1. STREAMFLOW and HYDROGRAPH ANALYSIS
Stream flow is one of the most important topics in engineering hydrology because it directly relate to water supply, flood control, reservoir design, navigation, irrigation, drainage, water quality, and others.

2. Stream Flow Measurements
Serves as the basis for many water resources engineering designs
Two approaches
Measurement of water stage;
Measurement of flow velocity
Measurement of Water Stage
Water stage: the elevation above some arbitrary datum of water surface at a station
Types of Gages Measuring River Stage:
Staff gage – vertical or inclined
Suspended – weight gage
Recording gage
Crest – stage gage ( used to indicate high water mark)
Misc (Table 1).

3. Figures of Stream Gauges

4. List of Stream Flow Measurement Methods

5. Stage-Discharge Relation
When water stages are measured, we need additional information to estimate the flow rates (or discharges) Q H t
Stage Hydrograph
Stage-Discharge Curve
or Rating Curve
Discharge Hydrograph

6. Stage-Discharge Relation
Typical relationship: Q = a(H +b)c
The function relationship between H & Q has to be calibrated locally for different stations

7. Storage Hysterisis
In natural rivers, the H-Q relationship in general appears to be a loop, rather than single-valued.

8. Devices for Flow Velocity Measurement
Current Meters
Cups
Propellers
V = a + bN
where V = flow velocity; a = starting velocity to overcome mechanical friction; b = equipment calibration constant; N = revolutions/sec.
Pitot Tubes: Suitable only for clean water
Floats: Suitable for straight channel, V = L/T

9. Current Meters

10. Mean Flow Velocity Estimation
Velocity Profile

11. Measurement of Stream Flow Discharge - 1
Stage – Discharge (Rating) Curve
Mid-Section Method

12. Measurement of Stream Flow Discharge - 2
(c) Mean-Section Method

13. Measurement of Stream Flow Discharge - 3
(d) Area-Slope Method

14. Measurement of Stream Flow Discharge - 4

15. Hydraulic Structures for Discharge Measurement

16. Regime Theory W = aQb ; D = cQf ; V = kQm Q = VWD  ack = 1.0
subject to b+f+m = 1.0
   Generally applicable up to mean annual discharge. For flows larger than the mean annual discharge, different relationships exist.

17. Extension of Rating Curve
- During the event of large flood, it is impossible or impractical to measure discharge directly. More often than not, the flood stage goes beyond the range of the data range used to define the rating curve. Therefore, extrapolation of the ration curve is needed when water level is recorded below the lowest or above the highest level.
- Large errors can result if the functional form of rating curve, Q = a(H+b)c, is extrapolated beyond the recorded gauge discharges without consideration of the cross-section geometry and controls
- Graphical extension or by the fitted Q-H relationship is adequate only for small extension
- For large extrapolation beyond the active channel cross-section, hydraulic formula can be used to estimate the stage-discharge relation.

18. Steven Extension Based on Chezy formula,
with A = flow cross-section area; C = Chezy Coefficient; R = hydraulic radius, A/P; and S = channel slope.
For a given section, = constant whereas for a wide channel (W>10D) RD. Therefore,

19. Example (Extension of Rating Curve)