Cecily Irene occurred was also plotted (Figure 2). Figure



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A section of
the Raritan River in Manville, New Jersey was analyzed in this Mini-Project. Two
GIS maps, Figures in Appendix 1, were generated in ArcMap, showing the
flowlines and gages in the selected section of the Raritan River. In these 2
maps, green lines represent the WBD
(watershed boundary dataset) lines; purple lines represent the flowlines; and
blue dots represent the gages.

A model of the
Raritan River at Manville and its neighboring towns was constructed in HEC-HMS
Program, shown in Appendix 2. For hydrologic analysis, the change in discharge
of the Raritan River due to Hurricane Irene was analyzed. Hurricane Irene was a
huge storm struck New Jersey on August 28, 2011. It led to intense rainfall
within the following days and resulted in floods that affected the entire state.
Hurricane Irene caused enormous damage to New Jersey, from infrastructure to
economy: it destroyed 200,000 homes and buildings, resulting in a total damage
cost of $1 billion. Among all of the rivers in the state, the Raritan River was
selected because it is the major river of central New Jersey, and also was one
of the rivers affected the most by the hurricane. In order to study how Hurricane
Irene affects the section of Raritan River in Manville, discharge of the section
was plotted corresponding to date of the year (Figure 1). As shown, several
other large rain events also happened during the year of 2011, and each of them
led to an increase in the discharge to over 10,000 cfs. However, this value was
only 1/3 of the discharge after Hurricane Irene.

Figure 1: Discharge of Raritan
River, Manville in 2011

To better
compare the river conditions before and after Hurricane Irene, the discharge
data within the week Hurricane Irene occurred was also plotted (Figure 2). Figure
2 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 hurricane
occurred, representing that there was a large amount of rainfall.

Figure 2: Discharge of Raritan
River, Manville in Late August, 2011

Hurricane Irene was one of the most destructive hurricane in the state’s
history, an event frequency analysis was conducted. The daily discharge data of
the Raritan River in 2011 was obtained from the USGS website. The log 10 value
of each discharge was computed in Excel Spreadsheet, and then the mean,
standard deviation, and skew can also be simply calculated (Appendix 3). Assume
Hurricane Irene is a 200-year event; and therefore, the KT value
corresponded to a skew coefficient of 0.8 and an exceedance probability of
0.005 would be 3.312. By using Equation 1, logQ200 can be determined.
And finally, Q200 was calculated to be 17,010 cfs according to the
value of log Q200.

 (Equation 1)

which is the computed daily discharge for a 200-year storm, was much less than
the daily discharge of 33,200 in the Manville section of the Raritan River on
August 22, 2011 when Hurricane Irene happened. However, the KT
Values for Pearson Type III Distribution Table didn’t provide the KT
value for lower exceedance probability, the event frequency of Hurricane Irene
can only be estimated. Since 33,200 cfs was almost doubled comparing to 17,010
cfs, Hurricane Irene can be approximated as an at least 500-year storm event.

In order to
study the relationship between precipitation and infiltration at Manville, the
NRCS runoff analysis was also conducted. The land classification information of
Manville was obtained from the Web Soil Survey (Appendix 4). The sum of the
percentage 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 was
neglected for the calculation. Since Manville is a residential town and the
land use can be described as row houses and town houses, the curve numbers used
for soil groups A to D were 77, 85, 90, and 92, respectively. Thus, the curve
number of the whole area of interest was determined to be 82.8, which is a
relatively high value, indicating low infiltration rate and high runoff
potential of the selected area.

USGS has reported
that the precipitation at Newark was 8.92 inches. Same precipitation was used
for Manville because it is not far away from Newark. The potential maximum
retention S can be calculated using Equation 2. Then the initial abstraction
before 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 and
5, 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 three
components of the total rainfall.

Based on the analyses above, some conclusions can be drawn from the
Mini-Project. Manville was one of the many towns that got flooded due to Hurricane
Irene, which was a 500-year event that brought huge precipitation to New
Jersey. Besides the intense rainfall due to the hurricane, another major factor
that cause the disastrous flooding was the highly impervious land cover.
Because of this, rain water could not infiltrate into the ground and thus, led
to the large rain excess and flood. A possible solution to this problem is to
decrease the impervious. For example, larger average lot sizes of residential
land and more open spaces such as parks and golf courses would help to increase
the infiltration potential and make the flood less destructive.