Stormwater Final Draft

The report describes an effort that began in March 2004 to create a shared vision with concerned residents and elected officials as to what solutions are needed to relieve the flooding problems that have faced the town for generations.  More importantly, the report provides a “road map” for how to successfully improve drainage throughout the Town of Round Hill in the years to come.

Background:

Early last year, the Town of Round Hill adopted a streetscape master plan, which established the new minimum standard for your roadways; to include improved sidewalks, traffic calming, and roadside beautification strategies through landscaping.  The streetscape master plan also identified the need for a storm water master plan to address drainage problems in the town.

Round Hill has a system of roadways that lack adequate drainage capacity.  Some runoff flows down sidewalks, across roadways and into yards and basements.  Other runoff simply stands along the edge of pavement in localized depressions.  These problems occur even though the Town of Round Hill has sloping terrain and is surrounded by naturally preserved waterways with good capacity for storm water runoff.

This report has been developed to describe drainage problems through a project formulation, compare alternative solutions, and make recommendations in the sections of the report that follow.

Project Formulation:

The first step in master planning was to develop base mapping of the town.  For this purpose, we obtained Loudon County GIS data and coverages that include buildings, topography, land cover, streams, floodplains, parcels and other relevant information.  We then

prepared base mapping on a “watershed basis”, meaning that the study area was extended to high points and low points in the terrain just beyond the Town’s municipal boundaries.  Mapping was then converted to AutoCAD format for use in this study. This existing conditions map of the town is enclosed as Exhibit A.

The second step in project formulation was to distribute a survey to town residents that was designed to solicit input on known drainage problems.  Surveys that were returned to the Town have been included in Appendix A.

As surveys were received, the opportunity to provide comments directly to our engineering team was also provided through an informal town meeting, held on April 28, 2004.  Several residents took this opportunity to describe drainage problems, and to provide additional survey responses, which are also included in Appendix A.

As a follow up to the town hall meeting, a field review was conducted on April 29, 2004.  Town residents helped the engineering team locate identified drainage problems, obtain additional field data, and further document the condition of the town’s drainage systems.

Survey responses, discussions with town residents, field reports, and mapping were then used to develop a plan showing identified drainage problems.  This is included as Exhibit B of this report.  Problems have been grouped and described in basic terms on this exhibit in order to provide an overview of the identified areas of concern.  It is important to note, however, that more detailed field notes and photographs are being kept and developed into the solutions that will follow.

Alternative Analysis:

Based on the coordination and determination of drainage problems, some general guidelines governing the selection of solutions were established.

• The large response to the survey and town meeting indicate that drainage problems are a significant concern by town residents and that the problems are widespread;
• All identified drainage problems should be addressed in the master plan we develop, but choosing cost effective solutions and prioritizing improvements will help address the limited funding that is available;
• Environmental impacts will need to be addressed; and
• The solutions will not include storm water detention because open space has not been designated in land use plans for the construction of detention basins in the town, and because the anticipated pipe sizing is relatively small.  If ponds or dry basins are built with developer projects, this will offset potential increases in peak discharge and will be consistent with this master plan.

With these initial guidelines in mind, we developed a preliminary engineering analysis of the identified drainage problems as follows:

Hydrology:  Using topographic mapping, the town was broken into watershed areas that create concentrated discharge at 9 primary locations under existing conditions (a map showing the drainage basins is included in Appendix B).  Some small areas of sheet flow were outside of the areas with identified drainage problems and were not evaluated.  Within these primary drainage areas identified, watershed response time was then measured in a “time of concentration” and watershed land uses were evaluated to determine a “c-factor”.  These hydrology factors were then used to determine peak discharge estimates based on the “modified rational equation”.  The supporting hydrology calculations are included in Appendix B.

Hydraulics:  Hydraulic calculations for sizing pipes were based entirely on the continuity equation (Q=AV) and the assumption that the velocity of discharge through the pipes for a 10-year storm event will average at least 4 feet per second (fps) due to sloping terrain. 3fps is the minimum velocity desired for pipes to be “self-cleaning”, meaning that larger storm events will cause sediment deposited in the pipes to wash away. In reality, the sloping terrain in Round Hill will probably allow flow velocities to be higher and pipe sizing in some areas to be slightly smaller than indicated in this master plan.

Budget:  Pipe and ditch lengths were measured from the alternative solution plans, included as exhibits to this report.  These measurements are approximate and are only provided as a basis for cost estimating.  Cost estimating is also approximate and was determined as follows:

1. Pipe Cost = Avg. Pipe Size x $2 per inch diameter x Pipe Length
2. Ditch Cost = Equivalent Pipe Cost / 3
3. Inlets and Incidentals = Additional 30% of the Pipe & Ditch Costs

Other costs for engineering design, drainage easements, utility conflicts, and other potential aspects of the drainage improvement projects will need to be treated as additional costs when determining the final budgetary needs for each solution.  Costs for street improvements such as paving, curbing, lighting, utilities, landscaping and beautification are not included in these budgetary estimates, but could cost three to five times as much as the related drainage improvements considered in this study.  This should be considered if the town decides to package drainage improvements with streetscape improvements.

Alternative Solutions:

Based on our analysis, we have developed three (3) plans showing alternative drainage solutions.  Each plan is based on the hydrology calculations in Appendix B and the hydraulics and cost estimating methods described above.  The following tables summarize the results for each alternative and should be used along with map exhibits for comparison.