The top of the dam is important when modeling conditions that may cause water to flow over the top of the dam. Dam tops can be added to reservoirs that use the outflow structure routing method. These represent the top of the dam, above any spillways, where water goes over the dam top in an uncontrolled manner. In some cases a dam top can be used to represent an emergency spillway. Up to 10 independent dam tops can be included in the reservoir. There are two different methods for computing outflow through a dam top: level or non-level.
Level Dam Top
The level dam top assumes flow over the dam can be represented as a broad-crested weir. The calculations are essentially the same as for a broad-crested spillway, but they are included separately for conceptual representation of the reservoir structures.
(8-6)
where:
Q = Discharge over the dam top.
C = Discharge coefficient; accounts for energy losses as water flows over the dam. Typical values will range from 2.6 to 3.3, depending upon the shape of the dam.
L = Length of the dam top.
H = Upstream energy head above the dam top.
The crest elevation of the dam top must be specified in order to allow for the determination of head.
The length of the dam top should represent the total width through which water passes, excluding any amount occupied by spillways.
The discharge coefficient accounts for energy losses as water approaches the dam top and flows over the dam. Depending on the exact shape of the dam top, typical values range from 1.45 – 1.84 in SI units (2.63 – 3.33 in US Customary units). 1.10 to 1.66 in System International units (2.0 to 3.0 US Customary units). Civil Engineering Reference Manual says 1.45 – 1.84 2.63 – 3.33
Non-Level Dam Top
In order to model a non-level dam top, the structure can be represented by a cross section with eight station-elevation pairs. A separate flow calculation is carried out for each segment of the cross section, and the modeler can specify different coefficients. The broad-crested weir assumptions are made for each segment.
An eight point cross section should be developed to represent the dam from one abutment to the other but excluding any spillways. Multiple dam tops can be used to represent the different sections of the dam top between spillways. The cross section should extend from the dam top up to the maximum water surface elevation that will be encountered during a simulation.
The same discharge coefficient value is used for all segments of the dam top. Typical values range from 2.6 to 4.0 depending on the exact shape of the dam top. Users enter the coefficients. Check the RAS manual for recommendations.
The tailwater is calculated.
Pool – dam elev
Then tailwater and recalculate pool-tailwater and if the head is smaller, you will go with the smaller flow. No reverse flow is allowed.