Hydrodynamics of Lake Karhijärvi - An Experimental Study of the Wind Forcing Effects on a shallow, eutrophic lake

Introduction  
 
 2
1 Introduction 
 
1.1 General overview on Finland’s water body 
 
Finland is a land of lakes and forests.  
Finland’s water bodies are both numerous and labyrinthine in shape. Most of 
the old towns grew up on or near the coast or along the inland waterway used 
from centuries as traffic arteries, both in summer and winter time. 
Forest cover the greater part of the country, but some 10% of its surface of 
altogether 338.145 km² is covered by inland waters, both large and small, for a 
volume of about 235 km
3 
. The number of lakes larger than one hectare is 56.000. 
Thousands of lakes in Finland are joined together by rivers and rapids forming 
watercourses which finally empty into the brackish Baltic Sea (Finnish 
Environmental Institute). 
Finland’s lakes are mostly shallow, with an average depth of seven meters, 
and circulation water velocities are rather small, so that any pollutants can 
likewise take a long time to get out from a water body. 
It cannot be forgotten that Finland is a land of ice and snow, too. A quarter of 
the country is situated north of the Arctic Circle, which makes Finland one of the 
northernmost countries in the world. The weather conditions are, however, 
favorable with respect to its position, thanks to the Gulf Stream. The lakes are ice-
covered for five to seven months a year.  
Waterways have played a considerable role in the development of habitation 
as well as trade and industry in Finland. Today the importance of the aquatic 
environment is continuously increasing as a resource of recreation as well as an 
invaluable source of biological and landscape diversity. Finnish lakes have a wide 
range both on flora and fauna, sometimes very typical and rare elsewhere. Their 
size runs from small pond to large extension, and their characteristics may be 
from oligotrophic to very rich in nutrients. 
Introduction  
 
 3
On the other hand, the lakes, rivers and reservoirs and the investments therein 
comprise valuable national assets for water supply and various other uses such as 
hydro-electric power production, fisheries and transport as well as flood control. 
 
Shallowness and small retention time mean that this kind of lake is highly 
vulnerable to pollution. Moreover at high latitude, like in Finland or in mountain 
areas as Alps regions, the fact that they are cold and frozen during the winter is an 
additional problem. 
During the last decades a renewed sensibility to environment has asked to pay 
more attention to water resources, and lakes are a main one. 
The present work approaches the study of a shallow lake and its 
hydrodynamic. Currents induced by wind resulted to be a dominant factor. Field 
measurements strongly supported this hypothesis, and also were used to test a 
computer model of wind induced circulations. Simulation and possible paths of 
hypothetical pollutants were drowned, and speculations on hydrobiology and 
hydrodynamic correlations have been advanced. 
 
Number of ponds and lakes larger than 0.0005 km
2
 
 
188.000
Number of lakes larger than 0.01 km
2
  
 
56.00 
Number of lakes than 100 km 
2 
47 
 
Largest lake Great Saimaa4.380 km
2
  
 
1.2 From nutrients to eutrophication 
 
About four-fifths of Finnish lake are healthier, and most of the other are 
classed as poor-quality due to eutrophication.  
Tab 1.1 Water 
resources statistic - 
Lakes in Finland 
(Finnish 
Environmental 
Institute) 
Introduction  
 
 4
High nutrients rate and consequently eutrophication is a typical problem of 
water body in Finland. Sources of nutrients were mainly urban sewerage works 
and pulp mills, but now agricultural and forestry play an important role as well. 
Eutrophication is a natural process taking place in water. The process is 
characterized by a development towards an environment rich in nutrients and a 
proliferous plant production.  
Visual effects of eutrophication include muddled and discolored water, 
excessive algae and bacteria production, and aquatic organisms killed by oxygen 
depletion. Thus, biodiversity in these aquatic ecosystems is often disturbed and 
depleted .  
Man-made eutrophication of inland water courses and shallow coastal waters 
is caused by excessive discharge of nutrients, especially phosphorus (P) and 
nitrogen (N) in the form of PO
4
 and NO
3
. In Scandinavian Country the nutrients 
come primarily from agricultural activities, sewage and fish farms along the coast. 
Several studies on the impact of pollution from nutrients (phosphorous and 
nitrogen) on water quality have been supported from several and multi-country 
institutes, and in most of the cases content of algal chlorophyll is adopted as a 
measure of the plant production and is often used - together with for example, the 
phosphorus content, nitrogen content and light penetration - to evaluate the water 
quality as regards the extent of eutrophication. Under natural conditions a low 
amount of nutrients should normally be fed to any lake from its watershed, even 
to watersheds with low human activity. This low level of nutrient supply will only 
support a limited growth of microscopic plants, such as algae or phytoplankton, in 
lake waters. 
The increase of human activities, including agriculture, industry and sewage, 
gives rise to increased nutrient loading and hence increased plant growth in lakes. 
Due to this influx of nutrients, changes in lake food-chains often occur; masses of 
phytoplankton develop with no corresponding increase in zooplankton, which 
normally controls the level of phytoplankton through grazing. The result of this 
Introduction  
 
 5
can be a deterioration of water quality, oxygen depletion in deep-waters, bad taste 
and smell, and even the production of toxic substances. 
Excessive phosphorus from runoff and erosion can fertilize surface waters. In 
this process, microscopic floating plants, known as algae, rapidly multiply when 
fertilized by phosphorus. These algae cloud the water making it difficult for most 
of submerged aquatic vegetation (SAV) to get light enough. The SAV may 
dieback reducing available habitat of aquatic animals. When algae themselves 
eventually die they start to decompose. During decomposition dissolved oxygen is 
removed from the water. Lowered oxygen levels make it difficult for other aquatic 
organisms to survive. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Moreover, phosphorus, attached to sediments derived from soil erosion, may 
accumulate in the sediments of lakes and streams. This phosphorus may be 
recycled slowly or released more rapidly when these sediments are disturbed, for 
example during a storm or flood. Pollution from phosphorus is therefore a long-
term problem and one of the main reasons of eutrophication. 
Fig.1.1 Impact of 
Phosphorus on aquatic 
life. 
Introduction  
 
 6
 
 
1.3 Water management in Finland 
 
The value of the use of Finnish water resources is estimated to be over 22 
billion EURO and it covers a primary role in the economy of the Country. 
Sophisticated models of multiobjective decision-making are applied to 
develop interactive management practices to regulate lakes and rivers. However, 
the most essential input for these models comes from the grass roots, from the 
people and from the users of the watercourses. 
Within the filed of water resources management the activities of the Regional 
Environment Centers, under the guidance of the Ministry of Agriculture and 
Forestry, cover water supply and sewerage, flood prevention, drainage and 
irrigation, restoration of watercourses also used for timber-floating, multipurpose 
regulation of river systems, permit holders’ obligations, maintenance of hydraulic 
structures, dam safety, combating sudden flood and ice jams, ditching procedures, 
and other water resources management issues. 
The use of water resources for a long time has been subject to license in 
Finland. The most of water areas are privately owned, but there are, however, 
general usage rights to which everyone is entitled. These include the right of 
passage in a watercourse and on its ice cover, the right to float timer on it, the 
right to swim in it and the right to draw household water from it.  
The utilization of waterways and their protection is regulated by the Water 
Act. As well as the economic and technical aspects, environmental and social 
concerns and considerations of equity have been incorporated into the planning 
and legal procedures, and thus into the management practices of the permit 
holders. The precautionary principle combined with use of the best available 
techniques have for long been applied in practice by Finnish water management.  
Introduction  
 
 7
Some 87% of Finland's population are served by public waterworks owned by 
the municipalities and their water-supply companies. The cost of public water 
services is covered by collecting water and wastewater charges. 
Although the Helsinki metropolitan area and some other major towns use 
surface water, groundwater constitutes an essential resource for water supply. The 
share of groundwater and artificial groundwater as a source for public water 
supply has been constantly increasing and is now almost 60% of the water 
distributed.  
The State has provided economic incentives for investments in water supply 
and sewerage in both rural and urban communities. The main objectives are to 
provide potable water of high quality for the entire population at reasonable cost 
and to minimize the adverse environmental impact. Recently investments in line 
with the objectives of European Union structural funds have also been co-
financed by the European Regional Development Fund.  
Some 200 flow regulation projects have been carried out in natural lakes or 
man-made reservoirs. Half of these serve primarily hydro-electric power 
production, a quarter flood control and the last quarter other purposes. As a rule, 
the regulation of lakes and river system benefits several purposes simultaneously. 
Efforts are made to develop sustainable management of regulated water systems. 
Most of the regulation activities aim, along with other goals, at the prevention 
of flood damage. Flood-combating measures have to be taken almost every year 
in some part of Finland due to ice jams, snow-melt and occasionally also flooding 
caused by torrential rain. Therefore, operational flood control plans have been laid 
out for all major flood-prone water systems. 
Although special legislation on dam safety was not enacted before 1984, 
regular monitoring of the safety of major dams was already started in 1962. 
Ongoing development projects cover risk assessment as well as preparedness for 
accidents and rescue arrangements. Early warning for flood and dam safety 
operations are expected to benefit from a more effective utilization of modern 
 
Introduction  
 
 8
 information and modeling technology combined with new applications of 
weather radars and weather forecasting (Ministry of Agriculture and Forestry). 
Environment policies are not supposed to know boundaries. In the field of the 
collaboration, Finland has signed agreements with its neighbours Sweden, 
Norway and Russia to ensure proper management of transboundary watercourses. 
As a member of the European Union, Finland is actively working together with 
other Member States in the field of water policy. European collaboration is 
rapidly expanding within research and development, too. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
In water management Finland has a long tradition of international co-
operation, especially in the Baltic Sea area. Several water supply and sanitation 
development projects have been carried out as far away as Africa and Asia. 
Finnish-Chinese undertakings in such sectors as flood control, dam safety and 
management of ice conditions are further examples of a successful collaboration 
(Finnish Environmental Institute). 
 
 
Fig. 1.2  Lake Pyhäjärvi, Tampere 
Introduction  
 
 9
 
 
1.4 The lake as a “system” 
 
Lakes are a classical subject of limnology studies, but only recently the 
physical approach has been adopted. 
Stratification, flows, mixing, temperature and light conditions are all factors 
of fundamental importance and have to be coupled with chemical and biological 
behaviors to understand the true nature of the “lake system”.  
The problem of the scale is still somehow unresolved. Biology and chemistry 
may easier find a compatible scale range more than physic and chemistry or 
biology. So that biological studies have been often treated separately to physics 
ones. 
Some recent results of comparative hydrobiology and hydrodynamics 
investigation studies (M.Straskrabe, 1998) have empathized the role of physic 
processes on phytoplankton and zooplankton, and once more the bias of the 
knowledge of all components of “lake system” is emerged together with the high 
demanding target to link such different fields. 
This work is done in the perspective of joint physical and some simple 
biological aspect to better describe the environment of the lake. 
“Circulation” is defined as “the rectified effects (derived from averaging the 
possibly non-linear processes over some predefined space and time) of all 
possible thermo-hydrodynamic processes”(J.Kielmann and T.J.Simons, 1980). 
When circulation is considerate in a small temporal scale (e.g. hours or few days) 
as in the present paper, then it often refers to wind forcing effects. 
Mixing results primarily from physical processes as the mixing energy from 
inflows and outflows and the transfer energy across the air-water interface due to 
wind. 
Variation in the intensity of the wind mixing seems to be related to a major 
effect on the seasonal succession of plankton (Reynolds, 1980, 1984, 1990). The 
Introduction  
 
 10
stability of the water column strongly influence the plankton packing type both on 
short and seasonal period (Reynolds 1984; Streinberg and Hartman, 1988). Such 
different phenomena scale make complex the analysis of mechanic processes and 
plankton behavior. Moreover the physical factors run in a broad range that results 
in a field of nested eddies where turbulent energy is gradually dissipated to reach 
viscous state. 
Eddies diffusion coefficients are often used to represent in a simple way the 
complex mixing process both in vertical and horizontal plane. 
While theory of vertical mixing is rather developed, less attention is often 
paid to the dispersion in the horizontal plane. If the object of dispersion is 
plankton, then the system to be analyzed become more and more complicated 
because of “living” nature of plankton. 
This paper summarizes a summer analysis of a medium, shallow water lake in 
the middle region of Finland where horizontal mixing and transport is not 
neglected and of primary importance to know for the recreational destination of 
the lake. Field tests have included experiments running from the simple drifters 
tracking to water currents measurements by Acoustic Doppler Instrument and 
plankton patchiness surveying. 
 
 
 
1.5 Lake model alternatives 
 
Lakes may be modeled in different ways according to the physical 
characteristics of the water body and its usage destinations. 
Basic distinction in models classification is between circulation and 
simplified model. The first simulates two or three dimensional flows, the further 
has from zero ( lake as a fully mixed reactor ) to one dimension, vertical or 
horizontal. One-dimensional vertical models consider the lake to be horizontally 
Introduction  
 
 11
homogeneous and varying on the vertical structure. Horizontal 1-D model better 
describes long narrow lakes but also shallows ones. 
Circulation models basically consider wind, bottom friction and in - outflow. 
They may be a single layer where assumption of a unstratified vertically 
homogeneous lake is made. Multi - layers models may predict both free surface 
elevation and current as well as stratified vertical structure.  
Single layer model are often successfully used in shallow lakes where the 
assumption of vertical homogeneity hold well (P.Shenehen, D.R.F. Hartemann, 
1996). 
 
Hydrodynamics of lakes 
 
 12
2 Hydrodynamics of lakes 
 
2.1 Characterization of lakes and reservoirs  
 
A primary difference between rivers and lakes is their speed of water 
currents. Classic literature defines rivers lotic systems, characterized by running 
water, while lakes and reservoirs are treated as lentic, with the meaning of almost 
totally standing water. New research in this field shows how important is to 
consider lakes as a systems of moving water, with defined hydrodynamics 
governing rules. Transports and mixing processes in rivers are regulated by the 
natural flowing, that causes complete mixing over the depth and the width. In 
lakes the standing nature causes water and water quality constituents and 
contaminants to move slowly, and under certain conditions they tend to stratify 
rather than mixing. Main factors dominate mixing are in-outflow, wind and solar 
heating. Related to lentic nature is also the tendency to store water for relatively 
long periods. With increased time of storage, internal cycling and matters 
originating within the lake (autochthonous materials) gain importance if 
compared to materials originating outside of the standing water (allochthonous 
materials). Often loads run periodically with seasons, and transports and mixing 
occur in similar time. Vertical variations are very important in lakes and 
reservoirs: light does not penetrate in all lakes’ layers, so that the heat exchange 
and productivity are limited to surface layers. Many factors allow to vertical 
stratification, with large vertical gradients in temperature, density and water 
quality, while few factors working for mixing, and often lakes do not totally mix 
vertically over long periods of the year. 
Currents in lakes may be not well defined, and may change in direction also 
over a short period of time, transport and mixing are slow and not homogeneous 
(L.Martin & McCutcheon, 1998), with a prevalence in horizontal direction 
compared to vertical, so it is not generally likely possible to describe the system 
with a one-dimensional longitudinal model. The pattern of flows or mixing in a 
Hydrodynamics of lakes 
 
