Question: We want to build a dam on a river to store water to meet the water needs of an area. In what ways can we estimate how much water is flowing in the river?
1. Measurement (Hydrometric Station)
2. Calculate
Discharge Measurement
Area-Velocity Method
Q = A ∙ V
Runoff = Section Area x Flow Velocity
Problem: Uneven velocity distribution across the cross section
Simple Solution: Vaverage ≈ V0.4 ή Vaverage ≈ Vs ή Vaverage ≈ (V0,2 + V0,8)/2 κτλ. (βλέπε πιο πάνω εικόνα)
- Current Meters (propeller type, cup type)
- Συντελεστές διόρθωσης (correction factors)
- Measurement of rotations during recording N
Stage-discharge curve
When several recordings of the water supply are made for small and large flows then it is possible to construct the Level-Flow curve for the specific location. Having the Level-Flow curve then we easily record the flow of the river by simply measuring the level from the stage.
The recording of the level can be done on a daily basis or it can be a continuous recording with the use of a weighbridge (counter – counterweight) or ultrasonic level meters, pressure gauges etc.
- Cyprus: operation of 56 water meter measuring stations since 1960.
- Nile: Nilometer, measurements from 500 μx
Flow measurement in small watercourses
In small rivers hydraulic structures are constructed to measure the flow (discharge measurement weirs). A stable cross-section is formed with sufficient height. No experimental training of supply level curve is required.
Surface runoff assessment
Hydrograph: Gives the diagram of the rain-runoff relationship.
Unit hydrograph: hydrograph from rain 10 mm high of a certain duration and evenly distributed on the surface of the catchment area
