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Rating Curve

Rating Curve

Rating Curve

(OP)
Hello everyone,

I have attached a rating curve which i developed from HECRAS and Excel for a lateral structure (spillway) which drains into a culvert, the crest elevation of spillway is 333 meter and that of the roadway is 335 m.
I took a C value of 1.44 (SI) assuming the spillway as broad crested and C value of 2.18 (SI) assuming the spillway as Ogee crested.

I could not understand the shape of the curve however. Is it the effect of the culvert downstream? i.e. the effect of pressure/weir flow? If so how would i incorporate this in Excel?

RE: Rating Curve

(OP)
SMIAH,

but why is the shape of the curve different in excel and HEC?
at some point in HEC the C value seems to be less and the curve rises rapidly for less discharge, which is not the case for Excel.

RE: Rating Curve

Are you overtoping the crest of the inline structure at +/- 335.4 m? Or submergence effects at such discharge?  

RE: Rating Curve

Checking it again, it really looks like downstream water rising near or above the weir crest elevation produces a submerged weir condition. If so, I would use the rating curve calculated by RAS as calculating this exactly in Excel is... time consuming.

 

RE: Rating Curve

(OP)
I have attached the Excel sheet of HEC calculations, which shows that at profile 8 the the WSEL is above 335 m for the lateral structure but for the culvert it is still below 335 m so there is no weir flow.
There is no submergence effect coz the d/s water level is well below the crest(335m).
when i remove the culvert and did let the flow from the lateral structure to out of the system then the curve is just like that calculated from excel.:S

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