Civil Engineering Storm water design

Design Problem

Use the Figure (map on the next page) to determine the peak discharge for the

forested drainage basin located in Bath County, VA for the 25-year storm.

You will have to measure and estimate the:

? Drainage area ? Length and slope of Overland Flow ? Length and slope of Shallow Concentrated flow ? Length and slope of the open channel

? Assume the stream is a trapezoidal channel with a base width of 4 feet,

having side slopes of s = 2.5 and the Mannings roughness coefficient is .035 @ a depth of flow of 2 feet.

Updates:

1. You need to find the longest flowpath in your watershed. I am showing to you to

options on the sketch. One of them is correct and one is wrong. You need to tell me which one is the correct.

2. Along the flowpath, assume that the first 200 refer to overland flow. Calculate the slope of the ground and then use the following chart to find the velocity (you have forested area). Then you need to think how you can calculate the time of overland flow knowing the velocity and length.

3. The shallow concentrated flow starts at the 200 and ends before the channel.

Measure the length and determine the slope. Use the graph above to determine the velocity and think how you can determine the time of the concentrated flow (similarly to the previous one)

4. Use the information provided and Mannings equation to determine the time for the channel flow. You will have to measure the length of your channel. The channel was provided to you on the original sketch. See below.

5. After you find all the times add them together. This is the time to concentration.

Then use the following graph to find the correct rainfall intensity for the 25-yr storm (you should not use the one I originally gave you).

Channel

6. Use the ciA formula to calculate the peak runoff rate at the outlet of your

watershed. To find the area A either use Autocad or break your watershed into smaller areas based on the scale that I gave you. I would probably break it into 200×200 squares as shown below.

7. Use a table to determine the runoff coefficient c for forested area.

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