state of utah - riverdale city homepage · 2017-12-08 · state of utah department of natural...

GARY R. HERBERT Governor

SPENCER J. COX Lieutenant Governor

State of Utah DEPARTMENT OF NATURAL RESOURCES

1594 West North Temple, Suite 3110, PO Box 146100, Salt Lake City, UT 84114-6100 telephone (801) 537-3300 facsimile (801) 537-3400 TTY (801) 538-7458 geology.utah.gov

MICHAEL R. STYLER Executive Director

Utah Geological Survey RICHARD G. ALLIS State Geologist/Division Director

December 6, 2017 Rodger Worthen, City Administrator City of Riverdale 4600 South Weber River Drive Riverdale, Utah 84405 Re: Spring Creek Road Landslide Geologic Emergency Response, Report of Recent Visual Assessment of Surface Features and Results of GNSS Monitoring, Riverdale, Utah Dear Rodger Worthen: In response to your request of assistance due to the recent Spring Creek Road landslide, the Utah Geological Survey (UGS) performed geologic emergency response. The intent of this letter is to summarize our response and geologic observations of the Spring Creek Road landslide during several visits to the site from November 21 to December 5, 2017. Introduction The Spring Creek Road landslide is located on a steep, approximately 200-foot-high thickly wooded west-facing bluff above a terrace of the Weber River east of Spring Creek Road. The purpose of this geologic emergency response and field reconnaissance was to determine the hazard potential of the landslide and provide recommendations for subsequent public safety and investigations. The scope of work included a review of published geologic maps, and aerial photographs (1937). Field visits were performed on November 21, 2017 with Rodger Worthen (Riverdale City); November 24, 2017, with Richard Giraud (UGS); November 27, 2017, with Greg McDonald and Ben Erickson (UGS); November 29, 2017, with Steve Bowman and Ben Erickson (UGS); and December 5, 2017, with Ben Erickson and Jessica Castleton. The landslide initiated on November 19, 2017, as a water-rich, quickly-moving debris slide involving the lower half of the slope that dislocated the upper 10 to 15 feet of organic-rich soil and unconsolidated silty sand. The evacuated area extended about 250 feet up the slope east of Spring Creek Road (located at the base of the slope) and deposited material on the pasture below extending approximately 270 feet to the west of the road to an average depth of about 4-7 feet. After initiation, several major and many minor releases continue to expand the upper extent of the landslide and add to the deposit below (figure 1). Expansion of the landslide in the upper part of the slope, resulted in Riverdale City announcing a mandatory evacuation of three houses at the top of the slope along the west side of South 600 West on November 28, 2017.

December 6, 2017 Subject: Re: Spring Creek Road Landslide Geologic Emergency Response, Report of Recent Visual Assessment of Surface Features and Results of GNSS Monitoring, Riverdale, Utah


Figure 1. Extent of the Spring Creek Road landslide, based on boundary data provided by Weber County surveyors. Base map from 2017 Google imagery. Physical Setting and Geology The Spring Creek Road landslide is on a steep, west-facing bluff above the Weber River floodplain (figure 2). The bluff top was farmed before subdivision development as shown on 1937 aerial photography (project AAJ-AAK). Slope vegetation consists of dense trees and abundant brush and grass. The upper part of the slope is dominated by oak brush and the lower part by more water dependent trees, such as cottonwoods. The slope steepness averages about 45 percent.

The Weber River has incised into lake (lacustrine) sediment that was deposited in Lake Bonneville as part of the pre-historic Weber River delta. Yonkee and Lowe (2004) mapped two different lacustrine units in the vicinity of the landslide. Late Pleistocene age (126,000 to 11,700 years before present) regressive deltaic deposits of silty sand comprise the upper part of the slope and underlie the land to the east in Washington Terrace. Exposures in the scarp reveal that the deltaic silty sand is underlain by lacustrine clays. Holocene (younger than 11,700 years before present) stream alluvium of clast-supported, moderate- to well-sorted, pebble and cobble gravel,



gravelly sand, and silty sand, deposited along modern channels and flood plains of the Weber River underlie the pasture at the base of the bluff. The slope itself is mapped as unsorted, unstratified mixtures of mostly sand, silt, and clay redeposited by single to multiple slides, slumps, and flows; deposits display hummocky topography, but lack fresh scarps and are mostly inactive. Soil mapping by Erickson and others (1968) shows that native soils along the bluff have a high runoff potential and erosion hazard.

Figure 2. Spring Creek Road landslide on bluff above the Weber River floodplain. Photo date November 29, 2017. The slope crest near the landslide, as shown on 1937 aerial photographs, shows several scallop shapes, suggesting previous landslide activity in the area. In addition, the slope surface itself has hummocky topography suggestive of past landsliding. The bluffs on both sides of the Weber River have been the site of numerous recent and historic landslides, and future landslides are anticipated along the bluffs.



