Floods, rockfalls, landslides; geologists map hazard areas along SR-9 Corridor

A rockslide in Rockville that destroyed a home and killed two people, Rockville, Utah, Dec. 12, 2013 | Photo by Dan Mabbutt, St, George News

ST. GEORGE — Anyone who has lived in Southern Utah long knows that Washington County homes have washed down rivers, been crushed by boulders, and slid off hillsides. There are higher risks involved with building in certain areas of the county and a new study released by the Utah Geological Survey maps out specific areas that are the most susceptible to various local geological hazards.

House near the town of Virgin damaged by flash flood along North Creek, Virgin, Utah, Aug, 2007 | Image courtesy of Utah Geological Survey
House near the town of Virgin damaged by flash flood along North Creek, Virgin, Utah, Aug, 2007 | Image courtesy of Utah Geological Survey

According to the UGS, the newest study, released in March, reveals significant geological hazards in the most recently mapped areas of Springdale, Rockville, Virgin and LaVerkin. The study lists nine significant hazard categories. Of those, flooding is the most prevalent and destructive geological hazard in this survey area although rockfalls and landslides have also caused significant destruction.

The UGS has been gradually mapping out hazardous areas in Washington County for years. It released its first study of this type in 2008, focusing on the more populous areas surrounding St. George and Hurricane. This newest study is on a previously unmapped area that the UGS labeled “the State Route-9 Corridor.”

Hazards in your area

To find which hazards are most prevalent in a given area, the maps are categorized at the end of this report and published here. You can also find previously published maps of the more populated areas of St. George and Hurricane at the end of this article and on the same Web page.

Remains of house at 368 West Main Street, Rockville, Utah | Courtesy of the Utah Geological Survey
Remains of house at 368 West Main Street, Rockville, Utah, Dec. 12, 2013 | Courtesy of the Utah Geological Survey

In the newest study area between LaVerkin and Springdale, development continues to ensue. In fact, according to the UGS, in this study area, the population is expected to grow six times what it is now by 2035.

These maps are not meant to drive growth, but to assist future builders, homeowners, developers and city planners to develop the land in a more informed way. They are what UGS Geologist Tyler Knudsen calls “red flag maps,” for citizens to inspect before building in an area. If the map shows a high probability of geological hazard, Knudsen said, builders should take it a step further and hire a consultant to do a site-specific study for the lot.

Based out of Cedar City, Knudsen along with Senior UGS Geologist William Lund compiled the report and did lots of the mapping.

The Watchman landslide in Springdale was first noticed in 2005 and continues to move intermittently, Springdale, Utah, 2005 | Image courtesy of Utah Geological Survey
The Watchman landslide in Springdale was first noticed in 2005 and continues to move intermittently, Springdale, Utah, 2005 | Image courtesy of Utah Geological Survey

Between LaVerkin and Springdale, several hazards of note have caused damage and even fatalities in recent history. The study cites among others three historical landslides, the most recent of which was a 4-acre slide that occurred over approximately three weeks in spring 2005 – the wettest winter/spring season on record in Springdale. This slide occurred close to residences and is listed as one of the many “landslide susceptible” areas in and around Springdale.

Areas near to Springdale also have serious rockfall hazards according to the study, as seen in recent destructive boulder slides like the fatal one in neighboring Rockville on Dec 12, 2013.

Recognizing a hazardous rockfall area isn’t always as cut and dry as you might think.  Although slopes covered with debris are indicators of a high rockfall hazard, other less obvious areas may also be vulnerable according to the report. Also, surprisingly, in many cases, a specific triggering event – like an earthquake or tremor – is not apparent during a rockfall. Furthermore, slope modification for development such as road cuts, building pads, or slope vegetation can increase or create a local rockfall hazard, according to the report.

Besides rockfalls and landslides, the March study covers nine geological hazards and maps out where these high-hazard building areas are. The maps are also very useful for locating some of the more subtle geological hazards, such as clays or collapsible soils, which are more difficult to recognize, Knudsen said.

LaVerkin-Springdale map folio:

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St. George-Hurricane map folio:

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