1. for Fresh and Tidal waters
Mixing zones
for Fresh and Tidal waters
Dju Bijstra
Centre for Watermanagement

2. Table of contents Possible approach Fresh water mixing zones
Distribution pattern
Dimensions of the mixing zone
questions

3. Possible approach Ready (1) (2) Emission data Watersystem parameters
(worst case)
Simple assessment
(i.e. Based on Fisher equations) No
Ready
Problem (Y/N)?
Extra input:
bed topography;
specific substance
related properties
(volatility, biological
degradation…etc)
Yes
Complex approach:
(3D, Cormix or
other model…..)
(2) No
Problem (Y/N)?
Yes
Possible approach:
As a first step it is useful to evaluate whether or not a discharge is ‘problem’ in terms of water quality. In order to do so a simple test can be very helpful for distinguishing potential ‘problems’ from ‘non problems’. A simple approach, based on the Fisher equations, can provide such an instrument. If a worst case approach does NOT LEAD to a problem you can be sure that the discharge will not lead to EQS exceedance. If the simple approach leads to a problem, a more complex approach based on more complex models can be followed (optional). If not, measures have to be taken to reduce the discharge.
Additional measures

4. Water quality standards
Mixing zones in fresh waters
Check-
point L
~~~~~~
i.e. distance = 10 x width water body
(max. L m)
Qeffl
Ceffl
MAC at distance
of …(25?) m
EQS b Cb
~~~~~~
Fresh water mixing zones:
When a discharge takes place at a water body (i.e. river) a mixing zone can results as a consequence of the discharge (mostly the effluent concentration exceeds EQS). The mixing zone is determined by the flow characteristics, water body dimensions, flow of the discharge and the emitted concentration. In order to evaluate whether or the emissions leads to a problem the concentration in the near vicinity of the discharge point and the checkpoint at distance L of the discharge point is of special interest. To avoid toxic effects in the mixing zone the concentrations may not exceed the MAC value at a certain distance x from the discharge point.
In order to prevent migration limitations a maximum cross-section of the mixing zone of 25% can be suggested as a limit value for i.e. emissions of heat, acids etc….
Testing of
Water quality standards
flow
depth
Cross-section
Max. 25%?
F-dilution

5. Concentration at checkpoint L:
Distribution pattern
X- axis
Concentration at checkpoint L:
CL=EQS C
Distribution pattern:
In this slide the distributions pattern as a function of the distance to the discharge point is shown. Mixing in the Y-direction rectangular to the flow direction increases when the distance to the discharge point increases.The EQS is given by the red rectangles. The red dotted line gives the concentration as a function of the distance to the discharge point at the river bank.At checkpoint at a distance L from te point of discharge the conecntration in the surface water has to meet EQS.
depth
Y-axis

6. Dimensions of the mixing zone (1)
- fresh waters and tidal waters -
As a consequence of the tidal movement accumulation occurs!
This has to be taken into account!
Qflood-netto
flow
Qebb-netto
Dimensions of the mixing zone in tidal waters and fresh waters:
Due to the tidal movement the direction of the flow can change in as a consequence also the position of the mixing zone. During the ebb period the mizing zone is positioned downstream the discharge point and during the flood period the mixing is positioned upstream of the discharge point, provided that the flow due the tidal movement exceeds the netto river flow. In this way a distance upstream (Lu) and a disctance downstream (Ld) can be indentified. These paramters can be used to describe the position of a mixing zone in tidal waters and in fresh waters. The following equation can be used for this purpose. Lu Ld L

7. Dimensions of the mixing zone (2)
Dimensions (lenght) of the mixing zone can be given by a general equation applicable for fresh waters as well as for tidal waters:
Lu = (Qflood/Qebb) * L/2
With L = maximum length of the mixing zone
Qflood = total tidal volume during flood period
Qebb = total tidal volume during ebb period

8. Dimensions of the mixing zone (3)
- emission of several priority substances - C=
MAC1 C=
MAC2 C=
EQS1 C=
EQS2
depth
Avoid toxic effects in the mixing zone! x1 x2 L1 L2
Cross-section
Max. 25%?
L  x * (MAC/EQS)2
how to define x (waterbody related or a fixed distance)?
how to define L (substance related or based on a (fixed) ratio
MAC/EQS and proportional to x)?
Dimensions of a mixing zone in the situation of a discharge of several priority substances:
In this example an emission contains two priority substances (1 and 2). The distance where EQS-1 is met at the river bank is L1 and the distnace where EQS-2 is met at the river bank is L2. The distance where MAC-1 is exceeded is x1 and the distance to the discharge point where MAC-2 is exceeded is x2. The central base principle to be followed is to avoid toxic effects, if possible, in the mixing zone. Central question wich remains is “how to define the mixing zone in such a way that this principle can be met in the major part of the mixing zone?” The maximum length (L) is proportional to the the distance x and the square of the ratio (MAC/EQS). The definition of x and the ratio (MAC/EQS) is crucial for the definition of the dimensions of the mixing zone.

