§ 6.06.103. Cross drains.

1.

Hydrologic analysis: Acceptable hydrologic methods for cross drain design include USGS regression equations, the rational equation, and NRCS methodology, provided the project meets published limitations of the methodology selected.

2.

Hydraulic design: All cross drains shall be designed in accordance with the requirements of this ordinance, 23 CFR 650, Subpart A, and the National Flood Insurance Program.

a.

The design of all cross drain structures shall be based on an analysis of the design flood, base flood and greatest flood. The required design flood frequency depends upon roadway classification as described below. The base flood is defined as the 100-year flood frequency. The greatest flood is defined as the lesser of the overtopping flood or the 500-year flood frequency.

Facility Classification	Design Flood Frequency
Arterial Streets and Evacuation Routes	50-Year
Collector Streets	25-Year
Local Streets	10-Year

 

b.

All cross drains shall be designed to have sufficient hydraulic capacity to convey the appropriate design flood without overtopping or damage to the structure and approach embankments, with consideration given to the effects of greater floods. The frequency of the overtopping event shall be documented.

c.

For purposes of cross drain design, the structure shall be considered to overtop if backwater from the structure (a) reaches any travel lanes or (b) crosses a drainage basin divide.

d.

The methodology used for hydraulic analysis shall be based on the techniques presented in FHWA Hydraulic Design Series #5.

3.

Backwater: All cross drains should be designed to minimize any increase in backwater resulting from the proposed project. In the event a significant increase in backwater occurs, such increase shall not significantly affect land use values, unless flood rights are acquired.

4.

Minimum pipe size: The minimum allowable pipe diameter for cross-drains shall be 18 inches or the equivalent section for arch or elliptical pipe, unless otherwise approved by the county.

5.

Minimum length of pipe: The minimum length of pipe to be used, including end treatment, shall be the length necessary to provide adequate roadway shoulder width and adequate clear zone requirements.

6.

Minimum pipe cover: Unless otherwise approved, minimum pipe cover shall be 18 inches measured from the outside top of pipe to the top of the roadway base at any point in the roadway cross-section.

7.

Side streets: Culverts under intersecting side roads shall be considered as cross drains and shall be designed using cross drain criteria.

8.

End treatments: Cross drains shall be installed with county approved end treatments. Permissible end treatments shall include mitered ends, headwalls and "U" type mitered end walls. Headwalls shall be placed outside of the roadway clear zone and used only if restrictions exist preventing installation of mitered end sections.

9.

Materials: Accepted materials for piping and structures are those included on the approved FDOT materials list. Piping within public right-of-way shall be reinforced concrete unless approved otherwise by the county engineer.

(Ord. No. 11-01, § 2, 1-18-11)