It will definitely help that your two beaches ARE NOT perpendicular to the long-axis of your pond!
Wave power is strongly dependent on the fetch of the wind. (A constant 20 mph wind will create MUCH stronger waves at the downwind shore when blowing along the long axis of the lake, compared to a wind direction across the short axis.)
The science of wind-driven waves on sand beaches is very complicated. Under certain wind conditions, your beach will probably encounter some constructive waves. These waves will actually add sand from the deeper water up onto your beach shoreline.
However, most of the time, you will probably have conditions with destructive waves that are moving sand from the shoreline into deeper water.
Therefore, any type of berm you build of solid materials should aid in reducing your sand movement from the beach into deep water. Hopefully, some of the experts on the forum can give you some advice on the best method and materials to build that berm.
I am only throwing in the "science" to help with the task above, and to mention that your beach may also be subject to longshore drift. Due to your beaches being parallel to the long axis of your pond, there will be lots of times where the wind will be creating waves that are moving at an angle close to parallel to your beach. These conditions will also move sand away from your beach. However, instead of moving the sand into deeper water, the sand will be moving to the adjacent shorelines beyond the designed perimeters of your beaches.
The best cure for that problem is to build a peninsula perpendicular to the shore. That would significantly reduce your sand losses AND make for much calmer waters for kids swimming in the waters at your beach area.
Would it help your overall lake design to build a peninsula of land between your beach and the proposed new dock? That would make for calmer conditions in the swimming area AND in the area of the dock. If the dock was pinned to the peninsula (instead of the existing shoreline), it would be easier to quickly transition from shallow water to deep water at the end of the dock.
I hope you enjoy your big lake project!
