Research ArticleIdentification of Manningrsquos Coefficient Using HEC-RAS ModelUpstream Al-Amarah Barrage

Sarmad A Abbas Ali H Al-Aboodi and Husham T Ibrahim

Department of Civil Engineering College of Engineering University of Basrah Basrah Iraq

Correspondence should be addressed to Husham T Ibrahim drhushamibrahimgmailcom

Received 16 November 2019 Revised 23 February 2020 Accepted 5 March 2020 Published 3 April 2020

Academic Editor Chuan-Yu Wu

Copyright copy 2020 Sarmad A Abbas et al is is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

In understanding the hydraulic characteristics of river system flow the hydraulic simulation models are essential tools is studysubmits the results of the proposition of a hydraulic model in order to determine the roughness coefficient (Manningrsquos coefficientn) of the Tigris River along 35 km within theMaysan Governorate south of Iraq HEC-RAS software was the simulation tool usedin this studye HEC-RAS model was adopted calibrated and validated in adopting two sets of observed water levels Graphicaland statistical approaches were used for model calibration and verification Results from this investigation showed that a value ofManningrsquos coefficient of 0025 gave an acceptable agreement between observed and simulated values of water levels

1 Introduction

In predicting flood the mapping and estimation of floodvolume and various hydrodynamic models are developed andapplied to several rivers with the aid of computers and nu-merical techniques In forecasting and warning of a flood theriver stage and discharge are considered as the variablesAmong various hydraulic parameters especially in hydraulicmodelling the roughness coefficient (or Manningrsquos coefficient)is an important parameter Knowing Manningrsquos coefficient ofroughness in open channels is of great importance in achievingany modelling that aims to study a variable of river variablessuch as flooding sediment transport and others

e parameters from models practiced in the engineeringof hydraulics may be grouped into two groups physical pa-rameters and empirical parameters e physical parametersdescribe the physical properties of materials usually they areconstants while the empirical parameters are established onmathematical models Because of the complexities of hydraulicengineering specific values for the empirical parameters suchasManningrsquos coefficient n are often uncertain As an empiricalparameter the roughness coefficient (Manningrsquos n) includesthe elements of roughness of channel surface bed materialvegetation channel alignment and irregularities channel shape

and size stage and discharge suspended sediment load and bedsediment loads etc [1] Several empirical formulas for theestimation of the value of n in practical problems have beenproposed in the past [2]

Determination of ldquonrdquo value presents an influential andcreative task in the hydraulics of open channel flows Due tochanges in time and space in the value of the coefficient nthe determination of the coefficient n becomes a verycomplex problem is change in n value depends ongeometric geomorphological and hydraulic parameters ofwater current and river or channel beds [3]

A number of researchers suggested several methods forthe determination of n Ramesh et al determined a singleManningrsquos n value for the flow in open channel using op-timization method they took the boundary conditions asconstraints [4] Ali et al investigated the roughness value ofthe Parit Karjo channel in Malaysia using Manningsequation ey found that the roughness value was between004 and 048 [5] Ding et al presented a numerical methodfor estimating Manningrsquos coefficient in modelling of flow inshallow waters [2] Abdul Hameed investigated four for-mulas to estimate Manningrsquos n (Manning Bajorunas Ein-stein and Kennedy) at Falluja regulator on the EuphratesRiver in Iraq en he was noticed that Einstein and

HindawiJournal of EngineeringVolume 2020 Article ID 6450825 7 pageshttpsdoiorg10115520206450825

Bajorunas gave values closer to the exact than the others did[6] Parhi used the HEC-RAS model to calibrate and validatethe value of the roughness coefficient ldquonrdquo forMahanadi Riverin Odisha (India) He considered the floods for the years2001 and 2003 in the calibration and verification [7]Shamkhi and Attab also employed the HEC-RAS model toinvestigate and estimate the value of ldquonrdquo downstream of KutBarrage in Wasit Iraq [8]

e target of this study is to calibrate and estimate thevalue of Manningrsquos roughness coefficient (n) in areach ofTigris River upstream of Amarh Barrage because there is noprevious information aboutManningrsquos n in this reach of TigrisRiver in addition to importance of the barrage in regulatingdischarges to Basrah provincee importance of knowing thevalue ofManningrsquos coefficient in the study reach becomes clearwhen you know that the study reach suffers from sedimen-tation bars due to the low water speed in the river due to thelow discharges in this reach in most times of the year isleads to a decrease in the depth and a continuous change in thesection of the river However the study reach may be exposedto floods coming from Iran for a short period of the yearwhich may cause flooding in the city erefore it is necessaryto know Manningrsquos coefficient in order to conduct morestudies to analyse the risk of flooding as well as to know theamount and locations of sediment deposition

2 Model Description

e United States Army Corps of Engineers developed theHydrologic Engineering Centrersquos (HEC) River AnalysisSystem (RAS) model e HEC-RAS model version 41 al-lows operating the hydraulics calculations of one dimen-sional steady and unsteady river flow It is regularly used tocalculate water surface profiles and energy grade lines in 1Dsteady state and gradually varied flow analysis

e model used empirical Manningrsquos equation in theform of equation (1) to supply the relationship betweenthe river discharge hydraulic resistance river geometryand the friction energy loss In the case of changing inchannel geometry energy losses were assessed using co-efficients of contraction or expansion multiplied by thechange in velocity head Head loss between two sectionswill be computed from equation (2) while the watersurface will be calculated from the energy equationequation (3) [1]

In this study HEC-RAS has been used to accomplish onedimensional steady and hydraulic calculation for a reach ofTigris River at the upstream of Al-Amarah Barrage tosimulate the flow and to distinguish the suitable value ofManningrsquos roughness coefficient (n)

Q KS12f (1)

he LSf + Cα1v212g

+α2v222g

1113888 1113889 (2)

H Z + y +α v2

2g (3)

where Q flow rate K conveyance of the channelSf energy slope g acceleration due to gravity he energyhead loss C expansion or contraction coefficient α1 andα2 velocity weighting coefficients v1 and v2 average ve-locities Hwater surface level above a specified datumZ bed elevation y depth of flow α kinetic energycorrelation coefficient and v average velocity

3 Reach of Study

e reach of study is a segment of the Tigris River 35 km inlength located in the center of Amarah city Maysan

Figure 1 Location of study reach

2 Journal of Engineering

province Iraqis reach is upstream Al-Amarah barrage Itis located between latitudes 31865degN and 31850degN andlongitudes 47115degE and 47155degE Figure 1 shows the lo-cation of the reach of the study

4 Methods

41 Geometric and Hydrologic Data e geometry of theriver reach boundary conditions upstream and downstream

CS1 CS2CS3

CS4

CS5CS6

CS7 CS8 CS9CS10

CS11

CS12

CS13

CS14

CS15

CS16CS18

CS17

Direction of flow The barrage

US

DS

Figure 2 Locations of sections over study reach

Figure 3 Study reach in the HEC-RAS model

Journal of Engineering 3

are necessary parameters for implementing a simulation offlow using HEC-RAS Special thanks to Khassaf and Hassan[9] who collected the data used in this paper and for thepermission to use these data ey used the ADCP (acousticDoppler current profiler) technology SonTek river trackersurveyor (mounted on a boat) in determining area of crosssections water velocity and river discharge ey used VanVeenrsquos grab in collecting samples from the riverbed etotal number of cross sections observed over the reach waseighteen cross sections as shown in Figure 2 a cross sectionevery 200 to 250m intervals extending over an entire lengthof 35 km of the river

