Author: Brian September 2, 2015

Its not unusual to see local flooding around a house, or on streets, during exceptionally heavy rainfall.

If you are unlucky you might even see a manhole ‘Pop It’s Top!’  like this one!

This is because the council drains don’t have the capacity to deal with these really heavy rains!

Why It Happens

The roof drainage to get the water off your roof will be designed for very severe storms.

This is because the risk to your property from gutters and downpipes not being able to cope is very high. (see this link: Roof Risk)

Some localised shallow flooding around the house, that will be drained away over an hour or two, however is low risk.

Council Design Considerations

To design the public drains to cope with extreme events would have huge costs.  . . Even if the pipes were big enough downstream flooding of rivers would be made worse.

For this reason although the roof drainage will be designed for a storm that may occur only over every 20, or even more years apart; the public drains may be designed for no more than a storm that occurs every 5 years.

The difference in additional rainfall between a 1 in 5 year storm and a 1 in 20 year storm can be an extra 50%. (See this link: Rainfall Intensity)

This is the water that could cause short term flooding around your house.

What You Can Do

Basically the best advise I can give is to avoid buying blocks at the bottom of slopes (These will be the worst affected by temporary flooding)

If you do get some localised flooding in your garden that either

• Doesn’t drain away as the rain eases; or
• Occurs every time it rains.

You should contact the council as it could be a sign of a problem.