1. Glenn E. Moglen Department of Civil & Environmental Engineering Virginia Tech Introduction to NRCS/SCS Methods (continued) CEE 5734 – Urban Hydrology and Stormwater Management Lecture 5

2. For next time: Please install VTPSUHM program available at Scholar under, “Resources: Week 3” Review from Lecture #4 Basic Equations Curve Number Questions from last time? Continue NRCS Methods Time of Concentration SCS Graphical Peak Discharge Method Begin SCS Tabular Method Design Storm Construction Today’s Agenda

3. SCS Method – Basic Concept P IaIa F Q time S = potential storage Premise: ratio of filling of storage ( F/S ) equals ratio of outputs ( Q ) relative to inputs ( P-I a )

4. P [=] inches F [=] inches S [=] inches I a [=] inches Q [=] inches (note: NOT Q [=]ft 3 /s) CN [=] inches -1 SCS Method – A Note on Units

5. Actual retention is: Substituting into earlier equation: Re-arranging to solve for Q : SCS Method – Basic Concept

6. Initial abstraction is often assumed to be a fixed percentage of storage: Substituting in equation for runoff: Recent research suggests f may be as little as 0.05: SCS Method – Basic Concept

7. So the central quantity is “ S ” (potential storage). How do we determine “ S ”? Enter the “Curve Number” ( CN ) Value between 0 and 100 Higher CN indicates less infiltration CN varies spatially CN = f (Land Use, Soil Type, Soil Wetness) Relationship between Curve Number and Storage SCS Method – Curve Number

8. Table 9-5: Urban part of overall CN table

9. Ex: TIA=P imp =20%, R=0.5. CN p =75 – yields CN c =78 Urban Areas: Unconnected Imperviousness Example (Nomograph)

10. > TIA=P imp =20%, R=0.5. CN p =75 Urban Areas: Unconnected Imperviousness Example (Equation) (1-0.5R)

11. Time of Concentration Def: Time of concentration ( T c ): is the time for runoff to travel from the hydraulically most distant point (see Point “A” below) of the watershed to a point of interest (Point “D” below) within the watershed. SCS segments the flow into: Sheet Flow (AB) Shallow Concentrated Flow (BC) Channel Flow (CD)

12. Time of Concentration – Total Travel Time Time of Concentration is just the sum of all the individual segment travel times T t, sheet T t, shallow T t, channel sheet shallow channel TcTc

13. Time of Concentration – Sheet Flow First L (<100) feet of longest flow path T t,sheet : travel time [hrs] n : from table at right L : length(<100) [ft] P 2 : 2-year, 24-hour precip [in.] S : land slope in [ft/ft]

14. T t,sheet : [hrs] n : woods, some underbrush: 0.4 L : 100 feet P 2 : 2.64 inches (from Blacksburg, NOAA 14) S : 0.01 in [ft/ft] Time of Concentration – Sheet Flow - Example Solve for T t,sheet >

15. Time of Concentration – Shallow Concentrated Flow Velocity, v shallow [ft/s] is a function of slope and surface conditions (what do you think of these significant figures?!) k = 16.1345 (unpaved) k = 20.3282 (paved) Travel time, T t, shallow [hrs] is then just length of shallow concentrated flow path divided by velocity

16. Time of Concentration – Shallow Concentrated Flow - Example Unpaved surface S = 0.04 L = 1000 feet Result (from graph): v = 3.2 ft/s Solve for T t, shallow >

17. Time of Concentration – Channel Flow Manning’s equation is used to determine the velocity, v channel [ft/s] in the channel flow portion of the longest flow path: Travel time, T t, channel [hrs] is then just length of channel flow path divided by velocity

18. 4 ft Time of Concentration – Channel Flow - Example Use Mannings n = 0.011 S = 0.005 ft/ft L channe l = 5000 ft Solve for T t, channel > 1.5 ft

19. Time of Concentration – Summing Travel Times T t, sheet : 0.52 hrs T t, shallow : 0.09 hrs T t, channel : 0.16 hrs Solve for T c >

20. Q p : peak discharge [ft 3 /s] q u : unit peak discharge [ft 3 /(s-in-mi 2 )] A : drainage area [mi 2 ] Q : runoff [in] F p : pond & swamp factor SCS Graphical Peak Discharge Method

