Within a few hours of emailing this letter to Mayor Laura Rosenthal of Malibu, I receieved this response from her:

Thank you Mr. Riles. I am so glad that you enjoyed a lovely day at the beach in Malibu. All of the beaches in Malibu are owned and operated by the County of Los Angeles Department of Beaches and Harbors so you might want to add them to your contact list.
And thanks for the excellent feedback on why our local beaches seem to be losing so much sand every year!
Best,

Laura Z. Rosenthal
Mayor, City of Malibu