Versickerung und Retention/en

Infiltration and retention

In water management, experience asserts that rainwater should be infiltrated at the place where it accumulates. If this is not possible, then in many cases the temporary storage (attenuation or retention) of rainwater is required in attenuation volumes in order to protect the drainage systems from overloading and to limit their dimension.

Advantages of rainwater infiltration

For end users:

• sealed surface fees are saved
• the local microclimate is improved

For municipalities:

• lower costs for flood protection / flood prevention
• lower costs for sewer construction, sewer rehabilitation and sewage plant operation
• lower connection costs for new developments
• protection of the groundwater supply

Advantages of rainwater retention:

• limiting area discharge, reducing flood risk
• lower costs in sewer construction, sewer rehabilitation and sewage plant operation
• connection of new developments to existing, full-capacity drainage systems
• relief of overloaded sewer networks

Basic principles

Quality of rainwater runoff

The runoff from paved surfaces are classified into the categories of non-hazardous, tolerable and intolerable according to 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 (e.g. in trenches) without pretreatment measures through the unsaturated zone (below the root zone and above the groundwater level).

Tolerable rainwater runoff

Tolerable rainwater runoff can be infiltrated through the unsaturated zone after suitable pretreatment or using cleaning processes (sedimentation system, rainwater cisterns, overgrown soil, etc.).

Intolerable rainwater runoff

Intolerable rainwater runoff can only be infiltrated after pretreatment.

Surface / Area	Qualitative evaluation
Green roofs, fields and cultivated land; roof surfaces without the use of uncoated metals (copper, zinc and lead), terrace surfaces in residential and similar commercial areas	non-hazardous
Roofs with usual proportions of uncoated metals, cycle tracks and footpaths in residential areas, calm traffic areas; lawns and car parking without frequent vehicle changes; as well as lightly used vehicle areas (up to DTV 300 vehicles); streets with DTV 300 - 5,000 vehicles, e.g. access roads, residential and district streets; airport tarmac; roofs in commercial and industrial areas with significant air pollution, see DWA-A138.	tolerable
Lawns and streets in commercial and industrial areas with significant air pollution; for special zones see DWA	intolerable

Source DWA-A138, DTV = average daily traffic intensity

Soil composition

Infiltration capacity of the soil

Overview of different kf-values for soils

The underground composition is of crucial importance for rainwater infiltration. The permeability coefficient (kf-value) is a measure of water permeability of the soil. The permeability coefficient should be between 10^-3 and 10^-6 in order to ensure the functionality of the infiltration system.

In order to avoid over-dimensioning of the system, the kf-value should be determined as accurately as possible through investigation. There are professional geotechnical experts for this purpose.

Quick test for soil composition

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

Test pit

1. Dig a 50 x 50 cm wide and approx. 30 cm deep pit. Important: Do not enter the pit to avoid compression!
2. Cover the soil with a gravel layer to prevent soil flotation. Insert a measuring rod into the ground. 10 cm above the pit bottom place a mark on the measuring rod.
3. Now fill the pit with water and replenish for 1-2 hours regularly (e.g. garden hose).
4. Fill water up to the mark. After 10 minutes, fill as much water as necessary to raise the water level back to the mark using a measuring bucket. The soil permeability can be estimated from the quantity of refilled water.
5. Repeat step 4 as many times (at least 3 times), until a consistent value is established.

Evaluation: Water quantity < 1.5 litres in 10 minutes: little infiltration possible (silt)
Water quantity = 1.5 litres in 10 minutes: infiltration possible (silty sand)
Water quantity > 3 litres in 10 minutes: good infiltration possible (sand, gravel)

Cleaning options for precipitation water

The contamination of underground and surface water from rainwater from roofs and traffic areas can be considered qualitatively and quantitatively by using simple assessment procedures (ATV DVWK-M153). Depending on the result, various measures for handling rainwater must be taken to ensure adequate water protection.

For discharge into a trench, minimum protection requires coarse filtration.

Important: with rainwater harvesting cisterns

According to DIN 1989-1, underground infiltrations systems (trenches) are equivalent to infiltration systems in active soil areas in terms of qualitative aspects, provided the inlet water comes from a rainwater harvesting system with non-metallic roof areas.

