|
Ponds can be constructed for fishing, swimming, landscaping,
animal and wildlife watering, or water quality management. The
more diverse its intended use, the more complex are the factors
to consider for its design and management. Farm/amenity ponds
have less stringent standards and conditions governing their
design and construction than do typical stormwater management
ponds. They can vary considerably in size depending on the owners’
preferences, the purpose for which they are intended, and the
available space. It is best to limit the number of expected
uses.
Preliminary studies for locating, sizing, and designing the
shape of a pond have a lot to do with the landform, soil type,
impact on downstream properties vulnerable to water damage in
case of dam failure, and the adequacy of water supply or the
drainage area for the pond. A pond should be designed to blend
well with existing ecological features. For those ponds that
are not intended to trap nutrients or other pollutants, the
location should not be in the path of runoff pollution.
The two main types of pond structures are excavated (dugout)
ponds and embankment ponds (ponds constructed by damming a channel/depression).
Excavated ponds generally are found in relatively flat areas
with a high groundwater table, while embankment ponds are common
in terrain with more defined relief.
The soil type of an area is important in determining the location
for a pond during the feasibility studies. Sites with average
to high clay content are ideal for ponds because they provide
a tight base that reduces water loss, whereas those with sandy
soil or shallow depth to bedrock do not. To minimize pond maintenance
during the life of the pond, it is best to locate the pond off-line
rather than in-line of a stream. Trained staff of the Northern
Virginia Soil & Water Conservation District can provide
assistance with pond feasibility studies.
Based on the physical features and the rainfall intensity of
Fairfax County’s geographic region, ponds that depend on
surface runoff as their main source of water supply must be
sized such that there are at least two to three acres of
drainage area for every acre-foot of pond storage. Pond
storage capacity is measured in acre-feet. If a pond with a
water surface of 1 acre is 6 feet deep at its deepest point,
its storage capacity in acre-feet is calculated as 1 x 6 x 0.4
= 2.4 acre-feet (0.4 is a constant value based on the standard
shape of pond bottoms). A pond with a small storage capacity
is more vulnerable to the impacts of water pollution.
Constructed ponds must have adequate water and depths to sustain
water losses due to seepage, evaporation, and lowered groundwater
table especially during seasonal drought conditions. In this
geographic region the recommended pond depth is 6-7 feet at
the deepest area. This depth also guarantees survival of
fish during freezing conditions of the winter months.
Call Miss Utility at 1-800-257-7777 to
ensure that there are no underground utility wires, pipes, or
cables in the area for the proposed pond site.
Ponds are best designed so that they can successfully pass a
100-year storm event through their riser and emergency spillway
without overtopping the dam.
Request for permission to build a farm/amenity pond may be
granted a “By Right” status (i.e., free of all requirements)
if the plan meets the following conditions:
- The total area of clearing and disturbance for the
purpose of building the pond is 2,500 square feet (232.25 square meters) or
less.
- The proposed pond is located outside of a major floodplain,
a Resource Protection Area (RPA), or a jurisdictional wetland
(the latter being defined by applicable wetland permitting
regulations).
- The pond poses little or no risk to life or downstream property,
in case of dam failure or flooding.
- The proposed location is in an area with ideal soils
and topographic features for constructing a pond.
- There are no proffers (associated with property rezoning)
or development conditions (associated with special exceptions,
special permits and/or variance) that would affect pond construction.
After the site for a pond has been identified, the next step
is to perform a detailed survey to plan and design the
dam, the pond’s storage capacity, the emergency spillway,
and the pipe water outlet.
One of the most critical structures of the pond is its dam.
A mixture of coarse and finer textured soils makes a good foundation
material. This combination of particle sizes provides enough
bonding strength to keep the dam stable. Excessive seepage of
water through the dam is prevented by proper compaction of the
dam material, especially at the point of contact with existing
land forms and around the horizontal outlet pipe that goes through
the dam.
