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. 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 different climate scenarios will be embedded to the municipality’s urban planning procedure. This allows urban planners to evaluate the impact of any development plan on climate resilience.

» RTC evaluation in Söderhamn – NOAH O3.3 (PDF)

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 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 contaminant and nutrient migration into the Baltic Sea. The Suntinoja ditch was originally designed for drainage of agricultural areas, so its capacity may not be 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. 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 since the outflow from the system is not restricted as the outlet pump has large capacity, and since 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.

» RTC evaluation of Pori – NOAH O3.3 (PDF)

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 river Selja 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. Movable weir with Real-Time Control (RTC) is installed for the Soolikaoja creek to reduce flooding in the downstream city by using the storage capacity in an upstream lake. The construction works of the system will be completed by the end of 2020. Extreme Weather Layer (EWL) as a planning tool for more flood resilient urban space is the second outcome of the modelling. It will be implemented with a dynamic feature allowing the municipality to create maps for flood risk for different development scenarios and climate projections.

» RTC setup in Rakvere – NOAH O3.3 (PDF)

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. The existing depreciated dam of the wetland, which is the buffer for rainwater outflow before the 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. Implementing Real-Time Control (RTC) can (1) prevent seawater backflow in case the sea level rises higher than level in the wetland (= flood protection), (2) ensure sufficient retention time of the urban stormwater in the wetland before letting the water to the sea (= water treatment).

» RTC setup in Haapsalu – NOAH O3.3 (PDF)

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, landscape can be characterized as rather flat. Roofs constitute around 9.5% and paved roads up to 14% of the total catchment area. The rainfall 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 storm water catchments, chosen 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 AHS is to develop a better process control and management system in regard to the city’s storm water and waste water system as well as their potential interaction, especially during rain events. This includes not only the modelling but also precipitation, storm water level and waste water flow measurements as well as storm water sampling.

» RTC setup in Jurmala – NOAH O3.3 (PDF)

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 NOAH installations support.

NOAH actions  Storm Water Management Model (SWMM) is created for the area. Measurements of the Ogre riverbed upwards from Daugava river water reservoir will be performed (including measurement data processing and cartographic material preparation). Sensory locations as well as the technical design of the Automated Hydrological Stations (AHS) are identified and evaluated. Three automated hydrological stations with a non-return tidal gate and a pump are constructed and installed.

» RTC setup in Ogre – NOAH O3.3 (PDF)

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. Low-rise residential buildings mostly occupy the area, impervious surfaces consist of roofs (42%) and paved roads are (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 weir wall system or usage of a tidal check valve, which would allow stormwater flow only in one direction. Additional pilot activities are performed in the northern part of Liepaja nearby Tosmare lake. This part of the city is enclosed by Cietokšņa channel. There is a plan to install two water level sensors in 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 are flooded.

» RTC setup in Liepaja – NOAH O3.3 (PDF)

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 overflows. The modelled network has one main outflow from which the wastewater and storm water is pumped to the WWTP. The excess which cannot be pumped is stored in the retention tank upstream 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 level of filling 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 has the potential to do this by controlling the outflow from the basin based on downstream water level sensors in the vulnerable areas.

» RTC setup in Slupsk – NOAH O3.3 (PDF)