
  5. Repeat steps 2-4 for the endpoint and midpoint of the float trial.
  6. Sum the three cross-sectional areas and divide by 3 to calculate the average cross-sectional area.

Measuring Cross-Sectional Area (A)

  • Measure by subsection and sum.
  • Divide the area into discrete units based on a fixed interval or natural breaks in the slopes.
  • Apply simple geometry: width x depth for each subsection, then sum the areas.

Calculating Discharge

  • Discharge (Q) equals velocity (V) multiplied by cross-sectional area (A).
    Q=V×AQ = V \times A
  • Measured in cubic meters or cubic feet per second: m3s\frac{m^3}{s} or ft3s\frac{ft^3}{s}

Fluvial Geomorphology Terms

  • Hydrologic floodplain
  • Bankfull width
  • Bankfull depth
  • Bankfull elevation

Geomorphic Terms

  • Abandoned floodplain (terrace)
  • Active floodplain
  • Bankfull stage
  • Active-channel stage
  • Thalweg
  • Natural levee

Plan View

  • Stream margin
  • Riffle: Fast, shallow flow over boulders and cobbles.
  • Pool: Areas of slow-flowing, deep water, often on the outside of bends.
  • Run: Smooth, unbroken flow connection riffles and pools.

Channel Roughness and Manning’s n

  • Manning's n: Represents resistance to flow due to vegetation, channel shape, sedimentation, channel bed material, and other obstructions.

Channels and Floodplains

  • Resistance to flow depends on roughness; Manning’s n represents roughness.
  • Roughness changes with time (vegetation growth, culvert deterioration).

Manning's Equation for Discharge

  • Q=1nAR23S12Q = \frac{1}{n} A R^{\frac{2}{3}} S^{\frac{1}{2}}
    • Q = discharge (cms)
    • A = channel area (m2m^2)
    • R = hydraulic radius (A/Pw), ~ depth (m)
    • S = water surface slope
    • n = Manning's roughness coefficient
  • Q=1.49nAR23S12Q = \frac{1.49}{n} A R^{\frac{2}{3}} S^{\frac{1}{2}}
    • If English units are used

Hydraulic Radius (R)

  • Hydraulic Radius: ratio of the cross-sectional area of a channel to the wetted perimeter.

Water Surface Slope (S)

How measuring slope?

Manning’s n: Guidance

  • Corrugated Pipes: 0.024
  • Concrete pipes, open channels: 0.013
  • Small channels, clean: 0.03
  • Large channels (width > 100’): 0.025
  • Floodplains (natural vegetation): 0.06-0.1
  • Roughness n is determined by characterizing the flow surface and looking up appropriate tabulated values and correction factors.

Change in Floodplain Features

  • As n decreases, v increases, resulting in more flow in the floodplain over time.
    • Trees, Shrubs, Grass in 1900: n = 0.1
    • Marsh in 1956: n = 0.05
    • Open Water in 1992: n = 0.03

Aquatic Ecosystem

  • An aquatic ecosystem is more than just water: all compartments must be considered.
  • Bank, Water, Bed.
  • Water quality: chemical, physical, and (micro)biological.

Water Quality Requirements

  • No specific requirements: navigation water or power generation.
  • Defined “minimum standards”: irrigation water, fisheries, and recreation water.
  • “Undisturbed quality”: ecosystem functioning.
  • Water quality expresses the suitability of water for various uses or processes.

Water Quality Standards

  • Human consumption: MCLG (non-enforceable public health goals) and MCL (enforceable standards).
  • Restoration and ecosystem stability: micropollutants (heavy metals, pesticides, PCBs, etc.) from industrial sources.

Natural Water Quality

  • Depends on environmental factors:
    • Occurrence of soluble minerals
    • Distance to the coastline
    • Precipitation/river run-off ratio
    • Peat bogs and wetlands
  • No “average” natural water quality can be given.

Spatial and Temporal Variations

  • Rivers and lakes have different water quality.
  • Spatially: source and mouth, surface and deepest layers, banks, and middle.
  • Temporally: day/night, seasons, years, centuries.

Discharge Regime

  • Understanding discharge is important for interpreting water quality measurements.
  • The discharge of a river often determines the concentration of dissolved substances via dilution.
  • Load L (g/s) = Discharge Q (m^3/s) * Concentration C (g/m^3).

Water Quality Parameters

  • Dissolved Oxygen
  • Temperature
  • Ammonia/Nitrite/Nitrate
  • pH
  • Alkalinity/Hardness
  • Salinity
  • Carbon Dioxide
  • Solids, EC, and TDS

Dissolved Oxygen

  • Dissolved Oxygen is the amount of gaseous oxygen (O2O_2) dissolved in water.
  • Enters the water by direct absorption from the atmosphere or as a byproduct of photosynthesis.
  • Levels below 5.0 mg/L cause stress to aquatic life.

pH

  • pH expresses the intensity of the acidic or basic characteristic of water.
  • Optimum pH for freshwater aquatic animals: 6.5 to 9.0.
  • Seawater: 8.0-8.5
  • Freshwater: 6.5-9.0

Carbon Dioxide

  • High CO2 concentrations reduce respiration efficiency and decrease tolerance to low dissolved oxygen.
  • CO2 is highly soluble in water.
  • Concentration in pure water: 0.54 mg/L at 20°C.
  • Groundwater concentrations range from 0-100 mg/L.

Solids

  • Three categories: settleable, suspended, and dissolved.
  • Upper limit: 25 mg TSS/L.
  • 10 mg/L for cold water species.
  • 20-30 mg/L for warm water species.

TDS and EC

  • EC (Electrical Conductivity) is measured in mS/cm.
  • TDS (Total Dissolved Solids) is measured in PPM.
  • TDS is acquired by taking the EC value and performing a calculation to determine the TDS value.
  • Both measure the amount of dissolved ions.

Nitrates

  • High levels are usually considered contaminants.
  • Sources: agricultural activities, human wastes, or industrial pollution.

Water Quality Testing: Field

  • pH
  • EC and TDS
  • Dissolved oxygen
  • Turbidity
  • Temperature
  • Biological testing
  • ORP

Water Quality Testing: Lab

  • Wet chemistry
  • Mass Spectrometry

Calibration Standards and Data Quality

  • How are you calibrating your equipment before, during, and after testing?
  • How can you be sure your samples have not been contaminated?

Precision and Accuracy

  • Understanding the accuracy of different measurement methods.