PRIVATE VERSUS PUBLIC CONSTRUCTION IN RELATION TO CONTRIBUTING SEDIMENT IN THE DOG RIVER WATERSHED

Shawn M. Gable, Department of Earth Sciences, University of South Alabama, Mobile, AL 36688. E-mail: smg302@jaguar1.usouthal.edu.

The Dog River Watershed, located within Mobile County in Alabama, has seen considerable growth in the past ten years. During this period, turbidity levels in Dog River have risen at an alarming rate. This has led to a substantial decrease in the river’s water quality. Major contributors to this problem are construction sites in the river’s watershed. In order to meet the demands of a growing metropolis, the county must grow and adapt as well. This includes new construction projects such as new or bigger schools and improved roads. When faced with these projects, the Mobile County government has its “hands tied.” The county must go with the lowest bidder. Although often going to the lowest bidder as well, private sites have the luxury of going to whomever they feel is most qualified. The focus of this study is to determine if Mobile County sites contribute more sediment to the watershed than private sites. The study uses qualitative rankings of three sites from each sector based on each site’s efforts to prevent the contribution into the watershed by using and maintaining Best Management Practices (BMP’s). The evidence given in this study indicates that although neither sector maintains their BMP’s properly, the private sites make much more of an effort to contain their sediment. As a result, Mobile County construction sites appear to be contributing larger amounts of sediment to the watershed than private sites.

Keywords: Best Management Practices, Watershed, Dog River

Introduction

Sediment build up is the most significant pollution problem in the Dog River Watershed (Hicks, 2003). A large amount of this sediment comes from the various construction sites spread throughout the before mentioned watershed. Over the past few years, Mobile has seen considerable growth in all areas. There is proof that significant environmental consequences result from the urban development of entire watersheds (Wolman 1967). This new growth and urban development has led to new construction projects which when poorly managed can contribute large amounts of dirt into the watershed (Williams 2003). Large mounds of dirt are often left uncovered on these sites. These mounds easily erode with the help of wind and rain. Mobile’s location and climate does not help the situation. The county receives an average of over sixty inches of rain a year. There is a direct correlation between annual rainfall and sediment loads that are deposited in rivers (Langbein 1958). Locally heavy downpours help to erode sediment and transport it more easily. In addition, it has been found that wind increases erosion. This is especially true of strong vertical winds such as those found in the updrafts and downdrafts of strong thunderstorms (Chepil 1945). The eroded material then makes its way into the waterways that spread throughout the city and eventually ends up in Dog River. This results in the increase of turbidity as well as harms the living organisms that inhabit the waters (Julien, 1995).

This pollution can be greatly reduced by using measures to keep the sediment contained within the site. The law requires developers and contractors to use Best Management Practices (BMPs). BMPs are voluntary or required practices designed to lessen the environmental damage from nonpoint sources such as runoff from construction sites (Alabama Water Watch 2002). Examples of BMP’s include silt fences and haybales that are meant to contain and filter out sediment from the runoff. However, these practices do little if they are not installed properly or not maintained.

In order to meet the demands of a growing metropolis, the county must grow and adapt as well. This includes new construction projects such as new or bigger schools. When faced with these projects, the Mobile County government has its “hands tied.” The county must go with the lowest bidder. Although often going to the lowest bidder as well, private sites have the luxury of going to whomever they feel is the most qualified. The focus of this project will be to determine if Mobile County sites contribute more sediment to the watershed than private sites.

Research question

Are County construction sites less well managed than private ones in regards to the prevention of sediment deposition in the Dog River Watershed? Is this problem a result of the fact that public construction projects go to the lowest bidder? Is the difference enough to drastically affect water quality? What can be done to make management better?

