Sedimentation ponds approved by the regulatory authority for retention as permanent (5) Any point-source discharge of water from underground workings to to meet State and Federal effluent limitations; (C) Contain or treat the year. Are the City of Buda's Retention & Detention Ponds Maintained? Yes. . PUBLIC MEETING ON PROPOSED DISTRICT LINES RESCHEDULED The Big Picture - Week of April 30, City of Buda Annual Report - Fiscal Year - . Early Voting Starts Today - Two Early Voting Dates in Buda. 2S + mOAt = 5+0 (1/2A/) = S + (O), Ai = seconds. The following suggested specifications should meet these objectives under most conditions. long-term simulations using many years of actual rainfall records for the area of interest (such The following values were extrapolated from extensive wet detention pond.
Distance from a private well - 50 feet Distance from the property line feet. Investigate soils, depth to bedrock, and depth to water table at a site before designing a dry detention basin. At sites where bedrock is close to the surface, high excavation costs may make dry detention basins infeasible.
If soils on site are relatively impermeable such as Soil Group Da dry detention basin may experience problems with standing water. In this case, building a wet basin may be more appropriate. On the other hand, if the soils are highly permeable, such as well-drained sandy and gravely soils Soil Group Ait will be difficult to establish a shallow marsh component in the basin.
The maximum depth of dry detention basins typically ranges from 3 to 12 feet. The depth of the basin may be limited by groundwater conditions or by soils. Locate dry detention basins above the normal groundwater elevation i. Investigate the effects of seepage on the basin if the basin intercepts the groundwater table.
Investigate the effects of a dry detention basin on wetland resources. Mitigate altered wetland resources according to local, state and federal regulations. Like all stormwater BMPs, dry detention basins may not be constructed in wetland resource areas except for bordering land subject to flooding, isolated land subject to flooding, land subject to coastal storm flowage, and riverfront areas. Embankments or dams that store more than 15 acre- feet or that are more than 6 feet high are regulated by the state Office of Dam Safety.
Design The critical parameters in determining the size of the basin are the storage capacity and the maximum rate of runoff released from the basin. Design dry detention basins to store the volume required to meet the peak rate attenuation requirements of Standard 2 for the 2-year and year hour storms. In some cases, compliance with Standard 2 may require flood storage volume to prevent an increase in off-site flooding from the year hour storm.
Design a multiple stage outlet structure to control peak discharges for the 2-year and year hour storms. Provide an emergency spillway.
III-3.2.1 Detention Ponds
Build the spillway in the existing ground--not in the embankment. Make the interior embankment slopes no greater than 3: Provide access for maintenance. Design embankments to meet safety standards.
Stabilize the earthern slopes and the bottom of the basins using seed mixes recommended by the NRCS. Design of Stormwater Pond Systems. Center for Watershed Protection. For steel fabric fences, consider the following aesthetic features: Vinyl coating that is compatible with the surrounding environment e.
All posts, cross bars, and gates may be painted or coated the same color as the vinyl clad fence fabric. For metal baluster fences, Uniform Building Code standards apply. Wood fences may be used in subdivisions where the fence will be maintained by homeowners associations or adjacent lot owners.
Wood fences should have pressure treated posts ground contact rated either set in 24 inch deep concrete footings or attached to footings by galvanized brackets.
Rails and fence boards may be cedar, pressure treated fir, or hemlock. Signage Detention ponds, infiltration ponds, wetponds, and combined ponds should have a sign placed for maximum visibility from adjacent streets, sidewalks, and paths. An example of sign specifications for a permanent surface water control pond is illustrated in Figure III Right of Way Right-of-way may be needed for detention pond maintenance.
It is recommended that any tract not abutting public right of way have 15 20 foot wide extension of the tract to an acceptable access location. Setbacks It is recommended that facilities be a minimum of 20 feet from any structure, property line, and any vegetative buffer required by the local government.
The detention pond water surface at the pond outlet invert elevation must be set back feet from proposed or existing septic system drainfields. However, the setback requirements are generally specified by the local government, uniform building code, or other statewide regulation and may be different from those mentioned above.
Seeps and Springs Intermittent seeps along cut slopes are typically fed by a shallow ground water source interflow flowing along a relatively impermeable soil stratum. These flows are storm driven and should discontinue after a few weeks of dry weather. However, more continuous seeps and springs, which extend through longer dry periods, are likely from a deeper ground water source.
When continuous flows are intercepted and directed through flow control facilities, adjustments to the facility design may have to be made to account for the additional base flow.
III Detention Ponds
Planting Requirements Sod or seed exposed earth on the pond bottom and interior side slopes with an appropriate seed mixture.
