The Contribution of
Small Storms to Annual Runoff Volume
Evaluated in a Sample
Watershed using the Curve Number Method
Ralph G. Mastromonaco,
P.E.
November 2004
As modern storm water regulations emphasize the capture or isolation of
early portions of the storm water runoff cycle for treatment and removal
of pollutants, it is advantageous to understand the contribution small
storms make to the annual runoff budget as capturing runoff for treatment
may reduce the total volume of runoff available to streams and wetlands.
Determining the magnitude of the impacts of treating storm water depends
on gaining an understanding of the annual amount of runoff that would be
diverted from the natural system by the treatment device.
For this analysis we developed a continuous runoff model as a composite
distribution of storms covering all possible rainfall events from 0.2 inch
to about 20 inches per storm for Yorktown, NY. We also developed a
means of extracting the contributions of ranges of storm frequencies to
the annual runoff volume as well as a means of relating annual runoff to
SCS curve number.
Rainfall Record Data
A
specific rainfall distribution model is constructed based on 33 years
(Oct. 1970- Jan. 2003) of NOAA rainfall data that was tabulated in 15
minute intervals for Yorktown, New York. A frequency distribution of
this data was prepared over the entire record as noted in Table 1.
Table 1: Yorktown, NY Distribution of NOAA Rainfall Data 1970-2003
|
Rainfall
(inch) |
33 year Exceedance
Frequency |
Annual
Exceedance
Frequency |
Return Frequency
(year) |
|
0 |
2810 |
85.15 |
0.012 |
|
0.1 |
2810 |
85.15 |
0.012 |
|
0.2 |
1919 |
58.15 |
0.017 |
|
0.3 |
1489 |
45.12 |
0.022 |
|
0.4 |
1200 |
36.36 |
0.028 |
|
0.5 |
1001 |
30.33 |
0.033 |
|
0.6 |
816 |
24.73 |
0.040 |
|
0.7 |
678 |
20.55 |
0.049 |
|
0.8 |
562 |
17.03 |
0.059 |
|
0.9 |
465 |
14.09 |
0.071 |
|
1 |
398 |
12.06 |
0.083 |
|
1.1 |
335 |
10.15 |
0.099 |
|
1.2 |
289 |
8.76 |
0.114 |
|
1.3 |
249 |
7.55 |
0.133 |
|
1.4 |
212 |
6.42 |
0.156 |
|
1.5 |
182 |
5.52 |
0.181 |
|
1.6 |
153 |
4.64 |
0.216 |
|
1.7 |
125 |
3.79 |
0.264 |
|
1.8 |
114 |
3.45 |
0.289 |
|
1.9 |
98 |
2.97 |
0.337 |
|
2 |
75 |
2.27 |
0.440 |
|
2.1 |
63 |
1.91 |
0.524 |
|
2.2 |
53 |
1.61 |
0.623 |
|
2.3 |
51 |
1.55 |
0.647 |
|
2.4 |
43 |
1.30 |
0.767 |
|
2.5 |
33 |
1.00 |
1.000 |
|
2.6 |
30 |
0.91 |
1.100 |
|
2.7 |
28 |
0.85 |
1.179 |
|
2.8 |
23 |
0.70 |
1.435 |
|
2.9 |
20 |
0.61 |
1.650 |
|
3 |
17 |
0.52 |
1.941 |
|
3.1 |
15 |
0.45 |
2.200 |
Other available data for individual storms is shown below on the Table 2:
Table 2: Various Sources of Local Storm Frequencies versus 24 hour
Rainfall (in).
|
Frequency
(year) |
NYC DEP (NWS TP-40) |
Westchester Soil and Water Board |
Thaler WHCGLHV |
Rainfall Data Used |
|
2 |
3.5 |
2.6 |
3.00 |
3.1 |
|
5 |
4.5 |
3.3 |
- |
3.55 |
|
10 |
5.0 |
5 |
4.80 |
4.71 |
|
25 |
6.0 |
5.77 |
6.40 |
5.5 |
|
50 |
7.0 |
6.3 |
7.00 |
6.5 |
|
100 |
7.5 |
7.2 |
9.00 |
7.2 |
|
PMP - 500yr 24 hr |
- |
- |
- |
19.5 |
The data for each storm was evaluated from a variety of sources only the
data with a “best-fit” continuous progression was used (5). The
other rainfall depth for these storms is shown on the chart to indicate
the range of values and their source.
Hydrologic Model
There are a few models that relate annual runoff to annual rainfall.
These are described as follows:
1.
Simple Method (Schueler, 1987), based on impervious area,
precipitation and fraction of storm events providing runoff. Model
is too general and too imprecise for our function.
2.
L-THIA (Long term hydrologic impact assessment) by Harbor, J.,
Grove, M., Bhaduri, B. and Minner, M., 1998, Long-Term Hydrologic Impact
Assessment (L-THIA) GIS. Public Works, 129, p.52-54. No information
on he model mechanics are provided by the author.
3. HSPF USGS - Hydrological Simulation Program—Fortran: HSPF simulates for
extended periods of time the hydrologic, and associated water quality, processes
on pervious and impervious land surfaces and in streams and well-mixed impoundments.
HSPF uses continuous rainfall and other meteorological records to compute stream
flow hydrographs and pollutographs. Very complex, requiring much data input.
To predict the contributions of ranges of storm frequencies we developed a
mathematical model of all rainfall, from the lowest rainfall to the
greatest precipitation possible. The model relies upon fairly
representing all rainfall over time as a series of individual one-day
storms, each having a relative probability of occurrence and a discrete
rainfall amount. This hydrologic model is authenticated and
correlated to the historic record in terms of (1) annual rainfall, (2) the
number of storms per year and (3) annual runoff.
Annual Rainfall - The 33 year Yorktown data record indicates annual
rainfall of 41.34 inches, however, we expect the rainfall to range from
43.15 (Table 3 Northeast United States) to
43.9
inches per year based
on the recent range from 1996 to 2003 and as reported by weather sites.
Table 3: Northeast US Annual Rainfall – 33-Year and Annual Rainfall
Amount (inches)
Similarly, the annual rainfall for Albany, NY is 38.37 inches and NYC is
49.88 inches. Yorktown is between the two NOAA stations, and the
average of NYC and Albany rainfall is 44.12 inches per year, providing
further indication of the annual rainfall amount.
Number of Storms per Year
From the NOAA (Table 1) data we know that there are 85 storms per year
when storms that register at least 0.1 inch are counted. Thaler reports
96 to 122 storms per year of greater than 0.01 inches in the area of our
study - Yorktown, NY, from the year 1930.
Annual Runoff
The USGS stream data indicates an average runoff in the locality of about
22.28 inches, as noted in Table 4. Annual runoff should range
between 19 and 26 inches or about 50% of annual rainfall based on USGS
records.
Table 4: USGS Records of Annual Runoff Near Yorktown, NY
|
Location |
Record Period |
Annual Runoff (in) |
