Infiltration and Inflow Homework Assignment
The attached spreadsheet file contains sewer flow data and rainfall data for a sewershed in a City in CT. The data include flow meter and rain gauge output for 15-minute intervals over a 5+ week period during the spring of 2010. From these data, calculate the following:
1. Peak Infiltration Rate (MGD)
2. Peak Inflow Rate (MGD)
3. Average Domestic Sewerage Flow Rate (MGD)
Your deliverable for this assignment is a technical memorandum (2-4 pages) and accompanying figure(s), probably a graph. The memorandum should:
• Be addressed to the City Council
• Describe what infiltration and inflow are and why they are of concern for the City (thus demonstrating to me that you understand the concept and that you can explain it to a non-technical audience). You may use graphics as long as the source is cited.
• Summarize the data that was evaluated
• Explain the method that you used to analyze the data, including any assumptions that you made
• Describe any calculations or graphical interpretations that you make, attaching the detailed calculations (if any) as an appendix.
• If you used a graph (hint), then it should be referenced and attached
• Conclude by reporting the three values that you’ve calculated above. While a real technical memo would also probably include recommendations, you do not need to make recommendations for this assignment.
Peak infiltration rate is defined as the average of the minimum flow rate observed over a period of several days, during high groundwater conditions which usually occur in early spring, after snow melt and spring rains, and is generally measured during times when domestic sewerage flows are low (midnight-6 a.m.).
Peak inflow rate is defined as the largest inflow rate determined over a one hour period which is generated from a single storm event. The peak inflow rate is the largest rate difference between the storm event hydrograph and the dry weather hydrograph (usually for the day before the rain event, at the same time of day).
Average domestic sewerage flow rate is not specifically defined. You need to figure out how you would estimate this and make that methodology clear in your write up. If you have trouble, see me during office hours.
To get started, I suggest making another table that summarizes the minimum, average and maximum flow for each calendar day and then plotting those on a graph (with days on the x axis), yielding three lines. On the same graph, superimpose the rainfall data (using a different y axis), usually done with a bar graph. This should help you answer questions 1 and 3.
To answer question 2, you will need to focus on hourly data for the days when it rains and the day before.
You will be applying engineering judgement to data and therefore, each of you may interpret differently. If the methodology is correct and your assumptions are sound, then you will get a “correct” answer.
This report is something you should save as it might be useful when interviewing for a job.
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