erosion&sedimentcontrol

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1 Auckland Reginal Cuncil Sediment Cntrl: 2 1 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin 2.1 Sediment Retentin Pnd Plate Sediment Retentin Pnd Shwing Decant Systems Definitin A temprary pnd frmed by excavatin int natural grund r by the cnstructin f an embankment, and incrprating a device t dewater the pnd at a rate that will allw suspended sediment t settle ut. Purpse T treat sediment laden runff and reduce the vlume f sediment leaving a site, thus prtecting dwnstream envirnments frm excessive sedimentatin and water quality degradatin. Applicatin Sediment Retentin Pnds are apprpriate where treatment f sediment laden runff is necessary, and are generally cnsidered the apprpriate cntrl measure fr expsed catchments f mre than 0.3 ha. It is vital that the Sediment Retentin Pnd is maintained until the disturbed area is fully prtected against ersin by permanent stabilisatin. The lcatin f the Sediment Retentin Pnd needs t be carefully cnsidered in terms f the verall prject, available rm fr cnstructin and maintenance and the final lcatin f any permanent strmwater retentin facilities that may be cnstructed at a later stage. Anther majr cnsideratin is whether drainage wrks can be ruted t the Sediment Retentin Pnd until such time as the site is fully stabilised. This eliminates the prblem f installing and maintaining Strmwater Inlet Prtectin thrughut the latter stages f a develpment. The general design apprach is t create an impundment f sufficient vlume t capture a significant prprtin f the design runff event, and t prvide quiescent (stilling) cnditins which prmte the settling f suspended sediment. The Sediment Retentin Pnd design is such that very large runff events will receive at least partial treatment and smaller runff events will receive a high level f treatment. T achieve this, the energy f the inlet water needs t be lw t minimise re-suspensin f sediment and the decant

2 Auckland Reginal Cuncil Sediment Cntrl: 2 2 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin rate f the utlet als needs t be lw t minimise water currents and t allw sufficient detentin time fr the suspended sediment t settle ut. Specific design criteria are discussed belw, but can be summarised as the fllwing. Generally use Sediment Retentin Pnds fr bare areas f bulk earthwrks f 0.3 ha r greater. Restrict catchment areas t less than 5.0 ha per Sediment Retentin Pnd. This limits the length f verland flwpaths and reduces maintenance prblems. Lcate Sediment Retentin Pnds s as t prvide a cnvenient cllectin pint fr sediment laden flws frm the catchment area. This will require strategic use f cut-ffs, Runff Diversin Channels and Cntur Drains. Lcate Sediment Retentin Pnds t allw access fr remving sediment frm the pnd. Wherever pssible, lcate Sediment Retentin Pnds t allw the spillway t discharge ver undisturbed, well vegetated grund. Keep the Sediment Retentin Pnd life t less than tw years. If a lnger term is required then further measures t ensure stability and effectiveness are likely t be needed. D nt lcate Sediment Retentin Pnds within watercurses. Embankment and spillway stability are generally the weak pint in Sediment Retentin Pnd cnstructin. Crrect cmpactin particularly arund emergency spillways, discharge pipes and antiseep cllars, will keep the system rbust. Figure Sediment Retentin Pnd

