1. Detainment Bunds (DBs)
Rotorua P-Project:
Detainment Bunds (DBs)
The performance of DBs for attenuating phosphorus and sediment loss from pastoral farmland in sub catchments of Lake Rotorua
D Clarke, J Paterson, D Hamilton, J Abell, R Moore, M Scarsbrook, K Thompson and A Bruere
I will be giving a brief overview of my MSc project which assess the performance of DBs for at……
Rotorua , July 2013

2. Sediment and nutrient highway
flow over usually dry paddocks

3. Ephemeral streams (overland flow)
Water runs off pastoral farmland during heavy rainfall
Ephemeral streams can transport soil and nutrients from paddocks to waterways
Transport a disproportionate amount over a short period of time
The predominant pathway for P and sediment export from pastoral farmland to freshwaters

4. Detainment bunds temporarily pond ephemeral streams
Nutrient and sediment laden runoff
water velocity arrested at pond
Pond allows settling for up to 3 days
EARTH BUND

5. An earth bund built to retain water for P mitigation
What is a DB
An earth bund built to retain water for P mitigation
Flick thru fast

6. MSc research: The Performance of DBs
Study Sites

7. Methods ‘Storm chasing’ to collect water samples from
-Incoming ephemeral streams & outflow from the DB
Sediment mats were deployed to catch sediment deposited in the DB Basin- Analysed for P, metals, and particle size of sediment.
Soil samples taken from a historic ponding area of a detainment dam.
Water level was recorded to help derive deposition estimates
- total and dissolved phosphorus (P) & nitrogen (N) nutrient samples were taken.
- Suspended sediment; particle size, total suspended solids.

8. Sediment sampling
Briefly point to the sediment mats deployed based on elevation, to derive a mass sediment deposition estimate

9. Results
What ran off depended on what was happening in the contributing catchment. Different for each event.
>50% Dissolved P
>95% Particulate P
Winter forage crop

10. Total suspended sediment in outflow from DB
Day Day Day 3
There are obvious of tss in water
Hauraki DB, March, 2012

11. Sedimentation rates in the DBs
Each event & site is different
Depending on quantity and composition of sediment

12. Organic material can stay suspended
Variable trends in PP observed between sites/ events
- Ranged from 36% reduction in PP concentration in water over 20 h
ponding, to no trend over ponding period.
PP reduction depended on suspended material characteristics (size of particles, amount of suspended sediment, organic content
DRP inversely correlated with TSS (when lots of suspended sediment- it adsorbed most of the dissolved P)

13. Sediment deposition estimates
The largest retention of sediment and P (2,749 kg and 6.08 kg respectively) during the study period occurred when a large rainfall and runoff event in July coincided with the recent complete removal of vegetation in an upstream paddock sown with a winter forage crop.

14. 2.7 t sediment deposited extreme case
Note the squeaky clean ephemral stream flow path- that sediment that is on the grass would have likely made it into L. Rotorua - you can see how the DB has promoted sediment settling – Note the brown smudge of sediment sticking to the grass and the very distinct ‘high tide’ mark on the bund wall – exactly level with the top of the riser.
( these first few slides will be ripped through fast)

15. P retained
Max: 6 kg P
There is a range – not un- common as winter cropping as a regular practice

16. Integrating DBs into farm systems
Aim to maintain the productive potential
of ponding area
Optimal ponding time is a compromise between:
- maximising water treatment
maintaining pasture quality
-3 days works

17. Summary
DBs can be used as one of the P mitigation tools to reduce P loads to Lake Rotorua
DB Co- benefits – Flood control on SH 36 (funding), improve downstream mitigations such as wetlands as water flow slowed down
16 DBs Built in Lake Rotorua catchment so far, Rerewhakaaitu catchment scoping underway
DB user guidelines handbook out soon- based on the 1st year of implementation

18. Thanks to MSc Supervisors:
John Paterson, Prof David Hamilton, Dr Jonathon Abell
Land owners/managers for integrating DBs in farm systems
Waitetī Trust & Murray Scott, Jamie & Chris Paterson, Daniel and Kim, Tony & Margo Cairns, Moses Beckhan, Vicky, Peter and Gisele Schweizer
Bay of Plenty Regional Council
University of Waikato
DairyNZ
Rotorua Primary Producers Collective
NZTA
Andy Bruere, R Moore, Mike Scarsbrook, Kevin Thompson,
C Putt, D Guinto, G Ewert, R McDowell, Alastair MacCormick, Greg Corbett, Roz Hensman, A Woolhouse, S Anderson, M Hawke, Tabea Merz, J Peryer-Fursdon, Dr. Deniz Özkundakci , Dr Grant Tempero, Lee Laboyrie, Kohji Muraoka, Matt Allen, Arianto Santoso, Hannah Muriel, Josh De Villers, Rebecca Eivers, Sascha Kochendörfer, Dan Prat, Alex Port, Steve Woods, Steve Cameron, Annette Rogers, Cheryl Ward
Thanks John Paterson was the master mind behind the DB project
To my supervisors, the Land owners and industry support from Dairy NZ
and the many others who have been influencial in this project ( I wont red out)