Mean flow calculation for hydro scheme 1

[Bob1001]

I am undertaking a feasibility study for a hydro scheme in the UK. We have daily flow records for the last 40 years from a gauging station about 13Km downstream. Using the FEH CDI have SAAR, SPRHOST, AREA values ect. for the site and gauging station. The gauging station catchment area is about 50% greater than the site's catchment.

I have calculated the average flow rate at the gauging station (m3/s) from the daily flow records.

My question is how can I translate this flow rate at the gauging station to that at the site?

Any assistance in this would be greatly appreciated.

 

[BenJohnson]

You need to compare the factors you listed (plus others) for the known gauging station against the data from your watershed. If the lower gauging station is 50% parkland and your watershed is more urbanized, then you must make an a dramatic adjustment. However, if the two areas have similar watershed characteristics, then you've got the best data an engineer could ask for with the gauging station data.

 

[Bob1001]

Ben, Thanks for your help. The catchment descriptors are similar although with an increased SAAR due to an upland watershed. Are there any formula that can be used for this or guidance on the weighting of the various descriptors?

 

[BenJohnson]

There are people who spend a lifetime studying hyrographic factors related to runoff. A lot depends on how accurate you need to be. If the inaccuracies in the estimates you provide could result in destruction of property, etc. then you will need to use more accurate methods. For most situations, the standard Rational Method will give you reasonable accuracy. (see Chapter 5 of this drainage manual for further details).

As you can imagine, complex computer models are available for more accurate estimates... but they require a lot of data which is an extensive effort to gather.

With the rational method, runoff (Q) is directly proportional to the factors used to make the calculations. In other words, Q is proportional to the land area (A), and the rainfall intensity (i). So, in your case, you would be justified in simply using the stream gauge data you have and reducing that number proportionate to the smaller watershed area and increasing the annual rainfall rate. [Are your watersheds so far apart that the SAAR is different for each watershed? If yes, then a more rigorous analysis seems to be needed.]

If you decide to dive deeper into the calculations, then first try to get a method that accurately predicts the annual runoff to the gauging station. In other words, use the gauging station data to calibrate your assumptions. Then you have some good numbers to use to do calculations on the smaller watershed.

Ok, that's a start - but just the tip of the iceberg compared to the complexities involved.

 

[cvg]

I would not recommend the rational method for estimating flow rates for a hydro-electric plant, except maybe for designing drainage improvements around the generating station. Rational method will be unable to address base flow and storage routing that you find in a river system. you will need to dive a little deeper.

Perhaps a better method is to get some flow data from your proposed site and correlate it to your downstream gage. This would be far better than any desktop method.

 

[Bob1001]

Ben and cvg,
Many thanks for your help on this.

Ben,
The SAAR and SPR do vary slightly.
What you suggested confirms what I had originally planned:
Q1/(A1xSPR1xSAAR1)=Q2/(A2xSPR2xSAAR2)

CVG
I reconise that this is a very simplistic approach but the flow calculation is for a feasibility study not detailed design so is only an estimate.

 

[BenJohnson]

Good idea to set up a gauging station now, at the point where need data, to check (and calibrate) your assumptions as work is proceeding on the project.

 

[dik]

Draw a graph of your total flow vs. time and the slope of this should give you an idea of the maximum design flow... and the flow outside the curve will give you a measure of the pondage required.

Dik

 

[cvg]

assuming this is to be a hydro-electric generation station installed at a new dam, maximum and minimum flows in the river are of interest but not particularly useful. You need to be able to estimate reservoir storage and water elevations in wet and dry years and based on your constant or scheduled withdrawal at some design flow rate, that should be close to the average flow rate in the river. In addition, you will need to estimate flood flows for design of the spillways and outlet works. These include potentially the 100-year and the probable maximum flood and maybe smaller floods as desired or required by the regulating agencies. As a start, you could use the ratio of watershed areas. You might also look at typical cross sections of the river upstream at bridges or other good measuring points, estimate high water levels and use that help validate your flow estimates.