Water Engineering_Variability in rainfall, and how to manage it

Published: 01 June 2017 | Civil Engineering

Variability in rainfall, and how to manage it

INTRODUCTION

Climate change projection models suggest that the variability in rainfall experienced in South Africa in recent years is likely to increase in future1, so we can expect a rise in the frequency and intensity of extreme rainfall events. The implications of this trend are substantial, especially as the country adjusts to  more stringent regimes of conserving water in the light of growing demand from households and  industry.

For a country in the top 40 driest nations in the world,2 it has perhaps taken us too long to recognise the need for a more judicious approach to water use. Fortunately, most industry sectors are recognising the need to conserve and recycle water - and local government is gradually placing more pressure on households to do the same.

This scarcity is relevant to rainfall variability because it impacts the way we respond to this challenge. It certainly constrains our responses, and demands that we become much more innovative in how we balance water supply and demand in ways that do not undermine safety, water quality, environmental protection, and business continuity.

DOWNPOURS AND DRY SPELLS

One of the features of our rainfall varia­bility has been the occurrence of brief but very wet periods, separated by extended dry periods. At the Mokopane weather station in January this year, for instance, 259 mm of rain fell; of this,131 mm fell within one week in the early part of the month, and about 122 mm fell during the three to four days at  the end of the month.

When just about all the rainfall occurs in one third of the time period, it becomes difficult to know how much water should be held in storage facilities to balance the need for water stocks - especially as there is a danger of these facilities overflowing if heavy rains fall before enough is tapped off to its respective applications.

To illustrate the point, consider the example of the mining sector. Mine sites are under growing pressure to manage their water balance carefully, so that they draw as little as possible from the local municipal and regional supplies - as they compete increasingly with growing com­ munities or towns on their doorsteps.

For a country in the top 40 driest nations in the world, it has perhaps taken us too long to recognise the need for a more Judicious approach to water use. A vital element of the engineering response is also the modelling of rainfall patterns to establish the optimal levels of storage facilities that large water users need to manage.

In the past it was easier to regulate water storage facilities (such as return water dams), as levels could be held safely low - that is, with generous buffer levels - to allow capacity for heavy rains without risking overflows. If the water levels became too low, they could always be topped up with supply from external sources. Not so today; most mines aim to 'import' as little water as possible, with many looking to be essentially self-sufficient. Increasing variability in rainfall is making this commendable goal potentially very challenging.

It should also be remembered that the stakes are getting higher in industries like mining. On the one hand there is the substantial risk of environmental contamination if rainfall is higher than predicted - leading to insufficient capacity in dams, and resultant overflows of dirty water. On the other hand, leaving too little water in the dams could lead to operation-threatening water shortages if there are no affordable external sources from which to draw.

Similar predicaments face other sec­ tors. Property developers must find ways to ensure that new residential and com­ mercial facilities have adequate potable water supplies, but they may not always be able to rely on local government alone to supply this. Particularly in areas that include parl<s and other green zones - and especially where irrigation of grounds is required - water conservation measures must be developed and implemented that make use of recycled water or rainwater for non-drinking purposes. Again, ap­plying such measures when rainfall is becoming more variable, makes the task that much more challenging.

More variable rainfall - especially extreme rainfall events - also has significant effects on urban infrastructure such as roads. It causes rivers to overflow more often, leading to flooding of roads and presenting growing risk to life and property. Government has recognised the problem, and consideration of climate
change impacts are now part of more stringent disaster management regula­tions. Municipalities are now required to plan more proactively and in some detail, so that infrastructure and preventative measures are in place to effectively deal with extreme weather events that can result in disruptions and damage.

SOLUTIONS

There are a number of solutions to consider when responding to the impact of rising variability in rainfall. One of the avenues that has benefited mines is the design of 'buffer' storage facilities that can contain run-off from extreme rainfall events, reducing the danger of overflow and contamination. Such facilities can even be compartmentalised - to separate pit water, sewerage and stormwater, for instance - so that water of different qualities can be channelled to serve ap­ propriate purposes.

It is no longer enough for mines to try and  use existing facilities - like return water dams and effluent  dams - for ad­ditional storage. Rather, buffer facilities downstream of waste dams can  act asa safeguard to catch any contaminated water over-topping an existing facility; they can also act as water stores for dry periods, thereby addressing the risk of shortages and the high cost of sourcing off-site.

This concept also works for residential complexes, where attenuation dams can be used to contain water from wet times to be used for irrigation or other purposes as allowed by its level of quality. With suf­ficient early planning, stormwater drains could be linked to these storage facilities, allowing local containment of run-off for applications that do not demand potable standards. This in  turn would take some of the pressure off the local government's water treatment works, so that potable water provision could be extended  to more beneficiaries.

Of course, there is likely to be an economic  impact  for  property  devel­opers, who may have to sacrifice some space in a development for attenuation ponds - space that they  perhaps would have preferred to assign to a revenue­ generating  building  or  home. But  there are more innovative ideas which lessen the trade-off, such as the possibility of underground storage that may not take up surface space. Some building designs have even used tall structural pillars as water­ containing towers, as part of rainwater harvesting  (RWH) strategies.

Certainly, South Africa has not yet tapped fully into the potential of RWH systems - nor into decentralised waste­ water treatment (DWWT) systems. These are relatively easy to install and maintain, and provide potential cost savings, aswell as additional water resources to the immediate user.

A vital element of the engineering response is also the modelling of rainfall patterns to establish the optimal levels of storage facilities that large water users need to manage. While the Department of Water and Sanitation (DWS) pro­vides valuable models at national and regional levels, there is also the need for local-level modelling that is of practical value to particular users like mines and municipalities.

When it comes to municipal infrastructure, like stormwater drains, some responses to rainfall variability do not even require innovation; they just need a renewed focus on maintenance issues. In a high-rainfall country like the Democratic Republic of the Congo, maintenance and regular cleaning of stormwater pipes is a high priority and is well performed. South Africa needs to follow suit to ensure that blockages do not occur and to avoid the kind of highway flooding recently experienced in Johannesburg.

When considering how variable rainfall will need to be better managed in the future, South Africa has taken a first important step by recognising this phe­nomenon as one of the many disruptive impacts of climate change. Now we need to put our minds to implementing and testing the various options that could lead to a more sustainable approach to water management and conservation.

REFERENCES

1    https://www.researchgate.net/publica­ tion/226529754_Changes_in_Extreme_ Rainfall_Events_in_South_Africa (also see https://sites.google.com/site/witsgeoghon- ours/extreme-rainfall-events-in-johannes­burg-physical-and-social-dimensions)

2    https://africacheck.org/reports/is-south- africa-one-of-the-30-driest-countries-on-earth/   

 

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