Question

77. Calculate Q for a stream with weeds in the channel (n = 0.035). It has an average width of 4.5 feet at gaging station A. Its average depth at the gaging station is 1.5 feet. The gradient of the water surface is 1.7 ft in the vertical over a distance of 1000 ft in the horizontal. Calculate V and then Q.

78. Why is it important to measure soil moisture? Go into the direct and indirect ways soil moisture is critical for human activities, the economy, forecasting applications etc

Answer 1

77. The formula for stream velocity using Manning's formula is

V = (1.49/n)R^2/3S^1/2

where R is hydraulic radius and S is the slope.

Hydraulic radius is the ratio of area of cross section of water divided by the wetted perimeter. If the stream channel is considered to be a rectangle, the area becomes 4.5x1.5 = 6.75 sq. ft. while the wetted perimeter is 2x1.5 + 4.5 = 7.5, since the two edges of the stream as well as the bottom are wetted by the stream. Thus, hydraulic radius becomes 6.75/7.5 = 0.9

Thus, stream velocity is

V = (1.49/0.035)0.9^2/3(1.7/1000)^1/2 = 1.64ft/s

The discharge is given by the formula

Q = VA = 1.64x6.75 = 11.07ft³/s

78. Soil moisture is important for agriculture. Depending upon the amount of moisture in the soil, irrigation needs need to be assessed. This determines the amount of water that needs to be extracted from the reservoir, the size of the dam that needs to be constructed and so on.

Soil moisture also determines the relative safety of the top layer of soil and its suitability in engineering designs. For example, extremely moist top soil cannot support shallow structures such as temporary shelters, large tents and so on. After studying soil moisture patterns over several years for a region, the soil moisture can be used to predict the weather pattern i.e. temperature, relative humidity etc. This helps determine the amount of water that may need to be irrigated for varoius reasons.