Comparisons of HEC-RAS results with those from HEC-2 were performed using 97 data sets from the HEC profile accuracy study (HEC, 1986). Water surface profiles were computed for 10% and 1% chance floods using HEC-2 and HEC-RAS, both programs using the HEC-2 approach for computing overbank conveyance. Table C-1 shows the percentage, of approximately 2000 cross sections, within ±0.02 feet (±6 mm). For the 10% chance flood, 53 cross sections had difference greater than ±0.02 feet (±6 mm). For those sections, 62.2% were caused by differences in computation of critical depth and 34% resulted from propagation of the difference upstream. For the 1% chance flood, 88 sections had elevation differences over ±0.02 feet (6 mm), of which 60.2% resulted from critical depth and 36.4% from the upstream propagation of downstream differences. HEC-RAS uses 0.01 feet (3 mm) for the critical depth error criterion, while HEC-2 uses 2.5% of the depth of flow.

Table C- 1 Computed Water Surface Elevation Difference (HEC-RAS - HEC-2)

Difference (feet)

-0.02

-0.01

0.0

0.01

0.02

Total

10% Chance Flood

0.8%

11.2%

73.1%

11.2%

0.6%

96.9%

1% Chance Flood

2.0%

11.6%

70.1%

10.8%

1.3%

95.8%