- City identifies problem areas related to stormwater drainage
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By Myrna Trauntvein
Times-News Correspondent
A 25-year rain storm pelts an area with one-inch of rain in one hour.
If such a storm were to happen tomorrow in Nephi, there would be problems with flooding in certain areas of the community.
The city council has its work cut out for it in determining just what to do. They must plan for the infrastructure needed and also find a way to allow for future development in threatened areas.
"Unless we do something, this is going to shackle us for development," said Mark Jones, mayor. "We need to address the problem of potential flooding sooner than later. If not, we will have to turn development away."
A drainage impact fee may need to be established, said McKnight.
"We may need to establish a monthly user fee," said Randy McKnight, city administrator.
Denton Hatch, city attorney, could do the necessary research and could help the city with the legal documents needed to set up such a program.
Generating sufficient financial resources to cope with water quality issues and stormwater drainage infrastructure is an essential part of a stormwater management program. The needed infrastructure to handle such a storm would cost millions. There is really no way, with the city's budget, to handle needed construction of basins in the canyons.
"One financing option is the stormwater utility which relies on user fees rather than on traditional tax revenues to fund stormwater management," said McKnight.
The map that McKnight showed council members indicated that there were some spots on the north and on the south of the city that would need structures built. The large maps may be viewed at city hall.
In addition, storm water does not flow well the further west the water runs.
Some problems with stormwater come because there are very few culverts that go under the railroad tracks in the west part of Nephi City.
"The city grades flow away from the Big Hollow and from Salt Creek," said McKnight. "Because the radial contours of the alluvial fan convey water away from Salt Creek and Big Hollow, there are no major natural drainage corridors to collect and convey storm water runoff generated in most areas of the city."
Farmers west of the city have filled in irrigation ditches in favor of piped irrigation systems. That has created problems with drainage and its historical flow on its westward direction leading away from the city.
"Big Hollow is the only existing drainage channel that can convey runoff between the railroad and West Creek," McKnight said.
"In other areas, runoff that was previously conveyed onto agricultural lands via the open ditch irrigation system now causes flooding on private property because the open ditch system has been abandoned," he said.
The area also has collapsible soils. Collapsible soils are some of the most widely distributed and costly of geologic hazards. These soils are subject to changes in volume and settlement in response to wetting and drying.
Collapsible soils consist of loose, dry, low-density materials that collapse and compact under the addition of water. These soils are distributed in areas of young alluvial fans, debris flow sediments, and wind-blown sediment deposits.
Soil collapse occurs when the land surface is saturated at depths greater than those reached by typical rain events. This saturation eliminates the clay bonds holding the soil grains together.
"The council needs to consider some of the issues facing future development in the very near future," said McKnight.
In some areas of the community, if development is to be allowed, the developer should be required to contain all storm water on site.
If the city were to have full build out to the north or to the south, there would be storm drainage problems. It would be necessary to have drainage containment basins or dams. Those need to be built in some of the canyon areas to prevent water from washing into any future housing areas.
Building north of the current city boundaries should not be allowed unless the threat of flooding from storm events is mitigated by that type of construction.
A retention pond is designed to hold a specific amount of water indefinitely. Usually the pond is designed to have drainage leading to another location when the water level gets above the pond capacity but the pond will still maintain a certain capacity.
A detention pond is a low lying area that is designed to temporarily hold a set amount of water while slowly draining it to another location. These ponds are more or less around for flood control when large amounts of rain could cause flash flooding if not dealt with properly.
"We need to work with the irrigation system company so that Salt Creek can remain a channel for storm water to move through Nephi," said McKnight.
Property west of the railroad tracks needs to be preserved and ways developed to get water from east Nephi to West Creek.
Major drainage facilities such as Interstate-15 culverts, Salt Creek, Big Hollow and natural drainage channels should have the capacity to convey the 1-percent annual chance flood while minor drainage facilities, such as urban-only collection and conveyance facilities, should be designed to have the capacity to manage runoff from a 4-percent annual chance flood.
South of Nephi, detailed plans need to be made for addressing ways to contain storm water in the Pinery and Broad Canyons before development is allowed south of the current city limits.
Building restrictions need to be imposed also.
For example, reasonable widths for streets need to be set, he said.
Garage floors need to be built higher than ground level so that they will not fill during a storm and landscaping needs to drain away from homes.
"There is also a lack of adequate drainage facilities on Main Street and 100 North," said McKnight. "These are both state roads and they do not have adequate drainage facilities to manage stormwater runoff."
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