Fujita Technical Research
ReportNo.57 2021

Experimentally based designing of rainwater runoff control
for implementing of Rain Garden

Nobuyuki Kitajima
Abstract
This report shows that the basic approaches to be needed for implementation of rain garden as
green infrastructure. The fundamental of green infrastructure including raingarden is described as
replicating the natural hydrology and water balance to be achieved by measures i.e., infiltration,evapotranspiration, capture and reuse of rainwater. Then it is assumed that the raingarden
technique mainly focused on infiltration to be achieved with a partially replacing soils in landscapes
is easier acceptable measure to manage the extreme rainwater.
To develop an implementing process of rain garden, two tasks were provided: 1, selection of soils tobe suitable for rain garden; 2, verification of infiltration capacity of rain garden structure to be
proposed by mock-up test.
The characteristics of two soils to be selected for rain garden deliver the possibility of use for rain
garden as follows.
a) Conditioned waterworks sludge:
Suitable for rain garden surface layer with vegetation planted areas.
b) Crushed clinker ash:Material that can be used for rain garden without plants or as a drainage layer.
The observation result and data based on the mockup test demonstrated that the rain garden
structure of which the double-layer consisted of two selected soils provides better performance thanthat of single layer of masa-do (decomposed granite soil)” used as well drained material, like a sandy
medium, for landscape in the western region of Japan.

Keywords: Extreme rainfall event, Rainwater management, Reduction of rainwaterrunoff, Infiltration capacity of soil, Rain Garden, Selection of soil, Mock-up
test−71−㻙 㻝㻌 㻙Fujita Technical Research Report§1. Introduction

No.57

soils in landscapes, and hence applicable in both ofexisting planted area and newly landscaping area.

Global climate change brings multiple effects on

Therefore, in this research aimed to develop anthe global and local environment, society, andimplementing process of rain garden, two tasks

economy. An excessive imbalance of rainfall eventswere provided: 1, selection of soils to be suitable foris counted among the influences of climate change ofrain garden; 2, verification of infiltration capacity ofwhich it could be felt in the normal days in life.

rain garden structure to be proposed by mock-up

Basically, Japan as the eastern end of Asiantest.

monsoon has a plenty annual precipitation of which§2. Selection of soils to be suitable for rain garden

corresponds to be twofold of global average (880mm/year). While at the same time, the trends ofextreme rainfall have been clearly observed in aThe following two soil properties were assumed

climate monitoring data in Japan. In long term

prior to seeking the soils to be suitable for rain

aggregated data as 120 years (1901-2020), thegarden structure i.e.

annual number of days with rainfall more than 100

1, Porous medium with coarse-textured soil to1).

allow high infiltration rate and adequate water

Whereas the annual number of days with rainfall

retention required for plant growth, thus sandy

more than 1 mm have significantly decreased. Suchmedium may be excludable on this point

mm and 200 mm have significantly increased

changes in rainfall events being extreme should
makedifficulty

tocontrol

rainwater

2, Medium to maintain an infiltration capacity

runoff,againstcompacting

pressure

likethesandy

especially in the urbanized region with large area

mediums which are not necessarily negligible forcovered by impervious surface.

rain garden use

In recent years, a soil- and vegetation-based

Based on these assumptions, two materials of ameasures to manage extreme rainfall have been

conditioned waterworks sludge and a crushedadvocating in Japan. This kind of approach called asclinker ash were selected among the commercial

the green infrastructure could be expected as

products referring to information on the brochures

cost-effective, sustainable, and environmentallyand websites.

friendlymeasure.

green

Generally, the dewatered waterworks sludge

infrastructures should be defined as and built as

originated from suspended solids in river water,

mimicking or restoring of natural hydrologic cycle ofhave a good water and air permeability, because ofwhich

evapotranspiration,

its soil mass is hard to unstiffen via water resistance.

capture and reuse of rainwater. Under present

Then water retentivity of dehydrated sludge could

situation in Japan, the green infrastructuresbe ameliorable through an appropriate fracturing

implementation is presumably in early stage toand size controlling process. Such a conditioned

widespread use. Whereas under the condition ofwaterworks sludge had been picked up as thewhich the extreme rainfall and its threat are coming

possible material to be used to rain garden.

