**3. Results and Discussion**

*3.1. EPA-SWMM Model Simulation Results for Accumulated Manholes Overflows*

The accumulated manhole overflow from 342 manholes in the study area was simulated using the EPA-SWMM model, and a model input rainfall database was established using diverse extreme rainfall events. The extreme rainfall event database periods were 10-years, 50-years, 80-years, and 100-years. The total rainfall is shown in Table 1, over 1-h, 2-h, and 3-h periods (in 10 min intervals). Table 2 shows the total EPA-SWMM-modelsimulated overflow results for a duration of one hour over different periods in the same quartiles. The extreme rainfall data produced by the Huff method were used.

**Huff Quartile Period (Year) Total Overflow According to Different Exceedance Probabilities (m3) Average (m3) Minimum (m3) Maximum (m3 10% 20% 30% 40% 50% 60% 70% 80% 90% )** 1st 10 110.2 70.2 74.4 72.7 67.1 58.9 46.4 32.0 22.8 61.6 22.8 110.2 50 395.0 206.3 133.3 118.8 122.8 129.7 118.3 117.5 114.3 161.8 114.3 395.0 80 489.7 222.6 167.8 137.0 149.3 142.4 141.4 136.8 132.4 191.0 132.4 489.7 100 536.0 263.9 180.1 149.8 153.9 153.1 149.3 146.6 156.8 209.9 146.6 536.0 2nd 10 102.7 98.9 101.3 101.0 84.0 85.1 82.0 77.9 74.3 89.7 74.3 102.7 50 189.9 187.9 158.0 156.8 157.9 140.1 148.6 201.0 156.1 166.3 140.1 201.0 80 236.4 233.5 182.5 173.6 173.0 160.0 169.4 172.3 183.7 187.2 160.0 236.4 100 290.4 225.9 192.8 236.2 186.4 186.0 179.5 184.0 201.2 209.2 179.5 290.4 3rd 10 71.4 84.0 89.4 85.9 93.6 95.8 93.0 108.0 115.4 92.9 71.4 115.4 50 159.6 168.6 165.8 162.7 159.5 170.7 176.8 200.7 205.8 174.5 159.5 205.8 80 190.2 192.5 185.2 184.8 185.0 202.8 215.2 217.3 268.1 204.6 184.8 268.1 100 207.5 197.4 196.5 195.7 206.7 217.3 232.0 241.8 296.7 221.3 195.7 296.7 4th 10 43.4 54.2 65.5 77.5 88.3 93.8 97.1 111.0 133.2 84.9 43.4 133.2 50 113.7 122.2 119.7 132.2 134.2 153.5 179.1 263.5 486.8 189.4 113.7 486.8 80 139.2 149.1 153.1 154.8 167.7 189.7 241.1 371.7 639.1 245.1 139.2 639.1 100 153.0 152.2 156.8 166.0 184.1 203.0 368.3 447.4 718.2 283.2 152.2 718.2

**Table 2.** The EPA-SWMM-model-simulated 1-h total overflow results.

In a period of 10 years, the Huff method's simulated results for the 1st quartile showed a minimum total overflow of 22.8 m3, a maximum total overflow of 110.2 m3, and an average total overflow of 61.6 m3. In a period of 50 years, the Huff method 1st quartile simulated results showed minimum, maximum and average total overflows with significant increases of 91.5 to 284.8 m<sup>3</sup> relative to the 10-year simulated results. However, in a period of 80 years, the Huff method 1st quartile simulated results showed that the minimum, maximum and average total overflows increased by 18.1 to 94.7 m3 relative to the 50-year simulated results. In a period of 100 years, the Huff method 1st quartile simulated results showed that the minimum, maximum, and average total overflows were increased by 14.2 to 46.3 m3 relative to the 80-year simulated results. Additionally, in a period of 10 years, the Huff method 2nd quartile simulated results showed minimum, maximum and average total overflows of 74.3 m3, 102.7 m3, and 89.7 m3, respectively. In a period of 50 years, the Huff method 2nd quartile simulated results showed minimum, maximum and average total overflows with significant increases of 65.8 to 98.3 m3 relative to the 10-year simulated results. In a period of 80 years, the Huff method 2nd quartile simulated results showed that minimum, maximum and average total overflows were increased by 19.9 to 35.4 m3 relative to the 50-year simulated results. In a period of 100 years, the Huff method 2nd quartile simulated results showed that minimum, maximum and average total overflows were increased by 19.5 to 54.0 m<sup>3</sup> relative to the 80-year simulated results. Additionally, in a period of 10 years, the Huff method 3rd quartile simulated results showed minimum, maximum and average total overflows of 71.4 m3, 115.4 m3, and 92.9 m3, respectively. In a period of 50 years, the Huff method 3rd quartile simulated results showed minimum, maximum and average total overflows with significant increases ranging from 81.5 to 90.4 m3 relative to the 10-year simulated results. However, in a period of 80 years, the Huff method 3rd quartile simulated results showed that the minimum, maximum and average total overflows increased by 25.3 to 62.3 m3 relative to the 50-year simulated results. In a period of 100 years, the Huff method 3rd quartile simulated results showed the minimum, maximum and average total overflows to be increased by 10.9 to 28.6 m3 relative to the 80-year simulated results. Finally, in a period of 10 years, the Huff method 4th quartile simulated results showed minimum, maximum and average total overflows of 43.4 m3, 133.2 m3, and 84.9 m3, respectively. In a period of 50 years, the Huff method 4th quartile simulated results showed increases in minimum, maximum and average total overflow of 70.3 to 353.6 m3 relative to the 10-year simulated results. In a period of 80 years, the Huff method 4th quartile simulated results showed minimum, maximum, and average total overflows that were increased by 25.5 to 152.3 m3 relative to the 50-year simulated results. In a period of 100 years, the Huff method 4th quartile simulated results showed minimum, maximum and average total overflows to be increased by 13.0 to 79.1 m3 relative to the 80-year simulated results.

