Sediment Control: 2 erosion sedimentcontrol On earthwork sites with slopes less than 10% and ......

erosion&sedimentcontrolGuidelines for Land Disturbing Activities in the Auckland Region

Auckland Regional Council Sediment Control: 2 1

2.1 Sediment Retention Pond

Plate 2.1.1 Sediment Retention Pond Showing Decant Systems

Definition

A temporary pond formed by excavation into naturalground or by the construction of an embankment, andincorporating a device to dewater the pond at a rate thatwill allow suspended sediment to settle out.

Purpose

To treat sediment laden runoff and reduce the volumeof sediment leaving a site, thus protecting downstreamenvironments from excessive sedimentation and waterquality degradation.

Application

Sediment Retention Ponds are appropriate wheretreatment of sediment laden runoff is necessary, and aregenerally considered the appropriate control measure forexposed catchments of more than 0.3 ha. It is vital thatthe Sediment Retention Pond is maintained until thedisturbed area is fully protected against erosion bypermanent stabilisation.

The location of the Sediment Retention Pond needs tobe carefully considered in terms of the overall project,available room for construction and maintenance andthe final location of any permanent stormwater retentionfacilities that may be constructed at a later stage. Anothermajor consideration is whether drainage works can berouted to the Sediment Retention Pond until such timeas the site is fully stabilised. This eliminates the problemof installing and maintaining Stormwater Inlet Protectionthroughout the latter stages of a development.

The general design approach is to create animpoundment of sufficient volume to capture asignificant proportion of the design runoff event, and toprovide quiescent (stilling) conditions which promotethe settling of suspended sediment. The SedimentRetention Pond design is such that very large runoffevents will receive at least partial treatment and smallerrunoff events will receive a high level of treatment. Toachieve this, the energy of the inlet water needs to be lowto minimise re-suspension of sediment and the decant



rate of the outlet also needs to be low to minimise watercurrents and to allow sufficient detention time for thesuspended sediment to settle out.

Specific design criteria are discussed below, but can besummarised as the following.

o Generally use Sediment Retention Ponds for bareareas of bulk earthworks of 0.3 ha or greater.

o Restrict catchment areas to less than 5.0 ha perSediment Retention Pond. This limits the length ofoverland flowpaths and reduces maintenanceproblems.

o Locate Sediment Retention Ponds so as to providea convenient collection point for sediment ladenflows from the catchment area. This will requirestrategic use of cut-offs, Runoff Diversion Channelsand Contour Drains.

o Locate Sediment Retention Ponds to allow accessfor removing sediment from the pond.

o Wherever possible, locate Sediment RetentionPonds to allow the spillway to discharge overundisturbed, well vegetated ground.

o Keep the Sediment Retention Pond life to less thantwo years. If a longer term is required then furthermeasures to ensure stability and effectiveness arelikely to be needed.

o Do not locate Sediment Retention Ponds withinwatercourses.

o Embankment and spillway stability are generallythe weak point in Sediment Retention Pondconstruction. Correct compaction particularlyaround emergency spillways, discharge pipes andantiseep collars, will keep the system robust.

Figure 2.1.1 Sediment Retention Pond



Design – Size of the Pond

Calculate the volume of the Sediment Retention Pondusing the depth measured from the base of the SedimentRetention Pond to the top of the outlet riser. Thefollowing design criteria apply.

o On earthwork sites with slopes less than 10% andless than 200m in length, construct a SedimentRetention Pond with a minimum volume of 2% ofthe contributing catchment (200m3 for each ha ofcontributing catchment).

o On sites with slopes greater than 10% and/or 200 oin length, construct Sediment Retention Ponds witha minimum volume of 3% of the contributingcatchment (300m3 capacity for each ha ofcontributing catchment).

o The slope angle is determined by that slopeimmediately above the Sediment Retention Pondor by the average slope angle over the contributingcatchment, whichever is the greater.

o For sand soils (less than 8% clay and less than 40%silt) the size of the Sediment Retention Pond maybe calculated using the following formula –

- Pond Surface Area (square metres) = 1.5 x PeakInflow Rate (litres per second),

- calculate the inflow rates using the 5% AEPrainfall event. Ensure the Sediment RetentionPond has a minimum depth of 1.0m,

- alternatively, construct Sediment RetentionPonds with a minimum volume of 1% of thecontributing catchment (100 m3 capacity foreach ha of contributing catchment).

o On sites that are particularly steep or have sensitivedownstream environments, a greater SedimentRetention Pond volume may be required.

o Clean out Sediment Retention Ponds when thevolume of sediment accumulated within themreaches 20% of the design volume.

o Clearly show the Sediment Retention Ponddimensions necessary to obtain the requiredvolume, as detailed above, on the site’s Erosion andSediment Control Plan(s).