Sedimentation and filter chamber, sedimentation systems

Sedimentation and filter chamber

Systems with a settling chamber in which the flow conditions allow specific substances heavier than water sink and specific lighter substances float are referred to as sedimentation systems.

Collection and filter chambers consist of a sedimentation area in which heavy particles settle and a filter that prevents light coarse contaminants from entering the downstream storage. Even light materials are retained in the chamber with an immersion pipe. Depending on the amount of dirt, they must be cleaned regularly. The total 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.

Soil passages

Soil passages

Contaminants from flowing rainwater are retained and stored or degraded by physical, chemical and if necessary, biological processes with passage through soil layers or in trough-trench systems or unsealed surfaces such as grass pavers. Thus passage through overgrown topsoil is more effective than through a non-vegetated soil zone. The protective cover layers over groundwater must not be penetrated.

Flushable and camera-accessible trenches

Should contaminants penetrate into the trench despite pre-cleaning, it is very important that subsequent cleaning is possible. In many trenches e.g. box systems, only the flushing ducts can be cleaned afterwards. However fine contaminates pass through the slots in the flushing ducts and gradually clog the floors and walls of these trenches. Ultimately these can only be dug out completely if they have lost their infiltration capacity. With DRAINMAX Tunnel trenches for example, the critical walls and floors can be inspected with a camera through adequate connection chambers and are completely flushable. Contaminants either are retained in the coarse filter of the sedimentation and filter chamber or settle in the sedimentation area. The coarse filter can be removed and emptied after the flushing process. The parallel rows of trenches are additionally protected by the long settling section in the seepage pipe and the additional settling possibility in the inspection and flushing chamber. This guarantees a constant infiltration performance long-term.

Construction of an infiltration system

1. Distance to MHGW (mean highest groundwater level) from the bottom of the system: > 1 m
2. Soil permeability > 1 x 10^-6 (with lower values see retention)
3. Soil permeability < 1 x 10^-3 (with higher permeability too little treatment)

Trench infiltration with DRAINMAX Tunnel

1. DRAINMAX Tunnel	5. Topsoil
2. Tunnel side and top backfill	6. Sedimentation/filter chamber
3. Geotextile	7. Rainwater inlet
4. Tunnel overburden

Trough-trench infiltration with DRAINMAX Tunnel

1. DRAINMAX Tunnel	6. Infiltration trough
2. Tunnel side and top backfil	7. Rainwater inlet
3. Geotextile	8. Distance to groundwater
4. Tunnel overburden	9. Active soil zone
5. Topsoil	10. Maximum water level

DRAINMAX Tunnel System for commercial properties

1. DRAINMAX Tunnel	7. Sedimentation/filter chamber
2. Tunnel side and top backfill	8. Flushing chamber
3. Geotextile	9. Rainwater inlet
4. Tunnel overburden	10. Maximum water level
5. Topsoil	11. Geotextile composite bottom layer
6. Rainwater distribution

Construction of a retention system

Retention volume

Retention cistern with throttle discharge

Retention cistern with throttle discharge and usable volume

There are several options for the retention of rainwater:

• Storage with pure retention and throttle discharge
• Storage with combined retention and use and throttle discharge

The combination of rainwater harvesting and rainwater retention in a cistern is particularly interesting for smaller systems for single-family homes since the costs for excavation and delivery are incurred only once and the cistern is not significantly more expensive.

• Retention with approved partial infiltration and throttle discharge

With permitted partial infiltration, the DRAINMAX system with tunnel elements is an extremely interesting alternative. The low height offset between inlet and outlet in combination with great space flexibility and a very high storage volume are the advantages of this variant. If no water is allowed to enter the surrounding soil from the system, it can be sealed with an EPDM foil on site.

1. DRAINMAX Tunnel	6. Topsoil
2. Tunnel side and top backfill	7. Sedimentation/filter chamber
3. Geotextile	8. Throttle chamber
4. Enclosed sheet basin made from EPDM and geotextile	9. Discharge throttle
5. Tunnel overburden	10. Rainwater inlet

Throttle discharge

In a retention system the water is supplied into the drainage system with a throttled flow rate. The throttle discharge corresponds to the permitted outflow of the sealed area connected to the drainage system. Most of the time this discharge corresponds to the natural flow before sealing the area.