3 Auckland Reginal Cuncil Sediment Cntrl: 2 3 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin Design Size f the Pnd Calculate the vlume f the Sediment Retentin Pnd using the depth measured frm the base f the Sediment Retentin Pnd t the tp f the utlet riser. The fllwing design criteria apply. On earthwrk sites with slpes less than 10% and less than 200m in length, cnstruct a Sediment Retentin Pnd with a minimum vlume f 2% f the cntributing catchment (200m 3 fr each ha f cntributing catchment). On sites with slpes greater than 10% and/r 200 in length, cnstruct Sediment Retentin Pnds with a minimum vlume f 3% f the cntributing catchment (300m 3 capacity fr each ha f cntributing catchment). The slpe angle is determined by that slpe immediately abve the Sediment Retentin Pnd r by the average slpe angle ver the cntributing catchment, whichever is the greater. Fr sand sils (less than 8% clay and less than 40% silt) the size f the Sediment Retentin Pnd may be calculated using the fllwing frmula - Pnd Surface Area (square metres) = 1.5 x Peak Inflw Rate (litres per secnd), - calculate the inflw rates using the 5% AEP rainfall event. Ensure the Sediment Retentin Pnd has a minimum depth f 1.0m, - alternatively, cnstruct Sediment Retentin Pnds with a minimum vlume f 1% f the cntributing catchment (100 m 3 capacity fr each ha f cntributing catchment). On sites that are particularly steep r have sensitive dwnstream envirnments, a greater Sediment Retentin Pnd vlume may be required. Clean ut Sediment Retentin Pnds when the vlume f sediment accumulated within them reaches 20% f the design vlume. Clearly shw the Sediment Retentin Pnd dimensins necessary t btain the required vlume, as detailed abve, n the site s Ersin and Sediment Cntrl Plan(s). Design Dead Strage (Permanent Strage) Dead strage is the cmpnent f impundment vlume that des nt decant and remains in the Sediment Retentin Pnd. It is imprtant fr dissipating the energy f inflws. Ensure dead strage is 30% f the ttal Sediment Retentin Pnd strage by psitining the lwest decant m abve the invert f the Sediment Retentin Pnd. The apprved decant design detailed in these Guidelines allws the lwer decant arm t be raised as sediment depsitin increases, thereby maintaining the percentage vlume f dead strage. Design Live Strage (Decant Strage) Live strage is the vlume between the lwest decant utlet level and the crest f the Sediment Retentin Pnd primary spillway. Ensure that the live strage vlume capacity is 70% f the ttal Sediment Retentin Pnd strage. The apprved decant design detailed in these Guidelines allws the decant system t be raised as sediment depsitin increases, thereby maintaining the percentage vlume f live strage. Design Decanting/Outlet Dewatering Device Dewater the Sediment Retentin Pnd s as t remve the relatively clean water withut remving any f the settled sediment, and withut remving any appreciable quantities f flating debris. Varius dewatering devices are available, hwever the Auckland Reginal Cuncil prefers the use f a flating T-bar dewatering device, which allws fr the decanting f the cleaner surface water frm the tp f the water clumn. Substantiated perfrmance design will need t be submitted t the Auckland Reginal Cuncil fr decant systems ther than the flating T-bar dewatering device. The recmmended decant rate frm a Sediment Retentin Pnd is 3 litres/secnd/ha f cntributing catchment. This rate ensures that apprpriate detentin times are achieved.

