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Infiltration and Retention

In storm water management, the findings asserts that rainwater should be infiltrated at the place of access instead of discharged through sewer systems. If this is not possible, then in several cases, the temporary storage (retention or buffering) of rainwater is required in storage volumes in order to protect the drainage systems from overloading and to limit their dimension.

Basic principles

Quality of rainwater runoff

The runoff from paved surfaces are divided into the categories of non-hazardous, tolerable and intolerable in terms of their material concentration and thereby possibly associated potential hazards to groundwater in targeted rainwater infiltration.

Non-hazardous rainwater runoff

Non-hazardous rainwater runoff can be infiltrated (such as in infiltration trenches) directly without pre-treatment measures via unsaturated zone (below the root zone and above the groundwater level).

Tolerable rainwater runoff

Tolerable rainwater runoff can be infiltrated through the unsaturated zone according to suitable pretreatment or using the cleaning processes (sedimentation system, rainwater cisterns, covered areas, etc.).

Intolerable rainwater runoff

Intolerable rainwater runoff can only be infiltrated by a pretreatment.

Surface / zone Qualitative evaluation
Green roofs, lawns and cultivated land; roof surfaces without the use of uncoated metals (copper, zinc and lead), terrace surfaces in residential and commercial areas Non-hazardous
Roofs with usual proportions of uncoated metals, cycle tracks and footpaths in residential areas, traffic-calmed zones, courtyard areas and car parking spaces without frequent change of vehicles, as well as less used road zones (up to DTV 300 vehicles); streets with DTV 300 - 5,000 vehicles, such as residential street, building land and district roads, runways of airports; roofs in commercial and industrial areas with significant air pollution, see DWA A138. tolerable
Courtyard areas and street in commercial and industrial areas with significant air pollution; for special zones, refer DWA intolerable

Source DWA-A138, DTV = average daily traffic intensity

Options to clean rainwater

The load of underground and surface water can be considered qualitatively and quantitatively from rainwater from roofs and traffic zones by using simple evaluation process (ATV DVWK-M153). Depending on the result, various measures for treating rainwater must be grasped to ensure an adequate level of protection.

It must be additionally protected by a coarse filter while discharging into a tunnel.

Important: at rainwater harvesting tanks

According to DIN 1989-1, the underground infiltrations systems (infiltration trenches) can be equalized regarding the qualitative aspects with infiltration through a ground zone, provided the inflow water is derived from a rainwater harvesting system at non-metallic roof.

Sedimentation and filter chambers, sedimentation systems

Sedimentation and filter chambers

Systems with a sedimentation chamber should allow such flow conditions that particularly, materials heavier than water sink to the bottom and lighter materials, float, this is designated as sedimentation system.

The collection and filter chambers are included in the sedimentation area in which the heavy particles settle, and a filter, which prevents light coarse contaminants from entering the downstream tank. Even light materials are retained in the chamber via an immersion pipe. Depending on the dirt, it must be cleaned regularly. The whole water discharged from roof is filtered and supplied to the tank. In Germany the chambers are designed in accordance with ATV DVWK M153, corresponding to the expected amount of dirt and connected roof area.

Ground passages

Ground passages

The contaminants from flowing rainwater is retained and stored or degraded by physical, chemical and if necessary, biological process in the passage of ground layers, as with basin-trench systems or lawn grids. Thus a passage is more effective by overgrown topsoil than by unplanted soil zone. The top layer protecting groundwater must not be permeated.

Flushable and camera-accesible tunnels

In case pollutants enter tunnel despite preliminary cleaning, it is very important that cleaning must be possible later. In several trenches, such as with plastic boxes, mostly only the flushing channel can be cleaned later. However, the fine dirt particles enter through the slots of flushing channel and then gradually clogs the floors and walls of such trenches. This may ultimately be completely excavated only if their infiltration capacity is lost. In DRAINMAX tunnel trenches, the critical walls and floors can be inspected and completely rinsed with a camera from connection slots. The pollutants then enter the coarse filter of the sedimentation and filter slot or settles in the sedimentation area. The coarse filter can be removed and emptied after flushing. The parallel tunnel series are additionally protected by long sedimentation links in perforated pope and additional sedimentation options in inspection and flushing slots. Therefore the consistent infiltration capacity is guaranteed in the long term.

