How Big Should my Dam Be?

Perfect Dam Size

Many people ask the question – how big should my dam be?

Sounds simple right? But does it give you the right answer for a perfect dam size?

Well, here’s the rationale behind that simplistic formula

Dam size calculations

Let’s say I have a rural-based business that needs water all year round, and you have calculated you need 2.5 megalitres each year for operations, and it is also critical that you always have some water available – how big should my dam be?

This is how I come up with the answer to that question

Let’s assume you are a progressive environmentally knowledge person, and you understand that Australia loses 1.4 trillion litres of water straight up into the sky each year

Therefore, let’s assume that you have an evaporation management strategy and implementation plan – and you plan to introduce water plants to the dam to decrease evaporation losses by 90%.

If you implement this cheap organic evaporation strategy, it is important to consider the benefit lag

Massive evaporation savings won’t be realised for 2-3 years because that’s how long the organic evaporation barrier will take to establish full coverage across 90% of the dam surface

For the sake of this exercise, let’s also assume that the current long-term weather trends of decreasing precipitation (rain) and increasing temperatures will continue,  and it is extremely critical that you never run out of water – you would plan dam storage based on the 1 to 4 formula

That simply means that for every 1 unit of water usage you require, you should have four units of storage – e.g. 2.5 megalitres of water required for operations translates to a dam with the ability to hold 10 megalitres of capacity

This figure is accurate for a couple of important reasons

According to collected data by government agencies, a naked water dam north of the Victoria/NSW border will turnover its full water volume 3 times in a year due to evaporation losses.

This figure increases the closer you get to Latitude 0° (the Equator).

In the example we are using of 2.5 megalitres water usage per year – you simply multiply usage (2.5) x expected evaporation losses (3)

This gives you 7.5 megalitres required.

But this means you have to capture and store 7.5mgl of water just to stay even.

It does not account for usage.

Usage has to be added back into the formula, and it looks like this:

2.5 mgl usage x factor of 3 for evap losses + usage required of 2.5 megalitres

This tells you that you need a dam capable of storing 10 megalitres

The 1 to 4 formula is over-engineeered to ensure that at no stage your water levels fall below 30%

If the water level drops below 30% capacity, water quality starts to decline 

This formula is not an absolute.

It is a back of the envelope method for quick calculation. But that’s not everything

So what else is needed to find out the answer to ‘How Big Should my Dam Be’ question?

For precise calculations for the dam size, it becomes much more complicated, because local precipitation, wind & temperature history and trend-lines have to be incorporated into the formula

Also, each year the formula changes slightly because evaporation losses are lessened by the increasing effectiveness of the evaporation management strategies

For the first 3 years, evaporation is going to be a constant, although reducing the element of the dam size calculation. 

These are the things I take into account when planning and designing long-term mission-critical sustainable water strategies

In the end, the answer to the question ‘how big my dam should be’ depends on how important water is to you and your operations.

For some, there is no more critical element

For others, such dam size calculations not that critical and the calculations above can be a little more relaxed. 

So hope, that the question ‘how big should my dam be’ will trouble you no more.