Presentation Program Outline

Transcription

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2 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)

3 Structural Countermeasures Overview / Planning Process & Design Considerations

4 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

5 Riverine vs. Alluvial Fan - Structural Countermeasure Design Issues / Considerations Flow uncertainty Flow duration Seepage control Flow direction and path uncertainty Velocity Sediment deposition Impingement Alluvial Fan Riverine

6 Whole Fan Solutions vs. Localized Protection Structural Countermeasures

7 Whole Fan Solutions vs. Localized Protection Structural Countermeasures

8 Structural Countermeasures for Alluvial Fans Basic Building Blocks Collection Channels Conveyance Channels Dispersion Channels

9 Structural Countermeasures for Alluvial Fans Basic Building Blocks - Example

10 Standard Alluvial Fan Structural Countermeasures

11 Effectiveness of Alluvial Fan Structural Countermeasures for Different Hazards

12 Structural Countermeasure General Design Considerations Hydrology Bulking Factor (concentration of sediment = CC ss ) Hyperconcentrated flows high suspended sediment concentrations Typical bulking factor range from 1.1 to 1.5 for semi arid alluvial fans BB. FF. = CC ss 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 SSSS BB. FF. = , 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)

13 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 FF. BB. = dddddddddd ssssssssssssssssssss dddddddddd + VV

14 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)

15 Basic Countermeasure Planning and Design Process

16 Baseline Technical Analysis Key Foundation for Engineering Design

17 Countermeasure Levees / Dikes / Diversions

18 Examples - Alluvial Fan Levee Characteristics

19 Examples - Alluvial Fan Levee Characteristics

20 Examples - Alluvial Fan Levee Characteristics

21 Examples - Alluvial Fan Levee Characteristics

22 Examples - Alluvial Fan Levee Characteristics

23 Example - Alluvial Fan Dike vs. Levee

24 Example - Alluvial Fan Dike vs. Levee

25 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

26 Minimum Applicable FEMA Requirements Applicable NFIP Regulations Levees 44 CFR Ch Alluvial Fans 44 CFR Ch 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

27 Critical Levee Design Features and Components

28 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

29 Typical Levee Section Design Elements

30 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

31 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

32 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

33 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

34 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

35 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 = (Qn) 3/8 S 3/16 Fixed Bed Water Surface Profile Hydraulic Models (HEC-RAS) Cross Section Orientation 2 - dimensional Hydraulic Models (ie. FLO-2D, MIKE, HEC-RAS 2D)

36 Approximating Self-forming Channel Hydraulics of Alluvial Fan Dawdy Empirical Procedure Alluvial Fan Hydraulics Channel Width W = 9.5 Q 0.4 Procedure Assumes Critical Depth N F = 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 = Q 0.6 Shear Velocity = V * = (grs) 1/2

37 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 n = 0.04 d s 1/6 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

38 Multiple Orientations of HEC-RAS Cross Sections Normal to Contour Orientation Perpendicular Transverse Levee

39 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

40 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 Height Levee = Depth flow + Z deposition + Y superelevation + 1/2 Antidune Height + F.B. (eqn.1) Height Levee = Critical Depth + F.B. (eqn.2) Compare Calculated Height to Total Specific Energy

41 Definition of Levee Design Variables

42 Levee Height Design Variables Antidune Height: V 2 (Kennedy) If Calculated Antidune Height Exceeds Flow Depth, Use Flow Depth and (2) Self- Calculate Hydraulics for (1) Actual Flow Depths forming Channel Superelvation: 1.3 V 2 (b+2zd) / (gr) Minimum Radius Calculated From Topwidth Equal to Curve Tangent Aggradation Potential (see Impingement Analysis) If Depth plus Superelevation Exceeds Specific Energy then Use Specific Energy

43 Step 5- Levee Toe-down Protection Requirements Primary Failure of Mechanism of Bank Protection is Scour Below Revetment Toe Depth Toe Depth Evaluated Through Empirical Design Charts / Tables Toe depth Calculated From Summation of Individual Components: Bedform Bend Scour Low flow Incisement Contraction Scour (Applied only at Channel Entrance for Training Levees)

