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Floods are an increasingly significant hazard in the United States because of major changes to the hydrology of the landscape. Floods cause financially greater loss and major loss of life per year than any other natural hazard. There have been several significant floods within the past ten years that have had a tremendous impact on the landscape of Iowa. Geographic Information System (GIS) and Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) technologies were used to develop flood simulations to assess the conditions of the Bear Creek watershed in central Iowa. HEC-HMS is a modeling system designed to simulate the precipitation-runoff in watersheds. The model provides information about stream discharge rates that can then be used to determine potential flood distribution on the landscape. Information from this study can be used as input to evaluate the extent of flooding for a given storm event and to evaluate alternative strategies to minimize the impacts of flooding. Introduction The physical landscape has been influenced by gradual economic and social change within the last 150 years, in particular since the 1930's. Human impact on 18 streams and stream channels has been widespread throughout the period of habitation of the planet (Cole 1976). As populations continue to increase, the competition and conflict for uses of steams also continues to increase. Rivers and streams have been a major component of development over time, and as such, an understanding of the natural stability of rivers and streams is necessary if maintenance of their functions and health are to be secured (Rosgen 1994). A flood is the occurrence of a flow that overtops the streambanks. Hydrologic research concludes that floods occur when parts of a drainage basin is saturated and unable to absorb additional water, so the water runs onto and across the surface as overland flow (Leopold 1994). Bankfull stage-discharge has a great influence on stream morphology and flooding potential (Rosgen 1996). An accurate definition of "bankfull" is important because it helps determine how stream width, cross-sectional area and average channel depth are measured (Rosgen 1994). In a "natural" stream, flooding generally occur every 1 .5-2 years as determined using a flood frequency analysis (Dunne and Leopold 1978). The purpose of this research is to develop a technique for modeling stream discharge rates in the Bear Creek watershed by using Geographic Information System (GIS) and Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS). HEC-HMS, a hydrologic modeling system, is used to predict stream discharge in the simulation of an actual flood situation. The study specifically uses a GIS digital elevation model and Geo HEC-HMS software (Environmental Systems Research Institute 2000) for the initial analysis of the HEC-HMS model. From this research, the areas within the Bear Creek watershed that are prone to ponding and 19 the changes in discharge of the stream under different storm conditions have been determined. Hydrologic modeling is important because it provides an assessment of the Bear Creek watershed by modeling hydrologic processes. Hydrologic modeling can provide landowners with important information about the benefits of taking ponded areas out of agricultural production and possibly constructing wetlands in order to minimize the impacts of flooding. Description of Study Area Iowa has an interesting and diverse geological past, which shaped its landscape into what it is today (Prior 1991 ). Iowa contains about 14,500,820 hectares of land. The Bear Creek watershed in central Iowa is a small drainage basin covering about 6,940 hectares. The watershed is located within the Des Moines Lobe sub-region of the Western Corn Belt Plains eco-region. This area is one of the youngest and flattest ecological sub regions in Iowa (Griffith et al. 1994). Land use in the watershed is primarily row cropping, which is typical of the Corn Belt (Mohanty et al. 1994). Figure 2.1 shows the land cover of the Bear Creek watershed. Corn and soybeans make up about 85% of the land cover within the Bear Creek watershed. Flowing water has contributed to the formation of Iowa's valleys. In these valleys, flooding has been a major concern over the years because of the altered conditions of the streams and the landscape (Prior 1991 ). The condition of the soils in the watershed has been affected greatly by the changes in the landscape. 20 Many of the soils in north central Iowa are naturally poorly drained or somewhat poorly drained and contain excess water which could interrupt farm operations or ruin crops. Therefore, tile drainage systems are used to regulate the water level in the soils (Seigley 1999). Landcover • Farmsted Permanent grassway • Corn • Soybeans Set aside Roadway Cemetery • Lagoons • Forest Figure 2.1 Land cover of the Bear Creek watershed. 21 Table 2.1 indicates characteristics of some of the soils in the Bear Creek watershed. These soils range from well to poorly drained. Approximately 55% of the soils within the Bear Creek watershed are poorly drained or somewhat poorly drained. The drainage class information was obtained from the Iowa Soil Properties and Interpretations Database (ISPAID). Restoration research efforts began in the Bear Creek watershed in the early 1990's by the Agroecology Issue Team of the Leopold Center for Sustainable Agriculture. Within this project, a riparian management system was created along stretches of Bear Creek. A multispecies riparian buffer model was used that consists of a 33-foot-wide strip of four to five rows of trees, a 12-foot wide strip of one to two rows of shrubs and a 21-foot-wide strip of native, warm-season grasses (Isenhart et al 1997). Table 2.1. Characteristics of some of the Bear Creek watershed soils (Note: mixed drainage class means soils can range from well to ~oorl~ drained}. Soil Name Soil Map Area % of Drainage Slope(%) Symbol (ha) watershed Class Harps-Okoboji 956 52 2.56 very poor 0-2 Canisteo 507 1259 9.27 poor 0-2 Coland 135 97 8.84 poor 0-2 T al cot 32-40 559 19 2.57 poor 0-2 Webster 107 859 2.22 poor 0-2 Clarion-Storden 638C2 79 3.08 mixed 5-9 Coland-Terril 2018 122 7.40 mixed 1-5 Hanlon-Spillville 1314 10 5.20 mixed 0-2 Spillville-Coland 1585 129 14.02 mixed 0-2 Spillville 485 42 4.56 moderate 0-2 Farrar 2538 186 2.47 well 2-5 Lester 2368 81 2.94 well 2-5 22 Along with the buffer, the Bear Creek watershed restoration project incorporates soil bioengineering and grade control technologies for streambank stabilization and constructed wetlands. The objectives of these components are to minimize soil erosion and intercept surface and subsurface agricultural chemicals from adjacent crop fields, slow floodwaters, improve wildlife habitat and provide alternative, marketable products (Environmental Protection Agency 1999). The Bear Creek watershed empties into the Skunk River just north of Ames, Iowa. The Skunk River can produce major floods in Ames. Figure 2.2 is the location of the Bear Creek watershed located in portions of Hamilton, Hardin and Story counties. Methodology Figure 2.3 illustrates the relationship between GIS and HEC-HMS model and the steps used in this project. The first step of the process was the acquisition of the raw GIS data. A Digital Elevation Model (DEM) of the watershed area were processed and analyzed within GIS using a spatial hydrology database for the creation of the Triangulated Irregular Network (TIN) from the DEM. The data was then preprocessed using the HEC-GeoHMS. After the preprocessing, the hydrologic data, HMS Inputs and watershed characteristics were entered into HEC-HMS for the modeling processes. The results were viewed and displayed in HEC-HMS.

A hydrologic study of the Bear Creek watershed using GIS and the HEC-Hydrologic modeling system