Part 1: Beyond Hydraulics – Real World Water and Sewer System Design

Engineering schools teach the fundamentals of water distribution system design. Bentley’s training supplements that with training on the modeling software that does the hard work in terms of calculations. But there is more to piping design than the pipe and tank sizing and pump selection done in WaterGEMS and SewerGEMS.

Introduction

This blog series attempts to present some important considerations in system design that are important but generally are overlooked in schools. Experienced engineers will be familiar with these issues, but inexperienced engineers run into them frequently until they learn. (This blog is based on an article I wrote for Journal of Water Resources Planning and Management a few years ago called “Real-World Considerations in Water Distribution System Design.” While it was written for water distribution systems, most of it is applicable to the wastewater and stormwater collection systems as well.) 

Some of the considerations that are described in this blog include:

  • the types of water system planning
  • sources of funding and their effects
  • dealing with uncertainty
  • construction sequencing
  • other issues that must be considered in design

Note that this series focuses on practices common to the United States, but most of the content should be relevant in other locations.

System design is driven by the need to reliably provide adequate water to customers at appropriate pressure and quality while maintaining water rates (customer prices) at a reasonable level. In the early days of centralized water systems, this involved tedious manual calculations and conservative designs. As computer technology advanced, hydraulic analysis models relieved engineers from these calculations and enabled them to consider much more detail and perform more analyses to arrive at generally better designs. Hopefully, this series can point out some considerations that need to be accounted for in any analysis of water system design.

Types of System Design

There are several instances where water distribution design is accomplished. The three main instances are master planning, capital budgeting, and land development. Across all these different types of designs, the decision variables are what to build, where to put it, how big it should be, when to build it, how to operate it, and how to pay for it. Rather than minimizing cost at an extreme level by eliminating safety factors and any excess capacity, the objective is to provide reliable service at a reasonable cost while not wasting money. The nature of the decisions that are made and the basis for making them is different in each type of planning. The most typical use cases are described below.

Master planning studies (which may go by other names such as comprehensive planning studies or long-term planning) are usually performed every few years by water utilities. In them, the utility forecasts future demands and, using that information, sizes and locates improvements. The details of the improvements are not the foremost consideration. It is usually adequate to indicate that in year T, a D-in. Diameter pipe will be installed between points x and y along route z at a cost of C dollars based on both long-term and short-term considerations. These are not usually the final decision on these improvements as there will be changes in conditions between the publication of the master plan and the installation of improvements. The decisions are adjustable as the future reveals itself. However, having a plan ensures that improvements fit into some overall direction for the water system and facilitate financial planning.

Capital planning is usually performed annually during the financial planning and budgeting for the water utility, when the utility needs to decide on the amount of money it will need to borrow to fund the improvements. While the master plan is used as a starting point, recent demand forecasts are usually different from those that were used as the basis for the master plan, as the future never turns out exactly as planned. The exact size and locations of improvements must now be determined to a level of detail to make an accurate cost estimate and serve as the starting point for construction plans and specifications and land/right-of-way acquisition. It is no longer adequate at this point to indicate that a pipe must run from point x to point y; the utility must also decide on the exact route and understand the implications of each potential right-of-way. Issues such as road restoration, tie-ins with existing infrastructure, and location of isolation valves, which can usually be ignored in the master plan, become important and can significantly change the design. Design decisions are essentially irreversible at this time, barring some dramatic change in the utility's circumstances. These decisions must be based on both long- and short-term considerations, and capital budgeting limitations must be explicitly considered. It is not uncommon for worthwhile projects identified in the master plan to be deferred or modified at this juncture due to budgetary issues, as the key driving force at this time is how many improvements can be afforded without having significant water rate effects.

Developer improvements are projects that are implemented by and for land development ventures. These are usually funded by some combination of developer and utility sources and, as such, are somewhat outside the normal capital budgeting process. These projects usually consist of the smaller pipes, valves, and hydrants within subdivisions or commercial properties with limited effect on the larger transmission facilities. However, they can have a serious effect on the overall utility master plan if they involve new pump stations or tanks outside the existing service area and especially for any facilities above the elevation of existing pressure zones. Sizing of improvements is usually driven by fire flow requirements. Because the developer actually pays for these improvements, the water utility is interested, not so much in minimizing cost, but in providing a set of improvements that:

  • have more than minimal capacity
  • will become a part of the utility's facilities
  • fit into the master plan
  • will be easy to operate efficiently going forward

In general, developers will be gone in a short time, and it is essential to have them pay for as much capacity as is reasonable while they have funding.

Sources of Funding

Funding of capital improvements is critical. When reviewing distribution system improvements, upper management does not usually ask, "How big will the pipe be?" or even less likely, "What C-factor did you use?" Instead, the questions are, "How much will it cost?" and "Where will we get the money?" The source of funding can affect the size and timing of construction. The overall goal is to provide required facilities with minimal effect on the rate payers.

The capital budget is the source of funding for most major capital improvements. The utility usually borrows this money by issuing bonds. The utility needs to arrange to borrow the correct amount of money. Too little, and they need to issue additional bonds, which is difficult. Too much, and the utility is paying interest on money that is not being converted into facilities.

The operating budget is used to fund items that would be considered under maintenance and repair. This can include painting tanks, rehabbing pumps and finding and repairing leaks. Usually bond money cannot be used for this type of work, and it must be fully funded out of the current year's revenue.

Rehabilitation funding in some utilities comes from a different source and is used for replacement of existing facilities. In regulated utilities, these costs can be passed on to customers without public hearings normally associated with rate increases, although the types of projects that can be funded this way are limited. Working this area is usually driven only partly by hydraulic analysis as asset management considerations (e.g., leakage and pipe breaks) become more important.

Grants and revolving loan funds are highly desirable sources of money for government subsidized projects. Federal and state money may be available on a competitive basis for projects that meet specific criteria. In some cases, this money is available for the purpose of "economic stimulus." The grants are essentially free money, while the loans are provided at an interest rate that is less than the market rate for bonds. Cost minimization is not quite as important as making the best use of the available money.

For developer improvements, issues get complicated when facilities can serve the development but are also part of the utility master plan. If a certain size pipe is in need for the development but the utility needs a larger pipe as part of its master plan, the utility can pay for upsizing, pay for the entire pipe, or have the developer pay for all the cost of the larger pipe, depending on local rules. Utilities like when developers pay for facilities in that it does not affect existing rate payers because growth pays for itself. In some cases, where a facility is installed for future developments, developers using that capacity may be required to pay a capital recovery fee to reimburse the utility for its investment to serve them.

Stay tuned for the second part in the series! We will cover right-of-way considerations, phasing considerations, conflicts, and how system design affects demands.

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