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Text of Document: Water Supply Planning – University of Tennessee IPS/MTAS Page 1 of 5
Water Supply Planning
Ray Crouch, Sr.
Fire Management Consultant
University of Tennessee IPS/MTAS
Updated by MTAS Staff
City residents and municipal government have a strategic interest in water supply capabilities. Residents and business owners expect to have a level of water supply that will meet the potable water supply needs and provide water for fire protection. Industrial development puts a greater strain on the water supply for not only the industrial processes conducted, but also for the increased demand for available water for fire protection. In fact, 40% of the score on a community’s ISO classification is determined by the water supply available for fire protection. This presents a unique situation in many cities. Some municipal governments are not “in the water business,” therefore, strategic partnerships with the utility districts, development boards, and fire protection agencies should be developed to provide not only good potable water, but also adequate fire flow capabilities for long range planning.
Across the state, most cities have formed planning commissions and several conduct building and fire code plans review and inspections. This may create a dilemma for developers in regards to conflicting requirements between subdivision regulations, fire code requirements, and utility district regulations. Therefore, all parties must conduct joint planning meetings to establish adequate regulations that will satisfy all of the various interests (safe/dependable potable water supplies, adequate storage capacities, and fire flow requirements.) Through progressive planning, a relatively small investment on the front-end of development projects will yield financial benefits many times over the initial costs. Additionally, adequate fire flow capabilities can quickly be established that can mean the difference in life or death, or whether a factory that employs 200 workers will be able to re-open in a few weeks or stay closed forever, leaving a large hole in the tax base.
Fire Protection Water Supply Standards
An excellent resource for water supply planning in regards to fire protection is NFPA1142 Standard on Water Supplies for Suburban and Rural Fire Fighting. NFPA 1142 identifies a method of determining the minimum requirements for alternative water supplies for structural fire-fighting purposes in areas that lack a reliable water system capable of providing sufficient water for fire-fighting purposes. The standard addresses topics such as: calculating minimum fire flow requirements, developing alternative water supply sources, water hauling capabilities, the use of large diameter hose, and includes schematics on the proper construction of dry hydrants. NFPA 1142 can assist the fire protection authority with adequate fire protection planning and development of recommendations for subdivision regulations adopted by the planning commission.
The ISO publication Guide for Determination of Needed Fire Flow should also be used when planning for needed fire flow requirements.
Needed Fire Flow
A simple formula for suppressing fires is: correctly applying water in sufficient quantity to absorb the BTU’s of heat being produced. The amount of water needed is called the needed fire flow, (NFF) and it is expressed in gallons per minute (gpm). The NFF varies depending upon the type of building, its use, and its proximity to other structures. For commercial property this NFF is calculated on a building-by-building basis. For one and two family dwellings, a simple chart has been established:
Distance Between Structures
100’ or more
31’ to 99’
11’ to 30’
10’ or less
In many communities the planning commission has the responsibility of setting the distance between homes and establishing minimum water line sizes. Both of these factors must be considered when establishing a master development plan and/or zoning regulations for the safety and welfare, as well as the economic impact of the community.
Water Supply Components
Components of fire protection water supply can be very diverse depending upon fire department capabilities, public water availability, and geographical factors. The most reliable component of the water supply system is a properly designed public water system. This system will have adequately sized water mains, pumps, and above ground storage capabilities, as well as properly placed fire hydrant locations. Inside municipal boundaries, fire hydrant spacing should never exceed 1,000 feet between fire hydrants and should be placed within 500 to 300 feet of the route that hose would be laid by a fire engine to an accessible point of a structure. The number of hydrants needed depends upon the spacing and size of structures. In rural areas, fire hydrants should be placed as close to structures as possible with the driving distance from a hydrant to the structure not to exceed 500 feet if possible. The minimum size of a water main should be not less than 6”. Water mains should be designed for future fire protection in mind, not just to address the current needs for water. Systems should also be designed to meet the calculated fire flow requirements in the various land use zones. At no point in the water system should the fire flow of the water system supply less than 500 gpm at 20 psi residual pressure.