Design – Dead Storage (Permanent Storage)

o Dead storage is the component of impoundmentvolume that does not decant and remains in theSediment Retention Pond. It is important fordissipating the energy of inflows.

o Ensure dead storage is 30% of the total SedimentRetention Pond storage by positioning the lowestdecant 0.4 – 0.8m above the invert of the SedimentRetention Pond.

o The approved decant design detailed in theseGuidelines allows the lower decant arm to be raisedas sediment deposition increases, therebymaintaining the percentage volume of dead storage.

Design – Live Storage (Decant Storage)

o Live storage is the volume between the lowestdecant outlet level and the crest of the SedimentRetention Pond primary spillway.

o Ensure that the live storage volume capacity is 70%of the total Sediment Retention Pond storage.

o The approved decant design detailed in theseGuidelines allows the decant system to be raised assediment deposition increases, thereby maintainingthe percentage volume of live storage.

Design – Decanting/Outlet Dewatering Device

o Dewater the Sediment Retention Pond so as toremove the relatively clean water without removingany of the settled sediment, and without removingany appreciable quantities of floating debris.

o Various dewatering devices are available, howeverthe Auckland Regional Council prefers the use of afloating T-bar dewatering device, which allows forthe decanting of the cleaner surface water from thetop of the water column. Substantiated performancedesign will need to be submitted to the AucklandRegional Council for decant systems other than thefloating T-bar dewatering device.

o The recommended decant rate from a SedimentRetention Pond is 3 litres/second/ha of contributingcatchment. This rate ensures that appropriatedetention times are achieved.



o A standard T-bar design is detailed in Figure 2.1.5for various sized catchments. For simplicity, givepreference to producing a standard T-bar decant thatprovides a decant rate of 4.5 litres/second per decantwhich can be added in discrete increments toaccommodate various sized catchments.

o To achieve a decant rate of 4.5 litres/second per decant,drill 6 rows of 10mm diameter holes at 60mm spacings(200 holes) along the 2m long decant arm.

o For catchments of less than 1.5ha, seal off theappropriate number of holes to achieve a 3 litres/second/hectare discharge rate.

o Single T-bar decants must be able to operatethrough the full live storage depth of the SedimentRetention Pond.

o If two decant systems are required, ensure the lowerT-bar decant operates through the full live storagedepth of the Sediment Retention Pond. The upperT-bar decant is to operate through the upper 50%of the live storage depth of the Sediment RetentionPond only.

o If three decant systems are to be used, then thelower T-bar decant operates through the full livestorage depth and the second T-bar decant throughthe upper two thirds of live storage depth of theSediment Retention Pond. The upper T-bar decantoperates through the upper one third of live storagedepth of the Sediment Retention Pond as detailedin Figure 2.1.4.

o Ensure that the T-bar decant float is securelyfastened with steel strapping directly on top of thedecant arm and weight it to keep the decant armsubmerged just below the surface through all stagesof the decant cycle. This will also minimise thepotential for blockage of the decant holes by floatingdebris. The most successful method found to dateis to weight the decant arm by strapping a 1.8m longwaratah between the float and the decant(approximately 4kg of weight).

o Position the T-bar decant at the correct height bytying 5mm nylon cord through decant holes ateither end of the decant arm and fastening it towaratahs driven in on either side of the decant.

o Lay the discharge pipe at a 1 – 2% gradient, compactthe fill material around it using a machinecompactor and incorporate anti-seep collars withthe following criteria:

o install collars around the pipe to increase theseepage length along the pipe with a spacing ofapproximately 10m;

o the vertical projection of each collar is 1.0m;ensure all anti seep collars and their connectionsare watertight.

o Use a flexible thick rubber coupling to provide aconnection between the decant arm and the primaryspillway or discharge pipe. To provide sufficientflexibility (such as is required for the lower decantarm) install two couplings. Fasten the flexiblecoupling using strap clamps and glue.

o Where a concrete riser decant system is utilised,ensure the lower decant connection is positionedon an angle upwards from the horizontal so as tosplit the operational angle that the decant worksthrough. This will reduce the deformation force onthe coupling used.

