Reservoir Water Supply Planning for an Uncertain Future ......Keywords: water supply planning, reservoir water suppl y operation, climate change, uncertainty, risk Background Perspectives

Reservoir Water Supply Planning for an Uncertain Future

Reservoir Water Supply Planning for an Uncertain Future Dave Campbell, P.E., Schnabel Engineering1 ©2012

Abstract

Key factors related to understanding reservoir water supply planning technical and permitting issues are examined. Conducive reservoir site conditions to maximize safe yield and minimize environmental impacts are touched on for background, as are performance differences between on-stream and pumped diversion reservoirs. Additionally, opportunities and constraints related to direct reservoir withdrawal and flow augmentation operations (release for downstream water supply withdrawal) are discussed. Also, a general overview of planning issues and obstacles related to the permitability of a reservoir project will be presented.

Over the past three decades, the reservoir permitting process has consistently become more complex, more costly and more time consuming. The number of hoops that need to be jumped through continue to increase, and the hoops get smaller and are held higher above the ground. Twenty years ago, there was limited scrutiny of population projections, per capita demand estimates, yield analysis parameters, water system operations and drought contingencies. Some additional scrutiny was merited to cull out those that would otherwise abuse the process. However, as will be discussed later, the process has in many cases gone well beyond regulatory validation and has adopted elements of project control that add cost, complexity and time, undermine achievement of project purpose and unduly complicate the process.

Discussion includes the considerable uncertainties in projecting demand up to 50 years into the future, as well as the uncertainties in dealing with climate change. Whether due to more erratic climatic cycles exposed over longer periods or to long term warming trends, either natural or man-made, climate patterns have revealed greater levels of uncertainty, including more frequent drought recurrence and greater drought extremes. When the emotion of the issue is extracted, we don’t yet really know if the recent climate extremes represent a wider span of expected variation or a directed trend. Also, in light of greater uncertainty, designing water supply systems to fail on a one-in-fifty to one-in-one hundred year basis is questioned.

Keywords: water supply planning, reservoir water supply operation, climate change, uncertainty, risk

Background Perspectives

Due to the considerable complexities and costs involved, most reservoir water supply projects are necessarily planned and sized on the basis of a fifty-year planning period. Projections in demand are prepared using empirical demographic procedures that estimate service populations fifty years into the future, and then apply per capita water use rates and industrial demands that have also been estimated for application fifty years into the future. A 50 year planning period is justified on the basis that these projects can easily take from ten to more than twenty years to evaluate, plan, permit, fund, design, construct and make operational; and, as noted, the reservoir permitting process will likely be costly. Although it is a process critical to the on-going health and well being of growing communities, few water utility managers would choose to participate in the reservoir water supply planning and permitting process more than once during their career.

Demand projection analysis intuitively is recognized to entail a great deal of uncertainty. However, reservoir supply-side planning analyses have not been sufficiently recognized as also entailing significant uncertainties. Until very recently, no one in the reservoir water supply planning process gave serious consideration to the uncertainty that the non-fixity of climate can have on project sizing and layout, nor

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would an applicant have had a chance of securing regulatory approval for such a proposition. Climate2 considerations have always been tacitly adopted as a predefined constant within the planning phase. With weather recognized as changeable, but climate presumed to be fixed, using accumulated stream flow data has been the best and most appropriate approach for modeling the availability of water for the next fifty years. While the length of streamflow records vary, the median in most locales is about fifty years3. Whether by coincidence or by independent consideration, a fifty year period has historically been thought to embrace a sufficiently open consideration of both typical and atypical weather patterns and resulting stream flows to allow broad application for modeling the extended future. Additionally, while reservoir planning focuses on meeting demand growth for a specified period (typically fifty years), it is also important to recognize that project sponsors and the citizens they serve expect these facilities to be able to continue to perform unabated for much longer time frames.

