Land subject to flooding

[Kenneally]

I have a piece of property approximately 50-acres which is essentially a valley that slopes down to a culvert (no standing water). I know that under a 100-yr storm event the property is subject to flooding (approximately 20 ac of the 50 ac is underwater).

My question is related to the size of the ponded water and how it effects the calculated time of concentration and the CN.

Because the ponded area is fairly large (40% of the subcat.) How do I look at the time of concentration? Is it from start to culvert? or Start to edge of flooded area? or somewhere in between?
...and as far as the CN value for the land which is pasture that becomes flooded should the pasture CN value be adjuseted to account for the standing water?

 

[LHA]

CN and Tc will not be affected. Your point of study for the inflow hydrograph is the inlet of the culvert, the point of study for the outflow hydrograph is the outlet of that culvert. Calc the runoff from the 30-acre contributory subarea as pasture with actual Tc, plus a 20-acre impervious subarea with Tc = 5 minutes to the 20-acre storage volume, then route the stored volume through the culvert.

 

[bltseattle]

I agree with LHA regarding the use of CN and TC and modeling part of the site as impervious. I would consider a level pool routing to analyze the ponding flow attentuation to then determine flows downstream of the culvert. UNLESS, there is overtopping, in which case it may be appropriate to use the unrouted flows downstream of the culvert (simpler analysis, higher flow rates).