Include culverts in a peak runoff rate calculation?

[Hydro_TX-QC]

I am working on the hydrology/hydraulics portion of a construction project, a school expansion including the addition of new drop off loops on the school property.

My first step is to determine my existing conditions' peak flow rate at the drainage area's final drainage point.

One of the sub-catchment areas (DA-3) that drains into the main drainage area collects runoff from a roadside ditch that crosses several (6) culverts. The ditch's runoff is then conveyed downstream via two 24" conduits 565 of if length into the main drainage area.

My question is, do i need to model the culverts to determine my peak runoff rate? It seems to me that if I don't, I may overestimate my runoff rate. Are there other risks involved? What's the common practice? What are some reasonnable assumptions I can make to avoid doing so?

The total project drainage area is approximately 50 acres. The ditch's drainage area is 9 acres.

Thank you for reading.

 

[gbam]

It is unlikely that the culverts and ditch will impede/attenuate the runoff enough to change your computed flow rate. Typically, the only time we address items in the watershed is if there are regional or large detention basins; which will attenuate the inflow to the basin. I would say ignore them. Remember that many of the hydrologic methods use lumped parameter approach so there are assumptions in that approach. If you are using a FLO-2D or RAS-2D then you may want to code the culverts and ditch because these are 2-Dim models. Hope this helps.