includesBecauseinfiltrate,ofthefriendly

Meanwhile, the other recycled material, crushed

measures are required to be developed and become

clinker ash has known as the porous medium withwidely used.

lower density than that of sand. Its lightweight anduptosurface,

theenvironmentally

Then a raingarden technique might be easiersimilar particle size distribution with sand canacceptable measure to afford an extensive managing

produce high infiltration rate and sand-replacing

extreme rainwater, because of the raingarden

usability for raingarden structure.
Thencross-sectional structure with improved infiltration

next,

therelationshipbetweenthecompaction energy and the saturated hydrauliccapacity would be installed as a partially replacing
2−72−Experimentally based designing of Rain Garden

conductivity were investigated for two media toa) Conditioned waterworks sludge:

determine the adequateness as rain garden soil. A

Suitable for rain garden surface layer withgiven amount of soil was put into plastic cylinder

vegetation planted areas.

(inner diameter 50 mm) followed by a compaction

Permeability like sand, but it must be noted thatwith dropping of 2.5 kg rammer on the filled soil.

compaction

The compaction energy was stipulated by the

Suitable for planting because of its high organic

number of weight (water filled bottle or rammer)

matter content bringing high fertilizer retention

dropping. Then the saturated hydraulic conductivity

capacity.

of filled soil was measured as the permeabilityb) Crushed clinker ash: Material that can be used forunder condition of constant water head after waterrain garden without plants or as a drainage layer.

saturation of soil supplying from bottom of cylinder.

Same level of permeability as sand, and it can maintainFig.1 shows measurement results of the saturated

nocompaction

markedly.

Not suitable as a planting area due to its high pH andThe permeability of conditioned waterworkswithpermeability

permeability under increasing of compaction pressure.

hydraulic conductivity of each soil.
sludgereducescorresponded

low level of organic matter.

toapproximately to that of sand then decreased largely§3. Verification of infiltration capacity of rain

in response to the increasing compaction energy.

garden structure to be proposed by mock-up test

Whereas The permeability of crushed clinker ash
with no compaction was lower about 60% level of

As described above, the rain garden mock-up test

conditioned waterworks sludge, however it could

had been built to verify the infiltration capacity ofmaintain relatively higher permeability comparison

rain garden structure consisted of two soils aswith the trend of conditioned waterworks sludge.

double layer in which the conditioned waterworks

The characteristics of these two soils describedsludge and the crushed clinker ash were used asabove and description in the brochure deliver theupper and lower layer respectively. In this test, thepossibility of use for rain garden structure as

infiltration capacity of double layer was compared tofollows.

that of single layer with “masa-do (decomposedFigure 1. Changes of water permeability under different compaction conditions
1: Not compacted with weight (10 times tapping), 2: 35 kJ/m3, 3: 140kJ/m3, 4: 190 kJ/m3

㻙 㻟㻌 㻙−73−

Fujita Technical Research Report No.57

granite soil)” used as well drained material, like adouble soil layer using selected two soils, conditioned

sandy medium, for landscape in the western region

waterworks sludge for upper layer of 30 mm and crushedof Japan.

clinker ash for lower layer of 200 mm respectively, wasinserted into the left side of soil tank, and its surface wasshaped like groove with 100 mm lower level to collect

3.1 Construction of rain garden mock-up
The mock-up of rain garden was installed as lysimeter

rainwater runoff from left area of soil tank surface.

facility to assess the infiltration capacity of cross-sectional

Then other volume of that soil tank out of rain garden

soil structure built in it. More specifically, two rectangularstructure was filled with masa-do as reference soil

soil tanks of 28 m3 volume (Inside dimension: Wide 2 m,(hereafter named as “rain garden section”). On the otherLength 5 m, Height 1.4 m) soil tanks with drainage pipe

hand, non-rain garden soil tank was filled using masa-doon the bottom surface were constructed using a concrete

only that had the same shape groove as the rain garden

brick. Fig.2 illustrates the cross section of one soil tanksection (hereafter named as “non-rain garden section).

with rain garden structure. That structure consisted ofFigure 2. Mock-up of rain garden㻙 㻠㻌 㻙−74−

Experimentally based designing of Rain Garden

Figure 3. observation of puddle formation on the surface of soil
Up-pointing open triangles indicates continuous puddle formation on the non-rain garden section withina period between the triangles. Whereas down-pointing closed triangles indicates intermissive puddle
formation on the non-rain garden section within a period between the triangles. Red doted box indicatesperiod at a rainfall intensity of corresponding 37 mm/hr. Two photographs right side of graph were taken atthat point, upper; rain garden section without puddle appearance, lower; non-rain garden section.
3.2 Results of RG mockup test (1): Under natural rainfall

shaped area of each section with 5 minutes

conditioninterval.

