The Erosion and Sediment Control Plan

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The Erosion and Sediment Control Plan is designed to ensure the protection of soils and surface water in the surrounding construction activities area associated with the construction  project.  The  plan  contains  methods  and  layout  that will  be  used  to  prevent impact  during  construction  activities, recommendations  for  mitigation  and  monitoring plans to determine the effectiveness of mitigation.

The objectives of the erosion and sediment control plan are:

• Ensuring the operations are undertaken with due regard to sediment control;
• Ensuring the sediment control to prevent surface water due to unacceptable sediment loads;
• Preventing any additional undesirable impacts on surface water.

Definitions :

1. Surface water: Any stream drain or water body, including surface run-off which  remains  on  ground  surface  and water carried in culverts.
2. Erosion prevention: Any  practice  or  device that protects the soil surface and prevents  mobilization of  soil  particles  to   the  receiving drains, streams or rivers.
3. Sediment control: Any practice  or  device which separates or concentrates solid particles from water or supernatant
4. Supernatant: The  resulting  relatively  clean  water  which  exists once sediment materials have settled.

Implementation and Validity

This plan covers all foreseeable activities that could result in unacceptable erosion of soil, the  resulting  sediment  load  in  surface  drainage  waters  around  the  project  sites  and  its impact  on  downstream  receiving  waters.  The  plan  should  be  implemented  when undertaking all civil works and activities including:

• Land clearing
• Plant construction
• Construction of transmission infrastructure

The SOP shall become effective at the commencement of pre-construction/land clearing activities and shall remain valid until construction is complete and the site drainage plans and  water  quality  mitigation  and  devices  of  the  operational  phase  are  ready  to  be commissioned.

Procedure

The  primary  protection  at  the  site  should  be  erosion  prevention  at  source.  Secondary protection is provided by sediment control techniques.

Sources of sediment

Sediment can become en-trained in surface run-off in situations where there is enough energy  in  the  water  to  mobilize  soil  particles  and  hold  them  in  suspension. In  many cases the suspension process can be complex, since first wetting of soils can reduce the cohesiveness  of  materials  which  subsequently  aids  their  mobilization. Common situations on the construction project where sediment mobilization can occur are:

1. The  flow  of  surface  run-off  across  slopes (this  can  often  be  associated  with  the scarring of small rivulets; channels; and surface)
2. Intense rainfall on soil and spoil heaps which have been left exposed
3. Any  other  situation  the    exposed  soils  contact  with  moving  water,  from  storms  or casual release from construction activities.

Erosion Prevention

In all instances erosion prevention is the preferred method for management of sediment creation on all sites.  In some cases, although prevention is the ultimate objective, these measures may only result in minimization, since only a few measures are 100% effective. Measures for prevention include:

1. Water Diversion, In  many  cases  the  best  way  to  avoid  water  flowing across slopes  and  surfaces  is diverting  flows  around  the  exposed  soils. This is  achieved  by  alteration  of  the typography immediately around the area of concerned by either constructing a water barrier or a channel.  A water barrier will be a levee, or linear bank at the top of the slope that prevents water from flowing across the surface, and usually diverts it to a drainage channel.  In some cases it may be more effective to dig a drainage channel at the top of the slope which again diverts water to a main channel. Water diversion techniques are especially effective where slopes are likely to receive surface run-off from areas on the site which at greater elevation.  A weakness is that although water from upstream is diverted around the slope, storm water falling on the slope will still create  surface  run-off  for  particularly  steep  slopes,  or  those  with  substantial  area combination  of  diversion  techniques  with  other  preventative  techniques should  be considered.
2. Surface Coverage – Planting, The most effective method of stabilizing exposed soils is planting with ground cover plants  such  as  grasses  and  cannot  be  applied  to  either  storage  heaps  or  exposed slopes.  This technique can be used for any long-term land form which is either being left  permanently  or  perhaps for  a  longer  period of time  after  clearing before further work  is  undertaken. Planting  needs  to  be  undertaken  in accordance with the procedure  for  re-vegetation (SOP of Landscape  and  Re-vegetation Plan). The weakness  of  planting  as  a  technique  is  longer  period  of  time  that  it  takes for vegetation cover to become effective in rainstorms, and perhaps the employment of other interim measures may need to be considered.
3. Temporary Surface Cove-rag, A particularly effective technique for  protecting  heaps,  whether  short-term  or  long-term is covering with a physical barrier, such as a tarpaulin, or plastic sheeting. Such coverage needs to consider the nature of the land form, and the possibility that storm events  are  often  associated  with  high  winds  that  require  the  secured  cover.  This technique can be applied for short term spoil heaps, especially in rainy season when storm  events  can  be  predicted.    Weaknesses  of  the  technique  are  that  it  requires particular deployment of the cover in a timely manner, and that once on it will restrict access to the heap therefore may need to be removed frequently in the absence of storm events.  This technique should be considered for the creation of casuals spoil heaps,  where  there  is  limited  time  or  availability  to  either  move  or  tidy  before  rain event occurs.
4. Silt fences, Silt  fences  are  an  effective  method  of  limiting  sediment  transport,  they  provide  a barrier at the base of slopes which prevent the movement of soil and sediment, yet allows the flow of surface run-off. Installation of silt fences should be considered for almost all land clearing activities where large areas of exposed soils may be created. Where the mobilization of the sediments on the slope cannot stop, but have a major role  in  preventing  the  movement  from  the  slope.  Once  installed,  it need  to  be inspected and maintained regularly, particularly in periods of frequent rain such as the rainy  season,  as  soil  can  build  up  at  the  base  of  the  fence  rendering  it  ineffective. Instructions  for  the  installation  and  maintenance  of  silt  fences.
5. Erosion Matting, Erosion matting are a temporary open weave material which can be used for slope temporary stabilization, particularly during re-vegetation, temporary stabilization of the soils  can  be  used,  and  access  to  sunlight  and  air  to  promote  the  regrowth. It  is generally  applied  on  finished  land form  rather  than  temporary  works,  since  it  more difficult to recover and usually used in conjunction with re-vegetation activities.
6. Bank Suring, In  some  cases  the  temporary  ditches  and  drainage  channels  have  potential  to become eroded and promoting the mobilization of sediments from the banks.  Ditches and  drainage  channels  will  consider  the  suitability  of  the  soils  through  it  may  be necessary to assure the banks by using rip rap, rocks or geo-textile material to protect walls  and  floors  of  the  drains  from  being  scoured  by  higher  energy  water. Design must  consider  flows  in  storm  events  and  wherever  possible  avoid  sharp  bends, narrowing of channels and points of confluence where higher energy water will flow and  turbulence  can  exist.  In  particular,  concerning  of  the  banks  on  the  outside  of bends  where  scouring  can  be  pronounced. This  should  not  be  considered  for drainage on site, but discharge points to water receiver need to be considered also, it should  be  remembered  that  water  that  across  the  whole  site  concentrated  into  a single flow, at the point of discharge it flow greater than the normal condition for the receiving stream.  Banks on water receiver and reinforcing need to be considered.

Sediment Control

Sediment control measures represent the last line of defense for surface water leaving the  drainage  system  on  site  to  the  water  receiver.    As  mentioned  above,  no  single erosion prevention measure is 100% effective, and it is inevitable that some material will become en-trained in storm water.  Erosion prevention devices will prevent most potential sediment transport. it is the role of sediment control devices to ensure a final stage of water treatment prior to release to the water receiver.  Sediment control devices located in  strategic  location  within  the  site  drainage  system  as  a  method  of  containment.  Sediments become en-trained in surface water flows due to the kinetic energy created by the moving water as long as the water is flowing around turbulent conditions where the sediment will remain suspended.  However if flows can be slowed and the kinetic energy dissipated, then sediments begin to settle, plus sediment control devices seek to lower the kinetic energy of the flowing stream and promote settling of the solids.

Planning Measurement

Appropriate site planning and timing of  activities can also assist in the minimization of sediment  production. Some  examples  of  appropriate  planning  to  avoid  sedimentation include: 

1. An awareness of seasonality and potential for rain events, some civil works and land clearing activities can be scheduled around peak storm periods if at all possible.
2. Activity  management  during  rainy  season  periods  allowed  the  works  to  be  “made good” with covers or appropriate prevention devices before the onset of a storm.
3. Good site housekeeping.
4. Appropriate Design.