In the retention system the permissible throttle discharge is either supplied to a downstream drainage system by means of a lift pump or through a discharge throttle provided the height conditions allow. According to DWA-A 117, the arithmetic average of the values of the throttle curve is to be set for uncontrolled throttling (fixed throttle/vortex throttle).

Compared to the vortex or fixed throttle, continuous throttles make sure that the maximum permitted water quantity Q drains constantly, irrespective of the impounding depth H. As a result, the retention tank with continuous throttle can be dimensioned by 10% to 30% smaller than with fixed throttle discharge or vortex throttles.

Fixed throttle

Vortex throttle

Continuous throttle

Exp. throttle curve for maximum admissible water quantity of 31 L/s

1. Fixed throttle (arithmetic mean = 21 L/s)

2. Vortex throttle (arithmetic mean = 21 L/s)

3. Continuous discharge throttle (31 L/s)

Fixed throttle

The simplest form of a fixed or static throttle is a simple flow restrictor. The discharge value Q of the fixed throttle depends on the hydrostatic pressure resulting from the impounding depth H.

Vortex throttle

A spiral stream of variable strength with a central rotating air core is formed by the tangential feed in the vortex throttle depending on the water level. However this does not lead to a continuous throttle outflow. The vortex throttle has the advantage of requiring less space and lower risk of blockages due to the larger remaining cross-section compared to the other throttle types. These advantages are rarely relevant with decentralized rainwater retention.

Continuous throttle

The outlet flow is constant with the continuous discharge throttle irrespective of the impounding depth H. The float adjusts the restrictor opening at the impounding depth by means of a lever arm. Coarse pre-cleaning of rainwater is necessary for the trouble-free operation of the throttle.

Calculation example of required storage volume

The crucial rain yield r_D(n), the duration D and frequency n [L/s-ha] must be determined iteratively (see Dimensioning of infiltration or retention systems).

Verf = ((Ared x rD(n) x 10-4) – Qdr ) x D x 60 x 10-3
Verf	= required storage volume in m³
Ared	= connected paved surfaces in m² (5,000 m² in example)
rD(n)	= crucial rainfall in L/s-ha (e.g. KOSTRA-Data Aachen, see Dimensioning of infiltration or retention systems)
Qdr	= discharge throttle value in L/s (the arithmetic mean of the throttle curve with non-continuous throttles, see diagram of throttle curves, 21 L/s in the example)
D	= Duration in min (in example, 30 min with the fixed throttle and vortex throttle, 20 min with the continuous throttle)
fixed throttle = vortex throttle: Verf = ((5,000 x 104.8 x 10-4) – 21) x 30 x 60 x 10-3 = 56.6 m³
continuous throttle: Verf = ((5,000 x 131.7 x 10-4) – 31) x 20 x 60 x 10-3 = 41.8 m³ (- 26 %)

The larger the permitted throttle outflow in relation to the connected areas, the greater the difference. This difference leads to correspondingly lower total costs for the retention system.

Dimensioning of infiltration or retention systems

Also see Online Planner

Rainwater runoff

The calculation of rainwater runoff is based on the knowledge that heavy rains last short durations and low rains persist for longer. The rainfall yield declines at the same statistical frequency with increasing rainfall duration. The relationship between rainfall yield, duration and frequency is determined by the statistical analysis of precipitation registrations. Simple calculation methods in Germany are used in accord with DWA-A 117. For this a statistical rainfall with selected duration D and frequency n should be used as load case for calculation. For the determination of rain yield refer to the "amount of heavy peak rainfall in Germany - KOSTRA" (see table for a sample location).

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 sample location

Inflow to infiltration or retention systems

Q_zu = 10^-7 x r_D(n) x A_red (1.)

