sediment management - basin

7/29/2019 Sediment Management - Basin

1/16

N O V E M B E R 2 0 0 5 B - 1

EROSION CONTROL AND SEDIMENT MANAGEMENT

Page

B0. Purpose ................................................. B-1B1. Riser-Board Water Control Structures ...... B-2B2. Sediment Settling Basins

in all Ditches .......................................... B-3B3. Ditch Construction ................................. B-4

B4. Stabilize Bare Soils ................................ B-4B5. Ditch Bank Contours ............................. B-5B6. Ditch Bank Vegetation Maintenance ....... B-5B7. Protect Ditch Banks ................................ B-6B8. Vegetative Stabilization ......................... B-7B9. Aquatic Plant Management .................... B-8

B10. Ditch Maintenance, Cleaning,and Dredging ....................................... B-9

B11. Herbicide Applications ....................... B-10B12. Middles Management ........................ B-10B13. Grove Management/Renovation .......... B-11B14. Settling Basins .................................... B-11

B15. Water Furrow Drain Pipes .................... B-12B16. Water Furrow Maintenance .................. B-12B17. Construction and Temporary

Erosion Control Measures ................... B-12

B0. PURPOSE

Sediments or suspended solids are recognizedforms of water pollution and often result in theloss of ditch or canal capacity. Unlike manychemical pollutants, sediment is a natural com-ponent of water bodies and the resources they

support. Excessive amounts of suspended solidsor sediments are often a product of erosion fromun-stabilized or disturbed land areas. Thesesolids originate from four primary sources:

Soil-particles eroded into ditches

Soil-particles eroded from ditches

Plant material washed into the ditches

Plant and biological material growing within theditches and canals.

Excessive sediments deposited on stream bottomsand suspended in the water column can harm fishspawning and impair fish food sources, reducehabitat complexity, potentially harm public watersupply sources, and reduce water clarity. Reductionin water clarity can harm natural resources, suchas sea grasses and oysters, in the receiving estu-ary. Studies have shown one of the major causesfor reduction in water clarity in Charlotte Harbor issuspended solids.

In addition to potential downstream water qualityimpacts, the build-up of silts and sediments in thegrove/farm-level, secondary, and primary drainagecanals reduces ditch and canal cross-section. Thisreduction in cross-sectional area results in higherwater velocities, as compared to an unfilled ditch

or canal. This higher water velocity (comparedto unfilled ditches/canals) may induce greateamounts of erosion of fine and coarse particles fromditch and canal banks. The presence of shoals andsandbars are good indicators of soil losses. Fielderosion also results in site degradation resulting inincreased costs for ditch-cleaning and reshaping obeds and furrows. In order to minimize effects osediment transport in surface water, efforts shouldfocus on keeping soils in the fields and along canaand ditch banks.

Figure B0.1. Sediment buildup downstream of weir controstructure.

Minimizing downstream transport of sediments fromgroves and canal/ditch banks requires an integratedapproach of managing erosion at the grove-level,the secondary canal system level and primary canasystem level. Erosion control measures for the secondary and primary canal systems are beyond thescope of this document. This document focuses onpractices that can help reduce sediment losses a

the grove-level. However, many of these practicesmay be applicable for the primary and secondarycanal systems. It should be noted that maximumsediment losses from groves are expected duringconstruction of new groves or renovation of olderones. Losses from mature, well managed groveswill be much lower.

The following sections describe BMPs that areapplicable for water conveyances within citrus


2/16

B - 2 B E S T M A N A G E M E N T P R A C T I C E S F O R G U L F C I T R U S

groves. The selection and implementation ofparticular BMPs must be based upon site-specificcircumstances and management styles.

NOTE: Water conveyances within citrus groves aredefined in Figs. B0.2, B0.3, and B0.4.

B1. RISER-BOARD WATERCONTROL STRUCTURES

Place and maintain culverts with riser-boardcontrol structures at locations where runoff isdischarged off-site.

Water discharge structures are used to control watertable levels and surface water levels in drainageditches within flatwoods citrus groves. The typeof structure selected can significantly influence thequality of water discharges.