Landslide Features

The most prominent feature of the Spring Creek Road landslide is a roughly 650-foot-long west-facing scarp that involved most of the bluff (figure 1). The scarp results from downward displacement of the ground surface to the west and is the result of several major and minor movement episodes. The maximum scarp height is about 85 feet.

Below the scarp, slope material has been transported by the landslide to the base of the slope near Spring Creek Road, where a debris deposit has formed extending approximately 225 feet to the west and covering a part of the pasture. In addition to the early, debris-rich part of the landslide deposit, later activity, including failure of portions of the slope and erosion of landslide material, have formed a shallow silty deposit surrounding the earlier debris-rich portion.

Visual observations and survey data provided by Weber County indicate the scarp is enlarging both upslope and laterally to both sides since the initial failure on November 19, 2017, and currently extends to a gate in the northwest corner of the lot at 4851 South 600 West. Landslide Modeling Based on unmanned aerial vehicle (UAV) acquired video files from November 29, 2017, and provided by Riverdale City to the UGS on November 30, 2017, the UGS generated a three-dimensional (3D) model of the Spring Creek Road landslide using video editing and specialized structure from motion (SfM) software. Figure 3 shows the landslide looking up the landslide toward the east. This model may be compared to future models to analyze landslide changes, such as enlargement (size and shape), failure volumes, changes within the landslide due to erosion, etc. when the requisite data becomes available.

Visual distortion present in the image for heavily vegetated areas adjacent to the landslide is due to the limited flightpath of the UAV, where proper image angles for SfM processing were not available and from tree canopy movement due to wind. Groundwater Conditions Abundant springs along the bluff near the Spring Creek Road landslide were observed during our field reconnaissance. Local residents report that these springs flow all year round and have been active for many years. Many of the springs have morphological evidence suggesting landslide activity in the area where water is coming out of the slope.

The stratigraphy of the slope with deltaic silty sand in the upper part overlying finer lacustrine silt and clay, likely create a perched aquifer that discharges near the elevation of the base of the scarp. This elevation is similar to the location of other springs and landslides to the north and south along the bluff. A dense band of cottonwoods and water dependent vegetation is



found along the bluff at the approximate elevation of the contact between the silty sand and the clay. This is also the general location of abundant landslide activity.

Figure 3 Oblique view of the Spring Creek Road landslide from UAV-acquired video files. Imagery provided by Riverdale City on November 30, 2017.



Landslide Hazard Potential and Life Safety and Property Risk

The hazard posed by the 2017 Spring Creek Drive landslide should not be underestimated. The weak silty sand soils in the approximately upper two-thirds of the slope are easily erodible, and once they fail in movement episodes, they rapidly transform into a fast-moving mud slurry or debris flow. Based on eyewitness accounts by others, these movement episodes lasted 10 to 20 seconds, which would prevent anyone within the landslide boundary, or on part of the failed material, the time to escape, likely leading to death or very serious injury.

It is our opinion that continued and/or future landslide movement along the crown of the scarp is likely, due to of the overly steepened slope that is unstable in the present configuration. The houses at greatest risk are at 4839, 4851, and 4865 South 600 West (see figure 4). Future movement of the scarp will cause damage to these lots and probably damage the houses. Additional movement may also cause the landslide to enlarge across the slope and affect additional lots. The general topographic setting, weak geologic material, groundwater conditions, and historical slope instability all indicate that the bluff is susceptible to landsliding.

Once this landslide moved, it became more susceptible to future movement because of the reduction in shear strength of the soil, oversteepening of the slope, and erosion of the base by water flow. Groundwater emitting from the landslide includes both natural and anthropologic (human-caused) sources. Anthropologic sources include landscape irrigation, storm water drains, broken water lines, etc. Lateral migration of groundwater from elsewhere in the community above the bluff likely also adds water to the landslide. Thus, even if landslide movement stops, the potential for reactivation is high, due to the close spatial association of the landslide scarp and the springs. Landslide Monitoring As part of our geologic emergency response, we installed seven movement monitoring points above the bluff on November 30, 2017, for GNSS fast-static measurements. Four points (SCR03 to SCR06) were installed along the back edge of the yard of the houses most at risk at that time (at 4839, 4851, and 4865 South 600 West), and three points (SCR08 to SCR09) along the east side of South 600 West in the same area, as shown in figure 3. These points were revisited on December 5, 2017, and measurements made for comparison of their locations to those collected earlier, resulting in a record of movement.

