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 3/2 L mpm bf s 901/6 Regime / Sediment Transport ‐0.344 • Bray L 2 500.58 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)