Urban Hydrology for Small Watersheds: Estimating Runoff with TR-55 Method, Slides of Water and Wastewater Engineering

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Urban Hydrology for Small Watersheds 1 Docsity.com Simplified methods for estimating runoff for small urban/urbanizing watersheds • Ch 1 Intro • Ch 2 Estimating Runoff • Ch 3 Time of Concentration • Ch 4 Peak Runoff Method • Ch 5 Hydrograph Method • Ch 6 Storage Volumes for Detention Basins 2 Docsity.com TR-55 (General) • Whereas the rational method uses average rainfall intensities the TR-55 method starts with mass rainfall (inches-P) and converts to mass runoff (inches-Q) using a runoff curve number (CN) • CN based on: – Soil type – Plant cover – Amount of impervious areas – Interception – Surface Storage • Similar to the rational method--the higher the CN number the more runoff there will be 5 Docsity.com TR-55 (General) • Mass runoff is transformed into – peak flow (Ch 4) or – hydrograph (Ch 5) using unit hydrograph theory and routing procedures that depend on runoff travel time through segments of the watershed 6 Docsity.com Rainfall Time Distributions • TR-55 uses a single storm duration of 24 hours to determine runoff and peak volumes • TR-55 includes 4 synthetic regional rainfall time distributions: – Type I-Pacific maritime (wet winters; dry summers) – Type IA-Pacific maritime (wet winters; dry summers-less intense than I) – Type II-Rest of country (most intense) – Type III-Gulf of Mexico/Atlantic Coastal Areas • Rainfall Time Distribution is a mass curve • Most of upstate NY is in Region II 7 Docsity.com Limitations of TR-55 • Methods based on open and unconfined flow over land and in channels • Graphical peak method (Ch 4) is limited to a single, homogenous watershed area • For multiple homogenous subwatersheds use the tabular hydrograph method (Ch 5) • Storage-Routing Curves (Ch 6) should not be used if the adjustment for ponding (Ch 4) is used 10 Docsity.com Ch 2 Determine Runoff Curve Number Factors: • Hydrologic Soil Group • Cover Type and Treatment • Hydrologic Condition • Antecedent Runoff Condition (ARC) • Impervious areas connected/unconnected to closed drainage system 11 Docsity.com Hydrologic Soil Group 1. A-High infiltration rates 2. B-Moderate infiltration rates 3. C-Low infiltration rates 4. D-High runoff potential 12 Docsity.com Hydrologic Condition Poor Fair Good Description in table 2-2 b/c/d 15 Docsity.com Antecedent Runoff Condition (ARC) Accounts for variation of CN from storm to storm Tables use average ARC 16 Docsity.com Impervious/Impervious Areas • Accounts for % of impervious area and how the water flows after it leaves the impervious area – Is it connected to a closed drainage system? – Is it unconnected (flows over another area)? • If unconnected – If impervious <30% then additional infiltration will occur – If impervious >30% then no additional infiltration will occur 17 Docsity.com Modifying CN using Figure 2-4 • If impervious area < 30% but not connected then use Figure 2-4 20 Docsity.com Determining Q (runoff in inches) • Find rainfall P (Appendix B) • Find Q from Figure 2-1 • Or Table 2-1 21 Docsity.com Determining Q (Table 2-1 Table 2-1 = Runoff depth for selected CN’s and rainfall amounts a Runoff depth for curve number of— Rainfall 40 45 50 5b 60 65 70 TS ao ah a0) o5 98 inches —. 