Step 2

Stormwater is rainwater and meltwater that accumulates on constructed and impermeable surfaces, as well as drainage water from foundations. Stormwater is absorbed locally in areas with sufficient water-permeable surface area and suitable soil. If stormwater cannot be absorbed locally, stormwater must be channeled through stormwater drains and ditches to more suitable terrain and water bodies.

In the Hamina local plan area, stormwater is mainly channeled through stormwater drains or ditches to drainage ditches, wetlands and directly into the sea. In areas where the plots are larger and the buildings are old, (smaller) stormwater is absorbed directly into the soil. In the local plan area, a new stormwater drainage network will be built in connection with the water supply renovation. In this case, open ditches will be removed and embankments will be built in their place. There are no constructed wetlands or other stormwater delay/treatment solutions in Hamina. 

From a water protection perspective, local and regional treatment of stormwater should be increased. The project has developed spatial data on surface runoff routes and identified potential stormwater absorption and treatment sites. The suitability of the sites has been analyzed based on, among other things, the terrain profile and the size of the catchment area. The study has taken into account existing wetlands that should be preserved and road connections that are needed for maintenance. The analysis based on spatial data was limited to areas owned by the city. The data produced in the project can be utilized in planning the use of areas and construction and in catchment-specific stormwater planning.

The project has paid attention to conventional working and operating methods that have an impact on water quality. In spring 2024, the work target was the excavation of the Myllyojan ditch. The goal was to remove soil and vegetation accumulated in the ditch, so that the water flow improves and flooding along the ditch is reduced. As a result of flooding, for example, more solids and nutrients dissolve from the fields into the runoff water, which burdens the watercourse where the ditch discharges (Savilahti).

The Myllyoja stream carries water from a large catchment area to the sea. There is a motorway, a railway line and cultivated fields along the stream. Water is also discharged into the stream from small-scale industrial areas and commercial forests.

The first excavation was carried out on a section of about 100 meters in the lower reaches of the Myllyoja stream near the Salmenvirta discharge point. It was decided to do the work in winter, when the ground is frozen and can support the excavator better. Also, less damage is caused to the ground cover vegetation under the snow than during thawed soil. Efforts were made to preserve the trees along the ditch and only necessary trees and vegetation were removed from the excavator's path. The flow was improved by excavating the soil and vegetation accumulated at the bottom of the ditch. The edges of the ditch were not excavated, and the width or depth of the ditch were not changed.

Water samples were taken from Myllyoja before excavation work on March 5, 2024 and after excavation on March 27, 2024. After the excavation work, the water samples contained more solids and the water was turbid. Total nutrient and iron concentrations had also increased. However, the electrical conductivity of the water and the ammonium nitrogen, chlorine, and sulfate concentrations had decreased.

There was still a frosty period at the beginning of March, but the weather changed rapidly during March. On March 5.3, the flow was approximately 122 l/s and on March 27.3, approximately 200 l/s. After the excavation, the amount of snowmelt water increased noticeably, so the calculated load also increased. Excavation work always has immediate effects on water quality, but the extent of the harm can be influenced by the excavation method. Monitoring of the water quality of Myllyoja will continue and the next samples will be taken in spring 2025.

In the second phase of the project, qualitative research on stormwater continued in different areas of the city. Basic water quality parameters (pH, electrical conductivity, solids, salt, temperature and ORP value, i.e. oxidation-reduction potential) were measured with handheld meters from different parts of the city (31 different measurement points).

For laboratory analyses, samples were taken from four sites: Myllyoja, Lelu (before the construction of the new snow reception area), Merikatu stormwater well and the ditch in the Horjalviikki industrial area. The results of the laboratory tests showed that all samples contained high levels of nitrogen and iron. The Lelu and Horjalviikki samples had high levels of aluminum. E. coli bacteria were particularly abundant in the Merikatu stormwater well sample, which was taken in September 2024. Heavy oil fractions (C22-C40) and PAH compounds were also detected in the Merikatu sample. The water quality of the Merikatu stormwater well is affected by traffic emissions from the city center and the proximity of fuel stations.

The results of the hand-held meter and the results of laboratory analyses are in the relevant appendices.