44 Levee Toe-down Protection Requirements Hydraulics Evaluated for (1) Actual Flow Depths and (2) Selfforming Channel Hydraulics Z Toe Down = Z Bend + Z Contraction + Z Bedform + Z Low Flow Incisement Compare Calculated Maximum Bed change to Empirical Toedown Design Charts

45 Step 6 -Additional Levee Design Considerations Uncertainty of Levee Performance Levee Closures Long Term Maintenance Levee Crossings Vehicular Access for Flood-fighting Aesthetics Environmental Mitigation / Maintain Natural Flowpaths Freeboard Least cost-evaluation of Alternative Systems

46 Summary of Design Procedures for Levees on Alluvial Fans Evaluate both Rigid Boundary and Alluvial Channel Hydraulics Apply Simplified Alluvial Fan Hydraulics Development Horizontal Alignment to Minimize Impacts Levee Height Calculated to include Maximum Flow Depth and Aggradation Revetment Toe-down Depth Calculated Below Maximum Potential Scour Depth Slope Revetment Designed to Resist Maximum Tractive Force

47 Countermeasure - Channel / Conveyance Facility

48 Channelization Alternative Design Options

49 Alluvial Fan Rigid Channels Design Issues & Considerations Manning s Factor for Bed Load Sediment Deposition Bulking Factor Debris/Sediment Flow Regime CChange

50 Channelization Hydraulic Sizing / Capacity Initial sizing/geometry flexible or earthen channel whole fan channel inflows utilize self-forming channel width (Dawdy equation) Geometry also dependent on providing similar sediment transport capacity as upstream delivery Lateral inflows to supercritical channel Side spillway entrance preferred limit surface wave disturbances Completely submerged lateral inflow pipes Analyze for high and low roughness coefficient for stability and capacity Analyze streambed at (1) initial streambed elevation, and (2) long term stabilized streambed

51 Channelization Sediment Transport Capacity Deposition / Degradation Fully armored channel should have sediment transport capacity that exceeds the upstream sediment inflow to avoid deposition Partially armored or earthen channel should have sediment transport that is in general equilibrium or stable so that the sediment transport capacity equals the sediment inflow or delivery from the upstream Calculate equilibrium slope based the hydraulics of the proposed channel, sediment characteristics, and sediment bedload inflow Adjust channel dimensions and slope until equilibrium achieved

52 Equilibrium Slope Analysis Sediment Continuity Analysis Sediment continuity analysis Calculate incoming sediment load from supply reach Worst case evaluate reduced sediment inflow for flatter slope Adjust slope of channel reach with revised hydraulics until sediment transport rate equals the supply

53 Channelization Armoring / Erosion Protection Bank Revetments Only (earthen bottom) Hydraulic shear resistance and abrasion resistance at invert Abrasion resistance Sacrificial wearing surface (additional concrete thickness) Wear cones (measures Stable channel slope Toe down elevation (similar to levee) Degradation Local scour Bedform Freeboard Top elevation revetment (similar to levee) Super elevation Aggradation Bedform (dune height)

54 Countermeasure - Grade Control Structures

55 Grade Control Design Background Two Basic Types of Grade Control Structures 1. Invert hard point to resist erosion/degradation 2. Hydraulic control generating water surface upstream Design Overview Optimizing Hydraulics & Economics Location Type of structure Locations Spacing and net drop height key design assessment Other factors

56 Grade Control Structure Alternative Design Variations

57 Grouted Rock Grade Control

58 Reinforced Concrete Grade Control

59 Gabion Grade Control

60 Hydraulic Design of Grade Control

61 Horizontal Spacing / Siting Grade Control Structures Spacing limited by equilibrium slope (S eq ) and maximum allowable drop height (H max ) LL = SS 00 SS eeee HH H max governed by type of structure, hydraulic criteria, energy dissipation, structure stability/forces, costs, safety

62 Drop Structure Spacing - Channel Equilibrium Slope Analysis Upstream sediment supply is a controlling factor assessing channel response. Balance of incoming sediment supply and transport capacity Application of multiple procedures since most difficult to reliably define 1. Geomorphic Procedures Extrapolate historical trends 2. Sediment Transport - Empirical Equilibrium Equations Static equilibrium (incipient motion) Dynamic equilibrium 3. Sediment Balance/Continuity Analysis 4. Sediment Transport Modeling Long Term

