CE5504 – Surface Water Quality Modeling
Lab 2. Modeling Loads: The C/Q Approach Applied to Onondaga Lake
Reading:
Chapra, Surface Water Quality Modeling, Chapters 3 and 4
The C/Q (concentration/flow) approach is commonly applied to represent tributary and point
source loads to lakes. Here, we will calculate phosphorus loads from two major tributaries to
Onondaga Lake and apply them in calculating resulting in-lake phosphorus concentrations.
Software Development - Loads
The first step in this assignment is to calculate the loads. This requires (1) flow data for the 30year period of record (1973-2002), (2) a predictive relationship (C/Q plot) between flow (Q) and
TP concentration (C) and (3) supporting software. Using the worksheet provided:
1. Go to the USGS web site and extract flow data for Onondaga and NineMile Creeks for the
years missing from the period of record.
2. Develop C/Q plots for both tributaries, using the data provided for 4 years. Prepare plots of
your C/Q relationships with appropriate statistical characterization.
3. Develop software that, for a specified year, will read flows, calculate concentration and
output tributary flow, concentration and load to a worksheet. Prepare a plot of load vs. time.
Programming Hints
To read information from a worksheet other than that linked to the program –
variable name = Sheets("worksheet name").Cells(row,column)
To clear a specified area on a worksheet prior to the next model run –
Worksheets("worksheet name").Range("e.g. A1:B10 ").ClearContents
Software Development – Lake Model
1. Morphologic, kinetic and loading data for Onondaga Lake are as follows:
Volume = 1.31x108 m3
Surface area = 1.20x107 m2
TP settling velocity = 100 m∙yr-1
METRO flow = 80 MGD
TP Standard = 20 mg∙m-3
2. Develop software that will simulate in-lake phosphorus concentrations for a specific one-year
period, i.e. METRO and tributary loads for that year. Calculate the summer average (May
15th to September 15th) phosphorus concentration for that year.
3. Expand the calculation to determine the number of violations of the standard occurring in
each year of record for a matrix of conditions including 0, 10 and 20% nonpoint source TP
reduction and 20, 40, 60, 80, 100, 125, 150, 175, 200 and 300 mg∙m3 TP in the METRO
effluent. Develop a family of curves plotting frequency of violation (for the 30-year period
of record) versus METRO effluent TP for each reduction in nonpoint P source inputs.
Establish an acceptable % violation for this system and develop a program of nonpoint
source reductions and METRO effluent concentrations that would meet that objective.
In-Class Discussion Questions
1.
Describe the broad overview of the management objectives in developing this model.
2.
Why is it important to accommodate interannual variability in tributary loads?
3.
What is the physical meaning of the slope and intercept coefficients in the C/Q equations?
What do they tell you about differences and similarities in the two tributaries?
4.
Why do we select the May-Sep period in defining our lake-water phosphorus goal?
Would this hold for most lakes?
5.
If we consider achievement of the phosphorus objective as a step toward sustaining this lake
environment, how might we equitably partition the responsibility for load management?
What role might effluent trading play here?