These considerations alone provide ample food for thought. We shall first set the table by reviewing the basic framework and issues related to developing a new water supply reservoir. In planning for a reservoir source of supply, unmet demand must first be established. Total projected water supply demand at the end of the planning period is reduced by the reliable delivery rate of existing sources of supply to define unmet demand (conservation and efficiency measures are commonly incorporated into total demand). Where a new reservoir water supply source is found to provide the “least environmentally damaging practicable alternative”4 (preferred project), the reservoir and associated facilities are sized to satisfy the unmet demand.

The source water supply facilities to meet that need are defined by reservoir water supply storage, the reservoir watershed (direct runoff), diversion streamflow(s) and diversion pumping capacity(s), net reservoir evaporation and environmental flow requirements for all affected streams. The reliable amount of water that can be delivered from a reservoir project is commonly known as its ‘safe yield’. Simply stated, safe yield is the reliable withdrawal rate of water of acceptable quality that can be provided by a combination of stream flows and reservoir storage releases throughout a defined critical drought period. In the Eastern US, it is common for the critical drought to be either the drought of record or a computed 100-year drought, as obtained from analysis of streamflow gage data reasonably reflective of project area streamflows.

Common Reservoir Configurations and Operations

Figure 1 depicts a combination of common reservoir configurations. Where a reservoir’s watershed is of sufficient size to provide water supply for all modeled streamflows, with recharge of the reservoir during high flow periods and reservoir storage supplementation of supply during drought periods, diversion

2 Climate – defined herein as the average span and condition of weather cycles at a location over a period of many decades. 3 While some stations do provide stream flow data for 50 years or more, it is recognized that many do not. Also, in many cases, streams targeted for development do not include gaged steams, so streamflow data obtained from other regional locations must be transposed and calibrated for project use. 4 Memorandum: Appropriate Level of Analysis Required for Evaluating Compliance with the Section 404(b)(1) Guidelines Alternatives Requirements (http://water.epa.gov/lawsregs/guidance/wetlands/flexible.cfm)

Figure 1 – Common Reservoir Configurations


inflows are not needed and the reservoir is known as an on-stream project. Where supplemental flows are needed from streams outside of the reservoir watershed to achieve the needed safe yield, the reservoir is commonly known as a pumped diversion reservoir.

On-stream reservoirs have the benefit of natural recharge, capturing water from within the reservoir’s watershed, with stored water used for supplemental supply when streamflows are low. On-stream reservoirs, therefore, have lower operational costs. However, even under the best of circumstances, the safe yield potential for an on-stream reservoir is less than 0.5 million gallons per day (mgd) per square mile of watershed area, if operated as a direct water supply withdrawal.

Depending upon the physical setting of the reservoir project, the availability of outside streams for pumped diversion and the location of the water supply system’s primary demand center(s), projects can be operated as direct withdrawals from the reservoir or they can provide low flow augmentation releases to satisfy flow requirements at a downstream water supply diversion location. Water withdrawn directly from the reservoir generally provides higher quality source water because it is pre-settled and water quality can be better monitored and controlled (all supply originates within the reservoir watershed). As an example, Greensboro, North Carolina’s Lake Townsend project (Figure 2) is a direct-withdrawal, on-stream reservoir (with a recently added emergency backup diversion). The Lake Townsend project is a 6.3 billion gallon reservoir on a 105 sq. mi. watershed. It has a safe yield of about 35 mgd.

If a reservoir releases into a much larger receiving stream proximate to a major demand area, the project’s yield can be significantly enhanced. Because the large receiving stream will likely provide sufficient flow to meet water supply needs and environmental base flows much of the time, augmentation operation of the reservoir can be beneficial. Water supply needs can be directly drawn from the larger receiving stream when flows are high. Reservoir water supply releases need only be provided when the receiving stream flows are insufficient to meet water supply needs and the receiving stream’s environmental flow requirements. Since storage is released only when needed, stored water is better conserved for use during droughts, thereby increasing safe yield for a given reservoir size.

