ANALYSIS OF SOIL TYPES IN THE RABBIT CREEK SUB-WATERSHED

 

Philip Herron, Department of Earth Sciences, University of South Alabama, Mobile, AL 36688. E-mail: pwh301@jaguar1.usouthal.edu.

            Rabbit Creek connects the areas of Theodore and Tillman’s Corner to Dog River. These two urban areas are showing signs of rapid growth and development. This rapid development can lead to runoff of sediment from construction sites into Dog River. Sediment runoff is a big problem throughout the entire area because of evidence that areas in the river are slowly beginning to fill up and grow shallow. Analysis of the soil around Rabbit Creek will let us learn the characteristics of the soil types in the watershed. Factors such as erodibility will let us know how the different soils in the area react to the process of erosion. By knowing erosion properties of all the soil types, construction companies will be able to find the best location to build on in regards to the cost of keeping the soil on the site. Also, the construction companies will know how many of the BMP’s they will need to use on the site to keep the piles of dirt on the property and not let them erode into the watershed.

            Keyword: Dog River, erosion, soil types.

 

 

Introduction

Rabbit Creek is a major tributary that flows from the West and Southwest directly into Dog River. It is located in the southern part of Mobile, AL. It carries sediment from the fast developing area of Theodore and Tillmans Corner and releases that sediment along its confluence with Dog River. Evidence of this can be found in Gabriel Dean’s (2005) paper that states that sediment build up is greater along the three confluences of Dog River. His project suggested that the phenomenon of sediment build up is being produced by massive sediment runoff that is occurring in the Western parts of Mobile County. There are also many areas along the creek that are prone to erosion that can add sediment to an already turbid creek. Erosion is the soil runoff that occurs when rainfall intensity exceeds the infiltration capacity of the soil (Fullen 2004). Soil erosion can be classified as a two step process. First, there is a detachment of individual particles from the soil mass, and then there is a transport of these particles by erosive agents such as running water, and wind (Morgan 1995). Once the energy of those agents are no longer available to transport the loose particles then a final phase of deposition occurs. The main soil detaching agent is rainsplash. This occurs when raindrops strike a bare surface, which causes the soil particles to become loose from the surface and become thrown for a distance of a few centimeters (Morgan 1995). Exposed soil can become severely weakened when it is exposed to continuous rain showers. Other methods that allow soil particles to break free from the surface are tilling operations, trampling by people and livestock, wind, and running water (Hillel 1971). Another factor to judge how water moves through soil is by using the soil’s permeability values. Permeability enables the soil to transmit water, and is measured by the number of inches per hour that water can move through the soil (USDA 1980). Another way to look at the erodibility of the soil is to look at the soils K index which is a measure of the susceptibility of the soil to the erosion by water. The K values in Mobile County range from 0.10 to 0.43 with the highest value the most likely to erode (USDA 1980). The erosion that occurs in the basin is a major problem because it is helping to slowly fill in Dog River by the amount of sediment that it is bringing into the waterway. An analysis of all the soil types could be very beneficial for this area because it would help those involved with urban development locate the best locations to build on. It would make developers more aware of any problems that could occur with the soil eroding during the period of time in which they are working on their construction site.

Research Question

What are the different soil erosion factors, and how much erosion can occur with each soil type? Where are the most erodible soils within Rabbit Creek Watershed located? How big of a role does the soil permeability play as an erosion factor for the area?


Methods

To answer this question I used the National Resource Conservation Service soil survey, and the soil survey of Mobile County to actually find what the soil types are in the Rabbit Creek sub watershed. Once I found the different soil types I researched what the different characteristics of each individual soil type are. For example, what particles are the names of all the soil types in the watershed, how susceptible to erosion are the different soil types, and how permeable each of them are. Once I have did all of that I went to the computer lab at the University of South Alabama and made a soil map that covered the Rabbit Creek sub watershed using ArcMap software and the already digitized soil map that I used from the NRCS soil survey (Fig. 1). Once the NRCS soil map was loaded onto the computer I began the long process of digitizing the 16 soil types onto a new layer. After that step I had a discussion with Dr. Ryder and we decided that I needed to add a soil erodibility layer to the map. I again spent some long hours digitizing the erodibility polygons into its own layer. The erodibility layer broke the soil types up into four groups based on their runoff potential. The four group were labeled A,B,C, and D (USDA 1980). The A group are the soils having a high infiltration rate (low runoff potential) when thoroughly wet. The B group are the soils that have a moderate infiltration rate. The C group are the soils that have a slow infiltration rate, and the D group are the soils that have a very slow infiltration rate (Table 1). This allowed me to get a better grasp on where the soil types are located in reference to the river. After I have figured out where the soil types are and how they react with a heavy rainfall I looked at how permeable they were. I broke the permeability factor into seven groups. In comparing these soils' permeability I used less than .06 inches as very slow, 0.06 to 0.2 inches as slow, 0.2 to 0.6 inches as moderately slow, 0.6 to 2.0 inches as moderate, 2.0 to 6.0 inches as moderately rapid, 6.0 to 20 inches as rapid, and more that 20 inches as very rapid. The last erosion factor that I looked at was the K-index which told me the measure of the susceptibility of the soil to erosion by water (ADEM 1994). Finally I used the map and the information that I gathered from the two soil surveys to help me find out where the majority of the erosion is taking place within the Rabbit Creek Sub-Watershed.

