CHAPTER 7 FLOOD HYDRAULICS & HYDROLOGIC VIVEK VERMA
CONTENTS 1. Flow Classification 2. Chezy s and Manning Equation 3. Specific Energy 4. Surface Water Profiles 5. Hydraulic Jump 6. HEC-RAS 7. HEC-HMS 8. Hydrologic and Hydraulic (H&H) Model
Flow Classification Open Channel Flow Steady Flow = 0 Unsteady Flow 0 Uniform Flow = 0 Non-Uniform Flow 0 Gradually Varied Flow Eg. Upstream of obstruction Rapidly Varied Flow Eg. Hydraulic Jump
Geometric Channels Properties
Optimal Channels - Max R and Min P
Chezy s and Manning s Equation Chezy formula was developed in 1775. It relates channel velocity to roughness, hydraulic radius and channel V = C.(RS) 1/2 C factor is related to Darcy s friction factor f, used in pipe flow; C = (8g/f) 1/2 2 main assumptions in Chezy equation: 1. The frictional force is proportional to the square of velocity 2. Uniform flow assumption gravity force is balanced by the frictional resistance of the stream
Chezy s and Manning s Equation Manning s equation was developed in 1890. Describes roughness with Manning s roughness coefficient V =. 2/ 3. 1/ 2 (SI Unit)
Specific Energy Energy equation Alternate Depths Subcritical and supercritical section Critical condition occurs when Fr = 1
Specific Energy For rectangular channel, Y c = ( )( 1 / 3 ) E min = ( ).y c
Surface Water Profile Normal Depth UNIFORM FLOW Q = C V = C y = C S 0 = C A = C b = C n = C R = C b Prismatic Channel A P
Surface Water Profile Normal Depth Energy Equation H = z + y + '( ) Energy Equation H = z + y + *'( ) Where, α = ' 0.12 4 3.2
Bed Slope Types
Occurrence of critical depth
Surface Profile Classification
Surface Profile for Gradually Varied Flow ALL POSSIBLE PROFILES
Hydraulic Jump Supercritical to subcritical (downstream condition) Abrupt change in depth involves significant energy loss via turbulent mixing Hydraulic Length
Hydraulic Jump Classification https://www.youtube.com/watch?v=xsygodmmiam
HEC-RAS Hydrologic Engineering Center River Analysis System Developed by US Army Corps of Engineers (USACE) Download link: http://www.hec.usace.army.mil/software/hec-ras/
Overview of Capabilities 1. Steady flow water surface profile computations 2. Unsteady flow simulation 3. Movable boundary Sediment Transport Computations 4. Water Quality Analysis 5. Channel Controls/Obstructions
Applications Bridge/Culvert Modeling Inline Structures (Dams, Weirs and Gated Spillways) Lateral Structures (Weirs, Spillways) Flood Management (Floodplain encroachment, Flood Insurance Studies) Sediment Transport & Management Water Quality Analysis (Temperature, Algae, DO, BOD, etc) Numerous Flow Analysis
Computational Procedure HEC-RAS utilizes the Standard Step Method Procedure: 1. Assume initial water surface (WS) elevation at downstream 2. Based on assumed WS elevation, determine the corresponding total conveyance and velocity head 3. Compute S f avg. and solve for h e with values from step 2. 4. Solve 1D Energy equation at downstream with values from steps 2 and 3 5. Compare WS elevations of step 1 and step 4. Iterate until values agree to within 0.1 feet (0.003m) or to user-defined tolerance
Computational Procedure Conveyance Calculation From Manning Equation Q = 1 n 2 / 3 1 / 2 R h AS f K 1 = R n 2 / 3 h 2 1/ Q = KS f A
Computational Procedure Energy Loss, h e Friction Manning Equation Contraction/Expansion loss coefficient g V g V C LS h e 2 2 2 2 2 2 1 1 f α α + = g V g V C LS h e 2 2 2 2 2 2 1 1 f α α + =
Computational Procedure One Dimensional Energy Equation
Data Requirements Channel description length of reach channel roughness channel cross-section geometry Boundary conditions Structure geometry bridges culverts weirs
Boundary Conditions Ways to specify Boundary Conditions Known Water Surface Elevations Critical Depth Normal Depth Rating Curve Boundary Condition Requirements Mild to Steep Transition Uniform flow Control structure Upstream depth Downstream depth Upstream and Downstream Supercritical Flow Subcritical Flow Mixed Flow
River Reach 10 0.2 Tributary Upper Reach 0.1 9.9 River Stations Sutter 9.8 0.0 Numeric labels increase upstream 9.7 Lower Reach 9.6 9.5
Cross Section Data x-y coordinates of channel bottom distance to downstream cross-section Manning s n
Channel Cross Section Manning n for overbank areas usually higher than for main channel
Channel Section Interpolation Water surfaces are calculated at each river station If water depth changes too much between river stations then the calculations are imprecise Interpolate between rivers stations of known geometry
Program Structure Input Channel geometry Flows and boundary conditions for each profile Other Analysis Scour at bridges Output Cross Sections Profiles Computed Rating Curves 3-D Cross Sections Tabular Data Errors
Additional Capabilities Perform Channel Modifications Cut and fill calculations
HEC-RAS Summary HEC-RAS solves the energy and momentum equations to calculate water surface profiles Modeling natural rivers is made difficult by the need to obtain and enter the geometric data
HEC HMS Hydrologic Engineering Centre Hydrologic Modeling System Designed to stimulate watershed hydrology Three components 1. Basin Model elements of basin, their connectivity, runoff parameters etc 2. Meteorologic Model rainfall, evapotranspiration data 3. Control Specifications start/stop timing, calculation intervals for the run
Hydrologic and Hydraulic (H&H) Modeling HEC RAS HEC HMS GIS HEC-GeoRAS HEC-GeoHMS
THANK YOU!! Any questions?