CE55504 Surface Water Quality Modeling

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 30- year 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 METRO flow = 80 MGD Surface area = 1.20x107 m2 TP Standard = 20 mg∙m-3 TP settling velocity = 100 m∙yr-1

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 do we select the May-Sep period in defining our lake-water phosphorus goal? Would this hold for most lakes?

3. 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. For what intentions would we apply the C/Q and UAL approaches to load estimation?

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?