Cobb-Marietta Water Authority and the City of Canton, Georgia jointly own the Hickory Log Creek pumped diversion project (Figure 3). Hickory Log Creek is a 6 billion gallon reservoir just as is Greensboro’s Lake Townsend Reservoir. However, unlike Lake Townsend, it is on a small watershed (8 sq. mi.) and receives up to 39 mgd in pumped diversions from the 600 sq. mi. Etowah River (located within a mile of the reservoir). This project is designed to Figure 3 –Hickory Log Diversion/Augmentation Reservoir

Figure 2 – Lake Townsend On-Stream Reservoir


provide augmentation releases to a water supply intake located further downstream on the Etowah River. Since this reservoir is recharged during Etowah River high flows, and water supply releases are only made when flows in the Etowah River are low, this small watershed project yields about 40 mgd.

Most contemporary water supply reservoirs are constructed on smaller watersheds and provided with diversions from a larger nearby stream or river because damming of large streams or rivers will likely instigate significantly greater environmental impacts such as increased wetlands and stream corridor impacts, expanded aquatic species impacts, stream corridor separation (bifurcation) and other issues. It is advantageous to find a small tributary with the potential for accommodating a large storage volume reservoir located reasonably close to a large stream that can provide diversion flows to supplement natural runoff from the reservoir’s limited watershed. Reservoir storage, flows available from the diversion stream, and the diversion pumping capacity are commonly the primary determinants of safe yield. It is important to recognize that a reservoir with more storage (within reason) can reduce diversion impacts and better withstand drought conditions, even for droughts more severe than are used for defining safe yield.

Because each project setting presents a unique set of opportunities and constraints, the relative storage capacity of reservoirs, the size of and distance to viable diversion streams, and the project’s location relative to the primary demand center(s) always represent a unique combination that needs to be puzzled over with respect to technical merit (addressing the intended purpose), environmental impact (“least environmentally damaging”) and economic feasibility (“practicable project”). Added to this mix of essentials are ever-present legal issues, political and public interest considerations and special interest (advocacy group) involvement.

Planning Considerations

As a first principle, it is imperative for water providers to recognize that droughts do not announce their arrival, their intensity, or their duration with sufficient forewarning to allow for development of additional sources in response to that drought event. A water provider may be fortunate enough to have a neighbor that has both sufficient reserves and a willingness to assist during a severe drought. If not, or if you are not certain of that assistance, it is imperative to take care of your own needs. That means that you need to plan ahead. Recognizing that the planning, permitting, funding, potential legal challenges, design, construction and operational startup can take decades, it is imperative to plan well ahead. Also note that unless that neighboring water provider is willing to sell you a defined quantity of water under a defined pricing arrangement in perpetuity, it is merely a temporary option and cannot be counted on as a supply available for meeting your community’s future needs. As discussed in the opening, the reservoir permitting process is difficult, frustrating, time consuming, costly and, given the past thirty years of experience, it appears that it is going to become more so over time.

What is it that drives the need for reservoir water supply sources? Unmet water supply demand for a growing population, the availability of developable reservoir sites, nearby diversion sources for reservoir filling (most projects) and, very importantly, the lack of other viable, lesser impact water supply development options are all driving factors. For most water systems located in the Piedmont, options other than reservoirs are limited.

What factors will have the most impact on the reservoir selection process? Endangered species and protected streams top the list, and stream and wetland impacts and the mitigation required have escalated over time. Stream and wetland impacts have a significant effect on the definition of “least environmentally damaging”, and mitigation of those impacts is very costly. Minimum in-stream flow standards are also of critical importance. Over time, higher environmental flow requirements have led to higher costs from needed increases in reservoir storage and higher capacity diversion pumps and larger diversion pipes to provide a defined yield (unmet demand). Ironically, the larger reservoirs, pumps and pipes needed to fill the reservoir and provide a given yield increase the magnitude of stream and wetland


impacts that need to be mitigated. In brief, water supply projects involve considerable complexity and cost, both of which have escalated at a rate significantly exceeding the economy in general.