Results

When I began analyzing the different soil types I decided that it would be easier to break the soil types into three groups. I broke them into loamy sandy soil, mucky soil, and pits. This was the easiest way to break the soils down because each group has basically the same erosion factors associated with them.

 

Sandy Loam Soils

            Sandy Loam soils are soils that contain 7 to 27% clay, 28 to 50% silt, and less that 52% sand. There are 13 out of the 16 soil types in the watershed that are sandy loam; Bama, Benndale, Escambia, Harleston, Heidel, Lucedale, Malbis, Notcher, Osier, Pactolus, Poarch, Smithton, and Troup. All the soils look the same, dark grayish brown, at the surface. There are so many soil types in this category because there are lots of different combinations of the compounds that are located within these soils. These soils are mostly located inland near the headwaters and away from the mouth of the river. Loamy sand soils are typically good for urban development and have a moderate or better absorption rate which leads to less runoff. The exception to this rule are two soil types that are located near the mouth the river, and also along the river banks. These two soil types are Smithton and Osier. These two soil types are located along Rabbit Creek, and at the headwaters of Rattlesnake Bayou. They both have poor potential for urban development, low K-index values, and moderate to rapid absorption rates.

 

Mucky Soils

            Mucky soil is dark colored, finely divided, well decomposed organic soil material mixed with mineral soil material. The organic matter content in the soil is more than 20%. The three soil types in the watershed that are in this soil category are Johnston-Pamlico complex, Lafitte muck and Pamlico. These soils are located along both Rabbit Creek and Rattlesnake Bayou. These soils are mostly near the mouth of the rivers near the confluence of Dog River. These soil types are characterized with slow to moderately slow permeability index, and have a high K-index value. Some mucky soils contain large numbers of organic material which cause the soil to become more permeable and have a lower runoff rate than if it didn’t have any organic material. These soils also have poor potential for urban development because these soils are more susceptible to erosion.

 

Pits

These areas are open excavations that soil and other geologic material have been removed from the surface. All the pits in the watershed are located near and along Rabbit Creek. Erosion is a severe threat to these areas, especially to the pits that have piles of left over soil lying around the pit. Since most of them are located near the creek it could be a potential problem for loose sediment to enter the creek.

 

Conclusions

            After analyzing my map I found that the soils near the headwaters of Rabbit Creek are more permeable and experience less runoff which would cause less erosion to occur. The soils that are along Rattlesnake Bayou and at the mouth of Rabbit Creek are less permeable and have more runoff which would cause more erosion in the watershed. The soils that are located at the headwaters are coarse to moderately fine texture that allows water to absorb through it. A reason for some of the soils in the watershed being less permeable is because of plinthite. Plinthite is a highly weathered mixture of clay and quartz that turns into hard stone when exposed to wetting and drying, especially if it is exposed to the sun (USDA 1980). These conditions are common in this area which would keep the plinthite hard and causing water to runoff the soil easier.

 

Benefits

            Analysis of all the soil types could be very beneficial for this area because it would help those involved with urban development locate the best locations to build on. It would make developers more aware of any problems that could occur with the soil eroding during the period of time in which they are working on their construction site. It would help construction companies save time and money when setting up the BMPs that were needed on their sites. If companies were working on areas where the soils where easily eroded than they would need to spend more time setting up their BMPs than if they were working on a site with soil that doesn’t erode as easily. A follow up study could be conducted on my project by somebody maybe mapping out another sub-watershed in the Dog River watershed. Once all the sub-watersheds soils have been mapped out than someone can put together a soil map of the whole Dog River watershed.

 

References Cited

Alabama Department of Environmental Management. 1994. A Survey of the Dog River Watershed. Soil Characteristics of the Watershed. pgs 11-14.

 

Dean, Gabriel. 2005. “Sedimentation in the Dredged Channel in Dog River.” Available online: Dog River Watershed: Geography 480. Accessed: 22 Feb., 2006

<http://www.southalabama.edu/geography/fearn/480page/dogriver.html>

 

Fullen, Michael A., John A. Catt. 2004. Soil Management Problems and Solutions. New

York: Oxford University Press.

 

Hillel, Daniel. 1971. Soil and Water: Physical Principles and Processes. New York: Academic Press.

 

Morgan, R.P.C. 1995. Soil Erosion and Conservation. 2nd ed. New York: John Wiley & Sons, Inc.

 

National Resources Conservation Services. 2004. Web Soil Survey version 1.0.

< http://websoilsurvey.nrcs.usda.gov/app/> Accessed: April 1, 2006

 

U.S. Department of Agriculture, Alabama Department of Agriculture and Industries. 1980. Soil Survey of Mobile County, Alabama.