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The Application of Network Modeling in Drought Management: Reflections on the AWWA Drought Management Handbook

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Introduction

Water and life are synonymous. Communities expect to be protected from floods, to have a reliable and safe water supply, to be able to swim and fish in rivers and lakes. Stakeholders who share water ultimately compete for that water, with conflicts becoming more critical during periods of prolonged drought, when there is not enough water to go around and rationing is enforced. Population growth, industrial and economic expansion and climate change will inevitably lead to more pressure on a finite water resource.

In recent years many areas of the world have experienced drought and experienced the adverse effect this inevitably has on water supply systems.
 
A good example of this is the United States where virtually every region has experienced drought. Arguably the situation is nowhere more serious than in Texas, the home State of Wallingford Software Inc. The recent drought of 1996, which started in the fall of 1995, ultimately cost the State an estimated $1.5 billion, with reserves in reservoirs and aquifers falling, crops withering and livestock dying.

In response to these ongoing drought problems, the American Water Works Association (AWWA) has produced a Drought Management Handbook to help water providers to assess the possible impacts of drought and to develop plans to minimise these impacts. In this article Dan Stevens, a Water Distribution Specialist with Wallingford Software and a member of the AWWA, considers the issues raised in this publication and discusses the ways Network Modeling can help water providers to anticipate, plan for and manage drought conditions.

Network Modeling Overview

Many Water Engineers are familiar with the more typical roles for Network Modeling as a system planning & management tool. Examples of typical applications include planning new services or facilities, designing pressure management schemes or understanding and improving system performance. Many engineers understand the need to build and maintain Strategic and Detailed Network models and the business benefits this undoubtedly brings. However, to date few water providers have used network models to their full potential to plan for and better manage drought conditions. This has almost certainly resulted in many uneconomic or infeasible design and system modifications, as engineers struggle to establish drought mitigation measures with reference to poor data and using inadequate design tools. These challenges are inevitably exacerbated by the pressurised working environment that inevitably exists during times of drought and in the full glare of community and media exposure.

Network Modeling provides a key tool that can be used before, during and after a drought incident to define key indicators and appropriate responses, plan both temporary and permanent system modifications and provide the capability to quickly assess and respond to ever-changing supply and demand conditions.

The AWWA Drought Management Handbook defines a process to prepare a Drought Management Plan (DMP). The recommended process consists of a sequence of six steps which involve the following actions:

1. Obtaining public input and involvement

2. Defining goals and objectives

3. Assessing water supply and demand conditions

4. Defining drought indicators

5. Identifying and assessing drought mitigation measures

6. Developing a drought index and management strategy.

Let us consider each of these steps and assess where Network Modeling is appropriate and how it can best be used throughout the process.

Step 1 - Obtaining Public Input and Involvement

Public input and “buy-in” is essential to the ultimate success of any DMP. If the public are sceptical of the need for managing demand (by effectively adopting a voluntary demand reduction by changing their normal water usage) then perhaps the most powerful tool in any DMP is lost to the Engineer.

Network Modeling can help influence public opinion successfully in a number of areas.

Firstly, modern network modeling software is a strong visual tool. With its GIS integrated mapping and aerial photography, householders and business can relate to the broad problem facing a community at a highly personal level. Making modeling screen shots available to media organisations is a highly effective way to communicate. By showing a computerised assessment of areas where water will be scarce, or indeed run out, should demand continue at current rates, is a highly effective, if emotive, way to help to galvanise public support and co-operation. There are documented examples of communities pulling together to protect an area shown to be vulnerable.

Secondly, it can prove extremely beneficial to approach major consumers that have a significant effect on system hydraulics to discuss in a sympathetic way how this impact could be reduced by modifications to their water usage. For example, a major drinks producer could see a considerable rise in production prior to and during a drought. They may have an on-site storage facility fed by a valve, which opens fully when the tank drops to a set trigger level and closes fully when it fills. This sudden large demand coinciding with peak domestic demand could drop service reservoir levels to a point whereby they are unable to recover or service pressure could be adversely affected. The model may indicate that replacing this valve to ensure steady filling throughout the day would mitigate this effect and ensure adequate water in storage and/or adequate service pressure. The commercial consumer could be shown the model and gain considerable public support from being seen to play their part in helping to reduce the impact of drought on their local community.

