Protecting the Baltic Sea from untreated wastewater spillages during flood events in urban areas

Site description  Söderhamn is a coastal municipality located in the bay of Söderhamn, at the outlet of Söderala river. The pilot area was chosen because the central parts of Söderhamn are most severely affected when a heavy rainfall occurs. In addition, some densification of the area is expected to happen with new buildings, changes in park areas and streets. The Söderhamn pilot area consists of 11 sub-catchments. Four of them have outlets to a natural stream and the rest to the narrow bay of the Baltic Sea, most of them submerged. The stormwater from e.g. roofs is still directed to the sewer system and therefore there are several combined sewer overflow (CSO) structures. These CSOs are equipped with backflow valves to avoid seawater entering the sewer.

NOAH actions  Storm Water management model (SWMM) is created for the area and calibrated on the basis of flow measurements. No Real-Time Control (RTC) installations are made in the pilot site, since the stormwater system has enough capacity because of the multiple outlets. The real problems in Söderhamn are related to sea level rise, so the focus is on urban planning improvement. Extreme Weather Layer (EWL) as a planning tool to create more flood resilient urban space is the main outcome of the modelling. Flood risk maps for three different climate scenarios are embedded to the municipality’s urban planning procedure. This allows urban planners to evaluate the impact of any development plan on the area’s climate resilience. Additionally, green areas of the pilot area are mapped and are visible in EWL. 

Site description  Pori is a town on the south-west coast of Finland, located about 10 kilometers from the Gulf of Bothnia on the estuary of the Kokemäenjoki river. The NOAH pilot area is the Suntinoja ditch catchment area from where water exits into the Kokemäenjoki river, which is the 4^th largest waterbody catchment area in Finland. The catchment area consists of fields, forests and an urban residential area. The ground surface is flat, which increases drainage problems and stormwater and snowmelt flood threat. Besides the risk to property, flooding increases harmful contaminant and nutrient migration to the Baltic Sea. The Suntinoja ditch was originally designed for drainage of agricultural areas, so its capacity may not be large enough in heavy rainfall situations, and may begin to flood residential areas through stormwater drains.

NOAH actions  Main ditches of the area are mapped, and calibration measurements are carried out by the city of Pori. Storm Water management model (SWMM) is created for the area. The modelling determines the effects of different types of heavy rain events and can be used to examine the impact of residential construction on Suntinoja’s capacity. Extreme Weather Layer (EWL) as a planning tool for more flood resilient urban space is an outcome of the modelling. Water sampling is conducted to analyze the quality of the stormwater and for modelling it in different flood situations. No actual Real-Time Control (RTC) installations are made in the pilot site. The outflow from the system is not restricted, as the outlet pump has a large capacity, and the system performs similarly in terms of flooding throughout the city. The actual problem is more linked to the water level in the surrounding ditches and the related risk of flooding. 

Site description  Rakvere is a municipality in northern Estonia, 20 km south of the Gulf of Finland of the Baltic Sea. There are two waterbodies in Rakvere, Soolikaoja creek and Tobia mainditch. The Tobia mainditch flows down to the Soolikaoja creek, the Soolikaoja flows down to Selja river and the river flows to the Baltic Sea. The Tobia mainditch catchment area is 31,8 km² and the Soolikaoja catchment area 122,1 km². The selected pilot area of about 1 km² is located in the middle of the town. According to the climate scenarios, this area has the highest flood risk.

NOAH actions  Storm Water Management Model (SWMM) of the area is created, calibrated and validated based on measurements made on the site. A movable weir with Real-Time Control (RTC), is installed in the Süsta pond (section of Soolikaoja creek) to reduce flooding in the downstream city by using the storage capacity of the pond. The Smart Weirwall System also consists of two water level sensors: one is installed in a manhole and another is installed in Süsta pond. The weirwall height adjusts on the basis of these sensors. The construction works of the system were completed at the end of 2020, and the final inspection of the system took place in April 2021. Extreme Weather Layer (EWL) as a planning tool for more flood resilient urban space is the second outcome of the modelling. It is implemented with a dynamic feature allowing the municipality to create maps for flood risk for different development scenarios and climate projections. EWL maps for three climate scenarios are created for the Rakvere pilot site. 

Site description  Haapsalu is a town on West Estonia’s Baltic coast, located in an oasis typical of the north-west coast of Estonia. The town’s coastline length is 18 km and the area 10.6 km². A total of 67% of the town area is covered with greenery (parks, recreational areas etc.). Due to the coastline length and ground elevation, the city is open to seawater flooding. Old drainage systems, bottlenecks in pipelines and incomplete information on the town’s drainage system are contributing to rainwater flooding. The pilot area is divided into two, corresponding to actual stormwater system catchment areas.

NOAH actions  The stormwater systems are mapped, and water samples are taken from stormwater outflows. Extreme Weather Layer (EWL) is created for the area as a planning tool for more flood resilient urban space. The existing depreciated dam of the bioswale, which is the buffer for stormwater outflow before the Baltic Sea, is replaced with a new automatic weirwall (Smart Weirwall System). The system consists of a moveable gate and two sensors, installed into a new manhole. The sensors automatically adjust the position of the weirwall based on water levels. Implementing Real-Time Control (RTC) can (1) prevent seawater backflow in case the sea level rises higher than the level in the bioswale (= flood protection), (2) ensure sufficient retention time of the urban stormwater in the bioswale before letting the water flow to the sea (= water treatment).