Ground consistency

Infiltration capacity of ground

Overview of Kf values of different grounds

The consistency of underground is crucial for infiltration of rainwater. The permeability coefficient (kf value) is a measure of permeability of soil. A permeability coefficient should be between 10-3 and 10-6 in order to ensure proper functioning of the drainage system.

In order to avoid over-sizing of the system, the kf-value can be determined as accurately as possible through research. There are professional soil experts.

Brief test for ground consistency

If the kf-value is unknown, then an approximate possibility of underground infiltration can be located based on the following short test.

trial pit
  1. A 50 x 50 cm wide and approx. 30 cm deep pit is dug. Note: Do not enter the pit to avoid compression!
  2. Um ein Aufschwemmen des Bodens zu verhindern, wird er mit einer Kiesschicht abgedeckt. Ein Messstab wird in den Boden geschlagen. 10 cm oberhalb der Grubensohle wird eine Markierung am Messstab angebracht.
  3. Now the pit is filled with water and supplied water every 1-2 hours by regular replenishment (garden hose).
  4. Now water is filled up to the mark. After 10 minutes, it is filled with as much water as necessary to reach the mark again using a measuring bucket. The permeability of ground can be estimated from the quantity of refilled water.
  5. Repeat step 4 as many times (at least 3 times), until a constant value is established.

Assessment: water quantity < 1.5 liters in 10 minutes: infiltration hardly possible (silt)
Water quantity = 1.5 liters in 10 minutes: infiltration possible (silty sand)
water quantity > 3 in 10 minutes: Good possibility of infiltration (sand, gravel)

Installation of an infiltration system

  • Distance to MHGW (mean highest groundwater level) from the bottom of the system: > 1 m
  • soil permeability > 1 x 10-6 (with even worse values: see retention)
  • soil permeability < 1 x 10-3 (with higher permeability low cleaning)
  • Distance to the basement floor is at least 1.5 x h

Tunnel infiltration with DRAINMAX Tunnel

Tunnel infiltration with DRAINMAX Tunnel

1. DRAINMAX Tunnel 5. Top layer
2. lateral and top tunnel backfilling 6. Sedimentation/Filter pit
3. Geotextile fabric 7. Rainwater inlet
4. Tunnel cover

Basin trench infiltration with DRAINMAX Tunnel

Basin trench infiltration with DRAINMAX Tunnel

1. DRAINMAX Tunnel 6. Infiltration basin
2. lateral and top tunnel backfilling 7. Rainwater inlet
3. Geotextile fabric 8. Groundwater distance
4. Tunnel cover 9. active ground zone
5. Top layer 10. maximum water level

DRAINMAX Tunnel System for commercial object

DRAINMAX Tunnel System for commercial object

1. DRAINMAX Tunnel 7. 7. Sedimentation/Filter pit
2. lateral and top tunnel backfilling 8. Flushing pit
3. Geotextile fabric 9. Rainwater inflow
4. Tunnel cover 10. Groundwater distance
5. Top layer 11. Geo composite support
6. Rainwater distribution

Installation of retention system

Retention volume

Retention tank with throttle discharge
Retention tank with throttle discharge and useable volume

There are several options for the retention of rainwater:

  • Tank with pure retention and throttle discharge
  • Tank with combined retention and usage and throttle discharge


The combination of rainwater harvesting and rain water retention in a tank is particularly interesting for smaller systems in single family dwelling areas since the costs for excavation and delivery incur only once and the tank is not much expensive.

  • Retention with approved partial infiltration and throttle discharge


With approved partial infiltration, the DRAINMAX system with tunnel elements is an extremely interesting alternative. The low height displacement between the inlet and outlet in combination with great flexibility and a very high spatial storage volume are the advantages of this model. If water must not enter from system into the surrounding soil, then it can be sealed with an EPDM sheet.

