Cracking dams intro

Worksheet

for the advanced level
on gravity dams



1. Read step 1 of the Process on the web. Click on the link to the National Inventory of Dams and search for the Narrows Dam on the Little Missouri River. To search the database:

Click on the [+] next to Search in the left column.
Click on By General Info.
Fill in the Dam Name.
Move down the page and click on the Search button.
Once the results appear, find the dam you are looking for and click on its name to see more information about it.
 Fill this table in with information about the dam. There is a key for the symbols at the end of the worksheet.

dam name .
river .
city, state .
type .
purposes .
height (ft) .
d.s. hazard pot. .
emerg. action plan .




2. What are the purposes of the Narrows Dam? Use the key at the end of the worksheet ***.



Return to the Process on the web. What are two other services that dams can provide?



Of these services, what does your group think are the most important services a dam provides?




3. Return to the Process on the web. The Narrows Dam is causing salmon in the Little Missouri to die. What are some other dams that are also causing fish to die?



What are two examples of solutions to this problem?




4. Return to the Process on the web. The government tells you that the dam is cracked in two places and you will need to do a computer simulation to determine if the cracks will grow. You need to gather the tools to do this simulation. From the Gravity Dams section, you must find three tools:
  • Tool #1: how to draw a cross-section of your dam. A cross-section is a two-dimensional slice through the dam, such that the dimensions of the dam that you see are the height of the dam and the width of the dam (and not the length of the dam). You must find what the shape of the cross-section looks like.

    From the information you gathered about the dam, you know the height but not the width of the base or the crest. Find the simple formulas for calculating the base and crest widths (in feet) of the dam in the gravity dams section.

    base width =

    crest width =


    Now, sketch a scale model of your dam.

    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...
    ... .... ... ... ...



  • Tool #2: how is the dam attached to the ground. Find this tool in the Gravity Dams section. Show where the dam is attached to the ground on your sketch of the dam.

  • Tool #3: what are the forces on the dam. For this simulation, you should apply two forces, the weight of the concrete of the dam and the force of the water in the reservoir. From the Gravity Dams section, you should learn how to calculate both of these forces:

    weight of the concrete =





    force of the water =





    You should also see where these loads are applied on the dam in a simulation.

    Calculate the centroid of your dam:





    Show these forces on the sketch of your dam.


    •

    5. Return to the Process on the web. From the Cracks section, you must find two more tools for your simulation. The government tells you that one crack is on the downstream side of the dam, half way up. All they will tell you about the other crack is that it is the place where gravity dams usually crack.
  • Tool #4: where do gravity dams usually crack. You must find this location in the Cracks section. Once you have found it, sketch the two cracks on your dam. The cracks must start from an outside edge of the dam

  • Tool #5: what do the simulation results mean.

    What does the simulation return to you?



    What do these results tell you?



    •

    6. Return to the Process on the web. In the Simulation section, review the steps of performing a simulation of cracking in a dam. Then move on to the next page to do the simulation yourself. Be sure to watch the movies on how to perform each step.

    Attach the dam as you have attached it to the ground in your sketch.

    Add the loads that you have drawn on your dam.

    Add the cracks you have sketched on your dam.

    Simulate!


    7. Post a message on the bulletin board under the heading "Simulation Results" about the results of your simulation -- did the cracks grow? Also post a message under "Using Simulations" about why computer simulations might be helpful. There is a link to the bulletin board from the Process section step 7 of the Webquest.


    8. Return to the Process on the web. Find the population of the city nearest the Narrows dam in the National Census Bureau.

    What is the population?



    Return to the Process on the web. Use the city population as the number of people at risk in Brown and Graham's algorithm. Try out the algorithm for three different warning times: one hour, 2 hours, and some time great than 2 hours. Also try each one with a landscape of a canyon and then of a plain. Fill your estimates in the table below.

    Estimates of loss of life: Brown and Graham's formula
    .warning time
    landscape 1 hour 2 hours ____ hours
    canyon ...
    plain ...



    Which combination of time and place caused the greatest loss of life? The least? Why do you think this is so?



    What else would you take into account in the loss of life estimation for a dam failure?



    Post your answer on the bulletin board. There is a link to the bulletin board from #8 in the process section of the Webquest.


    9. Return to the Process on the web. What were the causes of cracking at Fontana Dam?





    What things were done to the Fontana Dam once the engineers realized it cracked? These are perhaps things that might be able to remedy the cracking in your dam as well.






    10. Considering:
  • the services the Narrows dam provides
  • the impact the dam has on salmon
  • the cracking in the dam
  • and the case history about another cracked dam,
    •  do you think the dam should be repaired or decommissioned?




    Post your message on the bulletin board. There is a link to the bulletin board from #10 in the process section of the Webquest.


    Key for the NID database
    Dam Type PurposeHazard Pot. Emerg. Action Plan
    RE - Rockfill I - Irrigation L - Low Y - yes
    PG - Gravity H - Hydroelectric H - High N - no
    CD - Buttress C - Flood and storm control S - Significant NR - not required
    VA - Arch N - Navigation . .
    MV - Multi-Arch S - Water supply . .
    CN - Concrete R - Recreation . .
    MS - Masonry P - Fire protection, small farm pond . .
    ST - Stone F - Fish & wildlife pond . .
    TC - Timber Crib Debris control . .
    OT - Other T - Tailings . .
    . O -Other . .