IOP Conference Series: Materials Science and Engineering [607691]

IOP Conference Series: Materials Science and Engineering
PAPER • OPEN ACCESS
Numerical Limitations of 1D Hydraulic Models
Using MIKE11 or HEC-RAS software – Case study
of Baraolt River, Romania
To cite this article: Armas Andrei et al 2017 IOP Conf. Ser.: Mater. Sci. Eng. 245 072010
 
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1234567890WMCAUS IOP Publishing
IOP Conf. Series: Materials Science and Engineering 245 (2017) 072010 doi:10.1088/1757-899X/245/7/072010

Numerical Limitations of 1D Hydraulic Models Using
MIKE11 or HEC-RAS software – Case study of Baraolt
River, Romania
Armas Andrei1, Beilicci Robert1, Beilicci Erika1
1 Politehnica University Timisoa ra, Department of Hydrotechnical Engineering,
Address, George Enescu str. 1/A, 300022 Timisoara, Romania
[anonimizat]
Abstract . MIKE 11 is an advanced hydroinformatic tool, a professional e ngineering software
package for simulation of one-dimensional flows in estuaries, r ivers, irrigation systems, channels
and other water bodies. MIKE 11 is a 1-dimensional river model. It was developed by DHI Water
• Environment • Health, Denmark. The basic computational proced ure of HEC-RAS for steady
flow is based on the solution of the one-dimensional energy equ ation. Energy losses are
evaluated by friction and contraction / expansion. The momentum equation may be used in
situations where the water surf ace profile is rapidly varied. T hese situations include hydraulic
jumps, hydraulics of bridges, and evaluating profiles at river confluences. For unsteady flow,
HEC-RAS solves the full, dynamic, 1-D Saint Venant Equation usi ng an implicit, finite
difference method. The unsteady flow equation solver was adapte d from Dr. Robert L. Barkau’s
UNET package. Fluid motion is controlled by the basic principle s of conservation of mass,
energy and momentum, which form the basis of fluid mechanics an d hydraulic engineering.
Complex flow situations must be solved using empirical approxim ations and numerical models,
which are based on derivations of the basic principles (backwat er equation, Navier-Stokes
equation etc.). All numerical models are required to make some form of approximation to solve
these principles, and consequently all have their limitations. The study of hydraulics and fluid
mechanics is founded on the three basic principles of conservat ion of mass, energy and
momentum. Real-life situations are frequently too complex to s olve without the aid of numerical
models. There is a tendency among some engineers to discard the basic principles taught at
university and blindly assume that the results produced by the model are correct. Regardless of
the complexity of models and despite the claims of their develo pers, all numerical models are
required to make approximations. These may be related to geomet ric limitations, numerical
simplification, or the use of empirical correlations. Some are obvious: one-dimensional models
must average properties over the two remaining directions. It i s the less obvious and poorly
advertised approximations that pose the greatest threat to the novice user. Some of these, such
as the inability of one-dimensional unsteady models to simulate supercritical flow can cause
significant inaccuracy in the model predictions.
1. Introduction
Investigated area is Baraolt area, surrounded by mountains on t he right bank Harghita respectively
Baraolt Mountains in the left bank.
In the area between Biborțeni and Baraolt, p Baraolt has a mino r channel openings 8 to 25 m (in some
erosion cuvettes touch 70 to 75 m) lower in town Baraolt (ca. 6 to 8 m), respectively downstream in

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1234567890WMCAUS IOP Publishing
IOP Conf. Series: Materials Science and Engineering 245 (2017) 072010 doi:10.1088/1757-899X/245/7/072010

some areas where the distance between the sides is reduced from 4 to 5 m. The plan view is presented
in Figure 1.

Figure 1. Plan view
The banks are generally asymmetric (except in designated areas inside or outside the village Baraolt)
generally steep and variable heights between 1.50 to 5.00 m dam s that meet the banks Baraolt heights
1.50 to 2.00 m and height sometim es still natural bank collapse s because they have affected.
Baraolt creek in some areas is d ecorated with gabions, pitching built concrete parapets concrete
revetment slabs concrete embankment. Floodplain has extensions ranging approximately between 800 – 1 300 m
The slopes of the two sides form ed of pre-Holocene formations a re asymmetric:
 slope as steeply somewhat smoother and continuity;
 left side shows steep slopes is more fragmented in the meeting of the landslide and the base to
limit the accumulation meadow as ponds, ponds with vegetation h ydrophilic from runoff and
underground springs.
Shares in the land of the banks a re summarized approximately 49 3 mdMN upstream and 464 mdMN
downstream. Baraolt is right tributary stream of Olt River, flowing in a NE – SW, has a marked sinuous meanders,
dead arms, Disentangling thres holds submerged, variations in th e flow section and the following
characteristics river:
 a length of 40 km;
 a basin area of 121 km
2;
 an average slope of 22 ‰;
 tortuosity coefficient of 1.17.
The stream has a high flow rate, force erosion and very high ra te of solid transport.
In some areas of minor riverbed is, due to erosion and alluvial erosion, funnel is created true major
diameter over 50 – 60 m.
During the year between Biborțeni and Baraolt, stream receives several courses, leaking rain upstream
and generally in areas where the stream based approach slopes. Hydro surface consists of coarse silt bed and major groundwater was mainly of free level or slightly
ascending. Hydrostatic level of groundwater is influenced by ra infall, the relationship that exists
between aquifer hydraulic floodplain coarse of geomorphological units and adjacent aquifers – deluvial,
proluvial and colluvium, terrace – on the one hand, and the rel ationship established with the main drainer
of hydraulic area – p Baraolt.