Results shown on figure 3 indicate only two points with movement values greater than the assumed instrumental error of 0.75 inches. UGS experience with GNSS monitoring of landslides sometimes show small movement values above the instrumental error for individual measurements, such as recorded at points SCR06 and SCR08, often in directions that are not consistent with downslope landslide movement. Generally, these small values are only recorded on one monitoring campaign and resolved after several different measurements have taken place.



Therefore, it is our opinion that our GNSS monitoring shows no conclusive evidence of landslide movement recorded at the monitoring points from installation up to December 5, 2017. However, this does not indicate that landslide activity has stopped. The timing of each future measurement campaign will be dictated by the level of landslide activity.

Figure 4. Map showing locations of GNSS landslide monitoring points and measured movement. Base map from 2017 Google imagery. See text for explanation of movement monitoring.



Discussion and Recommendations The UGS provides this information and recommendations, based on our geologic emergency response, limited field reconnaissance, and conversations with city officials:

1. Evacuation of the three homes at 4839, 4851, and 4865 South 600 West should be continued. Should the landslide increase in size, evacuation of additional homes will be required. The UGS will continue to monitor various points established to measure landslide movement described above, as needed. If any significant movement is observed by residents or city staff, please contact the UGS so additional monitoring may be performed.

2. Fencing and appropriate signage of the three homes evacuated at 4839, 4851, and 4865

South 600 West should be continued. Additional fencing and appropriate signage along the sides of the landslide at least one lot width away should also be performed, or at a minimum, appropriate signage to alert those who may be coming close to the landslide to the danger and reduce access to the landslide for life safety.

3. Continued monitoring of culinary and secondary water lines, along with sewer and storm

drains, will be needed to ensure these utilities are not contributing water to the landslide and adjacent area.

4. No one should access the immediate landslide area, particularly the top of the scarp area

or approaching from the lower pasture. When silty sand soils, exposed in the landslide fail, they transform into a fast-moving mud slurry or debris flow, and would likely prevent anyone the time to escape, likely leading to death or very serious injury.

5. Surface water should not be allowed to flow near or over the bluff edge and landslide

scarp, due to the near-vertical scarp, easily erodible silty sand soils, and to reduce water infilitration. Roof downspouts should be appropriately piped away from structures toward the street, preferably directly connected to sealed storm drains to reduce water infiltration into the soil.

6. The landslide was likely triggered by an excessive amount of groundwater, both natural

from infiltration upslope, and from excessive landscape irrigation. The highest amounts of groundwater in urban areas are commonly observed during the late summer and fall, due to landscape irrigation since early spring. While reducing the amount of naturally-derived groundwater is rarely feasible, it is relatively easy to reduce groundwater from excessive landscape irrigation. Reducing excessive landscape irrigation may generally be accomplished in a combination of three ways: (1) educating the public and landowners/business owners on the dangers of



overwatering, (2) the installation and use of smart landscape irrigation controllers, instead of typical timer-based controllers, and (3) the use of very-low water to xeriscape landscaping. In addition, communication and coordination with Washington Terrace will be required, as the watershed contributing to the landslide groundwater issue extends upslope beyond Riverdale city limits. Public outreach about reducing landscape irrigation to reduce landsliding could take many forms, including utility bill inserts, direct mailing, billboards, providing monthly water use records to users, etc. With each citizen and company reducing their landscape irrigation, over a large area, makes for a significant reduction in groundwater, wasted water, and likely in the associated landslide hazard. People should know that excessive irrigation can easily cause a neighbor near or on a slope to lose their home from a landslide. Smart irrigation controllers that adjust the amount of water applied to landscapes, based on weather, plant/turf, and soil data, can significantly reduce the amount of excess water that percolates through the soil as groundwater. The Weber River Water Conservation District has a rebate program (http://weberbasin.com/index.php/rebates/rebates) where half of the purchase price of a EPA Water Sense-approved smart water controller up to $150 is eligible. A city ordinance may be needed to require smart irrigation controllers and/or an additional city-sponsored rebate if voluntary public response is low.

7. Understanding the geologic materials and groundwater conditions present at and adjacent

to the landslide is critical in designing proper landslide mitigation. Part of gaining this understanding should be a slope-stability-specific geotechnical investigation. The investigation should include a determination of the current landslide boundary(ies), groundwater conditions (several monitoring wells would be helpful to monitor groundwater and the effectiveness of mitigation, such as inclined horizontal drains), and soil-strength parameters, so that a proper slope stability analysis can be performed. The investigation should follow the guidelines described in the Guidelines for Evaluating Landslide Hazards in Utah (Beukelman and Hylland, 2016) and the accompanying report and review guidelines by Bowman and Lund (2016), which are considered to be the recommended minimum acceptable level of effort. Should groundwater monitoring wells be installed, the UGS can monitor water levels with the use of recording transducers for the cost of the transducers, similar to other locations we monitor, record, and archive groundwater level data. It is critical that any subsurface exploration (such as drilling) extend to sufficient depths to penetrate through the geologic units involved in landsliding. Relatively shallow exploration will rarely provide sufficient information for a proper slope stability analysis. At the Spring Creek Road landslide, borings on top of the bluff should be drilled to a



depth of at least 200 feet (bluff top elevation to the elevation of Spring Creek Road at the bottom) to fully penetrate through the silty sand and clay soils, and to determine their extent. All borings should be logged to record geologic materials encountered with a maximum 5-foot sampling interval (less will often be required), and as needed, completed with a piezometer to record groundwater elevations long term.