If you are a water provider with a growing service area, recognize that deferring the costs and efforts needed to develop a new water supply will save you money only as long as your water supply holds up. However, with long term escalation of water supply projects exceeding overall market escalation by a factor of about 3 to 5, there is a significant price to pay in deferring the process. Let’s assume general market escalation at 3% annually and water supply reservoir escalation at 9% annually (factor of 3). Deferring the permitting and development process by five years would result in a 33% increase in the cost ‘per mgd’ of new safe yield compared to general market escalation (not counting intangibles and costs related to last minute shopping). Assuming an escalation multiplier of 5 (15% annual reservoir escalation), a five year deferment of water supply project development would increase ‘per mgd’ costs by about 75% compared to general market escalation. The longer the deferral in action, the greater the burden of compounding.

Ten years ago, I would, for my own purposes, commonly (and confidentially) use a rule of thumb of $2 to $3 million per mgd of safe yield for a ballpark cost of source of supply facilities (not to include transmission, treatment or distribution facilities). Today’s rule of thumb is about $8 to $10 million per mgd. That translates to a rule of thumb compounded escalation rate for reservoir water supply projects in the range of 10% to 17% (7% to 14% above general market escalation). Using the rule of thumb escalation, a five year deferral results in reservoir cost escalations exceeding general market escalation by 40% to 92%. While we all know that past performance is not a sole indicator of future trends, the general trends of the past three decades should not be ignored.

In the Absence of Nostradamus

Planning and permitting processes necessarily require many estimations, approximations and educated guesses. A few key elements and the considerable reach of the planning process will be touched on herein. To illustrate the magnitude of the process, let’s step back 50 years to 1962. Gunsmoke was the #1 TV show (Figure 4), Walter Cronkite became the anchor for the CBS Evening News and Hank Aaron and the Braves were still operating out of Wisconsin, along with a promising 22 year-old catcher named Joe Torre. Since many of you have never been there, let’s take a minute to think about what the world would look like from a 1962 perspective. Since that was a while ago, here are some of the events of the day:

The Cuban missile crisis created a global nuclear arms showdown (Khrushchev and Castro vs. Kennedy)

Johnny Carson debuted as the Tonight Show host (with sidekick Ed McMann)

The average new home price was $12,500

Alan Shepard became the first American in space

Barack Obama had his first birthday

The US population was 58% of today’s

The NC population was 47% of today’s

Now think about how accurately you could project service conditions to appropriately meet a water provider’s unmet demand for 2012? Having put ourselves in a 1962 perspective, would anyone have been able to picture today’s world with any clarity? Would we have estimated that the State of North

Figure 4 – 1962’s Number 1 TV Show i

Source: Wikipedia


Carolina would more than double its population by 2012 or that Georgia would grow to more than 2.5 times its 1962 population?

Remember that phones still had wires and a dial, and some of them still needed an operator interface to work. Most people would have laughed at the concept of a portable ‘personal’ computer (electronic calculators didn’t exist) and there would have been no doubt that our climate was firmly fixed within the bounds of weather cycles we had experienced. Would you have been able to predict 50 years of changes in population and demographics, economic cycles and growth, technological advances, lifestyles and consumer trends, changes in politics, or a $16,000,000,000,000 national debt?

Let’s now return to 2012 (Figure 5), and consider the challenges of being charged with planning for a water supply to meet needs through 2062. How accurately can we define what our grandchildren’s world will look like and what its needs will be? Looking from our current perspective, to plan well, we need to quantify a significant number of issues and develop educated guesses for many others. I’d be 114 in 2062, so it isn’t likely going to affect me personally. However, it will be critical for younger people and for their children! How well do you believe you could predict population and demographics, economic cycles and growth, technological advances, lifestyles, consumer trends and their influences, the evolution of politics and the effects of climate change on stream flows and water supply availability?

