Create a rating curve (depth Vs. Inlet Capture) for each inlet in the sag then convert the depths to ponding elevations.  If the volumes are great enough you can even storage route it.  However typically the resulting volumes are not large enough to get any attenuation.  I have had situations where the crossection of the road will contain depths beyond 1 foot deep.  I had to acount for significant clogging on the catch basins because a 1 foot depth is our maximum depth.

Have Fun With it!

If you are simply checking extreme flood capacity, just route it like a basin, with two grated inlets leading to separate outfalls, but put some conservative tailwater on the receiving pipes, or analyze each run first and use that TW.  The grate efficiency will almost always govern though.

Here in PA, PennDOT will not allow more than ½ of one cartway spread during design flow AND you must stay 1 inch from the top of curb.  Gutter flow and grated inlets efficiencies based upon FHWA HEC-12 will normally be accepted for any review.

Immediately prior to the sump, calc. spread thickness with the following modified Manning’s: T=(Q*n/.56/Sx^1.67/S^.5)^(1/2.67), where Sx = roadway cross slope and S = Longitudinal slope, both decimals.  Check in both directions, as Long slope is rarely the same coming both ways.

Then check depth:
Weir flow depth, ft = [Q/(Cw*P)]^(2/3)
Orifice flow depth, ft = [Q/(Co*A*2g)]^(1/0.5)
I always use the greater depth for conservative design, but there is a chart in HEC12.

A common mistake:  check for composite cross slopes.  If the gutter line is spec’d for a steeper cross slope than the roadway (and they often are), there is an adjustment to the T value which greatly helps the results.