Qzu	= Inflow to infiltration system in m³/s
rD(n)	= Rain yield for duration D and frequency n [L/s-ha]
Ared	= connected paved areas in m²

Discharge from the infiltration system

Darcy’s Law is used to calculate the discharge from an infiltration system:

Q_s = (b+0,5h) x L x ½ x k_f (2.a)

kf	= Permeability coefficient of the saturated soil in m/s
b	= Bottom width of the trench in m
h	= Height of the trench in m
L	= Length of the trench in m

Discharge from the retention system

Q_s = Q_D (2.b)

QD	= Throttle discharge in the case of a retention system

Continuity condition

V_erf = L x b x h x s_RR = (∑Q_zu - ∑Q_s) x D x 60 (3.)

Verf	= required storage volume in m³
D	= Rain duration in min

Infiltration: If only formulas 1. and 2.a are used for formula 3 to calculate L, then this will result in a significant trench length and volume.

$={\dfrac {A_{u}\cdot 10^{-7}\cdot r_{D(n)}}{{\tfrac {b_{R}\cdot h_{R}\cdot s_{RR}}{D\cdot 60\cdot f_{Z}}}+(b_{R}+{\frac {h_{R}}{2}})\cdot {\frac {k_{f}}{2}}}}$

S_RR = Storage coefficient of the trench

Retention: Here, only formulas 1. and 2.b are used in formula 3. V_er = (∑Q_zu - ∑Q_s) x D x 60 The significant rain yield rD(n) of duration D and frequency n [L/s-ha] must be iteratively determined.

Overflow frequency

For statistical determination of rainwater outflows, the assumed frequency of rain from the rain yield curve 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 centers, industrial and commercial areas with flood assessment
1 in 5	City centers, industrial and commercial areas without flood assessment
1 in 10	Underground traffic infrastructure, subways

Source: ATV A118

Flood verification DIN 1986-100:2016-09 (Germany)

Drainage systems for the discharge of precipitation water from small properties, so long as the sewer network provider has given no other guideline, without a more effective run-off surface, are sufficient for DN 150 connection to the sewer. The rule applies in the same sense to infiltration systems designed according to DWA-A 138 with T = 5 a and a dimensioning rainfall according to KOSTRA-DWD-2010. It is assumed that due to terrain condition and architectural building plans no backed-up water from the system will penetrate into the connected or neighbouring buildings and exceed any other official regulations.

Ground conduits from properties, according to DIN EN 752 from 200 ha A_ges i.e. from approximately 60 ha A_E,b, that drain larger, harmless flood-prone yard, park or other outdoor systems can be designed according to DWA-A 118:2006, Table 4. Here the yearly occurance of the dimensioning rainfall cannot be less once within two years.

Shortest normative rain period in relation to terrain slope and degree of sealing:

average terrain slope	Sealing	shortest rain period (according to this norm r2 in min)
< 1%	≤ 50%	15 min
	> 50%	10 min
1% bis 4%	-	10 min
> 4%	≤ 50%	10 min
	> 50%	5 min

Source: DWA-A-118:2006, Table 4

For the difference in the amount of rainwater accumulated on a sealed surface of a property, V_Rück in m³ (see Equation 20), between the minimum 30-year rain event and the 2-year dimensioning rain, proof of harmless flooding on the property must be provided. If an exceptional level of safety is required, the yearly occurance of the dimensioning rain is chosen as greater than 30 a. The harmless flooding can take place on the property, e.g. with curbs or troughs, or through other retention areas, like retention basins, if people, animals or material goods are not endangered, so long as the precipitation water is not discharged on other fields. The following flood verification for guidance is dependent on the local conditions and if necessary for parts of the drainage system (e.g. in the calming areas).

