Tank versus Dam

Local dams might be the answer to tanks.

Local dams also offer lots of lower risks than big central dams and other benefits neither the big dam or a local tank can offer.

Lets explore the three –

Tanks

Deploying tanks in every home is a great way to truly localise water supplies, particularly in suburban areas that have good roof catchment areas. So they definitely add value but are they the best solution for all situations, I’d say unfortunately tanks are not a 100% solution because –

  • rain falls unevenly across suburbs (some have too much that goes to waste in overflow and some have not enough)
  • water consumption is uneven across households
  • roof area (rain water catchment areas) by household vary according to many factors including type of dwelling e.g. high rise apartments (also called ‘flats’ in some countries)
  • the cost of plumbing overflow from millions of small household tanks back into a central collection point is prohibitive in time and money and disruption and in many low lying suburbs simple gravity means further negative impact on the environment with pumps.
  • tanks have a cost to the environment in construction and installation that must be multiplied by millions of households so there is a material cost but it is not as obvious as the cost of more centralised solutions

So something in between household tanks and big monolithic centralised city (MCC) dams (like Sydney’s Warragamba dam) might be better.

Big Dams

Of course big MCC dams are centralised and get economies of scale in many ways but they have their issues too –

  • Algae bloom (like Sydney is experiencing right now) means there is a concentration risk – if something happens to the one dam, millions of people suffer
  • Terrorism targets are worse – e.g. poisoning supply with MCC dams for the same reason of concentration risk although admittedly the security can be done better because they are usually more remote (surrounded by native bush/forest catchment areas) and centralisation enables more cost effective security
  • They are usually distant from the communities they serve – costs of pumping, routing are higher.
  • May be located according to rain fall that varies over time (such as when housing growth reduces natural green areas and this impacts rainfall) and there is another concentration risk there too – this one is particularly difficult to control.

Local Dams

So the local dam idea has some merit, imagine one in the nature reserve near you if it has a gully or valley –

  • Natural valleys are proven collection points, less expensive land acquisition and clearing.
  • They are often already clear with nature reserves, parks etc around them becaus ehte land is harder to build on and local councils have them declared as parks or nature reserves
  • Downstream usually has no issues with dwellings because it is already a stream or river and the outflow/overflow is less than an MCC by orders of magnitude.
  • There is still a degree of centralisation so the cost of pumping back into ‘the system’ is more sensibly shared across thousands of households (better than tanks) and is much lower capacity (and hence cost) than a full MCC style pumping station.
  • It adds the the local wild life’s ability to be sustained in good and bad times, same for flora.
  • Significantly reduced concentration risk because if the supply is compromised, assuming a city has 20 or thirty smaller dams the loss of one or two through algae bloom or poisoning or pumping station failure is still tiny relative to the total available.

My colleague Marc Lehmann also has some good points on this and prompted me to finally write down some of this stuff. I’ll do another post on this with more detail and the relationship with storm water harvesting some time too.

Cheers, Peter.

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