PRACTICAL UNIT 3 exercise task - iwhw.boku.ac.at · Ö Ö File Ö New Geometry Data Name: Praktikum_3 Ö File Ö Import Geometry Data Ö HEC‐RAS‐Format… open Naturgeometrie01

Practical Unit 3 3‐1

Exercise Course for Computer Based River Modelling

PRACTICAL UNIT 3 exercise task

Preparation of a hydraulic model with different roughness values and compound cross‐sections

The task of this practical unit is to prepare a hydraulic model with various roughness‐coefficients and compound cross‐sections for a hydraulic simulation. The main objective consists in the independent estimation of roughness‐coefficients and their implementation in the model. Furthermore, the implementation of levees and ineffective flow areas will be shown. Aims

Editing of a compound geometry with various roughness‐coefficients Estimation and definition of roughness‐coefficients Analysis and input of ineffective flow‐areas in the overbank flow areas Simulation of the entered input Interpretation and visualization of the results

Input‐Data

geometry (e.g. exported cross‐sections from a digital terrain model) information about land use (e.g. from aerial photographs or land development

plans) steady state flow data

estimation of roughness‐coefficients levees ineffective flow areas



PRACTICAL UNIT 3 approach

1. Opening of the existing geometry Creation of a new project:

File New Project filename: Praktikum_3

!!! Check if the model is set to SI‐Units!!! Import of geometry data:

File New Geometry Data Name: Praktikum_3 File Import Geometry Data HEC‐RAS‐Format…

open Naturgeometrie01 (import in SI‐Units!!!) The examination of the geometry shows a channel with compound cross‐sections with floodplains on both sides of river.

creation of a new project



2. Roughness of the floodplains Usually, estimations concerning the roughness coefficients of the floodplains are done by using information from literature. This information is normally derived from model experiments or from recalculation of observed flows. Calibration is most often very difficult, because floodplain roughness is only effective for higher discharges (Q > Qbankful), where calibration data is rare and uncertain. The 3D‐plot below shows the land use in this area: Abbr. Identification Description kst

chosen n chosen

F Field harvested maize‐field FF Floodplain Forest mainly grey alder HF High Forest fit for cutting C Confluent Creek with bushy embankment RF Riparian Forest narrow riparian forest consisting of alders and

willows

M Meadow extensively used PL Pasture Land bushy pasture

(n = 1 / kst) Note: theoretically seasonal changes (like the condition of vegetation) should be taken into account, which can be important for investigations and scientific research. But in the engineering practice this aspect is most often neglected.

Floodplain Forest

Confluent Creek

Riparian Forest

Field

Meadow

High Forest

Pasture Land

floodplain roughness



Cross‐ Section

LOB Channel ROB

1 0: F 184 276: RF 2 0: F 152: RF 184 258: RF 322: C 354: FF 3 0: F 165: RF 215 295: RF 426: C 453: FF 4 0: F 166: RF 224 340: RF 476: C 521: FF 5 0: F 213: RF 228 368: RF 463: C 529: FF 6 0: F 210: RF 235 402: RF 453: C 527: FF 7 0: M 288: RF 327.4 450: PL 517: C 592: FF 8 0: M 369: RF 407.1 500: PL 525: C 594: FF 9 0: HF 104: M 331: RF 375.7 450: PL 534: C 605: FF 10 0: HF 237: M 488: RF 594.5 677: PL 920: FF 11 0: HF 548.4 646: PL 1015: FF 12 0: HF 706 830: PL 1131: FF 13 0: HF 471.3 570: PL 865: FF Entering more than three roughness coefficients: Options Horizontal Variation in n Values

Besides „Station“ and „Elevation“ a third column should be activated in the cross‐section window. This allows for entering numerous roughness coefficients, each associated with a station of the cross‐section (each roughness coefficient ranges from its starting station to the next station with a defined roughness coefficient).

multiple roughness coefficients

Grain‐size distribution

grain size [mm]

Percen

tage

[%]



Estimation of roughness values (Literature):







3. Roughness of the channel For the estimation of the channel roughness a grain distribution curve of the bed sediments is used. Here, a step‐by‐step calculation of water levels under different roughness values has to be performed (calibration; see excel‐file).

a = 21,1 (Strickler) ds = d90 (Strickler) in [m]

Steady state flow data : Q = 500 m3/s Normal Depth Upstream / Downstream: can be calculated with the excel‐data (calculating the trend line, display formula) Starting the calculation and check of the results in longitudinal plot and cross‐sectional plots. The result was calculated without any error, nevertheless the cross‐section plot shows that the flow conditions are not given correctly, because

‐ floodplain flows occur without overtopping of the river banks ‐ flows take place through the confluent channel although it isn’t connected to

the main channel Note: 1D‐programs do not automatically calculate the „right“ flow. Therefore, the flow paths must be carefully modeled by the user by setting levees, ineffective flow areas and obstructions. Furthermore, it is important to note, that the flow paths can change with different discharges.

6s

st dak =

channel roughness

cross‐section 12 cross‐section 7

floodplain inundation

flow in the confluent creek



4. Adaption of geometry to real flow conditions Before doing any further calibration, the geometry must be adapted in a way, that the flow path is calculated correctly. Therefore, HEC‐RAS offers three possibilities:

Levees (Dyke): floodplain flow occurs not until this point is overtopped Ineffective Flow Areas: area can be filled with water, but no flow occurs

(stagnant conditions) Obstructions: blocked area where no water is stored

To avoid floodplain flow we set levees at the bank stations of every cross‐section and define the flow area of the confluent channel as ineffective: Options Levees Defaults (set to Bank Stations)

Options Ineffective Flow Areas Multiple Blocks

Entering of Station, Elevation and Permanence [yes]

levees ineffective flow areas obstructions

Levees Ineffective Flow Area



A check of plausibility of the flow path is given in the 3D‐plot. Now, calibration can be continued. Changing the channel’s roughness can be easily done in Tables Manning’s n or k values green shaded fields

The calculation results of each run for W.S.Elev. (Water Surface Elevation) can be copied from the result‐table and pasted into an excel‐file to compare the different roughness‐values. 5. Calculation of larger discharges The computation for larger discharges should be carried out with the following flow data: PF 1 = 500 m3/s PF 2 = 700 m3/s PF 3 = 1200 m3/s The longitudinal plot shows that the water level elevation for PF 1 and PF 2 is quite parallel due to the fact that for both profiles flow only occurs in the channel itself. In contrast PF 3, where the floodplains are inundated. Note: Using relative roughness values (Strickler, Manning) would theoretically require to define different roughness values for different discharges. E.g. taking roughness values of a MQ‐calibration and using them for a HQ‐calibration can lead to great bias.

larger discharges

Documents

PRACTICAL UNIT 3 exercise task - iwhw.boku.ac.at · Ö Ö File Ö New Geometry Data Name: Praktikum_3 Ö File Ö Import Geometry Data Ö HEC‐RAS‐Format… open Naturgeometrie01