Site description  Jurmala is situated on the southernmost shore of the Gulf of Riga, by the drainage basin of the Lielupe river. The pilot area consists mainly of forested areas and low-rise residential buildings. The landscape can be characterized as rather flat. Roofs constitute around 9.5% and paved roads up to 14% of the total catchment area. The stormwater collection system is a separate sewer system with several sanitary sewer connections from households. Run-off is conveyed by gravity pipelines and roadside ditches. System discharges into the Lielupe river. For NOAH, three sections of the area have been chosen as pilot sites – A, B and C (see the photos). The sections are actual stormwater catchments, chosen in order to study the city area evenly.

NOAH actions  Storm Water Management Model (SWMM) is created for the area. Through modelling, Real-Time Control (RTC) is implemented by transforming a node (joining point of pipeline and ditch) into a pumping station to see the changes in the system discharge during dry weather and during rainfall. The main investment in Jurmala is an automatic hydrological station (AHS). The purpose of an AHS is to develop a better process control and management system in regard to the city’s stormwater and wastewater system as well as their potential interaction, especially during rain events. This includes not only the modelling but also precipitation, stormwater level and wastewater flow measurements as well as stormwater sampling. Local meteo stations, automatic samplers with level sensors and flow meters are installed in the pilot site. Extreme Weather Layer (EWL) as a planning tool for more flood resilient urban space is the second outcome of the modelling.

Site description  Ogre is located alongside the Ogre river approximately 50 km from the Baltic Sea coastline. The pilot area of Ogre has been selected due to its significant flood problem and estimated future challenges caused by climate change. The focus is on the Loka street neighborhood (between AHS1 and AHS2 – see the photos), which has developed from a low swampy meadow. The surface water run-off has been organized with a network of open ditches along the streets, draining into the Ogre river. Due to intensive detached housing construction, part of the ditches have been arbitrarily filled or the culvert elevation marks have been misaligned. This has led to a loss of functionality of the existing drainage network. To control surface run-off, the municipality must provide rainwater drainage from the street and adjacent areas by creating a single network. Therefore, the municipality has started the gradual construction of a rain drainage piping system, which is supported by NOAH installations.

NOAH actions  Storm Water Management Model (SWMM) is created for the area. Measurements of the Ogre riverbed upwards from Daugava river water reservoir are performed (including measurement data processing and cartographic material preparation). Sensory locations as well as the technical design of the Automatic Hydrological Stations (AHS) are identified and evaluated. Three automatic hydrological stations with a non-return tidal gate and a pump are constructed and installed. Extreme Weather Layer (EWL) as a planning tool for more flood resilient urban space is another outcome of the modelling.

Site description  Liepaja is located in western Latvia, between the Baltic Sea and the lake of Liepaja. For NOAH, Tebras street catchment basin with a separate storm sewer is chosen for detailed inventory. The area of the catchment basin is approximately 19 ha. Most of the area is occupied by low-rise residential buildings, and impervious surfaces consist of roofs (42%) and paved roads (8% of total area). The stormwater sewer outlet of the catchment discharges water into the Lake of Liepaja. There is a problem related to backflow from the lake with water standing still all the way from the outfall to the pump.

NOAH actions  Hydraulic model of the Tebras street catchment basin is developed to (a) clarify how the sewer functions under different circumstances, (b) examine whether it is possible to add new connections to  this catchment basin in the  future, (c) understand the quality of water drained from the catchment basin, (d) examine the possibility of using project partner experience from Haapsalu in application of a Smart Weirwall System or usage of a tidal check valve, which would allow stormwater flow only in one direction. A tidal gate and a pump are installed in the outlet to prevent sea water from backing up into the drainage system. Flow meters are installed in two manholes closer to the city center. Additional pilot activities are performed in the northern part of Liepaja nearby Tosmare lake. This part of the city is enclosed by Cietokšņa channel. Two water level sensors are installed in the Cietokšņa channel, as the territories around the channel are potential flood areas and sensors are needed to indicate water level rise in the channel. The main problem is that if the Cietokšņa canal outlet into the Baltic Sea is clogged, the adjacent areas get flooded. Extreme Weather Layer (EWL) as a planning tool for more flood resilient urban space is another outcome of the modelling.

Site description  Slupsk is situated in northern Poland, about 20 km from the Baltic Sea coast, on the Slupia river. The pilot area covers the entire city (22 km²). The Slupsk sewer system includes 2% of combined sewer pipes which drain 7% of the analyzed area. Even though the share of combined sewer system is small, approximately 30% of the total flow originates from the stormwater entering the sewer system via unsealed manholes and pipes. This poses a risk to the wastewater treatment plant (WWTP) and to the Slupia River, which is the recipient of the overflows. The modelled network has one main outflow from which the wastewater and stormwater is pumped to the WWTP. The excess which cannot be pumped is stored in the retention tank upstream from the pump or discharged to the Slupia River.

NOAH actions  Storm Water Management Model (SWMM) is created for the area. To collect data, precipitation meters (rain gauges) with an automatic data archiving system and remote transmission are installed in six locations in the city. In addition, devices for measuring the water level of the main sewage channels are installed in 12 locations. In order to improve on the flooding issues in the city, there is a need for retaining and delaying water upstream of the affected areas. A Real-Time Control (RTC) scheme with a detention volume will control the outflow from the basin based on downstream water level sensors in the vulnerable areas. Extreme Weather Layer (EWL) as a planning tool for more flood resilient urban space is another outcome of the modelling.