Discharge

• streamflow: water transported out of catchment through channels
• discharge: volume of water which passes cross-section of river per unit of time

  • closely related to flow velocity and water level
  • determine where erosion and sedimentation occur and which aquatic flora and fauna can survive in river
• discharge determined by size and cross-section of channel, roughness caused by channel bed and slope of water table
• discharge is high when cross-sectional areas is large and flow velocity is high
• $Q =$$mean(v) *$$A$

  • Q (m3/s) = discharge
  • mean(v) (m/s) = average flow velocity of cross-sectional area
  • A (m2) = cross-sectional area
• flow velocity is lower closer to banks and bed where friction slows water down
• $mean(v) =$$C*$$[sqrt(S0*Rh)]$

  • C (m0.5/s) = Chezy constant
  • S0 = slope of channel bed
  • Rh (m) = hydraulic radius
• Chezy constant/coefficient: measure for conductivity of channel bed and banks

  • high C value = smooth river bed and banks = no slowing down of flow velocity
• $Rh =$$A/P$

  • A (m2) = cross-sectional area
  • P (m) = wetted perimeter
• discharge high in rivers with smooth beds and banks, large cross-sectional area, steep slopes and large hydraulic radius
• discharge capacity: amount of water a channel can transport
• flow velocity measured with propeller current meter at multiple locations measuring rotations or using acoustic doppler current profiler (ADCP) attached to river banks emitting sound waves measuring change in frequencies
• measure water level continuously using stilling wells: vertical tubes which are connected to river via small horizontal tube with attached floating device or pressure sensory to measure water level
• rating curve/Q-h-relation/stage-discharge relation: converting water level to discharge by measuring both high and low discharges
• stream restoration projects: length of brook increases while river water level decline does not increase