Dealing with Uncertainties – Reason Provides the Will to Push Back

We all recognize that water supply planning is of necessity long-term and it necessarily includes making learned estimates and dealing with uncertainty. The recognized uncertainties within reservoir planning and permitting processes that most commonly are opposed include projecting 50 year population projections, estimating per capita demand, the extent to which conservation and efficiency measures can reduce demand during stress conditions, and industrial demand forecasts. These demand side issues present a major arena for questioning and attack because the uncertainties are clearly recognized by all parties. Many environmental protection regulators and special interest groups unsupportive of reservoir water projects probe most directly at these issues because they know that the numbers can readily be called into question. Project opponents probe each of these elements and fight to minimize each.

There are also unrecognized or, at the least, unspoken uncertainties, with climate change and the viability of limited streamflow records to appropriately forecast future streamflow conditions at the top of the list. At this point, we all should recognize that our climate is dynamic, meaning that clear inferences have been developed from weather records and, for longer terms, from tree ring surveys, ice cores, sediment sampling and other techniques. These records all clearly reveal long term climatic cycles occurring over the centuries and the eons that expose extremes well beyond what our short term streamflow records and intuition tell us. It appears that the past two hundred years have been more quiescent than the longer term norm. If one takes the long view of the situation – say around 4.5 billion years – the implied age of our planet – it is easy to see that change has been with us from the very beginning, and there is no reason for us to believe that things have somehow settled out. The land we stand on was once in intimate contact with Europe. The seas have risen and fallen mightily, in concert with growing and shrinking ice caps. If the age of the earth is shrunken to represent a single day, there appear to have developed regular 2 hour super-cycles of fluctuating greenhouse and icehouse conditions, with the greenhouse conditions being longer lived. The last ice age ended 0.2 seconds ago, and a Medieval Warm Period occurred about 0.02 seconds ago. When viewed over long periods of time, climate change becomes a brilliant statement of the obvious.

To make the record clear, I admit to being a global warming agnostic, wherein I define ‘global warming’ as defining a dominantly man-made climatic influence, whereas climate change is reflective of wandering climatic conditions that have prevailed for the past two billion years. Jennifer Marohasy, a noted

Figure 5 –2012’s Top SitcomSource: Wikipedia


Australian biologist wisely stated: “A consensus of 60 or 60,000 scientists is not science. Consensus is the business of politics. Science, on the contrary, requires only one investigator who happens to be right, which means he or she has results that are verifiable with reference to the real world. There is a need to take environmentalism out of science. This is going to be even harder than moving beyond politics, because religion can be even harder than politics to deal with.”

If global warming is real, we can be assured that streamflows will be altered, both droughts and floods will be more frequent and more severe, higher temperatures will translate to increased evaporation losses and sea level will rise. If global warming is not real, we none the less have become learned enough to know that our climate (previously defined as the average span and condition of weather cycles at a location over a period of many decades) has a greater span and more diversity than previously considered. Our planning efforts need to recognize the greater level of uncertainty that climate change brings to drought intensity, primarily as it affects streamflows. Climate change is currently a battleground because these greater uncertainties are generally not part of the water supply planning and permitting conversation. The supply side has not generally been recognized as entailing significant uncertainty. It is too often tacitly accepted that the past 50 years of streamflow data fully captures the range of climatological events to be expected to occur over the next 50 years (planning period). It is interesting that many individuals most supportive of the genesis of global warming tend to be the least willing to entertain discussion of its impacts in planning 50 years out for water supplies.

