/ Public Works Engineering / Water, Sewer, and Stormwater Utilities / Water System Plan
Water System Plan
Residential Waterline Replacement
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History of Issaquah's Water System
The City's water transmission and distribution system serves residents in and around the City of Issaquah within a water service area of approximately 10 square miles that corresponds to the City limits and Potential Annexation Area (PAA) along the west, south and eastern boundaries. The Water System has approximately 3,800 customer accounts serving approximately 5800 households (single and multi-family) and 3.9 million square feet of non-residential customers. The Water System is managed by the Public Works Operations Department, and the financial reporting is provided by the Finance Department.
The Water System originated with the purchase of the East Watershed Supply in 1923. Today, the Water System has approximately 70 miles of pipe ranging in size from 3 to 16 inches in diameter, 12 reservoirs totaling 7.5 million gallons serving 18 pressure zones, 6 booster pump stations, 22 pressure reducing stations, and 4 production wells with a capacity of 3,080 gallons per minute combined. All of the City's water storage facilities are covered, ground level reservoirs or standpipes; there are no elevated tanks in the system. Water utility operations and maintenance is based at a new facility located at 670 1st Avenue NE.
The City has relied primarily on groundwater produced by wells for it's potable water supply. The City holds Department of Ecology certificated water rights to annually withdraw 2,800 acre-feet of groundwater with a maximum instantaneous withdrawal of 3,880 gpm (5.59 million gallons per day, or mgd). Wells currently in use include Risdon Numbers 1 and 2 and Gilman Numbers 4 and 5. In 1989 and 1990 the City approved wholesale water agreements with the City of Bellevue to serve the Montreux and Lakemont areas. In 1999 the agreement with Bellevue was modified to provide wholesale water service to Issaquah to meet the water supply demands of two new urban villages - Issaquah Highlands and Talus. A 24-inch transmission pipeline was constructed between Bellevue and Issaquah to provide that supply. This pipeline began operation in June, 2006.
The City and its surrounding purveyors depend on developer extensions for expansion of their systems. For new construction, water systems are designed to ensure that minimum flow requirements can be met principally through the adequate sizing of pipes and the looping of water mains.
Future Water Supply
The City is a member of the Cascade Water Alliance (CWA), a group of eight Eastside cities and districts representing over 50% of retail water sales in the region outside Seattle. CWA’s objective is to provide water supply to its members to meet current and future regional needs in a cost-effective and environmentally responsible manner.
Future water needs of the City will be provided by CWA. A Block Contract that CWA signed with Seattle in late 2003 replaced the City's previous wholesale supply agreement with Bellevue. CWA’s Action Plan 2002/2003 and its Plan for Regional Water Supply describe three major programs for the CWA system: the Block Contract with Seattle for initial supply, purchase of water from the Tacoma Second Supply Project Partners, and development of the Lake Tapps Reservoir as a 65 million gallons per day municipal water supply source. In order to make efficient use of existing and new supplies, CWA will also be acquiring and developing regional pipelines.
The City's annual average daily demand in 2000 was approximately 1.79 mgd, or about 7,500 “equivalent residential units” (ERU's) or the annual average daily water use per single family dwelling (209 gallons per day). The maximum day demand is approximately 2.5 times greater than the average day demand. Peak day demand flows within the City's service area are projected to increase from 4.8 mgd in 2000 to 9.9 mgd by the year 2020 (with planned annexation areas). It is estimated that the City will need to provide water for approximately 17,700 ERU's by the year 2020.
Water System Plan Update
The City adopts a Comprehensive Plan that references the Water System Plan Update. In 2002 a new Water System Plan Update was approved by the City Council. This plan was prepared in 2000 and 2001 based on 2000 data.
The Water System Plan Update document can be viewed or downloaded at the link provided at the left, or by clicking on this link:
The Water System Plan Update establishes the water utility policies and criteria in accordance with the City of Issaquah Comprehensive Plan framework, analyzes the existing water system and recommends improvements to correct deficiencies and meet future service needs. The plan will provide the City with a guide to evaluate the impacts of future proposed development and land use on the water system. Furthermore, it will assist the City as it faces a number of operational challenges including an addition of a regional surface water supply from the City of Bellevue, new operating areas and increasingly complex federal, state and local regulations.
As a result of growth and the City's implementation of the utility’s standards and criteria (primarily for fire flow), demand for water storage capacity and other improvements is increasing. Recommended improvements in the Water System Plan Update include:
- Construct new production wells when permitted and purchase capacity in the regional supply line for future growth.
- New storage reservoir projects are needed during the planning period to serve the Mt. Hood and the Forest Rim operating areas.
- Upgrading the existing Wildwood booster pump station
- Replace mains as necessary to improve the hydraulic operations of the existing system in accordance with the utility’s standards and criteria.
In addition to these improvements, new developer-build facilities will be added to the system as part of new developments.
The City's water service standards include sizing transmission and storage facilities to:
- Provide was supply that meets or exceed all minimum applicable standards and regulations for fire flow, storage, and peak use periods, except under emergency conditions created by major disasters, such as earthquake or flood. As resources become available, the City shall make water system improvements to meet the current fire flow criteria.
- Provide a minimum pressure within the distribution system of 40 psi at the top floor elevation, with the storage at the lowest operational volume elevation.
- Wherever practicable, provide 100 percent operational redundancy in the water supply system, allow flushing operations, and looping.
- Provide 400 gallons of standby storage for each “Equivalent Residential Unit” (ERU) within the hydraulic operating area, and...
- Provide a volume of equalizing storage in the amount equal to the average daily demand per ERU multiplied by the total number of ERUs.
In general, new development or redevelopment requires a minimum fire flow of 1,000 gpm for two hours in single-family residential zones (increased to 1,500 gpm if property line setbacks are less than eight feet) and 3,500 gpm for four hours in multifamily, commercial and high risk areas.
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