Drainage engineering is not usually on the top of the list of a potential site’s due diligence checklist. However, adding these items to your site due diligence can potentially save you thousands of dollars in unforeseen costs.
There are many things to consider when selecting an appropriate site for multi-dwelling development. Understandably, how the eventual dwellings will work from a drainage perspective is not usually top of mind. However, this can be the source of future strife during design and construction if there isn’t a straightforward way of getting storm water safely off the site. This can sometimes result in thousands of dollars of unforeseen costs.
In this article we will go over the basics concepts of drainage design and explore some simple checks you can make during site selection to ensure that drainage from your site does not end up adversely effecting your bottom line.
The Basics of Drainage Design
The ultimate goal of drainage design is to take water from any point on your site to a safe designated discharge point. During a typical storm event (the technical term for ‘when it rains’) water will land on concrete driveway or tiled roof. It is then directed through gutters and downpipes into an underground drainage network.
Most commonly, the underground drainage network is made up of a network of pits and pipes. These pits and pipes are designed to direct all of the site’s storm water to a common discharge point. This discharge point is dictated by the local council authority and is known as the legal or approved point of discharge.
The drainage design may also have to incorporate Water Sensitive Urban Design (WSUD) and onsite stormwater detention.
During the process of site selection we would recommend you consider two areas in relation to your prospective development’s drainage design; existing infrastructure and the slope of the site.
Existing Drainage Infrastructure
A good place to start, as with any engineering problem, is at the end. In the instance of drainage design this refers to where the water is going to go. The local council authority will provide a legal, or approved, point of discharge that will dictate the design. But prior to getting any of that, you can get a pretty good idea based on what you can (or cannot) see.
First thing to have a look at is where the water on the site is currently going. There are two common methods for an existing site:
1. A pipe direct to kerb and channel, or
2. To an underground pit in the side of the road or in the backyard.
If you are unable to identify either of these, then the council may ask you to construct an outfall drain. This will typically involve the construction of a series of pits and pipes to the nearest existing council pit.
Depending on the location of the existing council infrastructure this may end up being a very costly exercise. Recently we were involved in advising a client as they contested the council at VCAT over the requested construction of a 350m long outfall drain (with a prospective cost of $110,000)!
In any case, it’s always best to know that these are potential costs upfront, prior to site acquisition, rather than being surprised with them further down the development process.
Slope of the Site
The second consideration for drainage design during site selection is the slope of the site. The first rule of drainage design is that water flows downhill. In fact, the steeper the site is, the better it is for drainage. Of course, clearly, a steep site is not good for a for a prospective development for many other reasons. However, a really flat site is a nightmare for drainage design!
The slope of the site impacts the drainage design in two ways:
1. It determines how the water is going to get into the underground drainage network, and
2. It determines how big of an underground network is required to direct the flow off the site.
So we’re looking for the Goldilocks of site slopes; not too steep, not too flat. A good rule of thumb is for the site to have a minimum slope of 1:50. That is, for every 50m in site length, there should be at least 1m of site fall. In general, if by eye you can see that a site has a slope, that will be greater than 1:50.
In the instance where the site cannot drain storm water by gravity, a pump may be required. Just remember the second rule of drainage design: it’s expensive to push water uphill!
Assess Now, Save Later
When assessing a potential site for multi-dwelling development, consider both the existing drainage infrastructure, as well as the slope across the site as part of your regular due diligence. Ensuring that your proposed development can be effectively and efficiently drained during storm events will limit the risk of costly surprises during design and construction.
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.
