INTRODUCTION TO HEC-RAS

Transcription

1 INTRODUCTION TO HEC-RAS HEC- RAS stands for Hydrologic Engineering Center s River Analysis System By U.S. Army Corps of Engineers One dimensional analysis of : 1. Steady flow 2. Unsteady flow 3. Sediment transport (mobile bed, as opposed to rigid bed) 4. Water temperature modeling

2 ABOUT THE U.S. ARMY CORPS OF ENGINEERS Under the Department of Defense Doing engineering, design and construction managements : dam, canals and flood protection Established since 1775 (about 240 years)

3 HEC-RAS (THE ENGINE FOR STEADY FLOW AND UNSTEADY FLOW) Steady flow Energy equation or momentum equation are used Energy equation (refer previous notes) Application of momentum equation on flow where energy is not conserved 2 2 V1 V2 z1 h1 z2 h2 h 2g 2g Momentum equation QV PA QV PA force acting on control surface 2 1 e

4 Unsteady flow Continuity and momentum equation for unsteady flow are used The continuity equation for unsteady flow A t Q x 0

5 MOMENTUM EQUATION Momentum equation (unsteady flow) Q uq y ga s ga sin b t x x gr 2 Neglecting Reynold shear stress, u 2 Reynold's stresses Derivation skipped y t x x Q uq s ga ga S o S f

6 The continuity equation for unsteady flow A t Q x 0 y t x x Q VA Q VQ s ga ga S o S f VA V VA y t x t V VV ys g g S o Sf t x x s ga ga So S f

7 ST. VENANT S EQUATION (SHALLOW WATER EQUATION) Continuity equation A t Q x 0 Momentum equation V VV y t x x s g g So Sf Flood waves propagation which are adequately represented by this model is called the Dynamic Wave. So this model is also known as the Dynamic Wave model Shallow water equation Horizontal length scale >> vertical length scale, Therefore, vertical pressure gradient is almost hydrostatic

8 KINEMATIC WAVE THEORY - It is one of the many approximations from dynamic wave - Some insignificant terms in dynamic wave equations are neglected Momentum equation V VV y t x x s g g So Sf Insignificant due to long and flat wave approximation in kinematic wave theory S o S Balance between gravitational and frction forces! uniform flow f

9 KINEMATIC WAVE THEORY Q Continuity equation 0 Q constant x Uniform flow equations : 1) Chezy's equation Q C RS 1 1) Manning's equation : C R n 1 2/3 1/2 Therefore, Q= R So A n o 1/6

10 HEC- RAS : SIMULATION OF STEADY FLOW REQUIRED INFORMATION GEOMETRY DATA CHANNEL DIMENSION ETC FLOW DATA DISCHARGE BOUNDARY CONDITIONS (B.C.) DOWNSTREAM FLOW DEPTH, UPSTREAM FLOW DEPTH. DEPENDING ON FLOW TYPE (SUPER OR SUB CRITICAL), THE REQUIREMENT OF B.C ARE DIFFERENT SUBCRITICAL FLOW = DOWNSTREAM FLOW CONDITION IS REQUIRED SUPERCRITICAL FLOW = UPSTREAM AND DOWNSTREAM FLOW CONDITION ARE REQUIRED OTHER INFORMATION EXISTENCE OF STRUCTURES : EMBANKMENT, BRIGDE PIER, JUNCTION ETC

11 HEC- RAS : SIMULATION OF STEADY FLOW Geometric data Perform steady flow cal. (RUN) View cross section, profiles etc HEC-RAS MAIN WINDOW Flow data

12 Open example Critical Creek Example 1

13 Step 1 : Check geometric data Geometric data You will see this window showing the Geometric data

14 Step 1 : Check geometric data upstream Load other geometric data Reach length downstream You will see this window showing new Geometric data

15 Cross section data Summary of geometric data (can edit here as well) Step 1 : Check geometric data You will see this window showing the cross section data

16 Step 1 : Check geometric data LOB = left over bank, ROB= right over bank LOB ROB Main channel (btwn two red dots)

17 Step 1 : Check geometric data Save geometry data!

18 Step 2 : Check flow data HEC-RAS MAIN WINDOW Flow data

19 Step 2 : Check flow data Boundary condition

20 Step 2 : Check flow data Boundary condition For this example, normal depth with So=0.01 is set for downstream B.C.

21 Step 2 : Check flow data Save Flow data!

22 Step 3 : Run SIMULATION Give an ID Geometry data file Flow data file The flow type in the regime RUN

23 See Options Set critical depth always calculated

24 Step 3 : Run SIMULATION RUN STATUS Computation messages!!

25 Step 4 : Flow Output Review View profiles HEC-RAS MAIN WINDOW Flow depth is close or equal to critical depth from station 8 to 12

26 View summary errors, warning and notes

27 RS Velocity head has change more than 0.5m 2. Energy loss was greater than 0.3m between current and previous section 3. Reason of 1. and 2. are inter-related and are due to drastic change in velocity. Since velocity is influenced by cross section. For example, between two sections there is a expansion/contraction of channel section area, but only two sets of data on each sections are given (not enough to represent the expansion / contraction of the channel section) inadequate cross-section data!. Other reasons might be : cross section data inputted wrongly.

28 RS Energy equation could not be balanced program could not calculate enough energy losses to achieved subcritical flow upstream. Therefore, program choses to use critical depth (default in the program), and continued the calculation 1. Divided flow computed for his cross-section flow occurs at two or more separate portion in the channel

29 There is flow in this portion too!!

30 Step 5 : Modify and fix cross section! - Maybe cross sections information not adequate - Add more cross sections Before modification of cross section

31 Step 5 : Modify and fix cross section! - To add more cross sections for all reaches Interpolation feature

32 Step 5 : Modify and fix cross section! - To add more cross sections for all reaches Interpolation feature You can set other values Press this to interpolate

33

34 Before addition of cross sections After addition of cross sections

35 Correct interpolation between RS 11 and RS 10 Delete one interpolation Use this feature Re-interpolate

36 Step 6 : Change simulation conditions - In previous simulation, some subcritical depth could not be calculated, the program use the critical when this occurs. - Therefore, there might be possibility of mixed flow (subcritcal and supercritical in the reaches) - So, we change the simulation to mixed flow type - But, we need to add in boundary conditions to do mixed flow type simulation!

37 Upstream BC is set to Normal Depth with S=0.01m

38 Step 7 : Re-run with new data and simulation conditions

39 Step 8 : Examine results View of profile output table Water surface < Critical depth (supercritical flow) Froude number <1??! Fr e e V average velocity = Q / A, A flow area e e e e V gy y average depth at that section

40 View Detail output HEC-RAS MAIN WINDOW

41 Hydraulic jump

42 Hydraulic jump? Supercritical to subcritical