Length of Need for guardrail next to creek

[Alex R]

Hey all,

I've been trying to determine the length of need (LON) for a SBGR on an undivided highway which is adjacent to a creek in an urban setting. Here's the location

As per my roadside design standards (provided below), LON requires the distance from the edge of travel way to the back of an area of concern (B) and the distance from road centerline to the back of an area of concern (B').

I'm confused on how B or B' are defined for a water body like this. What does "back" refer to for measurement purposes?

 

[CarlB]

A waterbody does not fit the "length of need" scenario shown by the figure, as it is not a hard/fixed object. The "back" is the far point of the object from the roadway, that is to be shielded with a guardrail. In the case of a waterbody or steep slope, extend guardrail as needed to keep errant vehicle from reaching it. Isn't that how existing guardrail functions?