In the name of God
College of Agriculture
Department of Desert Region Management
Hydrology Assessment of Discharge Rate
Prepared by M. Anvar
There are three methods that are usual for determining the discharge rate passing
through an open channel, river and open flowing water. The methods are
1) Slope – Area method :
In this method the Manning formula is utilized. In this formula the roughness
coefficient of the channel, n; hydraulic radius, R; flow section area, A; channel slope,
S are used.
Q R3 S 2 A
The hydraulic radius of the flow can be determined as follow:
R = A/P
In which P is the wetted perimeter of the flow.
Determine a function for calculating the wetted perimeter of a trapezoidal channel
with 50 cm bottom width and 1:3 side slopes?
How it can be practically measured the channel slope? By using excel software, for
the trapezoidal brick channel with 50cm bottom width and 1:3 side slope draw the
flow rate in term of flow depth, for depth of 0.01 m to 2m and 0.002 slope.
2) Floating object:
A floating object can be thrown on the flowing water in an open channel. The velocity
of object can be determined by measuring the time (T) at which the object passes
through a certain distance along the channel length (X):
V is the velocity and X and T are the length and time respectively. The discharge rate
of water then can be determined by using continuity equation:
The velocity of the floating object can not be regarded as the average velocity of
the flowing water in channel, so it should be considered a reducing constant
c=0.8-0.95. The reducing constant must be multiplied to the object velocity to
determine the average velocity ( V ):
Different instruments to determine the velocity of water in an open channel
Why the velocity of an object floating on the water surface is not equal to the
average flow velocity?
In figure …, it is shown that the floating object can be set in different depths.
Assume that the velocity of the floating object be equal to the flow velocity at
Now, for the following velocity profile,
a- draw the average velocity profile
b- How much is the c constant for this profile if the floating object be at the
water surface, depth A, and Depth B?
Vertical velocity distribution profile of water
3) Current meter:
Current meter is a instrument by which the velocity in a flowing water can be
determined. Current meter has a propeller that rotates when it is in the flowing water.
The rate of rotation is proportional to the water flow velocity. The number of round
per second of the propeller, n, can be used for estimation of flow velocity. N can be
determined by counting the number of rounds of the propeller, N, in an interval of
time, t, by the following equation:
The following equations show the relation between the number of round per second of
propeller and the flow velocity. There is a linear relationship between the number of
round per second of the propeller (n) and the flow velocity (v) as follow:
v = 0.0869n + 0.082 for n<=2.46
v= 0.104 n + 0.04 for n>=2.46
1) In an trapezoidal plane(flat)concrete channel with the bottom width of
10(m) and side slopes of 1(V):1.5(H), water is flowing with the depth of
3(m). if the linear slope of the channel is 0.0003, determine(assign) the
discharge rate with the manning formula?
2) Assign the discharge flow in a specific part(section) of a trapezoidal
channel with SLOPE-AREA, FLOATING OBJECT and CURRENT
METER methods and compare these methods together then answer the
following questions: (with the lab data)
- In your opinion wich method has been calculated(estimated) the rate of
discharge better and more accurate than the others?why?
3) To transmit the water flow from a rectangular creek with brick cover,
whenever the depth and width are 30 and 56 (cm) respectively, and the
linear bottom slope is equall to 0.02%, determine the passing discharge
4) We have utilized a current meter to calculate the velocity in different
depths of a river. The width at the measurer point is 18 (m).
According to the(based on) following information, determine the discharge
of the river: