On the Influence of Wind-Driven Surges on the Pärnu River

I. A. Veldre, Candidate of Biological Sciences, and N. N. Tikhon

From the Institute of Experimental and Clinical Medicine of the Academy of Sciences of the Estonian SSR and the Republican Sanitary and Epidemiological Station

The Pärnu River is one of the largest rivers in the Estonian SSR. It is 134 km long and has a well-developed river system with a drainage area of approximately 6,900 km², of which 730 km² is covered by bogs. As the problem of a centralized water supply and sewerage system for the city of Pärnu is being addressed under the current seven-year plan, the need has arisen to study the sanitary condition of the Pärnu River and the effects of industrial wastewater on it.

In 1959–1960, the Institute of Experimental and Clinical Medicine of the Academy of Sciences of the Estonian SSR and the Republican Sanitary and Epidemiological Station conducted a sanitary and hygienic investigation of the lower Pärnu River, covering the 15 km from the town of Sindi to the river mouth.

This article focuses primarily on the influence of winds causing surges on the composition of the water in the Pärnu River.

The Pärnu River flows slowly in the section studied, at approximately 0.1 m/s. Its average width is 200 m, ranging from 160 to 400 m, and its depth is 2–8.5 m. The river is navigable along almost the entire section studied.

According to the hydrometeorological service, the river’s mean discharge is 50 m³/s. The maximum discharge occurs in April and is 160 m³/s; the minimum occurs in September. According to data from recent years, the river was ice-covered from mid-December to early April.

At all sampling cross sections (see table), samples were taken at three points: approximately 10 m from each bank and in the navigable channel. Water samples were taken simultaneously from depth (0.5 m above the bottom) and from the surface (0.30 m below the water surface).

Influence of winds on the composition of water in the Pärnu River

In addition to the cross sections listed in the table, water samples were taken from the Pärnu River’s tributaries—the Reiu and Sauga rivers—and from Pärnu Bay at various distances from the ends of the breakwaters.

The investigations revealed a considerable difference in the composition of the river water depending on wind direction. With south-

…with westerly winds (blowing from the sea), seawater is driven upstream as far as the confluence with the Reiu River.

The data presented in the table¹ show that the mean concentrations of chlorides and sulfates in river water rise sharply under southwesterly winds. The influence of these winds is evident as far as 10 km from the mouth of the Pärnu River. There, the chloride concentration is 10.9–12.1 mg/L during calm conditions or northerly winds, but 855.8–963.5 mg/L under southwesterly winds; the corresponding sulfate concentrations are 25 and 75–120 mg/L. At the landward end of the left breakwater, the chloride concentration in the river water is close to that in seawater. Southwesterly winds also increase the magnesium content of the river water.

The increase in salt concentration leads to an increase in total water hardness.

For comparison, it is instructive to cite data from a 1962 study by the Institute of Experimental and Clinical Medicine of the Academy of Sciences of the Estonian SSR. Under a southeasterly wind, chloride concentrations of 837.8–908.8 mg/L, sulfate concentrations of 70–84 mg/L, and total hardness of 10.2–18.4 mEq/L were found at the mouth of the Pärnu River, between the breakwaters. Thus, even under southeasterly winds, these measures increase in the river water relative to its salt composition during calm conditions or northerly winds (see table).

The data obtained led us to give an unfavorable assessment of the proposed treatment facilities and sewerage system for the city of Pärnu. The plan called for wastewater to be discharged into the mouth of the Pärnu River. Since southwesterly winds prevail in Pärnu, discharging wastewater at the river mouth would inevitably have polluted the river water as far as 10 km upstream. On the basis of this study, the proposed locations of the treatment facilities and wastewater discharge were reconsidered.

The influence of surge-producing winds also extends to the tributaries of the Pärnu River—the Reiu and Sauga rivers.

In the Reiu River, 1 km from its mouth, chloride concentrations under southwesterly winds reach 200 mg/L, sulfate concentrations reach 40 mg/L, and total hardness is approximately 8–10 mEq/L.

In the Sauga River, under the influence of southwesterly winds along the entire section studied, extending up to 3 km from the river mouth, chloride concentrations at some sampling cross-sections reach 1,500–2,000 mg/L, sulfate concentrations reach 100–150 mg/L, and total hardness is 10–15 mEq/L.

Thus, the data presented support the conclusion that the lower reach of the Pärnu River, up to 10 km from its mouth, and its tributaries, the Reiu and Sauga rivers, are affected by surge-producing winds and therefore cannot be used as sources of drinking water for the city of Pärnu. Artesian water is preferable for the city’s water supply; if the capacity of the groundwater aquifers is insufficient, water from the Pärnu River upstream of the Sindi dam, approximately 15 km from the river mouth, should be used.

Received December 17, 1962.

¹ The table includes only the measures most characteristic of the influence of seawater.