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The original item was published from 8/16/2019 4:23:00 PM to 8/16/2019 4:40:30 PM.

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Posted on: June 17, 2019

[ARCHIVED] Proposal to create the Lake Shendandoah Stormwater Control Authority

A creek going down a hill

Resolution to Create stormwater control authority 2019

Lake Shenandoah Stormwater Control Authority District Map

List of Tax Parcels and Addresses within the Lake Shenandoah Watershed

Lake Shenandoah Watershed Drainage Study


Q: Will there be more information made available?

A: A Public Hearing was held to consider the creation of the Authority at the Rockingham County Board of Supervisors meeting on July 17th, 2019, at 6:00 pm. A second informational session for the public will be held Wednesday, August 21st at 6 pm at the County Administration Center.

Q: Am I in the boundaries of the Lake Shenandoah Stormwater Control Authority?

A: See map here: Navigate to the “Map Layers” tab along the blue bar on top of the map, then open the “Hydrological Features” folder, then check the box next to “Lake Shenandoah Stormwater Control Authority”.

Q: Why is this Authority proposed?

A: Properties within the Lake Shenandoah Stormwater Control Authority area have been subject to urban flooding for several years. Property and structural damage has occurred frequently, with some recent years having multiple damaging runoff events. After extensive study by county staff, engineering consultants, and a citizen stormwater advisory committee, it was determined that Rockingham County Government should implement improvements to the capacity and function of stormwater infrastructure within this area.

Q: What types of improvements will be done?

A: Further engineering analysis will be conducted to determine the most cost-effective methods for improvements to the stormwater system but, in general terms, a combination of new detention facilities and increased capacity in conveyance (ditches and pipes) will be evaluated.

Q: Where will the improvements be made?

A: Plans have not been finalized for exact locations of improvements. Information will be made publically available once determinations are made. A copy of the drainage study commissioned for this area is available below. Further engineering analysis will be performed.

How is development regulated?

The County administers three separate programs which deal with construction and runoff:

  • Erosion and Sediment Control Program:  Enabled by state code. Requires developers to create and implement an Erosion and Sediment Control Plan on their sites during construction.  Measures in these plans (such as silt fence, sediment ponds, and rock check dams) are temporary and are removed once the site no longer has disturbed soil.  The aim of this program is to minimize the amount of sediment that is carried off of construction sites in rain events.  Even with correctly installed controls, it is extremely difficult to prevent all sediment from leaving a site, due to less than 100% efficiency of sediment removal practices, and a capacity that can be exceeded in large (“above-design”) rain events.

  • Stormwater Management Program: Enabled by state code. Requires developers to create and implement a Stormwater Management Plan on their sites.  Measures in these plans (such as bioretention basins, stormwater detention ponds, and filtering devices) are permanent and remain in place after construction ends.  The aim of this program is to mitigate both the quantity (rate of water coming off the property during rain events) and quality of stormwater runoff.  Stormwater management regulations have changed significantly over the past 10-15 years.  New developments are held to a higher standard than those built prior to 2014.

  • Floodplain Management Program: Enabled by federal code. Requires developers to meet specific regulations when developing in a Federal Emergency Management Agency (FEMA) designated flood hazard area.  Some areas cannot be developed at all per regulations; others can be developed with specific mitigation and construction requirements.  The regulated floodplain boundary maps are provided to the County by FEMA.  Not all areas have been mapped.  Some areas may be prone to flooding but not included in FEMA risk maps.

What have developers done to mitigate runoff? 

Subdivisions within the Lake Shenandoah drainage area were developed over many years under varying standards pertaining to stormwater management.  Some citizens asked why developers hadn’t done more to control flow from their site.  The fact is, these developments were designed to meet the regulations at the time they were constructed. Even under older regulations, many of the subdivisions were required to have stormwater detention ponds.  Those include:  Barrington, Highland Park, Kentshire, Lake Pointe Village, Massanetta Springs Conference Center (lake was not necessarily designed to slow runoff, but does function in that capacity),Preston Lake, Quarles Business Park, Spring Oaks, Sunnyside Retirement Community, Taylor Grove Developments I and II, and Taylor Springs.

