Landslide Hazards in Glacial Lake Clays - Tully Valley, New York

Introduction

Figure 1. Oblique aerial view of the Tully Valley landslide taken April 30, 1993, 3 days after the slide. Debris moved toward the viewer, in the process covering Tully Farms Road (dashed line) up to 15 feet deep with reddish remolded clay. Three people were rescued by helicopter behind the white house (lower left) from the rapidly advancing landslide. Springs located between red arrows.

Physical Setting Tully Valley is located in the Finger Lakes region of New York State. Like the Finger Lakes, Tully Valley is a glacially carved valley into which lake sediments were deposited. Tully Valley is approximately 6 miles long and on average about 1 mile wide along the valley floor. Onondaga Creek flows north through the valley and eventually drains to Lake Ontario. The valley walls generally consist of colluvium (weathered bedrock) and glacial till (dense soils) over bedrock. The valley floor consists of more than 400 feet of glacial lake deposits (gravel and sand grading upward to silt and clay at land surface). The valley floor terrain slopes gently (generally less than 10°) from the valley walls toward the center of the valley. Land use in Tully Valley is mostly agricultural and low-density residential. Brine mining (solution mining of salt) took place from 1889 to 1986 at the southern end of the valley. Along the west side of the valley, several older landslide areas have been identified (fig. 2). However, none of the previous landslides were known to local residents or were reported in newspapers or historical records dating back to about 1780. Therefore, the frequency of landslide events in Tully Valley has not been established reliably. As the 1993 landslide has shown, though, the potential damage from such an event can be catastrophic. Hydrogeology of the Tully Valley Profiles of soil stratigraphy before and after the 1993 landslide are shown in figure 3. A massive, red, soft to firm lacustrine silty clay deposit interfingers with coarse sandy soils and a varved clay sequence (fig. 4) that lies against the steeply dipping weathered shale bedrock. The upper thin cover of stiff silty clay traps artesian-pressured ground water in the coarse sand interfingers. Below the massive red clay and the coarse interfingers, a dense till confines and separates brackish water from the overlying freshwater in the coarse interfingers. A few local wells derive domestic water supply from this lower aquifer; however, the water quality has gradually degraded to a point where some wells were abandoned in the late 1980's.

Figure 2. Major physical features in Tully Valley, including the 1993 landslide, previous landslides, and the brine-mining area at the southern end of the valley.

During a normal year, highest ground-water pressures develop in the coarse interfingers during spring -- following snowmelt runoff and spring rains and preceding the development of foliage on the forested hillside. Once the trees leaf out, ground-water pressures decline rapidly. Review of winter 1992-93 weather records indicates that heavy winter snowfall was followed by a large snowstorm (the blizzard of March 1993) and above-normal rainfall in April. Melting winter snow increased the saturation of near-surface soil strata. A rapid melting of the blizzard's snow and nearly constant rainfall in April contributed to greater than normal surface and subsurface flow toward the slide area from the adjacent Bare Mountain.

Figure 3. Soil stratigraphy before and after the 1993 landslide.

Review of available data from the New York State Department of Environmental Conservation (NYS-DEC) and statements by local residents indicate that there were signs of developing instability months to years prior to the 1993 landslide. As early as 1990, evidence of ground cracks, bulging, and slumping was noted by the NYS-DEC near what would be the southwest corner of the slide. The foundation of one house was slowly being pushed into the cellar space and had to be repaired in 1992, and the surface of a wetland in what would become the southern extent of the landslide area rose and fell. Even if these features had been reported to local officials, they probably would not have been sufficient for accurate prediction of the landslide location or timing.

Figure 4. USGS and Syracuse University scientists using a vane shear device to measure undrained shear strength of an exposed stratum of glacial lake clay at the top of the 1993 Tully Valley landslide.

Landslide Susceptibility Mapping

Gerald F. Wieczorek, USGS, Reston, Va.; Dawit Negussey, Syracuse University, Syracuse, N.Y.; and William M. Kappel, USGS, Ithaca, N.Y.

Sources of Technical Information

Wieczorek, G.F., Gori, P.L., Jäger, Stephan, Kappel, W.M., and Negussey, Dawit, 1996, Assessment and management of landslide hazards near the Tully Valley landslide, Syracuse, New York, USA, in Proceedings of the Seventh International Symposium on Landslides, June 17-21, 1996, Trondheim, Norway: p. 411-416.