Figura 1. Um riacho fluindo na montanha.

Fluxo é o volume total de um fluido que passa por um ponto fixo em um rio ou córrego ao longo do tempo. É comparável à velocidade na qual um volume de fluido viaja, conforme visto na Figura 1. As taxas de fluxo volumétrico podem ser medidas em várias unidades de volume/tempo, como:

  • Litros por segundo (L/s)
  • Pés cúbicos por segundo (ft³/s)
  • Galões por minuto (gal/min)
  • Metros cúbicos por segundo (m³/s)

Ferramentas domésticas ou medidores especializados podem ser usados ​​para encontrar taxas de fluxo para tubulações, sistemas de esgoto e eletrodomésticos. As pessoas usam dados de fluxo para sistemas de microhidro , sistemas de águas residuais , captação de águas pluviais , auditoria de água , taxas de sedimentação, estatísticas de lençol freático e outras informações relacionadas à água . Para encontrar o fluxo de corpos d'água maiores, como grandes rios ou atrás de barragens, são usados ​​medidores. [1]

Esta página descreve métodos de baixa tecnologia para determinar o fluxo de pequenos córregos e rios, bem como outras ferramentas que podem ser usadas para esse fim.

Method 1: Bucket method

Figura 2. Encontrando a vazão usando um balde.

O método do balde é uma maneira simples de medir a vazão usando itens domésticos. Requer um cronômetro, um balde grande e, de preferência, duas a três pessoas. Para medir a vazão usando o método do balde:

  1. Meça o volume do balde ou recipiente. Lembre-se de que um balde típico de 5 galões geralmente tem menos de 5 galões.
  2. Encontre um local ao longo do riacho que tenha uma cachoeira. Se nenhuma for encontrada, uma cachoeira pode ser construída usando um açude (ver Figura 4).
  3. Com um cronômetro, marque quanto tempo a cachoeira leva para encher o balde com água. Inicie o cronômetro simultaneamente com o início do enchimento do balde e pare o cronômetro quando o balde encher. O balde não deve ser enchido segurando-o abaixo da superfície do riacho porque não é a vazão real.
  4. Registre o tempo que leva para encher o balde.
  5. Repita os passos dois e três cerca de seis ou sete vezes e tire a média. É uma boa ideia fazer algumas execuções de teste antes de registrar quaisquer dados, para que se possa ter uma ideia do tempo e das medições necessárias.
  6. Elimine os dados apenas se surgirem problemas graves, como detritos do fluxo que interferem no fluxo.
  7. A vazão é o volume do balde dividido pelo tempo médio que levou para encher o balde. [2]
Dados do método de balde para fluxo (exemplo)
Número de testeTime (seconds)Bucket Volume (gallons)
113.25
2145
314.55
4135
513.45
613.15

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}{\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}{\displaystyle t={\frac {13,2s+14s+14,5s+13s+13,4s+13,1s}{6tentativas}}=13,5segundos}

so t=13.5seconds{\displaystyle t=13.5seconds}{\displaystyle t=13,5 segundos} and V=5gallons{\displaystyle V=5gallons}{\displaystyle V=5 galões}

Q=Vt=5gallons13.5seconds=0.37gallonssecond{\displaystyle Q={\frac {V}{t}}={\frac {5gallons}{13.5seconds}}=0.37{\frac {gallons}{second}}}{\displaystyle Q={\frac {V}{t}}={\frac {5 galões}{13,5 segundos}}=0,37{\frac {galões}{segundo}}}

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

Figure 3. Finding the flow rate using a float and a meter stick.

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:

  1. Locate a spot in the stream that will act as the cross section of the stream.
  2. 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.
  3. 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.
  4. 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]
  5. Using a stopwatch, measure the time it takes the float to travel down the length of stream from step 4.
  6. 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.
  7. Divide the stream length found in step 4 by the average time in step 6 to determine the average velocity of the stream.
  8. 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.
  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.

Figure 4: An example of a V-notch weir.

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:

Further reading

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References

  1. Engineers Edge. (2000). Fluid Volumetric Flow Rate - Fluid Flow. Retrieved October 28, 2009, from Engineer's Edge website: http://www.engineersedge.com
  2. 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
  3. Wikipedia. (2009, October). Streamflow. Retrieved October 28, 2009, from Wikipedia website: http://en.wikipedia.org/wiki/Streamflow
  4. Geo-Scientific Ltd. (2001). Flow and Current Meters. Retrieved November 7, 2009, from Geo-Scientific Ltd. website: http://www.geoscientific.com/flowcurrent/index.html
  5. 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
  6. Geo Scientific Ltd. (2001). Global Flow Probe. Retrieved November 7, 2009, from Geo Scientific Ltd. website: http://www.geoscientific.com/flowcurrent/Flow_Probe.html
  7. Geo Scientific Ltd. (2001). Swoffer Current Meter. Retrieved November 4, 2009, from Geo Scientific Ltd. website: http://www.geoscientific.com/flowcurrent/Swoffer2100_CurrentMeter.html

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