Cross-sectional area A in the formula is the product of stream width multiplied by average water depth. To calculate the average cross-sectional area for the study stream reach, volunteers should determine the cross-sectional area for each transect, add the results together, and then divide by 2 to determine the average cross-sectional area for the stream reach.
Volunteers should time with a stopwatch how long it takes for an orange or some other object to float from the upstream to the downstream transect. An orange is a good object to use because it has enough buoyancy to float just below the water surface. It is at this position that maximum velocity typically occurs. The volunteer who lets the orange go at the upstream transect should position it so it flows into the fastest current.
The clock stops when the orange passes fully under the downstream transect line. Once under the transect line, the orange can be scooped out of the water with the fishing net. This "time of travel" measurement should be conducted at least three times and the results averaged--the more trials you do, the more accurate your results will be. The averaged results are equal to T in the formula. It is a good idea to float the orange at different distances from the bank to get various velocity estimates.
You should discard any float trials if the object gets hung up in the stream by cobbles, roots, debris, etc. Continuing the example in Fig.
The calculation of flow would be:. Some current meters record the number of rotations with a counter, while others make clicking noises for a set number of rotations. As mentioned above, stream gaging can be done by measuring the stage height and velocity at a series of points in a cross-section of a stream or by constructing a flume or weir and recording stage height.
Stage height can be measured using a ruler, or a pressure transducer or stilling well connected to a data logger. Stream gaging methods will be discussed in further detail below. Show Caption Hide This rating curve relates manual measurements of stage height and velocity to determine a discharge Q. Discharge values on the y-axis were derived by measuring the velocity of water in the stream channel at a set of points in a cross-sectional transect of the stream at several sampling times.
The velocity measured by a current meter in meters per second was multiplied by the channel cross-sectional area measured in square meters to determine discharge in cubic meters per second, or cumecs.
A best-fit line was applied to the points to derive an equation that can determine discharge based on stage height. Stream water velocity is typically measured using a current meter. Current meters generally consist of a propeller or a horizontal wheel with small, cone-shaped cups attached to it which fill with water and turn the wheel when placed in flowing water.
The number of rotations of the propeller or wheel-cup mechanism corresponds with the velocity of the water flowing in the stream.
Water flowing within a stream is subject to friction from both the stream bed and the air above the stream. Thus, when taking water velocity measurements, it is conventional to measure flow at 0. This work is in the form of bed scouring erosion , sediment transport bed and suspended loads , and sediment deposition.
Gaining effluent streams receive water from the groundwater. In other words, a gaining stream discharges water from the water table. On the other hand losing influent streams lie above the water table e.
Gaining streams are perennial streams: they flow year around. Losing streams are typically ephemeral streams: they do not flow year round. Some streams are gaining part of the year and losing part of the year or just in particular years, as the water table drops during an extended dry season.
Streams have two sources of water: storm charge, from overland flow after rain events, and baseflow, supplied by groundwater. Flood Erosion and Deposition: As flood waters rise, the slope of the stream as it flows to its base level e. Also, as stream depth increases, the hydraulic radius increases thereby making the stream more free flowing. Both of these factors lead to an increase in stream velocity. The increased velocity and the increased cross-sectional area mean that discharge increases.
As discharge and velocity increase so do the stream's competence and capacity. In the rising stages of a flood much sediment is dumped into streams by overland flow and gully wash.
This can result in some aggradation or building up of sediments on the stream bed. However, after the flood peaks less sediment is carried and a great deal of bed scouring erosion occurs. As the flood subsides and competence and capacity decline sediments are deposited and the stream bed aggrades again. Even though the stream bed may return to somewhat like its pre-flood state, huge quantities of sediments have been transported downstream.
Much fine sediment has probably been deposited on the flood plain. Stream Patterns Meandering Streams: At a bend in a stream the water's momentum carries the mass of the water against the outer bank.
Water piles up on the outer bank making it a little deeper and the inner bank a little shallower. We can blow out a candle at quite a distance, for example, by pursing our lips, whereas blowing on a candle with our mouth wide open is quite ineffective. In many situations, including in the cardiovascular system, branching of the flow occurs. The blood is pumped from the heart into arteries that subdivide into smaller arteries arterioles which branch into very fine vessels called capillaries.
In this situation, continuity of flow is maintained but it is the sum of the flow rates in each of the branches in any portion along the tube that is maintained.
The equation of continuity in a more general form becomes. The aorta is the principal blood vessel through which blood leaves the heart in order to circulate around the body. The aorta has a radius of 10 mm. When the rate of blood flow in the aorta is 5. Given that the average diameter of a capillary is 8.
Substituting the known values converted to units of meters and seconds gives. Converting all quantities to units of meters and seconds and substituting into the equation above gives. Note that the speed of flow in the capillaries is considerably reduced relative to the speed in the aorta due to the significant increase in the total cross-sectional area at the capillaries. This low speed is to allow sufficient time for effective exchange to occur although it is equally important for the flow not to become stationary in order to avoid the possibility of clotting.
Does this large number of capillaries in the body seem reasonable? Many figures in the text show streamlines. Explain why fluid velocity is greatest where streamlines are closest together. Hint: Consider the relationship between fluid velocity and the cross-sectional area through which it flows.
The heart of a resting adult pumps blood at a rate of 5. Blood is pumped from the heart at a rate of 5. Determine the speed of blood through the aorta.
Determine the flow rate and the volume that passes through the artery in a period of 30 s. At the gorge, the river narrows to 20 m wide and averages 20 m deep. Figure 3. A major artery with a cross-sectional area of 1. By what factor is the average velocity of the blood reduced when it passes into these branches?
0コメント