42 Development of HEC-RAS Model e main parametersfor the HEC-RAS hydraulic model are geometric and flow datae geometric data have been developed by drafting the riverschematically with the direction of flowis was done with theaid of the button of river reach in the HEC-RAS main menu

(window) the method was explained in detail in the softwaremanual [1] Figure 3 shows the schematic diagram of the ge-ometry of the Tigris River study reach en cross section dataincluding cross section coordinate down reach length Man-ningrsquos n values main channel and contraction or expansioncoefficients were entered via the cross section data editor buttonon the same window

Cross section coordinates were defined by entering theriver station and elevation points from left to right bank insequence along the river Eighteen cross sections were al-located over the river reach Figure 4 represents a samplecross section After the geometric data are entered and saveddata of discharge were defined for the calculation processand finalizing the model creation

43 Calibration and Validation of HEC-RAS ModelCalibration is a comparison between a known measurement(measured water levels) and themeasurement using themodel

(a)

(b)

Figure 4 A sample of cross sections (a) Cross section No 18 from the ADCP (b) Same cross section in the HEC-RAS

4 Journal of Engineering

(simulated levels) it checks the accuracy of the model eHEC-RAS model for the Tigris River was calibrated forpredicting water levels and hence determining a single value ofManningrsquos coefficient n In order to check the ability of thecalibrated model in predicting the water levels for differentriver flows the model should be validated (verified) usingmeasurement data other than the one used in calibrating themodel

e data published by Hassan are divided into twogroups of data for calibration the water level values for the18 cross sections corresponding to the 50m3s flow rate wereused while for the validation the water levels in section 18for 36 flow rates were used

431 Model Calibration In the calibration Manningrsquos co-efficient ldquonrdquo was altered continually until the variations betweenobserved and simulated water levels were within the acceptablelimits e procedure followed in the calibration is given in theflowchart in Figure 5e calibration procedure gave the correctvalue of Manningrsquos coefficient of the river reach

is model calibration was done using an observed flowof 500m3s at the upstream of the river reach at crosssection No 1 is flow rate (500m3s) represents thedischarge released from Kut barrage upstream of the studyreach this flow considered a low flow in the Tigris River andis the available flow in most parts of the year e calibrationwas performed by comparing the results of the water level ineach cross section obtained by the model with the observedlevels

From the literature reviewed previously [6 8] the initialvalue of n was elected to be in the range 0020 to 0035 withan increment of 0005 e flow in this reach of Tigris occursonly in main channel no flow occurs in banks therefore thecalibration of the HEC-RASmodel was achieved using singlevalues of n applied to the main channel of the river be-ginning from 0020 By starting with an initial value the nvalue was altered until the differences between observed andsimulation water levels became small as far as possible Sothe outcomes of the HEC-RAS model for distinct values of nwere compared with the observed water surface profiletabulated in Table 1 and represented in Figure 6

It is obvious from Figure 6 that the best choice forManningrsquos n value is 0025 because it gives a good matchingbetween observed and simulated water surface profile

To provide an additional accuracy measurement theroot mean square deviation (RMSD) was tested betweenobserved and simulated water surface elevations e rootmean square deviation is commonly used as a measure of thedifferences between predicted and observed values as pre-sented in the following equation

RMSD

1113936Ni1 Yisim minus Yiobs( 1113857

2

N

1113971

(4)

where N the number of data (total number of cross sec-tions) i the identification number of cross sectionYisim water level for cross section No i (simulated) andYiobs water level for cross section No i (observed) Valuesof RMSD are tabulated in Table 2 it is also clear that the best

Input geometric and required hydraulic data

Input boundary and initial conditions

Choose a trial value of Mannings n

Run HEC-RAS

Steady floweditor

River reach and cross

section editors

Comparison Are differences between

calculated and observed water levelswithin the allowable limits

Print HEC-RAS outputs

YES

NO

Figure 5 Calibration flowchart

Journal of Engineering 5

Table 1 Observed and calculated WL for different Manningrsquos n

CS No WL (observed)Water level (calculated)

n 002 n 0025 n 003 n 00351 10 974 1003 1033 10712 998 969 1003 1018 10623 996 965 1003 1015 1064 994 961 1001 1017 10555 994 964 1001 1015 10486 99 957 997 1013 10477 988 955 995 1011 1058 986 953 993 1009 1059 985 953 988 1008 104910 984 951 991 1011 104211 982 953 989 1005 104612 982 949 989 1005 104613 98 947 987 1003 104414 979 948 986 994 104315 976 943 983 999 10416 975 942 982 998 103317 975 942 982 998 103318 974 941 979 997 1025

90

95

100

105

110

0 400 800 1200 1600 2000 2400 2800 3200 3600

Wat

er le

vel (

m)

Distance from section no 1 (m)

Observed WLSimulated WL n = 02Simulated WL n = 025

Simulated WL n = 030Simulated WL n = 035

Figure 6 Observed and simulated water surface profile

Table 2 RMSD values for Manningrsquos n

Manningrsquos roughness coefficient 0020 0025 0030 0035RMSD 0318 0065 0231 0616

Table 3 Validation of model

No 1 2 3 4 5 6 7 8 9 10 11 12Q 50 47 45 42 40 39 38 37 35 33 64 80WL (Ob) 10 986 977 963 953 948 943 914 931 917 106 112WL (Sim) 1012 974 969 945 911 966 919 951 922 9 1056 11No 13 14 15 16 17 18 19 20 21 22 23 24Q 85 76 78 67 63 72 52 56 59 63 66 68WL (Ob) 1141 112 1115 1072 1056 1092 1009 1026 1039 1056 1078 108WL (Sim) 1106 11 1139 1038 1019 1121 951 1039 1035 1106 1064 106No 25 26 27 28 29 30 31 32 33 34 35 36Q 72 136 76 57 48 83 80 83 85 61 67 69WL (Ob) 1092 131 1117 1031 991 1134 1123 1134 1162 1048 106 108WL (Sim) 1087 138 1102 1027 987 1128 1105 1128 1136 1057 1068 102

6 Journal of Engineering

Manning roughness coefficient value for the study reach is0025

432 Model Validation ere are 36 measurementsavailable for the verification of the model Hassan did these36 observations in section No 18 Validation was performedin order to check the suitability of a single value for thecoefficient of 0025 with several values of flow rates ecomparison between observed and predicted values of waterlevel for these flow rates is shown in Table 3 and Figure 7Depending on the value of the coefficient of determination(R2) 091 the elected Manningrsquos n (0025) gives an ac-ceptable matching between observed and predicted waterlevels [10]

