basement drainage

Venturi pumps would be able to drain the water for you but it will not be very effective.

Say your opening is 2 x 5 m = 10 square meter. With every 25mm of rain you then will have 250 l to dispose of (this excludes seepage and runoff into the gap) . To 'pump' the 250 l would require more than 500 l of municipal water which will be lost into the drain. (In SA a definite NO NO)

Thanks for your reply! Your estimate is close to the size that I would be draining. I wonder if it would be possible to use a reservoir of the rain water from the rest of the roof area of the house to achieve the required pressure? The house is probably more than ten times as large as the new patio would be and currently all of the rain water is diverted into the same pipe that I would be using to drain the patio.

I would only add that it is wise to install a check valve on any piped connection to a storm sewer.

I am thinking that there is no way to run a gravity drain under the patio. That would be the best way, or if nothing else as a backup for the inevitable pump / power failure. Make sure you have cleanout access for any drains you install, they will plug with silt over time.

edykes is correct about the pump. That is indeed the most cost effective solution. However what about when the power goes out? Simply either buy one with a battery backup or buy a battery backup system. Most of the sump pumps are about 1/4 hp and a car battery can usually hold off a short power failure. Attached is a link to a place in OH that has a pump system with a battery backup. http://www.basementsystems.com/basement_waterproofing/waterproofing_products/sump_pump/battery_backup_sump_pump.php Notice the battery in the picture appears to be a car battery.

Thanks again for all the great comments. The reminder about the battery backup was especially sobering as the power goes off most frequently when it rains. I am living in Medellín, Colombia and it often rains really hard for brief periods of time; just when I would need the pump to be working at peak capacity. The other possibility is checking the depth of the storm sewer in the street, and if the depth permits, installing a deeper gravity drain line with the necessary cleanouts. This would be a much more expensive solution, but not impossible.

Be cautious about connecting to a storm sewer. During heavy rains or if there is a blockage, your below grade inlet in your patio can suddenly become the lowest exit from the storm sewer. So instead of worrying about a small drainage to go out, a large drainage area (the upstream drainage contributing to the storm drain in the street) will be trying to come in. A check valve could help alleviate the problem, but thay also have a failure rate. Good luck.

Common "car" batteries aren't as durable due to competitive price and size requirements. I would recommend a 'Marine" grade battery with higher Ah capacity and more durable lead plate system.

Also, the battery back-up 12VDC pump is not completely fool proof, and needs semi-annual attention to know it is ready to operate when needed. I have had simultaneous problems woith utility power and battery backup and suffered water in basement when this happens.

In hindsight, I would recommend a battery-inverter system to back-up the 120V pump supply instead of having a 12VDC pump sitting dry and waiting for months between use.