Design – Shape of the Pond

o Ensure the length to width ratio of the SedimentRetention Pond is no less than 3:1 and no greaterthan 5:1. The length of the Sediment Retention Pondis measured as the distance between the inlet andthe outlet (decant system). A 2:1 ratio may be usedif the pond depth is no greater than 1.0m.

o Maximise the distance between the inlet and theoutlet (including the emergency spillway) to reducethe risk of short circuiting and to promote quiescentconditions. If this can not be achieved by correctlypositioning the inlet and outlets, install baffles toachieve the appropriate length to width ratio design.

o Ensure that the Sediment Retention Pond has a levelinvert as described below to promote the even andgradual dissipation of the heavier inflow wateracross the full area of the Sediment Retention Pond.



Design – Level Spreader

o Incorporate a Level Spreader into the inlet designto spread inflow velocities, thereby allowing rapiddissipation of inflow energies. Combine the LevelSpreader with a well compacted and smoothed inletbatter (no steeper than a 3:1 gradient), stabilisedover its entire area. The essential design feature isto ensure the Level Spreader is level, non-erodibleand spans the full width of the Sediment RetentionPond.

o To ensure even inflows, install a trenched andpegged 150mm x 50mm timber weir or similaracross the full width of the inlet. Bund the edgeswith compacted earth to prevent outflanking. Thistimber weir also serves to toe in any geotextileprotection which may be required. Sedimentaccumulated behind the Level Spreader may requireperiodic removal.

o Position the top of the Level Spreader weir 100 –200 mm above the invert of the emergency spillway.

o The shape of the Level Spreader will vary with thedirection and volume of the inlet diversion, but theentrance to the Level Spreader must be widened andflared to promote even flow, as in Part B, Section1.10 of these Guidelines.

Design – Baffles

o Incorporate baffles in the Sediment Retention Ponddesign where the recommended pond shape cannotbe achieved. Extend baffles the full depth of theSediment Retention Pond and place them tomaximise dissipation of flow energy.

o Generally, baffles are in the form of a wing to directinflows away from the outlet and maximise thestilling zone. A series of compartments within thepond can be used to achieve this, although care mustbe taken to avoid creating in-pond currents andresuspension of light particulates.

Plate 2.1.2 Sediment Retention Pond Showing Baffle to Lengthen Flowpath and Create Correct Length:Width Ratio



o Baffles may be constructed from various materialsranging from solid shutter boards to bracedgeotextile curtains.

Design – Depth of Pond

o Sediment Retention Pond depths may be 1 – 2mdeep, but no deeper than 2m. Deeper ponds aremore likely to cause short circuiting problemsduring larger storm events and require specificallydesigned floating decant systems.

o The decant design in these Guidelines operatesthrough a maximum live storage range of 1.5m.

Design – Embankment

o Before building a Sediment Retention Pond, installsediment controls such as Silt Fences below theconstruction area and maintain them to a functionalstandard until the Sediment Retention Pond battersare fully stabilised.

o Thoroughly compact the Sediment Retention Pondembankment, with material laid in 150 mm layersand compacted to engineering standards.

o Where possible install the discharge pipes throughthe embankment as the embankment is beingconstructed.

o Fully stabilise the external batter face, by vegetativeor other means, immediately after construction.

o Ensure all bare areas associated with the SedimentRetention Pond (including internal batters) arestabilised with vegetation if the Sediment RetentionPond is to remain in place over winter.

Design – Primary Spillway

o For larger catchments (greater than 1.5ha) theSediment Retention Pond requires a piped primaryspillway (refer Figures 2.1.3 and 2.1.4).

o For catchments up to 1.5ha, decant flows can bepiped using the same diameter piping as the decantsystem (100mm PVC smooth bore) directly throughthe Sediment Retention Pond wall to dischargebeyond the toe of the Sediment Retention Pond wall.

o For contributing catchments between 1.5 and 3hain area, use a discharge and primary spillway pipediameter of 150mm.

o Where contributing catchments are 3ha or greaterand/or the long term stability of the SedimentRetention Pond emergency spillway is questionable(for example, built in fill) then consideration shouldbe given to incorporating a concrete manhole riserand larger diameter pipe outlet as a primary spillwaysufficient to accommodate the 5% AEP rainfallevent.

o If the Sediment Retention Pond is to operate overthe winter and the contributing catchment is fullystabilised, disconnect the T-bar decant to reduce thefrequency of emergency spillway activation andconsequent erosion.

o Where a primary spillway upstand riser is used,place the top of the riser a minimum 600mm lowerthan the top of the Sediment Retention Pondembankment and a minimum 300mm lower thanthe emergency spillway crest. Ensure the riser andthe discharge pipe connections are all completelywatertight.

o Where possible, install the piping through theembankment as the embankment is beingconstructed.