For high flow rates the model shows high errors this canbe enhanced by collecting more flow observations includingmultiple levels of flow (low medium and high) in thecalibration of themodel Also wemay do a value refinementmaybe in the range 0022 to 0028

5 Conclusions

HEC-RAS software can be used for hydraulic modellingerefore it can be used effectively in modelling andsimulating water surface profiles A hydraulic model wasperformed using HEC-RAS on the Tigris River within areach of 35 km in Al-Amarah city Maysan province inorder to identify the value of n coefficient e roughnesscoefficient (n) equals 0025 which gives an acceptablematching between observed and predicted water levels emodel can be further improved by using GIS with HEC-RASto produce an accurate channel geometry and hence anaccurate flow simulation In addition flood risk can beanalysed using HEC-RAS 20 using the result as a deter-mined value of Manningrsquos coefficient

Data Availability

e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

e authors declare that they have no conflicts of interest

References

[1] HEC-RAS User Manual Davis Version 40 US Army Corpsof Engineers Hydrologic Engineering Center Davis CAUSA 2008

[2] Y Ding Y Jia and S S YWang ldquoIdentification ofManningrsquosroughness coefficients in shallow water flowsrdquo Journal ofHydraulic Engineering vol 130 no 6 pp 501ndash510 2004

[3] E Zic M Vranjes and N Ozanic ldquoMethods of roughnesscoefficient determination in natural riverbedsrdquo in Proceedingsof the International Symposium on Water Management andHydraulic Engineering Ohrid Macedonia September 2009

[4] R Ramesh B Datta S Bhallamudi and A Narayana ldquoOp-timal estimation of roughness in open-channel flowsrdquo Journalof Hydraulic Engineering vol 126 no 4 pp 299ndash303 1997

[5] Z MD Ali N H Abdul Karim and M A M Razi ldquoStudy onroughness coefficient at natural channelrdquo in Proceedings of theInternational Conference on Environment (ICENV 2010)Penang Malaysia July 2010

[6] U H Abdul Hameed ldquoDetermination of manning roughnessvalue for Euphrates river at Al-Falluja barrages using differenttheoriesrdquo Iraq Academic Scientific Journals vol 2 no 2

[7] P K Parhi ldquoHEC-RAS model for Mannnigrsquos roughness acase studyrdquo Open Journal of Modern Hydrology vol 3 no 3pp 97ndash101 2013

[8] M S Shamkhi and Z S Attab ldquoEstimation of Manningrsquosroughness coefficient for Tigris river by using HEC-RASmodelrdquo WASIT Journal of Engineering Sciences vol 6 no 3pp 90ndash97 2018

[9] S I Khassaf and A A Hassan Sediment transport modelingfor the upstream of Al-Amarah barrage PhD esis Collegeof Engineering University of Basrah Basrah Iraq 2014

[10] J K Patel C H Kapadia and D B Owen Handbook ofStatistical Distributions John Wiley amp Sons New York NYUSA 1976

R2 = 091

9

10

11

12

13

14

15

9 10 11 12 13 14

Sim

ulat

ed w

ater

leve

l (m

)

Observed water level (m)

Figure 7 Observed versus simulated water surface level

Journal of Engineering 7

province Iraqis reach is upstream Al-Amarah barrage Itis located between latitudes 31865degN and 31850degN andlongitudes 47115degE and 47155degE Figure 1 shows the lo-cation of the reach of the study

4 Methods

41 Geometric and Hydrologic Data e geometry of theriver reach boundary conditions upstream and downstream

CS1 CS2CS3

CS4

CS5CS6

CS7 CS8 CS9CS10

CS11

CS12

CS13

CS14

CS15

CS16CS18

CS17

Direction of flow The barrage

US

DS

Figure 2 Locations of sections over study reach

Figure 3 Study reach in the HEC-RAS model

Journal of Engineering 3

are necessary parameters for implementing a simulation offlow using HEC-RAS Special thanks to Khassaf and Hassan[9] who collected the data used in this paper and for thepermission to use these data ey used the ADCP (acousticDoppler current profiler) technology SonTek river trackersurveyor (mounted on a boat) in determining area of crosssections water velocity and river discharge ey used VanVeenrsquos grab in collecting samples from the riverbed etotal number of cross sections observed over the reach waseighteen cross sections as shown in Figure 2 a cross sectionevery 200 to 250m intervals extending over an entire lengthof 35 km of the river

42 Development of HEC-RAS Model e main parametersfor the HEC-RAS hydraulic model are geometric and flow datae geometric data have been developed by drafting the riverschematically with the direction of flowis was done with theaid of the button of river reach in the HEC-RAS main menu

(window) the method was explained in detail in the softwaremanual [1] Figure 3 shows the schematic diagram of the ge-ometry of the Tigris River study reach en cross section dataincluding cross section coordinate down reach length Man-ningrsquos n values main channel and contraction or expansioncoefficients were entered via the cross section data editor buttonon the same window

Cross section coordinates were defined by entering theriver station and elevation points from left to right bank insequence along the river Eighteen cross sections were al-located over the river reach Figure 4 represents a samplecross section After the geometric data are entered and saveddata of discharge were defined for the calculation processand finalizing the model creation

43 Calibration and Validation of HEC-RAS ModelCalibration is a comparison between a known measurement(measured water levels) and themeasurement using themodel

(a)

(b)

Figure 4 A sample of cross sections (a) Cross section No 18 from the ADCP (b) Same cross section in the HEC-RAS

4 Journal of Engineering

(simulated levels) it checks the accuracy of the model eHEC-RAS model for the Tigris River was calibrated forpredicting water levels and hence determining a single value ofManningrsquos coefficient n In order to check the ability of thecalibrated model in predicting the water levels for differentriver flows the model should be validated (verified) usingmeasurement data other than the one used in calibrating themodel

e data published by Hassan are divided into twogroups of data for calibration the water level values for the18 cross sections corresponding to the 50m3s flow rate wereused while for the validation the water levels in section 18for 36 flow rates were used

431 Model Calibration In the calibration Manningrsquos co-efficient ldquonrdquo was altered continually until the variations betweenobserved and simulated water levels were within the acceptablelimits e procedure followed in the calibration is given in theflowchart in Figure 5e calibration procedure gave the correctvalue of Manningrsquos coefficient of the river reach

is model calibration was done using an observed flowof 500m3s at the upstream of the river reach at crosssection No 1 is flow rate (500m3s) represents thedischarge released from Kut barrage upstream of the studyreach this flow considered a low flow in the Tigris River andis the available flow in most parts of the year e calibrationwas performed by comparing the results of the water level ineach cross section obtained by the model with the observedlevels

From the literature reviewed previously [6 8] the initialvalue of n was elected to be in the range 0020 to 0035 withan increment of 0005 e flow in this reach of Tigris occursonly in main channel no flow occurs in banks therefore thecalibration of the HEC-RASmodel was achieved using singlevalues of n applied to the main channel of the river be-ginning from 0020 By starting with an initial value the nvalue was altered until the differences between observed andsimulation water levels became small as far as possible Sothe outcomes of the HEC-RAS model for distinct values of nwere compared with the observed water surface profiletabulated in Table 1 and represented in Figure 6

