For those that are using 2010 storm sewer extension here is some helpful information that I found for different types of design methods.
Analysis w/ Design
After the flows have been computed, this option analyzes and designs starting downstream and working upstream, and uses the existing data for the calculations, such as pipe sizes and invert elevations. Data options that are set to 0, are designed by Hydraflow Storm Sewers Extension. First Hydraflow Storm Sewers Extension calculates the pipe size based on the specified design velocity, (Q/V = A). Next it computes the corresponding invert slope based on Manning's Equation, and with the new pipe area. Based on the settings in the Design Codes, it sets the downstream crown (or invert) equal to the next downstream line's crown (or invert). If the inverts have been set to 0, then Hydraflow Storm Sewers Extension begins at the downstream end of Line 1, sets the crown elevation equal to the starting HG, and proceeds upstream. Hydraflow Storm Sewers Extension does not assign pipe sizes outside the range you specify in the Design Codes.
Enhanced Modeling System
Traditionally, storm sewers are designed to flow full because the pipes are sized to accommodate the design flows and velocities, and there are no conflicting numbers, so the time of concentration (Tc) matches the flow rate (Q).
When analyzing existing systems, the actual velocity is not known until the hydraulic grade line (HGL) is computed. The computed Qs and HGL are based on assumed pipe velocities. When the actual velocity is different from the assumed velocity, the computed Tc is incorrect and the computed Q and HGL are incorrect. Enhanced Modeling System solves the problem by directing Hydraflow Storm Sewers Extension to recompute the hydraulic grade line, based on actual flow rates and actual Tc. Hydraflow Storm Sewers Extension computes three system iterations so that the computed Tc matches what was assumed with reasonable accuracy.
Hydraflow Storm Sewers Extension first computes the HGL using the Tc based on the design velocity. Next it computes the storm sewer system a second time using Tc based on actual velocities. These new velocities are still incorrect because they are based on the original HGL calculation, however they are more accurate than those used on the first trial. Several system iterations cause the Qs, Tcs and resulting HGL to converge to correct values. Three iterations are the most practical balance between accuracy and the time required to produce the results.
With this option, Hydraflow Storm Sewers Extension resets all pipe sizes and invert elevations to zero and redesigns the entire system. Pipes are sized first based on the specified minimum/maximum pipe sizes and design velocity. It then computes the invert slopes using Manning's Equation and the computed pipe size. Next it sets the invert elevations as high as possible but always below the minimum cover specified. At junctions, the outlet invert elevation is fixed by the lowest Inlet/Rim elevation. This option requires all lines to have Inlet/Rim elevations.
When selecting pipe sizes, Hydraflow Storm Sewers Extension computes the required area based on the design velocity. All pipe sizes are within the range specified in the Design Codes.
This option has some design limits. For example, if a pipe size has been set to 0 for design, Hydraflow Storm Sewers Extension cannot size it because it does not know the flow rate, Q. It sets the pipe size equal to the minimum size specified in the Design Codes. Pipe inverts and slopes are set by the same methods as in the other calculation options, using Manning's Equation and the design velocity.