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# Date Peak Discharge (cfs) Rank Recurrence Interval [R] Annual Exceedence Probability

i need help from my geology class home work  . the excel is on the file

This exercise will introduce you to the techniques that geologists use to predict stream floods as well as experience in working with larger data sets in Excel. You will construct a flood-frequency curve for Buffalo Bayou from data supplied in the excel file on Blackboard. You will rank the discharges, calculate the recurrence interval for each flood, and plot the recurrence intervals against the discharges. Then, you will be able to answer questions about the predicted discharges for large floods and their expected frequency of occurrence.

Introduction: It is normal for streams to flood. Damage caused by stream flooding amounts to several billion dollars annually in the United States. These losses can be prevented if buildings are constructed outside the flood plain, or if dikes, artificial levees, retention ponds, and other drainage-control modifications are built to control flooding. Either solution requires that planners be able to predict the recurrence interval and discharge of stream floods. If this information is known, then geographic restrictions can be placed on construction inside flood plains, or flood-control structures can be built to the proper height to protect property already existing inside flood plains. A stream's flood discharge is simply the amount of water, measured in cubic feet per second (cfs) that passes a point on the stream during a flood. This amount is far greater than the normal stream discharge. For Buffalo Bayou, the normal discharge is between 100 and 200 cfs (lower during extended dry periods). The highest recorded discharge was ~40,000 cfs.

A stream's flood frequency is given by the stream's flood recurrence interval, which is defined as the average interval of time, in years, within which a flood of given discharge or larger will occur. For example, a flood having a recurrence interval of 10 years is one that has a 10% chance of occurring in any year. A flood having a recurrence interval of 100 years has a 1% chance of occurring in any year; a flood of this magnitude is called a 100-year flood, and would occur, on average when measured over many centuries, about once every 100 years. It is important to calculate the predicted discharge of a 100-year flood for any stream on whose floodplain structures will be built since planners usually use this number as a reasonable limit for flood plain management. The reasoning is as follows: It is reasonable and prudent to allow construction in areas that will only be flooded, on average, about once every 100 years Another way to state this is the following: There is only a 1% chance that structures will be damaged or destroyed if constructed on a 100-year floodplain, and this chance is a reasonable and prudent level of risk that is acceptable to governments and insurance companies. Of course, it would be even better to place structures outside of even this 100-year floodplain, but this is often not practical, especially in urban conditions.

Computations of flood frequency (recurrence interval) can be made in a systematic manner if records of the annual peak floods of a stream have been kept. Since a flood can vary greatly at different points along a stream as the flood moves downstream, "annual peak flood" refers to the peak discharge for each year at a recording station, and a major stream would have several recording stations. In our problem, we will consider only a single recording station over a period of 86 years. The recurrence interval R for a flood of a given discharge is calculated by the following equation:

R = (N + 1)/m

in which m is the rank of the annual peak stream discharge and N is the number of years of record. In our problem, N = 86.

What you need to do:

• Head to our Blackboard page, under Lecture Powerpoints download the file called “Buffalo Bayou Peak Discharges.xlsx. This file contains yearly peak discharge measurements for Buffalo Bayou, taken from USGS gauge 08074000 at the intersection of Shepherd Dr. and Allen Pkwy/Kirby Dr. The first column is the date of the peak discharge that year, and the second column is the discharge measurement in cubic feet per second (cfs).
• Assign a ranking for each of the 69 peak measurements (in order of 1 to 69). We have peak measurements for 69 of the 86 years. Missing years may be the result of no flooding or corrupted data. This ranking can be easily done for you in Excel by sorting the discharge data. Enter this value in the “Rank” column.
• Calculate the recurrence interval (R) for each of the years on record using the equation above. N is the number of years on record (86) and m is the ranking you assigned to that particular year. Enter this equation once and copy it to the remaining cells. Enter this in the “Recurrence Interval” column.
• If all goes well the graph on the right-hand side should automatically populate. Please note the x-axis is on a logarithmic scale. If this didn’t work for some reason, you will need to create the graph yourself (very easy). Create a scatter plot with recurrence interval as your x-axis and peak discharge as your y-axis. Change the x-axis scale to logarithmic which will help interpreting the data and answering the questions.
• Now you need to calculate the annual exceedance probability (P), this will tell you the probability of an event equal or greater than that size will occur in a given year.
• Answer the questions related to the graph you just made. Download the file Buffalo Bayou Flooding HW.docx to find a copy of this instruction sheet and the questions. Paste a copy of your graph below the questions you answer on the next page.
• Turn in a physical copy of your questions and graph by Friday 4/22 during class. Be sure to save the work you do on the excel file as I may request to see it during grading.

Answer the following questions based on the recurrence interval graph you made for Buffalo Bayou:

• What would be the discharge during a 20-year flood?

• Estimate the discharge during a 100-year flood.

• What is the recurrence interval for a flood with a discharge of 30,000 cfs?

• What is the recurrence interval of the flooding experienced during May of this year?

• What is the probability that a flooding event like May 2015 can occur each year?

[Insert you graph here]

Date Peak Discharge (cfs) Rank Recurrence Interval [R] Annual Exceedence Probability (P) R=(N+1)/m P=(1/R)*100 ### 19000 2 ### 40000 1 4/1/1937 792 69 ### 8490 18 ### 2530 61 ### 1390 67 ### 6220 34 7/7/1942 6220 35 ### 6810 28 ### 10900 7 ### 6400 33 ### 2130 64 ### 6500 32 ### 1700 66 ### 2380 62 ### 4800 50 8/2/1954 4080 54 2/8/1955 2310 63 ### 820 68 ### 7200 25 ### 6060 37 ### 7270 24 ### 5750 41 6/4/1962 4310 53 3/8/1964 2650 60 ### 5150 48 ### 2100 65 ### 4400 52 ### 2930 58 ### 7320 23 ### 9200 13 ### 8570 17 ### 6070 36 ### 5430 45 ### 5650 44 9/8/1977 2810 59 6/7/1978 5660 43 ### 9210 12 ### 3630 55 ### 8830 15 ### 5770 40 ### 8490 19 ### 6800 29 ### 5670 42 7/9/1987 5270 47 ### 3550 57 ### 9000 14 ### 4520 51 4/5/1991 4840 49 3/4/1992 12500 6 3/1/1993 6710 31 ### 8450 20 ### 5370 46 ### 7650 22 ### 13400 5 ### 3560 56 6/9/2001 14000 4 ### 8680 16 ### 10100 9 ### 6060 38 ### 6800 30 ### 9350 11 ### 10100 10 ### 10900 8 7/2/2010 7940 21 1/9/2012 7160 26 ### 6940 27 ### 5920 39 ### 17500 3 1 10 0 2 4 6 8 10 12 Flood Data Flood Data Recurrence Interval (years) Discharge (cfs)

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