If your proposed development has a surface level that is below your site’s point of discharge, you’re probably going to need a pump. Here is what you need to know.
The first two rules of drainage design are: (1) water goes downhill, and (2) it’s expensive to push water uphill. In the case where your proposed design involves levels that make it impossible for stormwater to reach its discharge point by natural means; for example, a basement, a pump may be required to get the job done.
There are typically other considerations when deciding on the levels of your proposed development than whether a pump will be required. However, this article is a primer on what you need to know about drainage pumps for residential developments to help you make holistic and well-informed design decisions.
How Drainage Pumps Work
Without getting too much into the technical side of how pumps work; a pump converts electrical power into hydraulic energy. This energy is used to ‘push’ water up a pressurised pipe to ground level where gravity can then take over. The main components of a typical pump for residential townhouse and basement developments are:
Pumps
This is the mechanism that actually generates the hydraulic energy to push the water back up to a higher level. A typical pump system will consist of at least two pumps (one as a redundancy), some electric power service (usually with a battery back-up), and several water level sensors to detect when the pump or pumps should be in operation. The capacity of the pump will be determined by both the volume of the water that needs to be pumped, and how high the water needs to go.
Pump well or reservoir
The pump system will be housed within a large pump well or reservoir, sometimes referred to as a ‘wet well’. The pump well is where the excess water gets collected prior to being pumped back up to a higher level.
The larger your pump’s catchment area, the larger the pump well will need to be. The relevant Australian Standard, AS3500.3, requires the pump well to be at least 3000L, with the size of the well increasing if the catchment area is greater than 300sqm.
Pressurised pipe
This is a flexible but heavy-duty pipe (usually around 80mm in diameter) that is used to pump water from the lower level of the pump well to a higher level, typically a pit at the natural ground level.
Overflow mechanism
In the circumstance where the pump system is not in a basement, there may also be an overflow pipe. This is a final redundancy in the instance where all pumps fail or when there is a storm event that overwhelms the capacity of the pump system. These are only feasible in situations where the pump well is above ground.
Things to Consider with Pumped Systems
There are three main considerations when thinking about pumped systems; cost, redundancy and authority requirements.
Consideration #1: Cost
A typical drainage pump for residential developments can cost between $10,000 to $15,000, with costs scaling if there are complex excavations, or larger pump wells or pump capacities. However, there are also costs associated with ongoing maintenance in the form of pump servicing, oil changing, battery replacement, and pump well cleaning.
Consideration #2: Redundancy
There are a number of reasons for why a pump system may fail. For example, if a storm is larger than what has been designed for, the pump well may not be large enough to contain the excess stormwater runoff, or if electricity is cut then the pumps may not be operational. In any case, a good designer will ensure that there is enough redundancy to offset the risk of having stormwater flood habitable areas.
Consideration #3: Council Requirements
The intricacies of navigating Council requirements could warrant a website of its own. Needless to say, you will want to make sure that your designer is well versed in navigating the particulars of the Council in which your development resides.
Most Councils will not allow you to pump directly to kerb and channel or legal point of discharge, others have specific regulations regarding pumping of groundwater (i.e. perched groundwater table, or water from behind basement walls).
To Pump or Not to Pump
Ultimately, the design decision to include lower-than-ground level areas should not be determined exclusively by the cost and complexity of a pump. However, if you do decide to have a development that requires a pumped system, it is important to recognise that the difference between a well-considered pump design and one that simply complies with Australian Standards can result in over design that costs thousands of dollars more.
If you are looking for structural and drainage design engineers for your next unit, townhouse or multi-dwelling development, please get in contact with us so we can provide you with a comprehensive fee proposal for our services. We look forward to partnering with you on your next project.