Plant all remaining areas of the tract with grass or landscape and mulch with a 3 inch cover of hog fuel or shredded wood mulch. Shredded wood mulch is made from shredded tree trimmings, usually from trees cleared on site.
The mulch should be free of garbage and weeds and should not contain excessive resin, tannin, or other material detrimental to plant growth. Do not use construction materials wood debris or wood treated with preservatives for producing shredded wood mulch. Landscaping Landscaping is encouraged for most stormwater tract areas see below for areas not to be landscaped.
However, if provided, landscaping should adhere to the criteria that follow so as not to hinder maintenance operations. Landscaped stormwater tracts may, in some instances, provide a recreational space. Follow these guidelines if landscaping is proposed for facilities: Do not plant trees or shrubs on berms meeting the criteria of dams regulated for safety. Do not plant trees or shrubs within 10 feet of inlet or outlet pipes or manmade drainage structures such as spillways or flow spreaders.
Avoid using species with roots that seek water, such as willow or poplar, within 50 feet of pipes or manmade structures. Restrict planting on berms that impound water permanently or temporarily during storms. This restriction does not apply to cut slopes that form pond banks, only to berms. Do not plant trees or shrubs on portions of water impounding berms taller than four feet high.
Plant only grasses on berms taller than four feet. Grasses allow unobstructed visibility of berm slopes for detecting potential dam safety problems such as animal burrows, slumping, or fractures in the berm.
Trees planted on portions of water impounding berms less than 4 feet high must be small, not higher than 20 feet mature height, and have a fibrous root system. These trees reduce the likelihood of blow down trees, or the possibility of channeling or piping of water through the root system, which may contribute to dam failure on berms that retain water. The internal berm in a wetpond is not subject to this planting restriction since the failure of an internal berm would be unlikely to create a safety problem.
Plant all landscape material, including grass, in good topsoil. Make native underlying soils suitable for planting by amending with 4 inches of compost tilled into the subgrade. Refer to BMP T5. Post-Construction Soil Quality and Depth for soil quality standards. Soil in which trees or shrubs are planted may need additional enrichment or additional compost top dressing. Consult a nursery, landscape professional, or arborist for site specific recommendations.
The landscaped islands should be a minimum of six feet apart, and if set back from fences or other barriers, the setback distance should also be a minimum of 6 feet.
Where tree foliage extends low to the ground, the six feet setback should be counted from the outer drip line of the trees estimated at maturity. This setback allows a 6-foot wide mower to pass around and between clumps. Evergreen or columnar deciduous trees along the west and south sides of ponds are recommended to reduce thermal heating.
Place trash racks at a shallow angle to prevent ice formation. Linear Highway Sites ED ponds are poorly suited to treat runoff within open channels located in the highway right of way, unless storage is available in a cloverleaf interchange or in an expanded right-of-way. Construction Sequence The following is a typical construction sequence to properly install a dry ED pond.
The steps may be modified to reflect different dry ED pond designs, site conditions, and the size, complexity and configuration of the proposed facility. An ED pond may serve as a sediment basin during project construction.
If this is done, the volume should be based on the more stringent sizing rule erosion and sediment control requirement vs. Installation of the permanent riser should be initiated during the construction phase, and design elevations should be set with final cleanout of the sediment basin and conversion to the post-construction ED pond in mind.
The bottom elevation of the ED pond should be lower than the bottom elevation of the temporary sediment basin. Appropriate procedures should be implemented to prevent discharge of turbid waters when the basin is being converted into an ED pond. Stabilize the Drainage Area. ED ponds should only be constructed after the contributing drainage area to the pond is completely stabilized.
If the proposed pond site will be used as a sediment trap or basin during the construction phase, the construction notes should clearly indicate that the facility will be dewatered, dredged and re-graded to design dimensions after the original site construction is complete. Assemble Construction Materials on-site, make sure they meet design specifications, and prepare any staging areas.
Clear and Strip the project area to the desired sub-grade. All areas surrounding the pond that are graded or denuded during construction must be planted with turf grass, native plantings, or other approved methods of soil stabilization. Install the Riser or Outflow Structure and ensure the top invert of the overflow weir is constructed level at the design elevation.
EXTENDED DETENTION (ED) POND
Construct the Embankment and any Internal Berms in 8 to inch lifts and compact the lifts with appropriate equipment. Construct the Emergency Spillway in cut or structurally stabilized soils.
Install Outlet Pipes, including downstream rip-rap apron protection. Stabilize Exposed Soils with temporary seed mixtures appropriate for the pond buffer. All areas above the normal pool elevation should be permanently stabilized by hydroseeding or seeding over straw. Plant the Pond Buffer Area, following the pondscaping plan see Section 8. Construction Inspection Multiple inspections are critical to ensure that stormwater ponds are properly constructed.
