Dams – Latest levels: Winelands and Cape Town
Latest report (updated 12th June 2017) on the main dams serving
Cape Town City and nearby Boland/Overberg towns
Net inflows commenced for the first time this year as recent storm rainfall run-off reached the dams last week. Total stored water in the main dams reportedly rose by 17 M.cuM (1.9% of capacity).
The combined effect of savings plus rainfall means that since 1st November 2016, together we have in total used 24.7 M.cuM of water less than in the same period the previous year. Well done all.
Assuming continued weekly consumption for the SW Cape of 6M.cuM, this result implies that the run-off into the dams from the recent rains amounted to gross 23M.cuM last week, reducing our prevailing water insecurity by stretching the existing water supplies out for longer.
How many days of water is left? Using the moving average estimated water consumption rate of 6.2 M.cuM per week from the last weeks, arithmetically calculated, the usable water left will now last 15.9 weeks, i.e. 111 days, through to until end-September 2017; a gain of 3 weeks due to rain run-off.
Taking into account planned interventions to access more water in Theewaterskloof Dam if needed, the available accessible water could now be stretched until end-October.
What is being planned at Theewaterskloof Dam? A coffer dam to enable water to be pumped up from the deeper part of TWK Dam to where the water is abstracted out and over to the Berg River.
Reportedly, the material for a planned coffer dam is all ready on site at TWK but DWS is waiting to judge the outcome of the rains before proceeding with further work. If the TWK dam level drops again to 11%-10% then they will build the coffer dam at that time and start pumping up water from the deep end of the dam. The expectation is to thereby access about half of the remaining water at that point.
On other hand, if the dam now continues to refill then DWS will hold off until the dam should ever drop to this level again at some future point, at which stage the coffer dam would then be built and the pumping plan executed.
Currently the “sloot” is reportedly bringing through sufficient flow from the deep side to the abstraction point, permitting the target flow of 4cuM/sec water to be transferred to the Berg River side.
Caution:
While recent rains and run-off into the dams is a positive development that is to be welcomed, relatively, the situation remains extremely grave because the real challenge is how to survive the following dry summer period where consumption typically increases sharply and no significant fresh rain may be relied upon.
To put it in perspective, in order to be minimally safe, we need to exit winter with water levels in the main dams standing at least at 60% of capacity. I.e. the dams must be 3x as full at they now are. That could mean rainfall from another 15-20 winter storms or some more storms and plenty of other heavy showers. At present the long-term forecasts offer no promise of such rains arriving.
We therefore must continue all possible water savings efforts. Restrictions will continue in effect and the goal remains to restrict water consumption to 100L per person per day.
What ought you to be doing?
We consistently recommend that all households who can afford it should install the most appropriately sized household water tank system that they can afford.
But how does one go about calculating the best options?
- Decide your own goals; e.g. financial savings, water security, water availability convenience, unlimited gardening, and so on.
- Calculate different alternative systems for your household to see which makes the most sense in terms of your goals; calculate the appropriate sized system using the Water Research Commission WRC CSAG Water Harvesting Tool here.
- Following the model below, then determine the approximate costs of your preferred option and calculate your likely annual water and sewerage costs both before and after implementing such a water harvesting and tank storage system.
Financially, what will the cost of an appropriate tank installation be? How long will it take to recover the capital? What would subsequent savings be over the life of the installation?
Here are three examples drawn up using the UCT WSC model to estimate water harvested and of the remaining water therefore still needing to be drawn off municipal supply, assuming houses of different roof areas located in Pinelands, and the installation of two different tank sizes – 2000L and 5000L.
Cost calculations will obviously vary depending upon how the installation is made and the material used. The following costs are approximate assuming a D-I-Y installation.
This shows three outcomes for a family of four using 12,000 litres of water each month.
Assuming that the tariffs implemented 1st June 2017 apply throughout, then the capital cost is paid back between 2.8 years and 4.1 years. Additional savings achieved over the remaining part of an assumed 20-year lifetime of the installation are also shown.