8. Inclined horizontal drains may be effective in reducing groundwater and subsequent

water flow along the slope, thereby reducing the potential for landsliding. These drains would likely be installed from just east of Spring Creek Road at a sufficient angle to fully penetrate the lower clay soil and extend into the overlying silty sand soil a sufficient distance beyond the bluff crest. Water exiting the drains should be piped a sufficient distance west of Spring Creek Road, so that potential or old landslide toe areas are not saturated that could contribute to future landsliding. A detailed slope-stability-specific geotechnical engineering investigation will be needed before drain installation, so that effective layout of the drains is accomplished, and alternatives may be studied.

9. Stabilization of the steep, upper slope may be possible by a variety of methods, such as the use of a soil-nail launcher and resulting ground surface treatments. Landslide stabilization and mitigation is a very specialized field, requiring specific professional and construction knowledge and expertise, where selection of the best consultant and contractor team is critical. Engineering geologists, geological/geotechnical engineers, and specialized contractors working as an integrated team, often produce the best results. Geologists, engineers, and contractors retained by Riverdale City, landowners, or others are required to be licensed as Utah Professional Geologists (PG), Professional Engineers (PE), and Contractors, respectively, per Utah statue. It would be worth contacting GeoStabilization International (GSI, http://www.geostabilization.com/) in Grand Junction, Colorado, about evaluating the slide. GSI has worked on numerous landsides in mountainous areas of the western U.S. in similar geologic materials and conditions, and was the developer of the soil-nail launcher method in the U.S. Note that the UGS does not recommend or endorse any commercial company, service, or product.

Closure Photographs taken of the landslide during our field reconnaissance are available at https://geodata.geology.utah.gov/pages/search.php?search=!collection315. We will add additional files as work continues in the future. Feel free to share this web link and use the images as needed.



References Beukelman, G.S, and Hylland, M.D., 2016, Guidelines for evaluating landslide hazards in Utah,

in Bowman, S.D., and Lund, W.R., editors, Guidelines for investigating geologic hazards and preparing engineering-geology reports, with a suggested approach to geologic-hazard ordinances in Utah: Utah Geological Survey Circular 122, p. 59-73, http://ugspub.nr.utah.gov/publications/circular/c-122.pdf.

Bowman, S.D., and Lund, W.R., editors, 2016, Guidelines for investigating geologic hazards and

preparing engineering-geology reports, with a suggested approach to geologic-hazard ordinances in Utah: Utah Geological Survey Circular 122, 217 p., http://ugspub.nr.utah.gov/publications/circular/c-122.pdf.

Erickson, A.I., Wilson, L., Hughie, V.K., Nielson, W., and Chadwick, R.S., 1968, Soil survey,

Davis-Weber area, Utah: U.S. Department of Agriculture, Soil Conservation Service, 149 p., https://geodata.geology.utah.gov/pages/view.php?ref=23983.

Yonkee, A., and Lowe, M., 2004, Geologic map of the Ogden 7.5-minute quadrangle, Weber and

Davis Counties, Utah: Utah Geological Survey Map 200, 42 p. pamphlet, scale 1:24,000, https://ugspub.nr.utah.gov/publications/geologicmaps/7-5quadrangles/m-200.pdf.

Additional Information Aerial Photography (including the 1937 AAJ-AAK project): http://geology.utah.gov/map-

pub/publications/aerial-photographs/ Geologic Hazards Information: http://geology.utah.gov/about-us/geologic-programs/geologic-

hazards-program/for-consultants-and-design-professionals/ Geologic Hazards Program: http://geology.utah.gov/about-us/geologic-programs/geologic-

hazards-program/ Landslides: https://geology.utah.gov/hazards/landslides-rockfalls/ Landslide Hazards in Utah Handout: http://files.geology.utah.gov/online/pi/pi-98.pdf Smart Irrigation Controllers: https://www.epa.gov/watersense/irrigation-controllers and

http://www.srpnet.com/water/smartirrigation.aspx Unpublished Geologic Data, Photographs, and Reports: https://geodata.geology.utah.gov

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