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1234567890WMCAUS IOP Publishing
IOP Conf. Series: Materials Science and Engineering 245 (2017) 072010 doi:10.1088/1757-899X/245/7/072010

Groundwater level was encountered generally 2.50 to 4.00 m dept h. There are areas in which a
hydrogeological relationship with volcanogenic sedimentary bedr ock aquifers and groundwater debited
under pressure with a high mineral content and CO
2.
This area has a sinuous river Baraolt marked by numerous bends of varying degrees of openness,
submerged sills, mea nders, arms, dead (ma gnitude and higher fre quency near the confluence with river
Olt where floods and prolongs its remuneration lead to training material saturated with low mechanical
strength of the sides), variations in the geometry and flow rat e, ramble, Disentangling [3].
2. Material and methods
Computational fluid dynamics can be defined as a branch of flui d mechanics that uses numerical
methods and algorithms to solve and analyse problems involving fluid flows. The term “CFD model” is
commonly used to refer to a high-order numerical model capable of solving complex flow situations
with relatively few simplifications. In reality, all numerical models are CFD models (even a simple spreadsheet solution of the backwater
equation). There are generally considered to be two methods of analysing fluid motion: by describing
the detailed flow pattern at every point in the flow field (sma ll scale or differential analysis), or by
examining a finite region and determining the gross effects of and on the region (finite or control-volume
analysis). The complexity of real fluid flo w makes it impossible to solve the governing equations without making
some form of simplifying approximation, even with the use of co mplex models and fast computers.
Common practices include: simplification of the spatial and geo metric properties, assumption of steady
or quasi-steady flow conditions, neglect of fluid properties th at would have negligible influence in the
circumstances being investigated, and use of empirical formulae to approximate flow characteristics [1].
The basic computational procedure of HEC-RAS for steady flow is based on the solution of the one-
dimensional energy equation. Ene rgy losses are evaluated by fri ction and contraction / expansion. The
momentum equation may be used in situations where the water sur face profile is rapidly varied. These
situations include hydraulic jumps, hydraulics of bridges, and evaluating profiles at river confluences.
For unsteady flow, HEC-RAS solves the full, dynamic, 1-D Saint Venant Equation using an implicit,
finite difference method. The unsteady flow equation solver was adapted from Dr. Robert L. Barkau’s
UNET package.
Hydraulic models may be categorized by the spatial and temporal simplifications that the model
employs. Each category has associated with it a number of fluid properties and dynamic assumptions.
A less obvious simplification common to many numerical models ( HEC-RAS, MIKE 11) is to assume
that the grade of the channel is small, nominally less than 1:1 0, and therefore the sine and cosine of the
channel slope can be assumed equa l to zero and unity respective ly.
HEC-RAS is equipped to model a network of channels, a dendritic system or a single river reach. Certain
simplifications must be made in order to model some complex flo w situations using the HEC-RAS one-
dimensional approach. It is cap able of modelling subcritical, s upercritical, and mixed flow regime flow
along with the effects of bridges, culverts, weirs, and structu res.
HEC-RAS is a computer program for modelling water flowing throu gh systems of open channels and
computing water surface profiles. HEC-RAS finds a particular co mmercial application in floodplain
management and flood insurance studies to evaluate floodway enc roachments. Some of the additional
uses are: bridge and culvert design and analysis, levee studies , and channel modification studies. It can
be used for dam breach analysis, though other modelling methods are presently more widely accepted
for this purpose. HEC-RAS has merits, notably its support by the US Army Corps of Engineers, the future enhancements
in progress, and its acceptance by many government agencies and private firms. It is in the public domain
and peer-reviewed. The use of HEC-RAS includes extensive docume ntation, and scientists and
engineers versed in hydraulic analysis should have little diffi culty utilizing the software.
Users may find numerical instability problems during unsteady a nalyses, especially in steep and/or
highly dynamic rivers and streams. It is often possible to use HEC-RAS to overcome instability issues