With riser-board control structures, water is forcedto flow over the top of the boards. This flow pathcreates a low current area towards the bottom ofthe structure, which facilitates the deposition ofsediments and their accompanying nutrients or

pesticides, essentially removing them from the dis-charges. Conversely, screw-gates structures do notcreate this dead-current zone. Since they open fromthe bottom, sediments and their accompanying loadare swept out along with the discharge water.

References

Code 537. NRCS Conservation Practice Standard.

SS-409. Flatwoods Citrus Best Management Practice:

Riser-board Structures, http://edis.ifas.ufl.edu/SS409.

Figure B0.2. A water furrow is typically a 2-3 ft deep gentlysloping sides that allow tractors and equipment to travel be-tween beds. Water furrows are used to transport water awayfrom root zone in to lateral ditches.

Figure B0.3. Lateral ditches are cut at right angles to thebeds. They accept drainage from the water furrows.

Figure B0.4. Collector or perimeter ditches collect water fromseveral lateral ditches.

Figure B1.1. Riser-board water control structure.

Figure B1.2. Note the accumulation of aquatic weeds atthe riser-board control structure. Sediments generated by thedecay of these plants will settle to the bottom of the ditch andneed to be periodically removed.


3/16

N O V E M B E R 2 0 0 5 B - 3

B2. SEDIMENT SETTLING BASINSIN ALL DITCHES

Create and maintain localized settling basins(sumps) to trap sediments at field ditch connec-tions to lateral canals, at lateral and collectorditch connections, and prior to water dischargepoints from the grove.

Successful sediment traps require site-specificdesigns, with the following points to consider:

Determine runoff volume and intensity.

Determine transport and settling rates for sedi-ments of concern.

Size traps to allow adequate residence timefor natural settling to occur - include consider-ations for allowable storage (fill-up) of trappedsediments.

Make provisions for materials removed from the ditches

Figure B1.3. Riser-board structure with two sinker boardsinstalled. The sinker boards should be brightly colored tofacilitate retrieval if they are accidentally dropped in the ditchor misplaced around the structure.

Figure B1.4. An example of a sinker board weighted withmetal straps used with riser-board structures. The weight of thesinker board helps keep the other boards in place.

Figure B2.1. Settling basins can be created by digging outa basin on the upward side of the culvert and/or by placingculverts above the bottom of the ditch on the upstream side.

Figure B2.2. Example of a settling basin upstream of thepump intake in a collector ditch.

Figure B2.3. Illustration of settling basin at the intersection olateral and collector ditches.


4/16


Maintenance access to settling basin area shouldbe provided.

When sediments are removed, materials needto be placed in a manner that prevents materialfrom sloughing back into the waterway.

Sediment excavation and removal should beconducted during low stage conditions or duringthe dry season. This will reduce the likelihood ofincreasing turbidity and suspended solid loads.

Settling basins or settling ponds are a quick andsimple way to remove sediments out of runoffwater. Settling basins simply slow down the water,allowing sediments to settle out of the water beforethe water returns to the receiving water body.

NOTE: Existing detention impoundments may func-tion as sediment settling basins.

B3. DITCH CONSTRUCTION

Construct ditches and canals with side-slopesconsistent with soil types.

Refer to Engineering Tables in local County SoilSurveys for information on preferred side slopes forspecific soil types.

References

NRCS engineering tables & local county soil survey

manuals.

B4. STABILIZE BARE SOILS

Stabilize bare soils and canal or ditch banksby encouraging coverage by noninvasivevegetation.

Vegetation types selected should be adapted togrove conditions and should provide maximum

stabilization by roots and foliage. Vegetative bufferstrips can also serve to reduce the erosion of soilparticles. Whenever practical, plant or encourageestablishment of native species.

References

Code 342.NRCS Conservation Practice Standard,

Critical Area Planting.

Code 391a. NRCS Conservation Practice Standard,

Riparian Forest Buffer.

Figure B3.1. Relationships between side slopes and top widthsof typical grove ditches.

Table B3.1. Side slopes for open channels.

Soil

Side Slopes

Shallow channels(up to 4 ft)

Deep channels(over 4 ft)

Peat and muck Vertical 0.25:1

Heavy clay 0.5:1 1:1

Clay or silt loam 1:1 1.5:1

Sandy loam 1.5:1 2:1

Loose sandy 2:1 3:1

Reference: Land Drainage and Flood Protection, by Etcheverry, copyright, 1931,McGraw-Hill Book Co.

