INTENSE RAINFALL AND POORLY PLANNED CONSTRUCTION SITES

Timothy Williams, Department of Earth Sciences, University of South Alabama, Mobile, AL 36688. Email: krythan@hotmail.com.

Mobile, Alabama is subjected to numerous intense rainfall events throughout the year – enough to make it the wettest city in the United States. Construction sites are also numerous in Mobile; the city is rapidly expanding, and new homes and businesses are constantly being built. However, not all construction sites are properly maintained, such as the site located at Christ United Methodist Church. Sediment from this site is being allowed to flow directly into a nearby stream, and is obviously influencing its turbidity. After an intense episode of rainfall, the amount of sediment contributed dramatically increases. I monitored this stream (dubbed “Sugar Creek”) and recorded the fluctuation of turbidity after intense rainfall events. Samples were taken at various points on the stream from beginning to end to create a longitudinal survey in order to observe how far sediment can travel during intense rainfall.

Keyword: intense rainfall, turbidity.

Introduction

Throughout the year, Mobile is subjected to enormous amounts of rainfall – enough to make it the wettest city in the United States with a yearly average of 67.08 inches (Williams, 2002). Of course, that rain has to go somewhere, and that “somewhere” includes the Dog River Watershed. Consequently, the rain carries all sorts of things into the streams and rivers that drain the Dog River Watershed, and the thing that would seem to be the least harmful is dirt. However, because of the intensity of the rainfall in Mobile, tons of dirt and soil can be washed into our waterways, decreasing the quality of water and having adverse effects on the local aquatic life.

How does something as unassuming as dirt harm an aquatic ecosystem? To answer this, we will define the term “turbidity”, and observe and explore the effects it has on aquatic life. Turbidity simply refers to how clear the water is. Turbidity is measured in nephlometric units, or NTU’s, and gives a good approximation of the total suspended solids in a water sample. The murkier the water, the higher its turbidity(FFC, 2003). Turbid water results from suspended particles, such as silt or clay. A high concentration of such particles alters the habits of fish and other aquatic organisms the longer it persists. Prolonged exposure to turbid water may lead to gill damage, altered breeding habits, reduced growth rates, and even death (NSF, 2002).

Over the past few years, Mobile has seen considerable growth in both business and residential areas. This new growth leads to new construction projects, and when poorly managed, construction sites can contribute large amounts of dirt to the waterways. Large mounds of dirt are often left uncovered, and when rained on, these mounds easily erode; the loose sediment then makes its way into a nearby stream, thus increasing turbidity and raising the stress level of aquatic life (Julien, 1995).

Because of its location, Mobile is often subjected to numerous intense rainfall events throughout the year. The bulk of such events occurs during the summer months as the Gulf Coast sea breeze spawns thunderstorms across the area (Ahrens, 2000) . These thunderstorms are often fairly strong, and contain heavy downpours that are capable of causing major erosion, especially where loose dirt is present. Such events also occur during the spring and winter months. As cold fronts approach from Canada, the frigid air clashes with warm Gulf air to produce some rather nasty weather (NWS, 2002). Mobile is also susceptible to the most powerful storm on Earth – the hurricane. These storms bring tremendous amount of torrential rain, and when the storm system is a slow moving one, rainfall totals may reach several feet in extreme cases. (Ahrens, 2000) However, such rainfall events can be planned for, and when one occurs, proper action can drastically reduce erosion near construction sites, in turn reducing turbidity of our local waters.

Research Question

Because of the intensity of rainfall here in Mobile, a poorly managed construction site can contribute a huge amount of sediment into nearby water systems, thus, raising turbidity levels. But is the sediment from a construction site really enough to drastically alter a nearby water system? Also, how does the intense rainfall associated with Mobile’s climate worsen the problem?

Methods

My target construction site is at Christ United Methodist Church on Grelot Rd. The BMPs at this site were not installed properly and have not been maintained, even though construction at the site is far from over. As a result, every time it rains, large amounts of sediment are swept directly into a nearby stream. This stream, dubbed “Sugar Creek”, is a tributary to Milkhouse Creek, which, in turn, is a tributary of Dog River. So, anything that goes into Sugar Creek will eventually find its way into Dog River. Figure 1 shows the location of Sugar Creek and the sampling points.

By monitoring turbidity throughout Sugar Creek, I will be able to graphically show how far sediment can travel during an intense rainfall event. This test will be done several times during periods of minimal rainfall, and then directly after a significant rainfall event. My sample sites are located along the length of the stream; the first one begins approximately one mile from the construction area. By comparing the two sets of tests, I can determine whether or not the intense rainfall has significantly affected turbidity levels, as well as how far the sediment has been carried.

I photographed the construction site, and most of my selected sample sites and have also taken before and after pictures to show the increase of turbidity after rainfall. I visited my sample sites every Tuesday and Thursday and collected water for testing. The first set I collected serves as my comparison set since the area had been without rain for a couple of weeks, and the water was fairly clear because of this. I have a total of six sample sites located from the beginning to end of Sugar Creek, as shown in Figure 1. The head of the creek is the outlet for a drainage system that tunnels under the Winn-Dixie shopping center at the intersection of Hillcrest and Grelot; I chose this location as my first sampling site. Sediment from my target construction site enters directly into this drainage system during rainfall, and exits directly into Sugar Creek, eventually making its way into Milkhouse Creek. The rest of the sampling sites are located along the length of Sugar Creek, with the final sample site in Milkhouse Creek near the confluence of Sugar Creek and Milkhouse Creek.