Finally, the public perception of the water provider can improve by being seen using the latest technology to help manage the drought conditions.

Step 2 - Defining Drought Mitigation Goals and Objectives

The setting of goals and objectives is often a difficult and contentious issue. What is acceptable in one person’s eyes is often totally unacceptable in another’s. The use of network models can help greatly in this area, as a large number of possibilities can be investigated and uniformly acceptable solutions can be implemented wherever possible. If no such solution can be found, the least contentious alternatives can be further investigated and hopefully a consensus achieved.

The Drought Management Handbook describes how engineers, and any others elected to a specially convened “Task Force” under the terms of the DMP, should address a number key issues relating to the priorities of water use. The key question here is “Which water uses and users should be restricted during drought conditions? Which water uses can tolerate drought conditions better than others?.”

Network models can be used to assess whether there is any need for wide scale restrictions, and if so, the total extent of water use reduction that will maintain an adequate system performance and keep sufficient water in storage for essential purposes and emergency conditions.

Using an InfoWorks WS network model to identify supply deficiencies.

The model can then be used to identify priority areas for continued supply and areas where water usage restrictions should apply. For example, reducing non-essential water usage such as leisure activities could help maintain essential services such as medical and public health uses. Using a detailed model with accurate and comprehensive demand modeling can help enormously. If the model is linked to billing databases and the demand allocation is at a property level in the model then detailed analysis can be undertaken of the impact of reducing supply to individual customers or groups with similar usage. If the model has a simplified demand allocation then the use in this area may be limited to generalisation and assumption, which may prove inadequate and ultimately unacceptable. Clearly, the more comprehensive and detailed your network model, the greater the potential benefit and the accuracy of decisions based upon it.

Step 3 – Assessing Water Supply and Demand Conditions

The process of constructing a detailed network model with comprehensive demand allocation provides an invaluable insight into the water usage in an area. Water resource models and additional data can help provide a good overview and long-term perspective, but only a detailed hydraulic model can provide a direct link between the customer behaviour and the hydraulic performance of the system.

Network models give a true picture of the capability to produce and distribute water throughout the system. If a source has daily, weekly, monthly and annual abstraction licences (perhaps with maximum permissible rates) then the model can be used to assess how best to optimise the use of that source against other potential sources. Only a model which represents the true hydraulic capability of the system will give a reliable picture of this. For example, it may not be physically possible to extract the maximum permissible rate as the pump performance and downstream pipe condition may have deteriorated over time, and the model will help the engineer to assess the need for refurbishment or system modification to help obtain the maximum yield from the source. Also, as ground water levels are lowered over time during an extended drought, pump performance will fall off due to the increased lift required, and hence additional measures may need to be identified and tested using the model.

If insufficient water is available from the sources supplying an area, a network model can be used to help identify potential solutions for transferring additional water from neighbouring areas, which may be water-rich or certainly less water-scarce. This could be via opening an existing boundary valve and/or installing new pipework and control facilities to transfer water in from non-adjoining areas.

A series of model runs can be undertaken to assess the effect of modifying demand patterns or increasing individual categories or indeed individual customers. A whole series of scenarios can be investigated to help the decision making process. Historical drought events can be reproduced using the model, and the actions taken can be re-assessed against a series of alternatives. This will help to develop better response procedures in order to maintain supplies and remain within source abstraction licences in the future.

In certain circumstances the engineer may be forced to consider moving large quantities of raw water from one reservoir to another and temporary pipework and pumping plant may be required. A network model is by far the most effective way to assess this requirement and identify appropriate pumping plant and pipework capacity. By investing a relatively small sum in modeling the engineer could potentially save many hundreds of thousands of dollars by optimising the design by correctly sizing, positioning and specifying new pipework, storage and pumping plant. Any drought mitigation scheme is likely to be very high profile, and failure to achieve the solution will undoubtedly result in poor publicity and a reduced credibility for the water supplier, and this should not be underestimated.