Figure B4.1. Ditch banks stabilized with grasses reduce ero-sion potential.

Figure B4.2. Erosion of topsoil caused by high-intensity rainsand bare soil. Excess width of herbicide bands should beavoided.


5/16

N O V E M B E R 2 0 0 5 B - 5

Code 393. NRCS Conservation Practice Standard,

Filter Strip.

HS-802. Weed Control in Subtropical Conditions in

Florida Citrus.

B5. DITCH BANK CONTOURS

Contour ditch bank top edges or berms to divertwater away from the drainage ditch.

This practice will minimize overland flow of storm-water directly down the banks.

B6. DITCH BANK VEGETATIONMAINTENANCE

Broadleaf weed control using herbicides ormaintenance mowing of slopes and ditch banksincreases grass cover and decreases the prolif-eration of shade-producing shrubs and weeds,thus reducing erosion from wind and rainfall.

Points to Consider:

Mechanical mowing does not uproot vegetationand expose soil.

The use of herbicides should be conducted withcaution and precision to avoid excessively largeareas of bare soil.

Selective herbicides should be used in order tomaintain desired vegetation (e.g. remove broad-leaf vegetation while maintaining grasses).

Ditch Bank Brush Identification and Contro

The following describes several common plants thatmay be in or around ditches and canals within citrusgroves. The reader is directed to IFAS PublicationSP-168 and Circular 707 for further informationand recommendations for control.

Brazilian Pepper (Schinus terebinthifolius)

Description:An extremely fast growing shrub foundpredominantly in disturbed areas of south FloridaThis aggressive nonnative species produces large

Figure B5.3. A berm created by a good road management planRough roads can cause erosion and are hard on equipment.

Figure B6.1. Well-maintained vegetative cover on ditchbanks.

Figure B6.2. Herbicide applications to ditchbanksshould be made with caution to prevent bare soil conditions.

Figure B5.1. Slope tops of ditch banks to drain away from theditch or canal.

Figure B5.2. Roads adjacent to canals and ditches should besloped to drain away from the ditch or canal.


6/16


quantities of seeds contained in a red fruit usuallyabout 1/4-inch in diameter. Reproduction is byseed.

Wax Myrtle (Myrica cerifera)

Description: Shrub or small tree usually 10 feettall. Leaves are alternate, pale green, and lance-

shaped. When crushed, leaves emit a pleasantaroma. Close inspection of the leaves will revealnumerous small dark scales on top and brightorange scales below. Reproduction is by seed.

Water primrose (Ludwigia species)

Description: Small shrub attaining height of upto 6 feet with multiple branching stems. Leaves arelance-shaped with small soft hairs on both sides.Flowers are yellow with four symmetrical petals.

Reproduction is by seed and underground stems.

Small shrub attaining height of up to 6 ft with mul-tiple branching stems. Leaves are lance-shapedwith small soft hairs on both sides. Flowers areyellow with four symmetrical petals. Reproductionis by seed and underground stems.

Willow(Salix species)

Description: Fast growing shrub, which canbecome a tree in a short period of time. Leavesare alternate and lance-shaped with finely toothedmargins. The fruit capsule contains many smallhairy seeds, which drift in air currents.

References

CH-181. Aquatic Weed Management in Citrus Canals

and Ditches, http://edis.ifas.ufl.edu/CH181.

B7. Protect Ditch BanksProtect canal and ditch banks from erosion inareas subject to high water velocities.

Figure B7.1. Hi-energy areas such as culverts can be stabi-lized with concrete.


7/16

N O V E M B E R 2 0 0 5 B - 7

In areas where water is constricted (usually atdischarge points) or at ditch intersections wherevelocities are high, rip-rap, concrete, headwalls,

or other materials that buffer turbulence should beused to protect ditch banks and reduce sedimenttransport.

B8. VEGETATIVE STABILIZATION(WATER FURROWS)

Plant noninvasive vegetation and/or maintain

desirable vegetation within all water furrows toprevent/minimize erosion and trap sedimentsthat may result from stormwater runoff or irriga-tion drainage.