To verify the ability to supress of the rainwater

The observation under cumulative precipitation

runoff on the rain garden section by comparison

134 mm within 14 hours provided that the firstwith the non-rain garden section, an observation of

puddle formation on the non-rain garden section

puddle formation on the surface of soil wasappeared at cumulative precipitation of 3.5 mm.

conducted under natural rainfall event. The

Whereas on the rain garden section, the puddle

time-lapse

SANWA

formation was not observed until cumulativeSUPPLY Co. Japan) were placed at the upper chest

precipitation reached 33 mm. The puddle on thewall of soil tank to take a picture of groove-like

non-rain garden section had appeared continuously

cameras(400-CAM066,

Figure 4. Comparison of leaching rate from drainpipe between the rain garden section and non-rain garden
section ock-up of rain garden㻙 㻡㻌 㻙−75−

Fujita Technical Research Report No.57

until rain stopped, whereas that on the rain garden

The efforts of the people involved in the issue ofsection

toclimate change, will maintain lowering the risks

disappearance within a period of 30 minutes

including derived by extreme rainfall event. The(Fig.3).

implementation of the green infrastructure, one ofwasintermissiveandturnedsuch trials, could provide the countermeasures
based on the natural hydrology and water balance.

3.3 Results of RG mockup test (2): Under artificial
rainfall condition
The above observation result of puddle formation on theReferences

rain garden section was directly evident reducing[1] Japan Meteorological Agency: Climate change

rainwater runoff in comparison with that on the non-rain

monitoring report 2020, 2021 (in Japanese)garden section. Whereas to assess an infiltration capacity[2] Yaneshita M. et al., Physical property evaluation ofof rain garden structure, it is relatively difficult to produce

artificial lightweight soil under different compacting

a quantifiable evaluation on given soil layer underconditions. J. Jpn. Soc. Reveget. Tech., 44 (1) ， pp.

natural rainfall condition due to its instable intensity of

225-228，2018 (in Japanese).

rainfall.

[3] Uto Y. et al., Relation between soil-water

Therefore, the infiltration capacity of soil layer on eachcharacteristic curve and soil compaction curve. J.

section had been measured under an artificial rainfall

Applied Mechanics, 22, pp. 519-524, 2009 (in Japanese)condition. The self-made artificial rainfall equipment was[4] Ezaki K, Concept, and specialty on designing for

installed at near side of groove shaped area of eachlysimeter facility, Bulletin of the Faculty of Agriculture,

section, and drainage water from pipe laid on the bottom

Meiji University, pp.13-14, 2005 (in Japanese)of soil tank was collected into a plastic container. Then
Short comment

amount of drained water was measured as water rising

The rain garden is way to address rainwater runoff as ausing the level sensor (FL-001, KEYENCE Co., JAPAN)

sheet flow to be infiltrated into soil,

to be hold in the container.

unlike by a conventional piping andFig. 4 shows comparison of leaching rate fromconveying. Thus, in the first phase ofdrainpipe between the rain garden section and non-rain

rain garden planning that a developing

garden section. The doble-layer consisted of two soils

area should be partitioned into theto be selected for rain garden section had provided

watersheds based on a rational manner.

approximately 6-fold higher leaching rate than that ofNobuyuki

single-layer of masa-do under this artificial rainfall test.

Kitajima§4. Overview
The observation result and data based on the mockup
test to be referred to above descriptions. The rain garden
structure of which the double-layer consisted of two
selected soils provides better performance than that ofsingle layer of masa-do (decomposed granite soil)” used aswell drained material, like a sandy medium, for landscapein the western region of Japan. This means the rain
garden structure to be proposed based on theexperimental results of this approach demonstrated highability to reduce rainwater runoff.

㻙 㻢㻌 㻙−76−

Then experimentally based designing ofrain

gardenshouldbeideally

conjugated with a landscape design.