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Hydraulics of Grade Control Structures – Vertical Drop

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Hydraulics of Grade Control Structures – Vertical Drop 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 Grade Control Key Design Element for Environmentally Limited Solutions Grade Control Structures Benefits • Mitigation measure for hydromodification • Preventing erosion of existing structures • Protection of alluvial streambed long term degradation • Alternative to channel lining • Minimizing footprint impact in floodplain Negative Aspects of Grade Control • Potential barrier to fish passage • Impact to environmental regulatory streambed • Public safety Grade Control Design Process • Sediment • Mapping • Historical • Hydraulics • Conceptual Locations Geomorphic • Geomorphology • Alternative Systems Data • Fluvial Analysis • Local Adjustments • Alternative Spacing • Drop Structure Costs • Drop Hydraulics/Sizing • Revised Channel Lining • Drop Heights Costs • Optimization Baseline Technical Analysis Key Foundation for Engineering Design GRADE CONTROL DESIGN CHARACTERISTICS Grade Control Structure Alternative Design Variations Grouted Rock Grade Control Reinforced Concrete Grade Control Gabion Grade Control Sheet Pile Grade Control Soil Cement / Roller Compacted Concrete Grade Control Boulder Weir Structures • Intended for small creek restoration/stabilization • Concentrates energy at crest • Flanking common failure mode • Undermining failures Common Operational Issues / Failure Modes ENGINEERING DESIGN ANALYSIS OF GRADE CONTROL STRUCTURES Hydraulic Design of Grade Control Horizontal Spacing / Siting Grade Control Structures • Spacing limited by equilibrium slope (Seq) and maximum allowable drop height (Hmax) • Hmax governed by type of structure, hydraulic criteria, energy dissipation, structure stability/forces, costs, safety Horizontal Spacing / Siting Grade Control Structures • Local conditions generally prevent constant spacing intervals, results in structures with different drop heights • Variable slope lining cutoff depth profile 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 Channel Equilibrium Slope Analysis – Empirical Equations Incipient Motion ∗ • Shields diagram and ( 3/4 • Schoklitsch method L s bf • Meyer‐Peter Muller 1/6 3/2 L mpm bf s 90 Regime / Sediment Transport ‐0.344 0.58 • Bray L 2 50 0.75 • BUREC L 50 bf 1.15 –0.46 • Henderson L 90 Channel Equilibrium Slope Analysis – Design Aids Guidance • Design aid tools for equilibrium slope guidance • Offers tentative guideline and brackets stable slope • US ACOE EM 1110‐2‐ 1418 • Regional geomorphic based stability curves versus drainage area Equilibrium Slope Analysis – Sediment Continuity Analysis Sediment continuity analysis • Calculate incoming sediment load from “supply reach” • Adjust slope of channel reach with revised hydraulics until sediment transport rate equals the supply General Suggested Guidelines / Criteria for Grade Structure Sizing Maximum Net Drop Height (Hd) • 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 (yc) – “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 y2) • 1 or 2 feet • Difference between sequent depth and downstream tailwater Horizontal Siting Limitations • Not within horizontal curved reaches, minimum 200‐feet upstream or downstream of curve. Downstream/Upstream Flexible Armoring • 10‐feet Hydraulics of Grade Control Structures – Vertical Drop • Hydraulics uses dimensionless Drop Number (Dn) . . . . Hydraulics of Grade Control Structures – Sloping Drop • Follows hydraulics analysis procedures for stilling basin with Free Hydraulic Jump (trapezoidal or rectangular) • HEC‐RAS not necessarily correctly calculating hydraulic losses / steep slope not accurate 1. Estimate total drop height includes initial estimate of sill height plus net drop height 2. Calculate depth at brink of chute (Yc) 3. Calculate depth at base of chute (Y1) based on energy balance between top of chute ignoring energy losses Hydraulics of Grade Control Structures – Sloping Drop 4. Calculate subcritical sequent depth (Y2) for hydraulic jump in stilling basin using momentum balance (P+M)1 = (P+M)2 5. Calculate height of sill = where y3 = tailwater or yc 6. Determine Length of Stilling Basin = 60% x hydraulic jump length (6 y2) Engineering Tool Hydraulic Analysis of Sloping Chute Drop Structures Hydraulics of Grade Control Structures – Stepped Chute • Hydraulics of stepped chute function of step height, step length, inclination/slope (h/L),and unit discharge (q) • Results in either (1) nappe flow, (2) transition flow, and (3) skimming flow Nappe Flow . / . • Achieves 85‐95% energy dissipation total energy of drop Skimming Flow • Develop uniform flow down chute because steps act as roughness elements . . . . Where: . / Local Scour Analysis of Structure Vertical Drop Scour • Veronese method (USBR) . • Maximum scour depth occurs at distance downstream 6 x scour depth • Scour hole extend a distance downstream approximately 12 time scour depth Local Scour Analysis of Structure Sloping Chute Scour • Bormann and Julien . . . • ADOT (4:1 Sloping Chute) . ** Additional reference drop scour “Scour downstream of Grade‐Control Structures” (1991, Julien and Bormann) Foundation Design / Seepage Control for Structure Structural Design • Sliding • Overturning • Seepage Uplift • Buoyant Weight • Bearing Pressure • Hydrodynamic Impact Seepage Control • Underseepage analysis from underflow net • Piping occurs when upward seepage force at downstream toe exceeds submerged weight of material • Provide upstream cutoff at crest to minimize piping underflow • Provide weephole drain to relief seepage pressure DESIGN CONSIDERATIONS FOR SITING GRADE CONTROL STRUCTURES Critical Design Siting Considerations Hydraulic Considerations • Stable channel hydraulics Geotechnical Considerations • Streambank Stability / Foundation Flood Control Impacts • Local Drainage • Water Surface / Flood Protection Level Environmental Considerations • Fish and Wildlife Passage Barrier Safety Considerations • Drop Height • Vector Control Physical Constraints / Limitations • Existing Utility • Local Drainage • Existing Channel Slope Lining • Existing Structures Economics Horizontal and Vertical Siting Economics Impacted by Existing Channel Lining • Range of Drop Heights – Constant Spacing • Alternative Layouts Plan / Profile • Comparison to existing structure locations and channel profile • Impacts to existing lining toedown and channel freeboard • Cost considerations not only include drop structure construction but also modifications to channel slope lining • Raising or lowering of channel slope lining impact siting of drop structures AUTOMATED PLANNING DESIGN TOOL Drop Structure Automated Design Alternative Planning Tool • Drop Structure Alternative Assessment System Tool Development • Drop Height • Geometry / Construction Materials • Hydraulics Parameters • Scour • Structure Dimensions • Results in Costs function of drop height • Additional costs (environmental/dewatering) Channel Drop Structure Planning Matrix Tool – Initial Automated Siting Automated Drop Structure Costs Calculation Tool – Drop Height/Type • Two part planning tool for (1) drop hydraulics and (2) costs of facility • Automated sizing of drop structure basics of “type” of structure, height, and channel dimensions • Includes estimate of scour depth for drop type and height • Evaluate straight drop with and without stilling basins Analytical Engineering Drop Structure Planning Tool .
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