Figure B7.3. Canal section stabilized with rock and concretesections.

Figure B7.2. Rip rap, rocks, and broken concrete can helpstabilize ditch banks. However, materials should be placed toensure complete overlapping coverage of areas to be protected.

Figure B7.4. Fabriform lining used to stabilize ditchbanks androadways at a high-energy road crossing.

Figure B8.1. Healthy, uniform vegetative cover maintainedover water furrows

Figure B8.2. Water furrow erosion due to lack of protectivecover.


8/16


B9. AQUATIC PLANT MANAGEMENT

When removing vegetation from ditch bottoms,avoid disrupting side slopes.

If a backhoe without a vented bucket is used toremove aquatic plants from grove ditches, specialprecautions should be taken to prevent washouts.Once a bucketful of vegetation is picked up, thebucket should be raised to allow most of the waterto drain out over the deeper part of the ditch.The boom should be swung far enough over theditch bank so that when the vegetation is dumped,remaining water will flow away from the ditch.

Aquatic Plant Identification and Control

The following describes several common plants thatmay be in ditches and canals within citrus groves.The reader is directed to IFAS Publication SP-168and Circular 707 for further information and rec-ommendations for control.

Cattail (Typha species)

Description: Erect perennials (up to 9 feet) that canreproduce by seed or creeping rootstalk. Grasslikeleaves are flat and smooth to the touch. Flowerslook like a cats tail and can be found in a tightlypacked spike usually 6-8 inches long.

Hydrilia (Hydrilla verticillata)

Description: Long stemmed, branching plant that is

rooted to the bottom and often forms large surfacemats. Leaves grow in a whorl with toothed mar-gins that feel rough. Hydrilla can spread by plantfragments, underground stems, seed, leaf buds, orbuds located on the underground stems.

Torpedograss (Panicum repens)

Description: Narrow leaves (less than 1/4-inchwide), with stems often several feet in length.Torpedograss creeps horizontally by undergroundstems and forms large floating mats. Reproductionis by seed and creeping stems.

Figure B9.1. Erosion resulting from water in the backhoebucket running down side of ditch when aquatic plants weredumped too close to the ditch bank

Figure B9.2. Preferred backhoe buckets for ditch cleaning areslotted or are cross-drilled to allow water to seep out beforematerials are placed on ditch banks.


9/16

N O V E M B E R 2 0 0 5 B - 9

Water Hyacinth (Eichhomia crassipes)

Description: Plants are several inches to two feetin height. Smooth leaves are attached to spongybulb-shaped stalks. Reproduction is primarilythrough the production of daughter plants.

References

CH-181. Aquatic Weed Management in Citrus Canals

and Ditches.

B10. DITCH MAINTENANCECLEANING AND DREDGING.

Develop and implement a systematic manage-ment plan for removing sediments from canalsand farm ditches on a regular basis.

Figure B10.1 Improper placement of dredged sedimentsand vegetation on the sides of the canal may result in materialwashing back into the canal system.

Maintenance dredging of existing ditches, canals,and intake and discharge structures should includethe following:

Spoil material should be removed and depositedon a self-contained, upland spoil site and not

placed in a delineated floodplain. This will pre-vent the movement of the water and excavatedspoil material into wetlands or other surface

waters.

Do not remove any more material than is neces-sary to restore the original design specificationsor configurations.

No significant impacts should occur to previouslyundisturbed natural areas.

Erosion and sedimentation control devices (e.g.turbidity screens) should be used to prevent bankerosion, scouring, and to prevent turbidity fromdischarging into adjacent waters during mainte-

nance dredging.Removal of excess sediment to the originallydesigned and constructed cross-sectional areagenerally increases the canal cross-sectional areaand reduces water velocities (compared to samewater volume in filled-in systems), thus reducing thepotential for bank scouring. Caution should be con-sidered as ditch maintenance, cleaning and dredg-ing beyond the originally designed and constructedcross-sectional area may result in upstream and/ordownstream adverse water resource impacts.

Figure B10.3 Sediment should be removed from ditches andcanals to maintain design cross-sectional area.

Figure B10.2 Ditches and Canals need to be periodicallycleaned to ensure adequate drainage at maximum velocity.


