Directions: You are an energineer performing a hydrologic analysis for a community development in Tafton,
PA. Using the area estimated for the watershed in Homework #1, you are to estimate the runoff, Q (in cfs),
using both the Rational Method and the TR-55 Method. You are to comment on whether the Rational
Method is a reasonable first estimate compared to the more in-depth TR-55 Method. The following
assumptions or additional directions should be followed for each method:
Rational Method: Assume the area is comprised of residential lots (> 1/2 acre) and parkland. You are to
determine what the percentages of cover are for a weighted C value. For intensity, i, use the 100-year
storm with a duration of 24 hours (Figure 1.6 in the text).
TR-55 Method: Assume the cover description is 1 acre residential districts and woods in good condition.
You are to determine what the percentages of cover are for a weighted CN value. Use TR-55 Manual
Tables 2-2a and 2-2c. You are to determine the hydrologic soil group (HSG) using the NRCS Web Soil
Survey (WSS). The link is: http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm. Click the “Start
WSS” button when you are ready to search. You must define an “Area of Interest (AOI)” on the map by
clicking the AOI button and highlighting the area you want information for. Then click on the “Soil Map” tab
to view the breakdown of the hydrologic soil groups. Use you judgment to select the best HSG. Print and
include the soil map as part of your submission. Assume a time of concentration of 30 minutes.
Grading:Rational Method = 2 (out of 2 points)
TR-55 Method = 4 (out of 4 points)
NRCS Soil Map = 1 (out of 1 points)
Comparison of both methods = 1 (out of 1 points)
More explaination:
Homework #2: Runoff Analysis
Purpose: To perform a runoff analysis for the Tafton, PA watershed delineation (HW 1).
Problem: Now that we have delineated the watershed for Fairview Outlet, we will estimate for runoff, the
fraction of water from precipitation that empties into uncontrolled surface streams and other bodies of
water. Runoff may be measured directly from stream gauges or estimated using rainfall runoff equations:
the Rational Method (watersheds < 200 acres or 0.3mi2) and the TR-55 Method (watersheds > 200 acres
or 0.3mi2). Although your watershed area is larger than 200 acres, we will practice estimating runoff using
both of these methods and compare the two results.
Estimate runoff for a 100-year storm over a 24 hour duration for the Fairview Outlet watershed. We can
assume a time of concentration, tc, of 30 minutes. The Fairview Outlet watershed near Tafton, PA is in Pike
County, PA – this will help you determine the rainfall distribution type, Hydrologic Soil Group(s), and rainfall
intensity.
Refer to your area (A) in acres that was estimated from your watershed delineation (from HW 1, Google
Planimeter).
Interpolate for rainfall intensity (i) from the 100-year storm for a 24 hour rainfall (in the TR-55 Manual – also
available on Blackboard).
Estimate land uses within your watershed to find the runoff coefficient (c) and the Curve Number (CN). Use
the online NRCS soil survey to determine the Hydrologic Soil Group(s) (HSG) found within your watershed.
Keep in mind that this is an estimation based on the information that is available. You may also find helpful
information on NRCS and online by looking for maps or county information regarding the different types of
land use in this area (remember this is an estimation). Use the TR-55 manual for guidance on completing the TR-55 Worksheets 2 and 4.
Submission: Each student is required to submit the following on Blackboard:
A cover sheet with your full name, class name, homework number, and the date.
Runoff estimation using both the Rational Method and the TR-55 Method. Calculations are to be done by
hand on 8.5” x 11” engineering paper. Show all work.
Compare your Runoff estimation results. On engineering paper, include your final results and a brief
response to the following questions. Were the results close? Which method do you think is more accurate?
How come?
All supporting documents, including a printed copy of your NRCS soil survey, and completed TR-55 Method
worksheets 2 and 4.
Notes: Be sure to follow the instructions on the main page of the NRCS soil survey application –
https://websoilsurvey.nrcs.usda.gov/app/. The NRCS soil survey map will tell you more about the HSG,
types of land coverage, ponds/wetlands, and more. Use this information to guide you for your estimations.
NRCS Soil Survey tips: Look up the county and state of your watershed, then zoom in. Click on the map
icon to define your Area of Interest (AOI) by polygon, and before doing so, click on the topographic map
overlay in the Legend (this will help you define your watershed the same way you did for HW 1 using
ACME Google Planimeter). Double click once you’ve defined all of the points of your AOI to delineate the
watershed. You can then begin to explore the data and overlay information about soil types, land uses, and
more. Be sure to save copies or screenshots of your work.
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