5-Sediment Traps and Basins

8/7/2019 5-Sediment Traps and Basins

1/16

36 Technical Specifcations or BMPs

4.7 Sediment Traps and Basins

General In ormationThe purpose o a temporary trap or basin is to provide an area where muddy runo isallowed to pond, so sediment will settle out. Sediment traps and basins should be installedin selected drainage areas be ore excavation or ll work begins. Do not depend on sedimenttraps and basins alone to control sediment loss rom your construction site. Sedimentbasins and traps should ll with muddy runo during and immediately a ter a rain stormand drain down slowly over the next 12 days.

Containment or the ponding area can be provided by an excavation or a dike made o earthor stone. Low-lying sites on the downhill side o bare soil areas are ideal places to installtemporary sediment traps and basins. In general, sediment traps are designed to treat runo

rom about 1 to 5 acres. Sediment basins are larger, and serve areas larger than 5 acres. Basinsdraining areas larger than 10 acres require an engineered design and are o ten designed to

unction as a permanent stormwater treatment pond a ter construction is complete.

I easible, do not put sediment traps or basins in or next to fowing streams or otherwaterways. Make sure pooled water does not food buildings, roadways, utilities, or otherstructures. Construction o a permanent, stable outlet is key to long-term per ormance.

Temporary Sediment TrapsAny depression, swale, or low-lying place that receives muddy fows rom exposed soilareas can serve as a sediment trap site. Installing several small traps at strategic locations iso ten better than building one large basin. The simplest approach is to dig a hole or builda dike (berm) o earth or stone where concentrated fows are present. This will help todetain runo so sediment can settle out. The outlet can be a rock-lined depression in thecontainment berm.

Sediment BasinsSediment basins are somewhat larger than traps, but the construction approach is similar.Sediment basins usually have more spillway protection because o their larger fows. Mosthave risers and outlet pipes rather than rock spillways to handle the larger fows. Sedimentbasins are o ten designed to serve later as stormwater treatment ponds. I this is the case,agreements might be required assigning responsibility or long-term sediment removal andgeneral maintenance.

Small, temporary sediment traps intercept and detainconstruction site runo

so soil particles can settleout. Note how the outlet riser or this trap has beenwrapped with lter abric to increase detentiontime and trap suspended

sediment. Designing trapsand basins with long fow

paths between the inlet and outlet also helps toincrease sediment removal e ciency by extending thedetention time. Where spacerestrictions prevent longbasin designs, barriers placed in the basin can lengthendetention times by creatinga serpentine fow pathbetween the inlet and outlet.


2/16

Kentucky Construction Site BMP Planning and Technical Specifcations Manual 137


4.7.1 Temporary Sediment (Silt) Traps

DefnitionA temporary sediment or silt trap is ormed by excavation or by constructing a smallembankment o stone, stone- lled bags, or other material to retain sediment. Sedimenttraps are considered temporary structures and o ten placed at the site on an as needed basisby eld personnel. They should not be placed in fowing streams.

PurposeSediment traps pond and settle sediment rom muddy runo . Traps are used wherephysical site conditions or other restrictions prevent other erosion control measures romadequately controlling erosion and sedimentation. Sediment traps can be used downslope

rom construction operations that expose areas to erosion.

Design CriteriaBermed sediment traps con ned by rock, rock- lled ber bags, or other material arepre erred over excavated traps or those with soil berms. Traps are placed in converging fowareas (i.e., where ruts or washouts can orm) or in ditches, where they are o ten called ditchchecks or check dams. All traps are sized according to a design volume o 3,600 cubic eetper disturbed acre in the upstream drainage area. Multiple sediment traps constructed in aseries are needed when the storage volume o each cannot meet this design requirement.

Sediment traps are generally used to treat a drainage area o 5 acres or less. When thetotal drainage area to a single structure exceeds 10 acres, an engineered sediment basinis necessary. Traps cannot be placed in blue-line streams or other regulated waters unlessspace limitations or design limitations provide no other easible option. A USACE CleanWater Act (CWA) section 404 permit is required in these cases. Sediment traps must becleaned out be ore they are hal ull o sediment.