10/16

B - 1 0 B E S T M A N A G E M E N T P R A C T I C E S F O R G U L F C I T R U S

In order to reduce the potential for misunderstand-ings with regulatory agencies and adjacent propertyowners, growers are highly encouraged to initiallyschedule a site visit with a local SFWMD representa-tive prior to conducting routine ditch maintenanceactivities.

References:

Chapter 403.813, Florida statutes. Rule 40D-4.051, Florida Administrative Code.

B11. HERBICIDE APPLICATIONS(WATER FURROWS)

Restrict the area of tree-row applied herbicidesto within the canopy dripline of the citrus trees.

The restricted herbicide band width will maximizethe width of grassed water furrow slopes. Grassedwater furrows serve as filters, preventing sedi-ment movement from the fields into the drainage

systems.

See Section C12 for proper handling and applica-tion of herbicides.

For young plantings, minimize the width of tree-rowapplied herbicides and establish vegetation in thewater furrows. Smaller band widths will reduce thequantity of herbicides applied, thereby reducingmaterial costs while minimizing potential of soilerosion into the drainage systems. As the trees

increase in canopy width, the herbicide band widthcan be increased to match canopy size.

Figure B11.3 An example of an excellent herbicide programfor young trees, with a herbicide band that allows a vegeta-tion free area near the trees, but provides adequate protectionagainst erosion.

Figure B11.4 Excessive herbicide band on young trees thatextends well into the water furrow side of bed.

References

SP-43. Florida Citrus Pest Management Guide: Weed

Management, Section HS-107.


Florida Citrus.

B12. MIDDLES MANAGEMENT (HERBICIDE)

Suppress undesirable vegetation on bed topsand in water furrows.

Figure B11.1 An example of an excellent herbicide programfor mature trees, with the herbicide band at the canopy line.

Figure B11.2 Excessive herbicide band widths may result inappreciable erosion of soil particles from exposed bare soilareas during high intensity rainfall events.


11/16

N O V E M B E R 2 0 0 5 B - 1 1

Figure B12.1 Using a wiper to apply herbicides can be aneffective way of suppressing undesirable vegetation.

Figure B12.2 The use of tree sensing systems and precisionequipment that can apply both contact and residual productsprovides effective control of undesirable species.

References

SP-43. Florida Citrus Pest Management Guide: Weed

Management, Section HS-107.

HS-869. Vegetation Management in Row Middles in

Florida Citrus.


Florida Citrus.

B13. GROVE DEVELOPMENT/RENOVATION

Upon completion of the soil bedding processwithin citrus groves, all bare soil areas (excepttree rows) should be planted with grass or othervegetation species to minimize soil movementfrom rain and/or wind.

Bare soil surface, during windy conditions, can pro-vide sufficient soil to blast the bark of young treesand allow movement of soil into water furrows andother drainage systems.

Figure B13.1 Bare soil areas should be planted as soon as possible after beds are formed in order to minimize soil erosion. Whenestablished, plant roots help hold soils in place during windy andrainy conditions. Note erosion already beginning in photo.

Figure B13.2 Grass established on bed tops and watefurrows will hold soil in place against strong winds and highintensity rainfall.

B14. SETTLING BASINS (SUMPS)

Dig out and maintain small settling basins in fronof the drainage inlets within water furrows.

Figure B14.1 Maintaining a sediment trap can improvedrainage and prevent sediment suspended in runoff water tobe captured prior to discharge.


12/16


Figure B14.2 Water furrow sediment traps should be cleanedout in the dry season to allow time for grass to re-establish itselfbefore summer rains begin.

B15. WATER FURROW DRAIN PIPES

Use PVC drain pipe or flexible pipe to connect allwater furrows or field ditches to lateral ditches.

Extend the pipe on the downstream side awayfrom the ditch bank to prevent bank scouring.

Figure B15.1 PVC drain pipe extended from the ditchbank toprevent bank scouring.

Figure B15.2 Corrugated polyethylene used for water furrowdrain.

B16. WATER FURROW MAINTENANCE

Use water furrow drain pipes with managedvegetation in furrows to reduce surface watertransfer velocity from the furrows to the drainageditches and canals.

