AGENDA #5e
MEMORANDUM
TO: Mayor and Town Council
FROM: W. Calvin Horton, Town Manager
SUBJECT: Report on Storm Drainage Issues Involving 507 and 509 Colony Woods Drive
DATE: February 11 , 2002
This report describes the history of storm drainage issues involving 507 and 509 Colony Woods Drive, including actions the Town has taken to mitigate identified problems. In August of 2001, water entered the crawl space beneath the house at 507 Colony Woods Drive as a result of a local thunderstorm. Subsequently, the property owner, Mr. Denny Cook, petitioned the Town for assistance with the problem.
We recommend that the Town continue to maintain the public drainage facilities serving these properties but make no further improvements to the system.
BACKGROUND
The two properties located at 507 and 509 Colony Woods Drive have a history of storm drainage problems dating back to 1983, according to our records, when the problems were first brought to the attention of the Town. Drainage problems have continued to arise periodically at this location. Attachment #1 provides a history of the drainage issues relating to these properties.
The existing drainage system includes grassed ditches at the rear of both lots, and a concrete lined ditch located generally on the property line between the houses at 507 and 509 Colony Woods Drive that is connected to an 18 inch concrete pipe that runs under Colony Woods Drive. The ditches and pipe receive water from the drainage area behind the houses along the east side of Colony Woods Drive including the Newton Road development in Durham County and the Montessori School off Pope Road. The Newton Road development was designed and constructed under Durham County authority and regulation. During certain heavy rains, the ditch and pipe system cannot fully contain and convey the stormwater runoff from the contributing drainage area. Under some circumstances, stormwater overtops the ditches to a level between the houses where water encroaches under and sometimes into portions of the houses at 507 and 509 Colony Woods Drive.
The Colony Woods subdivision was constructed in accordance with regulatory standards at the time of construction, and is a product of now obsolete design concepts and requirements. If this subdivision were proposed today, an analysis of the drainage system would be required to ensure that the system could adequately collect and convey stormwater runoff in accordance with current design standards. Additionally, if lots were permitted adjacent to drainage-ways such as this, it would be required that they be graded to protect crawlspaces and finished floors in the event that the drainage system design was exceeded by unpredictable weather events.
In 1999, in response to continuing drainage complaints, the Town lowered a segment of Colony Woods Drive in front of these properties and provided pipe, headwall, grading and flood-proofing improvements in an attempt to mitigate the drainage problems these properties were experiencing. At the time these improvements were installed, we advised the property owners that, because of the proximity to the drainage ditches and elevations of the crawlspaces and floors of the houses, we were able to mitigate the frequency and intensity of drainage system overflows in this area but could not protect them from flooding that might result from intense rain storms concentrated in the immediate upstream watershed.
We believe that such a storm caused the flooding in August 2001 that resulted in the present request from the owners of 507 and 509 Colony Woods Drive.
DISCUSSION
Based on their observations during the August 2001 thunderstorm, the property owners suggested that the curb opening on the west side of Colony Woods was not large enough to carry stormwater water away from the street, and they also questioned whether the detention basin built by the Montessori School was functioning properly.
In response to these comments, the Town enlarged the curb opening on the west side of Colony Woods Drive and inspected the Montessori School stormwater detention basin. We noted that the basin was partially filled with sediment and did not appear to be functioning as designed. Upon notification from the Town, the Montessori School had the basin cleaned out and improved, and the school’s Engineer certified that the basin is now functioning as designed. Attachment #2 is a copy of the Engineer’s letter. The Town will provide a suggested basin maintenance schedule to the Montessori School, and will periodically inspect the condition and function of the basin.
Another concern expressed by the property owners involved the safety of the inlet to the 18-inch diameter pipe from the concrete lined ditch. Under certain conditions, the pipe entrance is completely submerged, causing a potentially hazardous condition. We believe that the inlet headwall and pipe system design is appropriate and that it functions properly. We agree with the property owners that caution should always be exercised around any submerged drainage structure during flooding events.
We are unable to suggest practical and affordable options for further reducing the flooding risk involving these properties. A larger drainage system could be installed to collect and convey a greater volume runoff past these properties and on downstream. However, this would in effect only move the flooding problem downstream in the drainage system. Further enlargement of the Colony Woods system would require commensurate improvements downstream at a number of piped road crossings and open channels at significant cost. Our experience suggests that, regardless of the extent of additional improvements in the drainage system adjacent to 507 and 509 Colony Woods Drive, conditions are such that certain storm events would still exceed system capacity with resultant flooding.
Site specific drainage improvement measures that the property owners may want to pursue include placement of earthen berms, elevation of mechanical and other equipment, installation of sump pumps, and waterproofing of walls. Although such improvements would not eliminate periodic flooding of these properties, they could mitigate the detrimental effects of such flooding. The Town’s Engineering Department can provide the homeowners with more information about these measures, if requested.
CONCLUSION
The 507 and 509 Colony Woods Drive properties are located along a natural drainage-way which was modified as part of the development of the Colony Woods subdivision. The Town has constructed significant drainage improvements in response to citizens’ complaints about drainage problems affecting these properties. These improvements have reduced both the frequency and severity of flooding associated with this drainage system and these properties.
We recommend that the Council direct no further improvement of this specific drainage system, but that the Town continue to maintain system elements within public rights-of-way and public drainage easements. Town staff will provide interested property owners with technical assistance regarding onsite mitigation measures to protect their properties from flood damages to the extent practicable; will provide the Montessori School with a suggested maintenance schedule for the school’s detention basin; and will periodically inspect the basin condition and function.
ATTACHMENTS
1. History of Colony Woods Drive drainage issues (p. 4).
2. Montessori School engineer’s letter dated November 28, 2001 (p. 7).