Streamflow

• Streamflow


  Runoff phase of the hydrologic cycle, most important basic data for hydrologic studies
• Stream


  Flow channel into which the surface runoff from a specified basin drains
• Hydrometry


  Science and practice of water measurement
• Streamflow measurement techniques


  • Direct determination of stream discharge
  • Indirect determination of streamflow
• Direct determination of stream discharge


  • Area-velocity methods
  • Dilution techniques
  • Electromagnetic methods
  • Ultrasonic method
• Indirect determination of streamflow


  • Hydraulic structures, such as weirs, flumes and gated structures
  • Slope-area method
• Classification of Streams


  • Perennial Streams
  • Intermittent Streams
  • Ephemeral Streams
• Stage


  Water surface elevation measured above datum
• Manual Gauges


  • Staff Gauge
  • Wire Gauge
• Automatic Stage Recorders


  Overcome the basic objection of manual staff gauges and provide continuous plot of stage vs time
• Float-Gauge Recorder


  Most common type of automatic stage recorder, uses a float operating in a stilling well balanced by a counterweight
• Bubble Gauge


  Compressed air or gas is made to bleed out at a small rate through an outlet at the bottom of the river, pressure gauge measures the water column above the outlet
• Methods of Streamflow Measurement

  • Volumetric method
  • Gravimetric method
  • Slope-Area Method
  • Velocity-area method
  • Control-section method
• Slope-Area Method


  Uses Manning's Rational equation to calculate velocity and discharge based on channel geometry and roughness

  
• Velocity-area method


  Determines velocity using floats or current meters, then calculates discharge from velocity and channel cross-section
• Control-section method


  Uses hydraulic structures like weirs, flumes and orifices to provide a unique control section where discharge is a function of water surface elevation
• Perennial Streams

  -Water flows in these streams throughout the year. The primary water source can be from ground water, surface water, or a combination of both.
• Intermittent Streams - Intermittent streams flow for part or most of the year but may carry no water during the dry season.
• Ephemeral Streams - These streams flow only for a short time, usually after a large storm or snowmelt when there is an increase in water runoff. Ephemeral streams are very small and normally have a dry channel during the year.
• The stage of a river is defined as its water surface elevation measured above datum. This datum can be mean-sea level (MSL) or any arbitrary datum, connected independently to the mean sea level.
• Volumetric method and gravimetric method

  • The first two methods are applicable if the channel or the stream cross sectional area is small, and the flow is relatively slow/laminar. It is the simplest and quickest method of determining the flow, thus may cause inaccuracies on flow measurement of large channels.
• Slope-Area Method

  • This method is most used by civil engineers in the design of channels (canals and drainage systems). It follows the Manning’s Rational equation wherein the following figure represents the cross-sectional area of the channel:
• Velocity-area method

  1. Float method
  2. Current meter
• Control-section method

  -The use of these structures is that these structures provide a unique control section in the flow such that the discharge will become a function of the water surface elevation measured from the specific datum.
• There are different categories of hydraulic structures:

  thin plate structure (orifice)

  long base weirs (broad crested structures)

  flumes (made of concrete, masonry, metal sheets)