The best hydraulic section of an open channel is characterized by provision of maximum discharge with a given cross sectional area. In fact, the choice of best hydraulic section also possesses other advantages than hydraulic performance. For instance, for a given discharge rate the use of best hydraulic section could guarantee the least cross sectional area of the channels. Substantial savings could be made from the reduction in the amount of excavation and from the use of less channel linings.
To obtain the best hydraulic channel section, some variables have to be determined. For a flow equation, e.g. Manning, the channel section contains two variables (rectangular and triangular sections) or three variables (trapezoidal and round-bottom triangular sections). For a given flow, the best hydraulic channel section can be obtained by minimizing the wetted perimeter (or the cross-sectional area). Any flow equation, e.g. Manning, can be used. There are many conventional methods given in the textbooks.
In this paper, Manning equation will be used to get the best hydraulic section for a trapezoidal section for open channels with different side slopes. Also, Microsoft Excel software is employed as a spread sheet to obtain the required best hydraulic sections. Definitely, samples are included to obtain the best hydraulic section for trapezoidal channel with side slopes of 1:1, 3:2, and 2:1.
Beside the basic solution obtaining the best hydraulic section, an additional solution is included. This additional solution is characterized by modifying the bed width (b) to the nearest 5 cm, and then it gives the modified water depth for the same flow area.
Also, a second additional solution may be used concerning the velocity of water through the trapezoidal best hydraulic section. This additional solution is characterized by calculating the velocity of the water through the channel section. For earth channels, the water velocity is likely non-silting non-scouring velocity. That is the velocity is not less than 0.3 m/sec to avoid silting, and is not more than 0.9 m/sec to avoid scouring. The second additional solution calculates the velocity, and then it modifies the area of flow to keep the velocity, and finally it obtains the corresponding values for the bed width and the water depth.
It is found that this design of best hydraulic sections is efficient, accurate, easy and simple. It can be widely used by students and engineers to obtain the best hydraulic section for any trapezoidal channel in hydraulics, irrigation canals, and drains.
It is recommended to apply the same technique to obtain the best hydraulic sections for other triangular and circular open channels. Also, it is recommended to investigate the application of this technique to design the non silting non scouring sections for canals and drains of irrigation projects.
|
