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flood risk assessment and drainage strategy advice

flood risk assessment and drainage strategy advice

flood risk assessment and drainage strategy advice

Esteemed colleagues

I’m about to start a dialogue with the local council as spokesperson for flood action group, a group of residents whom have repeatedly being flooded (one is my mother in law).

I’m a mechanical engineer in oil and gas so have no idea what I’m talking about when it comes to civils or drainage, however, using common sense and speaking with the residents we have an idea of a possible cause.

The area in question is at the bottom of a hill and is directly opposite a small beck (stream) that in turn goes underground into the surface water drainages system.

Floods only commenced in 2000 after a housing estate was developed (c. 1998) at the top of the hill, this previously was a green field site and heavily wooded with trees of all kinds.

Serious floods have occurred 5 times with the most resent last weekend.

My question to the council is what calculations should they have done to determine the amount of water the 3 hectares field would have absorbed, a quick google suggested the area during 50mm’s of rainfall would receive 1500 metres cubed or 1500 tons (330,000 gallons) of water.

Secondly what calculations did they do to determine the added flow from above can be accommodated by the existing “X” diameter drainage system?

As a subsea engineer we often consider waves of a 50 year return or sometimes even 100 year to ensure the equipment can withstand what could be thrown at it!, 5 floods in 17 years and only after this land was developed sound like something is not right.

If anyone in the know can shed some light on the above or give name to the appropriate calcs or work that should have been carried out so I at least sound like I know what I’m talking about, that would be appreciated.



RE: flood risk assessment and drainage strategy advice


The engineer of record should have developed a storm water runoff calculation. Typically when going from green spaced to paved parking and buildings, runoff will go up as the soil cannot absorb the water. To mitigate this many areas have storm water detention basins that serve as a capacitor to stop overloading the storm sewer.

You need to ask for storm water runoff calculations, and calculations assessing capacity of existing storm water pipe flow. I'd check calculations based on
-paved area calculation assumptions vs. field conditions (could include whether permeable pavement was specified but not installed)
-roof area calculation assumptions vs. field conditions (where do gutters go, is there more roof area then the original engineer expected)
-make sure any storm detention basins/ponds are shown on plans.

Now real world stuff:
-I'd also have city/town come out and do a CCTV inspection of stormline piping. They may not be cleaned or could be full of sediment from construction runoff (poor construction practice). I've see 36" storm water lines half full of sand and sediment that washed in via runoff. In one case the mason's pile of sand was getting washed into the storm drain Took days to clean the lines out.

Settlement of sediment is a function of velocity, which is a function of slope and size.
-If developer is at top of hill he could have much bigger slope coming toward you vs. going away
-If all the sediment is downstream of your flood action group, you're choking off design flow, and water will of course find the next path of least resistance (storm drains next to mother and law's house) and will begin to flood.

Hope this helps

Pipe Stress Analysis Engineer

RE: flood risk assessment and drainage strategy advice

Generally speaking in the US, the developer (landowner) is responsible for releasing stormwater in it's historic condition and quantities, which is why detention ponds/systems are installed. Do you know if there is a visible pond (likely with some concrete outlet structure) at the lower end of the new development? If not, that is most-likely the source of the flooding problem. But it's also possible that the local council did not require stormwater detention (in 1998) and they may claim governmental immunity.

RE: flood risk assessment and drainage strategy advice

Thanks Jeff, a very comprehensive reply indeed. I will definitely use this during my meeting!

Hi Civilman72, I’m hoping that their due diligence with regards to what Jeff mentions above has NOT been done properly/thoroughly and we at least have some recourse against someone.

Thanks again guys.



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