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Protecting a slope from a nappe

Protecting a slope from a nappe

(OP)
I have a situation along a shoreline where an existing 8-inch pipe ends at bulkhead, approximately 3 feet above grade.  The grade below is a rip-rapped slope @ roughly 20%.  An engineer wants to upsize the pipe to a 12-inch and increase the amount of runoff discharging to the outfall.  

Given that the outfall is tidally influenced and sometimes submerged, the engineer has designed his system based on high tide and determined that no rip-rap improvements are necessary.  I've argued that for erosion control the system also should be modelled during low tide situations when both the pipe and rip-rap slope are exposed.

Using the engineers design storm and performing my own calculations, I get approximately 5.1 cfs at 6.6 fps discharging from the 12-inch pipe.  Since the discharge is above grade, we're looking at vertical drop situation and would need to consider the nappe.  The head scratcher is that the landing surface is on an incline.  There is a desire to keep rip-rap as the protection of choice, however I worry about it becoming dislodged and shifting from place.   Any thoughts?

RE: Protecting a slope from a nappe

angular riprap on 5:1 slope should perform well. This is not a lot of flow and I would think 8 or 12 inch riprap would be sufficient.

RE: Protecting a slope from a nappe

About two years ago I downloaded "Standard for Soil Erosion and Sediment Control in New Jersy, New Jersey Soil Conservation Committee,1980 as copied from the Rhode Island Erosion and Sediment Control Handbook".   It is a 17 page PDF document and is very instructive--an easy read. I did not know much about RipRap design when I got involved in a project and this document helped a lot.

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