Rating Curve

[WATS2]

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?

 

[SMIAH]

The ogee curve has such a shape because the C is increasing as the WSE is increasing.

See from page 8/24 in the document attached.

 

[SMIAH]

One could also use the equation presented in Dam Hydraulics from Vischer et Hager at pages 44-45 (1998).

Excel sheet available, and should be validated in more detail.

C will be of :
1.71 when H/Hd = 0
2.19 when H/Hd = 1
2.44 when H/Hd = 2

 

[WATS2]

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.

 

[SMIAH]

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

 

[SMIAH]

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.

 

[WATS2]

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

 

[WATS2]

here's the profile plot for culvert