KYTC Silt Trap Types A, B, and C

The KYTC speci es three types o temporary sediment or silt traps. Type A is an excavatedbasin with or without a soil berm constructed in a ditch or drainageway. Type B is oneor more small berms o rock (KYTC No. 2 or shot rock) placed in a drainageway or ditch,with a geotextile underliner covered by 4 inches o KYTC No. 4 stone. A 12-inch overfowdepression appears in the middle o the berm(s). Type C traps are berms constructed o porous abric bags lled with crushed aggregate (e.g., KYTC No. 57), placed individually orin a series to create small ponding dams around drop inlets, curb inlets, or to orm checkdams in a drainageway or ditch.

Simple traps or checks with rock bermcontainment structures can be installed as needed by eld personnel with or without speci c notations on plandocuments. Standard notes on plans

should call or installation o temporary traps in concentrated fow areas subject to rutting on an as-needed basis. Make

sure containment berms are designed or overfow in the center o the berm, to

prevent sidecutting and bypasses. Install traps in a series to control sediment romlarge upland areas.


3/16


General Construct traps o rock (KYTC No. 2 mixed with smaller stone), rock- lled ber bags,

or use approved commercial sediment trap products installed and spaced accordingto manu acturers instructions.

Site sediment traps in areas where they can be maintained (i.e., sediment removed).

Set traps back rom property lines or water bodies as much as possible.

Do not site sediment traps at culvert or pipe outlets i possible.

Minimum sediment storage capacity is 3600 cubic eet per acre o upland areadrained by the trap. Where space restrictions exist, install multiple traps in a series atleast 50 eet apart.

Maximum drainage area is 5 acres.

Basin fow length should be at least two times the fow width.

Recommended trap depth or open areas is 2 eet at the inlet and 4 eet at the outlet.

Trap height must be 1.5 eet minimum in ditches, 35 eet in open area drainageways.

Trap berm width at base must be su cient to support 2H:1V berm.

Trap length must be su cient to tie into upper banks in ditches or high enough toprevent side bypasses in drainageways. Overfows must be in the center o the berm.

Construct the trap, seed and stabilize be ore clearing and grading work begins.

Embankment requirements Maximum height o 5 eet.

Maximum inside and outside slopes o 2:1.

Side slopes, containment berms, and infowing ditches should be seeded andmulched or blanketed as soon as possible a ter construction.

Outlet requirements The outlet must consist o an overfow spillway wide made o stone (KYTC No. 2

minimum).

Construction Specifcations Construct initial series o sediment traps be ore general site clearing and grading.

The area to be excavated or ponded must be cleared o all trees, stumps, roots, brush,boulders, and debris. All topsoil containing excessive amounts o organic matter mustbe removed.

Seeding, ertilizing, and mulching o the material taken rom the excavation mustcomply with the applicable soil stabilization sections o this manual.

Any material excavated rom the trap must be uni ormly spread to a depth notexceeding 3 eet and graded to a continuous slope away rom the trap.

Field-approved installations should be noted on weekly inspection reports and onplan documents within 7 days.


4/16


Inspection and MaintenanceThe trap must be inspected weekly and a ter every rain all greater than one-hal inch.Sediment must be removed rom the trap be ore the capacity is reduced to 50 percent o the design volume. Plans or the sediment trap must indicate the methods or disposing o sediment removed rom the pond.

Temporary sediment traps are removed upon stabilization or cover o the upland drainagearea with vegetation, pavement, and so on. The trap area should be graded, seeded, andmulched or blanketed. Excess sediment should be spread and stabilized where it will not

enter the drainage system.

Design sediment traps withlong fow paths i possible.Make sure overfow area is

protected with rock or other armoring. For best results,

seed trap and upland areasimmediately a ter construction.

Good trap placement and per ormance; poor maintenance. Removeaccumulated sediment be ore trap is hal ull.Spread material removed in a vegetated upland

area or other site where it will not wash into nearby sur ace waters.

Make sure overfow outlet or riser isdesigned or maximum detention times.Note the rock berm around riser, whichensures maximum detention or muddy fows a ter small storms.

In areas where space is restricted, use multiple traps in a series to meet thedesign goal o 3600 cubic t per acre o upland drainage. Get to nal grade,

seed and mulch as soon as possible to reduce trap maintenance and upkeep.


5/16


6/16



7/16



8/16



4.7.2 Sediment (Detention) Basins

DefnitionA sediment basin is a pond created by excavation and construction o an embankment anddesigned to retain or detain runo su ciently to allow excess sediment to settle out.

