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| HEC-RAS is a completely new software product. None of the computational routines in the HEC-2 program were used in the HEC-RAS software. When HEC-RAS was being developed, a significant effort was spent on improving the computational capabilities over those in the HEC-2 program. Because of this, there are computational differences between the two programs. The following describes all of the major areas in which computational differences can occur. |   
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| Both HEC-RAS and HEC-2 utilize the Standard Step method for balancing the energy equation to compute a water surface for a cross section. A key element in the solution of the energy equation is the calculation of conveyance. The conveyance is used to determine friction losses between cross sections, the flow distribution at a cross section, and the velocity weighing coefficient alpha. The approach used in HEC-2 is to calculate conveyance between every coordinate point in the cross section overbanks (Figure 1). The conveyance is then summed to get the total left overbank and right overbank values. HEC-2 does not subdivide the main channel for conveyance calculations. This method of computing overbank conveyance can lead to different amounts of total conveyance when additional points are added to the cross section, with out actually changing the geometry. The HEC-RAS program supports this method for calculating conveyance, but the default method is to make conveyance calculations only at n-value break points (Figure 2). | Back To Top |
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| | | Figure 1
HEC-2 conveyance subdivision | | | Figure 2
HEC-RAS default conveyance method |
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| 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 1O% and 1% chance floods using HEC-2 and HEC-RAS, both programs using the HEC-2 approach for computing overbank conveyance. Table 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. | Back To Top |
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| Discussion of the new features in HEC-RAS version 3.1.3, including unsteady flow modeling. | Click for details |
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| Detailed description of HEC-RAS’ steady and unsteady riverine modeling capabilities, including floodplain encroachments, bridges, culverts, channel modifications, split flow, subcritical & supercritical flow, and more. | Click for details |
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| We are offering BOSS HEC-RAS 3.1.3 for the special low price of only $95 to anyone visiting our web site. | Click for details |
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| Download free HEC-RAS tutorials, documentation, and support utilities. | Click for details |
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| | | Table 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% |
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| The two methods for computing conveyance will produce different answers whenever portions of the overbanks have ground sections with significant vertical slopes. In general, the HEC-RAS default approach will provide a lower total conveyance for the same elevation and, therefore, a higher computed water surface elevation. In order to test the significance of the two ways of computing conveyance, comparisons were performed using the same 97 data sets. Water surface profiles were computed for the 1% chance event using the two methods for computing conveyance in HEC-RAS. The results confirmed that the HEC-RAS default approach will generally produce a higher computed water surface elevation. Out of the 2048 cross section locations, 47.5% had computed water surface elevations within 0.1O feet (30.5 mm), 71 % within 0.20 feet (61 mm), 94.4% within 0.40 feet (122 mm), 99.4% within 1.0 feet (305 mm), and one cross section had a difference of 2.75 feet (0.84 m). Because the differences tend to be in the same direction, some effects can be attributed to propagation. The results from these comparisons do not show which method is more accurate, they only show differences. In general, it is felt that the HEC-RAS default method is more commensurate with the Manning equation and the concept of separate flow elements. The default method in HEC-RAS is also more consistent, in that the computed conveyance is based on the geometry, and not on how many points are used in the cross section. Further research, with observed water surface profiles, will be needed to make any final conclusions about the accuracy of the two methods. | Back To Top |
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