Classwork 5 Using HEC-RAS for computing water surface profiles

M.Pilotti: Classwork of Environmental Hydraulics
Classwork 5
Using HEC-RAS for computing water surface profiles
(in collaboration with Dr. Ing. Luca Milanesi)
M.Pilotti: Classwork of Environmental Hydraulics
Why classwork 5 ?
This lecture will give us the possibility to make our first acquaintance with Hydrologic Engineering Centers
River Analysis System (HEC- RAS), which is a worldwide renowned standard for computing steady and
unsteady flow. HEC-RAS allows you to perform one-dimensional steady flow, unsteady flow, sediment
transport/mobile bed computations, and water temperature modelling. In this applications we shall limit to
explore some of the capabilities of HEC RAS in steady flow, partly by repeating the classworks that we have
previously done numerically (see classwork 4), partly by studying a real case, that is relative to the final part of
the Oglio river entering Lake Iseo. Finally, we shall explore the effect of a bridge on high flow. Being able to
manage HEC RAS will give us a considerable professional boost in the field of open channel hydraulics, since
this is a tool that is used from every practitioners and consultant in the world working in the field of hydraulics
engineering.
Final targets of Classwork 5
•
•
•
Widening our programming skills with a real engineering problem
Starting to use a world-wide standard for free surface flow computations: HEC-RAS.
Appreciating the effects of interactions between hydraulic structures and flow
The content of classwork 5
•
As far as HEC RAS is concerned, first download it at http://www.hec.usace.army.mil/software/hecras/hecras-download.html and install it on your laptop.
CASE 1
The first case will simply be a repetition of the numerical classwork 4, so that we shall compute the
sequence S2 – hydraulic jump – S1 – M2, in order to verify with this simple case whether our naïve
implementation of the standard step algorithm has been sufficiently accurate to be comparable with
the results provided by HEC RAS.
Here are the data for the first case. Let us consider a channel of rectangular cross section with width
B= 3 m and a discharge of 15 m3/s; the channel is composed by two stretches, the bed slope of the
upper one is 0.01 m/m and 0.001 m/m for the lower one. The roughness of the channel is n= 0.0154
s/m1/3 As shown in Classwork 4, the upper stretch will be steep and the lower one will be mild. We ask
to compute the sequence S2 – hydraulic jump – S1 – M2. Both the upstream and the downstream
boundary condition is the critical depth.
Under the guidance of your trainer, let us follow the following steps:
Run HEC RAS. First control that under OPTIONS – UNIT SYSTEM is set to SI. Then click FILE and
and create NEW PROJECT, selecting a name and a location for the project you are creating. Then
click on the icon of the geometry set up (see following Figure for the icons). From there select the icon
of river reach, that will allow you to draw a single reach of arbitrary shape. Usually this command is
used to draw a river with a planimetric layout similar to the one of the real river you are going to study.
On this layout the cross-sections we shall put in are superimposed. In our case the layout is arbitrary.
We shall simply draw a segment and then it will be asked us to give an arbitrary name to the river and
to the reach (see figure 1 below). Then we shall click on the icon to edit or create cross-sections, in
order to create two cross sections of rectangular shape as required. Select OPTIONS – CREATE A
NEW CROSS SECTION (see figure 2 below). Let us call it 10 (remember in HEC RAS that numbering
increases form outlet to the source) and let us insert the data shown in figure 3. Remember that the xcoordinates points (stationing) of the cross section are entered from left to right looking downstream.
M.Pilotti: Classwork of Environmental Hydraulics
Then we shall insert a second cross-section (let us call it 20), located 100 m upstream, with the same
geometry of the first (you can use OPTIONS – COPY CURRENT CROSS SECTION – ADJUST
ELEVATIONS, adding 100*0.001= 0.1 m to the elevations). With the same procedure we can insert
the third cross-section section (let us call it 30), located 100 m upstream, with the same geometry of
the first and an increase to the elevation of 1 m (100*0.01= 1 m). Now we have three cross sections
spaced 100 apart. In the GEOMETRY DATA, select TOOLS – XS INTERPOLATION, creating, within
a reach, a set of interpolated section every 1 m. Now let us save the file containing this geometrical
information with the FILE command of the same mask. Now we have a rectangular channel that is 200
m long, has a slope of 0.001 m/m in the 1st stretch and 0.01 m/m in the 2nd one and has cross section
every 1 m.
Let us now return to the main mask where under the EDIT command, STEADY FLOW DATA, we
select the discharge (see figure 4; under Profile Names and Flow Rates) and the boundary conditions
referred to the zero level (under Reach Boundary Conditions, where we shall select the KNOWN W.S.
- 0.99*Y_crit = 1.98 m + 1.1 m = 3.08 m for upstream and 1.01*Y_crit = 2.02 m for downstream). We
save the file with these boundary conditions and we revert to the main mask where we RUN a
STEADY FLOW ANALYSIS, selecting a Mixed Flow Regime (see figure 5). Finally (see Figure 6),
either we select VIEW – WATER SURFACE PROFILES from the main menu, or we click on the
corresponding icon, and we visualize the computed profile.
In order to compare the profile with the one we computed in previous classwork, we have to write it
into a file. To this purpose in the main menu let us select VIEW – PROFILE SUMMARY TABLE, where
the select a Table where Cumulative Channel Length is present (e.g., Users Table – MyTable2). By
clicking on FILE command, we can save it as a text file for the following comparison.
