- Nephi Watershed Protection and Storm Drainage Master Plan is now in the hands of council members
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SHOWING THE NEPHII WATERSHED PROTECTION MASTER PLAN • Tim Jones of Jones & DeMille Engineering shows off the Nephi Watershed Protection and Storm Drainage Master Plan to the Nephi City Council.
By Myrna Trauntvein
Times-News Correspondent
There have been major floods throughout the history of Nephi which occurred as a response to snow melt in the spring and from cloudburst events that usually come in the summer and fall.
Craig Bagley of Bower Collins & Associates and Tim Jones of Jones & DeMille Engineering, presented the report that has been in the making for more than two years to the city council and to interested citizens in attendance at the meeting.
The final summary of the Nephi Watershed Protection and Storm Drainage Master Plan is now complete, as of Oct. 6, and is in the hands of council members.
Present in addition to council members were members of the county commission who are major stakeholders in the plan development because the city plan is confined to the reaches of the city and the stormwater problems also affect county residents.
Jones, the first to present to the council, said pioneers who settled the community, for obvious reasons, built a community close to the nearest water sources.
Jones said that the study limits extend beyond the city boundaries on all sides. It is generally bounded by the mountains on the east, 500 West on the west, just south of I-15 and gravel pit on the south and just south of Exist 228 and Highway 41 to the north.
“While of benefit to the early community, that also created some challenges,” he said.
“Nephi City is located on an alluvial fan,” Jones said. “Rocky and clay soils and steep slopes in the mountain watersheds east of the city make the Nephi area susceptible to flash flooding.”
In general, he said, the storm drain facilities evaluated as part of the study are located within the city limits.
“In order to thoroughly evaluate the Nephi City storm water conveyance and detention facilities, it was necessary to take into account all contributing drainage areas, including mountain watersheds that drain through the city from east of the city,” said Jones.
Flood waters follow the radial contours of the land to Salt Creek and Big Hollow but there are no major drainage corridors to collect and convey storm water runoff in most areas of the city. He said that historical newspaper articles record multiple records of cloudburst events that caused major damage to areas of the city.
Damaging flash floods occurred in 1935, 1943, 1952, 1955 and 1956 which prompted city officials and the Natural Conservation Service (NRCS) to detention basins in Miller and Bigelow Canyons in 1961.
Then in 1983 and 1984, some flooding occurred along the reach of Big Hollow as a result of the quickly melting large snowpack.
“Salt Creek conveys runoff from a 95 square mile mountain watershed to the mouth of Salt Creek Canyon located just east of the city,” said Jones.
Further contributing to the problem of flash flooding, was the construction of Interstate-15 by the Utah Department of Transportation (UDOT). Culverts were installed where natural drainage existed to convey runoff across the right-of-way.
“Since then, development has occurred adjacent to the I-15 right-of-way, creating potential flooding problems where building are located near the outlets of the cross drainage culverts,” Jones said.
Prior to the recent development of a pressurized irrigation system in the area, he said, runoff was collected in a network of irrigation ditches that existed throughout the city.
However, since the construction, the ditches have been filled in or abandoned leaving no means of conveying storm runoff west of the railroad which bisects the western portion of the community from north to south.
“This has resulted in increased flooding on agricultural fields, residential property and business developments,” he said. “The problems created by the abandonment of the historic irrigation/drainage system and the installation of pavement and other impervious surfaces associated with recent development have created drainage problems that did not previously exist.”
The city’s primary objectives of the study were and are to resolve flooding problems associated with I-15 culverts, identify improvements to resolve existing and future urban drainage problems, identify needed drainage corridors west of the city, develop a drainage system capital improvements plan, develop a flood control plan for Salt Creek and involve stakeholders and other interested parties in identifying problems and recommended solutions.
Larger amounts of storm water runoff are generated from paved surface, curb and gutter.
“There is a lack of major drainage corridors west of Main Street,” said Jones. “Existing cross drain culverts on Main Street drain onto farm land, residential lawns or business parking.”
Jones said that there are no functioning drainage corridors to West Creek and there are multiple culverts on Main Street, I-15 and other locations in the city that are damaged or have sediment or debris built up in them.
The city lacks drainage facilities to convey storm water from east of I-15.
In addition, said Jones, much of the urban area has collapsible soils making it difficult for sumps to operate.
Increase storm water runoff from development combined with the inadequate capacity of existing storm water collection and conveyance facilities can potentially lead to flood-related damage and resultant claims against the city.
The arid climate and general infrequency of flood events in the western region of the country means that may cities tend to assign a low priority to storm drainage.
In the next article in this series, the plan of how to handle the problems the city faces will be discussed.
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