# Determine the Hydrologic Soil Group

## Determine the Hydrologic Soil Group

Detailed Instructions

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 AnalysisPurpose: 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).

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