The output shows some criticals in the first stretch; as we can see the program doesn’t produce the S2
profile that we would expect and the whole sequence does not appear correct; in order to overcome
this problem we can try adding an upper mild stretch (Rectangular geometry, L=10000 m, B=3 m,
Sb=0.001 m/m, n=0.0154 s/m1/3 ) in order to induce at section 30 a continuous transcritical transition.
As before, we have to interpolate cross-sections every 1 m and we shall impose the normal depth as
upper boundary condition.
In spite of these modifications nothing changes in the output. We can suppose that the reason of
these problems are due to an excessively coarse mesh or to some parameters of the numerical
method. So we can both refine the mesh and reduce the tolerances (RUN STEADY FLOW
ANALYSYS, selecting OPTIONS and SET CALCULATION TOLERANCES).
The right combination of parameters will give the correct output (Low tolerances on Water surface and
Critical depth calculation, coarse mesh and low number of iterations).
You can compare the results of the Classwork 4, the ones of the first case of this classwork without
the upper stretch; the profiles appear slightly different. The reason of this divergence is very likely
linked to different approaches used in the computation of the slope friction.
M.Pilotti: Classwork of Environmental Hydraulics
figure 1
figure 2
figure 3
M.Pilotti: Classwork of Environmental Hydraulics
figure 4
figure 5
M.Pilotti: Classwork of Environmental Hydraulics
figure 6
CASE 2
Now we shall study the back-water effect and the draw-down effect induced by different downstream
water levels in Iseo lake and by different discharges on a geometrically simplified Oglio river. We shall
assume trapezoidal cross-sections (b=75 m and side slope s=0. 218 m/m) with constant n on the
section.
Since the river Oglio is not a straight line we need a more detailed sketch; to this aim we can import
(see figure 7) a topographical map and draw the line on that base.
M.Pilotti: Classwork of Environmental Hydraulics
figure 7
As in the previous exercise we have to draw the line from upstream to downstream and we now have
to follow the watercourse represented as a blue line. The first upstream cross-section of our interest is
the confluence of the Dezzo river into the Oglio river; this point is marked by a red dot.
We have now to edit all of the cross-sections as in the previous exercise. All geometrical data
(sections, Manning’s roughness and reach length) can be found in the file
Oglio_trapezoidal_sections.xls, we just have to copy-paste them in the cross-section editor. If you
want to import an already existing geometry data file, you have to open the GEOMETRIC DATA
EDITOR, click FILE-> IMPORTE GEOMETRY DATA and choose the file format you need (e.g., HECRAS Format).
A steady flow analysis in condition of subcritical flow will be performed. It will consider 4 different
levels of the lake and 3 river discharges as shown in the following table; all the possible combinations
will be studied.
Condition
T = 10 y
Max
Zero
Min
Y_lake [ m a.s.l.]
186.6
186.25
185.15
184.85
Condition
Max
Mean
Min
Oglio_discharge [m3/s]
655.8
55
10
As shown in figure 8, the profile plot can show at the same time all the profile we need in order to compare
them.
M.Pilotti: Classwork of Environmental Hydraulics
figure 8
CASE 3
Finally we shall consider the effect of a bridge on the flow in the case that the water depth is in contact
with the upper low chord of the bridge (High flow). In such a situation the bridge will cause a backwater effect and an M1 profile will be reconstructed.
The description of the bridge needs 4 input cross sections, already provided in your geometry file, as
shown in figure 9
M.Pilotti: Classwork of Environmental Hydraulics
figure 9 (Source: Hec-Ras Users Manual)
In particular the cross-section number 1 is located sufficiently downstream from the structure so that the flow is
not affected by the structure. Cross-section number 2 and number 3 are located at a short distance,
respectively downstream and upstream the bridge, in order to describe ineffective flow areas. Cross-section
number 4 is an upstream cross-section where the flow lines are approximately parallel and the section is fully
effective.
A cross section has to be indicate as location of the bridge (remember that cross-sections numbering
increases form the outlet to the source); in our case the bridge will be located between cross-sections 005.012
and 005.013, so the structure cross-section number will be 005.0125.
The structure of the bridge has to be edited in Hec-Ras geometric data panel as shown in figure 10.
M.Pilotti: Classwork of Environmental Hydraulics
figure 10
As shown in figure 9 the bridge deck is 4 m thick (difference between the high and the low chord), 13 m wide
and there are no piers; the distance between the upstream side of the bridge deck and the cross-section
immediately upstream of the bridge is 2 m.
The bridge modelling approach needs to be defined by the user; as shown in figure 11 two main situations
may happen: low flows or high flows. The first one is verified if the flow doesn’t raise above the upper point of
the low chord of the deck elsewhere the second one is verified. We will focus on high flows only and we will
compare the results obtained through the energy method and the pressure/weir method (see figure 12).
figure 11
M.Pilotti: Classwork of Environmental Hydraulics
A steady flow analysis will be performed under the hypotesis of subcritical flow with the following flow data:
Q=400 m3/s and Y_lake=186.57 m a.s.l. (corresponding to a 10 y return period). As figure 12 shows the
interaction with the bridge deck causes a significative backwater effect on the incoming flow. As soon as the
flow starts interacting with the lower chord of the deck the water depth increases of about 0.3 m.
193
192.5
W.S. [m a.s.l.]
192
191.5
191
190.5
190
Bridge_pressure
No-Bridge
Bridge_energy
189.5
189
188.5
3000
3200
3400
3600
3800
x [m]
figure 12
4000
4200
4400