Open Stormwater Management, Stormwater Utilised as a Resource

Groups: 
  • Design-Build Contracts
Category: 
  • Ecology and Surface Water
  • Environment
  • Buildings

Basic

Requirements and Criteria Type: 
  • Technical Specification (Requirement Specifications)

Main Objective: 

Motive

The challenges associated with stormwater generally involve closed drainage systems and our urban areas. Increased precipitation intensity is also a contributory factor to flooding and its consequences.

Argument

Open stormwater management can provide greater flexibility in relation to the volume of stormwater that can be managed and led away from a site since there are fewer bottlenecks such as pipes, drains, tanks, etc. From a LCC perspective, operation and maintenance are often simpler and can be less expensive than closed stormwater management.  Open systems can provide added value such as aesthetic bonuses, recreation, green spaces and greater biodiversity.

 

Requirement Specification: 

More than 50% of stormwater from smaller volumes of rain (table 8.22.1) must be managed via open systems. Vegetation should be used locally in combination with infiltration, evaporation, etc. such that the water's natural cycle is maintained and to exploit nature's capacity for self-purification.

Smaller volumes of rain are defined here as the volume of precipitation one must manage to catch up to 95% of annual precipitation, stated in table 8.22.1. When determining the dimensions of stormwater measures, dimensions must be calculated for all rain durations of between 10 and 1,440 minutes. The dimensions must be based on the largest volume of precipitation.

Table 8.22.1: Calculated smaller volumes of rain [mm] for managing 95% of annual precipitation (calculated data from eklima.no).
Rain duration [minutes] 10 15 20 30 45 60 90 120 180 360 720 1440
95% of annual precipitation caught in mm 1,4 1,8 2,2 2,9 3,6 4,4 5,5 6,7 8,3 12,2 17,3

24,0

This must be documented via the preparation of a post-inspection report:

  • The work must be performed by a person with relevant knowledge of the standardised methods for calculating stormwater, e.g. a water and wastewater engineer.
  • The inspection must be conducted before construction works, including preparatory works, start on the site.
  • The report and assessments associated with stormwater must be coordinated with any other documentation requirements such as ecology reports, landscape descriptions and outdoor plans, geotechnology, etc.

As a minimum, the following factors must be assessed in the report:

  • Regulations and previous plans
  • Assessment of ground conditions
  • Description of the current situation
  • Placement and choice of stormwater management measures
  • Stormwater system for future situation

 

Solutions must be built that manage more than 50% of the stormwater from smaller volumes of precipitation.

Information about the Requirement Specification: 

Here, it is assumed that prior design of the open storm water management has been carried out. If not, the contractor must do so. Here, you must consider whether the text matches what has been projected. Depending on what has been done in the design project, there may be a need to adjust this text. If nothing is planned, you can use the requirement specification in the design project.

These criteria will be calculated by a water and wastewater engineer in the design project, who should analyse alternative retention measures in a report. The report should contain some of the factors listed in the requirements, which can be elaborated on:

  • Regulations and previous plans: For example, a short review of the regulations in the zoning or municipal master plan associated with stormwater management.
  • Assessment of ground conditions: For example, an assessment of the soil composition, groundwater level and infiltration capacity.
  • Description of the current situation: For example:
    • Area delineation
    • Description of the inspection (time, date and weather conditions)
    • Existing stormwater system
    • Assessment of natural water balance
    • Condition of nearby watercourses (hydrogeology and water quality)
    • Existing flood routes
    • Any historic stormwater challenges
    • Green structures
    • Run-off from adjoining areas
  • Placement and choice of stormwater management measures: For example:
    • Calculation of stormwater volumes based on future land use
    • Placement and determining the dimensions of stormwater management measures that satisfy the criteria level.
  • Stormwater system for future situation: For example, a description of how the ambition level for this requirement will be achieved

In general, the following challenges are associated with closed stormwater management systems:

  • Discharges to public wastewater systems: The systems often assume stormwater will be discharged into the municipal wastewater system, which in many cases have limited capacity.
  • Water balance: Precipitation is transported out of the area without the opportunity for infiltration, evaporation or absorption by vegetation, which could otherwise ensure a natural water balance. The groundwater level is reduced with the consequent subsidence damage.
  • Foreign water: When transported to the wastewater system, clean stormwater will be supplied to the wastewater purification system. This unwanted water that is supplied to the wastewater purification system is often referred to as "foreign water".
  • Capacity: The capacity of a system is limited by drains, pipe diameters, tanks, etc.
  • Makes nature-based purification difficult: Combining closed stormwater systems with the nature-based purification of polluted stormwater can be challenging.
  • Purification: With the exception of sand traps and oil separators, the number of purification measures that can be combined with a closed system is limited.
  • Operation and maintenance: It is difficult to identify any need for necessary operation and maintenance since the systems are below ground.
  • Added value: A closed system provides little added value. The only function of closed underground stormwater systems is to retain and forward stormwater.
  • Volume needs below ground level: Closed retention solutions for stormwater require volumes below ground level. In urban areas this could take up volumes that, in the future, we will want to use for other infrastructure.

Densification often results in pristine terrain being replaced with impermeable surfaces such as roofs, asphalt and concrete. Together with climate changes and the expected increase in precipitation intensity, densification will result in increased volumes of stormwater that must be managed locally so they do not overload wastewater systems or watercourses.

To meet the current and future challenges associated with stormwater, Norwegian Water recommends that stormwater be managed according to a three-step strategy (figure 1). Compared with the original three-step strategy, a step 0 has been added to the figure which covers the comprehensive planning intended to ensure the subsequent steps are implemented.

Figur 1: Tretrinnsstrategi for håndtering av overvann basert på Norsk Vann. I forhold til opprinnelig figur er det lagt til et trinn 0 som omfatter den planlegging som er nødvendig for å sikre vannhåndtering gjennom tre trinn.
 
 

Analyses from Oslo show that 95% of the annual precipitation typically amounts to around one third of the volumes of precipitation with 2-year frequency intervals. As illustrated by figure 2, the measures in step 1 will manage the vast majority of the water on an annual basis but have limited effect for heavy rain.

•	Figur 2: Illustrasjon av hvor vannmengdene håndteres (fordeling på ulike trinn) ved ekstremnedbør (venstre) og på årsbasis (høyre) i henhold til tre-trinnsstrategien.
 
 

 

Example:

An area is going to be developed.

  • For advanced ambition level, 50% of heavy rain must be managed via open systems.
  • Stormwater from the area is discharged into the municipal wastewater system. The municipal system can cope with a maximum of 10 l/s.
  • According to table 8.22.2, we must manage 10-year rain in the area locally (in this example 10-year rain represents a large volume of precipitation).
  • Go to eklima.no and download the local IVF statistics for the plan area and get data for 10-year rain. Multiply the data by a climate factor, expected relative increase in precipitation intensity. (Norwegian Water recommends using a climate factor of between 1.3 and 1.5 for determining the dimensions of new wastewater systems with a lifetime of 100 years).
  • Calculate the total retention volume for the entire area (e.g. 100 m³) using the method described in report 193.
  • At least 50 m³ of the retention volume must be above ground (managed via open systems on roofs, rain gardens, etc.).
  • The remaining 50 m³ can in principle be managed below ground via large pipe reservoirs.

 

Advanced

Requirements and Criteria Type: 
  • Technical Specification (Requirement Specifications)

Main Objective: 

Motive

The challenges associated with stormwater generally involve closed drainage systems and our urban areas. Increased precipitation intensity is also a contributory factor to flooding and its consequences.