 13
lake is largely affected by bathymetry, thermal structure, inflow, outflow, wind 
mixing and solar heating. By the way, many flow models for lakes have as a basic 
assumption that the mixing in horizontal plane is complete, so that is possible to 
define layers and analyze only vertical variation in mixing and water quality. 
Lakes are defined by the British Encyclopedia as “ a mass of still water 
situated in a depression of the ground without direct communication with the sea”. 
Natural lakes are those found in natural depressions such as ponds, not excavated 
by humans or resulting from the construction of a dam. Unlike natural lakes, a 
reservoir typically results from the building of engineering structures to impound 
a stream and create an artificial basin for different uses (e.g. flood control, power 
generation, water supply, recreation and navigation). Often words as “lakes” and 
“reservoir” are used indifferently, but for some purpose is necessary to underline 
the artificial nature of the latest. It is also important to know the differences in the 
origin and degree of regulation because affect physical, chemical and biological  
aspects. 
In order to classify lakes, the most common way is the lake-type approach 
based on lakes origins. Major types of lake basins are the following: 
 
a) Tectonic basins : result from disastrophics events such as earthquakes, tilting 
or faults, or upheavals forming basins without drainage. This kind of basins 
usually have elongate shapes, steep side and great depths (e.g. Dead Sea in 
Israel, Lake Baikal in Russia, Great Salt Lake in Utah) 
 
b) Volcanic basins : often results from dams due to lava flow, or in the craters of 
extinct volcanoes and in cavities formed by subsidence (Wetzel 1975). 
Usually they are low nutrient content and of high depth (e.g. Crater Lake in 
Oregon) 
 
Hydrodynamics of lakes 
 
 14
c) Glacial basins : include moraines formed by the debris from glaciers, cirques 
and tarns, respectively formed in the eroded head of a glacial valley by 
freezing and thawing (e.g. Laurentian Great Lakes) 
 
d) Solution basins : formed by the collapse of caves; they are common in 
limestone regions (e.g. Indiana, Florida, Tennessee) 
 
e) Wind basins : formed in depressions created by wind. 
f) Streams basins : are in someway linked with streams and their erosion 
character. Fluvial lakes directly result from stream erosion or behind deposit 
and obstruction in streams. Plunge-pool lakes made by erosion below 
waterfalls, while oxbow lakes result formed by the cut-off of a stream 
meander (McCutcheon, 1999). 
Reservoirs are manmade in order 
to supply different purposes. They 
are characterized by the possibility 
to regulate the volume and the 
outflow, factors that largely affect 
the transport and mixing within the 
reservoir, and in turn water quality 
aspect. 
Purposes range of artificial lakes 
includes irrigation, flood control, 
hydroelectric power generation, 
water supply, navigation, 
recreation, fisheries and water 
quality maintenance. 
Multiple uses are often envisaged at the same time, but often the primary 
purpose of the reservoir excludes any other use like in flood control where the 
flood-mitigation pool capacity should be large and storage kept to a minimum in 
Fig.2.1 Stratification in a reservoir 
(from J Martin,  S. McCutcheon,1999) 
Hydrodynamics of lakes 
 
 15
order to reduce the peak outflow. Reservoirs for hydroelectric generation power 
have to be located where it is possible to have a sufficient head drop, and may be 
considered that many facilities are peaking operating to satisfy power demand, so 
that the outflow may vary both in time and amount. Reservoir, dams and 
withdrawal structure often affect the water quality in the down streams 
proportionally to the use of the basin and environmental factors. In natural lakes 
the surplus are usually withdrawal from the surface water, but sometimes in 
reservoirs dams do not have a selective withdrawal structure, and released water 
come from a certain layer with strong characterization in quality often worse than 
inflow water. 
 
 
 
 
 
 
 
 
 
 
 
 
2.2 Zonation of lakes 
 
In lakes and reservoirs it is possible to locate different zone formed by 
physical processes. Distinction between areas is often not well defined, but 
presence or absence of certain zones may change the choice of the model used to 
study a certain lake. 
First distinction may be done between the littoral and the limnetic zone. The 
littoral zone is the area between the beach and the open water, and often is the 
Fig. 2.3 Longitudinal zoonation in lakes (from J Martin,  S. 
McCutcheon,1999)