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Welcome to CivilGEO Knowledge Base
Welcome to CivilGEO Knowledge Base
Welcome to CivilGEO Knowledge Base
HEC‑RAS and HEC‑2 are both software programs that are used for computing water surface elevations for rivers, streams, and other open channel networks. While both software programs were developed by the Army Corps of Engineers, the two software programs are completely different. 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 improving the computational capabilities of the HEC‑2 program. Because of this, there are computational differences between the two programs.
This article describes all of the major areas in which computational differences can occur between the two software programs.
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 overbank (as show in the first figure). 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, without 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, as shown in below given figure.
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. The below table shows the water surface difference in percentage, for approximately 2000 cross sections, within ±0.02 feet (±6 mm). For the 10% chance flood, 53 cross sections had differences greater than ±0.02 feet (±6 mm). For those cross 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 cross 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: Computed Water Surface Elevation Difference (HEC‑RAS and HEC‑2)
Difference (feet) | ‑0.02 | ‑0.01 | 0 | 0.01 | 0.02 | Total |
10% Chance Flood | 0.80% | 11.20% | 73.10% | 11.20% | 0.60% | 96.90% |
1% Chance Flood | 2.00% | 11.60% | 70.10% | 10.80% | 1.30% | 95.80% |
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 0.10 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.
During the water surface profile calculations, each of the two programs may need to calculate critical depth at a cross section if any of the following conditions occur:
The HEC‑RAS program has two methods for calculating critical depth:
The HEC‑2 program has one method computing critical depth, which is very similar to the HEC‑RAS parabolic method. The parabolic method is computationally faster, but it is only able to locate a single minimum energy. For most cross sections there will only be one minimum on the total energy curve; therefore, the parabolic method has been set as the default method for HEC‑RAS (the default method can be changed from the user interface). If the parabolic method is tried and it does not converge, then the HEC‑RAS program will automatically try the secant method. The HEC‑RAS version of the parabolic method calculates critical depth to a numerical accuracy of 0.01 feet, while HEC‑2’s version of the parabolic method calculates critical depth to a numerical accuracy of 2.5 percent of the flow depth. This, in its self, can lead to small differences in the calculation of critical depth between the two programs.
In certain situations it is possible to have more than one minimum on the total energy curve. Multiple minimums are often associated with cross sections that have breaks in the total energy curve. These breaks can occur due to very wide and flat overbanks, as well as cross sections with levees and ineffective flow areas. When the parabolic method is used on a cross section that has multiple minimums on the total energy curve, the method will converge on the first minimum that it locates. This approach can lead to incorrect estimates of critical depth in that the returned value for critical depth may be the top of a levee or an ineffective flow elevation. When this occurs in the HEC‑RAS program, the software automatically switches to the secant method. The HEC‑RAS secant method is capable of finding up to three minimums on the energy versus depth curve. Whenever more than one minimum energy is found, the program selects the lowest valid minimum energy (a minimum energy at the top of a levee or ineffective flow elevation is not considered a valid critical depth solution).
Given that HEC‑RAS has the capability to find multiple critical depths, and detect possible invalid answers, the final critical depth solutions between HEC‑2 and HEC‑RAS could be quite different. In general the critical depth answer from the HEC‑RAS program will always be more accurate than HEC‑2.
A vast amount of effort has been spent on the development of the new bridge routines used in the HEC‑RAS software. The bridge routines in HEC‑RAS allow the modeler to analyze a bridge by several different methods with the same bridge geometry. The model utilizes four user-defined cross sections in the computations of energy losses due to the structure. Cross sections are automatically formulated inside the bridge on an as needed basis by combining the bridge geometry with the two cross sections that bound the structure.
The HEC‑2 program requires the user to use one of two possible methods, the special bridge routine or the normal bridge routine. The data requirements for the two methods are different, and therefore the user must decide prior to defining the model on which method to use.
Differences between the HEC‑2 and HEC‑RAS bridge routines are addressed below.
The largest computational differences will be found when comparing the HEC‑2 special bridge routine results to the equivalent HEC‑RAS bridge results. The following list details these differences:
In general importing HEC‑2 normal bridge data into HEC‑RAS should not cause any problems. The HEC‑RAS program automatically selects the appropriate energy‑based method for low flow and high flow conditions, which is equivalent to the HEC‑2 normal bridge method. The following list details possible differences that may occur:
The culvert routines in HEC‑RAS and HEC‑2 were adapted from the Federal Highway Administrations Hydraulic Design of Highway Culverts publication, HDS No. 5 (FHWA, 1985). The following is a list of the differences between the two programs.
The floodway encroachment capabilities in HEC‑RAS were adapted from those found in HEC‑2. For the most part, encroachment methods 1‑3 in HEC‑RAS are the same as methods 1‑3 in HEC‑2. The following is a list of the differences between the two programs.
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