PurposeThe sediment basin is intended to collect and store sediment rom sites that are cleared orgraded during construction or or extended periods o time be ore permanent vegetationis reestablished or be ore permanent drainage structures are completed. It is intendedto intercept and trap sediment be ore it leaves the construction site. Some basins aretemporary, with a design li e o 12 to 18 months, and are to be maintained until the sitearea is permanently stabilized. Basins that will serve as permanent stormwater treatmentponds o ten require modi ed outlet risers during construction to ensure adequate pondingtimes and sediment removal.

Basins should be located at the stormwater outlet rom the site, not in any natural orundisturbed stream. Use o temporary dikes, pipes or channels might be necessary to divert

runo rom disturbed areas into the basin and to divert runo originating rom undisturbedareas around the basin.

Design CriteriaSediment basins must be designed by a pro essional engineer licensed in Kentucky. Thebasin should be designed using SEDCAD or other computer program. The design criteriaare listed below:

GeneralSite sediment basins where they will provide the best treatment (longest fow path betweeninlet and outlet, longest settling times) or the greatest area o the site. It is recommendedthat dams be located in a natural drainageway in a deep constriction that has a wide area

upstream or ponding detained stormwater. Do not locate dams where a ailure would result in severe property damage or danger

to human li e.

Sediment basins should be designed or modi ed to drain down slowly or 24 daysa ter a storm event. Modi y the outlet i necessary to achieve the maximum detentiontime.

Well-stabilized detention basin witherosion control blankets protecting

sidewalls during grass seed germination.Note the temporary stone berming in

ront o outlet, which increases detention

time and promotes maximum settling o soil particles. Design o this basin could be greatly improved by adding a temporary ba fe or barrier between the inlet and outlet, which would orce infows romthe culvert around the ba fe. The longer fow path and settling time would improve soil removal.


9/16


Minimum design storage capacity is 3600 cubic eet per acre o upland area drained.The maximum capacity or the impoundment must not exceed 10 acre- eet. I moreimpoundment capacity is needed, install basins in a series or site them to intercepttributary drainage areas.

Construction phase per ormance goal is to reduce the total suspended solids by80 percent or the 10-year, 24-hour storm, or provide a detention time o 24 to 48hours or the 10-year, 24-hour storm.

Minimum drainage area is 5 acres; the maximum drainage area is 120 acres.

Basin fow length should be at least two times the fow width; the longer, the better.Ba fes constructed o lter abric and metal posts can be used inside the basin tocreate a longer (e.g., serpentine) fow path between inlet(s) and the outlet.

Construct the basin be ore clearing and grading work begins.

Basins, side slopes, berms, inlets, and downstream outlet channels must be seededand mulched or blanketed immediately a ter construction.

Basins that drain more than 10 acres can be designed as retention (rather thandetention) basins (i.e., wet ponds). Design outlet to drain top o the pool arthest away

rom muddy infows. Incorporating a sediment collection orebay is recommended toaid in maintenance.

Embankment requirements Dam height should not exceed 20 eet

Maximum inside and outside slopes o the dam must be 3H:1V

Minimum 1 oot reeboard during the 100-year, 6-hour storm

Antiseep collars around discharge pipe are required

Minimum top width o the dam must be 12 eet

Principal spillway (riser and barrel) requirements

Use a subsur ace drain, a solid riser pipe,or both, with su cient dewatering holes toprovide su cient detention time. Risers withone-hal inch holes every 3 to 6 inches apartare recommended.

No large holes or slots should appear inthe lower two-thirds o the riser. Riserswith large openings can be modi ed asdescribed below or wrapped with lter

abric to cover lower openings during theconstruction period.

During construction, risers should bemodi ed with an inlet protection dike,pile o stone at the riser base, or otherstructure to provide longer ponding times

or small fow events.

Operational design goal is to reduce thepeak fow to predevelopment levels orthe 2-year and 10-year, 24-hour storms.

KY Division of Water DamSafety Requirements

The sediment basin might haveto be designed in accordancewith dam sa ety requirements o the KY Division o Water. A damis defned as any impounding

structure that is either 25 eet in height, measured rom thedownstream toe to the crest,

or has a maximum impoundingcapacity o 50 acre- eet o water. Structures that do not meet these criteria but have the

potential to cause signifcant property damage or pose athreat to loss o li e in thedownstream area are regulated in the same manner as dams.