It is obvious from Figure 6 that the best choice forManningrsquos n value is 0025 because it gives a good matchingbetween observed and simulated water surface profile

To provide an additional accuracy measurement theroot mean square deviation (RMSD) was tested betweenobserved and simulated water surface elevations e rootmean square deviation is commonly used as a measure of thedifferences between predicted and observed values as pre-sented in the following equation

RMSD

1113936Ni1 Yisim minus Yiobs( 1113857

2

N

1113971

(4)

where N the number of data (total number of cross sec-tions) i the identification number of cross sectionYisim water level for cross section No i (simulated) andYiobs water level for cross section No i (observed) Valuesof RMSD are tabulated in Table 2 it is also clear that the best

Input geometric and required hydraulic data

Input boundary and initial conditions

Choose a trial value of Mannings n

Run HEC-RAS

Steady floweditor

River reach and cross

section editors

Comparison Are differences between

calculated and observed water levelswithin the allowable limits

Print HEC-RAS outputs

YES

NO

Figure 5 Calibration flowchart

Journal of Engineering 5

Table 1 Observed and calculated WL for different Manningrsquos n

CS No WL (observed)Water level (calculated)

n 002 n 0025 n 003 n 00351 10 974 1003 1033 10712 998 969 1003 1018 10623 996 965 1003 1015 1064 994 961 1001 1017 10555 994 964 1001 1015 10486 99 957 997 1013 10477 988 955 995 1011 1058 986 953 993 1009 1059 985 953 988 1008 104910 984 951 991 1011 104211 982 953 989 1005 104612 982 949 989 1005 104613 98 947 987 1003 104414 979 948 986 994 104315 976 943 983 999 10416 975 942 982 998 103317 975 942 982 998 103318 974 941 979 997 1025

90

95

100

105

110

0 400 800 1200 1600 2000 2400 2800 3200 3600

Wat

er le

vel (

m)

Distance from section no 1 (m)

Observed WLSimulated WL n = 02Simulated WL n = 025

Simulated WL n = 030Simulated WL n = 035

Figure 6 Observed and simulated water surface profile

Table 2 RMSD values for Manningrsquos n

Manningrsquos roughness coefficient 0020 0025 0030 0035RMSD 0318 0065 0231 0616

Table 3 Validation of model

No 1 2 3 4 5 6 7 8 9 10 11 12Q 50 47 45 42 40 39 38 37 35 33 64 80WL (Ob) 10 986 977 963 953 948 943 914 931 917 106 112WL (Sim) 1012 974 969 945 911 966 919 951 922 9 1056 11No 13 14 15 16 17 18 19 20 21 22 23 24Q 85 76 78 67 63 72 52 56 59 63 66 68WL (Ob) 1141 112 1115 1072 1056 1092 1009 1026 1039 1056 1078 108WL (Sim) 1106 11 1139 1038 1019 1121 951 1039 1035 1106 1064 106No 25 26 27 28 29 30 31 32 33 34 35 36Q 72 136 76 57 48 83 80 83 85 61 67 69WL (Ob) 1092 131 1117 1031 991 1134 1123 1134 1162 1048 106 108WL (Sim) 1087 138 1102 1027 987 1128 1105 1128 1136 1057 1068 102

6 Journal of Engineering

Manning roughness coefficient value for the study reach is0025

432 Model Validation ere are 36 measurementsavailable for the verification of the model Hassan did these36 observations in section No 18 Validation was performedin order to check the suitability of a single value for thecoefficient of 0025 with several values of flow rates ecomparison between observed and predicted values of waterlevel for these flow rates is shown in Table 3 and Figure 7Depending on the value of the coefficient of determination(R2) 091 the elected Manningrsquos n (0025) gives an ac-ceptable matching between observed and predicted waterlevels [10]

For high flow rates the model shows high errors this canbe enhanced by collecting more flow observations includingmultiple levels of flow (low medium and high) in thecalibration of themodel Also wemay do a value refinementmaybe in the range 0022 to 0028

5 Conclusions

HEC-RAS software can be used for hydraulic modellingerefore it can be used effectively in modelling andsimulating water surface profiles A hydraulic model wasperformed using HEC-RAS on the Tigris River within areach of 35 km in Al-Amarah city Maysan province inorder to identify the value of n coefficient e roughnesscoefficient (n) equals 0025 which gives an acceptablematching between observed and predicted water levels emodel can be further improved by using GIS with HEC-RASto produce an accurate channel geometry and hence anaccurate flow simulation In addition flood risk can beanalysed using HEC-RAS 20 using the result as a deter-mined value of Manningrsquos coefficient

Data Availability

e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

e authors declare that they have no conflicts of interest

References

[1] HEC-RAS User Manual Davis Version 40 US Army Corpsof Engineers Hydrologic Engineering Center Davis CAUSA 2008

[2] Y Ding Y Jia and S S YWang ldquoIdentification ofManningrsquosroughness coefficients in shallow water flowsrdquo Journal ofHydraulic Engineering vol 130 no 6 pp 501ndash510 2004

[3] E Zic M Vranjes and N Ozanic ldquoMethods of roughnesscoefficient determination in natural riverbedsrdquo in Proceedingsof the International Symposium on Water Management andHydraulic Engineering Ohrid Macedonia September 2009

[4] R Ramesh B Datta S Bhallamudi and A Narayana ldquoOp-timal estimation of roughness in open-channel flowsrdquo Journalof Hydraulic Engineering vol 126 no 4 pp 299ndash303 1997

[5] Z MD Ali N H Abdul Karim and M A M Razi ldquoStudy onroughness coefficient at natural channelrdquo in Proceedings of theInternational Conference on Environment (ICENV 2010)Penang Malaysia July 2010

[6] U H Abdul Hameed ldquoDetermination of manning roughnessvalue for Euphrates river at Al-Falluja barrages using differenttheoriesrdquo Iraq Academic Scientific Journals vol 2 no 2

[7] P K Parhi ldquoHEC-RAS model for Mannnigrsquos roughness acase studyrdquo Open Journal of Modern Hydrology vol 3 no 3pp 97ndash101 2013

[8] M S Shamkhi and Z S Attab ldquoEstimation of Manningrsquosroughness coefficient for Tigris river by using HEC-RASmodelrdquo WASIT Journal of Engineering Sciences vol 6 no 3pp 90ndash97 2018

[9] S I Khassaf and A A Hassan Sediment transport modelingfor the upstream of Al-Amarah barrage PhD esis Collegeof Engineering University of Basrah Basrah Iraq 2014

[10] J K Patel C H Kapadia and D B Owen Handbook ofStatistical Distributions John Wiley amp Sons New York NYUSA 1976

R2 = 091

9

10

11

12

13

14

15

9 10 11 12 13 14

Sim

ulat

ed w

ater

leve

l (m

)

Observed water level (m)

