LOGO Self-Cleansing of Storm Drain through Incipient Motion Design Concept and Flushing Gate River Engineering & Urban Drainage Research Centre (REDAC)
Sedimentation Problem Sediment deposits in drainage system have been found to be one of the causes of flash flooding. Sediment deposits reduces the hydraulic capacity of drains besides causing blockages and potential pollution to existing water. Periodic removal of sediment manually is effective but requires man-power and costly.
Self-Cleansing Design Concept
Design Concept based on Incipient Motion Conventionally a single minimum flow velocity or shear stress is adopted. For example 0.75 m/s for storm sewer and 1.0 m/s for combined sewer in UK. 3 4 N/m 2 for storm sewer in US. However, this single appear to develop from experience and not taking account of sediment and channel characteristics. Incipient motion equations take into account some characteristics of sediment and channel.
Design Concept based on Incipient Motion Existing Incipient Motion Equations
Development of Incipient Motion Equation Experimental work
Development of Incipient Motion Equation Definition of Incipient Motion used in the experiment. Kramer (1935) based on visual observation: a) No movement. b) Weak movement few smallest particles moving at isolated spots. c) Medium movement many particles of mean diameter moving but discharge is small. d) General movement particles of all sizes are moving at all points on the bed at all time.
Development of Incipient Motion Equation Video of incipient motion (top view)
Development of Incipient Motion Equation Video of incipient motion (side view)
Development of Incipient Motion Equation Experimental range
Development of Incipient Motion Equation Comparing with the Shields Diagram
Development of Incipient Motion Equation Characteristics parameter for incipient motion
Results of Incipient Motion Incorporating just the effect of sediment thickness
Results of Incipient Motion
Results of Incipient Motion Further analysis using more characteristics parameter produced the following best fit equations
Use of Flushing for Sediment Removal in Sewers Though various techniques have been developed to solve sedimentation problem, hydraulic flushing was probably the most widely applied. Among the techniques for hydraulic flushing are temporary dams, flushing gates and flushing tanks with valves or siphons.
Flushing: Temporary dam Flush waves created by lifting of a locking device can be compared with dam-break waves resulting from a sudden opening of barrier.
Flushing: Gates Water is stored upstream of the gate; when the gate is opened, flush wave will be released. Flushing effect could reach a distance of 200 m for some gates.
Flushing: Flushing Tank with Valve Could be manual where the valve was opened by hand or automatic which used counterweights or floating valves. Effect of flushing distance between 150 m to 300 m.
Tipping Sediment Flush Gate (REDAC) Stores water to certain amount before releasing into drainage system, thus generating high-energy flushing waves to scour sediment. The sediment deposit could be pushed by the flushing waves to a desired location or sump where it can be extracted manually, instead of requiring manual worker to clean the entire stretch of the drainage system.
Tipping Sediment Flush Gate (REDAC) Operational Concept The hinge were positioned in decentralized position that the difference between the hydrostatic pressure on the top and bottom of the gate will tilt the gate to open. As the water level recede, the water pressure will act at the bottom of the gate to close it.
Tipping Sediment Flush Gate (REDAC) Forces on gate Force on upper portion Force on lower portion Weight of gate Taking moment at hinge
Tipping Sediment Flush Gate (REDAC) Substituting the forces Simplifying By using trial and error the location of the hinge could be determined.
Tipping Sediment Flush Gate (REDAC) Gate dimensions
Tipping Sediment Flush Gate (REDAC) Onsite Flushing at Taman Pekaka, Penang The gate was designed to open when water level behind the gate reach around 0.475 m.
Tipping Sediment Flush Gate (REDAC) Video of onsite gate flushing
Tipping Sediment Flush Gate (REDAC) Video of onsite gate closing
Tipping Sediment Flush Gate (REDAC) Video of experimental flushing
Tipping Sediment Flush Gate (REDAC) Before adding sand bed (natural sediment) After adding sand bed
Conclusions The improved incipient motion equations could be used as self-cleansing design criteria for storm drain. To improve the self-cleansing characteristics of the storm drain, hydraulic structure such as flushing gate could be installed in the system.
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