4 Auckland Reginal Cuncil Sediment Cntrl: 2 4 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin A standard T-bar design is detailed in Figure fr varius sized catchments. Fr simplicity, give preference t prducing a standard T-bar decant that prvides a decant rate f 4.5 litres/secnd per decant which can be added in discrete increments t accmmdate varius sized catchments. T achieve a decant rate f 4.5 litres/secnd per decant, drill 6 rws f 10mm diameter hles at 60mm spacings (200 hles) alng the 2m lng decant arm. Fr catchments f less than 1.5ha, seal ff the apprpriate number f hles t achieve a 3 litres/ secnd/hectare discharge rate. Single T-bar decants must be able t perate thrugh the full live strage depth f the Sediment Retentin Pnd. If tw decant systems are required, ensure the lwer T-bar decant perates thrugh the full live strage depth f the Sediment Retentin Pnd. The upper T-bar decant is t perate thrugh the upper 50% f the live strage depth f the Sediment Retentin Pnd nly. If three decant systems are t be used, then the lwer T-bar decant perates thrugh the full live strage depth and the secnd T-bar decant thrugh the upper tw thirds f live strage depth f the Sediment Retentin Pnd. The upper T-bar decant perates thrugh the upper ne third f live strage depth f the Sediment Retentin Pnd as detailed in Figure Ensure that the T-bar decant flat is securely fastened with steel strapping directly n tp f the decant arm and weight it t keep the decant arm submerged just belw the surface thrugh all stages f the decant cycle. This will als minimise the ptential fr blckage f the decant hles by flating debris. The mst successful methd fund t date is t weight the decant arm by strapping a 1.8m lng waratah between the flat and the decant (apprximately 4kg f weight). Psitin the T-bar decant at the crrect height by tying 5mm nyln crd thrugh decant hles at either end f the decant arm and fastening it t waratahs driven in n either side f the decant. Lay the discharge pipe at a 1 2% gradient, cmpact the fill material arund it using a machine cmpactr and incrprate anti-seep cllars with the fllwing criteria: install cllars arund the pipe t increase the seepage length alng the pipe with a spacing f apprximately 10m; the vertical prjectin f each cllar is 1.0m; ensure all anti seep cllars and their cnnectins are watertight. Use a flexible thick rubber cupling t prvide a cnnectin between the decant arm and the primary spillway r discharge pipe. T prvide sufficient flexibility (such as is required fr the lwer decant arm) install tw cuplings. Fasten the flexible cupling using strap clamps and glue. Where a cncrete riser decant system is utilised, ensure the lwer decant cnnectin is psitined n an angle upwards frm the hrizntal s as t split the peratinal angle that the decant wrks thrugh. This will reduce the defrmatin frce n the cupling used. Design Shape f the Pnd Ensure the length t width rati f the Sediment Retentin Pnd is n less than 3:1 and n greater than 5:1. The length f the Sediment Retentin Pnd is measured as the distance between the inlet and the utlet (decant system). A 2:1 rati may be used if the pnd depth is n greater than 1.0m. Maximise the distance between the inlet and the utlet (including the emergency spillway) t reduce the risk f shrt circuiting and t prmte quiescent cnditins. If this can nt be achieved by crrectly psitining the inlet and utlets, install baffles t achieve the apprpriate length t width rati design. Ensure that the Sediment Retentin Pnd has a level invert as described belw t prmte the even and gradual dissipatin f the heavier inflw water acrss the full area f the Sediment Retentin Pnd.

5 Auckland Reginal Cuncil Sediment Cntrl: 2 5 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin Design Level Spreader Incrprate a Level Spreader int the inlet design t spread inflw velcities, thereby allwing rapid dissipatin f inflw energies. Cmbine the Level Spreader with a well cmpacted and smthed inlet batter (n steeper than a 3:1 gradient), stabilised ver its entire area. The essential design feature is t ensure the Level Spreader is level, nn-erdible and spans the full width f the Sediment Retentin Pnd. T ensure even inflws, install a trenched and pegged 150mm x 50mm timber weir r similar acrss the full width f the inlet. Bund the edges with cmpacted earth t prevent utflanking. This timber weir als serves t te in any getextile prtectin which may be required. Sediment accumulated behind the Level Spreader may require peridic remval. Psitin the tp f the Level Spreader weir mm abve the invert f the emergency spillway. The shape f the Level Spreader will vary with the directin and vlume f the inlet diversin, but the entrance t the Level Spreader must be widened and flared t prmte even flw, as in Part B, Sectin 1.10 f these Guidelines. Design Baffles Incrprate baffles in the Sediment Retentin Pnd design where the recmmended pnd shape cannt be achieved. Extend baffles the full depth f the Sediment Retentin Pnd and place them t maximise dissipatin f flw energy. Generally, baffles are in the frm f a wing t direct inflws away frm the utlet and maximise the stilling zne. A series f cmpartments within the pnd can be used t achieve this, althugh care must be taken t avid creating in-pnd currents and resuspensin f light particulates. Plate Sediment Retentin Pnd Shwing Baffle t Lengthen Flwpath and Create Crrect Length:Width Rati

