Problem 1. (50 pts) An urban watershed in Miami is shown in Figure 1 below, alon

Question

Problem 1. (50 pts) An urban watershed in Miami is shown in Figure 1 below, along with the travel paths from the most remote points in each subarea The details of the subareas are given in Table 1 below, Determine the 25 year peak flow for the drainage outlet G. Use the IDF chart for Miami found on page 342 of your textbook. Use the NRCS Method (TR55) to determine time of concentration and estimate runoff coefficients using Table 15.2.3 (provided in Lesson 14 Notes) Figure 1 Table 1 Nol Area Type Path Length Sl acres 1 14Grass, Fair Condition AE 1600 4 212.5 Forest 31 Asphalt BF 1490 3 CE 1280 2 EF 1300 1.5 DF 1250 2 48.5 Concrete

Explanation / Answer

SOLUTION :

Time Concentration = Tc = Ts + Tc + To

Tc = time of concentration, hours

Ts = travel time for sheet flow, hours

Tc = travel time of shallow concentrated flow, hours

To = travel time for open channel flow, hours

We here with to find using Sheet flow only, to find Time of concentration

Sheet Flow time concentartion

Tt = 0.007 x ( n x l )0.8  / (P20.5 x S0.4)

where:

Tt = travel time, h

n = Manning’s roughness coefficient

= sheet flow length, ft

P2 = 2 year, 24 hour rainfall, in

S = slope of land surface, ft/ft

FOR AE - Gross Fair condition -

Mannings value n = 0.05

L = 1600 ft

s = 0.04

P2 = 5 in

Tt = 0.007 x ( 0.05 x 1600 )0.8  / (50.5 x 0.040.4) = 0.52 hrs

FOR BF - Forest -

Mannings value n = 0.202

L = 1490 ft

s = 0.03

P2 = 5 in

Tt = 0.007 x ( 0.202 x 1490 )0.8  / (50.5 x 0.030.4) = 1.74 hrs

FOR CE - Asphalt -

Mannings value n = 0.011

L = 1280 ft & 1300 ft

s = 0.02 & 0.015

P2 = 5 in

Tt = 0.007 x ( 0.011 x 1280 )0.8  / (50.5 x 0.020.4) = 0.18 hrs

Tt = 0.007 x ( 0.011 x 1300 )0.8  / (50.5 x 0.0150.4) = 0.21 hrs

FOR DF & FG - Concrete -

Mannings value n = 0.011

L = 1250 ft & 1510 ft

s = 0.02 & 0.015

P2 = 5 in

Tt = 0.007 x ( 0.011 x 1250 )0.8  / (50.5 x 0.020.4) = 0.18 hrs

Tt = 0.007 x ( 0.011 x 1510 )0.8  / (50.5 x 0.0150.4) = 0.24 hrs

So Total time of Concentraction = 0.52+1.74+0.18+0.21+0.18+0.24 = 3.07 hrs

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