Activity 12.6: Land Subsidence from Groundwater Withdrawal Name: Course/Section:

Question

Activity 12.6: Land Subsidence from Groundwater Withdrawal Name: Course/Section: Date: A. Santa Clara Valley, California. 1. Based on Fig. 12.11, where are the areas of greatest subsidence in the Santa Clara Valley? 2. What was the total subsidence at San Jose (Fig. A12.6.1) from 1934 to 1967 in feet? Figure A12.6.1 Subsidence at benchmark P7 in San Jose, California Total Subsidence (feet) from 1912 level Year 1912 1920 1934 1935 1936 1937 1940 1948 1955 1960 1963 1967 0.0 0.3 4.6 5.0 5.0 5.2 5.5 5.8 8.0 9.0 11.0 12.7 3. What was the average annual rate of subsidence for the period of 1934 to 1967 in feet/year? 4. Analyze Fig. 12.11 At what places in the Santa Clara Valley would subsidence cause the most problems? Explain your reasoning 5. Would you expect much subsidence to occur in the darker shaded (tan) areas of Fig. 12.113? Explain 6. By 1960, the total subsidence at San Jose had reached 9.0 ft. (Fig. A12.6.13). What was the average annual rate of subsidence (in feet/year) for the seven-year period from 1960 through 1967? Show your work. 7. Refer to Fig. A12.6.2, which shows the variation in several factors relevant to the subsidence of the Santa Clara Valley since~ 1915

Explanation / Answer

1) graph shows the maximum contour value of 8 ft at the centre,at santa claraand san jose, so this region has subsided maximum.

2) subsidence in 1967= 12.7 ft ; 1934=4.6 ft

total subsidence from 1934 to 1967= 12.7-4.6= 8.1 ft

3)1934 to 1967= 33 years

total subsidence=8.1 ft

average subsidence=8.1/33= 0.2454 feet/year.

4)the regions of santa clara and san jose subsided maximum, which means that when the excess amount of water pumped the pressure in the aquifer system reduced and which caused the alluvial sediments to sank, that means the volume of the aquifer is decreased. when the water level increases the alluvial sediments remains as it is and hence the water storing capacity of the aquifer reduces. so these areas will affect lots of problems due to the large subsidence.

5)dark shaded regions include the santa cruz mountaes and Diablow ranges, formed during the cenozoic era and these regions are less prone to subsidence.

6)subsidence in 1960= 9 ft; in 1967=12.7 ft

so total subsidence in this seven year= 12.7-9 = 3.7 feet

average annual rate of subsidence=3.7/ 7= 0.5285 feet/ year

a) since 1970 the change in elevation is very low and it is nearly one feet.

there is no large variations in elevation it seems to be constant.

b)in 1965 the water table depth has reached a maximum of around 240 feet, correspondingly we can see the maximum pumping has takenplace during the same time.

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