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The best hydraulic section of an open channel is characterized by provision of maximum discharge with a given cross sectional area. As such, channels with circular shape is the best hydraulic sections while a rectangular channel with channel width being equal to two times the height of channel is the best hydraulic section among all rectangular sections.
For checking of self-cleansing velocity for pipes, there is another criterion to check design flow Q to full bore flow Qfull> 0.5. If this criterion is met, it can be deduced that the design flow is always greater than self-cleansing velocity.
In the design of gravity drainage pipes, full bore flow capacity is normally adopted to check against the design runoff. However, one should note that the maximum flow rate does not occur under full bore conditions. The maximum discharge occurs when the water depth in circular pipes reaches 93.8% of the pipe diameter. Therefore, the use of full bore discharge is on the conservative side though the pipe’s maximum capacity is not utilized.
Darcy-Weisbach equation combined with the Moody Diagram is the accepted method to calculate energy losses resulting from fluid motion in pipes and other closed conduits. However, the Moody Diagram may not be suitable for usage in some conditions.
Manning’s formula was proposed by Robert Manning (an Irish engineer) to calculate uniform flow in open channel. It is probably the most widely used uniform-flow formula around the world. Its extensive usage is due to the following reasons:
(i) The majority of open channel flows lies in rough turbulent region;
(ii) It is simple in form and the formula is well proven by much practical
experience.
Manning’s Equation is commonly used for rough turbulent flow while Colebrook-White Equation is adopted for transition between rough and smooth turbulent flow.
Rational Method is suitable for small catchments only because the time of concentration of small catchments is small. In Rational Method the peak runoff is calculated based on the assumption that the time of concentration is equal to the rainfall duration. For small catchments, this assumption may hold true in most circumstances. One of the assumptions of Rational Method is that rainfall intensity over the entire catchment remains constant during the storm duration. However, in case of a large catchment it stands a high probability that rainfall intensity varies in various part of the large catchment. In addition, for long duration of rainfall, it is rare that the rainfall intensity remains constant over the entire rainstorm and a shorter duration but a more intense rainfall could produce a higher peak runoff. Moreover, a reduction of peak runoff is also brought about by the temporary storage of stormwater like channels within the catchment.
Computation of runoff is a complicated matter which depends on many factors like the ground permeability, rainfall duration, rainfall pattern, catchment area characteristics etc. Basically, Rational Method is a means to find out the maximum discharge suitable for design purpose. In this method, it is assumed that the rainfall duration is the same as the time of concentration and the return period of rainfall intensity is the same as the peak runoff. Time of concentration refers to the time required for the most remote location of stormwater inside the catchment to flow to the outlet. When the time of concentration is equal to the rainfall period, the maximum discharge occurs and rainfall collected inside the catchment comes to the same outlet point.
If sharp edged inlet to pipes connecting to a pump is adopted, flow separation will occur. Flow separates from sharp edges and a recirculation zone is formed. Moreover, turbulence shall form at downstream when the flow at vena contracta subsequently expands to fill up the unfilled void. Flow separation leads to significant head loss.
Pumps can be installed as dry well or wet well. The wet well is commonly used because of its simplicity and low cost. However, this type of pump arrangement has the potential problem of maintenance. For instance, it requires the de-watering of sump and removal of pumps out of the sump, which is suitable for stormwater pumping station which does not require pumping for most of the time. For dry well, the pumps could be assessed and maintained all of the time.
In general “turned-down” bellmouth of pump inlet is more popular because of the following two reasons:
