Ground Water Pressure
Data collection at the Woodway landslide ceased on October 19, 2006
- Interpreting Data
- Data
Data
Summary, median daily pressures at all sensors
Pressure head at 46 ft in B-1, last 4 days
Pressure head at 75 ft in B-2, last 4 days
Pressure head at 88 ft in B-2, last 4 days
Pressure head at 92 ft in B-3, last 4 days
Interpreting Ground-Water Data
Graphs linked to this page show pressure head the height to which water would rise above the sensor in a well or borehole. Interpreting graphs of ground-water pressure head requires several kinds of information. Knowledge of precipitation and artificial sources of ground-water recharge are essential to interpreting changes in pressure head. Knowledge of an instrument's installation, depth, sensitivity, accuracy, and range also enter the interpretation. Other measurements or direct observations in open boreholes may be needed to verify the pressure measurements. Final interpretation depends on analysis of the data using numerical models. The annotated graphs below describe some of the features of pressure-head data to aid the user in understanding the graphs linked to this page.
Noise and Short-Term Pressure Changes
Two kinds of noise
Data from the sensor at 75 ft in Boring 2 (above) at the Woodway landslide, show two kinds of noise. The long downward spikes occur frequently in data from this sensor and occasionally in data from other vibrating-wire sensors at this site. In final data processing, these values will be removed. A second type of noise that occurs in data from all the pressure transducers is visible as tiny wiggles in the line traced by the data points.
Short-term pressure changes
After the noise is removed, the trend of the data follows a smooth, gently undulating curve. The average pressure head is slightly negative, indicating that the sensor is in an unsaturated zone (above the water table). Detailed analysis of the data is needed to identify the cause(s) of such short-term changes.
Annual Cycles
Seasonal changes related to annual cycles
Data from several years of observations at the Woodway landslide reveal a pattern of seasonal water-level fluctuations. Water levels measured 46 ft below the surface in B-1 rise from about December until April and fall thereafter until about December of the following year when they begin rising again. Annual precipitation appears to influence the amount of rise and fall. Small wiggles in the curve reflect short-term changes in pressure head. Some of the larger ones might follow significant precipitation events.
Short-term Response to Precipitation
Response of shallow ground water to rainfall
Rainfall can trigger surprisingly rapid rises in shallow ground water. This data from a landslide in Honolulu, Hawaii, shows that pressure head was gradually declining before a rain storm on December 8 and 9, 1990 (in this graph, the date is shown at the end of each day). After about 12 hours of rainfall, the pressure head began to rise, and it continued rising until the end of the storm. Pressure head declined rapidly for about a day after the storm ended and thereafter it declined at a gradual, steady rate.
So far, similar short-term changes in pressure head have not been confirmed at Woodway. Several observations indicate that pressure head might respond more slowly to precipitation at Woodway than at Honolulu. Sensors at Woodway are 45 to 90 feet deep below partially saturated soils compared with 11 feet at Honolulu. Below-average precipitation was recorded at stations near Woodway during the 1997-1998 rainy season. The Woodway landslide occurred about two weeks after a rainfall and rapid snow melt event, indicating that pressure head changes at the depth where ground failure occurred may lag several days or more behind precipitation.