DRAINMAX tunnel system

1. DRAINMAX Tunnel 6. Top layer
2. lateral and top tunnel backfilling 7. Sedimentation/Filter pit
3. Geotextile fabric 8. Throttle slot
4. Sheet trough made of EPDM and Geotextile fabric 9. Discharge throttle
5. Tunnel cover 10. Rainwater inflow

Throttle outflow

continuous throttle

With a retention system, the water is routed via a throttled volume flow to the drainage system. The throttle flow corresponds to the admitted outflow of the sealed area into the drainage system. Usually this outflow corresponds to the natural outflow before sealing.

The permitted throttle outflow is routed to the downstream drainage system either with a lifting pump or directly via the throttle outflow if the height difference allows.

In comparison to fixed throttles, continuous throttles ensure that the maximum permitted quantity of water is discharged irrespective of the water level. Thus, the retention tank can be designed up to about 30 % smaller than fixed discharge throttle.

Dimensioning of infiltration and retention systems

which see online planner

Rainwater discharge

The calculation of the rainwater discharge is based on the knowledge that heavy rains last for short duration and low rains persist longer. The rain yield factor declines with similar statistical frequency for increasing rainfall duration. The relationship between rain yield factor, duration of rainfall and frequency is determined by the statistical analysis of rainfall registrations. Usually the simple calculation methods in accord with DWA-A 117 are used in Germany. Therefore a statistical rain with selected duration D and frequency n should be used as load case for calculation. The "amount of heavy rainfall for Germany - KOSTRA" (see sample table for Aachen) is to be referred for determining the rain yield factor.

Rain duration D r D(1) l/(s*ha) r D(0,2) l/(s *ha)
5 min 135,0 243,0
10 min 113,0 183,9
15 min 97,2 152,6
20 min 85,3 131,7
30 min 69,5 104,8
45 min 52,9 81,2
60 min 43,1 66,8
90 min 32,3 49,7
2 h 26,4 40,3
3 h 19,8 29,9
4 h 16,1 24,3
6 h 12,1 18,0
9 h 9,1 13,4
12 h 7,4 10,9
18 h 5,4 7,9
24 h 4,3 6,5
48 h 2,6 3,7
72 h 2,1 2,9

Kostra data Aachen

Inflow to infiltration and retention systems

Qzu = 10-7 x rD(n) x Ared
Qzu = Inflow to infiltration system in m³/s
rD(n) = Rain yield factor for duration D and frequency n [l/sha]
Ared = connected paved area in m²

Discharge from infiltration system

The law of Darcy will be used to calculate the discharge from an infiltration system:

Qs = (b+0,5h) x L x ½ x ksub>f</sub>
kf = Permeability coefficiant of saturated base in m/s
b = Bottom width of trench in m
h = Height of trench in m
L = Length of trench in m

Discharge from retention system

Qs = QD
QD = Throttle discharge in case of retention system

Continuity condition

Verf = L x b x h x sRR = (∑Qzu - ∑Qs) x D x 60
Verf = required storage volume in m³
D = Rain duration in min

Infiltration: If only the formulas 1 and 2. a are used in formula 3 and resolved as per L, then this results in significant length of trench and trench volume.

RWV Formel.jpg

Retention:Here, only the forumal 1, and 2 b are used in formula 3. Ver = (∑Qzu - ∑Qs) x D x 60 The significant rain yield factor r D(n) of duration D and frequency n [l/sha] must be iteratively determined.

Frequency of overflow

For statistical determination of discharge of rainwater, the likely frequency of rain yield factor line is crucial. This value depends on the economic importance of the area and is related to the frequency with which the proposed system is congested.