9. Dimensions of the mixing zone (4): possible options
1) a chosen fixed value
2) a value depending on the width of the water body
3) a fixed value depending on the composition of the effluent
4) a fixed value depending on the composition of the effluent and the width of the water body
5) a substance related value
6) a substance and water body related value

10. Dimensions of the mixing zone (4): possible options
1) a chosen fixed value
2) a value depending on the width of the water body
3) a fixed value depending on the composition of the effluent
4) a fixed value depending on the composition of the effluent and the width of the water body
5) a substance related value
6) a substance and water body related value
Possible options:
For the distance x two options are possible: a fixed chosen value or a water body related value (i.e. relative to the width of the water body). For the ratio (MAC/EQS) three options are possible: 1. A fixed value based on the average value of the ratio MAC/EQS for priority substances. 2. Fixed value based on the lowest ratio MAC/EQS in the effluent (bases on priority substances). 3. A substance related ratio based on the ratio MAC/EQS for each compound in the effeluent. This means that for all sunstances in the effluent this ratio has to be determined. This also includes that for all compounds in the effluent the maximum lenght has to be determined. This means also that the dilution factor has to be determined for ecah substance seperately (more work!). The definition of the mixing zone becomes more complex and less transparent.

11. Dimensions of the mixing zone (4): possible options
1) a chosen fixed value
2) a value depending on the width of the water body
3) a fixed value depending on the composition of the effluent
4) a fixed value depending on the composition of the effluent and the width of the water body
5) a substance related value
6) a substance and water body related value

12. Dimensions of the mixing zone (4): possible options
1) a chosen fixed value
2) a value depending on the width of the water body
3) a fixed value depending on the composition of the effluent
4) a fixed value depending on the composition of the effluent and the width of the water body
5) a substance related value
6) a substance and water body related value

13. Dimensions of the mixing zone (4): possible options
1) a chosen fixed value
2) a value depending on the width of the water body
3) a fixed value depending on the composition of the effluent
4) a fixed value depending on the composition of the effluent and the width of the water body
5) a substance related value
6) a substance and water body related value

14. Dimensions of the mixing zone (4): possible options
1) a chosen fixed value
2) a value depending on the width of the water body
3) a fixed value depending on the composition of the effluent
4) a fixed value depending on the composition of the effluent and the width of the water body
5) a substance related value
6) a substance and water body related value

15. Dimensions of the mixing zone (4): possible options
1) a chosen fixed value
2) a value depending on the width of the water body
3) a fixed value depending on the composition of the effluent
4) a fixed value depending on the composition of the effluent and the width of the water body
5) a substance related value
6) a substance and water body related value
Judgement of the consequences:
Besides the starting point for choosing a value for the distance x and the ratio MAC/EQS also the consequences of possible choices are important for the evaluation. Aspects wich have been taken into account are simplicity, ecology and applicability. Simplicity reproduces the data necessary to determine L, and the transparency. Ecology gives information related to the risk that toxic effects can occur in the mixing zone. Applicability reproduces the uniformity, simplicity and the amount of work needed to work with a definition of the mixing zone. When looking to the table options 1 and 3 seem to score the best. All aspects are valued with the same weighing factor.

16. Dimensions of the mixing zone (5):
- consequences of choices to be made -
Range (L):
40 – 400 m
Range (L): m
Range (L):
250 – 2500 m
Dimensions of the mixing zone(5):
For a list of priority substances where MAC and EQS values are available the ratio of MAC/EQS is given in the table in this slide. For some substances the ratio MAC/EQS is given by a range. This leads to a range for the average value of the ratio MAC/EQS of 4,1-4,8 for priority substances. The value 2 can be seen as a low value (the value 1,4 is an exception) and 4 and 5 as average values.The consequences for the maximum length (L) of the mixing zone is given in the figure. In the figure the L is given as a function of the distance x and the ratio MAC/EQS. For a ratio MAC/EQS of 2 the maximum length of the mixing zone varies between 40 (x=10 m) and 400 m (x=100 m). For a ratio 4 the maximum length of the mixing zone varies between 160 m (x=10 m) ans 1600 m (x=100 m). For a ratio of 5 the maximum length of the mixing zone varies bewteen a value of 250 m (x=10 m) and 2500 m (x=100 m). Conclusion: The chosen distance x and the ratio to be used have great impact on the maximum length of the mixing zone. What to do?

17. Maximum length (L) of mixing zone
Questions
How to define the area in the near vicinity of the discharge point where MAC may be exceeded:
(distance x)
the ratio MAC/EQS
Maximum length (L) of mixing zone
Can the worst case ‘simple’ approach, based on Fisher equations, be
supported?
Fresh waters
Tidal waters (accumulation due to tidal movement has to be taken into account)
Tidal waters: specific situations (harbours and open sea)
Harbours: when stratification occurs: density flows (extra mixing) have to be considered
Open sea: approach based on fixed maximum volume of the mixing zone?