[Financially adept readers will recognise that these calculations may be considerably refined by applying accurate time-cost of money, cost escalations, write-off provisions and the like. My intention was to keep this example simple just to illustrate a point.].
When one takes into account all the very tangible advantages of creating one’s own water storage as well as the intangible benefits provided by attaining water security, then the decision appears to become very straight-froward.
What are the authorities doing now?
The Think Water Exhibition of water conservation, rainwater harvesting, storage and related technologies arranged by the City of Cape Town took place at Canal Walk last weekend. It was well worth a visit to see the different technologies and ideas on offer. The many of you that missed this exhibition will, I understand, have more opportunities later when the exhibition visits other malls across the city. Keep an eye open for announcements of events.
The City emphasises that due to the severity of the drought, above-target consumption, as well as the unpredictability of climatic conditions, Level 4 water restrictions remain in place indefinitely over the long-term and could yet be intensified if warranted.
All water users are warned not to increase consumption because of the rain that has been experienced. Residents should continue to do all they can to keep their daily usage under 100 litres per person per day whether they are at home, work, school or elsewhere.
Read main details of level-4 water restrictions here.
Will the weather help?
The deep depression that moved through last week certainly produced some serious winter weather across the entire SW Cape, with 106mm measured at Berg River Dam and 111mm at Wemmershoek Dam. No surprise then that both these dams have started to rise. Snows on the higher mountains has stored up water for later release when it melts.
The South Atlantic high pressure cell has unfortunately moved back into position to the west of the Cape, once more “shepherding” low pressure fronts past the tip of the country.
This cumulative 3-day rainfall forecast chart from NOAA/CPC indicates 5-15mm rain in the south-western Cape over the coming three days. Hopefully it will concentrate around Berg River and Wemmershoek Dams, also extending to the Steenbras dam area.
What else may we expect?
Some light rain is forecast for 15th and 17th June as shallow cold fronts held down by the resurgent high pressure will flick past the Cape. Over the medium term some 5-15mm showers are expected every few days through into July which we hope will be enough to maintain a positive net inflow in the dams.
The next spell of unstable weather is predicted for the 5th-7th July when we may expect more brisk rain accompanied by thunderstorms.
NOAA/CPC, is recording a flattening in Pacific surface temperature patterns. It now predicts a neutral El Niño through the 2017 southern winter/spring. Seems likely El Niño will have no material effect on our winter rains
How much rain has fallen in the dam catchment areas? As expected, Berg River Dam and Wemmershoek Dam were the main recipients, but good rains also fell near the other dams, with only Voelvlei not having received much (but it is still benefiting from good rain in the area a week earlier). So that large storm of last week to some extent made up for the April and May rainfall shortfall.
There is a good chance that most of the dams will finally receive more than the average June rainfall.
However, the rest of June does not promise much rain, so we will be reliant on water working its way off the mountains through the rivers into the dams to keep them recharging over coming weeks.
In Summary: last week net inflows occurred for the first time in 2017. Happily the combined water stored rose by 2% last week up from 19.1% to 21.1% of capacity. This may have extended our water reserves but the dams still remain nearly empty. Every savings effort must be continued.
Here are the levels of the six main dams that serve the Cape Town/Overberg/Boland area. Also shown is the total water storage level of all the dams in the whole greater Western Cape.
Read about a D-i-Y household rainwater harvesting and tank system here
Our water-wise landscaping and water-wise gardening suggestions.
Read about South Africa’s desalination policies and initiatives.
Cape Town water tariff effective 1st June 2017 for 2017/2018 here.
Click here to see Winelands Weather forecast of possible near-term rain.
Report instances of piping failures and incorrect use to municipalities.
In Cape Town the contact details of the 24-hour Technical Operation Centre are: Email: waterTOC@capetown.gov.za Telephone: 0860103089 (choose option2: water-related faults) or SMS: 31373 (max 160 characters).
Note: To calculate number of days of water left we first reduce reported water stored volume by 10% of capacity (i.e. 89 Million cuM) to allow for water that usually can’t be extracted or processed and then divide the remainder by the net outflow rate during the previous week.
Tom.
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