Table 3 shows the EPA-SWMM-model-simulated overflow results for a duration of two hours. Similarly, in a period of 10 years, the Huff method 1st quartile simulated results showed minimum, maximum and average total overflows of 28.1 m3, 203.5 m3, and 101.1 m3, respectively. In a period of 50 years, the Huff method 1st quartile simulated results showed minimum, maximum and average total overflows with significant increases of 128.1 to 470.4 m3. In a period of 80 years, the Huff method 1st quartile simulated results showed minimum, maximum and average total overflows were increased by 76.7 to 247.7 m3. In a 100-year period, the Huff method 1st quartile simulated results showed the minimum, maximum, and average total overflow was increased by 42.6 to 129.0 m3. In a period of 10 years, the Huff method 2nd quartile simulated results showed minimum, maximum and average total overflows of 93.8 m3, 182.1 m3, and 141.9 m3 respectively. In a period of 50 years, the Huff method 2nd quartile simulated results showed minimum, maximum, and average total overflow with significant increases of 228.9 to 296.1 m3. In a period of 80 years, the Huff method 2nd quartile simulated results showed that the minimum, maximum and average total overflows increased by 96.0 to 177.8 m3. In a period of 100 years, the Huff method 2nd quartile simulated results showed that minimum, maximum and average total overflows increased by 40.8 to 83.1 m3. In a period of 10 years, the Huff method 3rd quartile simulated results showed minimum, maximum, and average total overflows of 121.2 m3, 201.9 m3, and 159.5 m3, respectively. In a period of 50 years, the Huff method 3rd quartile simulated results showed minimum, maximum and average total overflows with significant increases of 250.1 to 326.2 m3. In a period of 80 years, the Huff method 3rd quartile simulated results showed minimum, maximum and average total overflows to be increased by 104.3 to 124.6 m3. In a period of 100 years, the Huff method 3rd quartile simulated results showed minimum, maximum and average total overflows were increased by 46.01 to 74.6 m3. In a period of 10 years, the Huff method 4th quartile simulated results showed minimum, maximum and average total overflows of 74.3 m3, 361.5 m3, and 177.0 m3, respectively. In a period of 50 years, the Huff method 4th quartile simulated results showed minimum, maximum and average total overflows with significant increases of 181.6 to 624.2 m3. In a period of 80 years, the Huff method 4th quartile simulated results showed that the minimum, maximum and average total overflows increased by 90.5 to 281.2 m3. In a period of 100 years, the Huff method 4th quartile simulated results showed that the minimum, maximum and average total overflows increased by 47.7 to 127.3 m3.

**Table 3.** The EPA-SWMM-model-simulated duration 2-h total overflow results.


Finally, Table 4 shows the EPA-SWMM-model-simulated results for a duration of three hours. In a period of 10 years, the Huff method 1st quartile simulated results showed minimum, maximum and average total overflows of 27.3 m3, 286.5 m3, and 123.5 m3, respectively. In a period of 50 years, the Huff method 1st quartile simulated results showed minimum, maximum and average total overflows with significant increases ranging from 271.0 to 630.8 m3. In a period of 80 years, the Huff method 1st quartile simulated results showed that minimum, maximum and average total overflows increased by 152.6 to 354.1 m3. In a period of 100 years, the Huff method 1st quartile simulated results showed that minimum, maximum and average total overflows increased by 68.4 to 176.7 m3. Similarly, in a period of 10 years, the Huff method 2nd quartile simulated results showed the minimum, maximum and average total overflows were 100.0 m3, 256.8 m3, and 166.7 m3, respectively. In a period of 50 years, the Huff method 2nd quartile simulated results showed minimum, maximum and average total overflows with significant increases of 428.2 to 544.1 m3. In a period of 80 years, the Huff method 2nd quartile simulated results showed minimum, maximum and average total overflows to be increased by 133.5 to 290.3 m3. In a period of 100 years, the Huff method 2nd quartile simulated results showed that minimum, maximum and average total overflows were increased by 77.1 to 141.7 m3. In a period of 10 years, the Huff method 3rd quartile simulated results showed minimum, maximum, and average total overflows of 174.1 m3, 371.3 m3, and 231.6 m3, respectively. In a period of 50 years, the Huff method 3rd quartile simulated results showed minimum, maximum and average total overflows with significant increases of 426.3 to 463.1 m3. In a period of 80 years, the Huff method 3rd quartile simulated results showed that minimum, maximum and average total overflows increased by 193.9 to 253.3 m3. In a period of 100 years, the Huff method 3rd quartile simulated results showed minimum, maximum and average total overflows were increased by 93.7 to 144.1 m3. In a period of 10 years, the Huff method 4th quartile simulated results showed minimum, maximum, and average total overflows of 119.5 m3, 459.9 m3, and 269.3 m3, respectively. In a period of 50 years, the Huff method 4th quartile simulated results showed minimum, maximum and average total overflows with significant increases of 349.7 to 996.7 m3. In a period of 80 years, the Huff method 4th quartile simulated results showed that minimum, maximum and average total overflows increased by 164.1 to 391.3 m3. In a period of 100 years, the Huff method 4th quartile simulated results showed minimum, maximum and average total overflows were increased by 70.1 to 179.3 m3. The simulated total overflow results for different periods in the same quantiles showed a gradual increase in simulated overflow over the different periods, with significant increases concentrated in a 50-year period.

**Table 4.** The EPA-SWMM-model-simulated total overflow results for a 3-h duration.

