Minimum Top Establishment
The roadway embankment elevation should be measured at the point of the low shoulder (crown line), as shown in Figure 12-1.
Figure 12-1. Reservoir Freeboard Requirement
The minimum top of embankment elevation should be set no lower than the elevation created by the higher of the following conditions:
- the 2% AEP reservoir surface elevation for the entire reservoir, plus a minimum freeboard of 3-feet,
- the elevation of the 2% AEP flood backwater curve as depicted in Figure 12-2, plus a minimum 3-feet freeboard to the low chord elevation of any structure,
- the elevation of the 0.2% AEP flood backwater for interstate highways and evacuation routes.
At times the crossing may be located on a tributary to the reservoir. Structures so located may at times operate independently from the reservoir, in which case the water surface elevation should be determined according to Chapter 9, Section 4,
. In cases where the crossing is located on a tributary, the minimum top of embankment should be set to the higher of the reservoir requirements above and the results of the procedure in Chapter 9.
Figure 12-2.