REPORT:
An Updated Numerical Simulation of the Ground-Water Flow System for the Castle Lake Debris Dam, Mount St. Helens, Washington, and Implications for Dam Stability Against Heave

Table of Contents

• Abstract
• Introduction
  • Previous Investigations
  • Scope and Methods of Investigation
  • Acknowledgments
• Description of Debris Dam
  • Topography and Geology
  • Drillhole Data and Pre-1980 Model
    • 1983 Drillholes
    • 1990 Drillholes
      • Site DH1
      • Site DH2A, B
      • Site DH3
      • Site DH4A, B, C, D
      • Site DH5A, B
      • Site DH6
  • Vegetation
• The Ground-water Flow System
  • Alternative Conceptual Models of the Flow System
  • Hydraulic and Engineering Properties
  • Precipitation
  • Water Levels in Piezometers
  • Seep Discharge
  • Evapotranspiration
  • Castle Creek
  • Hydraulic Head Gradients and Factors of Safety
    • Factors of Safety: Gradient Method
    • Factors of Safety: Overburden Method
• Numerical Simulation of the Ground-water Flow System
  • Approach
  • General Features of the Model
    • Model Geometry
    • Boundary Conditions
      • Upper and Lower Boundaries
      • Castle Lake
      • Castle Creek and Spillway
      • Western Boundary
      • Northern Boundary
    • Natural Conditions to be Simulated by the Model
      • Precipitation
      • Evapotranspiration
      • Seep Discharge
        • Seep Locations and Conductances
        • Correction of Seep Discharge Measurements for the 1991 Water Year
      • Piezometer Water Levels
    • Free Parameters and Calibration
  • Uniform Model
  • Modifications Tested for the Uniform Model
    • Permeable Material Beneath the 1980 Deposits
    • Vertical Anisotropy
    • Decrease of Hydraulic Conductivity with Depth
    • Evapotranspiration Parameters
    • Variation of Recharge with Surface Slope
  • Nonuniform Calibrated Model
    • Large-scale Hydraulic Conductivity Variations
    • The Area Around Piezometers DH4A, B, C, D
    • Crest Conductance
    • Other Local Conductivity Changes
    • Creekbed Conductance
    • Drain Conductances and Elevations
    • Fit to Observations
    • Hydraulic Conductivity Distribution
    • Water Budget
  • Limitations on the Use of the Numerical Models
• Stability of the Debris Dam Against Heave
  • Factors of Safety
  • Effect of Lowering Castle Lake
  • Effect of Artificially Draining the Dam Crest
• Conclusions
• References Cited

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