10/16


Minimum diameter o pipe outlet is 12 inches; anti-vortex ba fe and trash rack arerequired

Minimum one oot reeboard required rom top o riser to crest o emergency spillway

Emergency spillway requirements Designed to pass the 100-year, 6-hour post development peak fow

Crest elevation at least one oot above the tip o the riser pipe

Minimum one oot reeboard during the 100-year, 6-hour storm to the top o theembankment

Rock used or the emergency spillway must be KYTC No. 2 or larger, depending onfow volumes and spillway slope (see sections on rock-lined channels and outletstabilization energy dissipator)

Emergency spillway energy dissipator must be extended at least 4 eet beyond the toeo the dam

Construction Specifcations Construct the basin by excavating or building an embankment dike be ore any

clearing or grading work begins. Areas under the embankment and any structural works must be cleared, grubbed and

stripped o any vegetation and rootmat as shown on the erosion and sediment controlplan.

To acilitate cleanout and restoration, the basin area must be cleared, grubbed andstripped o any vegetation.

A cut-o trench must be excavated along the centerline o the earth llembankments. The minimum depth must be 2 eet. The cut-o trench must extendup both abutments to the riser crest elevation.

Fill material or the embankment must be clean, low-permeability, mineral soil ree o

roots, woody vegetation, oversized stones, rocks, or other objectionable material. Fill material must be placed in 6 inch li ts, continuous layers over the entire length

o the ll. Compacting must be obtained by routing the hauling equipment overthe ll so that the entire sur ace o each layer o the ll is traversed by at least onewheel or tread track o the equipment or by the use o a compactor. Each layer mustbe compacted to 95 percent o maximum density and +/ 2 percent o optimummoisture content.

The embankment should be constructed to an elevation o 10 percent higher thanthe design height to allow or settlement i compacting is achieved with haulingequipment. I compactors are used or compacting, the overbuild may be reduced tonot less than 5 percent.

The principle spillway riser must be securely attached to the discharge pipe bywelding all around. All connections must be watertight.

The pipe and riser must be placed on a rm, smooth soil oundation. The connectionbetween the riser, and the riser base must be watertight. Pervious materials such assand, gravel, or crushed stone must not be used as back ll around the pipe or anti-seep collars.


11/16


The ll material around the pipe spillway must be placed in 4-inch layers andcompacted under the shoulders and around the pipe to at least the same density asthe adjacent embankment. A minimum o 2 eet o compacted back ll must be placedover the pipe spillway be ore crossing it with construction equipment.

Risers might require a rock berm or other fow restrictor during the constructionphase to ensure that muddy fows are detained su ciently to promote settling o sediment.

Steel base plates must have at least 2.5 eet o compacted earth, stone, or gravel overthem to prevent fotation.

An emergency spillway is required, and must not be installed in ll. Appropriateoverfow channel lining and energy dissipator must be constructed.

Ba fes, i used, must be constructed o 4 inch by 4 inch posts and o 4 oot by 8 oothal -inch exterior plywood. The posts must be set at least 3 eet into the ground, no

arther apart than 8 eet center to center, and must reach a height 6 inches below theriser crest elevation. Silt encing with metal posts can also be used i fow velocitiesin the basin are low and ponding heights during the 2-year, 24-hour storm will notexceed 5 eet.

The embankment, emergency spillway, incoming channels, and other site eatures

must be stabilized with vegetation and mulched or blanketed immediately ollowingconstruction.

Construction operations must be carried out in such a manner that erosion and waterpollution will be minimized.

Local and state requirements must be met concerning encing and signs warning thepublic o hazards o so t sediment and foodwater.

Inspection and MaintenanceInspect the sediment basin weekly and a ter each rain all greater than one-hal inch. I incoming fows are exiting the basin quickly because o large holes in the outlet, modi y thelower portion o the riser with a stone berm, lter abric, or other fow restrictor that retainsincoming fows or at least 1224 hours.

All damages caused by soil erosion or construction equipment must be repairedbe ore the end o each working day.

Remove sediment when the sediment storage zone is hal ull. This sediment mustbe placed in such a manner that it will not erode rom the site. The sediment mustnot be deposited downstream rom the embankment or in or adjacent to a stream orfoodplain.

When temporary structures have served their intended purpose and the contributingdrainage area has been properly stabilized, the embankment and resulting sedimentdeposit must be leveled or otherwise disposed o according to the approved erosionand sediment control plan.

I the sediment basin is designed to unction as a permanent stormwater treatmentpond, the basin and riser will be con gured to that mode upon stabilization o theupland drainage area. Temporary fow restrictors on risers and other constructionphase modi cations must be removed.


