I have a question about the evaluation of expansion reach lengths for bridges. I am reviewing the HEC-RAS 5.0.4 Applications Guide, and looking through Chapter 2: Beaver Creek – Single Bridge. On page 2-23, the guide provides equation 2-1 for an approximation for estimating the expansion length. I have plugged this into excel, along with the data from the example, and cannot seem to produce the results they calculate.
The equation I am have set up in excel is Le=-298+257*(Fr2/Fr1)+0.918*(Lobs)+0.00479*(Q), where Le is the expansion length, Fr2 is the Froude number at cross section 2 (just downstream of the bridge), Fr1 is the Froude number at cross section 1 (fully expanded flow), Lobs is the average length of obstruction caused by the bridge in ft, and Q is flow in cfs. I’m using the values listed in the guide as well, namely Fr2 = 0.37, Fr1 = 0.30, Lobs – 740 ft, and Q = 14,000 cfs. The example in the Application Guide produces an Le = 778 ft. My spreadsheet produces an Le = 765 ft. Any insights into where I’m going awry? Rounding error? Operator error? Thanks in advance!