4
1234567890WMCAUS IOP Publishing
IOP Conf. Series: Materials Science and Engineering 245 (2017) 072010 doi:10.1088/1757-899X/245/7/072010

on river problems. HEC-RAS is a 1-dimensional hydrodynamic mode l and will therefore not work well
in environments that require mu lti-dimensional modelling. Howev er, there are built-in features that can
be used to approximate multi-dimensional hydraulics [5].
MIKE 11 is an advanced hydroinf ormatic tool, a professional eng ineering software package for
simulation of one-dimensional flows in estuaries, rivers, irrig ation systems, channels and other water
bodies. MIKE 11 is a professional engineering software package for the simulation of flows, water quality and
sediment transport in estuaries, rivers, irrigation systems, ch annels and other wate r bodies. MIKE 11 is
a user-friendly, fully dynamic, one-dimensional modelling tool for the detailed analysis, design,
management and operation of both simple and complex river and c hannel systems. With its exceptional
flexibility, speed and user friendly environment, MIKE 11 provi des a complete and effective design
environment for engineering, water resources, water quality man agement and planning applications.
The Hydrodynamic (HD) module is the nucleus of the MIKE 11 mode lling system and forms the basis
for most modules including Flood Forecasting, Advection-Dispers ion, Water Quality and Non-cohesive
sediment transport modules. The MIKE 11 HD module solves the ve rtically integrated equations for the
conservation of continuity and momentum, i.e. the Saint Venant equations.
The MIKE 11 is an implicit finite difference model for one dime nsional unsteady flow computation and
can be applied to looped networks and quasi two-dimensional flo w simulation on floodplains. The model
has been designed to perform detailed modelling of rivers, incl uding special treatment of floodplains,
road overtopping, culverts, gate openings and weirs. MIKE 11 is capable of using kinematic, diffusive
or fully dynamic, vertically integrated mass and momentum equat ions. Boundary types include Q-h
relation, water level, discharge, wind field, dam break, and re sistance factor. The water level boundary
must be applied to either the upstream or downstream boundary c ondition in the model. The discharge
boundary can be applied to either the upstream or downstream bo undary condition, and can also be
applied to the side tributary fl ow (lateral inflow). The latera l inflow is used to depict runoff. The Q-h
relation boundary can only be a pplied to the downstream boundar y. MIKE 11 is a modelling package
for the simulation of surface runoff, flow, sediment transport, and water quality in rivers, channels,
estuaries, and floodplains. The computational core of MIKE 11 i s hydrodynamic simulation engine, and
this is complemented by a wide range of additional modules and extensions covering almost all
conceivable aspects of river modelling [4].
Fluid motion is controlled by three basic principles: conservat ion of mass, energy and momentum.
Derivatives of these principles are commonly known as the conti nuity, energy and momentum
equations. These principles are among the first taught in basic fluid mechanics, and they form the
foundation of the field of hydraulic engineering. However, as s ituations become increasingly complex,
we lose track of these essential principles. Basic equations ar e replaced by empirical approximations,
and mathematical calculations with numerical models. Determinin g an equivalent surface roughness of
a floodplain is far more difficult than estimating an equivalen t roughness height or a Manning’s
roughness coefficient; solving a backwater equation for an irre gular channel would be an arduous task
without the assistance of a numerical model. Numerical models come in a wide range of shapes and flavours – one, two or three dimensions, steady
or unsteady flow conditions etc. All are based on derivations o f the basic principles. All are required to
make some form of numerical approximation to solve these princi ples. All have their limitations.
The objective of this paper is to promote a basic awareness of how numerical models operate and to
draw attention to some of the m ore common limitations that are implicit to this operation, in the hope
that this may encourage these models to be used in (and only in ) the manner for which they are intended.
3. Results and discussions
To exemplify of numerical modelling with MIKE11 and HEC-RAS hyd roinformatic tools was
considered a sector of Baraolt River, located in central Romani a. The considered sector has a length of
11 km; representative cross sections are considered between Bar aolt and Olt River confluence (Figure
1). Cross sections have been raised by the Romanian Waters, Olt , Water Basin Administration.

5
1234567890WMCAUS IOP Publishing
IOP Conf. Series: Materials Science and Engineering 245 (2017) 072010 doi:10.1088/1757-899X/245/7/072010

The input data are: area plan with location of cross sections ( Figure 2); cross sections topographical data
and roughness of river bed (Figure 3); flood discharge 1 %, 2 % , 5 % and 10 % in section79 [2] and
Bridge culvert data in cross section 9.2, 23.2, 28.2, 30.2, 40. 2, 42.2 and 45.2.
After simulation with software MIKE11 and HEC-RAS, a longitudin al profile was obtained, presenting
water levels along the river (Figure 4).

