ABE459 Drainage and Water Management
Development of a Stream Rating Curve
Due September 9, 11:59 pm.
OBJECTIVES
The objectives of this exercise are to:
Determine Roughness Coefficient from observation of field conditions.
Develop a rating curve for a section of an open channel
Develop a routine for using two depth measurements to solve for flow in natural channels.
Develop a routine for solving Manning’s Equation in rectangular and trapezoidal channels.
EQUIPMENT
Experiments will be performed in a stream. You will be supplied with, Engineer's Level, Surveying Rod, Measuring Tape, and Flags.
FIELD PROCEDURES
- Establish a temporary benchmark to serve as a common datum for all measurements.
- Observe the physical condition of the stream channel. Note especially, the vegetation (if any) in the stream, the size of the stones in the stream, any obstacles to flow, and the conditions of the banks. Based on your observations, use resources in the literature and the class notes to estimate the Roughness Coefficient. Write a justification of your choice.
- Use the tape and rod to determine the channel shape at the given cross-section. Use up to seven points to define the cross-section.
- Use the level and rod to determine the slope of the channel by measuring the change in the elevation of the water surface, or the channel bed, over a fixed length of flow. Use the average of three measurements.
- If the stream is flowing, determine the depth of flow. In addition, determine the velocity of flow at the cross-section using the float method. To do this, determine the time it takes a small floating object to traverse a known length of stream. Choose the flow path so that the cross-section is in the center of the reach. Multiply the velocity obtained by 0.8 to account for flow variations with depth. Based on the measured velocity, the flow area, and the channel slope, determines the roughness of the channel.
COMPUTATIONAL EXERCISES
- Develop a rating curve (flow rate vs. depth) for the flow section assuming that the Manning Equation always applies.
- Using data from two cross-sections, develop a routine for using two depth measurements to solve for flow in natural channels. Use the routine to determine flow over this past spring and early summer. You will be supplied with depths at two cross sections.
- Develop a routine for solving Manning’s Equation in rectangular and trapezoidal channels.
- Bonus : Develop a protocol for using data from three cross-sections to determine flow in natural channels, without assuming that the flowrate is between the values at the three cross-sections.
Manning Equation:
where
Q = flow rate [ft3/s]
n = roughness coefficient of the channel
R = A/P, the cross-sectional area divided by the wetted perimeter [ft]
A= cross-sectional area of flow [ft2]
s = hydraulic gradient (slope of the channel or water surface)
= 1.49 for English units
A Protocol for Estimating Flow from Adjacent Rating Stations
Consider a site consisting of two cross sections, C and D, at which flow depths are measured. If these sections are close together, without concentrated inflow between them, the flow at both may be considered to be equal. Using the Manning equation, the flow at each is given by
And the flow ratio, Qr, is given by
A, R and s are based on actual measurements across and along the cross sections, while n values are based on estimates from the flow conditions. In addition, because the Manning equation is based on the assumption of normal depth flow, rating curves are typically looped with different effective n values on the rising and falling limbs of a hydrograph.
If the flow ratio is not equal to one, then the n values can be tweaked to make the flow equal. Assuming the true flow is between the two values, the flows can be made equal by slightly increasing the n value at one cross section, and slightly decreasing the n value at the other.
If the n values are changed by the same fraction, , to equate the flows, then
Thus
By using two measurements, flow is more accurately estimated, and flows on both limbs of a looped rating curve can be obtained.