Figura 1. Un ruscello di montagna che scorre.

Il flusso è il volume totale di un fluido che scorre oltre un punto fisso in un fiume o torrente nel tempo. È paragonabile alla velocità alla quale viaggia un volume di fluido come mostrato nella Figura 1. Le portate volumetriche possono essere misurate in varie unità di volume/tempo come:

  • Litri al secondo (L/s)
  • Piedi cubi al secondo (ft³/s)
  • Galloni al minuto (gal/min)
  • Metri cubi al secondo (m³/s)

Strumenti domestici o misuratori specializzati possono essere utilizzati per trovare portate per tubi, sistemi fognari ed elettrodomestici. Le persone utilizzano i dati di flusso per i sistemi microidro , i sistemi di acque reflue , la raccolta dell'acqua piovana , il controllo dell'acqua , i tassi di assestamento, le statistiche della falda freatica e altre informazioni relative all'acqua . Per trovare il flusso di corpi idrici più grandi come grandi fiumi o dietro dighe, vengono utilizzati i contatori. [1]

Questa pagina descrive metodi a bassa tecnologia per determinare il flusso di piccoli corsi d'acqua e fiumi, nonché altri strumenti che possono essere utilizzati a tale scopo.

Metodo 1: metodo Bucket

Figura 2. Trovare la portata usando un secchio.

Il metodo Bucket è un modo semplice per misurare la portata utilizzando articoli per la casa. Richiede un cronometro, un grande secchio e preferibilmente da due a tre persone. Per misurare la portata utilizzando il metodo della benna:

  1. Misura il volume del secchio o del contenitore. Tieni presente che un tipico secchio da 5 galloni è spesso in realtà inferiore a 5 galloni.
  2. Trova una posizione lungo il torrente che abbia una cascata. Se non se ne trova nessuno, si può costruire una cascata utilizzando uno sbarramento (vedi Figura 4).
  3. Con un cronometro, misura il tempo impiegato dalla cascata per riempire il secchio d'acqua. Avvia il cronometro contemporaneamente all'inizio del riempimento del secchio e ferma il cronometro quando il secchio si riempie. Il secchio non dovrebbe essere riempito tenendolo sotto la superficie del flusso perché non è la vera portata.
  4. Registra il tempo necessario per riempire il secchio.
  5. Ripeti i passaggi due e tre circa sei o sette volte e prendi la media. È una buona idea fare alcune corse di prova prima di registrare qualsiasi dato in modo che si possa avere un'idea dei tempi e delle misurazioni richieste.
  6. Elimina i dati solo se sorgono problemi importanti come detriti dal flusso che interferiscono con il flusso.
  7. La portata è il volume del secchio diviso per il tempo medio impiegato per riempire il secchio. [2]
Dati del metodo bucket per il flusso (esempio)
Numero di provaTime (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}{6prove}}=13.5secondi}

so t=13.5seconds{\displaystyle t=13.5seconds}{\displaystyle t=13,5 secondi} and V=5gallons{\displaystyle V=5gallons}{\displaystyle V=5galloni}

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 {5galloni}{13,5 secondi}}=0,37{\frac {galloni}{secondi}}}

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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