Heber City is rethinking an update to its stormwater master plan for the North Village area after discovering planned embankments may pose a risk to human life during a severe and improbable flood.
The North Village Overlay Zone constitutes several developments north of downtown. These include the 8,000-acre community Jordanelle Ridge on the east side of U.S. 40 and The Slope on the west side. The latter, 39-acre development broke ground in April in the North Fields, a freshwater wetland area that has been used as agricultural land for generations.
The stormwater master plan exists in part to protect these developments, which are in the path of stormwater runoff, from debris flows, also called mudslides.
An update to the plan would have included several embankments designed to hold stormwater runoff.
City engineer Russ Funk said Utah Division of Water Resources representatives indicated that the largest of these embankments, planned for near Coyote Canyon Parkway, would likely be classified as a high hazard dam, and several other embankments would also likely be classified as high hazard — meaning it poses a risk to human life if the dam fails.
Funk had anticipated the embankments being classified as moderate or low hazard.
High-hazard dams must be engineered to endure the maximum probable flood, the largest flood that could physically occur in the area. Maximum probable floods are astronomically unlikely.
In the North Village, the maximum probable flood would have a discharge of 1,200 cubic feet of water per second.
The embankment near Coyote Canyon Parkway is designed to handle a 100-year flood, a flood with a 1% chance of being equaled or exceeded in a given year. That means the embankment can handle a flood with a discharge of 160 cubic feet of water per second.
“Is (re-engineering and constructing the dam to handle 1,200 cubic feet per second) even feasible without completely cutting out another big, huge section of the mountainside there?” Funk said. “Is there a way to do it? Probably. Is it going to be expensive? More expensive than it would have been if it was moderate.”
North Village developers would build the embankments and be reimbursed by the city, which would own the embankments and be liable if they failed.
“This is a real risk to the city, putting these dams above the city,” said City Manager Matt Brower. “Santa Clara, Utah, in (2012), had a failure of a dam similar to that. It flooded a third of their historic downtown, causing damage to all those properties within it.”
There is currently no risk to human life in the North Village area because these embankments have not been built.
These embankments were designed in response to public outcry about letting stormwater flow to the North Fields.
Stormwater in the area has historically flowed southwest to the North Fields from higher elevations on hillsides on the east side of U.S. 40.
“Prior to the valley being developed and farmed … you can see (from) the natural topography (that) when a big storm would come, it would direct water down these natural drainages (and) would make its way through the North Fields to the Provo River,” explained Funk.
The construction of the Timpanogos and Wasatch canals in the late 1800s altered the flow of stormwater runoff, and the construction of U.S. 40 in the next century further changed it.
That’s why most stormwater runoff does not flow into the North Fields like it once naturally did, Funk explained.
During the two-year process between 2020 and 2022 to develop the first stormwater master plan for the North Village, Funk and city staff pitched the idea of allowing stormwater runoff flow into the North Fields, which received significant public opposition.
That’s because there are two kinds of stormwater runoff, natural and developed, the latter having been polluted by flowing through roads and development. Both types would have been permitted to flow into the North Fields.
The city then devised a plan that separated natural and developed runoff, allowing only natural runoff to flow to the North Fields.
Both kinds of runoff would first be detained at basins. Developed runoff would be treated and retained. Natural runoff would seep into the ground.
Any natural runoff that did not infiltrate the ground would be released into the North Fields. But that plan also received pushback from the public, Funk said. So, the city came up with the idea to hold the leftover natural runoff in embankments, the high hazard dams in question.
These embankments would be about 25 feet tall and would have significant aesthetic impacts.
The Division of Water Resources has not yet officially classified the hazard levels of the dams. Funk hopes they are ultimately classified as moderate or low hazard. In that case, the city would likely stick with the current plan.
But if any of the dams are high hazard, Funk would like to return to the design that permits leftover natural runoff to flow into the North Fields instead of being retained.
City councilors did not vote, but all were amenable to the idea because natural runoff has historically flowed to the North Fields, and developed runoff would not be permitted to flow there.
How long it might take to work out the kinks in the North Village stormwater plan is unclear.
For Franco, the fiasco represented “the cost of doing business” in “sensitive land areas” like the North Fields.