Sediment pool of the Mustjõe Stream, Tallinn/Estonia

Arvo Käärd, Ain Valdmann, Tarmo Soomere

Thursday 2 july 2015

12:39 - 12:42h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: Poster pitches: 11B. River Engineering

The purpose of constructing a sediment pool for the Mustjõe Stream (Mustjõe oja) is to provide a solution for fluvial sediment flux on the lower reaches of this stream before it enters the sea and to provide opportunities to prevent seawater from rising into the stream bed during high water level events. The solution entails raising the banks of the Mustjõe Stream and constructing a hydraulic plant. The Mustjõgi is neither public nor in public use. According to the geological mapping, the area under study and its immediate surroundings are located in a deep (ca 120 m) buried ancient valley (Kopli clint bay (Kopli klindilaht)). This valley was eroded into Vend-Cambrian minerals (sandstone, aleurolite and clay) and filled with Quaternary sediments. The Mustjõe Stream is located above its western bank, where the topsoil thickness is 30-40 metres. Due to being close to the coastline, the area’s ground water regime depends on the water level of Kopli Bay. Strong storm winds from west and north-west may raise the water level to 1.5 metres above the long-term mean, flooding most of the mouth area of the Mustjõe Stream and its surrounding region with seawater. On average, the water level at the mouth of Kopli Bay reaches up to 96 cm once per year, up to 112 cm once in three years, up to 125 cm once in five years and up to 142 cm once in ten years. Wave-driven set-up in storms with unfavourable wind direction may theoretically increase the aforementioned values by up to 38 cm at the mouth of the Mustjõe Stream. The proposal is to construct soil walls (with a height of 2.0-2.5 m) on both sides of the sediment pool until the Paldiski maantee (mnt) culvert, in order to prevent rising seawater from entering the stream bed. The hydraulic plant and the pump station are to be constructed on the sediment pool’s outlet. This alternative would protect the sediment pool and the flow bed of the Mustjõe Stream until the Paldiski mnt culvert against rising seawater.