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Hydrological Assessment
Urbanization quickly increases impermeable land surfaces and leads to a greater surface runoff. Because of more roads, parking lots and other impervious areas, the surface roughness decreases and water quickly flows on smooth impervious surfaces therefore increasing the probability of flooding during intense storms. In addition, urbanized areas usually suffer from a relative lack of vegetation and urban soils exhibit low rates of infiltration therefore contributing to higher runoff. LEAM combined land use simulation results and r.hydro.CASC2D hydrology model at the HUC 10 watershed level to create a more refined method to identify the impacts of urbanization on surface runoff and to predict potential flood areas.
The CASC2D model is a physically based, distributed, raster hydrologic model, widely used for commercial and educational purposes. It originated as a two-dimensional overland flow routing algorithm written by Prof. P. Julien at Colorado State University and it was incorporated into GRASS GIS by Saghafian and Ogden. The CASC2D calculates cell by cell during a given time step and it updates each cell on the basis of previously calculated parameters as well as the parameters of the neighboring cells at the moment.
For this project, LEAM selected to simulate infiltration and interception losses in addition to surface runoff. LEAM took into account the following parameters: Digital Elevation Model (used to identify land slope); spatially distributed Manning values map (used to identify surface roughness of various land uses); vegetation interception coefficient map and vegetation storage capacity map (used to calculate interception); soil saturated hydraulic conductivity map, soil capillary pressure head at the wetting front map, soil effective porosity map and initial soil moisture map (used to calculate infiltration). Soil moisture was assumed to be uniform at 29% throughout the whole area. The intensity of the rain was simulated at 0.4 in/hour during a 100-year, 24-hour storm event and the total precipitation depth thus equaled to 9.5 inches. The length of the total simulation depended on the size of the subwatershed and lasted from three to 14 days, which allowed to trace the movement of surface water after the storm event was over.
Sample Analysis (Des Plains Subwatershed 403)
Subwatershed 403 would experience a greater surface flow and a peak flow will arrive four hours earlier and it will be approximately 50% higher in Reference scenario. Second peak arrives about 20 hours later after the end of the storm indicating a danger of a second flood event. Due to the loss of cultivated areas in Reference scenario, the amount of intercepted water is expected to be slightly lower.
