Summary of preliminary 2D inundation modeling for three Hattian landslide dam breach scenarios

of 2D Inundation Modeling for Three Hattian Landslide Dam Breach ScenariosOn October 8, 2005, a M 7.6 earthquake near Muzafarrabad, Pakistan (Figure 1), triggered a landslide that dammed the Karli River and one of its tributaries about 4 km upstream of the confluence of the Karli and Jhelum rivers near the town of Hattian Bala.The smaller dam on the tributary of the Karli River has been artificially breached and is no longer a hazard.When the larger dammed lake on the Karli River has filled enough to flow over the landslide blockage, it will have impounded about 60 million cubic meters of water.This lake will drain through the landslide dam as it breaches during the spring runoff or during the monsoon season in early summer.The inundation associated with the Karli River landslide dam breach endangers a substantial downstream population, particularly the population located in the vicinity of Hattian Bala at the confluence of the Karli and Jhelum rivers.To help mitigate this hazard, we used an accurate two-dimensional flow model to simulate dambreak flows associated with three breach-rate downcutting scenarios, and estimated inundation depths and peak flow velocities.We superimposed inundation extents and other attributes on photographic images of the region to provide clear delineation of potential impacts on populated areas near the confluence of the Karli and Jhelum rivers.The numerical simulations are done on a two-dimensional mesh of the topography formed from square cells 5m on a side and derived from a high-resolution post-landslide digital elevation model (DEM).Using pre-and post landslide DEMs, we reconstructed the landslide mass blocking the Karli River, shown in Figure 2. We modified the breach to reflect the artificial cut made to breach the smaller tributary dam, and we cut a 7-m-deep spillway through the lowest elevation portion of the Karli River landslide dam to approximate remediation activities to reduce downstream impacts of the landslide dam breach.It appears that an 18-m-deep spillway may instead be implemented prior to the dam breach, so our simulations would likely overestimate impacts if the 18-m-deep spillway excavation is achieved prior to overtopping of the dam.To initiate flow over the dam, we flooded the region upstream of the Karli River landslide dam to a water surface elevation 0.2 m higher than needed to flow over the top of the landslide dam.We simulated inundations associated with three dam-breach downcutting rate scenarios as water flows over the top of the dam.The highest breach-rate scenario was used to simulate a potential liquefaction of the blockage and its rapid incorporation into a flow.The two lower breach rate scenarios are associated with stable downcutting of the slide.These three breach scenarios bracket the range of maximum inundation conditions associated with two classes of breach behavior.A breach rate of 1000 m/hr was used to represent maximum inundation associated with rapid liquefaction and integration of the slide mass into the flow over a period of 8 minutes.A breach downcutting rate of 100 m/hr was used to represent the maximum inundation associated with stable downcutting of the slide, consistent with the maximum estimated downcutting rates for landslide dams (Walder and O'Connor, 1997).A lower breach downcutting rate of 50 m/hr was used to illustrate the reduced inundation associated with stable downcutting of the slide, at a rate consistent with the median estimated downcutting rates observed for landslide dams (Walder and O'Connor, 1997).For all three breach rate scenarios, we used initial conditions of 100 m 3 /s baseflow in the Karli River and 2000 m 3 /s in the Jhelum River obtained by looking up baseflow conditions on the world wide web.These baseflow conditions, particularly in the Jhelum River, can significantly impact the inundation near the confluence of the Karli and Jhelum Rivers as well as along the Jhelum River both upstream and downstream of its confluence with the Karli River.We assumed these values, and actual flows may be quite variable, particularly during the monsoon season.The

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