THE DISCHARGE COEFFICIENT

In December 1993, on my second trip to India to work at the National Institute of Hydrology, I met a colleague who confided to me that he was having trouble modeling a dam-breach failure1 using a well-known model. He mentioned that his results appeared to be off by a factor of two.

I asked him what value of discharge coefficient was he using.

He responded: "Greater than 3, as given in Chow." 2

I said: "You must be using SI units."

To which he responded: "Of course."

Then I said: "That is your problem. You are using a discharge coefficient applicable to U.S. customary units. You should be using the SI equivalent, which is roughly about one-half."


1 Ponce, V. M., A. Taher-Shamsi, and A. V. Shetty. (2003). Dam-breach flood wave propagation using dimensionless parameters.   Journal of Hydraulic Engineering, ASCE, Vol. 129, No. 10, October, 777-782.

2 Chow, V. T. (1959). Open-channel hydraulics. McGraw-Hill, New York.

 

Failure of Teton Dam, on the Snake River, Idaho, on June 5, 1975 (USBR photo).

Failure of Teton Dam, on the Snake River, Idaho, on June 5, 1975 (USBR photo).