1. ICL Landslide Teaching Tools PPT-tool 4.064-1.7 (1)
Quantifying the benefits for floodplain management of targeted reforestation of landslide-prone terrain in New Zealand
Mike Page
GNS Science (Lower Hutt, New Zealand)

2. Location of Waipaoa Catchment in northeast North Island, New Zealand
ICL Landslide Teaching Tools　
PPT-tool (2)
Location of Waipaoa Catchment in northeast North Island, New Zealand
3D perspective of
Waipaoa Catchment topography

3. ICL Landslide Teaching Tools　
PPT-tool (3)
The Waipaoa Flood Control Scheme is losing capacity through channel aggradation and sediment deposition within stopbanks
What is the source of this sediment?
What is the most effective “area for benefit” reforestation strategy to reduce loss of capacity?
Why is it aggrading?

4. ICL Landslide Teaching Tools　
PPT-tool (4)
Te Arai River
Sediment deposition within stopbanks following mm rainfall in 2002

5. Waipaoa River Lower floodplain following Cyclone Bola
ICL Landslide Teaching Tools　
PPT-tool (5)
Waipaoa River
Lower floodplain following Cyclone Bola

6. riverbeds and roads are also sources of sediment.
ICL Landslide Teaching Tools　
PPT-tool (6)
The Waipaoa catchment has a diverse range of terrains and associated erosion processes:
gully
landslide
earthflow
sheetwash
streambank
tunnel gully
riverbeds and roads are also sources of sediment.

7. ICL Landslide Teaching Tools　
PPT-tool (7)
Erosion Terrains

8. Sediment sources in the Waipaoa Catchment
ICL Landslide Teaching Tools　
PPT-tool (8)
Sediment sources in the Waipaoa Catchment
Gully Terrain - Tarndale Slip
(most of the sediment comes from a few severely eroding sources)

9. ICL Landslide Teaching Tools　
PPT-tool (9)
Outline
The contribution that landslides make, both to long-term sediment yield, and to sediment yield for a large storm. 
2. The contribution of landsliding to overbank (berm and floodplain) sedimentation. 
3. The effect targeted reforestation has on reducing landslide-derived sediment.

10. Sediment sources in the Waipaoa Catchment
ICL Landslide Teaching Tools　
PPT-tool (10)
Sediment sources in the Waipaoa Catchment
Landslide Terrain - Cyclone Bola 1988

11. 1. Calculating landslide contribution to sediment yield
ICL Landslide Teaching Tools　
PPT-tool (11)
1. Calculating landslide contribution to sediment yield  

12. ICL Landslide Teaching Tools　
PPT-tool (12)
August 2002 Storm

13. Storm rainfall-landslide density relationships
ICL Landslide Teaching Tools　
PPT-tool (13)
Storm rainfall-landslide density relationships

14. ICL Landslide Teaching Tools　
PPT-tool (14)
Sediment Delivery

15. ICL Landslide Teaching Tools　
PPT-tool (15)
Sediment Delivery

16. Landslide contribution to sediment yield
ICL Landslide Teaching Tools　
PPT-tool (16)
Landslide contribution to sediment yield
Mean annual suspended sediment yield
(1972–2000)
15.2 x 106 t t km2/yr
landslides x 106 t t km2/yr ~15%
Cyclone Bola suspended sediment yield
32.0 x 106 t
landslides x 106 t ~64%

17. ICL Landslide Teaching Tools　
PPT-tool (17)
2. Contribution of landsliding to overbank (berm and floodplain) sedimentation

18. Landsliding contributes majority of sediment within stopbanks
ICL Landslide Teaching Tools　
PPT-tool (18)
Landsliding contributes majority of sediment within stopbanks

19. ICL Landslide Teaching Tools　
PPT-tool (19)
3. The effect targeted reforestation has on reducing landslide-derived sediment (The most effective “area for benefit” reforestation strategy)

20. ICL Landslide Teaching Tools　
PPT-tool (20)

21. ICL Landslide Teaching Tools　
PPT-tool (21)
Summary
Landslides contribute ~15% of the long-term suspended sediment yield
In Cyclone Bola, landslides contributed ~64% of the suspended sediment yield
76% of suspended sediment is transported at lower than bankfull discharge
Overbanking (recurrence interval of 4–35 years) occurs in large storms above the threshold for landsliding

22. ICL Landslide Teaching Tools　
PPT-tool (22)
Summary - continued
Both overbanking and landslide-derived contribution increase with increase in storm magnitude
Therefore, to prolong the life of the flood control scheme, planting of the most susceptible landslide-prone terrain is an option
A 50% reduction in landslide-derived sediment could be achieved from targeted reforestation of 12% of the Waipaoa catchment

23. But consider ICL Landslide Teaching Tools PPT-tool 4.064-1.7 (23)
The Waipoa River catchment has one of the highest natural erosion rates in New Zealand
Prior to the imposition of river-control works, the deposition of over-bank deposits was spread across the entire width of the natural flood plain
Deposition is now confined to an unnaturally narrow channel
Channel aggradation is now a major problem
It will not be controlled by reducing the high rate of natural erosion
It will be “controlled” by learning to live with a high rate of aggradation
The benefits of catchment management need to be balanced against the costs from the source of the problem – the river-control works.