I am discharging from the site into a flow spreader and then sheet flowing across the native ground. How should I model the flow spreader so that i can input all the critical data regarding length, width, etc?
For the overland flow, you can generally use a reach with a wide, flat bottom, together with a high Manning's value appropriate to the shallow flow. For details see
As for the spreader itself, it may not appear in the model at all. If there is enough storage volume to create a dentention effect, you could model this as a pond with a weir outlet.