Methods

I used three Mobile County construction sites and three private construction sites in my research. I recorded these sites with a designation of a letter (Site A, Site B, Site C…) on a map along with the waterways to which these sites drain (Fig. 1). I also recorded the dates I visited the sites as well as which of those visits occurred during or immediately after rainfall. Two significant rainfall events occurred during the study period allowing a great opportunity to observe the BMP’s being tested. First, I noted the measures taken by each site to prevent the contribution of sediment into the waterways. I also recorded thorough notes on each site to see if they were being maintained. Second, I took pictures of each site for comparisons. I monitored both the preventive measures as well as noting any changes on each visit. After the data was gathered, I analyzed and compared the notes and pictures. I used the notes and pictures to rank each site on a qualitative basis. The ranking criteria consisted of the preventative measures taken with BMP’s, the maintenance of the BMP’s, and their effectiveness during or immediately after a moderate rain. Values of zero through ten were given for each site for each of the criteria. Ten was tantamount to being perfect while zero meant that the sites had nothing to show for that criterion. The sites were judged based on literature obtained from the Alabama Department on Environmental Management. The rankings then indicated the answer to the research question.

Results

Site A was a rather large site with a moderate slope of around 600 feet to a creek.

Silt fences were in place on the downward slope side of the site and served to contain runoff from a very large mound of dirt. However, several smaller mounds of clay, sand, dirt, and building materials surrounded the fence, uncovered to the outside. The fence was not being maintained nor was any other BMP installed to help contain runoff. Large amounts of sediment also poured through several spots in the fence into storm drains. During the rainfall events, runoff flows nearly uninhibited down the slope and into the drains. This was a Mobile County construction site and was ranked the second worst overall.

Site B was a smaller site than site A. The site also was on a flatter piece of land. Silt fences and several bales of hay surrounded the site. They were never maintained, although they were for the most part well installed and held up pretty well. The contactors created a mini-retention pond surrounded by a grassy mound in the back corner of the site. Most of the runoff appeared to divert to this area. Nevertheless, sediment did find its way out in a few locations during the rainfall events. One of these leaks occurred right over a storm drain. This was a result of uncovered mounds of dirt eroding during the heavy rains. Site B was the worst private site and third worst overall.

Site C was a very large site that sat at the top of a substantial 250-foot grassy slope that flattened out through around fifty feet of woodland before reaching a creek. The actual construction at the top of the slope was pretty well contained with silt fences and hay bales. The problem with site C was that large mounds of uncovered dirt sat several hundred feet away, at the base of the slope, piled up against silt fences that began to give way after significant rainfall. This was a bad place for the long mound because erosion by surface flow is most apparent at the base of a slope where the greatest amount of surface flow concentrates (Ellison 1944). No maintenance of the fences occurred until their removal near the conclusion of the study. The lack of maintenance was not a problem, at first, because the woodlands served to filter the sediment before it reached the creek. However, as the outside construction ended, several changes occurred at the base of the slope. Loose hay was spread along the mound, sod was laid down in a few spots, and a drain was installed in the corner on the upslope side of the mound. Also, the woodlands were cleared in the corner with the drain to make an area for the pipe leading from the drain. Concrete now sits in the corner supporting the pipe. Soil appears to be pouring from either side into the hole in the woodlands. The concrete allowed soil to easily wash to the creek below. The pipe itself appeared not to allow much through it because of sod that sits in front of the drain in an effort to filter out any sediment. Overall, site C was ranked as the fourth worst.

Site D was the smallest of the projects in the study. However, it sat on a short, but relatively steep slope no more than ten feet from a large ditch that feeds a larger canal. A silt fence and hay bales lined the edge of the ditch all the way down the property. Small mounds of dirt and building materials sat uncovered on the property. Yet, runoff from the mounds was small compared to the other sites during the rainfall events. The BMP’s began to deteriorate after several weeks. By this time, however, most the heavy construction was finished. As a result, properly installed BMP’s were no longer necessary. Site D was a private site and ranked fifth.

