FRA

Very topical these days, FRA stands for flood risk assessment.
Our work is very much to gather the information about the ground as it stands at this moment; contours, spot heights, ridges, valleys, water courses and man-made features like roads and buildings. I continue to learn with interest from the experts who are the end users of our maps and data. They can be architects, engineers or like today a flood risk expert.
I can’t really say much about our specific job and I doubt this photograph offers any clue to location but this image may form part of our deliverables. We have already measured this culvert; its diameter inlet and outlet position and “invert” level. Invert level is simply the height of the bottom of a pipe above some datum ie the lowest point that water can run at along the internal base of the pipe. It is not an intuitive term and I don’t know its origins. We have also measured the road level above so if the pipe was running full and started to back up the effect of the overtopping to the road could also be accounted for.
Apparently the hydrologist will run our data through a flood modelling program –he talks about the one in X hundred year event- but he still needs to influence how the application will interpret the model by adding other parameters and this is what I learned today.
Pipes and even stream beds of the same dimensions and gradients will not necessarily allow the passage of water with similar ease. The roughness of the surfaces will set up turbulence and resist the flow. Maybe it’s obvious when you think about it. He specifically asked for the photo. This image will allow him to take an educated judgement on how this corrugated pipe may influence an answer.
A slight throttling of the water could be good or bad. If you live downstream of the partial obstruction you may welcome the restriction in flow as it may limit flooding. If you are above it then you are almost certainly wanting the water to discharge as freely and as quickly as possible.