Important features of a dam are the water outlets that
control the flow of water out of the pond. The primary outlet
feature is the vertical pipe (riser) connected to a horizontal
pipe (barrel) that actually passes through the dam. The primary
outlet carries water during normal flows to maintain the pond
water elevation. Pipes made of smooth metal, corrugated metal,
or polyvinylchloride (PVC) plastic materials may be used.
The other important water outlet feature is the emergency
spillway. During flood events this structure carries flood
waters so that the water does not overtop the dam and wash it
away. This feature can be designed and constructed in one of
several ways. The basic criterion is that the structure should
be large enough to adequately carry floodwaters. Its elevation
must be set higher than the riser and about two feet lower than
the top of the dam. If the emergency spillway is in the form
of an open channel, a “rule of thumb” is that the
bottom width of the spillway channel can be estimated by adding
15 feet to one-half of the total drainage area in acres. For
instance, a pond with a 20-acre drainage area should have a
spillway with a bottom-width of about 25 feet; one with a 50-acre
drainage area should have a spillway with a bottom-width of
about 40 feet.
Introduce the correct combination and species of fish to make
a balanced pond ecosystem. In the Virginia area, a ratio
of 1:1:3:7 of the following species are known to co-habit well:
Largemouth Bass : Catfish : Sunfish : Bluegill, respectively.
The catfish, sunfish and bluegill are best stocked in August
or September. Bass are best stocked a year after the bluegill
are stocked. Together, the bluegill and sunfish can be stocked
at a rate of 500 per acre and the other species determined accordingly.
If you have questions about pond construction or maintenance,
e-mail
NVSWCD's Willie Woode or call him at 703-324-1430.
For information about E&S permits for pond construction,
call the Engineer-of-the-Day at 703-324-1575.
- Erosion control measures and small area grading permits:
Chapter 104 (Erosion and Sediment Control) of the Fairfax
County Code requires prior approval of an Erosion and Sediment
(E&S) Control Plan before clearing, grading, filling or
otherwise disturbing natural terrain on any area greater than
2,500 square feet (232.25 square meters). There is a minimum charge of $400
dollars for reviewing a “rough grading plan” for
a potential pond that will be about 1/4 of an acre in size.
The bigger the pond, the higher the fee.
- In like manner, section 2-601 of the Zoning Ordinance requires
approval of E&S control and grading plans before the
removal or addition of soil in excess of 18 inches (457.2mm)
in an area exceeding 2,500 square feet (232.25 square meters).
- Article 2, part 7 of the Zoning Ordinance sets forth requirements
that apply in common open space areas. Construction of
ponds in common open space areas would be subject to the
approval of the Department of Public Works and Environmental
Services (DPWES).
- Submitted designs for all farm/amenity ponds must meet the
minimum geotechnical design standards for structures
with small dams classified in the Public Facilities Manual—Plate
64-4, as a “C” reservoir, unless waived by the Director
of DPWES during the review and approval process.
- Under the Chesapeake Bay Preservation Ordinance, the Zoning
Ordinance, and in the Public Facilities Manual (PFM), such
ponds should not be located within a designated Resource
Protection Area (RPA), major floodplain, or storm drainage
easement. However, under certain conditions the Director
of DPWES may permit such pond construction, at the recommendations
of the Environmental and Facilities Review Division and Site
Permits.
- If runoff from a 70-acre drainage area flows through the
pond, a drainage study must be submitted together with
grading plan details.
- For ponds that are planned to be located within a jurisdictional
wetland or involve damming a continuously flowing stream,
additional approval is required from both the Virginia Department
of Environmental Quality (a state approval), and the US Army
Corps of Engineers (a federal approval). A written waiver
must be obtained if these agencies choose to waive this requirement.
- Ponds must be consistent with any proffers or development
conditions that apply to the property(ies) in question.
- The Department of Conservation and Recreation must approve
all dams of ponds that will store 15 acre-feet of water and
are higher than 25 feet, as well as dams that store more than
50 acre-feet and are higher than 6 feet.
RETURN to paragraph about construction
standards
|
