Erosion Control on Beach Outlet

[nzo]

hi, i was wondering if anyone had some advice on erosion control on a beach where a newly constructed ditch/swale will be constructed through sand.

the situation is this: a drain pipe will be installed down a bluff with an approx. 30% grade. the top of the bluff is about 14 metres (45') higher then the beach below. once at the bottom of the bluff, the pipe will change grade to about a .25%, not 25%, then outlet into a newly constructed small ditch to convey it from the bottom of the bluff into the lake. the ditch will be contructed through a sand dune, a fairly high, large one so my concern is if the sand will maintain the section of the ditch with the high velocity flows going through it. it obviously wouldn't without any protection, rip-rap or cable concrete or heavy vegetation but even with any of that protection, is wind filling in the ditch a big concern? i haven't designed a ditch through sand before but with the heavy winds along beaches and waves, i would think that the ditch closing in on itself and carving its own path to the lake is a possibility by infringing on adjacent properties along it's way. apparently though the 100 year water level isn't higher then the proposed streambed elevation so i'm not too sure if upsrush or waves are a big concern.

also, any suggestions on energy dissipators? i've seen some variations of the concrete blocks at the outlet to help slow the water down but not much else.

i've contacted the lake authority and they didn't have too much info, surprisingly.

does anyone have any links or design guides for such thing? thanks for any help and sorry for the long post.

 

[civilperson]

Try a flat bottom concrete paved ditch with 1:5 side slopes with the width and depth to maintain 3 fps velocity. The transition length from pipe to ditch should be 3 x top width of the ditch with bagged concrete slope paving and rough broom finish bottom. The energy dissipation will occur in the form of a hydralic jump, (supercritical flow to sub-critical velocity), in the transition region. The sand fill from waves and wind will be self correcting when large volumes of flow scour the ditch back open.

 

[cvg]

I've done something similar before and used a plunge pool for energy dissipation. Transition from pipe to concrete channel is not recommended as the hydraulic jump will be quite high. You may want to read up on energy dissipaters - check out Engineering Monograph No. 25, Hydraulic Design of Stilling Basins and Energy Dissipators, United States Department of the Interior, Bureau of Reclamation for several types.

 

[blueoak]

I am not sure you want to build a channel so much as some barriers to meandering. Looking at outlets on coasts a lot of time you see buried concrete walls pretty far apart. I think with a dune and if the beach has any wave action you are going to get continual filling and cutting of your channel. Maintenance of a concrete channel could be intense unless you make it wide enough that it doesn't matter.

I think some riprap for a short distance initially that will get covered by sand (but who cares if your rock gets covered) with some barriers to contain the meander. I am sure the US Army Corps of Engineers Coastal Engineering Manual has some good design info. Beaches are interesting in that sometimes erosion is the last of your concerns.