Chapter 5 Multiple choice questions

. The Reynolds number is a relevant number when describing:

.
Figure ch05q02.jpg
Specific energy diagram for a specific discharge qw of 0.5 m2 s-1
For subcritical surface water flow the average water flow velocity v is

. The discharge of a stream Qstream is determined with salt dilution gauging using the constant rate injection method. The initial or upstream salt concentration of the stream is Cstream. A salt solution is added with discharge Qadd and salt concentration Cadd. Cmix is the constant salt concentration measured at the downstream monitoring location with complete mixing. All discharges are in litre s-1; all concentrations are in mg litre-1. The following holds:

. The natural salt concentration or base concentration of a stream equals Cb mg litre-1. The discharge of a stream is determined by instantaneously adding 1200 gram of salt to the stream. At a downstream location where the salt addition and stream water are completely mixed, the salt concentration Cd (mg litre-1) is monitored. A wave of salt water passes the monitoring site between t1 and t2. The following measurement results are obtained:
Figure ch05q04.jpg
The discharge Q of the stream in litre s-1 equals

.
Figure ch05q05.jpg

The stream discharge Q in litre s-1 is related to the water level
H - H0 in cm by the following equation: Figure ch05q05a.jpg
: a regression line (log Q along the vertical axis; log (H - H0) along the horizontal axis) is established in the above figure; a in the above equation =

. The river-water intake of a beer brewery has to close when the river discharge falls below 320 m3 day-1. At the beginning of a long period with no rainfall the river discharge amounts to 625 m3 day-1. Exactly one week later the river discharge equals 500 m3 day-1. No river water is lost to the underground and the recession curve of the river discharge with time perfectly fits a negative exponential curve. How many weeks after the start of the rainless period does the water intake have to be closed?

. In the vicinity of Larochette, Luxembourg a drainage basin with an area of 5 km2 receives 25 mm of rainfall. The losses due to interception and evaporation during the rainfall event amount to 5 mm. On tracks near the stream overland flow occurs; 2% of the rain falls directly into the stream as channel precipitation. A very large part of the rainfall infiltrates into the sandy soil and does not contribute quickly to the discharge of the stream; 4×104 m3 of water leaves the drainage basin at the outlet; Figure ch05q07.jpg of this water is baseflow. What is the runoff coefficient (%)?

. In the vicinity of Berdorf, Luxembourg a drainage basin with an area of 10 km2 receives 25 mm of rainfall. On tracks near the stream overland flow occurs; 2% of the rain falls directly into the stream as channel precipitation. A very large part of the rainfall infiltrates into the sandy soil and does not contribute quickly to the discharge of the stream; 3×104 m3 of water leaves the drainage basin at the outlet; Figure ch05q08.jpg of this water is baseflow. The amount of overland flow in mm equals:

. The soils in a very small drainage basin near Nommern in Luxembourg are characterized by a clayey B horizon at 25 cm depth. At the beginning of a rainy day in September (with precipitation = 22 mm and losses by interception and evaporation = 7 mm) the soil moisture deficit in the upper 25 cm of the soil equals -2 mm. At the end of the day the upper 25 cm of soil is water-saturated. The contribution of throughflow to the brook discharge is 12 mm day-1. All parts of the drainage basin contribute to the brook discharge and the B horizon may be regarded as water-impermeable. The amount of overland flow in mm during the day equals:

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