CecilyWang04/19/2016 Mini-ProjectReportA section ofthe Raritan River in Manville, New Jersey was analyzed in this Mini-Project. TwoGIS maps, Figures in Appendix 1, were generated in ArcMap, showing theflowlines and gages in the selected section of the Raritan River. In these 2maps, green lines represent the WBD(watershed boundary dataset) lines; purple lines represent the flowlines; andblue dots represent the gages.A model of theRaritan River at Manville and its neighboring towns was constructed in HEC-HMSProgram, shown in Appendix 2.
For hydrologic analysis, the change in dischargeof the Raritan River due to Hurricane Irene was analyzed. Hurricane Irene was ahuge storm struck New Jersey on August 28, 2011. It led to intense rainfallwithin the following days and resulted in floods that affected the entire state.Hurricane Irene caused enormous damage to New Jersey, from infrastructure toeconomy: it destroyed 200,000 homes and buildings, resulting in a total damagecost of $1 billion. Among all of the rivers in the state, the Raritan River wasselected because it is the major river of central New Jersey, and also was oneof the rivers affected the most by the hurricane. In order to study how HurricaneIrene affects the section of Raritan River in Manville, discharge of the sectionwas plotted corresponding to date of the year (Figure 1). As shown, severalother large rain events also happened during the year of 2011, and each of themled to an increase in the discharge to over 10,000 cfs.
However, this value wasonly 1/3 of the discharge after Hurricane Irene. Figure 1: Discharge of RaritanRiver, Manville in 2011To bettercompare the river conditions before and after Hurricane Irene, the dischargedata within the week Hurricane Irene occurred was also plotted (Figure 2). Figure2 indicated that the discharge was only around 800 cfs before the hurricane;and it rose rapidly to above 30,000 cfs after August 28, when the hurricaneoccurred, representing that there was a large amount of rainfall. Figure 2: Discharge of RaritanRiver, Manville in Late August, 2011SinceHurricane Irene was one of the most destructive hurricane in the state’shistory, an event frequency analysis was conducted. The daily discharge data ofthe Raritan River in 2011 was obtained from the USGS website. The log 10 valueof each discharge was computed in Excel Spreadsheet, and then the mean,standard deviation, and skew can also be simply calculated (Appendix 3). AssumeHurricane Irene is a 200-year event; and therefore, the KT valuecorresponded to a skew coefficient of 0.
8 and an exceedance probability of0.005 would be 3.312. By using Equation 1, logQ200 can be determined.
And finally, Q200 was calculated to be 17,010 cfs according to thevalue of log Q200. (Equation 1)Q200,which is the computed daily discharge for a 200-year storm, was much less thanthe daily discharge of 33,200 in the Manville section of the Raritan River onAugust 22, 2011 when Hurricane Irene happened. However, the KTValues for Pearson Type III Distribution Table didn’t provide the KTvalue for lower exceedance probability, the event frequency of Hurricane Irenecan only be estimated. Since 33,200 cfs was almost doubled comparing to 17,010cfs, Hurricane Irene can be approximated as an at least 500-year storm event.In order tostudy the relationship between precipitation and infiltration at Manville, theNRCS runoff analysis was also conducted. The land classification information ofManville was obtained from the Web Soil Survey (Appendix 4).
The sum of thepercentage of each hydrologic soil group was computed for the area of interest.The urban area was considered as a group D soil, and the water area wasneglected for the calculation. Since Manville is a residential town and theland use can be described as row houses and town houses, the curve numbers usedfor soil groups A to D were 77, 85, 90, and 92, respectively. Thus, the curvenumber of the whole area of interest was determined to be 82.
8, which is arelatively high value, indicating low infiltration rate and high runoffpotential of the selected area. USGS has reportedthat the precipitation at Newark was 8.92 inches. Same precipitation was usedfor Manville because it is not far away from Newark. The potential maximumretention S can be calculated using Equation 2. Then the initial abstractionbefore ponding (Ia), the rainfall excess (Pe) and continuing abstraction (Fa)were determined to be 0.415 in, 6.
84 in, and 1.67 in based on Equation 3, 4 and5, respectively. Sample calculations are shown in Appendix 5. (Equation 2) (Equation 3) (Equation 4) (Equation 5)As expected, the rainfall excess was the largest portion out of threecomponents of the total rainfall. Based on the analyses above, some conclusions can be drawn from theMini-Project. Manville was one of the many towns that got flooded due to HurricaneIrene, which was a 500-year event that brought huge precipitation to NewJersey.
Besides the intense rainfall due to the hurricane, another major factorthat cause the disastrous flooding was the highly impervious land cover.Because of this, rain water could not infiltrate into the ground and thus, ledto the large rain excess and flood. A possible solution to this problem is todecrease the impervious. For example, larger average lot sizes of residentialland and more open spaces such as parks and golf courses would help to increasethe infiltration potential and make the flood less destructive.