Frequency of dimensioning system (once in n years) Location
1 in 1 Rural area
1 in 2 Residential area
1 in 2 City centres, industrial and commercial areas with overflow assessment
1 in 5 City centres, industrial and commercial areas without overflow assessment
1 in 10 Underground traffic infrastructure, subway

Source: ATV A118

Sample calculations on infiltration with DRAINMAX Tunnel

a. with value D=15min and n=0.2 = Exampple for various regions abroad

Location: Aachen
Ared = 100 m²
Measured rainfall: r15,n=0,2 = 152,6 l/(s*ha)
kf = 1*10-4 m/s (Medium sand)
srr = 0,56 (DRAINMAX Tunnel installation as per German Institute for Civil Engineering)
fz = 1,1

Sample calculation for INTEWA DRAINMAX Tunnel in grit block:

B = 1,85 m, H = 1 m, L = 2,25 m
Lerf,rigole = 1,31 m
Verf,rigole = 1,36 m³ (= B x H x Lerf,rigole x srr = 1,85 m x 1 m x 1,31 m x 0,56)
Required number of DRAINMAX Tunnel: LLerf,rigole / L = 0,82

b. With Iteration = Example for Germany

Location: Aachen
Ared = 100 m²
Measured rainfall: r15,n=0,2 = 152,6 l/(s*ha)
kf = 1*10-4 m/s (Medium sand)
Duration D [min] Rain yield factor r [l/sha] Lerf,rigole [m] Verf,rigole [m³]
5 243,00 0,75 0,77
10 183,90 1,09 1,13
15 152,60 1,31 1,36
20 131,70 1,46 1,51
30 104,80 1,64 1,69
45 81,20 1,74 1,80
60 66,80 1,76 1,83
90 49,70 1,70 1,76
120 40,30 1,62 1,68
180 29,90 1,46 1,51
240 24,30 1,33 1,38
360 18,00 1,12 1,16
540 13,40 0,91 0,95
720 10,90 0,78 0,81
1080 7,90 0,60 0,62
1440 6,50 0,51 0,52
2880 3,70 0,3 0,31
4320 2,90 0,24 0,25

c. Table for rough estimation of small systems with r15,n=0,2

kf (m/s) For example, Aachen (D) site r15,0,2=152,6 l/(s*ha) For example, Berlin (D) site r15,0,2=213,1 l/(s*ha)
A=100 m2 A=150 m2 A=200 m2 A=100 m2 A=150 m2</sub> A=200 m2
1*10-4 Volumen in m3 1,36 2,04 2,72 1,90 2,85 3,79
1*10-5 Volumen in m3 1,49 2,24 2,99 2,09 3,13 4,79
1*10-6 Volumen in m3 1,51 2,26 3,02 2,11 3,16 4,21

Rough estimation of retention volume

The following method of calculation can be used for a rough estimation of required retention volume with specified duration of rain.

Sample calculation:

Permitted discharge from land: 1,5 l/s x ha
Size of land: 0,105 ha
Rain yield factor r15(1) = 108 l/s x ha
Rain yield factor r15(2) = 193 l/s x ha
Surface x Run-off coefficient x Rain yield factor = Qr15(2)
231 m2 x 1 x 0,0193 l/s x m2 = 4,46 l/s
114 m2 x 0,8 x 0,0193 l/s x m2 = 1,76 l/s
Total rainwater discharge Qrges = 6,22 l/s
Zulässige Approved discharge quantity Qab = 0,105 ha (land size) x 1,5 l/s x ha
= 0,158 l/s
Rainwater quantity for retention: Qs = Qr15(0,2)ges - Qab
= 6,22 l/s – 0,158 l/s = 6,06 l/s
Required backed-up volume Note: (The retention system must accommodate Qs for 15 Min.).
Verf = Qs x 60 x 15 = Qs x 900
= 6,06 l/s x 900 s = 5,5 m3

Exact calculation of a tunnel or retention system with designing software

Since calculation of the required tunnel volume is done iteratively, it is most appropriate to use a designing software such as RAINPLANER.

