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City Wide Drainage Needs Assessment Report (March 2007)
City of Portsmouth
Citywide Drainage Needs Assessment

Executive Summary

The City of Portsmouth, like its neighboring Hampton Roads localities, experiences frequent and sometimes severe flooding in many of its residential neighborhood locations. Following intense rainfall events, flooding occurs in areas lacking proper drainage infrastructure, locations with inadequate pipe sizes, clogged or caved in pipes, filled in street side ditches, and in low lying areas which are also subject to tidal flooding. Other neighborhoods within the city contain backyard and side yard ditches which function adequately but are frequent targets for requested aesthetic improvements. In the spring of 2005, responding to City Council’s direction a citywide drainage needs assessment was undertaken. The project team, including members from the City Department of Public Utilities/Works, the Department of Engineering and Technical Services, and HDR developed the following Mission Statement to guide this project:

"To develop an expansive picture of the City of Portsmouth's current stormwater drainage conditions including areas that lack drainage systems, areas with inadequate drainage, and areas where the drainage is satisfactory but needs aesthetic improvement. Upon assessment, these areas will be graded for effectiveness and adequacy so that improvement requirements and corresponding costs can be derived.“

Graphic 1In order to effectively evaluate the citywide drainage needs, the project team identified 38 drainage assessment areas (DAA), which consist of one or more neighborhoods and subdivisions throughout the City. Excluded from this study were new neighborhoods with adequate drainage, commercial/industrial areas, private drainage systems, and the Downtown/Olde Towne Area (which is included in an independent study by others).

Graphic 2Based on the observed drainage conditions, different levels of improvements were assigned to each DAA depending on the drainage needs. Recommended drainage improvements varied from replacing small diameter pipes that tend to clog easily to improving neighborhood streets that currently have only roadside ditches as a collection system with widened streets with curb and gutter and sidewalk. Existing roadways with roadside ditches generally include small pipes underneath driveway entrances and tend to have ponded water conditions for an extended period of time due to the flat topography in many of the City’s neighborhoods. While roadside ditches can be an effective drainage collection system, they are generally undesirable due to their tendency to drain slowly resulting in stagnant water which provides a source for mosquito breeding.

The results of the assessment indicate that many areas lack appropriate drainage. The cost of necessary drainage improvements vary from $173,800 for the Swimming Point DAA to $22 Million for the Highlands DAA. Assessment areas such as Highlands or Park Manor which require replacement of roadside ditches with a wider street section, curb and gutter and sidewalk generally have a much higher improvement cost. Older neighborhoods with curb and gutter but with inadequately sized pipes and outfalls also have a higher improvement cost. The citywide cost estimates from the Decision Matrix are summarized in the following table:

Graphic 3


* For the purpose of his study, betterment improvements are defined as “replacing backyard and side yard ditches that are hydraulically adequate for aesthetic reasons”.

Since 1997, the City Capitol Improvement Plan (CIP) allocates $800,000 annually for neighborhood street and drainage improvements. Four neighborhoods are selected and improved in phases corresponding to available annual funding levels. These neighborhood construction phases rotate every four years until the entire neighborhood improvements are completed. The following two neighborhood examples illustrate the impact of this funding:

Graphic 4


The drainage and street improvement cost for Ebony Heights and Park Manor is approximately $6.0 million and $16.1 million, respectively. At present day costs with the four neighborhood rotation cycle, it will take 8 cycles and 32 years to complete all the improvements proposed in Ebony Heights and 20 cycles and 80 years to complete all the improvements proposed in Park Manor. This duration does not include the impact of inflation in the construction costs over time.

The recent cost escalation in construction projects also has an impact on the financing levels. For instance, the $800,000 annual budget set in 1997 has a present day value of approximately $400,000 in 2006. The yearly fee increases for stormwater utilities charged to each household is allocated for maintenance only. Even with customary yearly fee increases, there is a current budget shortfall of approximately $4 million per year in maintenance funding.

This citywide drainage needs assessment has identified the overwhelming need for drainage improvements throughout the city. The Decision Matrix is a useful tool for providing a snapshot view of existing conditions, proposed recommendations, and associated costs for improving the streets and drainage infrastructure within the identified assessment areas throughout the city. The current city system was built over a long period of time and it will require a long term commitment and increased funding to implement these improvements. By identifying compatible funding sources, the matrix can be used to help develop a long term plan for improving the City’s neighborhoods and drainage infrastructure for the current residents and future generations. The city may wish to scale back on the definition of what defines an aesthetically pleasing neighborhood, thus stretching funding.


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