Equation 20

V_rück = ( r(D,30) x A_ges – ( r(D,2) x C_Dach + r(D,2) x A_FaG x C_FaG)) x D x 60 / (10,000 x 1000)

V_Rück the to-be retained rainwater amount, m³

r_(D,2) rain event with period D and 30-year return time

D die kürzeste maßgebende Regendauer, in Minuten, für die Bemessung der Entwässerung außerhalb der Gebäude nach DWA-A118, Tabelle 4, sonst D = 5 Minuten

C der Abflussbeiwert

A_Dach die gesamte Gebäudedachfläche, in m²

A_FaG die gesamte befestigte Fläche außerhalb der Gebäude, in m²

A_ges die gesamte befestigte Fläche des Grundstücks, in m², d. h. A_ges = A_Dach + A_FaG

Sind die Grundleitungen nach DWA-A118:2006, Tabelle 4, bemessen, so kann statt des Bemessungsabflusses der – meist größere – maximale Abfluss der Grundleitungen bei Vollfüllung Q_voll angesetzt werden nach Gleichung (21):

Gleichung 21

V_rück = (( r_(D,30)) x A_ges/ 1000)– Q_voll x D x 60/1000

V_Rück die zurückzuhaltende Regenwassermenge, in m³

D D = 5, 10 und 15 Minuten. Der größte dieser drei Werte für V_Rück ist maßgebend*

Q_voll max. Abfluss der Grundleitungen bei Vollfüllung in l/s

A_ges die gesamte befestigte Fläche des Grundstücks, in m², d. h. A_ges = A_Dach + A_FaG

Sollten die Regeneinzugsflächen des Grundstücks weitgehend aus Dachflächen und nicht schadlos überflutbaren Flächen (z. B. > 70 %, hierzu zählen auch Innenhöfe) bestehen, ist die Überflutungsprüfung in Verbindung mit der Notentwässerung für das 5-Minuten Regenereignis in 100 Jahren nachzuweisen (r_(5,100)).

Für den Fall der Begrenzung der Einleitung ist zusätzlich zum Überflutungsnachweis die Berechnung des erforderlichen Rückhaltevolumens (Regenrückhalteraum (RRR)) entsprechend DWA-A 117 mit dem „einfachen Verfahren“ durchzuführen. Hierbei wird vereinfachend vorausgesetzt, dass die Jährlichkeit T des Berechnungsregens (einheitlich bezogen auf die gesamte abflusswirksame Fläche des Grundstücks), der der zulässigen Überschreitungshäufigkeit des RRR entspricht. Die Einleitungsbeschränkung muss den Drosselabfluss in l/s und die Jährlichkeit T der zulässigen Überschreitung enthalten. Für die Berechnung volumenbezogener Bemessungsaufgaben, wie die Bemessung von Niederschlagswasserrückhalteräumen, sind für die Ermittlung der abflusswirksamen Fläche mittlere Abflussbeiwerte C_m nach Tabelle 9 zu verwenden. Für die Dimensionierung des Regenrückhalteraums müssen entsprechend DWA-A 117:2013 die zum Entwässerungssystem gelangenden Abflüsse sowohl von der befestigten Fläche A_E,b als auch von einer nicht befestigten Fläche (Tabelle 9, Nr. 3) mit Zufluss zu einem Ablauf in die Entwässerungsanlage berücksichtigt werden. Die ermittelten Flächenarten werden in dieser Norm vereinfachend als A_FaG bezeichnet, mit den mittleren Abflussbeiwerten C_m multipliziert und zu einem Rechenwert Au zusammengefasst. Das erforderliche Speichervolumen V_RRR wird aus der maximalen Differenz der in einem Zeitraum gefallenen Niederschlagsmenge und dem in diesem Zeitraum über die Drossel weitergeleiteten Abflussvolumen ermittelt.

In Anknüpfung an DWA-A 117 gilt für Grundstücksentwässerungsanlagen für die Bemessung des Rückhalteraumes (RRR) Gleichung (22).

Gleichung 22

V_RRR = A_u x r_D,T / 10,000 x D x F_z x 0.06 – D x f_z x Q_Dr x 0.06

Die Gleichung 22 entspricht der Berechnung des erforderlichen Rückhaltevolumens auf der Basis einer Einleitbeschränkung entsprechend DWA-A 117 mit dem „einfachen Verfahren“ (Formel s. Kapitel 3.2 Drosselabfluss).