Argument

Open stormwater management can provide greater flexibility in relation to the volume of stormwater that can be managed and led away from a site since there are fewer bottlenecks such as pipes, drains, tanks, etc. From a LCC perspective, operation and maintenance are often simpler and can be less expensive than closed stormwater management.  Open systems can provide added value such as aesthetic bonuses, recreation, green spaces and greater biodiversity.

Requirement Specification: 

More than 50% of stormwater from smaller volumes of rain (table 8.22.1) must be managed via open systems. Vegetation should be used locally in combination with infiltration, evaporation, etc. such that the water's natural cycle is maintained and to exploit nature's capacity for self-purification.

Smaller volumes of rain are defined here as the volume of precipitation one must manage to catch up to 95% of annual precipitation, stated in table 8.22.1. When determining the dimensions of stormwater measures, dimensions must be calculated for all rain durations of between 10 and 1,440 minutes. The dimensions must be based on the largest volume of precipitation.

Table 8.22.1: Calculated smaller volumes of rain [mm] for managing 95% of annual precipitation (calculated data from eklima.no).
Rain duration [minutes] 10 15 20 30 45 60 90 120 180 360 720 1440
95% of annual precipitation caught in mm 1,4 1,8 2,2 2,9 3,6 4,4 5,5 6,7 8,3 12,2 17,3

24,0

This must be documented via the preparation of a post-inspection report:

  • The work must be performed by a person with relevant knowledge of the standardised methods for calculating stormwater, e.g. a water and wastewater engineer.
  • The inspection must be conducted before construction works, including preparatory works, start on the site.
  • The report and assessments associated with stormwater must be coordinated with any other documentation requirements such as ecology reports, landscape descriptions and outdoor plans, geotechnology, etc.

As a minimum, the following factors must be assessed in the report:

  • Regulations and previous plans
  • Assessment of ground conditions
  • Description of the current situation
  • Placement and choice of stormwater management measures
  • Stormwater system for future situation

In addition, more than 50% of the stormwater from large volumes of precipitation, e.g. by using retention ponds, infiltration-based measures, green roofs and spaces designed for controlled flooding.

This will include physical measures that temporarily retain, i.e. hold back, stormwater before discharge into a watercourse or municipal wastewater system. The thresholds for what are regarded as large volumes of precipitation depend on the type of area, see table 8.22.2.

Table 8.22.2: Norwegian Water's recommended minimum frequency intervals for determining dimensions. Data obtained from Norwegian Water (2008).
Frequency intervals for determining dimensions for precipitation Type of area
5 years Areas with low damage potential (outskirts, rural municipalities, etc.)
10 years Residential areas
20 years City centres/industrial areas/business districts
30 years Subways/areas with very high damage potential

Local precipitation statistics must be used (e.g. from eklima.no) when calculating large volumes of precipitation (step 2 in figure 2). The precipitation for determining dimensions must be multiplied by a climate factor in order to address the expected increase in precipitation due to global warming. The climate factor should be chosen based on the latest updated estimates, e.g. klimaservicesenteret.no.

Information about the Requirement Specification: 

Here, it is assumed that prior design of the open storm water management has been carried out. If not, the contractor must do so. Here, you must consider whether the text matches what has been projected. Depending on what has been done in the design project, there may be a need to adjust this text. If nothing is planned, you can use the requirement specification in the design project.

These criteria will be calculated by a water and wastewater engineer in the design project, who should analyse alternative retention measures in a report. The report should contain some of the factors listed in the requirements, which can be elaborated on:

  • Regulations and previous plans: For example, a short review of the regulations in the zoning or municipal master plan associated with stormwater management.
  • Assessment of ground conditions: For example, an assessment of the soil composition, groundwater level and infiltration capacity.
  • Description of the current situation: For example:
    • Area delineation
    • Description of the inspection (time, date and weather conditions)
    • Existing stormwater system
    • Assessment of natural water balance
    • Condition of nearby watercourses (hydrogeology and water quality)
    • Existing flood routes
    • Any historic stormwater challenges
    • Green structures
    • Run-off from adjoining areas
  • Placement and choice of stormwater management measures: For example:
    • Calculation of stormwater volumes based on future land use
    • Placement and determining the dimensions of stormwater management measures that satisfy the criteria level.
  • Stormwater system for future situation: For example, a description of how the ambition level for the basic requirement will be achieved

