All of the questions please.

Figure 1: Hydrograph showing the difference between floods in a natural environment and urban setting.

The Sandy River has been monitored by a gaging station for the last 99 years. Over that time, the average stage of the river is 619.3 feet. Bank-full level through the city is at approximately 621.0 feet with flood stage at 623.5 feet. The first flood maps for the city were produced 25 years ago (after 74 years of data had been collected). At that time, scientists used the discharge and stage data to estimate the size of a 100-year flood for the river in Mosston. The graph below (Figure 2) shows a best fit line on a recurrence interval vs stage graph for the Sandy River (based on the first 74 years of data).

Figure 2: Graph of estimated discharges based on recurrence interval for the Sandy River in Mosston.

5. Examine Figure 3 (map of Mosston). Using a colored pencil, outline and shade in the land area that would be under water at flood stage (623.5 feet). Any land along the river that is lower in elevation than the flood stage level lies within this zone. Carefully examine the contour lines on the map and locate the position of the flood stage elevation relative to the plotted contours. Draw lines in the appropriate locations along both sides of the rivers. Be sure to fill in the color used in the appropriate box on the key beneath the map.

6. Based on the best-fit line on Figure 2, what is the estimated stage of the 25-year flood?

The 25-year floodplain represents the land that would be underwater with the predicted discharge of a flood with a 25-year recurrence interval. When these studies were conducted, city officials decided to prohibit the placement of new homes or businesses within the 25-year floodplain.

7. Using a regular pencil, draw in the estimated location of the 25-year floodplain on the map.

8. Do any structures currently exist within the 25 year floodplain? If so, what?

9. Based on the best-fit line on Figure 2, what is the estimated stage of the 100-year flood?

10. How high above flood stage is the predicted stage of the 100-year flood?

11. Using a different colored pencil, outline and shade in the additional land area that would be under water during a 100-year flood. Fill in the key with the correct color.

In the 25 years since the last flood maps were produced, the population of Mosston has nearly tripled. More forests and farmland have been removed and turned into residential, business, and industrial zones.

12. Is the flood map that you’ve produce (Figure 3) based on the first 74 years of river data still reliable? Why or why not?

13. Do you think the 100-year floodplain will be smaller, the same size, or larger when including all 99 years of data?

14. Examine the map of Mosston (Figure 3). Which buildings do you think will be impacted by the change to the floodplain (circle them on the map)? Why?

The growing population has forced the city to look for new sites (A, B, C, D) to expand. However, city officials have noted that 8 of the 10 largest floods in the 99-year record of the Sandy River have occurred in the last 20 years. Based on the increased frequency of floods, officials realized a need for an updated flood map. They hire your company to analyze the data, produce a new flood map, and make recommendations for the expansion of the city. To do so, you need to reassess the data and see how the size of the 100-year floodplain has changed.

To help in your assessment, you need to calculate the recurrence intervals for the ten largest floods (listed in Table 1). Recurrence interval is calculated using the following formula:

where: T = recurrence interval (in years)

n = the number of years of record

m = the rank of the annual flood discharges/stages from the greatest (1) to the

smallest for the number of years of record

Table 1 lists the peak stages of the ten largest floods that occurred on the Sandy River in the last 99 years. As noted above, 8 of these 10 occurred within the most recent 20 years.

15. Place an “X” in the “Exceeded flood stage” column for all peak stages that were greater than flood stage.

16. Rank the stages from greatest (1) to smallest (10).

17. Calculate the recurrence intervals for these ten largest stages. For the calculations here, “n” will be 99 since you have 99 years of data recorded for this river (even though you are only recording the calculations of ten here).

Table 1: Rank and recurrence interval for the ten largest recorded stages on the Sandy River in the last 99

years.

18. Based on your calculated recurrence intervals above, what is the new stage for a 100-year flood?

19. Using a different colored pencil, outline and shade in the land area that would be included in the new 100-year floodplain. Fill in the key with the correct color.

20. Based on your new flood map, rank the sites officials are proposing for new development from best (1) to worst (4).

A___________ B___________ C___________ D___________

21. Describe why the site you ranked as 1 the best for new development.

22. Describe why the site you ranked as 4 the worst for new development.

23. The city takes your advice and decides to convert the site you selected as 4 to one of the following uses: farmland, baseball fields, or a nature preserve. Which of these three proposed uses do you think is best for this land? Describe at least two reasons why your choice is better than the other two choices.

24. A new major industry wants to move to Mosston. Despite your findings, the owners of the industry are pushing to place its industrial complex at the site you ranked as 3. The owner proposes that the city construct a levee system with a height 5 feet above the 100-year flood level. If the levee system is not built, the owner will likely choose a different city to build the industrial complex. How do you respond?

Yes, we will build the levees. or No, we will not build the levees.

Provided a detailed justification for your response.

Part 2: Flooding along the St. Croix River, Minnesota

The topographic map provided shows portions of the Stillwater, MN-WI quadrangle along the St. Croix River. This map was published in 2013. The contour interval of this map is 10 feet. Flood stage for the river in Stillwater is 687 feet. Once the water level rises to 680 feet, an action stage is declared. A stage of 688 feet is considered a moderate flood and a stage of 689 feet is considered a major flood in this region.

25. Describe the topography and level of development along the Wisconsin side of the St. Croix.

26. Describe the topography and level of development along the Minnesota side of the St. Croix in the Stillwater area.

27. Describe the topography and level of development along the Minnesota side of the St. Croix in the Bayport area.

28. Rank the three locations you describe above for their flood risk with 1 being the greatest risk. Be sure to examine the contour lines to determine where the flood stage elevation exists in each of those locations to help you determine the ranking.

________ Wisconsin ________ Stillwater ________ Bayport