تأثير المنطقة ذات الكثافة السكانية العالية على عمل نظام الصرف الصحي المشترك: دراسة حالة، مدينة النجف

Rp_The Effect of Densely Populated Area on the Operation of the Combined Sewage System: Case Study, Al-Najaf City.pdf

Flooding in the sewer systems may be caused by urbanization when increasing the impervious areas and decreasing infiltration rate, land-use changes, or climate change that cause increasing the rainfall intensity or the misuse of the inhabitants, all that affect the performance of these systems.
The present study aims to search the effect of a densely populated area on operating the combined sewage system under higher rainfall intensities using the Storm Water Management Model (SWMM) by simulating a future increase in rainfall intensities for reducing the combined sewage system overflow in the study area. The present study selected AL-Rashadiya quarter that locates in Najaf city, as a case study.
The first step in the present study was generating the Intensity-Duration-Frequency (IDF) curves by finding the empirical equations of these curves based on historical precipitation data of the interval from 1989 – 2018, three statistical distributions were used to predict the highest rainfall intensities for the return periods of 2, 5, 10, and 25 years. These distributions include Gumbel, Log Pearson, and Lognormal distribution, where the higher rainfall intensities obtained were by using Gumbel distribution, where these rainfall intensities have been used in the SWMM modelling to estimate the maximum flooding volumes in the sewer systems of the study area.
The available data represents only one event of precipitation, on 28/11/2020, which was used for calibration purposes of the quantity model, where the rainfall depth was 55 mm in Al-Najaf city.
Also, comparisons were conducted between the design flow rates, the event model flow rates and models flow rates using the different rainfall intensities, using the indicators of the Normalized Mean Square Error, coefficient of determination, and Root Mean Square Error. These indicators help to estimate the modelling performance and obtain the nearest rainfall intensity to the rainfall event, which was the rainfall intensity for the return period of 5-years. This rainfall intensity was used in the SWMM modelling of stormwater quality in the study area to estimate the pollutants volumes.
The estimated flooding volumes in the modelling of the combined sewage system are 3817 m3, 6912 m3, 8929 m3, and 13434 m3, using the rainfall intensities for the return periods of 2, 5, 10 and 25 years, respectively. Obtained flooding quantities were utilized as an external flow rate from the combined sewage system nodes to the nearest nodes in the stormwater system.
The flooding result using the stormwater system modelling was 526 m3, 2669 m3, 3316 m3, and 5568 m3, using the rainfall intensities for the return periods of 2, 5, 10 and 25 years, respectively, where these flooding volumes were the final flooding volumes in the study area.
Also, laboratory tests were conducted on the stormwater at the disposal point at the end of the stormwater system in dry and wet weather. The obtained tests result during the rainfall event were used in the calibration process of the stormwater quality model to estimate the pollutants Buildup and Washoff factors in the study area.
The quantities of pollutants produced at the outfall point of the stormwater system were 1589.34 kg of TSS, 250.302 kg of BOD5, 0.514 kg of Cu, 0.227 kg of Pb, 0.165 kg of Cr and 0.309 kg of Mn, by using simulation of the SWMM quality model for 2 hours.
The suggested scenarios include increasing diameters of some pipes in the combined sewage system, where the obtained results showed a flooding reduction percentage in the combined sewage system of 53.52 %, 39.65 %, 33.3 %, and 24.41 % from the total flooding, where this scenario an ability to reduce the pollutant quantity that comes from the combined sewage system overflow during the rainfall events when this overflow enters the stormwater system inlets.
The other scenario includes using a compact unit at the end of the stormwater system to treat pollutant stormwater, which considers an un-controlling problem, with a treatment capacity of 500 m3/h.