Calculating surface infiltration

As = Ared / ( kr x sf x 107 / 2 x r D(n) –1)
Ared = connected paved surface
sf = Joints of permeable paved area (0 < sf =< 1)
kr = Permeability coefficient in considered infiltration level
rD(n) = crucial rain yield factor
Beispiel:
Ared = 300 m2
sf = 1 (INTEWA Rasengitterplatten)
kr = 2 x 10-4 m/s
r D(n) = aus KOSTRA Tabelle bei n=0,2/a und D=10 min: r10(0,2) = 204,60l/s ha
As = 300 / ( 2 x 10-4 x 1 x 107 / 2 x 204,6 –1) = 77 m2

Dimensioning infiltration tunnels behind small sewage water systems

According to DIN 4261-1, version 2002, the water discharged from small sewage water plants can be infiltrated through trenches at ground with kf = 5 x 10-7 to 5 x 10-3 m/s. As the base of infiltration system gets loaded with time, only the side areas remain effective in the long run. A large retention volume is an advantage for variable infiltration efficiency such as to buffer during frost or uneven loading of the trench. The below mentioned simplified dimensioning methods shall apply according to DIN:

Trenches behind small waste water treatment plants
Required wall area(m2/single family dwelling value PE):
1 m2 / PE bis 1,5 m2 / PE with: Sand-grit mixture, sand, light silty sand
2 m2 / PE to 2,5 m2 / PE to: Silt (even slightly clayey), Sand-silt mixture, stone clay mixture
Required number in the example of DRAINMAX Tunnel:
Ground element 2,25 m length x 0,8 m Height x 2 Sides
As = 3,6 m2 each tunnel without front walls
PE to 1,5 m²/PE to 2,5 m²/PE
4 2 pcs. 3 pcs.
8 4 pcs. 6 pcs.
12 5 pcs. 9 pcs.
16 7 pcs. 12 pcs.

A specified calculation must be done with other floor ratios and higher PE values.

Legal framework conditions in Germany

The current versions of following regulations must be considered while planning and installation of an infiltration or retention system:

Regelungsbereich Regelwerk Inhalt
Wasserversorgung Arbeitsblatt DWA-A 138 Planung, Bau und Betrieb von Anlagen zur Versickerung von Niederschlagswasser
ATV-DVWK-M 153 Handlungsempfehlungen zum Umgang mit Regenwasser
ATV-A 121 örtliche Niederschlag / Starkregenauswertung nach Wiederkehrzeit und Dauer
DWA-A 117 Bemessung von Regenrückhalteräumen
Kostra Starkniederschlagshöhen für Deutschland
DIN 4261-1,Kapitel 9 Kleinkläranlagen, Verbringung von biologisch behandeltem Abwasser in den Untergrund
EN 752 Entwässerung außerhalb von Gebäuden...
ATV A 118 Hydraulische Bemessung und Nachweis von Entwässerungssystemen
ATV A 118 Richtlinien für die Bemessung von Regenentlastungsanlagen in Mischwasserkanälen

Obligations to notify and obtain licens

Regelungsbereich Regelwerk Inhalt
EU-Recht EG-Richtlinie 76/464/EWG / 1976

EG-Richtlinie 80/68/EWG / 1979 || Verschmutzung infolge der Ableitung bestimmter gefährlicher Stoffe in die Gewässer der Gemeinschaft Schutz des Grundwassers gegen Verschmutzung durch bestimmte gefährliche Stoffe

Bundesrecht Wasserhaushaltsgesetz WHG Versickerungsanlagen sind nach dem WHG erlaubnispflichtig, die Länder können seit 1996 die Erlaubnispflicht aufheben, Grundwasserverordnung
BauGB Baugesetzbuch
Landesrecht Landesbauordnung Angabe der Systemart und Größe im Bauantrag, die meisten Landesbauordnungen fördern oder verlangen die dezentrale Niederschlagswasserversickerung inzwischen
AVBWasserV §3 Antrag auf Teilbefreiung vom Anschluss- und Benutzungszwang an die öffentliche Abwasseranlage Anzeigepflicht vor Errichtung der Anlage beim kommunalen Wasserversorger
Landeswassergesetz evtl. Pflicht zur Versickerung von Niederschlagswasser
Landeswassergesetz evtl. Erlaubnis der unteren Wasserbehörde bei Versickerung
kommunale Abwassersatzung evtl. Antrag auf Teilbefreiung vom Anschluss- und Benutzungszwang beim kommunalen Wasserentsorger

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