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Alluvial Fan Hazards & Design Issues for Design

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Presentation Program Outline Overview Countermeasures / Planning Process Levees / Dikes / Diversions Channelization / Conveyance Grade Control Structures Detention Basin / Debris Basin Case Study – Localized Subdivision Protection (THOUSAND PALMS, CA) Case Study – Whole Fan Facilities (INDIAN WELLS, CA) Structural Countermeasures Overview / Planning Process & Design Considerations Alluvial Fan Hazards & Design Issues for Design • Uncertainty of flow depths (R&U analysis) • Inundation extents / flow direction / impingement • Sediment deposition • Scouring and undermining • Impact forces • Channel avulsions and entrenchments • Hydrostatic and buoyant forces • High velocities • Unpredictable flow path (R&U analysis) • Flooding from both debris and water flows “Riverine” vs. “Alluvial Fan” - Structural Countermeasure Design Issues / Considerations • Flow uncertainty • Velocity • Flow duration • Sediment deposition • Seepage control • Impingement • Flow direction and path uncertainty Alluvial Fan Riverine “Whole Fan” Solutions vs. Localized Protection – Structural Countermeasures “Whole Fan” Solutions vs. Localized Protection – Structural Countermeasures Structural Countermeasures for Alluvial Fans – Basic Building Blocks Collection Channels Conveyance Channels Dispersion Channels Structural Countermeasures for Alluvial Fans – Basic Building Blocks - Example Standard Alluvial Fan Structural Countermeasures Effectiveness of Alluvial Fan Structural Countermeasures for Different Hazards Structural Countermeasure – General Design Considerations Hydrology – Bulking Factor (concentration of sediment = ) • “Hyperconcentrated” flows – high suspended sediment concentrations • Typical bulking factor range from 1.1 to 1.5 for semi arid alluvial fans . = . • Sediment concentration can be estimated from calculating sediment transport capacity of self forming channel or HEC-RAS average floodplain hydraulics applied to sediment transport relationships − • Additional simplified empirical methods for bulking factor . = + , Where SY = sediment yield/production in cubic yards per square mile • Sediment yield can be estimated through multiple empirical methods: • US Army Corps of Engineer, Los Angeles District Debris Method (regression equations) • Modified Universal Soil Loss Equation (MUSLE) • Fire and Resource Assessment program (FRAP) Structural Countermeasure – General Design Considerations Freeboard • Additional items: • Surface waves • Sediment accumulation • Bedform • Superelevation • Flow regime change • Sediment ramping • Minimum levee requirements per FEMA • ACOE guideline minimum freeboard . = + ퟓ − Structural Countermeasure – General Design Considerations “Drainage Law” Basic Requirements An upper landowner is entitled to discharge surface water from his land as the water naturally flows. If he modifies the natural flow, he is liable for any damage done to a lower landowner unless the lower landowner had acted “unreasonably” in altering the natural drainage over his land. The determination of reasonable or unreasonable is a question of fact to be determined by the court in each case • No diversion of flows to adjacent or downstream property owners • Cannot increase level of flooding to adjacent property owners • Cannot increase erosion downstream • Must return flows at the downstream end of property to same as the existing conditions (both depth and velocity) Basic Countermeasure Planning and Design Process Baseline Technical Analysis Key Foundation for Engineering Design Countermeasure – Levees / Dikes / Diversions Examples - Alluvial Fan Levee Characteristics Examples - Alluvial Fan Levee Characteristics Examples - Alluvial Fan Levee Characteristics Examples - Alluvial Fan Levee Characteristics Examples - Alluvial Fan Levee Characteristics Example - Alluvial Fan “Dike” vs. “Levee” Example - Alluvial Fan “Dike” vs. “Levee” Critical Design Objectives for Levees on Alluvial Fan • Minimize Potential for Overtopping • Prevent failure of Embankment Slope Revetment • Eliminate failure from Erosion Critical Design calculations: • Levee Height • Revetment Toedown Depth Minimum Applicable FEMA Requirements • Applicable NFIP Regulations • Levees 44 CFR Ch. 1 § 65.1 • Alluvial Fans 44 CFR Ch. 1 § 65.13 • Minimum Riverine Levee Freeboard 3.0 feet and 4.0 feet at Bridge Structures • Supercritical hydraulics • Inspection and Maintenance program • Settlement • Interior drainage • Embankment geotechnical requirements • Embankment protection • Upstream Termination Freeboard 3.5 feet Critical Levee Design Features and Components Additional Critical Levee Design Elements 1. Alignment • Orientation Angle / Horizontal Alignment 2. Termination Points 3. Embankment Cross Section Geometry 