1) Green roofs can help manage stormwater but often flooding still occurs due to heavy rainfall exceeding infrastructure design capacities. Implementing green infrastructure like green roofs may help reduce flooding by retaining water. 2) Studies show biochar amendment to green roof substrates can significantly improve nutrient retention and water holding capacity. Biochar reduced nitrogen and phosphorus loads in runoff by 40% and 35% respectively in one study. 3) Preliminary research also found crushed concrete substrates amended with biochar and limestone showed potential for good nutrient retention and water holding. Further research on biochar and mycorrhizal amendments in crushed concrete is warranted.

1. How to improve water and nutrient
retention in green roofs?
Kirsi Kuoppamäki, PhD, Docent
Department of Environmental Sciences, LAHTI
kirsi.kuoppamaki@helsinki.fi
Green Building – Building Green in Cities
25.8.2016, Helsinki, Finland

2. Stormwater management
among the important ecosystem services
provided by green infrastructure
2
Heavy rainfall flooded into city museum in Helsinki Thundershower resulted in a flood in the centre of Mikkeli

3. Mikkeli waterworks (Länsi-Savo 4.7.2016): ”Stormwater drainage
built for the management normal rainfall and 5-year design storms.
However, flooding occurs every year. Thus, more stormwater
conveyances are needed as there is no empty space to retain water
due to the densely built city centre.”
3
Photo: Otto Kärhä, Länsi-Savo 3.7.2016
Mikkeli:
54 724 inhabitants
land area 2 548 km2
21 people/km2 (in Helsinki 2 944 pers./km2)
Grey infrastructure could be
complemented by green infrastructure

4. Implementing green infrastructure do
contribute to stormwater management
4
Stormwater green infrastructure* can
reduce the magnitude, frequency,
duration, variability of flashiness and
nitrogen loading at catchment scale
 encouraging news to stormwater
managers
*)detention ponds, shallow marshes, wet ponds,
sand filters, infiltration trenches/basins, bioretention
basins, swales, underground storages, porous
pavement, green roofs
Municipalities of Baltimore, Montgomery and Washington DC
However, no reduction in phosphorus exports
Pennino et al. 2016, Sci. Tot. Env. 565: 1044-1053

5. 5
Cyanobacterial bloom
Lake Vesijärvi, Lahti 29.6.2016
Smith & Schindler 2009, TREE 24: 201–207
Schindler et al. 2008, PNAS 105: 11254-11258
Green infrastructure (GI):
management of both
quantity and quality of urban water
(GI should not become a new source of pollution)
Building green...
but not in water bodies!

6. Nutrient retention in green roofs?
6
Total nitrogen
concentration (mg/l)
Total phosphorus
concentration (mg/l)
Solutions to nutrient leaching?
amount or type of organic matter or fertilisers (Harper et al. 2015)
amend inert substrates (Oberndorfer et al. 2007)
various reactive aggregates (Karczmarczyk et al. 2014, Bus et al. 2016)
BIOCHAR

7. 7
10%
35%
Biochar improves nutrient retention
Total nitrogen load (mg m-2 6 months-1)
40%
20%
Biochar was made of birch
wood (Betula sp. including bark)
and had been pyrolysed in a
continuous retort at 380–420oC
for a holding time of 2 h
control = biochar absent
Total phosphorus load (mg m-2 6 months-1)

8. 8
Biochar amended in crushed brick substrate
20 green roof platforms, each 2 m2 and 10 cm thick
10 Pre-grown vegetation mats, 10 planted with plug plants and seeds 5 replicates
Half of the roofs amended with biochar (10% by volume)
(85% recycled brick, 5% compost, 5% peat, 5% crushed bark)

9. 9
Biochar reduces nutrient load
no substrate no substrate

10. Biochar improves
water retention
cumulative runoff and rainfall measured with
rain gauge tipping buckets (Decagon ECRN-100)
summer-autumn 2014
10
biochar
absent 62%
biochar
present 73%
End of growing season in late October
Rainfall
Runoff from pre-grown
green roofs
unpublished data

11. Biochar and dense vegetation
improve water retention
30 mm rainfall in three days in August 2014
11
80 %
90 %
93 %
95 %
biochar absent
biochar present
Furthermore, biochar improves plant
water supply and acts as a carbon sink
unpublished data

12. 12
Crushed concrete as substrate
(+compost, crushed bark, reed) 12 plots, each 18 m2 and 8-10 cm thick

13. Nutrient concentrations in runoff
from experimental green roofs
August 2014
13
biochar
absent
biochar
present
Substrate
thickness
cm
biochar+
limestone
limestone
concrete
Crushed brick substrate
biochar
absent
biochar
present
biochar+
limestone
limestone
concrete
Crushed brick substrate
Total nitrogen Total phosphorus
unpublished data

14. 14
So how to improve water and nutrient retention in green roofs?
Biochar is part of the solution. Concrete-based substrate is promising…
Amendment of biochar and mychorriza in crushed concrete?!
PhD Marleena Hagner

15. 15
To be continued. . .
Results applicable also in other constructed green infrastructure, nature-based solutions...

16. 16
Thank you!
Fifth Dimension - Green Roofs in Urban Areas
Funding: Academy of Finland (URCA project), Helsinki University Centre
for Environment, HENVI (ENSURE-project), Uudenmaan MAKERA