Water flow is measured to assess water supply and flow through systems. Discharge is the rate of water flow passing through a cross section per time unit. It is calculated by multiplying water velocity by the cross-sectional area. Surface water flow measurement methods include the velocity-area method and weir gauging. Rating curves relate river discharge to stage. Flood frequency analysis uses historical flood records to predict future flood magnitudes and recurrence intervals.

1. Water Measurement

2. Water Measurement
Water flow is measured to assess
how much water is available for a
supply and to check the quantity of
water flowing through a system

3. Water Measurement
Discharge is defined as the rate of flow or
the volume of water that passes through a
channel cross section in a specific period of
time.
Q = V A
Measured in units of cubic feet per second, (ft3/s
or cfs) or its metric equivalent of liters per second
(Lps or m3/s)

4. Example
If the width of the channel is ten feet, the
depth is one foot, and the velocity is two
feet per second, then
A = 10 ft x 1 ft = 10 ft2
Q = 2 ft/s x 10 ft2 = 20 cfs

5. Water Measurement
Different sources of water require
different methods of flow measurement
and can be divided as follows:
 Surface water
 Springs
Wells

6. Surface Water Sources
Surface water flow is simply the continuous
movement of water in runoff or open
channels.
The two most common methods of flow
measurements of surface water are:
 Velocity-Area Method
 Overflow Weir Gauging

7. Velocity - Area Method
Figure 3.11 The
water velocity is
found using a flow
meter, which looks
like an
anemometer

8. Figure 3.12 One cubic foot per second, cfs (or one
cubic meter per second, cms) is equivalent to one
cubic foot (or meter) of water flowing past a given
point in a one-second time interval.

9. Gauging Weirs Method
Figure 3.13 A weir is
an obstruction in an
open channel which
constricts the flow
and causes it to fall
over a crest.

10. Figure 3.14 The hydrograph of a river can look
similar to this example after a brief but intense
rainfall event.

11. Stage
Stage is the depth of flow (m or ft) which
most commonly measured the elevation of
the water surface relative to an arbitrary
fixed point.
To relate the discharge to the river stage, a
Rating Curve is used.

12. Rating Curve
Figure 3.15 To
relate the
discharge to
the river stage,
a Rating Curve
is used.

14. Flood Events

15. Flood Frequency
Flood Frequency is the likelihood that a
large flood will happen.
Flood Frequency Analysis uses historical
records of peak flows to produce
guidance about the expected behavior of
future flooding.

16. Flood Frequency Analysis
Two primary applications of flood
frequency analyses are:
 To predict the possible flood magnitude
over a certain time period
 To estimate the frequency with which
floods of a certain magnitude may
occur.

17. Return Period
Two primary applications of flood
frequency analyses are:
 To predict the possible flood magnitude
over a certain time period
 To estimate the frequency with which
floods of a certain magnitude may
occur.

18. Return Period
Return Period also referred to as the Recurrence Interval
is the time period over which it is likely that a particular
magnitude flood will occur.
100-year flood
A flood that can occur on average once every 100 years or 1%
chance of occurring in a given year
*** 100 years is considered the return period

19. Exceedance Probability
Exceedance Probability or the Probability of Occurrence
is the chances over a given time period that a flood will
reached or exceed a specific magnitude.
100-year flood
A flood that can occur on average once every 100 years or 1%
chance of occurring in a given year
***0.01 considered the exceedance probability

20. Figure 3.16 Flood damage can be predicted based
on the intensity of a storm and the topography of a
region.

21. • Probable Maximum Precipitation (PMP)
• The most extreme rainfall possible
• Used for estimating the effects of extreme
weather
• Probable Maximum Flood (PMF)
• The most extreme flood possible
• Used for estimating maximum extent of
flooding
Extreme Events

22. GIS Mapping
Geographic Information Systems (GIS) is
used to organize spatial information. These
information are stored in the computer.
Its GIS layers include topography, soils,
hydrography, vegetation, land use, etc.