AGENDA #(9-19-01, 9:04 pm)6
MEMORANDUM
TO: Mayor and Town Council
FROM: W. Calvin Horton, Town Manager
SUBJECT: Follow-up Report on Lake Ellen Watershed
DATE: September 24, 2001
The purpose of this follow-up report is to provide the Council with additional information concerning the condition of sediment and erosion control measures within the watershed above Lake Ellen and to describe the watershed forces and dynamics related to the generation of sediment (suspended and deposited soil particles), and the sediment transport characteristics of Booker Creek in Lake Ellen. As a part of this follow-up report, Dr. Larry Band, Viot Gilmore Professor of Geography, UNC-Chapel Hill make a presentation on this topic.
On August 27th, the Council received a preliminary report describing the existing conditions of the watershed above Lake Ellen. This report was in response to a July 2nd petition from the Lake Ellen Homeowner’s Association concerning on-going sediment and turbidity (small suspended soil particles) problems in the lake.
On August 3rd, Orange County Erosion Control staff and Town staff inspected the watershed to determine if development activity was complying with the soil erosion and sedimentation laws and to make observations and recommendations about possible improvements. We discovered some areas under development in need of improvement or maintenance of soil erosion and sedimentation control measures, as well as other areas where the urbanized characteristics of the watershed is causing erosive forces to existing stream banks resulting in the suspension of sediment to the receiving waters.
During the field inspection, we also noted that in some cases best management practices exceeding existing regulations (installation of silt fence) were being used on single lots and that construction crews were cleaning sidewalks and street gutters when sediment was deposited on them.
Upon re-inspection approximately one week later, we found that the soil erosion and sedimentation control problems had been satisfactorily corrected.
Lake Ellen is a man-made lake. It’s dam was constructed directly on top of Booker Creek as an “in-stream impoundment.” The nature and characteristics of the watershed above Lake Ellen has changed significantly over the last one hundred years. Land use has changed from primarily agricultural use to forest use and subsequently to its current use as a developed watershed. All of these land-use changes have resulted in changes in the streamflow dynamics and the amounts and types of materials suspended in the runoff.
The agricultural period yielded significant amounts of sediment in the runoff, due to soil exposure. This historical sediment loading is evident in Chapel Hill’s riparian areas today. The period of the primarily forested watershed reduced sediment loading and created a changed dynamic in the stream systems. This reduction of sediment laden water caused more scour and down-cutting to the streambanks, which were, in part, comprised of the deposited agricultural sediment.
With urbanization, the net increases in runoff volume, velocity and rate has increased down-cutting, scour and erosion, and this situation has created additional sediment loads into the surface waters. These sediment loads, coupled with erosion from construction projects is the primary cause of turbidity and sediment in our creeks and man-made lakes, including Lake Ellen.
The Town’s Erosion Control Ordinance and the permitting and inspection efforts of Orange County Erosion Control staff is one of the most effective programs in North Carolina. According to the four-year study at UNC-Chapel Hill. The issue of turbidity reduction in stormwater runoff in this region is inherently difficult due to the clay content of the upper soil layer. However, turbidity is being controlled in Chapel Hill with the most effective best management practices and design standards available, which exceed State design standards and regulations. As described in the UNC report, the most effective method for reducing sediment laden runoff from construction sites is through continuous inspection and enforcement activities.
We believe the Town can best work to reduce post-construction runoff, which is a more long-term stormwater runoff pollutant control issue. This work is underway through the development of design standards for best management practices, added development ordinance provisions for stormwater management and the planning of a program to meet the mandated NPDES Phase 2 permit requirements. One of the minimum measures under this permit will require the Town to set standards that reduce the total suspended solids from stormwater runoff by 85 percent. This may be achieved through adequate best management practices selection, design, construction and maintenance. Another long-term solution for the Town to consider will be to identify and eliminate existing sources of sediment and turbidity loads such as stream bank stabilization, retrofitting existing facilities to improve pollutant removal efficiency and buffer restoration projects.
On September 11th, Town and Orange County staff met with members of the Lake Ellen Homeowner’s Association, represented by Robert Loomis, , Alan Neebe, Blair Pollock and Paul Wilson regarding the watershed, development issues in the watershed and lake management issues. We inspected construction sites as a follow-up to the previous inspections to ensure that the areas in need of corrective action were indeed corrected and to discuss other related issues.
The Association members stated that the lake has been slowly filling in with sediment over the years, especially the last ten years or so. They showed topography and aerial photos as a comparison to the existing conditions. It was apparent that sedimentation has indeed occurred there at the mouth of the lake, where the creek enters into it. Also, dense vegetation has taken root in the sediment areas blocking the view and reducing the area of open water. We discussed the likely sources of this sediment as a combination of construction site runoff and eroded streambanks and beds. Staff also described the natural process of sediment transport in a stream system and the settling effects when an artificial dam is placed across a stream.
The group then went to several sites in the watershed and found areas where no erosion control was used because single family lot (less than one-half acre disturbed) is not required to have it, and where erosion control facilities were effectively working. The other sediment pollutant source, scoured streambeds and banks, were observed and discussed.
It was determined that the effectiveness of the Orange County Erosion Control Permit and the requirements of the Resource Conservation District was significant, yet other areas remained which the Town may wish to consider to further reduce sediment loads. These areas include:
1. Consideration of single lot erosion control requirements through added regulation.
2. Consideration of addressing/mitigating the altered hydrology to streams in the developed watersheds, causing scour, bank erosion and downstream sediment loads through a comprehensive stormwater management program.
The Town staff suggested to the representatives of the Homeowners Association that a lake management program should be considered by the property owners as a means to assess the alternatives available to improve the conditions to meet their goals. If the Association requested, the Town would provide them with preliminary technical assistance as a means to get the lake management process started. This could include grant information and contacts as well as preliminary engineering advice.
The staff will continue to respond to and assist the Association as future needs arise.
1. Agenda Item #5d, dated August 27, 2001 (p. 4).