El flujo es el volumen total de un fluido que pasa por un punto fijo en un río o arroyo a lo largo del tiempo. Es comparable a la velocidad a la que viaja un volumen de fluido, como se ve en la Figura 1. Las tasas de flujo volumétrico se pueden medir en varias unidades de volumen/tiempo, tales como:
- Litros por segundo (L/s)
- Pies cúbicos por segundo (ft³/s)
- Galones por minuto (gal/min)
- Metros cúbicos por segundo (m³/s)
Se pueden usar herramientas domésticas o medidores especializados para encontrar tasas de flujo para tuberías, sistemas de alcantarillado y electrodomésticos. Las personas usan datos de flujo para microsistemas hidroeléctricos , sistemas de aguas residuales , captación de agua de lluvia , auditoría de agua , tasas de sedimentación, estadísticas de nivel freático y otra información relacionada con el agua . Para encontrar el flujo de cuerpos de agua más grandes, como ríos importantes o detrás de represas, se utilizan medidores. [1]
Esta página describe métodos de baja tecnología para determinar el flujo de pequeños arroyos y ríos, así como otras herramientas que pueden usarse para este propósito.
Sumario
Method 1: Bucket method
El método del balde es una forma sencilla de medir el caudal utilizando artículos domésticos. Requiere un cronómetro, un balde grande y preferiblemente de dos a tres personas. Para medir el caudal usando el método del balde:
- Mide el volumen del balde o recipiente. Tenga en cuenta que un balde típico de 5 galones a menudo tiene menos de 5 galones.
- Encuentra un lugar a lo largo del arroyo que tenga una cascada. Si no se encuentra ninguno, se puede construir una cascada usando un vertedero (ver Figura 4).
- With a stopwatch, time how long it takes the waterfall to fill the bucket with water. Start the stopwatch simultaneously with the start of the bucket being filled and stop the stopwatch when the bucket fills. The bucket should not be filled by holding it below the surface of the stream because it is not the true flow rate.
- Record the time it takes to fill the bucket.
- Repeat steps two and three about six or seven times and take the average. It is a good idea to do a few trial runs before recording any data so that one can get a feel for the timing and measurements required.
- Only eliminate data if major problems arise such as debris from the stream interfering with the flow.
- The flow rate is the volume of the bucket divided by the average time it took to fill the bucket.[2]
| Trial Number | Time (seconds) | Bucket Volume (gallons) |
|---|---|---|
| 1 | 13.2 | 5 |
| 2 | 14 | 5 |
| 3 | 14.5 | 5 |
| 4 | 13 | 5 |
| 5 | 13.4 | 5 |
| 6 | 13.1 | 5 |
Here is an example using data found for the flow rate of the Jolly Giant Creek on Cal Poly Humboldt grounds: Using this data, the volumetric flow rate (Q) is equal to the volume of the bucket (V) divided by the average time (t).
Q=v/t{\displaystyle Q=v/t}
where t=13.2s+14s+14.5s+13s+13.4s+13.1s6trials=13.5seconds{\displaystyle t={\frac {13.2s+14s+14.5s+13s+13.4s+13.1s}{6trials}}=13.5seconds}
so t=13.5seconds{\displaystyle t=13.5seconds}
Q=Vt=5gallons13.5seconds=0.37gallonssecond{\displaystyle Q={\frac {V}{t}}={\frac {5gallons}{13.5seconds}}=0.37{\frac {gallons}{second}}}
So the flow rate is 0.37 gallons/second or Q = 0.37 gal/sec * 60 sec/min = 22.2 gallons/minute.
Therefore the flowrate (Q) is 22.2 GPM.
Method 2: Float method
The float method (also known as the cross-sectional method) is used to measure the flow rate for larger streams and rivers. It is found by multiplying a cross sectional area of the stream by the velocity of the water. To measure the flow rate using the float method:
- Locate a spot in the stream that will act as the cross section of the stream.
- Using a meter stick, or some other means of measurement, measure the depth of the stream at equal intervals along the width of the stream (see Figure 3). This method is similar to hand calculating a Riemann sum for the width of the river.