Why have the recommendations from the Timmons plan not been implemented?

The recommendations in the Timmons plan were based on preliminary assessments of the drainage area and hydrology.  None of the infrastructure recommendations are complete to a level of detail that would allow them to be constructed at this point.  Further engineering and analysis will be needed to determine the most cost-effective approach as well as specific design details.  The formation of an Authority (which was also recommended by the Timmons report) would facilitate a funding mechanism to pay for the engineering needed as well as to actually construct and maintain new infrastructure.

 What fees will be charged?

Fee rates and collection mechanisms, if the Authority were to be established, would be determined by the Board of the newly formed Authority.  Currently, this is proposed to be the members of the Board of Supervisors.

My property doesn’t flood, why should I be charged for infrastructure upgrades?

Infrastructure improvements are designed to accommodate runoff from all contributing areas.  Even if your property doesn’t experience flooding, the water that flows from your roofs, mowed lawns, and parking areas will make its way into the stormwater conveyance infrastructure.  Runoff from all properties accumulates in the system, causing flooding in low-lying areas where the existing infrastructure is inadequate to handle the cumulative volume of runoff.

Is grant funding available to fund some of the upgrades?

Grant programs exist through various agencies for funding of stormwater infrastructure improvements.  The grants are fairly competitive, and application does not guarantee funding.  The County is currently pursuing multiple grant funding opportunities for the Lake Shenandoah drainage area.

What about flooding in other areas?

Some other areas of the County are subject to periodic flooding.  It is important to make a distinction between “Urban Flooding” and “Riverine Flooding”, as different approaches are more appropriate depending on the situation.

Urban flooding:  Flooding that occurs when stormwater runoff flowing from developed impervious surfaces (buildings, parking lots, roads) overwhelms infrastructure (pipes, ditches) created to carry the water.  Urban flooding often occurs outside of mapped floodplain boundaries, and is generally localized within development patterns.

In areas that experience Urban Flooding, infrastructure upgrades (ponds, culverts) are the primary approach to reduce flooding.  Due to the localized nature of urban flooding, establishment of a local Authority can facilitate implementing infrastructure upgrades.  If other areas of the County outside of the Lake Shenandoah drainage area are determined to have inadequate infrastructure, a similar Authority could be established for those areas.

Riverine Flooding:  Flooding that occurs when runoff from large rain events accumulates in streams and rivers causing the water to overtop the banks of the channel.  FEMA, the Army Corps of Engineers, and the Virginia Department of Emergency Management are among the agencies that typically respond to severe riverine flooding events.

For areas that experience Riverine Flooding, infrastructure upgrades are not generally a key component of mitigation.  Development regulations, removing debris from streams and around bridges, and clean-up or post-disaster assistance are the primary tools for response to Riverine Flood risk.


10-year 24-hour storm:  For Lake Shenandoah area, rainfall of 3.88” within a 24-hour period; also described as a storm with a 10% chance of occurring in each 12-month period.  Stormwater infrastructure is currently designed to accommodate this amount of rain without flooding.

100-year 24-hour storm: For Lake Shenandoah area, rainfall of 6.12” within a 24-hour period; also described as a storm with a 1% chance of occurring in each 12-month period.  Where it has been mapped, regulated floodplain is defined as the areas that would be flooded in a 100-year storm.

2018 Rainfall Events

The total amount of rainfall for the whole year of 2018 far exceeded the typically annual rainfall amount for our area – approaching 150-200% of expected rainfall total depending on the area, with several individual storm events exceeding 5-, 10-, and even 25-year rainfall amounts.  Compounding the problems of significantly more rain falling was the fact that it rained so frequently that the ground conditions remained saturated with moisture for most of the summer.  This limited the soil’s ability to infiltrate rain, and led to even more runoff.   The most severe flooding occurred in May and June, but smaller flooding events continued to occur through the remainder of 2018, and above-average precipitation continued through early 2019.

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