Figure B16.1 Re-working water furrows when they get rut-ted can improve drainage and prevent ponding, resulting inhealthier trees and more uniform drainage. Water furrowsshould be re-worked during the dry season to allow adequatetime for re-vegetation prior to the wet season.

Figure B16.2 Properly sloped and maintained furrows willenhance grove drainage while minimizing adverse effects ofexcess rainfall.

B17. CONSTRUCTION AND TEMPORARYEROSION CONTROL MEASURES

Special measures and/or temporary erosion con-trol measures should be taken during construc-tion and renovation of groves, when culverts andcontrol structures are replaced or repaired, andwhen there is a major disruption of establishedvegetation such as during irrigation system instal-lation or when buried water lines are repaired.


13/16

N O V E M B E R 2 0 0 5 B - 1 3

Figure B17.1 Road grading, reworking beds or furrows allrequire turning the soil. It is this time when soil is unbound byvegetation and most prone to runoff.

Erosion control measures are used to minimizesediment transport and protect the quality of waterbodies that receive runoff from disturbed areas.The most common temporary erosion control toolsinclude straw or hay bale barriers, silt screens, and

silt fences; however, more permanent control canbe obtained through the use of specialized blanketsand mats, gabions, and other systems used for soilstabilization.

The cost of erosion control options are highly vari-able and agricultural producers are encouragedto consider economics and site-specific conditionswhen selecting the most appropriate erosion controlsystem for a particular action. When selecting anerosion and sediment control method, it is recom-mended that a NRCS representative, engineer, and/

or a SWFWMD Ag-Team member be consulted.

Straw or Hay Bale Barrier: A temporary sedi-ment barrier consisting of a row of entrenchedand anchored straw or hay bales.

Installation Considerations:

Install bales below disturbed areas that are sub-ject to sheet or rill erosion.

Use bale barriers when effective control is neededfor less than 3 months.

Install bales in a continuous row, perpendicular tothe direction of water flow, and place bales tightly

together to prevent water from passing betweenthe bales.

Entrench bales approximately 4 inches, andsecure by driving two stakes through each bale.

Use double rows spaced a minimum of five feeapart when the disturbed area is within 50 feet ofa receiving water body.

If bales are used in conjunction with silt screensbales should be placed adjacent to the silt screenon the upslope side.

Things to Avoid:

Do not construct straw or hay bale barriers instreams or swales where hydraulic forces wilcause a washout.

Do not use straw or hay bale barriers for sedimencontrol when site conditions exceed the applica-tion specifications identified below.

Avoid bare ground situations during construc-tion, and sod, seed and protect as necessary tominimize adverse impacts from erosion.

Applicable Technical Criteria:

Drainage area should not exceed acre per 100feet of barrier length.

Slope length behind barrier should not exceed100 feet, and slope gradient behind barriershould not exceed 5%.

Operation and Maintenance:

Inspect barriers after all significant rainfall events

and replace bales as needed.

Remove sediment deposits if they reach one halfthe height of the barrier.

Silt Fence:A temporary barrier consistingof a filter fabric (burlap or synthetic material)stretched across and attached to supportingposts and, in some cases, wire fence mate-rial. Small silt fences, which are designedwith lighter materials and without the use of

Figure B17.2. Placement of hay bales for temporary sedimentbarrier.


14/16


supporting wire, are often referred to as filterbarriers.

Installation Considerations:

Install silt fences perpendicular to the direction ofwater flow, below disturbed areas that are subjectto sheet or rill erosion.

Use filter barriers when effective control is neededfor less than 3 months, and use silt fences whenlonger application periods are needed.

Drive posts or stakes securely into the ground(minimum depth of 12 inches for posts and 8inches for stakes).

Excavate a 4-inch by 4-inch trench along the line

of posts and on the upslope side. If wire fence material is used to provide extra sup-

port, fence should extend 1inch into the trench,be securely attached to the posts using heavygauge wire staples, and should not extend morethan 36 inches above the original land surface.

Securely fasten filter fabric to the posts or fencematerial with 8 inches of the fabric extending intothe trench.

Backfill the trench and compact over the filter

fabric. Install two parallel silt fences (located at least

three feet apart) if needed to provide additionalsediment control. When within 50 feet of surfacewaters, utilize two rows of silt fences spaced atleast 5 feet apart.