12/16


For best results, seed basin sidewalls and upland drainage areasas soon as possible. Make sure outlet structure does not allow rapid fow through the basinuse a rock berm, lter abric, or other means to maximize ponding and detention time.

This is a well-constructed sediment basin. Notethe rock fow restricter around outlet riser, which

lters and detains infows. Basin sidewalls should be seeded immediately a ter construction.

The fow path through this basinhas been lengthened by using

lter abric ba fes constructed to create a serpentine fow path.Note the rock pile around outlet riser to maximize detention and

grass on sidewalls.

This outlet riser intakehole has been modi ed with a hal round

section o pipe with1-inch holes on 6-inchcenters and rock berm.This temporary dike

provides additional detention during theconstruction phase, whichimproves soil removal.

This shows a very well designed detention basin, eaturing long

fow path between inlet and outlet and V-notched outlet riser, which provides longer detention times

or low fow events while still accommodating larger storms.

Operation o this basin during theconstruction phase, however, is

very poor. Note the lack o grasson sidewalls; no temporary dikein ront o the outlet. This basinappears to be lling rapidly and

requires sediment removal.


13/16



14/16



15/16



4.7.3 Dewatering Devices

DefnitionDewatering is the pumping o stormwater or groundwater rom excavation pits or trenches.The sediment-laden water must be pumped to a dewatering structure or sediment removal

be ore it is discharged o -site.

PurposeThe purpose o a dewatering device is to remove sediment rom the water be ore it isdischarged o -site.

Design CriteriaDewatering operations should not discharge to a ditch, pipe, or other conveyance thatleads to a regulated water body (e.g., stream, river, wetland, lake) except as authorized by aKPDES permit.

There are several types o dewatering structures or devices that can be used. A fat, well-stabilized, vegetated area can serve as a ltering structure i it can withstand the velocityo the discharged water and in ltrate or assimilate it without erosion. The minimum lterradius or length must be at least 75 eet.

It is recommended that sediment basins or temporary sediment traps receive sediment-laden water rom bore pits and trenches. This will ensure that the 80 percent trappinge ciency goal will be upheld. Take special care to ensure that pumping this water does notcause the sediment control structure to ail. Also take care at the outlet o the hose rom thepump to ensure that erosion does not occur because o high concentrated fows.

Another option is to use an in ltration trencha shallow, excavated trench back- lledwith stoneto orm a reservoir. This reservoir can contain subsur ace drainage pipe or juststone. This trench allows water to lter through the stone and then be diverted to a suitabledischarge point. The soils and the depth to the water table must be suitable or this sort o dewatering. Typical trench depths range rom 2 to 8 eet. The stone ll material consists o washed aggregate 1.5 to 3 inches in diameter.Other methods that can be used include a portable sediment tank, a silt ence pit, or acommercial sediment lter bag or sock . The structure must be sized to allow pumped waterto fow through the structure without overtopping.

This shows a dewatering sediment lter bag (center) in use at residential

construction site. Muddy water pumped into the bag is physically

ltered, with clear water passing

through the bag abric. Pumpingmuddy, un ltered water directly intocurb drains (center le t) or sur ace

streams constitutes a direct KPDES permit violation.


16/16


Construction SpecifcationsSee the speci cations in this manual or sediment traps and basins. Follow themanu acturers recommendations or commercial products.

Inspection and MaintenanceInspect the dewatering structure or device requently to ensure that it is unctioningproperly and not overtopping. Accumulated sediment should be spread out on site andstabilized, or disposed o o -site.

Silt ence enclosures and commercial sediment lters will likely require cleaning to removene particles and restore per ormance. This can be done with a sti brush when the lter is

dry, or via other manu acturers recommendations.

Containment structuresor sediment-laden water

can be made o rock or lter abric. Standard notes

should require monitoring tomake sure the containment

structure is not breached during dewateringoperations.

Large bags or socks made o lter abric provide excellent sediment

removal and are extremely versatile. Site ltration structuresaway rom sur ace waters i

possible. Dispose o sediment

collected in a fat vegetated areaor other site where it will not wash into sur ace waters.

Large sediment lter bag in operation.Note the row o straw bales around thebag providing additional treatment or clari ed fow oozing out o the bag. A

silt ence could also be used.

When dewatering sediment or other ponds, wait until several days a ter the last rain i possibleto allow or settling o sediments.Pump rom the upper portion o

pond, where water is clearer.

Documents

5-Sediment Traps and Basins