It has become common practice for many regulators, resource agencies and anti-reservoir advocacy groups to scrutinize and question every parameter and consideration in the planning process with no room for safety factors or contingencies to reflect unknowns and uncertainties, and to vocally demand that each parameter within each analysis framework be minimized. For its part, the engineering profession has been passive in ceding ground and removing factors of safety from an analysis process that is filled with uncertainties. Clearly, integrating a set of minimized considerations results in output that is likewise minimized and, therefore, has a marginally small probability of meeting planning period needs. As currently applied, this minimization approach translates into projects that won’t meet the sponsor’s needs over the adopted planning period.

The time has come to reassert a recognition of the considerable uncertainties that are implicit in water supply planning and permitting on both the demand and the supply sides. The broad acknowledgement of climate change (increased climatic unpredictability) over the past decade provides a resounding ‘shot across the bow’ regarding the need to reestablish both reason and caution to the long-term water supply planning and permitting processes.

Further Discussion of Supply Side Uncertainties

For the Georgia reservoir site illustrated by Figure 6, drought of record yield can readily be shown to have diminished through the past three decades, with reservoir safe yield measurably impacted the end of the last decade. Yield was assessed holding all variables constant except the limits applied to the available flow record. None of us can unambiguously define whether the flows used to develop this disturbing yield histogram reflect a definitive climate change trend or erratic longer term hydrologic behavior that expresses itself periodically as the amount of available data continues to grow.

Figure 6 – Decline in reservoir yield with flow record


While the reason for the decline may be arguable, what isn’t arguable is the volatility of climate and the considerable risk posed by reliance on 30 to 60 year streamflow records, whether applied directly or transposed to define a ‘drought of record’ or mathematically massaged to develop a simulated 100-year drought. For the river gaging data used to develop Figure 6, the one-year river flow volume from February 2007 to February 2008 was 54% of the lowest 365-day flow series for the previous 55 years of streamflow record. Over the 27 year period from 1982 to 2009, added streamflow data changed this project’s ‘drought of record’ derived safe yield from 33.6 mgd to 18 mgd.

Each of the lines on Figure 7 represent safe yield over time for a fixed set of project facilities and operating conditions, and each had at least 30 years of streamflow record in 1982. The project discussed above has experienced the most extreme drought and the most extreme yield change over time. The only site with an extended record showing reasonable resistance to significant decline in safe yield is a proposed project which would be located near the mountains of North Georgia.

The trends for the other plotted projects vary, with the overall average of the five projects pointing ominously downward. The average decline in yield of about 1% per year may be approaching a steady state, but given the linearity of the average decline, the data provide a reasonably defensible argument for on-going yield deterioration.

For the same five projects reflected in Figure 7, a plot of the ratio of current yields vs. 1982 yields, as presented in Figure 8 (next page), clearly highlights the focal point of the recent severe West Georgia drought. The approximate contour lines indicate the ratio of safe yield based on currently available flow records to those available in 1982 (having a minimum of 30 years of flow data at that time).

Figure 7 – Composite Yield Vs. Data Record for Five Georgia Sites

Average of 5 Sites: Loss of 1% of Original Yield per Year

Significant

drought events


The assessment illustrated in Figures 7 and 8 is not meant to be a comprehensive research study, but rather a preliminary assessment using data that was readily available. However, it provides enough perspective to beg the question of why West Georgia was so hard hit while the adjacent Atlanta Metro Area saw significant, but less severe, droughts? Until about 10 years ago, the historic streamflow records in West Georgia and Central North Georgia (Atlanta Metro Area) had been remarkably similar, and there don’t appear to be any transitional factors to explain the extent of differences in the distribution of recent drought events. These findings appear to predominantly be a reflection of the chance distribution of precipitation and runoff. If this is true, an Atlanta area drought of West Georgia magnitude (2007-2009) appears to be no less probable than what occurred in West Georgia.

This provokes serious concerns about the veracity of currently accepted yield analysis procedures, but more importantly, it implies that the actual safe yield of many existing water supply reservoirs may be significantly less than the permitted values on file. Also, because there exists a relative climatological homogeneity over a much broader Southeastern piedmont and coastal plain geography, this assessment provides a consequential harbinger for Carolinians as well.

