1.6 Discharge

Discharge or stream flow - the amound of water flowing through a channel (known as runoff as well)

• water flows perpendicular to a cross section (cross section is below)

• the colors indicate the flow velocity

  • lowest near the bed and banks because there its slowed down by friction

  • highest in the middle of the surface - furthest away from the bed and banks you can get

    

• flow velocity - measured in meteres per second

  • discharge (in cubic meters/second) = flow velocity (average velocity in meters/s) x cross sectional area ( squared meters)

    • volume of water that passes this cross seciton in a certain amount of time

    • what happens when there is an obstruction and water slows down:

      • the discharge is determined by the catchment drainage (how much water flows into the channel)

      • if the discharge remains the same and velcotiy decreeases - the cross sectional area increases

        • the bed and banks are fixed - the cross sectional area has to rise

• factors that determine how fast water flows

  • Chezy equation - steady uniform flow * discharge does not change in time and space

    

    • used as an approx in other situations as well

    • chezy’s constant - the smoothness of the bed and banks

      • whne there are large boulders or lots of aquatic vegetation - chezy’s constant is small and the flow velocity is small as well

      • the hydraulic radius = cross -sectional area/ wetted perimeter

        •   wetted perimeter is the part of the circumference that touches the banks and bed ( where friction is encountered)

          • do not count the part of the water surface (as air does not slow the water down)

        • if the hydraulic radius is big then the average distance from a point in the cross section to the bed and banks is large

          • the friction of the vegetation and boulders has less effect and the flow velocity is higher

          • the flow velcoity is higher

      • the slope of the channel bed = the elevation drop/ horizontal distance

        • how much the bed drops over a certain horizontal distace

        • the slope of the water table - parallel to the channel bed

        • when the slope of the channel bed is steep - water flows fast beacuse gravity

      • helps to predict hte effect of measures

        • when a straightened channel is being remandered - the slope decreases beacuse the elevation drop remains the same and the length increase

          • the flow velcoity and the discharge capacity will decrease

    • discharge = velcoity x cross sectional area

      • estimate the discharge capatiy of a channel - how much a channel can transport before it overflows

      • can do it with a propeller current meter or other instruments

    • meaursing water elevesl is easier than measuring flow velocities

      • the measurment of water levels is done through a stilling well - a vertical tube connected to the surface water which dapens out the waves

      • inside the stilling well there is a device recording the water level continuously