Figure 7 Observed versus simulated water surface level

Journal of Engineering 7

are necessary parameters for implementing a simulation offlow using HEC-RAS Special thanks to Khassaf and Hassan[9] who collected the data used in this paper and for thepermission to use these data ey used the ADCP (acousticDoppler current profiler) technology SonTek river trackersurveyor (mounted on a boat) in determining area of crosssections water velocity and river discharge ey used VanVeenrsquos grab in collecting samples from the riverbed etotal number of cross sections observed over the reach waseighteen cross sections as shown in Figure 2 a cross sectionevery 200 to 250m intervals extending over an entire lengthof 35 km of the river

42 Development of HEC-RAS Model e main parametersfor the HEC-RAS hydraulic model are geometric and flow datae geometric data have been developed by drafting the riverschematically with the direction of flowis was done with theaid of the button of river reach in the HEC-RAS main menu

(window) the method was explained in detail in the softwaremanual [1] Figure 3 shows the schematic diagram of the ge-ometry of the Tigris River study reach en cross section dataincluding cross section coordinate down reach length Man-ningrsquos n values main channel and contraction or expansioncoefficients were entered via the cross section data editor buttonon the same window

Cross section coordinates were defined by entering theriver station and elevation points from left to right bank insequence along the river Eighteen cross sections were al-located over the river reach Figure 4 represents a samplecross section After the geometric data are entered and saveddata of discharge were defined for the calculation processand finalizing the model creation

43 Calibration and Validation of HEC-RAS ModelCalibration is a comparison between a known measurement(measured water levels) and themeasurement using themodel

(a)

(b)

Figure 4 A sample of cross sections (a) Cross section No 18 from the ADCP (b) Same cross section in the HEC-RAS

4 Journal of Engineering

(simulated levels) it checks the accuracy of the model eHEC-RAS model for the Tigris River was calibrated forpredicting water levels and hence determining a single value ofManningrsquos coefficient n In order to check the ability of thecalibrated model in predicting the water levels for differentriver flows the model should be validated (verified) usingmeasurement data other than the one used in calibrating themodel

e data published by Hassan are divided into twogroups of data for calibration the water level values for the18 cross sections corresponding to the 50m3s flow rate wereused while for the validation the water levels in section 18for 36 flow rates were used

431 Model Calibration In the calibration Manningrsquos co-efficient ldquonrdquo was altered continually until the variations betweenobserved and simulated water levels were within the acceptablelimits e procedure followed in the calibration is given in theflowchart in Figure 5e calibration procedure gave the correctvalue of Manningrsquos coefficient of the river reach

is model calibration was done using an observed flowof 500m3s at the upstream of the river reach at crosssection No 1 is flow rate (500m3s) represents thedischarge released from Kut barrage upstream of the studyreach this flow considered a low flow in the Tigris River andis the available flow in most parts of the year e calibrationwas performed by comparing the results of the water level ineach cross section obtained by the model with the observedlevels

From the literature reviewed previously [6 8] the initialvalue of n was elected to be in the range 0020 to 0035 withan increment of 0005 e flow in this reach of Tigris occursonly in main channel no flow occurs in banks therefore thecalibration of the HEC-RASmodel was achieved using singlevalues of n applied to the main channel of the river be-ginning from 0020 By starting with an initial value the nvalue was altered until the differences between observed andsimulation water levels became small as far as possible Sothe outcomes of the HEC-RAS model for distinct values of nwere compared with the observed water surface profiletabulated in Table 1 and represented in Figure 6

It is obvious from Figure 6 that the best choice forManningrsquos n value is 0025 because it gives a good matchingbetween observed and simulated water surface profile

To provide an additional accuracy measurement theroot mean square deviation (RMSD) was tested betweenobserved and simulated water surface elevations e rootmean square deviation is commonly used as a measure of thedifferences between predicted and observed values as pre-sented in the following equation

RMSD

1113936Ni1 Yisim minus Yiobs( 1113857

2

N

1113971

(4)

where N the number of data (total number of cross sec-tions) i the identification number of cross sectionYisim water level for cross section No i (simulated) andYiobs water level for cross section No i (observed) Valuesof RMSD are tabulated in Table 2 it is also clear that the best

Input geometric and required hydraulic data

Input boundary and initial conditions

Choose a trial value of Mannings n

Run HEC-RAS

Steady floweditor

River reach and cross

section editors

Comparison Are differences between

calculated and observed water levelswithin the allowable limits

Print HEC-RAS outputs

YES

NO

Figure 5 Calibration flowchart

Journal of Engineering 5

Table 1 Observed and calculated WL for different Manningrsquos n

CS No WL (observed)Water level (calculated)

n 002 n 0025 n 003 n 00351 10 974 1003 1033 10712 998 969 1003 1018 10623 996 965 1003 1015 1064 994 961 1001 1017 10555 994 964 1001 1015 10486 99 957 997 1013 10477 988 955 995 1011 1058 986 953 993 1009 1059 985 953 988 1008 104910 984 951 991 1011 104211 982 953 989 1005 104612 982 949 989 1005 104613 98 947 987 1003 104414 979 948 986 994 104315 976 943 983 999 10416 975 942 982 998 103317 975 942 982 998 103318 974 941 979 997 1025

90

95

100

105

110

0 400 800 1200 1600 2000 2400 2800 3200 3600

Wat

er le

vel (

m)

Distance from section no 1 (m)

Observed WLSimulated WL n = 02Simulated WL n = 025

Simulated WL n = 030Simulated WL n = 035

Figure 6 Observed and simulated water surface profile

Table 2 RMSD values for Manningrsquos n

Manningrsquos roughness coefficient 0020 0025 0030 0035RMSD 0318 0065 0231 0616

Table 3 Validation of model

No 1 2 3 4 5 6 7 8 9 10 11 12Q 50 47 45 42 40 39 38 37 35 33 64 80WL (Ob) 10 986 977 963 953 948 943 914 931 917 106 112WL (Sim) 1012 974 969 945 911 966 919 951 922 9 1056 11No 13 14 15 16 17 18 19 20 21 22 23 24Q 85 76 78 67 63 72 52 56 59 63 66 68WL (Ob) 1141 112 1115 1072 1056 1092 1009 1026 1039 1056 1078 108WL (Sim) 1106 11 1139 1038 1019 1121 951 1039 1035 1106 1064 106No 25 26 27 28 29 30 31 32 33 34 35 36Q 72 136 76 57 48 83 80 83 85 61 67 69WL (Ob) 1092 131 1117 1031 991 1134 1123 1134 1162 1048 106 108WL (Sim) 1087 138 1102 1027 987 1128 1105 1128 1136 1057 1068 102

6 Journal of Engineering

Manning roughness coefficient value for the study reach is0025

432 Model Validation ere are 36 measurementsavailable for the verification of the model Hassan did these36 observations in section No 18 Validation was performedin order to check the suitability of a single value for thecoefficient of 0025 with several values of flow rates ecomparison between observed and predicted values of waterlevel for these flow rates is shown in Table 3 and Figure 7Depending on the value of the coefficient of determination(R2) 091 the elected Manningrsquos n (0025) gives an ac-ceptable matching between observed and predicted waterlevels [10]

