Side Slope Final Cover Characterization & ET Cover Modeling

The Calabasas Landfill is operated under a joint powers agreement with the County of Los Angeles and has accepted waste under multiple regulatory classifications since 1961. The facility transitioned from Class II operations to Class I, and later to Class III, and currently maintains a permitted disposal area of 305 acres with a 296‑acre refuse footprint. The landfill is projected to operate until approximately 2034 based on current disposal rates.

Verdantas characterized and modeled the performance of approximately 95 acres of existing interim final soil cover placed on side slopes at the landfill. The objective was to demonstrate that the existing cover could function as an evapotranspirative (ET) final cover, serving as an engineered alternative to the prescriptive Title 27 final cover system.

Characterization of As‑Constructed Side Slope Cover

Verdantas performed 47 auger borings across the 95‑acre side slope area and conducted laboratory testing to evaluate geotechnical and unsaturated hydraulic properties of the cover soils. Vegetation conditions were also assessed to support ET cover performance evaluations.

Unsaturated Flow Modeling

Unsaturated flow modeling was performed using the UNSAT‑H model to evaluate percolation performance at each test location. The modeling incorporated existing irrigation practices and vegetation types. Where modeling results did not meet Title 27 equivalency requirements, Verdantas developed and implemented mitigation measures.

Cover Equivalence Demonstration Report

The findings from the characterization and modeling were compiled into a Cover Equivalence Demonstration Report submitted to the Regional Water Quality Control Board (RWQCB). This report supported regulatory approval of the interim cover as a suitable alternative evapotranspirative final cover.