Beispielrechnung zurückzuhaltende Regenwassermenge nach Überflutungsnachweis

Standort: Aachen Angeschlossene Auffangflächen: Gebäudedachflächen: A_Dach = 1.250 m², Schrägdach Ziegel, C_Dach = 0,8 Auffangflächen außerhalb von Gebäuden: A_FaG = 4.445 m², Asphalt, C_FaG = 0,9 Gesamte befestigte Fläche des Grundstückes: A_ges = 5.695 m² (A_red = 5.000 m²) Mittlere Geländeneigung: < 1% Befestigung: > 50 %

Berechnung nach Gleichung 20

V_rück = ( r_(D,30) x A_ges – ( r_(D,2) x C_Dach + r_(D,2) x A_FaG x C_FaG)) x D x 60 / (10000 x 1000)

with: D = 10 Min (from DWA-A-118:2006, Table 4)

r_(D,30) = 273 l/sxha r_(D,2) = 148 l/sxha V_rück = 273 x 5,695 – (148 x 1,250 x 0.8 + 148 x 4,445 x 0.9) x 10 x 60 / (10,000 x 1000) = 48.9 m³

Berechnung nach Gleichung 21

V_rück = (( r_(D,30) x A_ges / 10000) – Q_voll) x D x 60 /1000

mit: Einzelnachweis der Bemessungsregenspenden:

a) r_(5,30) =377 l/sxha (aus DIN 1986-100 Tabelle A.1 Regenspenden in Deutschland)

b) r_(10.30) = 273 l/sxha

c) r_(15.30) = 223 L/sxha Q_voll = 100.0 L/s

a) V_rück = ((377 x 5,695 / 10,000) – 100.0) x 5 x 60 / 1000 = 34.4 m³

b) V_rück = ((273 x 5,695 / 10,000) – 100.0 ) x 10 x 60 / 1000 = 33.3 m³

c) V_rück = ((223 x 5,695 / 10,000) – 100.0 ) x 15 x 60 / 1000 = 24.3 m³

Der größte der drei Werte für Vrück ist maßgebend.

Berechnung nach Gleichung 22

Das Einstauvolumen aus der Regelbemessung (nach Einleitbeschränkung) ergibt sich nach Kap. 3.2.4 zu 41,8 m³.

Fazit:

Das sich aus den Berechnungen für den Überflutungsnachweis und für die Einleitungsbeschränkung ergebende größere Volumen ist maßgebend. Die maßgebende Größe des Rückhalteraumes ergibt sich somit nach Gleichung 18 zu 48,9 m³. Somit wird durch den Überflutungsnachweis das erforderliche Rückhaltevolumen um 7,1 m³ (17 %) erhöht. Spätestens dann, wenn das Überflutungsvolumen oberflächig nicht dargestellt werden kann, müssen unterirdische Speichervolumen größer ausgelegt werden.

Sample calculations for infiltration with DRAINMAX Tunnel

a) with value D = 15min and n = 0.2 = Example for various international regions

Location: Aachen

A_red= 100 m²

Measured rainfall: r15,n=0.2 = 152.6 L/(s*ha)

k_f = 1*10-4 m/s (medium sand)

s_rr = 0.56 (DRAINMAX Tunnel installation according to German Institute for Civil Engineering)

f_z = 1.1

Sample calculation for INTEWA DRAINMAX Tunnel in gravel bed:

B = 1.85 m, H = 1 m, L = 2.25 m

L_erf,rigole = 1.31 m

V_erf,rigole = 1.36 m³ (= B x H x L_erf,rigole x s_rr = 1.85 m x 1 m x 1.31 m x 0.56)

Required number of DRAINMAX Tunnels: LL_erf,rigole / L = 0.82

b) With iteration = Example for Germany

Location: Aachen

A_red = 100 m²

Measured rainfall: r_{15, n=0.2} = 152.6 l/(s*ha)

k_f = 1*10^-4 m/s (medium sand)

Duration D [min]	Rain yeld 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. TTable for rough estimation of small systems with r_15,n=0.2

kf (m/s)		e.g. location Aachen (D) r15,0.2=152.6 L/(s*ha)			e.g. location Berlin (D) r15,0.2=213.1 L/(s*ha)
		A=100 m2	A=150 m2	A=200 m2	A=100 m2	A=150 m2	A=200 m2
1*10-4	Volume in m3	1.36	2.04	2.72	1.90	2.85	3.79
1*10-5	Volume in m3	1.49	2.24	2.99	2.09	3.13	4.79
1*10-6	Volume 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 rain duration.