For high flow rates the model shows high errors this canbe enhanced by collecting more flow observations includingmultiple levels of flow (low medium and high) in thecalibration of themodel Also wemay do a value refinementmaybe in the range 0022 to 0028

5 Conclusions

HEC-RAS software can be used for hydraulic modellingerefore it can be used effectively in modelling andsimulating water surface profiles A hydraulic model wasperformed using HEC-RAS on the Tigris River within areach of 35 km in Al-Amarah city Maysan province inorder to identify the value of n coefficient e roughnesscoefficient (n) equals 0025 which gives an acceptablematching between observed and predicted water levels emodel can be further improved by using GIS with HEC-RASto produce an accurate channel geometry and hence anaccurate flow simulation In addition flood risk can beanalysed using HEC-RAS 20 using the result as a deter-mined value of Manningrsquos coefficient

Data Availability

e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

e authors declare that they have no conflicts of interest

References

[1] HEC-RAS User Manual Davis Version 40 US Army Corpsof Engineers Hydrologic Engineering Center Davis CAUSA 2008

[2] Y Ding Y Jia and S S YWang ldquoIdentification ofManningrsquosroughness coefficients in shallow water flowsrdquo Journal ofHydraulic Engineering vol 130 no 6 pp 501ndash510 2004

[3] E Zic M Vranjes and N Ozanic ldquoMethods of roughnesscoefficient determination in natural riverbedsrdquo in Proceedingsof the International Symposium on Water Management andHydraulic Engineering Ohrid Macedonia September 2009

[4] R Ramesh B Datta S Bhallamudi and A Narayana ldquoOp-timal estimation of roughness in open-channel flowsrdquo Journalof Hydraulic Engineering vol 126 no 4 pp 299ndash303 1997

[5] Z MD Ali N H Abdul Karim and M A M Razi ldquoStudy onroughness coefficient at natural channelrdquo in Proceedings of theInternational Conference on Environment (ICENV 2010)Penang Malaysia July 2010

[6] U H Abdul Hameed ldquoDetermination of manning roughnessvalue for Euphrates river at Al-Falluja barrages using differenttheoriesrdquo Iraq Academic Scientific Journals vol 2 no 2

[7] P K Parhi ldquoHEC-RAS model for Mannnigrsquos roughness acase studyrdquo Open Journal of Modern Hydrology vol 3 no 3pp 97ndash101 2013

[8] M S Shamkhi and Z S Attab ldquoEstimation of Manningrsquosroughness coefficient for Tigris river by using HEC-RASmodelrdquo WASIT Journal of Engineering Sciences vol 6 no 3pp 90ndash97 2018

[9] S I Khassaf and A A Hassan Sediment transport modelingfor the upstream of Al-Amarah barrage PhD esis Collegeof Engineering University of Basrah Basrah Iraq 2014

[10] J K Patel C H Kapadia and D B Owen Handbook ofStatistical Distributions John Wiley amp Sons New York NYUSA 1976

R2 = 091

9

10

11

12

13

14

15

9 10 11 12 13 14

Sim

ulat

ed w

ater

leve

l (m

)

Observed water level (m)

Figure 7 Observed versus simulated water surface level

Journal of Engineering 7

(simulated levels) it checks the accuracy of the model eHEC-RAS model for the Tigris River was calibrated forpredicting water levels and hence determining a single value ofManningrsquos coefficient n In order to check the ability of thecalibrated model in predicting the water levels for differentriver flows the model should be validated (verified) usingmeasurement data other than the one used in calibrating themodel

e data published by Hassan are divided into twogroups of data for calibration the water level values for the18 cross sections corresponding to the 50m3s flow rate wereused while for the validation the water levels in section 18for 36 flow rates were used

431 Model Calibration In the calibration Manningrsquos co-efficient ldquonrdquo was altered continually until the variations betweenobserved and simulated water levels were within the acceptablelimits e procedure followed in the calibration is given in theflowchart in Figure 5e calibration procedure gave the correctvalue of Manningrsquos coefficient of the river reach

is model calibration was done using an observed flowof 500m3s at the upstream of the river reach at crosssection No 1 is flow rate (500m3s) represents thedischarge released from Kut barrage upstream of the studyreach this flow considered a low flow in the Tigris River andis the available flow in most parts of the year e calibrationwas performed by comparing the results of the water level ineach cross section obtained by the model with the observedlevels

From the literature reviewed previously [6 8] the initialvalue of n was elected to be in the range 0020 to 0035 withan increment of 0005 e flow in this reach of Tigris occursonly in main channel no flow occurs in banks therefore thecalibration of the HEC-RASmodel was achieved using singlevalues of n applied to the main channel of the river be-ginning from 0020 By starting with an initial value the nvalue was altered until the differences between observed andsimulation water levels became small as far as possible Sothe outcomes of the HEC-RAS model for distinct values of nwere compared with the observed water surface profiletabulated in Table 1 and represented in Figure 6

It is obvious from Figure 6 that the best choice forManningrsquos n value is 0025 because it gives a good matchingbetween observed and simulated water surface profile

To provide an additional accuracy measurement theroot mean square deviation (RMSD) was tested betweenobserved and simulated water surface elevations e rootmean square deviation is commonly used as a measure of thedifferences between predicted and observed values as pre-sented in the following equation

RMSD

1113936Ni1 Yisim minus Yiobs( 1113857

2

N

1113971

(4)

where N the number of data (total number of cross sec-tions) i the identification number of cross sectionYisim water level for cross section No i (simulated) andYiobs water level for cross section No i (observed) Valuesof RMSD are tabulated in Table 2 it is also clear that the best

Input geometric and required hydraulic data

Input boundary and initial conditions

Choose a trial value of Mannings n

Run HEC-RAS

Steady floweditor

River reach and cross

section editors

Comparison Are differences between

calculated and observed water levelswithin the allowable limits

Print HEC-RAS outputs

YES

NO

Figure 5 Calibration flowchart

Journal of Engineering 5

Table 1 Observed and calculated WL for different Manningrsquos n

CS No WL (observed)Water level (calculated)

n 002 n 0025 n 003 n 00351 10 974 1003 1033 10712 998 969 1003 1018 10623 996 965 1003 1015 1064 994 961 1001 1017 10555 994 964 1001 1015 10486 99 957 997 1013 10477 988 955 995 1011 1058 986 953 993 1009 1059 985 953 988 1008 104910 984 951 991 1011 104211 982 953 989 1005 104612 982 949 989 1005 104613 98 947 987 1003 104414 979 948 986 994 104315 976 943 983 999 10416 975 942 982 998 103317 975 942 982 998 103318 974 941 979 997 1025

90

95

100

105

110

0 400 800 1200 1600 2000 2400 2800 3200 3600

Wat

er le

vel (

m)

Distance from section no 1 (m)

