Surfing Waves

How can you build a standing wave in a river on a beach?

At Waimea bay there is a lot of sand that gets pushed up on the beach by the waves. This often blocks the river mouth. Once the river water level gets high enough they then dig a trench from the ocean to the river mouth releasing the backed up river water which rushes out carving a larger opening as it does. Eventually rapids form and for a little while there are standing waves

I found a video of the process but there are "secrets" to the formula in how much water and how big to dig the trough

Looks easy yeah ? But before you try, look up Leptospirosis.

It's been a long time since I have been to Waimea Bay but seems to me I recall the tide flows up the river during high tide which will result in brackish water around the river mouth rendering it leptospirosis free except for little puddles of fresh water along the banks where leptospirosis can survive. Anyway I might be wrong but there are certainly risks in anything you do.even so if there is no leptospirosis riding a standing wave has it's own significant inherent risks

Seems fun!

I was reading the comments though, it seems like it's a nattural thing and actually good to the ecosystem in Hawaii to do this, but you should check if you can do the same without harming the environment there where you live.
A point that was brought up is that in places like California, where the drainage pollutes the water, doing so could bring harmful polluted water to the beach. If the information is true it seems like they could even arrest you for doing it there. It's also a place that have a water issue, and making a way for sweet water (is that term right in english?) to go back to sea could have a impact on the water distribution.

I'm not an expert or anything, no affirmations here, just readed some raw comments and thought it would be a good idea for you to make some better research before trying it!