6 Auckland Reginal Cuncil Sediment Cntrl: 2 6 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin Baffles may be cnstructed frm varius materials ranging frm slid shutter bards t braced getextile curtains. Design Depth f Pnd Sediment Retentin Pnd depths may be 1 2m deep, but n deeper than 2m. Deeper pnds are mre likely t cause shrt circuiting prblems during larger strm events and require specifically designed flating decant systems. The decant design in these Guidelines perates thrugh a maximum live strage range f 1.5m. Design Embankment Befre building a Sediment Retentin Pnd, install sediment cntrls such as Silt Fences belw the cnstructin area and maintain them t a functinal standard until the Sediment Retentin Pnd batters are fully stabilised. Thrughly cmpact the Sediment Retentin Pnd embankment, with material laid in 150 mm layers and cmpacted t engineering standards. Where pssible install the discharge pipes thrugh the embankment as the embankment is being cnstructed. Fully stabilise the external batter face, by vegetative r ther means, immediately after cnstructin. Ensure all bare areas assciated with the Sediment Retentin Pnd (including internal batters) are stabilised with vegetatin if the Sediment Retentin Pnd is t remain in place ver winter. Design Primary Spillway Fr larger catchments (greater than 1.5ha) the Sediment Retentin Pnd requires a piped primary spillway (refer Figures and 2.1.4). Fr catchments up t 1.5ha, decant flws can be piped using the same diameter piping as the decant system (100mm PVC smth bre) directly thrugh the Sediment Retentin Pnd wall t discharge beynd the te f the Sediment Retentin Pnd wall. Fr cntributing catchments between 1.5 and 3ha in area, use a discharge and primary spillway pipe diameter f 150mm. Where cntributing catchments are 3ha r greater and/r the lng term stability f the Sediment Retentin Pnd emergency spillway is questinable (fr example, built in fill) then cnsideratin shuld be given t incrprating a cncrete manhle riser and larger diameter pipe utlet as a primary spillway sufficient t accmmdate the 5% AEP rainfall event. If the Sediment Retentin Pnd is t perate ver the winter and the cntributing catchment is fully stabilised, discnnect the T-bar decant t reduce the frequency f emergency spillway activatin and cnsequent ersin. Where a primary spillway upstand riser is used, place the tp f the riser a minimum 600mm lwer than the tp f the Sediment Retentin Pnd embankment and a minimum 300mm lwer than the emergency spillway crest. Ensure the riser and the discharge pipe cnnectins are all cmpletely watertight. Where pssible, install the piping thrugh the embankment as the embankment is being cnstructed. Design Emergency Spillway An emergency spillway is essential fr all Sediment Retentin Pnds. Emergency spillways must be capable f accmmdating the 1% AEP event withut erding. The uter emergency spillway crest and batter requires a very high standard f stabilisatin with the fill material f the spillway batter well cmpacted. Cnstruct the emergency spillway as a stabilised trapezidal crss sectin with a minimum bttm width f 6m r the width f the pnd flr, whichever is the greater. When utilising getextile fr emergency spillway stabilisatin purpses, the batter face must be smth and all vids eliminated. If getextile is used, a sft neddle punch getextile is laid first and then cvered with a strng wven lw permeability getextile. Ensure the getextile is pinned at 0.5m centres ver the full area f the emergency spillway.

