Beyond the Design Storm: From Compliance Metrics to Adaptive Operations for Consecutive Minor Events in Dams
Authors: A.M. Cassa1, F.W. van Zyl1
Conference: SAIMM ESGS Conference 2026
Date: August 26-27, 2026
1 Knight Piésold (Pty) Ltd, Johannesburg, South Africa
ABSTRACT
Return water dams at mines are commonly assessed against single-event compliance criteria, typically the 1-in-50 year 24-hour storm, with probable maximum precipitation used as an upper check. Although useful for design verification, these criteria do not adequately represent the operational risk posed by sequences of smaller storms occurring over days to weeks. In practice, spills are often driven by cumulative inflow, antecedent wetness, reduced recovery time between storms, sedimentation and limitations in pumping and operational response.
This paper presents a sequence-based framework for assessing the performance of return water dams and linked tailings storage facilities under consecutive minor rainfall events. The framework uses continuous water balance simulation, rainfall sequencing, antecedent wetness response and climate stress testing to evaluate spill likelihood, time at risk for freeboard exceedance and recovery capacity. A case study based on observed rainfall at a South African mine site is used to illustrate the problem. During January 2026, 831 mm of rainfall was recorded over 11 days, equivalent to about 145% of the local mean annual precipitation. Within this sequence, daily events reached approximately the 5-year to 20-year range, while the 72-hour and 7-day accumulations approached the local 50-year design depths. The findings support a shift from a design storm philosophy towards a design sequence and adaptive operations approach.
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