Analysis of historical records and model runs for each supply zone will help the engineer to assess how each area varies under drought conditions. There are considerable differences in the way an area of dense housing with little garden or recreational area responds to a prolonged drought compared with leafy suburbs. Similarly, an area of principally agricultural demand will react differently from an area of commercial development or heavy industry. A network model can help the Engineer understand how these variations are reflected in the hydraulics of the system and identify where re-zoning or re-setting valve or pump settings may overcome deficiencies.

Step 4 – Defining Drought Indictors

The Drought Management Handbook refers to defining specific Drought Indicators, using any one of the commonly used indicators such as reservoir levels, streamflow conditions, groundwater levels and precipitation.

While network models are not strictly necessary for monitoring the above, and indeed are not relevant where streamflow or precipitation is concerned, they can be put to some use. A network model that has been linked into telemetry and SCADA systems can provide a form of on-line monitoring of reservoir and groundwater levels, and pre-determined levels could be set to determine the severity of the drought. The model could then be used to predict when the current status will change to a higher or lower drought index value.

Step 5 – Identifying and Assessing Drought Mitigation Measures

Having established a set of agreed Drought Indicators the next stage is to identify an appropriate response to each level of drought severity. As described in the Handbook, the responses should take account of basic fundamental concepts. Exceptional circumstances should not be seen as an excuse for poor, uneconomic or environmentally unsound engineering. Basic concepts that should be considered are sound economics, feasibility, environmental acceptability, legality, public acceptance and liability.

It is in the area of identifying operational changes, assessing the requirement for temporary or permanent system enhancements or re-zoning that network models are absolutely essential. A large range of scenarios can be tested and evaluated without affecting a single customer. It is also extremely important in the eyes of the public that the water provider is able to demonstrate clearly that they have identified and assessed all possible options before spending their customer’s money on mitigating works or imposing restrictions. Establishing and maintaining the public trust is essential in the battle against drought. Media coverage is likely to be extensive, and public meetings will have to be addressed by representatives of the water provider. It is essential that the engineer or their PR representative has evidence available to support their decisions. Showing the results of computer simulations (either live or in the form of plots, prints, maps and reports) can only enhance the credibility of the organisation and help maintain public approval.

Some of the key areas where models can help directly include:

1. Establishing potential savings in demand by introducing temporary pressure management thereby reducing leakage and pressure related demand

2. Establishing the potential for or limits to the bulk transfer of water between water rich and water scarce areas

3. Designing temporary pipework and pumping plant to transfer water or boost system performance

4. Assessing ways to optimise water source use within abstraction constraints, or assessing additional water requirements to apply for temporary increases to abstraction licences

5. Assessing the effect of shutdowns (water rationing) by pressure zone and the maintenance of water quality to prevent disease through poor management

6. Establishing the potential to maximise use of system storage possibly by “over-pumping” at night to fill reservoirs.

Also, it should be stressed that water providers should take the opportunity to monitor performance closely under these exceptional drought conditions. This will help to improve model calibration, enabling better model analysis and design work to alleviate supply deficiencies and improve future system performance. Only when the system is under stress can a true picture of system performance and capability be obtained.

Step 6 – Developing a Drought Index and Management Strategy

The Handbook lays out a clear and staged DMP based on recent experience in the US, which is designed to guide a water provider through the levels of action needed, based on the severity of the drought. It is clear from all the items discussed above that strategic and detailed network models can be used at various stages of a DMP, and it is advisable to highlight in the plan at which stages particular models could be used to provide details and results, and to test possible alternative actions.

Conclusions

It is clear from the points that have been addressed in this article that network models can play a major role in helping water providers to manage all aspects of drought, and that, along with publications such as the AWWA Handbook, network models should form a key element of the Engineers’ toolkit.

We have shown that all-mains models can be used before, during and after drought events to increase the effectiveness of the water provider in maintaining a safe supply of water even under the most severe shortages. Of central importance however, is that such strategic and detailed all-mains models are constructed accurately, using recent or current data and that they are adequately maintained. Whilst modern models can be built surprisingly quickly (and sometimes with 24 or 48 hours), the water provider with an existing and well maintained network model is far better placed to optimise its value in the management of a drought environment.

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