Keeping it all in context

With a growing awareness of climate change, whether natural or manmade, historic records alone have come to be recognized as inadequate for capturing longer terms climatic cycles. If human activity is measurably rebalancing the climate equation, it further compounds the difficulty in projecting available water supplies 50 years into the future. Even disregarding the dynamic nature of climate and the impact of recent scientific certifications regarding climate change, these major, costly, long-term projects merit the inclusion of factors of safety to cover a multitude of unknowns and unknowables inherent in the planning and permitting process. They require development of 50-year projections to provide a framework for execution of major capital investment programs to effectively serve for 50 years or more. In 1962, we had little idea of what our world would look like today, and we today have little idea of what our world will look like in 2062. Knowing that climate changes are likely to have a trajectory, the need to prepare protective 50 year projections requires that we include consideration of climate change in addition to reestablishment of traditional factors of safety in the reservoir planning and permitting processes.

Figure 8 – Yield Ratio Vs. Location for Figure 7 Reservoirs

Atlanta


It is imperative that all interested parties to the planning and permitting processes recognize that most of these critical, costly and vitally important source of supply development programs are to serve the needs of small to mid-sized water providers that work with moderate revenue bases, and have limited breadth and depth of staff expertise to direct towards these types of disruptive, infrequent and costly capital program considerations. For growing communities confronted with major economic and population growth, demand growth within a typical planning horizon can be up to an order of magnitude increase over current deliveries.

Planning, permitting, funding and implementation of a source of supply development program for these utilities is an overwhelming process that is further intensified by a complex of layered regulatory requirements. It is not a process to be taken on lightly. It is therefore imperative that all involved in the process partner for thoughtful protection of both the impacts to the environment and the needs of the applicants, including the citizens and communities that they represent. These developing communities are not considering build-out conditions within the planning period. Failure of their water supply due to under-sizing can present severe and unacceptable consequences to public health, fire protection and the economies and livelihoods of the communities that the water system serves. Moderate over-sizing of a source of supply merely defers the timing of need for an additional water supply source and provides the opportunity to assist a less proactive neighboring community during critical drought periods to come. For communities that are approaching build-out conditions, closer scrutiny can and should be considered.

Relative Risk - Sidebar Discussion

When examining levels of risk related to reservoirs, there appears to be a disconnect of epic proportion. Spillways for dams whose failure present a risk of loss of life or significant economic damage (including loss of water supply storage) are typically designed for a 10,000+ year event5. The prospect of a dam failing and washing away homes, roads and businesses is disturbing enough that high standards are uniformly required.

Reservoir water supply sources, which are vital to a community’s health, sanitation, fire protection, vitality and economic well being are typically designed for a 50 to 100 year event. With the recognition of climate change, it may in fact be a lesser standard. Admittedly, water supplies grow in steps. After a new project comes on line, the short term risk of a failure is temporarily lessened, and enforced water restrictions can compel an additional margin of safety.

However, the dichotomy between these seemingly similar failure impacts and the radically different perspective on level of required protection (orders of magnitude!) doesn’t add up. The only discernible difference is that a dam failure flood wave is as dramatic as a plane crash, while the cause and effect relationship between a severe drought (water supply failure), and its equally devastating impacts, is less dramatic. Prudence dictates that impacts, not the suddenness of their arrival, define appropriate safety standards. Because droughts do not announce their arrival, their intensity, or their duration with sufficient forewarning to allow for development of additional sources, greater levels of protection need to be granted to our nations community water supply systems.

5 Given that extreme floods are used to define needed spillway capacity, pinning down an accurate return period is not possible.

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Reservoir Water Supply Planning for an Uncertain Future ......Keywords: water supply planning, reservoir water suppl y operation, climate change, uncertainty, risk Background Perspectives