"堆石斜坡堤" can be translated as "rubble mound slope breakwater" or simply "rubble mound breakwater with a sloping face" in English, depending on the context and desired level of specificity.
"Rubble mound breakwater" is a common and general term used in coastal engineering to describe a type of breakwater constructed with loose stones (rubble) piled up to form a mound, which can have a sloping face.
If emphasizing the sloping feature is important, "rubble mound slope breakwater" or the more descriptive phrase "rubble mound breakwater with a sloping face" can be used.
In most cases, "rubble mound breakwater" should suffice.