Model Description

The CCHE FLOOD is a finite volume model based on the solution of the full dynamic shallow water equations in conservative form. CCHE-FLOOD exists in both one-dimensional and two-dimensional versions. The model is particularly adapted for simulating real flood propagation over complex topography. It can be used to determine arrival/receding times of flood waves, to delineate the inundated area and to plot flow-depth contours as a function of time, to carry out operational flood-risk analysis and mapping, etc. The 2D version of CCHE-FLOOD can be efficiently used for studying flood propagation due to dam-break, and embankment or levee breaching.

Main Features of CCHE-FLOOD

CCHE FLOOD uses a robust, shock capturing explicit scheme, which allows the presence of mixed flow regimes in the computational domain (e.g. supercritical flows, subcritical flows, transcritical flows, overland flows, and overtopping flows), and can resolve surge-type flow discontinuities. The solution scheme automatically handles wetting and drying nodes. The 2D CCHE-FLOOD can import GIS topographic data (e.g. USGS DEM, ASCII Raster, regular XYZ format, etc.) to prepare the mesh, and to assign the bed elevations to the nodes. The result file can be imported into a GIS program for post-processing.

CCHE-FLOOD has been extensively tested against various analytical, experimental and field data. Currently studies are underway for parallelization of the code for faster online simulation.

Application Examples of CCHE-FLOOD

Catastrophic Flooding due to Malpasset Dam-Break

In December 2, 1959, the 66,5m-high, double-curvature Malpasset arch dam located in a narrow gorge of the Reyran river valley, France, literally exploded. The resulting flood destroyed downstream villages and claimed more than 400 lives. The trace marks left by this catastrophic flood showed water depths of 20m above the original bed level. The 2D CCHE-FLOOD was used to simulate the flood resulting from the Malpasset dam-break.

The Malpasset Dam after the dam-break.

The area marked by transparent blue color indicates the extent of the inundation 14 minutes after the dam-break computed using 2D CCHE-FLOOD.

(a)

(b)

The water levels (a) and flood wave arrival times (b) predicted by the 2D CCHE-FLOOD are in good agreement with the observations.

Toce River Valley flood simulation

The Toce River Valley in the Northern Alps, Italy, is a a pilot area for studying catastrophic flood events. A 5km reach of the valley has been reproduced on a 1:100 scale physical model. The experimental data (topographic data, inflow hydrographs, hydraulic conditions and physical parameters and water elevations measured at 32 gauging stations) is available as benchmark data for testing numerical models. Two different dam-break test cases are available; one with the flood wave overtopping a reservoir located in the middle of the valley, and the other without overtopping. The case with overtopping of the reservoir was used to validate CCHE-FLOOD.

View of the physical model from downstream (notice the reservoir on the left side of the valley).

Simulation of dam-break in Toce Valley using 2D CCHE-FLOOD. It is important to note how the water first backs up behind the reservoir wall, and then overtops it to fill the reservoir.

View Animation

Stage-time hydrographs calculated at various points along the valley using 2D CCHE-FLOOD show excellent agreement with measurements in the physical model.

Landslide Generated Waves in Lake Sarez

In February 1911 a major earthquake triggered a massive landslide in Pamir Mountains, Tajikistan, at an altitude of over 3,000m. Approximately 2km3 of rock and debris rushed into the valley to form a 600m-high natural dam. The waters of Murgab River accumulated behind this natural Dam and formed Lake Sarez. The stability of this natural dam, and the safety of some 5 million people living at the downstream of the dam along the Bartang, Panj, and Amu-Darya rivers have long been a concern of engineers, scientists, and various governmental and international organizations. Various studies focused on the potential risk of a dambreak by internal erosion due to seepage or a massive overtopping and partial washout due to a huge wave generated by the failure of an unstable zone on the right bank slope located some 4km upstream of the dam.

The CCHE2D-FLOOD model was used to investigate the consequences of the landslide generated wave in the Lake Sarez. Two cases of computation, which were designed based on the “South Part” landslide scenario (the most active instable zone), were carried out. Case A has a slide speed of 20 m/s, and Case B 14 m/s. The landslide volume for both cases is 0.1 km3. In the computations, wave was generated by assuming that the slide mass moves inwards to lake and pushes entire water column in its immediate vicinity at a certain velocity during a certain time. Such treatment of wave generating process can take into account both water displacement effect and horizontal impulse force due to landslide; hence it should be preferable in the event that the angle between the slide surface and the horizontal plane is small.

A digital elevation map (DEM) in raster format may be directly imported into CCHE2D-FLOOD to define the computational mesh and the geometry of the terrain. For the present study the DEM of Lake Sarez elaborated under the supervision of USGS, within the framework of “UN/IDNDR Interagency Risk Assessment Mission to Lake Sarez” was used. This DEM covers an area of 10.506km×8.993km which includes only the western extremity of Lake Sarez, Usoy Dam and its immediate downstream region, as well as the Lake Shadau, whose water surface level is at the same altitude as that of Lake Sarez. It is composed of 618×529 cells, and has a resolution of 17m.

Fig.1 DEM of Lake Sarez and the surrounding area used for the computations is composed of 618×529 cells with a resolution of 17m. The white contour line shows the shoreline of Lake Sarez and Lake Shadau at the altitude of 3,260 m.a.s.l

The numerical simulations provided realistic predictions of the entire process of the wave movement, including generation, propagation, run-up and run-down slopes, and overtopping flows. The computational results show that for the Case A landslide generated wave overtops the lowest point in the mountain ridge separating Lake Sarez and Lake Shadau. The water passing over the ridge flows towards Lake Shadau. In Case B, which has a slower landslide velocity, the spill into the Lake Shadau is considerably less important. The overtopping of Usoy dam crest did not occur in either of the two cases. It is important to note that both simulations assume extreme landslide events involving monolithic movement of the rock mass, which according to the geologists, is not very likely. Geological studies rather indicate that during a landslide event, the sliding rock masses should at least split up into three parts.