In general, the following challenges are associated with closed stormwater management systems:

  • Discharges to public wastewater systems: The systems often assume stormwater will be discharged into the municipal wastewater system, which in many cases have limited capacity.
  • Water balance: Precipitation is transported out of the area without the opportunity for infiltration, evaporation or absorption by vegetation, which could otherwise ensure a natural water balance. The groundwater level is reduced with the consequent subsidence damage.
  • Foreign water: When transported to the wastewater system, clean stormwater will be supplied to the wastewater purification system. This unwanted water that is supplied to the wastewater purification system is often referred to as "foreign water".
  • Capacity: The capacity of a system is limited by drains, pipe diameters, tanks, etc.
  • Makes nature-based purification difficult: Combining closed stormwater systems with the nature-based purification of polluted stormwater can be challenging.
  • Purification: With the exception of sand traps and oil separators, the number of purification measures that can be combined with a closed system is limited.
  • Operation and maintenance: It is difficult to identify any need for necessary operation and maintenance since the systems are below ground.
  • Added value: A closed system provides little added value. The only function of closed underground stormwater systems is to retain and forward stormwater.
  • Volume needs below ground level: Closed retention solutions for stormwater require volumes below ground level. In urban areas this could take up volumes that, in the future, we will want to use for other infrastructure.

Densification often results in pristine terrain being replaced with impermeable surfaces such as roofs, asphalt and concrete. Together with climate changes and the expected increase in precipitation intensity, densification will result in increased volumes of stormwater that must be managed locally so they do not overload wastewater systems or watercourses.

To meet the current and future challenges associated with stormwater, Norwegian Water recommends that stormwater be managed according to a three-step strategy (figure 1). Compared with the original three-step strategy, a step 0 has been added to the figure which covers the comprehensive planning intended to ensure the subsequent steps are implemented.

Figur 1: Tretrinnsstrategi for håndtering av overvann basert på Norsk Vann. I forhold til opprinnelig figur er det lagt til et trinn 0 som omfatter den planlegging som er nødvendig for å sikre vannhåndtering gjennom tre trinn.
 
 

Analyses from Oslo show that 95% of the annual precipitation typically amounts to around one third of the volumes of precipitation with 2-year frequency intervals. As illustrated by figure 2, the measures in step 1 will manage the vast majority of the water on an annual basis but have limited effect for heavy rain.

•	Figur 2: Illustrasjon av hvor vannmengdene håndteres (fordeling på ulike trinn) ved ekstremnedbør (venstre) og på årsbasis (høyre) i henhold til tre-trinnsstrategien.
 
 

Example:

An area is going to be developed.

  • For advanced ambition level, 50% of heavy rain must be managed via open systems.
  • Stormwater from the area is discharged into the municipal wastewater system. The municipal system can cope with a maximum of 10 l/s.
  • According to table 8.22.2, we must manage 10-year rain in the area locally (in this example 10-year rain represents a large volume of precipitation).
  • Go to eklima.no and download the local IVF statistics for the plan area and get data for 10-year rain. Multiply the data by a climate factor, expected relative increase in precipitation intensity. (Norwegian Water recommends using a climate factor of between 1.3 and 1.5 for determining the dimensions of new wastewater systems with a lifetime of 100 years).
  • Calculate the total retention volume for the entire area (e.g. 100 m³) using the method described in report 193.
  • At least 50 m³ of the retention volume must be above ground (managed via open systems on roofs, rain gardens, etc.).
  • The remaining 50 m³ can in principle be managed below ground via large pipe reservoirs.

Related links: 

Published: 08. Feb 2018, Last modified: 18. Dec 2018