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

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).
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.