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Hec-RAS vs. WSPGW
By Dennis W. Jackson, P.E., Lead Software Architect for Joseph E. Bonadiman & Associates, Inc.
Overview
Hydraulic analysis of storm drains is a task that, since the 1960's, engineers have used computer pro-grams to perform the thousands of calculations required to determine channel flow characteristics. Hand calculations are too time consuming, and often the study requires re-accomplishment, because the design needs to be modified due to a change in flow rate, channel size, or some other factor. To speed the design process of a storm drain system, many computer programs are available to calculate the projected water surface profile, to make sure the system will properly function in a 100-year storm, or other required storm events.
The Water Surface Pressure Gradient (WSPG) program was originally developed by the Los Angeles County Flood Control District in the late 1970's as program number F0515, for the IBM® 370 mainframe computer. This program, was compiled to work as a DOS program on the personal computer in the late 1980's and has been adopted by nearly all Southern California county Flood Control Districts, and some CALTRANS , (California Department of Transportation Offices), as the primary computational tool for analyzing improved storm drains, either open channels or closed conduits. The Federal Emergency Management Agency (FEMA) has included the WSPG program in its list of accepted numerical models for determination of water surface elevations. In 1997 a Windows® 95/98/NT version of the WSPG program became available, and is named WSPGW, replacing the older DOS version, with many added features including multiple pipe channels, Metric or English unit options, and a drawing file output.
The U.S. Army Corps of Engineers Hydrologic Engineering Center has, over the years, developed many hydrology and hydraulics programs such as the HEC-2 Water Surface Profiles program. This has been used for many years to determine the flood levels used in the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps. This program is now being replaced by the HEC-RAS (River Analysis System) hydraulic program which is specifically design to work inside the Windows® 95/98/NT operating systems. The HEC-RAS program uses the same proven HEC-2 hydraulic analysis procedures, and, the Hydrologic Engineering Center have added many new features, thanks to the added computing power available with Microsoft Windows®.
Which Program is Best?
Why choose between HEC-RAS or WSPGW programs? The HEC-RAS (and HEC-2) computer programs have the capability of analyzing flood plains, bridges, culverts, scour design of bridge piers and abutments, ice flow, etc. However, the WSPGW program, was designed to analyze smaller, and sometimes more complex hydraulic problems, such as that shown in these photographs. The photos show a small portion of a 1.5 mile long improved storm drain, named Bradley Channel, located in Riverside County, California. The storm drain has long sections of either closed conduit, or open channel, with many junctions adding to the flow in both types of channels. Also, there are a number of bridges. The analysis shows pressure flow at the system outlet, with both super-critical and sub-critical flow regimes in different sections of the system.
Only the open channel portions of this channel could properly be modeled with HEC-RAS, or HEC-2. In the case of HEC-2, several data files would be required for sub-critical and super-critical flow analysis. The HEC-RAS program now allows a mixed profile analysis. However, it is impossible to model the long closed pipe sections that include junctions under either open channel or pressure flow.
The WSPGW program is designed to perform this type of analysis. It calculates the total water surface profile for the channel in steady, gradually varied flows, in the open and closed portion of the channel, with regular or irregular sections, in any sequence of open-channel, sub-critical, super-critical, or pressure flow conditions.
Computation Theory
The WSPGW program is based upon solving the Bernoulli equation for the total energy at each section using Manning’s formula for friction loss between the sections in a channel reach. The open channel flow procedure uses the standard step method. Confluences and bridge piers and wall entrances and exits are analyzed using the pressure and momentum theory.
If under pressure flow conditions, the Bernoulli formula takes into account minor losses from bends, angle points and manhole losses. After computing a lower stage profile from the system head-works down-stream to the system outlet (super-critical flow regime), and, an upper stage water surface profile in the (sub-critical flow regime), WSPGW will automatically create a composite water surface profile for the system.
Any hydraulic jump present will be located by equating the total energy from both stages at various sections. Also, in the reach analysis, the program automatically calculates the water surface profile at intermediate sections when the velocity head changes by more than 10%.
| Description |
WSPGW |
Hec-RAS |
| Multiple Profiles - Different Flows |
Yes |
Yes |
| Normal / Critical Depths, Froude |
Yes |
Critical Depth where supported, no normal depths. |
| Super-Elevation of Water Level due to a curved channel |
Yes |
No |
| Roughness Factor "N" |
Composite required at each section |
Varied Options with separate over-bank "N" values |
| Automatic Flow Regime determination |
Yes |
If "mixed profile" option selected |
| Junction Analysis |
Yes |
Yes |
| Hydraulics of tributary streams and laterals |
Only at junction entrance for force analysis of main stream |
Yes |
| Bridge Hydraulics |
Low flow and/or pressure flow |
Low flow, pressure and weir flow |
| Bridge Pier Analysis |
Pressure and Momentum |
Various Options |
| Culvert Analysis, wall entrance, wall exit, multiple pipes, pressure/non-pressure flow |
Yes |
Yes |
| Closed Storm Drain Hydraulics |
Yes |
No |
| Over-bank Flooding |
No |
Yes |
| Error and Warning Messages |
Yes |
Yes |
| Hydraulic Jump Location |
Yes |
Yes if "mixed flow" option used |
| Average Friction Slope |
Arithmetic Mean |
4 Options |
| Metric/English Units - input and output data |
Yes.
4 Input Options with flow rates as small as gallons per minute or liters per second.
Output option in Metric or English. |
Yes.
Option to "convert" the input data file to English or Metric Units. No output option unless input data is first converted by the program. |
| Interpolated Cross Sections |
Automatic when Velocity Head Changes by 10% in reaches |
Program requires user to specify added cross-sections, however, it will calculate the input data automatically. |
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Data Entry
As may be seen from the program comparison list, the HEC-RAS program has a great number of options for data entry. Therefore, the engineer must have a good under-standing of all of the options and correct procedures to be used, or the answers may be incorrect.
On the other hand, the WSPGW program has a more straightforward data entry with fewer options. The user of the program enters the data starting at the system outlet, then progressing upstream in a logical manner. The only options are a Reach, Transition, Junction, Wall or Bridge Exit, Wall or Bridge Entrance, and, at the end of the study; the System Head-works with flow rate , and the number of profile options.
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