7 Auckland Reginal Cuncil Sediment Cntrl: 2 7 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin Where pssible, cnstruct emergency spillways in well vegetated, undisturbed grund (nt fill) and discharge ver lng grass. If the emergency spillway is cnstructed n bare sil, prvide cmplete ersin prtectin by means such as gruted riprap, asphalt, ersin matting/ getextile r cncrete. Cnstruct the emergency spillway with a minimum f 300mm freebard height abve the primary spillway. Cnstructin Specificatins Cnstruct a fabric silt fence acrss the dwnslpe end f the prpsed wrks. Clear areas under prpsed fills f tpsil r ther unsuitable material dwn t cmpetent material. Large fill embankments may need t be keyed in. Use nly apprved fill. Place and cmpact fill in layers as per the engineer s specificatins. D nt place pervius materials such as sand r gravel within the fill material. Cnstruct fill embankments apprximately 10% higher than the design height t allw fr settlement f the material. Install apprpriate pipe wrk and antiseep cllars during the cnstructin f the embankment and cmpact arund these apprpriately. Install the emergency spillway. Install and stabilise the Level Spreader. Securely attach the decant system t the hrizntal pipewrk. Make all cnnectins watertight. Place any manhle riser n a firm fundatin f impervius sil. D nt place pervius material such as sand r scria arund the discharge pipe r the antiseep cllars. Install baffles if required. Check Sediment Retentin Pnd freebard fr differential settlement and rectify as necessary. Stabilise bth internal and external batters with vegetatin and the emergency spillway in accrdance with the site s apprved Ersin and Sediment Cntrl Plan. Pnd Maintenance and Dispsal f Sediment Clean ut Sediment Retentin Pnds befre the vlume f accumulated sediment reaches 20% f the ttal Sediment Retentin Pnd vlume. T assist in gauging sediment lads, clearly mark the 20% vlume height n the decant riser. Clean ut Sediment Retentin Pnds with high capacity sludge pumps, r with excavatrs (lng reach excavatrs if needed) lading nt sealed tip trucks r t a secure area. The Ersin and Sediment Cntrl Plan shuld identify dispsal lcatins fr the sediment remved frm the Sediment Retentin Pnd. Depsit the sediment in such a lcatin s that it des nt lead t a direct discharge t receiving envirnments. Stabilise all dispsal sites as required and apprved in the site s Ersin and Sediment Cntrl Plan. Inspect Sediment Retentin Pnds every day and befre every frecasted rainfall event. Inspect fr crrect peratin after every runff event. Immediately repair any damage t Sediment Retentin Pnds caused by ersin r cnstructin equipment. Safety Sediment Retentin Pnds are attractive t children and can becme safety hazards if nt apprpriately fenced and if safety rules are nt fllwed. Lw gradient pnd batters prvide an additinal safety measure. Check the safety requirements f the City r District Cuncil Authrity and the Occupatinal Safety and Health branch f the Department f Labur. Chemical Treatment Sme chemicals can be used successfully t prmte flcculatin (clumping tgether) f suspended slids in the Sediment Retentin Pnd t increase the particle mass and speed the rate f settling.

8 Auckland Reginal Cuncil Sediment Cntrl: 2 8 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin Recent trials have identified a simple and effective chemical dsing system that des nt require a pwer supply. This system uses ply aluminium chlride (PAC) and has been fund t be particularly effective in settling fine particulate such as fine silts and clays. Other chemicals may becme available as mre trial data is btained. Chemical dsing systems are likely t be required where the design Sediment Retentin Pnd vlume cannt be achieved because f site cnstraints and/r where a high level f treatment is required because f the sensitivity f the receiving envirnment. Chemical treatment is als mre likely t be required where the clay cmpnent is high r where the cumulative effects f sediment discharge are significant. Figure Sediment Retentin Pnd fr Catchments up t 1.5 ha

9 Auckland Reginal Cuncil Sediment Cntrl: 2 9 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin Figure Sediment Retentin Pnd fr Catchments Between 1.5 and 3 ha

10 Auckland Reginal Cuncil Sediment Cntrl: 2 10 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin Figure Sediment Retentin Pnd fr Catchments Between 3 and 5 ha

11 Auckland Reginal Cuncil Sediment Cntrl: 2 11 ersin&sedimentcntrl Guidelines fr Land Disturbing Activities in the Auckland Regin Figure Sediment Retentin Pnd Decant Detail