Things to Avoid:

Do not install silt fences in streams or swaleswhere the contributing drainage area exceeds 2

acres or where flows will exceed one cubic footper second.

Avoid bare ground situations during construc-tion, and sod, seed and protect as necessary tominimize adverse impacts from erosion.

Applicable Technical Criteria:

Drainage area should not exceed acre per 100

feet of barrier length.

Slope length behind barrier should not exceed100 feet, and slope gradient behind barriershould not exceed 5%.

Silt fences should not exceed 36 inches in height,and filter barriers should be between 15 and 18inches in height.

Posts (4 inch diameter wood or equivalent) for siltfences should be spaced a maximum of 10 feetapart. When extra strength fabric is used without

wire support, post spacing should not exceed 6feet.

Stakes (1-inch by 2-inch wood or equivalent) forfilter barriers should be spaced a maximum of 3feet apart.

Operation and Maintenance:

Inspect silt fences after significant rainfall events,and make repairs as needed.

Remove sediment deposits if they reach one half

the height of the barrier.

Erosion control blankets: Erosion controlblankets and mats are made of natural and/orsynthetic fibers that are woven to form a highlypermeable material. These blankets, which areoften photodegradable and/or biodegradable,are commonly used to provide permanent

Figure B17.3 Silt fences are designed to detain sediment andsheet erosion.

Figure B17.4 Erosion control blanket installed on slope toprevent erosion. Vegetation will grow through the porous mate-rial and stabilize the soil.


15/16

N O V E M B E R 2 0 0 5 B - 1 5

erosion control on ditch banks, berms, leviesand other areas with excessive slope. Erosioncontrol blankets protect soils prior to vegeta-tive establishment and encourage vegetativegrowth by capturing sediment and providingsoil reinforcement. Erosion Control Blanketscan be utilized in association with other types oferosion control methods where the feasibility ofutilizing sodding is cost prohibitive because of

the area of application.

Mulches are generally utilized to retard erosion byprotecting bare earth areas where sheet and rillerosion are likely to occur by reducing the velocityof overland flow and enhancing the effectiveness ofturbidity and erosion controls installed down slope.In addition, mulches can enhance the effectivenessof seeded areas by retarding the transport of seedsdown slope. Mulches should be free from weedsand/or nuisance and exotic plant material. Therecommended application rate for broadcasting

hay/straw is 1.5 to 2 tons per acre or 70 to 90 lbsper 1000 sq. ft. Mulches such as wood chips needto be anchored (i.e., netting, etc.).

For further information regarding the applicabilityand usage of specific products in this category,please contact your local Soil and Water Conserva-tion District, or USDA-NRCS.

Gabions:Gabions are partitioned, wire meshedcontainers that are filled with stone or concreterubble to form flexible, permeable, monolithicstructures such as retaining walls, weirs, and

channel linings. Gabions can provide effectivelong-term erosion control within water convey-ance features especially around water controlstructures. For further information regardingthe applicability and usage of gabions, please

contact your local Soil and Water ConservationDistrict, USDA-NRCS, or refer to informationprovided by product manufacturers, distributorsand/or dealers.

Floating Turbidity Barriers: Turbidity barriersor curtains are designed to minimize sedimenttransport from a disturbed area adjacent to orwithin a water body. Turbidity barriers are com-

monly used in addition to other sediment controlsto provide additional water quality protection.These barriers are designed to trap sediment inlow to moderate flow conditions. Turbidity barri-ers should not be installed across channel flowswhere higher water velocities can occur.

Figure B17.6 Typical floating turbidity barrier.

The standard practice in installing barriers is to keep

the lower edge of the barrier about 12 inches abovethe bottom as shown in Fig. B17.7. When the loweedge of the barrier is long enough to drape on thebottom, silt will build up as shown in Fig. B17.7. I

Figure B17.5 Rock-filled gabions used to stabilize slope atdrainage pump station.

Figure B17.8 Floatingsediment barriers installedwith the bottom too low mayresult in the barrier beingpulled below the surfacewhen sediments accumulate.

Figure B17.7 Floatingsediment barriers should beinstalled with the lower edgeabout 12 inches above thebottom of the channel.


