L4 stream flow
- 1. Hydrology CEN 363 Credit : 3.00 Md. Mehedi Hassan Masum Lecturer Department of Civil Engineering Port City International University Stream Flow
- 2. Introduction of stream flow •Stream - flow channel into which surface runoff from a specified basin drains • A river drains the water collected from a specific area, which is called its ‘catchment area’. • An area drained by a river and its tributaries is called a drainage basin. • The boundary line separating one drainage basin from the other is known as the watershed. • The catchments of large rivers are called river basins while those of small rivulets and rills are often referred to as watersheds. •Stream flow – runoff • Measurement - fairly accurate Department of CEN, PCIU
- 3. Stream flow measurement •Direct determination (time consuming and costly) • Area velocity • Dilution technique • Electromagnetic method • Ultrasonic method •Indirect determination • Hydraulic structures • Slope-area method Department of CEN, PCIU
- 4. Velocity measurement •Commonly used device is current meter •Float can also be used for approximate measurement •Current meter consists of a rotating element • Vertical-axis • Horizontal-axis Department of CEN, PCIU
- 5. • Calibration • Relation between stream velocity and revolutions per sec • Field use • Shallow stream (depth up to 3 m) • Moderately deep – two point measurement at 0.2 and 0.8 times of depth. Avg velocity • Rivers having flood flow, only surface velocity is measured within a depth of 0.5m below the surface. Avg velocity is K times vs. 6.0vv 2 8.02.0 vv v Department of CEN, PCIU Velocity measurement
- 6. Discharge measurement • Area – velocity method • Measurement of area of X-sec • Measurement of velocity • Method is suitable, when • Stream have a well-defined cross section and does not change in various season • Easily accessible throughout the year • -- Straight and stable reach • -- Gauging site should be free from back water effect Department of CEN, PCIU
- 7. •Cross section is divided into a number of sub sections by verticals •Avg velocity of these sub-sections are measured by current meter • Segment width should not be greater than 1/15 to 1/20 of river width • Q segment should be less than 10% of total Q • Difference in velocity in adjacent segment should not be more than 20% Department of CEN, PCIU Discharge measurement
- 8. For the 1st and last segment, those are triangular section 2-Nto2ifor 22 Q alith verticat thevelocityright)xavgthewidth to1/2 leftthewidth to(1/2xsegment)ithat the(depth segmenteithinischarge 1 i 1 1 i ii i i N i i v WW y dQ QQ 1 2 2 1 1 2 2 W W W W 111 * yWA Department of CEN, PCIU Discharge measurement
- 9. Department of CEN, PCIU Discharge measurement
- 10. •Moving-boat method for velocity measurement in area- velocity method • A special current meter of the propeller type in which the velocity and inclination of the meter to the boat direction θ in the horizontal plane can be measured, is selected. tv yy Q vW yy Q R ii i fi ii i .cossin 2 2 21 1 1 θ vb vf vR Department of CEN, PCIU Discharge measurement
- 11. •Dilution technique of stream flow measurement •Tracer should be • Not absorbed • Non toxic • Capable of being detected in small concentration • Not expensive • Types of tracer • Chemical (salt) • Fluorescent dye • Radioactive material Department of CEN, PCIU Discharge measurement
- 12. Indirect methods • Flow measuring structure • Notch, weir, flume, sluice gate etc. • Slope area method- Frictional loss, hf and length of the reach, L n = Manning’s roughness coefficient n KHQ HfQ )( 3 2 2 2 n 1 ChanneltheofConveyanceKWhere, slopeenergy AR K Q S L h f f Department of CEN, PCIU
- 13. Stage-discharge •First a stage-discharge relationship (rating - curve) is developed •Then only stage is measured, which is easy, inexpensive and continuous reading can be taken Department of CEN, PCIU
- 14. Stage measurement • Elevation measurement from a fixed graduated staff • Normally the staff is fixed to a rigid structure like abutment, pier wall • If one staff is not sufficient use of sectional staff • Wire gauge • From a bridge, a weight is lowered by a reel to touch the water surface • Automatic stage recorder • Float-gauge recorder Department of CEN, PCIU
- 15. Stage discharge relationship •Plot stage versus discharge • Gives a relationship representing integrated effect of a wide range of channel and flow parameter • Combined effect of these parameters are known as control • If control does not change with time permanent control • Otherwise shifting control •Easy to measure discharge Department of CEN, PCIU