Geoscience Reference
In-Depth Information
Fig. 2 Digital elevation model
Company (OPGK) in Gda´sk. The data needed for the description of the terrain
elevation and coverage were elaborated using the aerial photographs forming
the orthophotomap. Finally, the DEM with a resolution of 5
5 m was interpolated
(Fig. 2 ). Then, the main land cover categories were identified on the aerial photographs
and the Manning roughness coefficient distribution map was prepared. The following
values of the roughness coefficient were adopted in the land cover model: free surface
water bodies, like rivers, creeks, and lakes
0.025; meadows
0.035; arable
lands
0.15.
In the next step of numerical simulation of catastrophic flood, the location of
embankment failure must be identified or assumed and the water discharge hydro-
graph through the broken dike has to be estimated. In this test case, the location of
the breach was assumed in the middle of the dike. Then the computation of water
discharge from the reservoir was undertaken as a unique problem separated from the
flood routing. In order to estimate the outflow hydrograph, the reservoir was consid-
ered as a flat pond. It was impossible to analyze the hydrodynamics of the Kolbudy II
reservoir because of the lack of information about the bathymetry of the reservoir.
In order to predict the outflow from the reservoir it is necessary to describe the
failure mechanism. This problem is very complex for the earth dams. In this work it
was decided to omit the detailed definition of breach formation process. The dike
break was assumed to be sudden, instantaneous, and total. The breach width and
height were imposed constant ensuring the highest flood hazard. Following the
United States Bureau of Reclamation (USBR), estimating the breach width up to
five times longer than water depth above final breach elevation (Wahl 2004 ), the
breach was assumed 15-m long with the bottom at the elevation corresponding to
the total erosion of the embankment. The final breach bottom level was imposed
equal to 83.0 m a.s.l. and the initial water level in reservoir was 86.3 m a.s.l. The
hydrograph of the water outflow through the breach is presented in Fig. 3 . It was
estimated using known reservoir stage-area relation and applying the simple
broad crest flow equation for calculation of discharge inside the breach.
0.05; bushes and forests
0.11; roads
0.012; built-up areas
Search WWH ::




Custom Search