16/16

the build-up is allowed to collect over a long periodof time, the barrier may be pulled beneath the sur-face of the water. The build-up also makes it difficultto remove the barrier without stirring up the silt andsending it back it to the water course. For furtherinformation regarding the applicability and usageof floating turbidity barriers, please contact yourlocal Soil and Water Conservation District, USDA-NRCS, or refer to information provided by product

manufacturers, distributors and/or dealers.

Key Terms:

Sheet Erosion: Sheet erosion is caused by shallowsheets of water flowing off of the land. These broadmoving sheets of water are seldom the cause oferosion, but the flow transports sediments detachedby raindrop impact and splash. The shallow surface

flow rarely moves as a uniform sheet for more thana few feet before concentrating due to topographicirregularities.

Rill Erosion: Rill erosion develops as the shallowsurface flow begins to concentrate in low spots.The concentrated flow increases in velocity andturbulence, which in turn causes the erosion andtransport of more sediments. This action cuts tiny

well-defined channels called rills, which are usuallyonly a few inches deep.

References

The Florida Stormwater, Erosion, and Sediment

Control Inspectors Manual.

FDEP Storm Water Inspection Course.

SL-195. Flatwoods Citrus Best Management Practice:

Soil Stabilization.

Practice Definition Purpose Where Applicable

Straw Bale Dike

Temporary sedimentbarrier constructed froma row of entrenched andanchored straw bales.

To catch and retain sedimenton the construction site andprevent sedimentation.

May be used below disturbed areas subjectto sheet and rill erosion where temporarysedimentation control is needed.

TurbidityBarrier (staked

or floating)

Temporary sediment barrierconsisting of an impermeablematerial stretched acrossa surface water body.

To catch and hold sedimentplumes and floating debrisin surface waters.

May be used across ditches and canalsand in retention/detention areas orreservoirs adjacent to shores.

TemporarySediment Trap

A small temporary pondingbasin formed by excavationor by an embankment.

To detain sediment-ladenrunoff and to trap the sedimentand to protect receiving waterbodies from sedimentation.

May be used at the outlets ofdrains, diversions, ditches, andother runoff conveyances.

Temporary

Seeding

Planting fast-growingvegetation to provide

temporary erosion control.

To provide stabilization ofbare soil areas that will notbe brought to final form for aperiod of more than 30 days.

May be used on cleared, un-vegetated areaswhere temporary erosion control is needed.

MulchingUse of a protective blanket ofstraw, residue, gravel, or syn-thetic material on soil surface.

To protect soil surface fromforces of raindrop impacts andoverland or sheet water flow.

May be used on beds for temporary or per-manent seeding and on areas of bare soilwhen seeding or planting must be delayed.

Outlet Stabiliza-tion Structure

Structure designed to controlerosion at the outlet of achannel or conduit.

To prevent erosion by reducingwater velocity from the outletof a channel or conduit.

May be used at locations where water veloc-ity from a conduit or open channel exceedspermissible velocity of the receiving canal.

Fabric Drop InletTemporary fabric barrierplaced around inlets toditches or canals.

To prevent sediment from enter-ing canals during constructionactivities; allows use of the drain-age system during construction.

May be used where drain inlets areto be operational before permanentstabilization of the drainage area occurs.This method is used in areas that arenearly level, with slopes less than 5%.

TemporaryGraveled

Entrance/Exit

A gravel driveway or padlocated at a point wherevehicles enter and exita construction site.

Provides a suitable locationfor vehicles to drop mudand sediment before enter-ing public roads; controlserosion from surface runoffand helps control dust.

May be used whenever traffic leavesa construction site and enters apublic road or other paved areas.

Silt Fence

Temporary sediment barrierconsisting of filter fabricor burlap stretched across sup-porting posts and entrenched.

To catch and hold smallamounts of sediment fromdisturbed areas by reducingthe velocity of sheet flow toallow sediment deposition.

May be used below small disturbedareas less than 1/4 acre per 100 ft offence, and where runoff can be storedbehind the fence without damaging thefence or the area behind the fence.

Table B17.1. Potential temporary erosion control practices for use during grove construction and renovation and when major landdisturbances are make during repair operations.

sediment management - basin

Documents