Chapter 4 - Elements of Design

4.3. Vertical Alignments

4.3.7. Minimum Profile Elevation Above High Water

One major factor in establishing a vertical profile for either a roadway or a bridge is clearance over high water or a design flood. For roadways, this is important for two reasons:

• Pavement Protection - A major factor in a roadway’s durability is minimizing the amount of moisture in the base and pavement. Keeping the roadway base as dry as possible will help prevent or minimize pavement deterioration.

• Safety - A roadway with a profile set above the design high water will keep water from overtopping the roadway. Overtopped roadways are a hazard to moving vehicles and can effectively shut down a facility when they are needed most, i.e., a hurricane evacuation route.

For bridges, prescribed low-chord clearances must be maintained to protect the bridge superstructure from unanticipated lateral forces associated with high-velocity flood waters.

 

Table 4.8. Vertical Profile Clearance Based on High Water summarizes the required high water clearances for roadways and bridges in Georgia. A vertical profile that satisfies the worst-case situation for either clearance or overtopping shall be established.

Table 4.8. Vertical Profile Clearance Based on High Water
Facility	Designer’s First Priority Roadway Base or Bridge Low Chord Clearance		Designer Must Check Shoulder Break Point Clearance or Bridge Low Chord Clearance
	Required  Clearance	Design Flood Frequency	Required Clearance	Design Flood Frequency
Interstate	2-ft.	50-year	1-ft. below shoulder break point	100-year
Hurricane Evacuation Routes	2-ft.	50-year	1-ft. below shoulder break point	100-year
Roads Designed as State Routes	2-ft.	50-year	1-ft. below shoulder break point	100-year
Roads Not Designed as State Routes
ADT:  0 – 99	2-ft.	5-year	1-ft. below shoulder break point	10-year
ADT:  100 – 399	2-ft.	10-year	1-ft. below shoulder break point	25-year
ADT:  400 – 1,499	2-ft.	25-year	1-ft. below shoulder break point	50-year
ADT:  1,500 or more	2-ft.	50-year	1-ft. below shoulder break point	100-year
Driveways	2-ft.	25-year	Shoulder break point not overtopped	50-year
Temporary Detours	2-ft.	10-year	Shoulder break point not overtopped	25-year
Permanent Bridges	2-ft.	50-year	1-ft. low-chord clearance	100-year
Temporary Bridges
Local Road with ADT < 400	2-ft.	2-year	Low-chord not overtopped	5-year
All Other Roads	2-ft.	10-year	Low-chord not overtopped	25-year

Refer to the most current version of the GDOT Manual on Drainage Design for Highways (also referred to as the Drainage Manual) , which may be downloaded from the GDOT Repository for Online Access to Documentation and Standards (R.O.A.D.S.). For roadways, designers should be familiar with the concept of culvert hydraulics and be aware that head losses associated with culverts will generally produce a headwater greater than the design flood elevation of the natural conditions. A vertical profile that provides the prescribed clearances over either the headwater of the natural conditions or the headwater created by a culvert, whichever is greater, shall be developed.

For bridges, designers should be familiar with the concept of riverine hydraulics and coordinate the bridge profile with the results of the bridge hydraulic study. As bridges will tend to generate backwater, a vertical profile that provides the prescribed clearances over the backwater created by the bridge or other nearby influencing structures shall be established. For additional information on Bridge Hydraulic guidelines, please refer to the GDOT Drainage Manual.