VVAN30 - urban drainage systems

From Lecture 5 Design of Urban drainage and swedish guidelines, Lecture 6 Urban stormwater design, Lecture 7 Stormwater quality,

Shortly describe the difference between the urban stormwater system and the conditions between a street, city street, residental area, park?

Large differences in available space in and above the ground for stormwater management. Densly built areas leads to additional pipes, difficulties to implement infiltratrion measurements, safety distances to foundations to consider etc. City centers usually have older pipes, combined sewage system for example. Residential areas have more space, can implement SuDS.

What does the stormwater drainage system consist of? Give examples of both grey and green solutions

The stormwater drainage consists of pipes, gullies, manholes, basins for detention/retention/purification, ditches and swales, inlet/outlets, drainage. It can also consist of channels, wetlands, ponds, gravel filled ditches and basins, raingardens.

When planning for a new urban stormwater design, describe some of the existing conditions you must look at

Some exisisting conditions one must consider is topography, catchments, geotechical conditions (bedrock, soil, risk for erosion/landslides), hydrogeology (groundwater conditions and levels, trees, protecting environments, existing pipes and cables, other underground structures such as foundations for bridges and lightpoles

What are the differences between building pipes for flat and hilly areas?

For flat areas elevation driven flow can be difficult to implement. This means that the starting point for the pipe system should be as shallow as possible, then it will become deeper and deeper. Here one must consider the pipe strength, safety for maintenace workers. Maximum 4-5m ground over pipes. In a hilly site, elevation driven flow is not a problem, the pipe should be designed after the gradient of the ground level. If there are variations in elevations, there can be a problem with too shallow pipes. Benchmark should be 0.4-0.6 m below ground. 

What main pollutants can we find in stormwater? what are the sources?

Main pollutants are nutrients, pathogens, metals, petroleum hydrocarbon (from oil and grease), organic pollutans from pesticides.

How is the need for stormwater treatment assessed?

The requirement for treatment depend on a combination of the pollution load (i.e the area) and the sensitivity of the recipient. Industrial areas with much traffic will cause a high load of pollutants, while residental areas will result in lower loads. A less sensitive recipient will result in lower requirements for treatment and the other way around. High loads + very sensitive recipient calls for extensive stormwater treatment such as ponds, wetlands, while in other cases raingardens, swales etc can fulfill the treatment requirements.

what is EMC? how do we calculate it?

EMC is the event mean concentration, it is a flow-proportional average of the concentration representing the total runoff event. You calculate it by dividing the total pollution load with the total water volume. 

Describe the different strategies for stormwater quality control?

Strategies can vary from source control such as substitution of harmful substances, information control, laws, rules and regulations.Then there is private source control such as rainwater harvesting. Then on public land, we can have onsite control such as ponds, rivers, ditches etc.

Which are the common process to remove pollutants from stormwater?

Sedimentation, filtration, sorption, microbial degration/transformation, bioremidation

What is a raingarden “good” for? which processes are involved?

A raingarden can provide infiltration for water through soil and gravel which filtrates the water. Depending on design it can store and thus attenuate the water in gravel. Plants enhance water loss by evapotranspiration and might provide bioremidation by plants.

What are swales/ditches good for? which processes are involved? Where are they common?

Swales and ditches are mainly good for storage of water and conveyance (slowing down the transport). They can to some extent filtrate the water if it is infiltrated, and depending on the vegetation in the swale, there are increased levels of evatranspiration. Wet swales can provide sedimentation. 

Which design steps are there for stormwater sewage (to calculate for example pipe size)

First, decide on design storm using IDF curve. Then, the contributing area and its characteristics is determined (impermeable surfaces, surface type etc). Then, a method must be chosen to calculate the design flow. One can use for example the time-area method. Lastly the hydraulic constraints of pipes are considered and the pipe size, gradient, elevation and so on can be determined.

Which three safety levels are there in stormwater design?

The first one is the pipe capacity level, where the pipes are full. The second one is the surface flooding level, where stormwater reaches the ground.

The third one is the critical damage level where the stormwater reaches and causes damage on buildings

Give some recommendations for increasing flood protection

elevate building levels, extra detention facilities reduces peak flows and pollutant discharge, oversized pipes (must acess downstream capacity as well)

What are the key design elements of SUDS?

SUDS provides storage to reduce peak flows, sediment traps, water conveyance, floodable land to make space for water in a safe way. SUDS can increase infiltration and evaporation to decrease volume of runoff.  

Describe and discuss the multiple benefits provided by SuDS

How and to what extent are SuDS components able to improve the water quality of runoff?