- Once this data is gathered, multiply each depth by the interval it was taken in and add all the amounts together. This calculation is the area of a cross section of the stream.
- Decide on a length of the stream, typically longer than the width of the river, to send a floating object down (oranges work great).[3]
- Using a stopwatch, measure the time it takes the float to travel down the length of stream from step 4.
- Repeat step five 5-10 times and determine the average time taken for the float to travel the stream. Throw the float into the water at different distances from the shoreline in order to gain a more accurate average.
- Divide the stream length found in step 4 by the average time in step 6 to determine the average velocity of the stream.
- The velocity found in step 7 must be multiplied by a friction correction factor. Since the top of a stream flows faster than the bottom due to friction against the stream bed, the friction correction factor evens out the flow. For rough or rocky bottoms, multiply the velocity by 0.85. For smooth, muddy, sandy, or smooth bedrock conditions, multiply the velocity by a correction factor of 0.9.
- The corrected velocity multiplied by the cross sectional area yields the flow rate in volume/time. (Be sure to keep consistent units of length/distance when measuring the cross section and the velocity e.g. meters, feet)
Method 3: Weirs
Weirs are small dams that can be used in measuring flow rate for small to medium sized streams (a few meters or wider). They allow overflow of the stream to pour over the top of the weir, creating a waterfall, as seen in Figure 4. Weirs increase the change in elevation making the streamflow more consistent which makes flow rate measurements more precise. However, it is very important that all the water in the stream be directed into the weir for it to accurately represent the stream flow. It is also important to keep sediment from building up behind the weir. Sharp crested weirs work best. There are many different types of weirs which include broad crested weirs, sharp crested weirs, combination weirs, V-notch weirs and minimum energy loss weirs.
Method 4: Meters
Meters are devices that measure the stream flow by directly measuring the current. There are many different types of meters but the most common are the Pygmy meter, the vortex meter, the flow probe, and the current meter, described below:
Pygmy meter: a wheel is rotated by water flow and the rate of the rotation signifies the water velocity. It is primarily used in measuring discharge.[4]
Vortex meter: velocity is proportional to the downstream frequency of the vortex flow and is read on a digital readout. It is used for measuring flow in pipes.[5]
Flow probe: the flow turns a propeller that sends the water velocity data to a digital readout display in ft/s or m/s[6]
Current meter: electronic pulses determine water velocity. Can be used in large bodies of water like oceans to measure the current.[7]
Further reading
References
- ↑ Engineers Edge. (2000). Fluid Volumetric Flow Rate - Fluid Flow. Retrieved October 28, 2009, from Engineer's Edge website: http://www.engineersedge.com
- ↑ Trimmer, W.L. (1994 September). Estimating Water Flow. Retrieved October 29, 2009, from Oregon State University website: http://web.archive.org/web/20091122100921/http://extension.oregonstate.edu:80/catalog/pdf/ec/ec1369.pdf
- ↑ Wikipedia. (2009, October). Streamflow. Retrieved October 28, 2009, from Wikipedia website: http://en.wikipedia.org/wiki/Streamflow
- ↑ Geo-Scientific Ltd. (2001). Flow and Current Meters. Retrieved November 7, 2009, from Geo-Scientific Ltd. website: http://www.geoscientific.com/flowcurrent/index.html
- ↑ Cahner Publishing Company. (1984, November 21). Liquid Flowmeters. Retrieved October 28, 2009, from Omega Engineering website: http://web.archive.org/web/20170909023441/http://www.omega.com:80/techref/flowcontrol.html
- ↑ Geo Scientific Ltd. (2001). Global Flow Probe. Retrieved November 7, 2009, from Geo Scientific Ltd. website: http://www.geoscientific.com/flowcurrent/Flow_Probe.html
- ↑ Geo Scientific Ltd. (2001). Swoffer Current Meter. Retrieved November 4, 2009, from Geo Scientific Ltd. website: http://www.geoscientific.com/flowcurrent/Swoffer2100_CurrentMeter.html