Site E was the biggest of the projects. It sat a few hundred feet uphill on the same street as site D. In fact, storm drains in place all around site E feed the ditch that runs along site D. Clay, dirt, sand, and other building materials are spread throughout the site. It appears at first glance to be a clay pit. There are silt fences and bales of hay installed at numerous locations throughout the site. However, they do not appear to contain anything, as they are not in strategic positions. This was very evident during rainfall events in which “rivers” of clay poured from the site into the street, nearby residents yards, and the storm drain (Fig. 2). Most of the sediment was carried out completely uninhibited from the two main entrances to the site. The sediment is deposited everywhere, including the neighbor’s yards and the street. The fences deteriorated and the hay bales began to fall apart. This was a Mobile County project and was ranked the worst overall by far.

Site F was located at a major intersection. It sat on a moderate slope with storm drains that feed a large canal a half-mile down the hill. The site had properly maintained BMP’s throughout the period. Although uncovered, mounds of clay were each surrounded by its own set of hay bales (Fig. 3). Hay bales and silt fences reinforced the downward slope side and corner. During rainfall events, very little runoff was seen from the site. This runoff was noted after the extremely heavy rainfall event. Site F was a private site and by far was the best site in the study.

The Mobile County sites were ranked the first, second, and fourth worst in the study (Table 1). This indicates that they, in fact, are more poorly managed and contribute more sediment into the watershed than do the private sites. Furthermore, the same contracting group was in charge of all of the County sites. This an indication that perhaps that particular contracting group is cutting costs by neglecting best management practices.

What can be done?

Perhaps the most important aspect of the watershed strategy is to find the optimal placement and design of BMP’s so that their combined effect is most cost-effective (Zen, 2004). None of the sites in the study seemed perfect. However, there were examples of good and bad management. I believe the solutions to be simple. First, mounds of soil, dirt, and clay need to be covered. Next, BMP’s would be drastically more efficient if someone maintained them throughout the construction period after their installation at proper locations. They should be checked once a week and immediately before and after a significant rainfall event. BMP’s should also be installed one after the other in a series on the downward slope. If applicable, runoff should be diverted into a location within the site. These simple steps would not cost the contractors a substantial amount of money and would help to keep all the their materials at the site.

Conclusion

Although it appears that all construction sites contribute to the problems of the watershed, there is plenty of hope. Most of the sites in the study at least tried to contain the sediment within its boundaries. To answer all of the questions posed in the study, I believe this project should be repeated on a greater scale. Due to time constraints created by the end of the school semester, I was unable to perform the study under my ideal methods. Ideally, the study would include more sites as well as documenting their progress from start to finish. In addition, turbidity tests downstream of the sites before and after construction would be taken to help support the qualitative data. Nevertheless, I feel that this study will help people know what they are looking at when they drive by a “river” of clay coming from a site. I also feel this study will support efforts for further studies in an attempt to get the problem fixed.

References

Alabama Water Watch, 2002. Water Monitoring

Chepil, W.S., 1945. “Dynamics of Wind Erosion: l. Nature of Movement of Soil by Wind.” Erosion and Sediment Yield. Hutchinson Ross Publishing Company.

Ellison, W.D., 1944. “Studies of Raindrop Erosion.” Erosion and Sediment Yield. Hutchinson Ross Publishing Company.

Hicks, William R., 2003. “The Effectiveness or Ineffectiveness of Storm Water BMP’s in the Dog River Watershed.”

Julien, Pierre, 1995. Erosion and Sedimentation. Cambridge University Press

Alabama Water Watch, 2002. Water Monitoring

Langbein, W.B. and Schumm, S.A., 1958. “, S.A., 1958. “Yield of Sediment in Relation to Mean Annual Precipitation.” Erosion and Sediment Yield. Hutchinson Ross Publishing Company.

Williams, Timothy, 2003. “Intense Rainfall and Poorly Planned Construction Sites.”

Wolman, M. G., 1967. “A Cycle of Sedimentation and Erosion in Urban River Channels.” Erosion and Sediment Yield. Hutchinson Ross Publishing Company.

Xiao-Yue "Jenny" Zhen; Yu, Shaw L.; Jen-Yang Lin, 2004. “Optimal Location and Sizing of Stormwater Basins at Watershed Scale.” Journal of Water Resources Planning & Management. Vol. 130 Issue 4, p339