Always reactive. As if the annual rains come as a total surprise.

Posted by Herbert on June 16, 2015 12:51

I wish, that the authorities could sense, that water always flow downwards, thus sufficient drains the right places are the best weapon to quickly combat floods in Patong.

Posted by Sherlock on June 16, 2015 13:28

Phuket officials blame the rain, blame closed/blocked ditches, etc, but not themselve.
I am sure they are confident that the water problems are over once the raining season ends.

Posted by Kurt on June 16, 2015 14:45

The basics of all construction projects is good planning.
Monsoon rains expected, install drainage that can handle 10 year previous rain records....nothing more needs to be said

Posted by C.Eng on June 16, 2015 19:04

C.Eng,
And the basis for good planning is understanding the situation you are dealing with, having accurate data to produce a design, understanding what is technically feasible given the existing built up area and services, mapping the terrain to determine locational peak flow rates and the 'minor' issue of funding.
Then, the understanding that if you base the design on 10 year return frequency rain event it may not cope with a rain event the day after completion. Add the consideration that you are trying to drain a few sq km catchment with approx 2 meters of head to a discharge between 700-800 meters away.
We should ignore the low flow velocities generated in pipes or culvents having a fall of only 0.25% and lets also just assume a rain event will always occur at low tide so we can just ignore further velocity losses due to discharge resistances.
While we are at it, lets ignore the cross sectional area requirements of discharge piping or the limitations in head height restrictions to enable adequate inlet sumps.

Its not quite as simple as "nothing more needs to be said"

Posted by MoW on June 16, 2015 20:02

All European country faced same problems since engineers understood that " base the design on 10 year return frequency rain event it may not cope with a rain event the day after completion" and to close by cement pipes all the open channel is a crazy idea.. Flooding is the natural consequence.

Posted by dave on June 16, 2015 20:23

Dave,
It is neither feasible nor economical to design storm water systems for long term return frequency events. The difference between say a 1 in 3 year and a 1 in 100 year event is the duration of rain intensity. The difference in peak flow rates between these two design considerations is determined by terrain, distance to the extreme point of the catchment, effectiveness of progressive drainage and available height to induce flow.
Road design, gradient and camber need to be considered as part of the storm water design as these form the overflow path when the system design is exceeded. The problem is that grates over pits and collection sumps work well at low velocity flows but become useless trying to capture high velocity flow as most of the water will flow straight over it creating increased water volumes downstream.
Grates openings are narrow sized to avoid pedestrian injuries and this further reduces how well they collect water. They also become blocked by debris further reducing their capacity.

It would be very easy to design a system for a virgin development where all services could be integrated or considered in an overall design. It is difficult to envisage the cost and disturbance to install such a system in say Patong where medium and high density development has been allowed to continue in a mainly ad hoc uncontrolled fashion.

Posted by MoW on June 16, 2015 21:12

dave

MoW seems to explain us not European, but South Pacific doctrine on the subject, and its implementation within SE Asian territorial scope across various climatic zones.

Posted by Sue on June 16, 2015 21:29

Sue,
Rain is just water wherever you are. Local variables or characteristics required consideration as part of the overall design.

Posted by MoW on June 17, 2015 00:30

Estimate the pipes size is one of my work. And really I did it in Phuket since 10 years. I never faced problems: in emergencyy case to allow the channels flooding is a part of the project. But piping can't flood: instead when full of water can even stop. The incorrect sizing of the parts of any project, either hydraulic or electric (always flows) is the part where the thai technicians are more deficient. And when they can make the right calculations, decrease sections for greed of a 20% thinking it is not so much. This is because they think linearly, where 20% of 10 is 2. But in sections 20% less in diameter causes a loss of flow rate by almost 40% (1 mt diam sez.0.785, 80 cm diam section 0.5 without taking into account of friction). And 40% is no longer: " nid noi".

Posted by dave on June 17, 2015 14:53