Secondary water supply components can be used as: (1) a supplement to the public water supply, or (2) a back-up to the public water supply in case of catastrophic failure of the water system. Secondary water supply components can consist of fire department tankers, ponds, streams, and even (usually with portable pumps) swimming pools. An active approach to identify these water sources should be in place. Some jurisdictions create alternative water supply committees consisting of fire officials and planning personnel to locate and mark alternative water supplies in strategic locations. This should also be addressed in areas with public water as a “back-up” in case a catastrophic event disables the water system. These locations are then investigated further to verify drought resistance and the possibility of dry hydrant placement. Dry hydrants are a cost effective way of establishing a water supply in areas without public mains. A dry hydrant can be counted the same as a regular hydrant in jurisdictions that have a split ISO rating, as long as the dry hydrant is located within 1,000 feet of the structure and five driving miles of a fire station. Many sources of water are available beyond the accessible reach of a fire truck. The installation of dry hydrants can extend the “reach” of a fire truck in a year-round capacity.
Swimming pools can be a good source for water, but can be difficult to reach with a fire engine. An average size in-ground pool holds 15,000 gallons or more. If the pool is to be located beyond the fire vehicle access point, a connection to a dry hydrant at the road could be installed off of the main drain. This could even be incorporated into swimming pool regulations in cities that conduct building permits and inspections. The fire department could use a portable pump to pump water from the swimming pool to a fire truck located on the road on in a parking lot.
These alternatives can eliminate ISO split ratings and reduce insurance premiums by thousands of dollars per property owner over a 10-year period. Optimally, alternative water sources should be identified within one driving mile of all structures. This is about the furthest distance that an effective tanker shuttle can be established to meet the fire flow delivery requirements required by ISO.
Public Water System Planning
As a new development is being planned, water systems must be designed to meet the new and future demands on the system. Even if the water supply network going into a new development is inadequate, the future sights should be designed to handle the potable and fire flow projections for the next 20 to 25 years. Main sizes should be installed that reflect the anticipated demand for a minimum of the next 20 years. Projects designed to meet only today’s demand will have to upgrade all of the water mains, because they will all be undersized. Additionally, fire hydrants should be spaced and located appropriately, with additional hydrants being placed by the future development projects.
This creates a pay now or pay more later situation. To design and install a water line expansion for potable needs and fire protection versus potable water only adds only a small percentage to the costs of a new main. For any new water project the cost to design/engineer the system, the labor to dig the trench, and installation of the pipe are relatively the same, the only difference is the actual pipe cost. This same principle holds true for aboveground storage capacity. A small amount added to a water bill is many times off-set by the reduced insurance premiums over the course of a 10 to 15 year rating period.
The following information was been compiled by MTAS in 2002 from actual utility projects that occurred in the state:
PVC Pipe Installation Cost
Water Line Capacity
(with velocity equaling 5ft/sec)
Elevated Water Reservoir Cost
As you can see, the increased cost is not proportional to the increased capabilities of the system. A good rule of thumb in regards to hydraulic calculations: “As you double the size of the water line, the capacity increases by four fold.” The charts above show that the cost does not increase four fold with the doubling of the water line size. Therefore, it is essential that as new development and upgrades occur, the maximum feasible system capacity should be designed.
Water system and community development planning is a complex subject that a city government cannot afford to ignore. It is vital for the life safety and property conservation and for the continued economic growth of a region.
The water supply system should be designed to meet the demands for potable water and for fire protection. Main sizes should be installed that meet the demands for water today and for the next twenty years. Fire hydrants should be installed so that they will be within 300 to 500 feet of all structures and will meet the needed fire flow of the structure without taking the water system below 20 psi residual pressure. Planning, coordination, and cooperation are needed if water supply needs are to be met cost effectively. If construction standards for new water supplies are not designed with these criteria in mind, the water supply customers will pay much more in insurance premiums than they could ever save by having water that cost a few cents less per 1,000 gallons.