10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.08 O17 0.32 0.56 O7o 12 0 oo a0 00 00 a0 03 OT 15 27 AG 7: go 14 00 00 00 00 00 02 06 13 24 0 61 92 1.18 L6G Oo oo ap 00 o1 08 uu 20 et Ba 76 Lil 18 00 00 00 00 03 09 AT 20 -H 65 83 1.29 20 00 00 00 02 06 14 24 aS 56 80 1.09 148 25 00 00 02 08 7 30 46 6 a0 1.18 LS3 1.96 3.0 00 02 09 19 33 SL TL 6 1.25 150 Los 2.45 35 02 08 26 53 Th LOL 1.30 Lt 2.02 245 2.04 40 6 18 53 76 1.08 1.33 1.67 2.04 2.46 292 3.43 , 45 14 ao 74 1.02 1.33 167 2.05 2.46 2.91 S40 3.88 4.26 5.0 24 A 8 1.30 1.65 204 245 230 337 3.88 443 A76 6.0 50) 0 152 1.92 2.35 2.81 3.28 3.78 4.30 455 5.41 5.76 7.0 84 1.24 2.12 260 310 S082 4.15 4.60 5.25 5.82 6.41 6.76 a0 L265 L.74 2.78 333° 3.80 4.46 5.04 5.03 6.21 6.31 7.40 7.76 o.0 L71 2.20 3.49 410 472 5.33 5.05 6.57 TAS 7.79 (B40 B76 10.0 223 2.80 423 4.50 5.56 6.22 6.88 7.52 5.16 8.78 8.40 O76 11.0 278 362 4.26 5.00 5.72 6.43 713 TAL 348 9.13 O77 10.389 10.76 12.0 3.38 4.15 5.00 5.79 656 ‘7.82 &05 8.76 $45 1011 1076 1139 1176 12.0 4.00 4.80 5.76 6.61 742 B.21 a8 O71 W420 1110 1L76 1239 0 12.76 14.0 465 5.62 6.55 744 5.30 9.12 8.01 W687 LL 12.08 17S 188 1.76 15.0 5.33 6.38 7.35 8.29 9.19 10.04 10.85 1L63 0 1287 107 13874 1489) 14.76 L/ Intexpolate the values shown to obtain runoff depths for CN's or rainfall amounts not shown. 22 Docsity.com TR-55 Example Docsity.com Project Worksheet 2: Runoff curve number and runoff By Location Date Checked Dette Check one: Oo Present O Developed eam al 1g . 1 Soil name Cover description cn” Area Product and of hydrologic CN xarea group 7 ou 2 Z| Gacras (cover type, treatment, and hydrologic condition; parcart 3 g 2 ome? (appendix A) imparvious; unconnected/connected impervious area ratio) 2 2 P low © Use only ona CN source per lina Totals > CN (weighted) = total product _ = Use CN total area Storm #1 Storm #2 Stonm #3 Frequency Rainfall, P (24-hour) oes Runoff, @ (Usa Pand CN with table 2-4, figure 2-4, or equations 2-3 and 2-4) 26 Docsity.com Examples • Example 2-1 (undeveloped): – Impervious/Pervious doesn’t apply • Example 2-2 (developed): – Table assumptions are met • Example 2-3 (developed): – Table assumptions not met (Figure 2-3) • Example 2-4 (developed): – Table assumptions not met (Figure 2-4) 27 Docsity.com iweb 70.1 CN (weighted) = _ lal product _ 7010 = + Use CN 70 total area 700 » Storm #1 Storm #2 Storm #3. Frequency... w 25 Rainfall, P (24-hour)... h 6.0 Runof, @ h 2.81 (USSF and CH wih table 2-4, 1gur8 2-4, oF equator 2-4 and24} Table 2-1 Runoff depth for selected CN’s and rainfall amounts L’ as Runoff depth for curve number of— Rainfall 40 45 50 55 60 65 70 75 a0 55 90 95 O58 inches — Lo 0.00 o.00 0.001 6.001 0.00) o.00 0.00 0.03 0.08 O17 0.32 0.56 O.7o 12 00 00 00 00 00 00 08 OF 15 27 AG ae 9 14 00 Oo O00 00 00 2 06 3 24 38 61 a2 1.18 16 00 00 00 00 OL 05 ll 20 a4 52 76 Ld 1.38 18 00 00 00 00 03 00 17 29 44 85 3 120 LBS 20 O00 00 00 02 06 4 24 8 56 80 1.09 148 L.a7 25 00 00 02 OB 7 30 id 30 1.18 1.53 1.96 27 a0 oo 02 Oo 9 a3 1 TL 1.25 Loo 108 245 LTT 35 02 08 20 36 53 5 LOL Let 2.02 245 2.04 3.27 40 O06 18 a8 5a 76 1.03 1.33 204 2.46 2.02 3.43 3.77 45 14 a) 50 Wa 1.02 133 LeT 246 2.91 340 3.98 5 24 4 6a 8 1.30 L65 204 50 80) 1.14 L52 2.95 2.8L 3.78 450 485 5.41 34 Lod L6s 212 125 LT 2.25 2.78 3.10 3.62 415 4.60 5.25 5.82 6Al 4 280 S37 3.88 442 4 5 6. 89 4.46 5.04 5.63 621 6.81 7.40 7. Yuaus w& aaa a 30 Docsity.com Examples • Example 2-2: – Land is subdivided into lots – Table assumptions are met 31 Docsity.com Worksheet 2: Runoff curve Pa Heavenly Acres teat" Der County, Tennessee check one: Clrrvaert [El oavsoped TIS Acres residential 4. Runoff curve number sol name Cwyardaseripfon oN | area | Frodat and a ctokoghe CNxare sp a || a |ifame oover oa beats and iydrolople condone porceat alg § [om (appancte 8) Inparview; ucomodstternedsdinpoviasoreart} =| @ | & | |e " 25% impervious Memphis, B V2 acre lots, good condition 70 75 | 5250 : 25% impervious Loring, C V2 acre lots, good condition 80 too | 8000 Loring, C Open space, good condition 4 qq | 8550 7 may aa One 1 Usa only ana CH ecurca pr ina Totals m | 250) 13,800 CNiweighied) = alpedict 18,800 _ 75.2 Use CH i total area o Bea ‘Storm #1 Som a2 ‘Skm #3 Frequency .... 