Observed WLSimulated WL n = 02Simulated WL n = 025

Simulated WL n = 030Simulated WL n = 035

Figure 6 Observed and simulated water surface profile

Table 2 RMSD values for Manningrsquos n

Manningrsquos roughness coefficient 0020 0025 0030 0035RMSD 0318 0065 0231 0616

Table 3 Validation of model

No 1 2 3 4 5 6 7 8 9 10 11 12Q 50 47 45 42 40 39 38 37 35 33 64 80WL (Ob) 10 986 977 963 953 948 943 914 931 917 106 112WL (Sim) 1012 974 969 945 911 966 919 951 922 9 1056 11No 13 14 15 16 17 18 19 20 21 22 23 24Q 85 76 78 67 63 72 52 56 59 63 66 68WL (Ob) 1141 112 1115 1072 1056 1092 1009 1026 1039 1056 1078 108WL (Sim) 1106 11 1139 1038 1019 1121 951 1039 1035 1106 1064 106No 25 26 27 28 29 30 31 32 33 34 35 36Q 72 136 76 57 48 83 80 83 85 61 67 69WL (Ob) 1092 131 1117 1031 991 1134 1123 1134 1162 1048 106 108WL (Sim) 1087 138 1102 1027 987 1128 1105 1128 1136 1057 1068 102

6 Journal of Engineering

Manning roughness coefficient value for the study reach is0025

432 Model Validation ere are 36 measurementsavailable for the verification of the model Hassan did these36 observations in section No 18 Validation was performedin order to check the suitability of a single value for thecoefficient of 0025 with several values of flow rates ecomparison between observed and predicted values of waterlevel for these flow rates is shown in Table 3 and Figure 7Depending on the value of the coefficient of determination(R2) 091 the elected Manningrsquos n (0025) gives an ac-ceptable matching between observed and predicted waterlevels [10]

For high flow rates the model shows high errors this canbe enhanced by collecting more flow observations includingmultiple levels of flow (low medium and high) in thecalibration of themodel Also wemay do a value refinementmaybe in the range 0022 to 0028

5 Conclusions

HEC-RAS software can be used for hydraulic modellingerefore it can be used effectively in modelling andsimulating water surface profiles A hydraulic model wasperformed using HEC-RAS on the Tigris River within areach of 35 km in Al-Amarah city Maysan province inorder to identify the value of n coefficient e roughnesscoefficient (n) equals 0025 which gives an acceptablematching between observed and predicted water levels emodel can be further improved by using GIS with HEC-RASto produce an accurate channel geometry and hence anaccurate flow simulation In addition flood risk can beanalysed using HEC-RAS 20 using the result as a deter-mined value of Manningrsquos coefficient

Data Availability

e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

e authors declare that they have no conflicts of interest

References

[1] HEC-RAS User Manual Davis Version 40 US Army Corpsof Engineers Hydrologic Engineering Center Davis CAUSA 2008

[2] Y Ding Y Jia and S S YWang ldquoIdentification ofManningrsquosroughness coefficients in shallow water flowsrdquo Journal ofHydraulic Engineering vol 130 no 6 pp 501ndash510 2004

[3] E Zic M Vranjes and N Ozanic ldquoMethods of roughnesscoefficient determination in natural riverbedsrdquo in Proceedingsof the International Symposium on Water Management andHydraulic Engineering Ohrid Macedonia September 2009

[4] R Ramesh B Datta S Bhallamudi and A Narayana ldquoOp-timal estimation of roughness in open-channel flowsrdquo Journalof Hydraulic Engineering vol 126 no 4 pp 299ndash303 1997

[5] Z MD Ali N H Abdul Karim and M A M Razi ldquoStudy onroughness coefficient at natural channelrdquo in Proceedings of theInternational Conference on Environment (ICENV 2010)Penang Malaysia July 2010

[6] U H Abdul Hameed ldquoDetermination of manning roughnessvalue for Euphrates river at Al-Falluja barrages using differenttheoriesrdquo Iraq Academic Scientific Journals vol 2 no 2

[7] P K Parhi ldquoHEC-RAS model for Mannnigrsquos roughness acase studyrdquo Open Journal of Modern Hydrology vol 3 no 3pp 97ndash101 2013

[8] M S Shamkhi and Z S Attab ldquoEstimation of Manningrsquosroughness coefficient for Tigris river by using HEC-RASmodelrdquo WASIT Journal of Engineering Sciences vol 6 no 3pp 90ndash97 2018

[9] S I Khassaf and A A Hassan Sediment transport modelingfor the upstream of Al-Amarah barrage PhD esis Collegeof Engineering University of Basrah Basrah Iraq 2014

[10] J K Patel C H Kapadia and D B Owen Handbook ofStatistical Distributions John Wiley amp Sons New York NYUSA 1976

R2 = 091

9

10

11

12

13

14

15

9 10 11 12 13 14

Sim

ulat

ed w

ater

leve

l (m

)

Observed water level (m)

Figure 7 Observed versus simulated water surface level

Journal of Engineering 7

Table 1 Observed and calculated WL for different Manningrsquos n

CS No WL (observed)Water level (calculated)

n 002 n 0025 n 003 n 00351 10 974 1003 1033 10712 998 969 1003 1018 10623 996 965 1003 1015 1064 994 961 1001 1017 10555 994 964 1001 1015 10486 99 957 997 1013 10477 988 955 995 1011 1058 986 953 993 1009 1059 985 953 988 1008 104910 984 951 991 1011 104211 982 953 989 1005 104612 982 949 989 1005 104613 98 947 987 1003 104414 979 948 986 994 104315 976 943 983 999 10416 975 942 982 998 103317 975 942 982 998 103318 974 941 979 997 1025

90

95

100

105

110

0 400 800 1200 1600 2000 2400 2800 3200 3600

Wat

er le

vel (

m)

Distance from section no 1 (m)

Observed WLSimulated WL n = 02Simulated WL n = 025

Simulated WL n = 030Simulated WL n = 035

Figure 6 Observed and simulated water surface profile

Table 2 RMSD values for Manningrsquos n

Manningrsquos roughness coefficient 0020 0025 0030 0035RMSD 0318 0065 0231 0616

Table 3 Validation of model

No 1 2 3 4 5 6 7 8 9 10 11 12Q 50 47 45 42 40 39 38 37 35 33 64 80WL (Ob) 10 986 977 963 953 948 943 914 931 917 106 112WL (Sim) 1012 974 969 945 911 966 919 951 922 9 1056 11No 13 14 15 16 17 18 19 20 21 22 23 24Q 85 76 78 67 63 72 52 56 59 63 66 68WL (Ob) 1141 112 1115 1072 1056 1092 1009 1026 1039 1056 1078 108WL (Sim) 1106 11 1139 1038 1019 1121 951 1039 1035 1106 1064 106No 25 26 27 28 29 30 31 32 33 34 35 36Q 72 136 76 57 48 83 80 83 85 61 67 69WL (Ob) 1092 131 1117 1031 991 1134 1123 1134 1162 1048 106 108WL (Sim) 1087 138 1102 1027 987 1128 1105 1128 1136 1057 1068 102

6 Journal of Engineering

Manning roughness coefficient value for the study reach is0025

432 Model Validation ere are 36 measurementsavailable for the verification of the model Hassan did these36 observations in section No 18 Validation was performedin order to check the suitability of a single value for thecoefficient of 0025 with several values of flow rates ecomparison between observed and predicted values of waterlevel for these flow rates is shown in Table 3 and Figure 7Depending on the value of the coefficient of determination(R2) 091 the elected Manningrsquos n (0025) gives an ac-ceptable matching between observed and predicted waterlevels [10]