21. SCS Graphical Peak Discharge Method – Storm Distributions

22. Unit Peak Discharge, q u TcTc ququ See Figure 4-II in TR-55

23. Analytical form for q u : C 0, C 1, and C 2 depend on storm distribution: See Table F-1 Unit Peak Discharge, q u

24. “The F p factor can be applied only for ponds or swamps that are not in the T c flow path.” Pond & Swamp Adjustment Ponds & Swamps

25. Given: A : 4.3 mi 2 CN : 75.1 P : 10-year, 24-hour event =4.08 in T c : 0.77 hrs (from earlier) No ponds or swamps Solve for Q p > Calculating SCS Graphical Method Peak Discharge

26. Non-homogeneous sub-areas (differing geology, development, curve number, etc.) Need full hydrograph, not just peak Need hydrographs at one or more internal points within a watershed. SCS Tabular Method – Why do we use it?

27. When NOT to use Tabular Method

28. Q p : peak discharge [ft 3 /s] q u : unit peak discharge [ft 3 /(s-in-mi 2 )] A : drainage area [mi 2 ] Q : runoff [in] F p : pond & swamp factor Comparison – Graphical Peak Discharge vs. Tabular Method Q t : peak discharge [ft 3 /s] at time, t q t : unit peak discharge [ft 3 /(s-in-mi 2 )] at time, t A : drainage area [mi 2 ] Q : runoff [in]

29. Examining a sheet of q t values Comment on: q t ordinates, hydrograph time increments, travel time increments, T c values, I a /P values, visible trends in the q t ordinates.

30. Rounding “Rules” (Time) Round T c and T t separately to the nearest table value and sum. Round T c down and T t up to the nearest table value and sum Round T c up and T t down to the nearest table value and sum From these three alternatives, choose the pair of rounded T c and T t values whose sum is closest to the sum of the actual T c and T t. If two rounding methods produce sums equally close to the actual sum, use the combination in which rounded T c is closest to actual T c. Valid T c Values: {0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.75, 1.0, 1.25, 1.5, 2.0} Valid T t Values: {0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.75, 1.0, 1.25, 1.5, 2.0, 2.5, 3.0} Example: T c = 0.7, T t = 1.3,

31. Rounding “Rules” (cont.) Comment on interpolation of T x : - Don’t do it! I a /P : “The computed I a /P value can be rounded to the nearest I a /P value in exhibits 5-I through 5-III, or the hydrograph values (csm/in) can be linearly interpolated because I a /P interpolation generally involves peaks that occur at the same time.”

32. Sub-dividing a Watershed 1 2 3 12 3 Idealized System Actual System

33. Simple Example – Consider a Single Sub-Area CN = 75 S = 1000/(75) – 10 = 3.33 in I a = 0.2(3.33) = 0.67 in P = 3.05 in (arbitrary – for this example) Q =(3.05 – 0.67) 2 / (3.05 + 3.33 – 0.67) = 1.00 in I a /P = 0.67 / 3.05 = 0.22 (use 0.3) Assume Type II storm (see comment on Type II) Use T c = 0.5 hrs. Determine at outlet of subarea, T t = 0 hrs. From Exhibit 5-II (Sheet 5 of 10) ordinates ( q t ) are: {0, 0, 0, 1, 9, 53, 157, 314, 433, 439, 379, 299, 237, 159, 118, 95, 81, 71, 65, 56, 50, 46, 42, 38, 34, 31, 30, 28, 25, 22, 19, 0} T c = 0.5 hrs

34. Aside on Type II storm Based on TP-40 NOAA Atlas 14 suggests new design storm distribution How to build a design storm?

35. Aside on Type II storm 0.1 hour timestep data provided at class website. Question: How does Type II storm compare to symmetric 24-hr storm from NOAA Atlas 14?

36. Aside: How to build a design storm… AEP, 100-year depths for BLACKSBURG 3 SE, VIRGINIA (44-0766) are shown in above table (units are inches). 5 min10 min 15 min 30 min 60 min 120 min 3 hr6 hr12 hr24 hr 0.6230.9911.251.922.643.163.374.135.126.51 Example: Distribute two shortest durations around 12 hours.

37. Aside: How to build a design storm… Question: Do we get the same design storm at different locations? Question: Do we get the same design storm at same location for different frequencies? RP yrs 5 min 10 min 15 min 30 min 60 min 120 min 3 hr6 hr12 hr24 hr 20.3250.5220.6560.9061.141.321.411.722.082.54 1000.6230.9911.251.922.643.163.374.135.126.51 AEP, BLACKSBURG 3 SE, VIRGINIA (44-0766) (units are inches).