Dual "major" and "minor" drainage systems

Jon,

I simulate surcharging pipes that run down streets and other urban surfaces by adding a few feet to my true ground elevations (EXTRAN D1 lines) and adding elevated channels (EXTRAN C1 lines). The elevated channels often run parallel to my pipe/channel system, but not necessarily. I use broad, shallow channels with adjusted Mannings n's to simulate this street flooding. (I also document profusely with comment lines) My goal is to try to minimize surcharge losses (conservation of mass) during extreme events, which can be checked by noting the continuity error and the surcharge loss error, or by comparing event hydrograph volumes.

As for equivalent pipes for manholes, I let major manholes be represented by my nodes. In certain sections, I may use equivalent pipes, but only increase Mannings n values if there are important entrance or exit losses or other energy losses (like bends). In SWMM 4.4 (EXTRAN), these losses can be entered directly.

I would be curious to hear what others have to say about this approach.

Don Waye
NVPDC

I am an XP-SWMM user and am in the process of developing a major/minor drainage system for my model. I am uncertain as to how I can model the surcharged water exiting my manhole structures and entering my roadways.

The SWMM manual recommends using equivalent pipes for manholes. It goes further to suggest using orifices to simulate inflow constrictions.

To account for the head/friction losses, do I simply adjust my orifice discharge coefficient? The documentation is very slim in this area.

Furthermore, should the inflow of water from my streets and into my pipe network be modeled differently than the water surcharging from my pipe network and into the streets. ie. isn't node outflow modeled differently than inflow.

Has anyone set up this type of model that may have some insight?

Regards,
Jon Marshall

I was playing with dual drainage system in XP. I gave up as I found it extremely difficult, partially because of my lack of experience. Maybe I compare lemons and oranges, but have you ever seen OTTSWMM dual drainage system?

Jon,

We have modeled street flooding similar to Don's method, by adding wide trapezoidal channels with pavement "n" values at the manhole rim elevation. (You must therefore increase the node's ground elevation to be higher than the rim elevation so you don't lose water from your model if the manhole floods.) The flooding channels continue downslope to the next storm inlet, which may also be flooding. You will need to check topography at sumps to find out where your flooded manhole water will go (e.g. over the curb and into a stream or pond). Interestingly enough, in some cases (using EPA SWMM) when we have added flooding channels to manhole systems, the manholes have no longer flooded, and the flooding channels were dry!

In XP SWMM, if you open the conduit data window to enter/edit data, in the lower right corner is a conduit factors check box, which will allow you to enter entrance, exit, and expansion/contraction losses, etc. Typical values we use are 0.5 for entrance losses in flooding channels, 0.1 for entrance losses to pipes in manholes, and 1.0 for exit losses.

For bringing street flooding back into your pipe system, you have several options depending on the level of detail you wish to model, and the maximum number of nodes/links your XP license will allow. If you want coarse detail, you can bring your flooding channels right to the next manhole at the rim elevation. If you want more detail to model flow picked up by street inlets, you can bring your flooding channel to a node for the inlet throat/grate, and have flow enter through an orifice (bottom- or side-orifice), through a weir (specify length and RL1 and RL2 elevations using slot geometry), or both using a multi-link. This multi-link can go right to your manhole, or to another node simulating the inlet box, which will be connected to the manhole by a pipe.

All of your nodes in XP (and EPA) will have a default surface area - in XP EXTRAN mode, see Junction Defaults under Special, Job Control. Most modelers set the node surface area comparable to a 4-foot diameter manhole. You can specify surface area at individual nodes by checking the storage box and entering square footage if constant (i.e. for junction boxes), or entering a depth-area (acres) rating curve for detention basins. Unfortunately SWMM is clumsy in modeling both street inlets and detention basin outlets - your model will get very cluttered with nodes and links.

Best of luck, and happy SWMMing!
Bill Walker
hausbrau@erols.com