Example calculation:

Permitted discharge from property: 1.5 L/s x ha

Property size: 0.105 ha

Rain yield r₁₅₍₁₎ = 108 L/s x ha

Rain yield r₁₅₍₂₎ = 193 L/s x ha

Surface	x	Runoff coefficient	x	Rain yield	=	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 runoff				Qrges	=	6.22 L/s
Approved discharge quantity:		Qab	=	0.105 ha (property 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 backwater volume Verf: (The retention system must absorb Qs for 15 min.).
Verf	= Qs x 60 x 15 = Qs x 900
	= 6.06 L/s x 900 s = 5.5 m3

Accurate dimensioning of a trench or retention system with planning software

Since a calculation of the required trench volume is iterative, it is likely to carry it out with planning software such as the RAINPLANER.

Dimensioning surface infiltration

As = Ared / ( kr x sf x 107 / 2 x r D(n) –1)
Ared	= connected paved surfaces
sf	= Joints proportion in a permeable paved area (0 < sf =< 1)
kr	= Permeability coefficient in the considered infiltration layer
rD(n)	= Rain yield
Example:
Ared	= 300 m2
sf	= 1 (INTEWA grass grid pavers)
kr	= 2 x 10-4 m/s
r D(n)	= from KOSTRA table with n=0.2/a and D=10 min: r10(0.2) = 204.60 L/s-ha
As = 300 / ( 2 x 10-4 x 1 x 107 / 2 x 204.6 –1) = 77 m2

Dimensioning infiltration trenches behind small wastewater treatment systems

According to DIN 4261-1, version 2002, the water discharged from SWWTPs can be infiltrated through trenches in soils with k_f = 5 x 10^-7 to 5 x 10^-3 m/s. As the base of infiltration system can clog up with time, only the side areas remain effective in the long run. A large retention volume is an advantage for variable infiltration efficiency i.e. during frost or uneven loading of the trench. The following simplified dimensioning methods shall apply according to DIN:

Trenches behind small wastewater treatment systems

Required wall area (m2/inhabitant value PE):
1 m2 / PE to 1.5 m2 / PE with:		Sand-gravel mixture, sand, light silty sand
2 m2 / PE to 2.5 m2 / PE with:		Silt (also light clay), sand-silt mixture, stone-loam mixture
Required number for example of DRAINMAX Tunnel:
Base element	2.25 m length x 0.8 m heigth x 2 sides
As	= 3.6 m2 per tunnel without front walls
PE	to 1.5 m²/PE	to 2.5 m²/PE
4	1 pcs.	2 pcs.
8	1 pcs.	4 pcs.
12	3 pcs.	6 pcs.
16	4 pcs.	8 pcs.

A specified calculation must be done with other soil conditions and higher PE values.

Comparison between DRAINMAX Tunnel and pipe trench variants

According to EN 12566-3 for small wastewater treatment plants (SWWTPs) 150 L / day / inhabitant (PE) has the following daily distribution:

3h = 30%
3h = 15%
6h = 0%
2h = 40%
3h = 15%
7h = 0%

When using a classic pipe trench, the largest flow rate must be determined. This occurs within 2h with 40%. For a SWWTP with 5 PE this is calculated as follows:

40 % in 2 h from 750 L/day
=> 300 L/2h
=> 0.0417 L/s

When using the DRAINMAX Tunnel, the daily volume can be stored in the trench. The largest flow rate is then calculated as follows:

100 % in 24 h from 750 L/day
=> 750 L/24 h
=> 0.0087 L/s

=> this flow rate is 4.8 times smaller than with the pipe trench variant
=> the DRAINMAX tunnel can be dimensioned approx. 4.8 times smaller than the pipe trench

Rechtliche Rahmenbedingungen in Deutschland

Bei der Planung und Installation einer Versickerungs- oder Rückhalteanlage sind unter anderem die aktuellen Fassungen folgender Regelungen zu beachten:

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

Anzeige- und Genehmigungspflichten

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