Classwork 5 Using HEC-RAS for computing water surface profiles

Transcription

1 Classwork 5 Using HEC-RAS for computing water surface profiles (in collaboration with Dr. Ing. Luca Milanesi)

2 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 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 m 3 /s; the channel is composed by two stretches, the bed slope of the upper one is 0.01 m/m and m/m for the lower one. The roughness of the channel is n= s/m 1/3 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 x- coordinates points (stationing) of the cross section are entered from left to right looking downstream.

3 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 m/m in the 1 st stretch and 0.01 m/m in the 2 nd 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 *Y_crit = 1.98 m 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= s/m 1/3 ) 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.

4 figure 1 figure 2 figure 3

5 figure 4 figure 5

6 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= 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.

7 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., HEC- RAS 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 Y_lake [ m a.s.l.] Condition Oglio_discharge [m 3 /s] T = 10 y Max Max Mean 55 Zero Min 10 Min As shown in figure 8, the profile plot can show at the same time all the profile we need in order to compare them.

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9 CASE 3 figure 8 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

10 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 and , so the structure cross-section number will be The structure of the bridge has to be edited in Hec-Ras geometric data panel as shown in figure 10.

11 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

12 A steady flow analysis will be performed under the hypotesis of subcritical flow with the following flow data: Q=400 m 3 /s and Y_lake= 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. W.S. [m a.s.l.] x [m] Bridge_pressure No-Bridge Bridge_energy figure 12