Design – Emergency Spillway

o An emergency spillway is essential for all SedimentRetention Ponds.

o Emergency spillways must be capable ofaccommodating the 1% AEP event without eroding.

o The outer emergency spillway crest and batterrequires a very high standard of stabilisation withthe fill material of the spillway batter wellcompacted.

o Construct the emergency spillway as a stabilisedtrapezoidal cross section with a minimum bottomwidth of 6m or the width of the pond floor,whichever is the greater.

o When utilising geotextile for emergency spillwaystabilisation purposes, the batter face must besmooth and all voids eliminated.

o If geotextile is used, a soft neddle punch geotextileis laid first and then covered with a strong wovenlow permeability geotextile. Ensure the geotextileis pinned at 0.5m centres over the full area of theemergency spillway.



o Where possible, construct emergency spillways inwell vegetated, undisturbed ground (not fill) anddischarge over long grass.

o If the emergency spillway is constructed on baresoil, provide complete erosion protection by meanssuch as grouted riprap, asphalt, erosion matting/geotextile or concrete.

o Construct the emergency spillway with a minimumof 300mm freeboard height above the primaryspillway.

Construction Specifications

o Construct a fabric silt fence across the downslopeend of the proposed works.

o Clear areas under proposed fills of topsoil or otherunsuitable material down to competent material.Large fill embankments may need to be keyed in.

o Use only approved fill.

o Place and compact fill in layers as per the engineer’sspecifications.

o Do not place pervious materials such as sand orgravel within the fill material.

o Construct fill embankments approximately 10%higher than the design height to allow for settlementof the material. Install appropriate pipe work andantiseep collars during the construction of theembankment and compact around theseappropriately.

o Install the emergency spillway.

o Install and stabilise the Level Spreader.

o Securely attach the decant system to the horizontalpipework. Make all connections watertight. Placeany manhole riser on a firm foundation ofimpervious soil.

o Do not place pervious material such as sand orscoria around the discharge pipe or the antiseepcollars.

o Install baffles if required.

o Check Sediment Retention Pond freeboard fordifferential settlement and rectify as necessary.

o Stabilise both internal and external batters withvegetation and the emergency spillway inaccordance with the site’s approved Erosion andSediment Control Plan.

Pond Maintenance and Disposal of Sediment

o Clean out Sediment Retention Ponds before thevolume of accumulated sediment reaches 20% ofthe total Sediment Retention Pond volume. To assistin gauging sediment loads, clearly mark the 20%volume height on the decant riser.

o Clean out Sediment Retention Ponds with highcapacity sludge pumps, or with excavators (longreach excavators if needed) loading onto sealed tiptrucks or to a secure area.

o The Erosion and Sediment Control Plan shouldidentify disposal locations for the sediment removedfrom the Sediment Retention Pond. Deposit thesediment in such a location so that it does not leadto a direct discharge to receiving environments.Stabilise all disposal sites as required and approvedin the site’s Erosion and Sediment Control Plan.

o Inspect Sediment Retention Ponds every day andbefore every forecasted rainfall event. Inspect forcorrect operation after every runoff event.Immediately repair any damage to SedimentRetention Ponds caused by erosion or constructionequipment.

Safety

Sediment Retention Ponds are attractive to children andcan become safety hazards if not appropriately fencedand if safety rules are not followed. Low gradient pondbatters provide an additional safety measure. Check thesafety requirements of the City or District CouncilAuthority and the Occupational Safety and Healthbranch of the Department of Labour.

Chemical Treatment

Some chemicals can be used successfully to promoteflocculation (clumping together) of suspended solids inthe Sediment Retention Pond to increase the particlemass and speed the rate of settling.



Recent trials have identified a simple and effectivechemical dosing system that does not require a powersupply. This system uses poly aluminium chloride (PAC)and has been found to be particularly effective in settlingfine particulate such as fine silts and clays. Otherchemicals may become available as more trial data isobtained.

Figure 2.1.2 Sediment Retention Pond for Catchments up to 1.5 ha

Chemical dosing systems are likely to be required wherethe design Sediment Retention Pond volume cannot beachieved because of site constraints and/or where a highlevel of treatment is required because of the sensitivityof the receiving environment. Chemical treatment is alsomore likely to be required where the clay component ishigh or where the cumulative effects of sedimentdischarge are significant.



Figure 2.1.3 Sediment Retention Pond for Catchments Between 1.5 and 3 ha


Auckland Regional Council Sediment Control: 2 1 0

Figure 2.1.4 Sediment Retention Pond for Catchments Between 3 and 5 ha


Auckland Regional Council Sediment Control: 2 1 1

Figure 2.1.5 Sediment Retention Pond – Decant Detail

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