The water surface elevation at the start of the modeling is always a critical parameter in flood routing, and the volume of storage below this elevation will not affect the routing of a new surcharge inflow.  As the flood wave recedes, reservoir pool will gradually drop to the spillway elevation unless there is another means of discharge that is capable of drawing the pool down below the spillway crest.  It's important to have an appropriate "stage-storage" relationship for the reservoir that encompasses whatever range of reservoir elevations the flood will induce.  An appropriate spillway discharge rating curve ("stage-discharge") is also critical input. But unless the routing is being extended in time long enough to allow the pool to drop below the starting elevation, storage volume that exists below the initial water level does not affect the flood wave. 

Hi  Mohamed Elfeki 

Reservoir volume cannot affect the routing of a flood through a reservoir. The critical parameter is the surface area of the reservoir based on the basic routing equation

   Q*dt = A*dh

where Q = spillway discharge, A = surface area of reservoir, dh = change in stage or head, dt = time increment.                                                                                               The larger the value of A the lower the peak of the outflow and the greater the time lag (ie the time interval between the inflow and outflow peaks). Q depends on dh (height above spillway crest) and not on H the overall depth of water. In addition A is a function of dh in a variable x-section dam. The reservoir volume only affects the time before  commencement of  routing because routing can start only after the reservoir becomes filled.

Thank you all. Your answer explained my issue. Accordingly, I did deducted the reservoir capacity from the inflow and the rest of the hydrograh is routed over the spillway. This is because the dam reservoir was empty before the flood comes to the dam (a usual case in arid zones). See my results in the link below:

https://www.researchgate.net/publication/274927405_Reservoir_Routing_in_Arid_Regions

Research Reservoir Routing in Arid Regions

Dear Dr.Amro ElfeKi

Happy to get in touch again with you. About your question, "why the reservoir routing method does not account for the reservoir capacity". I think that the reason is the primary purpose of the method to get the maximum expected values for the flood flow rates downstream the dam. This goal can be achieved only by passing all the flood wave volume downstream. This requires full reservoir storage coincide with the spillway crest level at the initiation of the flood. Consequently no need to take the reservoir capacity through the calculations.

But, I agree with you that it is logic to consider the reservoir capacity when routing the flow at normal condition or for the case of partially full reservoir.

Thank you very much Dr. Hosaam. I am happy to hear from you. Please send me your e-mail and phone numbers.

Amro

A further correction that may be needed depending upon the situation is to take account of the surface gradient in the reservior if it is long - as may be the case with on-line flood storage reserviors.  In Muskingum routing this is the wedge storage effect.  I recently considered the performance of a flood storage scheme in a flood where this factor has some influence - it is likely to have the greatest effect on the rising limb of the flood. 

Thank you Paul

I understood that the wedge storage in Muskingum method is similar to what we are talking about. Which means the reservoir routing method assumes that there is base flow in the river. However in arid region we have empty reservoirs before the flood arrived.

see these images about a dam in KSA:

https://www.researchgate.net/publication/266136604_Stilling_Basin_Side_View?ev=prf_pub

https://www.researchgate.net/publication/266136608_Toe_of_Stilling_Basin_VIEW_1

https://www.researchgate.net/publication/266136596_End_Sill_of_Stilling_Basin?ev=prf_pub

https://www.researchgate.net/publication/266079473_Typical_Dam_Spillway_Crest_in_Arid_Zone_%28Wadi_Fatima_Dam_spillway_in_KSA%29

You may see more on the site,

Regards,

Amro

Data Stilling Basin Side View

Data Toe of Stilling Basin VIEW 1

Data End Sill of Stilling Basin

Data Typical Dam Spillway Crest in Arid Zone (Wadi Fatima Dam spi...

Happy to get in touch with you. About your question, "why the reservoir routing method, I recommend for you to read the reference (Hydrology,Principles,Analysis.Design, by Raghunath,H,M.2006. if you don't have a copy ask me later to upload the reference to you, my regard

Thank you dr. Ghazi.

I am familiar with the book. Anyway i shall have a look at it.

Amro