63 Channel Equilibrium Slope Analysis Empirical Equations Incipient Motion Shields diagram SS eeee = ττ cc γγ ww dd and Schoklitsch method S L = K s (D W bf /Q) 3/4 Meyer-Peter Muller S L = K mpm (Q/Q bf ) (n s /D 90 1/6 ) 3/2 D / d Regime / Sediment Transport Bray S L = Q D BUREC S L = ( D 50 W bf / Q) 0.75 Henderson S L = 0.44 D Q 0.46 ττ = ττ cc (γγ ss γγ ww )DD ss

64 Channel Equilibrium Slope Analysis Design Aids Guidance Design aid tools for equilibrium slope guidance Offers tentative guideline and brackets stable slope US ACOE EM Regional geomorphic based stability curves versus drainage area

65 General Suggested Guidelines / Criteria for Grade Structure Sizing Maximum Net Drop Height (H d ) Many agencies limit to 5-feet maximum Drops without energy dissipating appurtenances such as chute block/baffle blocks limit drop heights less than critical depth (y c ) low height drop Sloping Chute Slope 4(H) 1 (V) (maximum slope) flatter slopes assists in preventing reverse roller waves Stilling Basin Length 60% length of hydraulic jump Minimum End Sill Height 1/6 of sequent depth (1/6 y 2 ) 1 or 2 feet Difference between sequent depth and downstream tail-water Horizontal Siting Limitations Not within horizontal curved reaches, minimum 200-feet upstream or downstream of curve. Downstream/Upstream Flexible Armoring 10-feet

66 Local Scour Analysis of Structure Vertical Drop Scour Veronese method (USBR) yy ss = qq HH tt TTTT Maximum scour depth occurs at distance downstream 6 x scour depth Scour hole extend a distance downstream approximately 12 time scour depth

67 Countermeasure - Detention Storage / Debris Basin

68 Debris Basin Design Features Basin Minimum Design Elements Basin storage volume Multiple storm events including 100-year and several smaller events Embankment or debris retaining structure Low-level outlet tower Upstream inlet / training facilities Downstream outlet Primary Spillway Temporary storage/drying area Storage volume access ramps / roadway

69 Low-Level Outlet Tower Configurations

70 Debris Basin Design Consideration Sediment Yield / Delivery Watershed sediment production vs. sediment delivery (yield) Sediment yield either supply limited or transport limited Alluvial fans generally transport limited which is why depositional feature Watershed sediment/debris production calculated standard 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) Sediment delivery (yield) computed through the sediment transport capacity of upstream channel Calculate hydraulic rating curve for upstream inflow channel Apply appropriate sediment transport bedload equation to hydraulic rating curve Apply calculate sediment transport rate to flood hydrograph discharge ordinates Calculate volume (yield) under the sediment hydrograph

71 Debris Basin Design Considerations Sediment Trap Efficiency Large reservoirs deposition (ACOE) Brune s curve (not recommended dry) Brown s curve Churchill s curve (sediment index method)

72 Debris Basin Design Considerations Sediment Trap Efficiency Small reservoir / basin Sediment theory for removal of sediment dynamic conditions (EPA, 1986) RR dd = nn VV ss = settling (fall) velocity of particles (Rubey s eqn. or Stokes Law) n = turbulence or short circuiting coefficient (n=1 poor, n=3 good, n>5 very good, and n= infinity, ideal performance) QQ = peak flowrate AA = surface area of the basin RR dd = fraction of solids removed under dynamic conditions VV ss QQ/AA nn

73 Debris Basin Design Considerations Sediment Storage / Deposition Patterns Top-set slope sediment delta ½ upstream streambed slope Fluvial sediment transport models (HEC-6 / FLUVIAL12)

74 Debris Basin Design Features Layout / Geometry Length to width ratios should always exceed 2:1 avoid short circuiting Triangular shaped basins locate inlet at narrow end of basin and expands to outlet Maximum Design Water Surface / Freeboard Water surface profile for design storm on the maximum sediment storage profile in basin. Typical criteria is 100-year sediment storage plus water surface profile for design storm such as 100-year Assumes cleanout of basin after loss of designated storage volume