For high flow rates the model shows high errors this canbe enhanced by collecting more flow observations includingmultiple levels of flow (low medium and high) in thecalibration of themodel Also wemay do a value refinementmaybe in the range 0022 to 0028

5 Conclusions

HEC-RAS software can be used for hydraulic modellingerefore it can be used effectively in modelling andsimulating water surface profiles A hydraulic model wasperformed using HEC-RAS on the Tigris River within areach of 35 km in Al-Amarah city Maysan province inorder to identify the value of n coefficient e roughnesscoefficient (n) equals 0025 which gives an acceptablematching between observed and predicted water levels emodel can be further improved by using GIS with HEC-RASto produce an accurate channel geometry and hence anaccurate flow simulation In addition flood risk can beanalysed using HEC-RAS 20 using the result as a deter-mined value of Manningrsquos coefficient

Data Availability

e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

e authors declare that they have no conflicts of interest

References

[1] HEC-RAS User Manual Davis Version 40 US Army Corpsof Engineers Hydrologic Engineering Center Davis CAUSA 2008

[2] Y Ding Y Jia and S S YWang ldquoIdentification ofManningrsquosroughness coefficients in shallow water flowsrdquo Journal ofHydraulic Engineering vol 130 no 6 pp 501ndash510 2004

[3] E Zic M Vranjes and N Ozanic ldquoMethods of roughnesscoefficient determination in natural riverbedsrdquo in Proceedingsof the International Symposium on Water Management andHydraulic Engineering Ohrid Macedonia September 2009

[4] R Ramesh B Datta S Bhallamudi and A Narayana ldquoOp-timal estimation of roughness in open-channel flowsrdquo Journalof Hydraulic Engineering vol 126 no 4 pp 299ndash303 1997

[5] Z MD Ali N H Abdul Karim and M A M Razi ldquoStudy onroughness coefficient at natural channelrdquo in Proceedings of theInternational Conference on Environment (ICENV 2010)Penang Malaysia July 2010

[6] U H Abdul Hameed ldquoDetermination of manning roughnessvalue for Euphrates river at Al-Falluja barrages using differenttheoriesrdquo Iraq Academic Scientific Journals vol 2 no 2

[7] P K Parhi ldquoHEC-RAS model for Mannnigrsquos roughness acase studyrdquo Open Journal of Modern Hydrology vol 3 no 3pp 97ndash101 2013

[8] M S Shamkhi and Z S Attab ldquoEstimation of Manningrsquosroughness coefficient for Tigris river by using HEC-RASmodelrdquo WASIT Journal of Engineering Sciences vol 6 no 3pp 90ndash97 2018

[9] S I Khassaf and A A Hassan Sediment transport modelingfor the upstream of Al-Amarah barrage PhD esis Collegeof Engineering University of Basrah Basrah Iraq 2014

[10] J K Patel C H Kapadia and D B Owen Handbook ofStatistical Distributions John Wiley amp Sons New York NYUSA 1976

R2 = 091

9

10

11

12

13

14

15

9 10 11 12 13 14

Sim

ulat

ed w

ater

leve

l (m

)

Observed water level (m)

Figure 7 Observed versus simulated water surface level

Journal of Engineering 7

Manning roughness coefficient value for the study reach is0025

432 Model Validation ere are 36 measurementsavailable for the verification of the model Hassan did these36 observations in section No 18 Validation was performedin order to check the suitability of a single value for thecoefficient of 0025 with several values of flow rates ecomparison between observed and predicted values of waterlevel for these flow rates is shown in Table 3 and Figure 7Depending on the value of the coefficient of determination(R2) 091 the elected Manningrsquos n (0025) gives an ac-ceptable matching between observed and predicted waterlevels [10]

For high flow rates the model shows high errors this canbe enhanced by collecting more flow observations includingmultiple levels of flow (low medium and high) in thecalibration of themodel Also wemay do a value refinementmaybe in the range 0022 to 0028

5 Conclusions

HEC-RAS software can be used for hydraulic modellingerefore it can be used effectively in modelling andsimulating water surface profiles A hydraulic model wasperformed using HEC-RAS on the Tigris River within areach of 35 km in Al-Amarah city Maysan province inorder to identify the value of n coefficient e roughnesscoefficient (n) equals 0025 which gives an acceptablematching between observed and predicted water levels emodel can be further improved by using GIS with HEC-RASto produce an accurate channel geometry and hence anaccurate flow simulation In addition flood risk can beanalysed using HEC-RAS 20 using the result as a deter-mined value of Manningrsquos coefficient

Data Availability

e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

e authors declare that they have no conflicts of interest

References

[1] HEC-RAS User Manual Davis Version 40 US Army Corpsof Engineers Hydrologic Engineering Center Davis CAUSA 2008

[2] Y Ding Y Jia and S S YWang ldquoIdentification ofManningrsquosroughness coefficients in shallow water flowsrdquo Journal ofHydraulic Engineering vol 130 no 6 pp 501ndash510 2004

[3] E Zic M Vranjes and N Ozanic ldquoMethods of roughnesscoefficient determination in natural riverbedsrdquo in Proceedingsof the International Symposium on Water Management andHydraulic Engineering Ohrid Macedonia September 2009

[4] R Ramesh B Datta S Bhallamudi and A Narayana ldquoOp-timal estimation of roughness in open-channel flowsrdquo Journalof Hydraulic Engineering vol 126 no 4 pp 299ndash303 1997

[5] Z MD Ali N H Abdul Karim and M A M Razi ldquoStudy onroughness coefficient at natural channelrdquo in Proceedings of theInternational Conference on Environment (ICENV 2010)Penang Malaysia July 2010

[6] U H Abdul Hameed ldquoDetermination of manning roughnessvalue for Euphrates river at Al-Falluja barrages using differenttheoriesrdquo Iraq Academic Scientific Journals vol 2 no 2

[7] P K Parhi ldquoHEC-RAS model for Mannnigrsquos roughness acase studyrdquo Open Journal of Modern Hydrology vol 3 no 3pp 97ndash101 2013

[8] M S Shamkhi and Z S Attab ldquoEstimation of Manningrsquosroughness coefficient for Tigris river by using HEC-RASmodelrdquo WASIT Journal of Engineering Sciences vol 6 no 3pp 90ndash97 2018

[9] S I Khassaf and A A Hassan Sediment transport modelingfor the upstream of Al-Amarah barrage PhD esis Collegeof Engineering University of Basrah Basrah Iraq 2014

[10] J K Patel C H Kapadia and D B Owen Handbook ofStatistical Distributions John Wiley amp Sons New York NYUSA 1976

R2 = 091

9

10

11

12

13

14

15

9 10 11 12 13 14

Sim

ulat

ed w

ater

leve

l (m

)

Observed water level (m)

Figure 7 Observed versus simulated water surface level

Journal of Engineering 7