25 Raintal, P 24-hour. 60 PUNO tn 3.28 (Usa Panel Cher tatla 2-4, gure 24, or atere 23 and 24 Docsity.com Worksheet 2: Runoff curve number and runoff Heavenly Acres wr Paes fehacead Casal ™ Dyer County, Tennessee NM Pa yora/g5 checkonx Klprouot [i covotpea el Sol name Cover deseripton area Prog le at bydrodogie CNX afea me eo | ao | st | Gace [novor typo. raatnent, and hydrologic cnction porceat a 2 | onl iene} Mpogucewecmmmasst mean aaa) | 3 |B | B |S : 35K impervious Memphis, B 1/2 acre lots, good condition 74 75 | 3550 : 35% impervious Loring, € 1/2 acre lots, good condition 82 too | 8200 Loring, C Open space, good condition 74 15 5550 Y Ueeoriy cae cheat pa tha Totals | 250) 19,300 CH (weighted) = ta product 19.300. 772 total area 250 1 Use CN ‘Storm 41 tam 2 som #2 Frequency... w 25 Raiolall, P (24-foun) 6.0 (isan as ane, or mums sanz Docsity.com Using Figure 2-3 • Pervious CN’s were 61 and 74 – Open space; good condition; same as first example • Start @ 35% • Go up to hit CN 61 & 74 curves • Go left to determine new CN=74 & 82 36 Docsity.com Figure 2-3 Composite CN with connected impervious area. 100 Composite CN Pervious CN +90 20 30 40 50 60 70 Connected impervious area (percent) 80 90 100 Docsity.com Using Figure 2-4 • Pervious CN is 74 – Open space; good condition; same as first example • 50% unconnected • Start @ the bottom (right graph) @ 25% • Go up to 50% curve • Go left to pervious CN of 74 • Go down to read composite CN of 78 40 Docsity.com Figure 2-4 a Composite CN with unconnected impervious areas and total impervious area less than 30% 70 60 90 80 70 60 Composite CN 50 40 10 20 Total impervious area (percent) 30 0.0 0.5 e 2 = |e &\s Ele 1.0 plo 2\e 5 LE els els = 8\- 2 4 Docsity.com Example Comparison • Undeveloped • Developed (25% impervious connected • Developed (35% impervious connected) • Developed (25% impervious but only 50% connected) • Roff=2.81” • Roff=3.28” • Roff=3.48” • Roff=3.19” 42 Docsity.com Chapter 4: Graphical Peak Discharge Worksheet 4 • Inputs: – Drainage Area – CN (from worksheet 2) – Time of concentration (from worksheet 3) – Appropriate Rainfall Distribution (I/IA/II/III) App B – Rainfall, P (worksheet 2) – Runoff Q (in inches) from worksheet 2 – Pond & Swamp Adjustment Factor (Table 4-2) 45 Docsity.com Ch 4 Calculations • Find initial abstraction • Function of CN # • Find in Table 4-I • Calculate Ia/P 46 Docsity.com Ch 4 Calculations • Determine peak discharge (cubic feet per square mile per inch of runoff) from Exhibit 4- I, 4-IA, 4-II or 4-III by using the Ia/P ratio and the time of concentration 47 Docsity.com Compute Peak Flow • Peak flow=Unit peak flow * Inches of Runoff * Drainage Area * Pond/swamp Adjustment Factor 50 Docsity.com Worksheet 4: Graphical Peak Discharge method Heawerty Acres RH I [Shacked KE 4. Davia Drainage ars... mi? (perea!6 40} 75 Fi Rund curva number .. Ss (From worksheet 2, __! 153 = Time of concentration .. CSS Lohr (From workshest aj, FIGUT: Rainfall distribution Matai Pondaand swamp areas sprea throughout watershed ... == parcantct Am f acres or mi? covered) ‘Stem 41 | Storm a2 | Storm #3 & Frequency .. 25 2. Rainfall, P (24-hour} 6.0 4. Initial obetraction, lq n |_O667 {Usa CH with table 4-1) 5. CompulalyP on i i 270 Unit pank discharge, qu .... (Usa T. and I/F with sabibit 4 328 . Figure 2-6 & Pond and swamp adjustmant facta, Fe, Lo (Use percent pond and swamp area with table 4-2. Factor fs 1.0 for zam percantpendans swamp area.) @ Peak discharges, qp . . efs L345 (Where q, = 4,0, } 51 Docsity.com Limitations • Watershed must be hydrologically homogenous • One main stream (not branched) • No reservoir routing • Pond/swamp adjustment factor applied only if not in the time of concentration path • Can’t use if Ia/P values are outside range of 0.1-0.5 • Not accurate if CN<40 • Tc between 6 minutes and 10 hours 52 Docsity.com