Check RAS Routine Details
The NT Check program provides the warning if any of the following requirement is not met:
- The left overbank (LOB) and right overbank (ROB) 'n' values: 0.035 - 0.20
- The channel 'n' value: 0.025 -0.10
- Contraction Loss Coefficient (CLC) is 0.3 and Expansion Loss Coefficient(ELC) is 0.5 at sections 2, 3, and 4 of the structure.
- CLC is 0.1 and ELC is 0.3 at all other cross sections.
- Channel 'n' values at sections 2 & 3 of the structure is not less than the channel 'n' values at sections 1 & 4 respectively, by 0.005.
The XS Check program provides the warning if any of the following requirement is not met:
- Length across junction should not be zero.
- Both the LOB and ROB should not be greater than channel distance.
- All the distances, i.e. LOB, ROB and channel should not be zero.
- The ratio of ROB & channel distance and LOB & channel distance should not exceed 5:1.
- Between any two cross sections, all of the following should not occur simultaneously:
- -Change in velocity head exceeds 0.5.
- -K is outside the range of 0.7-1.4.
- -Water depth changes by a factor of 1.1.
- -Floodplain topwidth varies by a factor of 2.
- -Distance between two XS is more than 500 feet.
- The program identifies if any of the following are used:
- -Normal ineffective flow.
- -Multiple (blocked) ineffective flow.
- -Blocked obstruction (B)
- -Critical Depth (C)
- -Divided Flow (D)
- -Extended cross section (E)
- -Known or changing water surface or energy elevation (K).
- The cross section upstream of a critical depth XS should be at a distance less than twice the Top width of critical depth XS.
- Discharge should not decrease in the downstream direction.
- Discharge should vary along the profile length.
- Flowrate cannot be changed in the middle of the structure, i.e., same discharge should be used upstream and downstream of the structure.
- A value of zero discharge cannot shift between LOB and ROB between any two consecutive XSs.
- Average conveyance method is usually used. If other method is used and channel is more than 500 ft in length, the program will issue warning.
- The program will provide information on the downstream and upstream boundaries (Known water surface, energy slope, critical depth, elevation-discharge relationship).
The structure check program provides warning if any of the following requirement is not met:
- The distance between section 3 & upstream face of the structure and the distance between downstream face of the structure & section 2 should be less than the height of the opening.
- The flow method selected through structure should correspond to the type of flow
occurring through the structure.
Following examples provide some guidance for choosing flow methods for high flow conditions:
- When the bridge deck is a small obstruction to the flow, and the bridge opening is not acting like a pressurized orifice, the energy based method should be used.
- When the bridge deck and road embankment are a large obstruction to the flow, and a backwater is created due to the constriction of the flow, the pressure and weir method should be used.
- When the bridge and/or road embankment is overtopped, and the water going over top of the bridge is not highly submergedby the downstream tailwater, the pressure and weir method should be used. The pressure and weir method will automatically switch to the energy method if the bridge becomes 95 percent submerged. The user can change the percent submergence at which the program will switch from the pressure and weir method to the energy method. This is accomplished from the Deck/Roadway editor in the Bridge/Culvert Data editor.
- When the bridge is highly submerged, and flow over the road is not acting like weir flow, the energy-based method should be used.
- Culvert entrance coefficients should be specified properly.
- Range from 0.2 to 0.9 for pipe-arch and pipe culverts.
- Range from 0.2 to 0.7 for box culverts.
- 0.5 for sharp edged culvert entrance with no rounding.
- 0.2 for well rounded entrance.
- 0.3 to 0.5 for ConSpan culverts.
- Culvert exit coefficients should be 1.0. Although it can vary from 0.3 to 1.0, CheckRAS will provide message if it is not set to 1.0.
- Culvert roughness coefficient 'n' should be specified in accordance with the RAS reference manual guidance.
Following range of values are specified in the RAS reference manual. Please refer to the RAS reference manual for more details.