Storm drain advice, please

[Tapster]

Hello everyone,

I live in Rawai, Phuket.

About two months ago we had a lot of rain and one night, while I slept soundly, a flash flood occurred. I think it came from a hill approx 500m from the front of our house. The water came in via our front gate and overwhelmed the drainage system to the extent that two outside rooms at the back of the property were flooded to a depth of 20cm (approx. 8 inches). To flood the rooms the water flowed down both sides of the house, a distance of 15m (approx 50 feet) and still had enough force and volume to flood rooms with an area of approx 40 square metres, to a depth of 20cm.

That's a lot of water and it did a lot of damage as it was also full of fine silt. Fortunately it didn't flood our pool as that has a raised edge.

The next day I put sandbags around the front gate to stop it happening again. Since then there hasn't been any rain of that severity. I imagine that this is a one-a-year thing, maybe only one in every five years. Who knows? However, this must never happen again!

Here are some photos:

I'll run through what I plan to do and I welcome your input and the answer to one or two specific questions, thanks.

• I don't think I can do much outside our gate. There is already a concrete "speed bump" around the gate where the sandbags are in my photo. It's approx 10cm high and I can't make it higher because I have a low-slung motorbike which already touches the concrete as I leave the gate. The only thing I am going to do is to create concrete "wings" on each side of the gate to help direct water that runs along our soi, away from our gate. The flood water came directly at the front gate, across a waterlogged grassy area and I can't stop that without a flood barrier in place and that isn't practicable in my opinion.
  
• So, my big idea is to enlarge the existing storm drain which runs across the parking area inside the gate.

This is currently 22cm (9 inches) wide and drained by two 10cm (4 inch) diameter pipes, one at each end.

I plan to make it 50cm (20 inches) wide and replace the pipes with 20cm (8 inch) diameter pipes.

I think that this will take the bulk of any flood water and direct it immediately downhill and out through the back wall of the property. (The property slopes downhill, front to back)

Now, a question: is there any advantage in placing a concrete speed bump on the downhill side of the storm drain, to try and encourage the water into the drain? I'm thinking that a speed bump may make little or no difference if the amount of water is of flash flood proportions. What do you think?

• Outside the door to the outside rooms which were flooded, I plan to build a wide step across the entrance. This will be the same height (20cm/8 inches) and width as the pool surround (shown in one of the photos). This means that the door will only open inwards and there will be a step to step over, but I think it will stop any water that isn't dealt with by the storm drain.

OK, that's the plan. Does it sound like it will work? Do I need a wider/deeper storm drain? Any other thoughts?

Many thanks for any advice.

[Naam]

50cm width and doubling the diameter of the pipes should work. if the drain is deep enough i'd go for a single pipe, diameter a big as the drain. if depth is a problem you could make an "adapting" connection to the pipes where the drain ends.

note: there are big diameter (black colour) pipes available, don't know what material, but if i'm not mistaken they are cheaper than the common blue PVC-pipes.

[Pacificperson]

In the second picture, it looks like you need an outlet on the back side of the house. The back wall may be preventing any water that comes in the front gate from going out the back side.

Edited October 20, 2015 by Pacificperson