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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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• Critical Depth Calculations
• Bridge Hydraulic Computations
• HEC-2 Special Bridge Methodology
• HEC-2 Normal Bridge Methodology

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:

1. The supercritical flow regime has been specified by the user. 
2. The calculation of critical depth has been requested by the user. 
3. The current cross section is an external boundary cross section and critical depth must be determined to ensure the user-entered boundary condition is in the correct flow regime. 
4. The Froude number check for a subcritical profile indicates that critical depth needs to be determined to verify the flow regime of the computed water surface elevation. 
5. The program could not balance the energy equation within the specified tolerance before reaching the maximum number of iterations.

The HEC-RAS program has two methods for calculating critical depth: a "parabolic" method and a "secant" method. The HEC-2 program has one method, 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.

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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 need 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 appropriately which method to use.

Differences between the HEC-2 and HEC-RAS bridge routines will be addressed by discussing the two HEC-2 bridge methodologies separately.

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The largest computational differences will be found when comparing the HEC-2 special bridge routines to the equivalent HEC-RAS bridge methodologies. The following is a list of what is different between the two programs:

1. The HEC-2 special bridge routines use a trapezoidal approximation for low flow calculations (Yarnell equation and class B flow check with the momentum equation). The HEC-RAS program uses the actual bridge opening geometry for all of the low flow methodologies. 
2. Also for low flow, the HEC-2 program uses a single pier (of equivalent width to the sum total width of all piers) placed in the middle of the trapezoid. In the HEC-RAS software, all of the piers are defined separately, and the hydraulic computations are performed by evaluating the water surface and impact on each pier individually. - While this is more data for the user to enter, the results are much more physically based. 
3. For pressure flow calculations, HEC-2 requires the net flow area of the bridge opening. The HEC-RAS software calculates the area of the bridge opening from the bridge and cross section geometry. Because of the potential error involved in calculating the bridge opening area by hand, differences between the programs may occur for pressure flow calculations.
4. The HEC-RAS software has two equations that can be used for pressure flow. The first equation is for a fully submerged condition (i.e. when both the upstream side and downstream side of the bridge is submerged). The fully submerged equation is also used in HEC-2. A second equation is available in HEC-RAS, which is automatically applied when only the upstream side of the bridge is submerged. This equation computes pressure flow as if the bridge opening were acting as a sluice gate. The HEC-2 program only has the fully submerged pressure flow equation. Therefore, when only the upstream side of the bridge is submerged, the two programs will compute different answers for pressure flow because they will be using different equations.
5. When using the HEC-2 special bridge routines, it is not necessary for the user to specify low chord information in the bridge table (BT data). The bridge table information is only used for weir flow in HEC-2. When HEC-2 special bridge data is imported into HEC-RAS, the user must enter the low chord information in order to define the bridge opening. This is due to the fact that the trapezoidal approximation used in HEC-2 is not used in HEC-RAS, and therefore the opening must be completely defined. 
6. When entering bridge table (BT records) information in the HEC-2 special bridge method, the user had to enter stations that followed along the ground in the left overbank, then across the bridge deck/road embankment; and then along the ground of the right overbank. This was necessary in order for the left and right overbank area to be used in the weir flow calculations. In HEC-RAS this is not necessary. The bridge deck/roadway information only needs to reflect the additional blocked out area that is not part of the ground. HEC-RAS will automatically merge the ground information and the high chord data of the bridge deck/roadway.

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In general, when importing HEC-2 normal bridge data into HEC-RAS there should not be any problems. The program automatically selects the energy based methods for low flow and high flow conditions, which is equivalent to the normal bridge method. The following is a list of possible differences that can occur:

1. In HEC-2 pier information is either entered as part of the bridge table (BT data) or the ground information (GR data). If the user stays with the energy based methods in HEC-RAS the results should be about the same. If the user wishes to use either the Momentum or Yarnell methods for low flow, they must first delete the pier information from the BT or GR data, and then re-enter it as separate pier information in HEC-RAS. If this is not done, HEC-RAS will not know about the pier information, and will therefore incorrectly calculate the losses with either the Momentum or Yarnell methods.
2. The HEC-2 Normal bridge method utilizes six cross sections. HEC-RAS uses only four cross sections in the vicinity of the bridge. The two cross sections inside the bridge are automatically formulated from the cross sections outside the bridge and the bridge geometry. In general, it is common for HEC-2 users to repeat cross sections through the bridge opening (i.e. the cross sections used inside the bridge were a repeat of the downstream section). If however, the HEC-2 user entered completely different cross sections inside the bridge than outside, the HEC-RAS software will add two additional cross sections just outside of the bridge, in order to get the correct geometry inside of the bridge. This however gives the HEC-RAS data set two more cross sections than the original HEC-2 data set. The two cross sections are placed at zero distance from the bridge, but could still cause some additional losses due to contraction and expansion of flow. The user may want to make some adjustments to the data when this happens.
3. In HEC-2 the stationing of the bridge table (BT Records) had to match stations on the ground (GR data). This is not required in HEC-RAS. The stationing of the data that makes up a bridge (ground, deck/roadway, piers, and abutments) does not have to match in any way, HEC-RAS will interpolate any points that it needs.

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