Week 11: Erosion and Sediment Control Flashcards
What are the three processes in erosion?
- Detachment (erosive forces > cohesive forces)
- Transport (water or wind)
- Deposition (sedimentation)
What are the different types of erosion?
- Splash erosion
- Sheet erosion
- Rill erosion
- Gully erosion
- Tunnel erosion
- Channel erosion within streams
- Wind erosion
What are the factors influencing erosion?
- Weather
- Topography
- Soil characteristics
- Ground cover
- Duration of soil exposure
How might soil type affect erodibility?
- Texture/particle sizes
- Organic matter content
- Structure
- Soil permeability and porosity
- Soil moisture
What are the environmental impacts of erosion?
- Soil loss
- Impact on aquatic ecosystems (habitat loss, water quality decline, smothering of plants, insects and fish)
(Also, recreational impacts.)
What are the principles of erosion and sediment control?
(I.e., what are our main objectives?)
- Minimise disturbance
- Construction in stages
- Protect slopes
- Protect waterways
- Stabilise exposed areas quickly
- Consider weather
- Install perimeter controls and diversions
- Use sediment control tools
- Mix & match your tools
- Adjust the plan as needed
- Monitor and adjust tools
Describe the principle: minimise disturbance.
- Leave vegetation/ground cover where possible
- Fit development around sensitive areas
- Design to suit topography (rather than doing significant earthworks)
- Staging + rapid stabilistation of soils to minimise exposed areas
Describe the principles: do the construction in stages.
- Avoid bulk earthworks across whole site
- Break project into stages
- Expose only area that you are working on
- Progressively stabilise exposed areas as you go
- Allow for staging in project programme
Describe the principle: protect slopes
On slopes, topsoil/hydroseeding may not be enough. May need additional:
- Texturing slopes
- Compost
- Polymers
- Straw mulch
- Other protective surface covers
Diversion of clean runoff at the top of slopes is good too.
Describe the prinicple: protect waterways.
- Identify waterways and drainage areas
- Ephemeral streams and/or drainage areas also require protection
- Avoid earthworks in or adjacent to these areas
- Avoid stockpiling soil near drainage areas
- Where does the water go?
Describe the principle: stabilise exposed areas quickly.
- Stabilise soils as soon as earthworks are complete
- Stabilisation can be achieved using vegetation, compost, mulch, grassing, polymers, geotextiles, sealing of roads etc.
- Consider the time required for grass/vegetation to grow vs. weather window.
Describe the principle: consider the weather
- Consider weather in the programme
- Some parts of NZ only allow earthworks during summer to avoid heavy rainfall
- In Canterbury: hot dry winds (NW)
- Specific high-risk activities must be undertaken in fine weather
- Allow time for stabilisation & consider impact of weather on grass growth
Describe the principle: install perimeter controls and diversions.
- Divert runoff away from active earthworks areas, stockpiles and exposed areas
- Separate dirty & clean runoff
- Clean water diversions
- Perimeter bunds
Describe the principle: use sediment control tools.
Provide treatment of sediment-laden runoff before it is discharged to the receiving environment. Some tools are:
- Sediment retention ponds
- Decanting earth bunds
- Silt fences
- Silt socks
- Stormwater inlet protection
- Coagulants and flocculants
How to prepare an ESCP:
(Erosion and Sediment Control Plan)
- Site scoping
- Construction programme
- Identify tools & techniques
- Inspections plan
- Monitoring plan
- Emergency procedures
- Live document
What questions do you ask during the site scoping step?
(ESCP)
- Soil type? Where does the water go?
- Contaminated site?
- Groundwater depth?
What questions do you ask during the construction programme step?
(ESCP)
- What works are going to occur on site?
- How long will the works take?
- Where will the works occur?
What is a typical construction programme?
- Install erosion and sediment control works
- Strip topsoil
- Form roads
- Bulk earthworks + site shaping
- Install services (3 waters, power, comms)
- Hydroseeding/stabilisation of lots
- Seal the road
What questions do you ask during the identify tools & techniques step?
(ESCP)
- Mix & match erosion and sediment control tools
- Suit the site conditions and the construction works
What questions do you ask during the inspections plan step?
(ESCP)
- How often will you check the devices?
- When are the special checks required?
- Who is responsible for completing and signing off on checks?
- What actions should be taken as a result of inspections?
What questions do you ask during the monitoring plan & emergency response steps?
(ESCP)
- How will you prove that the devices are working?
- How will you check that the discharge meets consent limits?
- How will you prevent spills?
- How will you respond in the event of a spill, flood, slip, etc?
What were the main challenges in the Westmorland development?
(In terms of ESCP)
- Bulk earthworks across a hillside subdivision (Port Hills)
- No protection of exposed loess soils
- Massive discharge of silt
Explain the China Loess Plateau case study.
(ESCP)
Context
- One of largest and thickest loess plateaus in the world
- Some of most severe erosion rates in the world
Two main goals
- Restore agricultural productivity
- Reduce erosion
Key components
- Changing agricultural processes
- Terracing
- Forestation
- Planting shrubs
- Irrigation
- Sediment controls dams
- Grazing restrictions
Social & environmental transformation
- Sustainable crop production
- Planting for land stabilisation
- Substantially reduce sediment runoff
- Lifting millions out of poverty
