I was able to do that with environmental influence and fine tuning of the blend control. My scene was a stream bank, not a beach, but the altitude difference was not great.

Try using both slope and altitude and keeping the blend value very low (5% to 10%). With such a low blend number, the mixing control becomes very sensititve, so it's best to enter numbers manually, instead of using the slider, and use decimal fractions (e.g. 48.7%, rather than 49%, etc.).

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