Figure 2. Plan view with the network model
MIKE11and HEC-RAS

Figure 3. Cross sections (for example cross
sections 1, 10, 20, 30, 40, 50, 60, 70, 79)
MIKE11and HEC-RAS
baraol t
79
78
77
74
73
72
71
70
69
68
67
66
64
63
62
61
60
59
58
57
55
54
52
51
50
48
47
46
45
44
43
42
41
40
39
38
37
35
34
33
31
30
29
28
27
26
25
24
23.6
22
21
20
19
18
17
16
15
14
13
12
10
9.6
8
7.5
7
6
5
4
3
2
1
baraolt

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1234567890WMCAUS IOP Publishing
IOP Conf. Series: Materials Science and Engineering 245 (2017) 072010 doi:10.1088/1757-899X/245/7/072010

Figure 4. Longitudinal profile
0 2000 4000 6000 8000 10000 12000460465470475480485490495
Main Channel Di stance ( m)Elevation (m)Legend
EG 1 %
WS 1 %
EG 2 %
WS 2 %
EG 5 %
WS 5 %
EG 10 %
WS 10 %
Crit 1 %
Crit 2 %
Crit 5 %
Cri t 10 %
Ground
Left Levee
Right Lev eebaraol t baraolt

7
1234567890WMCAUS IOP Publishing
IOP Conf. Series: Materials Science and Engineering 245 (2017) 072010 doi:10.1088/1757-899X/245/7/072010

4.
Conclusions
Numerical models usually solve the backwater equation between a djacent cross-sections using an
iterative procedure called the standard step method. The primar y assumption of the integrated backwater
equation used in steady-state numerical modelling is that the f low is gradually varied (Henderson 1966).
This implies that changes along the channel, such as cross-sect ion shape, invert level, flow depth and
pressure distribution, are relatively small over short distance s.
The backwater equation has questionable or no accuracy in: are as of rapid acceleration or deceleration,
where the assumption of a hydrost atic pressure distribution is no longer valid, areas of large turbulence
and/or energy loss, and areas of large change in cross – sectio n property where the assumption that the
representative friction slope a nd contraction/expansion losses can be estimated by some combination of
the section properties at each end. While the backwater equatio n is based on a steady-state differential
form of the Energy equation, the Saint Venant equation based so lutions can model unsteady flow
conditions. Many software packages, including HEC-RAS and MIKE 11, adopt an algorithm that cannot
accommodate two boundary conditions at the same boundary. As a consequence, they cannot model
supercritical flow, for which both, discharge and water level, are controlled by the upstream boundary.
Instead, supercritical flow conditions are ‘solved’ by suppress ing the convective acceleration as the
Froude number increases. The study of hydraulics and fluid mechanics is founded on the t hree basic principles of conservation of
mass, energy and momentum. Real-life situations are frequently too complex to solve without the aid
of numerical models. There is a tendency among some engineers t o discard the basic principles taught
at university and blindly assume that the results produced by t he model are correct. Regardless of the
complexity of models and despite the claims of their developers , all numerical models are required to
make approximations. These may be r elated to geometric limitati ons, numerical simplification, or the
use of empirical correlations. Some are obvious: one-dimensiona l models must average properties over
the two remaining directions. It is the less obvious and poorly advertised approximations that pose the
greatest threat to the novice user. Some of these, such as the inability of one-dimensional unsteady
models to simulate supercritical flow can cause significant ina ccuracy in the model predictions.
Acknowledgment
Authors thanks for support to project: Development of knowledge centres for life-long learning by
involving of specialists and decision makers in flood risk mana gement using advanced hydroinformatic
tools, AGREEMENT NO LLP-LdV-To I-2011-RO-002/2011-1-RO1-LEO05-53 29.
This project has been funded with support from the European Com mission. This publication
[communication] reflects the vi ews only of the author, and the Commission cannot be h eld responsible
for any use which may be made of the information contained ther ein.
References
[1] Henderson, F.M. (1966). Open Cha nnel Flow. MacMillan Company, N ew York, USA.
[2] National Administration “Romania n Waters” (NARW), Banat, 1987-2 015. Data from various
documents and studies.
[3] S.C. AQUAPROIECT S.A., Project 94/4311, 2011.
[4] MIKE by DHI (2014) MIKE 11 A Modelling System for Rivers and Ch annels Reference Manual.
[5] USACE (20138). HEC-RAS River Analysis System Hydraulic Referenc e Manual, v4.0,
California, USA.