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Reach FAQ

This document discusses general issues about the use of a "reach" within HydroCAD.

What is a reach?

A reach is used to perform an independent hydrograph routing through an open channel based on normal Manning's flow, or a user-defined rating table.  A reach can also be used to model overland flow through a subcatchment.

What about flow through a pipe?

Although you can use a reach to model a pipe, this option is recommended only in very limited situations. (i.e. open-channel flow with no inlet restrictions.)  For other pipe modeling options please read about pipes.

Are there other ways to model a channel?

1) If the channel lies within a subcatchment, you may want to model it as a flow segment within the time-of-concentration. This works best when the reach receives runoff along it's entire length, but if you have a separate inflow hydrograph at one end, a separate reach routing is usually necessary.

2) If the end of the channel is blocked with a weir or other outlet control (so that a level-pool is created) you should model the channel as a pond.  Example here.

3) If the reach is completely "flooded out" by a downstream node, you may want to connect the upstream and downstream nodes directly, rather than using an intermediate reach.  This will allow automatic pond-to-pond tailwater capability, which would otherwise be "broken" by the intermediate reach.

Note that many models will not use a reach at all.  See the modeling tips for details.

How is the rating table determined?

Reach flow is based on Manning's equation, or on a user-defined rating table.  This means that the reach must have a consistent cross-section and constant depth of flow along its length, and must not be subject to tailwater effects.  If the cross section is variable, or there are tailwater effects, a different modeling approach may be required, such as HEC-RAS.

What kinds of shapes can be modeled?

A reach allows the direct entry of rectangular, vee, trapezoidal, parabolic, and circular shapes.  For any other geometry, you can enter a custom rating table of discharge vs. depth, or wetted-area vs. depth.  Starting with HydroCAD-8 you can also enter an arbitrary cross-section, with the ability to specify a unique Manning's value for each segment.  (See screen at right)  This approach can be used to model compound shapes, such as a low-flow channel cut into the bottom of a larger channel, or a main channel with a flood plain.

How do I model a channel with check dams?

Under high flow conditions, check dams can be accommodated by using a reach with a higher Manning's value.  For low-flow conditions, using a series of ponds may be more accurate.  Details here.

How is the routing performed?

For short reaches with minimal kinematic effects, the default Storage-Indication routing procedure is generally sufficient.  For longer reaches, HydroCAD can add an option time lag (translation) to approximate the kinematic effects.  This is done by selecting the "Stor-Ind+Trans" procedure.

With the introduction of HydroCAD-8, reach routing can also be performed with the Muskingum-Cunge routing procedure, as used in the current Win-TR-20 release.  For details please read about MC routing.

Why is the outflow less than the inflow?

The storage effects of a reach will normally cause some reduction in the peak flow.  The effect is usually very small for short reaches, but can be significant for a long reach, low slope, and/or high Manning's value.

What do the calculation messages mean?

[52] Hint: Reach Inlet/Outlet conditions not evaluated is generated for any reach.  This is a reminder that a reach always operates under normal-flow conditions, without consideration of possible inlet restrictions or tailwater.  If you wish to consider entrance losses, pressure flow, or other conditions for a pipe, it should be modeled as a pond with a culvert outlet.  You can convert a pipe reach to a pond/culvert by using the right-click menu on the routing diagram.

[55] Hint: Reach peak inflow is 1xx% of Manning's capacity means that the peak inflow exceeds the rated normal-flow capacity of the channel.  However, the storage capacity of the channel was sufficient to contain the maximum volume (assuming uniform depth along the channel), so this may be an acceptable condition.

[63] Warning: Exceeded reach INLET depth by x.x' @ x.x hrs means that the tailwater has exceeded the depth of flow at the reach inlet, and that tailwater or even reverse flow conditions must be considered.  The message indicates the maximum reverse-head condition, and the time at which it occurs. (See illustration below)

Important: The previous messages indicate that the user must evaluate the situation and take any necessary action.  Otherwise, the reach routing will continue to be performed based on normal, open-channel flow.  The program does not make any special adjustments when these messages occur.

[76] Warning: Reach detained x.xx AF means that the entire volume of a pipe reach has filled with water, and the excess volume is detained in an imaginary storage area at the pipe inlet until the pipe is able to accept the excess volume.  This results in a flat-topped outflow hydrograph, with the peak flow reduced to the pipe capacity, and the overall volume preserved (conservation of mass).  To accurately modeling this situation, you should use a pond with culvert outlet, instead of a pipe reach.  (You can perform this conversion using the right-click menu on the routing diagram.)

What do the tailwater messages mean?

HydroCAD automatically checks for tailwater conditions during the routing, and will issue a warning message if a possible tailwater condition is detected at any time.

The message always indicates the "worst case" condition that occurs during the routing, as shown in the illustration at the right.

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