Inspections are recommended during the following stages of construction: If the ED pond has a permanent pool, then to facilitate maintenance the contractor should measure the actual constructed pond depth at three areas within the permanent pool forebay, mid-pond and at the riserand they should mark and geo-reference them on an as-built drawing. This simple data set will enable maintenance inspectors to determine pond sediment deposition rates in order to schedule sediment cleanouts.
Maintenance Agreements Section 4 VAC of the regulations specifies the circumstances under which a maintenance agreement to must be executed between the owner and the local program. This section sets forth inspection requirements, compliance procedures if maintenance is neglected, notification of the local program upon transfer of ownership, and right-of-entry for local program personnel.
Access to ED ponds should be covered by a drainage easement to allow inspection and maintenance. It is also recommended that the maintenance agreement include a list of qualified contractors that can perform inspection or maintenance services, as well as contact information for owners to get local or state assistance to solve common nuisance problems, such as mosquito control, geese, invasive plants, vegetative management and beaver removal.
Maintenance Inspections Maintenance of ED ponds is driven by annual inspections that evaluate the condition and performance of the pond, including the following: Measure sediment accumulation levels in forebay. Monitor the growth of wetlands, trees and shrubs planted. Record the species and their approximate coverage, and note the presence of any invasive plant species.
Inspect the condition of stormwater inlets to the pond for material damage, erosion or undercutting. Inspect the banks of upstream and downstream channels for evidence of sloughing, animal burrows, boggy areas, woody growth, or gully erosion that may undermine embankment integrity. Inspect pond outfall channel for erosion, undercutting, rip-rap displacement, woody growth, etc.
Inspect condition of principal spillway and riser for evidence of spalling, joint failure, leakage, corrosion, etc. Inspect condition of all trash racks, reverse sloped pipes or flashboard risers for evidence of clogging, leakage, debris accumulation, etc.
Inspect maintenance access to ensure it is free of woody vegetation, and check to see whether valves, manholes and locks can be opened and operated. Inspect internal and external side slopes of the pond for evidence of sparse vegetative cover, erosion, or slumping, and make needed repairs immediately.
Based on inspection results, specific maintenance tasks will be triggered. The constantly changing water levels in ED ponds make it difficult to mow or manage vegetative growth. The bottom of ED ponds often become soggy, and water-loving trees such as willows may take over.
The maintenance plan should clearly outline how vegetation in the pond and its buffer will be managed or harvested in the future. Periodic mowing of the stormwater buffer is only required along maintenance rights-of-way and the embankment. The remaining buffer can be managed as a meadow mowing every other year or forest. The maintenance plan should schedule a shoreline cleanup at least once a year to remove trash and floatables that tend to accumulate in the forebay, micropool, and on the bottom of ED ponds.
Frequent sediment removal from the forebay is essential to maintain the function and performance of an ED pond. As noted above, the designer should also check to see whether removed sediments can be spoiled deposited on-site or must be hauled away. Sediments excavated from ED ponds are not usually considered toxic or hazardous, and can be safely disposed by either land application or land filling.
ED ponds tend to accumulate sediment and trash, which residents are likely to perceive as unsightly and creating nuisance conditions. Fluctuating water levels in ED ponds also create a difficult landscaping environment. In general, designers should avoid designs that rely solely on dry ED ponds. ED ponds should never be constructed within existing natural wetlands, nor should they inundate or otherwise change the hydroperiod of existing wetlands.
Designers can expect a great deal of neighborhood opposition if they do not make a concerted effort to save mature trees during design and pond construction. Designers should also be aware that even modest changes in inundation frequency can kill upstream trees Wright et al. ED ponds have less risk of stream warming than other pond options, but they can warm streams if they are unshaded or contain significant surface area in shallow pools.
If an ED pond discharges to temperature-sensitive waters, it should be forested, contain the minimum pools to prevent clogging, and have a detention time of no longer than 12 hours.
ED ponds are generally considered to be safer than other pond options, since they have few deep pools. Steep side-slopes and unfenced headwalls, however, can still create some safety risks. Gentle side slopes should be provided to avoid potentially dangerous drop-offs, especially where ED ponds are located near residential areas. The fluctuating water levels within ED ponds have potential to create conditions that lead to mosquito breeding. Mosquitoes tend to be more prevalent in irregularly flooded ponds than in ponds with a permanent pool Santana et al.
Designers can minimize the risk by combining ED with a wet pond or wetland. Urban Watershed Forestry Manual: Conserving and Planting Trees at Development Sites. Center for Watershed Protection.