Hello Chris;

Having no access to the doc Ron referred to, and to get a general idea
of the importance and the region of influence of the obstruction effect
on the system behaviour I would have done the followings:

1- define the culvert's us node as a storage junction.

2- set the headloss k values for the joining links on the C cards just
as there's no obstruction there.

3- run the model, examine and spare the result.

4- decide on a range of headloss k values judged to be appropriate for
the losses at the storage junction defined (Entrance,Exit & Other
losses).

5- run the model for the assumed max and min increase in the k range
decided on in step 4 above.

6- observe the sensitivity of the model results obtained in steps 3 and
5 to the assumptions made on k values.

7- Now see if you're content with the outcome so that you can peacefully
come to a plausible final decision on the k values, especially
considering the step 6 above OR, you should seek a higher level of info
(e.g., what Ron's proposed).

8- let other list members benefit from your interesting case,if you may
post a summary of what you've found.

Best of Luck!
Mostafa Bozorg-Zadeh