75 Case Study Palm Creek Ranch (Thousand Palms, CA) Subdivision Protective Countermeasure Application

76 Thousands Palms, CA Proposed Residential Subdivision

77 Proposed Residential Subdivision Layout Interception Channel Conveyance Channel Dispersion Channel

78 Applied FEMA 3-Phase Approach Alluvial Fan Flood Hazards Geomorphic / 2D MIKE Modeling Alluvial Fan Geomorphic Assessment Flowpath Uncertainty Virtual Levees

79 2D MIKE Floodplain Modeling Four Scenarios Virtual Levees Existing Conditions No.1

80 2D MIKE Floodplain Modeling Four Scenarios Virtual Levees Existing Conditions No.2

81 2D MIKE Model Established Existing Flow In and Out Project Boundary Divide into Segments

82 HEC-RAS Modeling Used to Compute Detailed Water Surface and Lateral Outflows

83 Case Study Lowe Reserve (Palm Desert, CA) Whole Fan Structural Control Measures

84 Project Vicinity Map

85 The Reserve Development Project - Location & Description Location in both cities of Palm Desert and Indian Wells, CA High-End luxury residential golf course development 500 acres of land adjacent to Ironwood Country Club Situated entirely on the active Deep Canyon alluvial fan Proposed 18-hole championship golf course 250 residential sites Encompass portion of Living Desert regional debris basin

86 Existing Conditions Alluvial Fan - Desert Hydrology

87 Aerial View - Pre Construction 1996

88 Aerial View - Post Construction 2005 Project limits

89 Aerial Schematic of Development Area

90 Watershed Description & Hydrology 46.4 square mile deep Canyon Watershed Tributary to living desert debris basin Deep canyon is tributary to Whitewater River Two independent drainage basins tributary to project boundary (A & B) Extremely rugged mountains and steep rocky canyons of Santa Rosa mountains Elevation variation extremes from 8,716 feet to 460 feet at project 17.7 miles watershed length Average slope 9%

91 Description of Channelization Facilities Twelve concrete grade control structures (Net Drop 6.5 feet) Flood Control Channel 5,000 feet long Plunge-pool channel outlet structure (Net Drop 18 feet) Small entrenched sediment basin 1,400 feet golf course channel Outlet water feature / plunge pool grade control

92 Proposed Reserve Development With Flood Control Improvements Transverse Levee Main Channel Existing Debris Basin Transverse Levee Outlet Structure Secondary Channel

93 Plunge Pool Channel Outlet With Aesthetic Treatment

94 Plunge Pool Channel Outlet With Aesthetic Treatment

95 Flood Control Primary Channel / Grade Control

96 Levee & Grade Control Post Cosntruction Transverse Levee System Grade Control Structure

97 Channel / Grade Control Currently

98 Physical Model Study for Grade Control Structure Model Objectives Evaluate the modification to the Erosion patterns with alternative design of grade control structures Investigate the hydraulics of different grade control geometric configurations Determine the effect to the local scour from an artificial horizontal armor blanket

99 Physical Model & Experiment Description Model construction and operated by PACE Experimental setup located outdoors under protected covered carport in Palm Desert 40-foot segment with plexiglass side Constructed ring-embankment storage reservoir 80-foot long and 24 wide flume 2-10 horsepower pumps (1,800 gpm at 16-ft head) Flume height 42 Sand bed material 60% medium to coarse sand, 20% pea gravel. 20% medium gravel

100 Hydraulic Model Scaling to Prototype Froude Law Scales selected to provide the minimum dimensions which erosion features could be adequately observed and measured Linear scale of Lr = 1:16 Discharge sable Qr = 1:1,024 Time scale Tr = 1:16 Velocity scale Vr = 1:16 Selection of model sand-bed material

101 Modeling Experiment Setup / Operation Model Setup With Drop Structure Before Water Grade Control Structure at Peak Flow / Scour Hole

102 Recommendation and Conclusions From Model Study Investigation Rip-rap armor blanket provided downstream from toe of grade control structure Length of armor blanket a minimum of 30-feet Blanket should be configured so it resembles shape of the scour hole Thickness of the armor blanket should be a minimum of 6.5-feet with 48 diameter rock Geometry of the rock blanket is important and should provide a 3-foot high thickened sill Minimum vertical height of the concrete lining for chute toedown is 16-feet compared to the original 24-feet from empirical equations. Cost Savings of $1M

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