Projects

Identifying flood generating areas in 8-Mile Creek watershed through a novel approach

End Date: 02/01/12

Abstract

Floods are the leading cause of natural disaster losses in the United States, costing approximately $50 billion in property damage in 1990s alone (NAP 2009). In addition to property damage, floods also kill about 140 people each year in the United States alone (USGS 2006). FEMA lists flooding as the most familiar and frequent natural disaster in Alabama. From 1998 to 2007, insured flood losses totaled more than $730 million (FEMA fact sheet 2008). Residents living on Alabama’s coast are at a heightened flood risk during hurricane seasons due to much of the area’s low lying elevation.

Although climate is the major driver for floods, land use and cover (LULC) change (especially urbanization) exacerbate the magnitude of floods. It is well established now that increased imperviousness, a consequence of urbanization, increases both frequency and magnitude of peak flows. Therefore, potential effect of future development plans on the hydrology of coastal watersheds needs to be critically evaluated.

An efficient and cost effective way to minimize the adverse impacts of urbanization on hydrology is to proactively develop flood management plans synergistically with land use management plans. Informed management decisions may benefit from the identification of the portions of a watershed that have the highest contribution to the increased peak flow rates. Efforts and resources can then be focused on those areas. Studies have shown that areas intuitively thought to have the highest impact on the increased high- or low-flows may not be critical at all. The interaction of different LULC at different parts of the watershed, flow travel times from those areas (mainly affected by topography and surface roughness), and soil types are important driving factors in the formation of the hydrograph.

In this project we propose an index-based methodology to identify flood generating areas in a watershed. The method relies on the use of distributed watershed models and is designed to help managers and decision-makers with their future development plans to minimize the adverse impacts of urbanization on flooding. In this method, a watershed is divided into smaller subunits and a flood index is computed for each using various land use/cover scenarios. The indexes of each subunit are compared to identify sensitive areas. The developed method is generic and can be utilized to address low-flow problems (e.g. drought) as well as water quality problems.

We will apply the index-method to 8-Mile Creek Watershed, which encompasses the city of Prichard in Mobile County, Alabama. Flooding is a serious issue in the area which is expected to grow substantially in the near future. We will work closely with the city of Prichard. Our ultimate goal is to help the city planning managers in making science-based decisions in their development plans. The City of Prichard is excited about this partnership and is looking forward to working with the grant team. The results of this grant will be shared with the City of Prichard and other stakeholders through a hands-on workshop that is a combination of classroom and field activities.

Objectives

Our overall objective is “to develop a generic method that ranks different parts of a watershed based on their contributions to flooding and test it in an urbanizing coastal Alabama watershed.” 

More specific objectives for year 1 are:

  1. To develop an index-based methodology to identify flood generating areas in a watershed
  2. To test the proposed index-based method in the Eight-Mile Creek Watershed in Mobile County, Alabama

More specific objectives for year 2 are:

  1. To prove that the intuitive judgment “all areas with steep slopes and high imperviousness have larger contributions to flooding” is not always necessarily true
  2. To provide science-based guidance to the City of Prichard to help in their future development plans and to educate city officials and watershed stakeholders on the benefits of watershed modeling techniques

Methodology

Year 1:
An index-based method is developed to identify areas in a watershed that contribute to flooding. The method relies on the use of a watershed model. A watershed is partitioned into smaller units and the relative impact of each unit on the flow hydrograph is determined through this index. The method will be applied to 8-mile Creek where the City of Prichard is part of. The model will provide input to the city’s new comprehensive plan, as well as, identify potential problem areas caused by upstream development. 

Year 2:
The spatial configuration of impervious or less permeable areas within a watershed and the complex interaction of various parts of the watershed due to differing travel times could have larger contribution to flooding then. A distributed watershed model will be utilized to answer this question. This information would be beneficial to city land use planners and stormwater management officials in locating new development sites, and would result in sustainable development in coastal areas. The model will provide input to the city’s new comprehensive plan, as well as, identify potential problem areas caused by upstream development.

Rationale

Urbanization and ensuing increase in impervious areas exacerbate the flooding problem. Knowing beforehand where to develop and where not to in order to reduce peak flows is extremely valuable information. The proposed method aims at answering this question. This proposed study is directly addressing MASGC research priority 4: “Determine the sustainable carrying capacity of coastal communities’ land, water, and other resources through resource assessments, scenario building, modeling, and other techniques” listed under focus area of “Sustainable Coastal Development.”

For More Information Contact: the MASGC Research Coordinator, Loretta Leist (Loretta.leist@usm.edu). 
Please reference the project number R/CCD-21.