4. Levee Height • Levee Vertical Profile 5. Revetment or Armoring Typical Levee Section Design Elements Special Design Considerations for Levees on Alluvial Fans • Variable Flow Paths • Direct Flow Impingement from Alternate Flow Paths • Sediment Ramping • Debris Deposition • Local Scour • Channel Entrenchments • High Velocity and Erosive Flows • Water Surface Runup / Super-elevation on Upstream Side of Levee • Impact Forces from High Velocity Flows Step 1 - Levee Alignment Selection • Levee Orientation Relative to Fan Geometry • Optimum or Maximum Acceptable Divergence Angle • Maximum Horizontal Limits of Levee • Entire Fan • Tie to Limits of Alluvial Fan • Limited by Risk Assessment and Historical Flow Paths Evaluating Effects of Alternate Flowpath Impingement • Direct Impingement Would Result in Alternate flow Path Parallel to Levee • Reduced Slope for Alternate Flow Path and Reduced Sediment Transport Capacity • Aggradation Occurs Along Alternate Flow Path to Re-establish Sediment Equilibrium • Area Between New Bed Profile and Existing Bed Profile Filled from the Differential Sediment Transport Rates Evaluating Effects of Alternate Flow Path Impingement • Calculate the Maximum Deposition Height at Levee: • Calculate the deposition volume from the Sediment transport rates and hydrographs • Deposition width from the self-forming channel width • Applying geometrical relations maximum deposition depth computed at contact point • Trial and Error Procedure Applied to Vary Contact Length along Levee for Maximum Depth Upstream Levee Termination Points • Extend to maximum limits of fan flooding • Increase freeboard per FEMA • Extend to hard-point for tie-in • Potential for “flanking” if Levee cannot be extended to maximum fan flooding limits • May not be economically feasible or environmental / jurisdictional limitations / property ownership • Estimate the amount of flow that will flank upstream limits of levee • Apply French’s procedure or Flowpath Uncertainty Analysis Step 2 - Alluvial of Alluvial Fan Hydraulics For Flood Protection • Limited Procedures Available for Evaluation of Alluvial Fan Hydraulics • Empirical Procedures to Evaluate Self-Forming Channel Hydraulics • Dawdy (Critical depth stabilize where decrease in depth results in two-hundred fold increase in width) W = 9.5 Q 0.4 • Edwards - Thielman (Application of Mannings Equation and Dawdy procedure) W = 17.16 (Qn)3/8 S3/16 • Fixed Bed Water Surface Profile Hydraulic Models (HEC-RAS) • Cross Section Orientation • 2 - dimensional Hydraulic Models (ie. FLO-2D, MIKE, HEC-RAS 2D) Approximating Self-forming Channel Hydraulics of Alluvial Fan • Dawdy Empirical Procedure Alluvial Fan Hydraulics • Channel Width W = 9.5 Q0.4 • Procedure Assumes Critical Depth NF = 1.0 • Can develop the following hydraulic relationships for “self forming” channel • Velocity = 1.5 Q 0.2 • Depth = 0.07 Q 0.4 • Unit Flowrate = q = Q / W = 0.105 Q0.6 1/2 • Shear Velocity = V* = (gRS) Alluvial Fan Water Surface Hydraulics for Flood Protection • HEC-RAS Hydraulics Analysis with Levee System • Mannings roughness value selection • Strickler & Anderson Equations for mannings based on bed material size 1/6 • n = 0.04 ds • Cowan’s procedure • Orientation of HEC-RAS cross sections with Levee 1. Parallel to fan contours 2. Normal to Levee orientation • Intent is to maximize the depth to prevent overtopping for worst case • Compare analysis with empirical fan hydraulics • 2-D analysis for validation only, NOT design Multiple Orientations of HEC-RAS Cross Sections Normal to Contour Orientation Perpendicular Transverse Levee Step 3 - Levee Revetment / Armoring Requirements • Historically Unarmored Levees are Ineffective • Armoring Selection Based Upon Tractive Force or Velocity Requirements • Alternative Revetments Available • Concrete Slope Lining • Grouted Stone • Soil Cement • Rock Rip-Rap • Evaluate Performance and Least Cost Step 4 - Evaluation of Levee Height Requirements • Levee Height Accounts for Moveable Bed and Rigid Boundary Conditions • Calculate Height from Summation of Individual Components: • Bedform Height • Superelevation • Flow Depth Compared to Critical Depth • Deposition from Alternate Flow Path HeightLevee = Depthflow + Z deposition + Y superelevation + 1/2 Antidune Height + F.B. (eqn.1) HeightLevee = Critical Depth + F.B. (eqn.2) • Compare Calculated Height to Total Specific Energy Definition of Levee Design Variables Levee Height Design Variables • Antidune Height: 0.027 V2 (Kennedy) • If Calculated Antidune Height Exceeds Flow Depth, Use Flow Depth • Calculate Hydraulics for (1) Actual Flow Depths and (2) Self- forming Channel • Superelvation: 1.3 V2 (b+2zD) / (gR) • Minimum Radius Calculated From Topwidth Equal to Curve Tangent • Aggradation Potential (see Impingement Analysis) • If Depth plus Superelevation Exceeds
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