Describe the three-point approach

The three point approach aims to be a communication tool for helping stakeholders making decisions when it comes to stormwater management and handling of rainfall. The claim of the 3PA is that we can never can build a solution that can handle extreme rains. The focus should be on taking care of the water, lead it somewhere, “water resilience”

describe the difference between fail-safe design and safe-fail design

Fail-safe design is designing for a system to never fail which makes the consequences large if it fails (such as designing stormwater systems with pipes). Safe-fail design is designing for a system that is safe if it fails.

Briefly describe the plan and building act (PBL) and its role in stormwater management

PBL regulates how land can be used and build on. It says that the land must be suitable for its purpose. When new zoning areas (detaljplan) are developed, stormwater management must be included if it there will be a problem with stormwater otherwise.

Briefly describe the environmental code (MB) and its role in stormwater management

In MB stormwater is considered wastewater if it is water that is diverted within a zoning plan (detaljplan). This means that according to MB, stormwaters should be regulated the same way as wastewater. There are also 16 environmental quality objectives (miljömål) that one can use when interpreting MB.

Briefly describe act on public water servies (LAV) and its role in stormwater management

LAV says that the municipality must meet the need for safe drinking water and safe disposal of wastewater within service areas. LAV regulates the property owner’s right to, for a fee, connect their pipes to the connection points

Briefly describe the Water framework directive on Environmental quality standards (MKN) for water

This directive states that all waters should be classified and their ecological and chemical status assessed. The aim of the directive is to push the environmental work for water quality forward, which means that the status of the recipient and the quality of the discharged stormwater must be taken into consideration.

What is the role of the municipality when it comes to stormwater management?

The municipality have monopoly on water and land planning (and is therefore responsible for it). It is the municipality that can own a public water facility. The municipality is obliged to meet the need of drinking water and sewerage

What is the role of the water utility company when it comes to stormwater management?

Thw water utility company is majority-owned by municipality. They construct public stormwater facilities according to industry standards (praxis) which depend on where the facility is located. In planning: should provide knowledge on appropriate elevation, propose appropriate sw systems etc IF the company according to LAV should do that. They have a right to charge a tariff for their services and they are responsible for operation and maintenance of plant - to connection point

What is the role of the county administrative board when it comes to stormwater?

The county administrative board have a supervisory and guiding role. They supervise that municipalities comply with LAV. They can annul zoning plans if deemed to exceed environmental quality standards

Briefly, what is the industry standard when it comes to capacity of stormwater management?

For separate systems, the responsibilites of utility company and municipality are larger. In city centres, the system should manage a 30-year return period of pressure head at ground level, but in sparse residential area, this is 10-year. Municipality is responsible for 100-year events. When it comes to combined systems, they should be designed for 10 years at basement level only. This means that combined systems are less protected than separate. 

Which actor is economic liable when it comes to floodings after heavy rainfalls?

For combined systems, the utility company should be able to show that their system can handle a 10 year event. If not, or if the rainfall causing the floodings exceeds 10 year event, utility is liable. Home insurance pays the owners for the damages and then insurance companies claim the same money from the water principal. 

What is the real estate owners/propery owners responsibilites when it comes to stormwater management?

The propery owners are responsible for the pipes/management after the connection point. Properties must be designed to withstand rain, leading water away from the buildings, having cleaned gullies etc. They are ultimately responsible for climate adaptations of their own properties.

What is inflow/infiltration to pipes and what is the difference between them

Inflow/infiltration is water that should not be in the system. Inflow is stormwater in sewage pipes coming from illegal missconnections in separates systems where for example garden gullies are connected to the sewage pipe instead of the stormwater pipes. Infiltration to sewers is water infiltrating through cracks etc from groundwater, springwater, seawater and water from other pipes.

Why is inflow/infiltration a problem?

Inflow/infiltration results in too high flows in the sewage system including the WWTP which leads to more frequent sewer overflows, overload of pumping stations (which also is energy consuming and results in shorter life spans), insufficient treatment of sewage in WWTP resulting in increased use of chemicals. During very heavy rainfalls it can also result in sewage water flooding basements.

What is fast rain impact?

It is the impact from stormwater runoff from impervious surfaces entering the wastewater sewage network (both combined and separate). This impact comes from both legitimate runoff (runoff that the system is designed after/for) and inflow. In combined systems, the fast rain impact on the use of CSO is large since stormwater and sewagewater is combined. In separate systems, the inflow water from misconnections results in fast rain impacts due to misconnections but also due to infiltration. 

What is slow rain impact?

Part of rainfall that infiltrates the pipe systems after the rain event due to infiltration in soil, elevated ground water levels etc.