ADEQUATE CHANNEL ANALYSIS (natural or existing man-made channels) :

A. Extent of analysis:

 Text Box: Extend adequate channel analysis all the way to the channel of the major floodplain, with a minimum of 3 cross-sections analyzed, 50’ apart.   Normally, from the minor floodplain to the major floodplain, visual inspection, narrative explanation, and 5’ topography can be used if there are no erosion problems or obstructions present.

B. Proving there’s a well-defined receiving channel:

Text Box: In order to prevent meandering flow paths which can lead to erosion, a min.  top width- to-normal depth ratio of 35:1 must be maintained throughout the channel cross-sections.  An exception to this rule may be a compound channel section (please see item I.C below) where the majority of the 2-year storm is contained within a well-defined channel within the broader, flatter cross-section and the velocity of flow is not erosive.   Color photos may help to depict these features of natural channels.

C. Shape of natural cross-section:

Text Box: Do not approximate natural channels by using prismatic sections when they have highly irregular shapes– especially if they are compound channel sections.  Define the actual geometry for those channels using surveyed cross-sections.

D. Characterize the channel.

Text Box: Derive the roughness coefficient, “n,” by using the range of values in Appendix 7D-1 in Chapter 7 of the VDOT Drainage Manual or the “survey of the channel” method found in Chapter V of the VESCH (use primarily with large, irregular natural channels with variable roughness).

E. “n” values and allowable velocities for selected grass-lined and riprap man-made channels:

* Lower by 25% in highly erodible soils.
** Riprap velocities adapted from Ishbash equation / curve. 

F. Erodibility of soils must be considered in the determination of allowable velocity for grass or earthen channels.

Text Box: Determine the erodibility, “k”, of the soil comprising the channel based upon the soils map / County soils information.  If k ≥ 0.36, a 25% reduction in the allowable velocity must be applied.

G. Transition from manmade channel or pipe system to natural receiving channel .

Use the first two strategies to safely reach the receiving channel if no defined channel exists all the way to the receiving channel (or normal water surface elevation for perennial streams) and construction of the same is not possible or desirable (e.g., pipe exits into flat floodplain area, or construction of channel would impact wetlands).

 

BACK