Rainfall samples were collected a few hours after the rainfall event on April 7^th-8^th. Since the tunnel drains the retention pond beside Winn-Dixie, I collected a water sample there to determine whether or not the retention pond contributes a significant amount of turbidity; I also collected a sample of the water flowing from the construction site and compared the two. Two days later, I returned to collect samples and to determine how much the turbidity had decreased.

Discussion of Results

Upon first visiting the construction site, it was obvious that large amounts of dirt were washing away from the main site and into a nearby retention pond. The pond has a drainage pipe that directs the water into a roadway ditch, where it then enters the Winn-Dixie tunnel. The water that escapes from the retention pond is not being filtered properly; all that separates the water from the ditch is a tattered piece of cloth. After a significant rain, the water can easily flow over and through this cloth (Fig. 2).

During periods of little or no rainfall, the construction site doesn’t contribute very much sediment to Sugar Creek. This is shown in Figure 3; samples for this graph were taken during a period when Mobile had not received rain for about two weeks. The maximum reading on this graph is at sample site #5, and is 13 NTU. This turbidity spike is due to algal growth, not sediment.

Beginning around 7:00 p.m. on April 7^th, and lasting until about 12:00 p.m. on April 8^th, Mobile experienced a large amount of rainfall due to a low pressure system. This system slowly moved over the area, and spawned continuous rainfall as it did so. I was able to access the construction site, as well as my sample sites, a few hours after the rainfall ended, and the values recorded differ drastically from the comparative values. The water at the construction site retention pond was red with sediment, and had a turbidity reading of 130 NTU. I also took a sample of the flow into the Winn-Dixie retention pond, just to be sure that it was not a major contributor; a value of 17 NTU was recorded there. Visibly, Sugar Creek had turned from clear to reddish-orange, and the construction site was clearly the cause of it (Fig. 4). At the head of the stream, a reading of 60 NTU was recorded, just under half the reading from the construction site’s retention pond. I conclude that the cleaner flow from the Winn-Dixie retention pond is mixing with the flow from the construction site, and is actually reducing the total turbidity of the water that enters Sugar Creek. Figure 3 also shows the turbidity readings along the creek after intense rainfall.

Readings decreased as I moved down the creek, and seemed to average about 35. This number may seem low, but by comparing this number to Figure 5, a turbidity reading of 35 will begin to alter stress levels of fish within the first few hours. Over a period of just two days, respiratory problems, reduced feeding rates, and avoidance behavior begin. Two days later, I returned to the site and recorded a maximum reading of 13 at the head of the stream, and minimum reading of 4 at Milkhouse Creek, as shown in Figure 3. This indicates that Sugar Creek was able to recover fairly quickly, but doesn’t necessarily mean that the construction site has not harmed the local environment.

One thing that I was quick to notice about Sugar Creek is the absence of visible aquatic life throughout most of its length. I have visited the area several times, and have only seen aquatic life, such as minnows, frogs, and crawfish, near the far end of the creek where it merges with Milkhouse creek. There are plenty of moderately deep, calm pools throughout the stream that would seem to be perfect habitats for aquatic creatures, but for some reason there are none. I attribute this to the sediment contribution of the construction site. The course of the stream has been altered in areas where sediments have accumulated; flow is being greatly restricted in some areas, making it nearly impossible for fish to travel upstream. Also, turbidity increases as you move closer to the head of the stream. Aquatic life would naturally avoid areas where survival would be a challenge.

The quick recovery of Sugar Creek may seem surprising at first, but I believe that it is a derivative of the current season. In early spring, rain systems come and go, and usually do not persist for more than six to twelve hours. Also, periods between rain systems may range anywhere from one to three weeks. During this time, the ground becomes very dry, and when it rains again, the water is quickly soaked up. As soon as the rain ends, water levels in creeks and rivers quickly decrease within a few hours since the ground is not yet saturated. As a result, turbidity contribution soon ceases, and levels begin to decrease. However, during the summer, Mobile experiences sea breeze thunderstorms, which are an everyday occurrence. If loose sediment were to be washed into Sugar Creek daily, then it would not be able to recover as quickly, and raised turbidity levels may persist. Referring back to Figure 5 , it can be seen that moderate levels of turbidity that persist for days and weeks could have a more significant impact on aquatic life than higher levels that quickly diminish. This is simply a theory, though, and summer research of the creek is needed to prove it.

Conclusion

At this point, I can easily conclude that the construction site at Christ United Methodist Church is the main contributor of increased turbidity of Sugar Creek, and, therefore, is also increasing turbidity in Milkhouse Creek. Longitudinally, the construction site significantly increased turbidity over approximately two miles. This is an obvious underestimate, since my sample sites stop at Milkhouse Creek. There is also evidence that the sediment from the site has altered the flow of Sugar Creek. Although I cannot be certain, I will venture to say that the site is the reason for the absence of aquatic life in upper portions of the stream.

Mobile’s intense rainfall is the main factor of increased turbidity throughout Sugar Creek, but is not to be blamed for it. Poorly maintained BMPs by the construction company are the culprit here. Properly installing silt fences and hay bales could really help reduce the amount of sediment runoff, but a few simple, cheap tarps placed over exposed dirt mounds would make the most difference. Covering the dirt keeps erosion from occurring, and , in turn, keeps our waterways a little cleaner.

References

Ahrens, Donald. 2000. Meteorology Today, 6^th Edition.

Friends of Five Creeks (FFC). 2003. Water Quality Monitoring. http://www.fivecreeks.org.

Julien, Pierre, 1995. Erosion and Sedimentation

National Science Foundation. Water on the Web. http://wow.nrri.umn.edu/wow/index.html.

National Weather Service, Mobile/Pensacola Office. Local Climate Data. http://www.srh.noaa.gov/mob.

Williams, Aaron. 2002. Mobile Weather and Marine Almanac.