Krallis, George A.

Lennon, Gerard P.

Weisman, Richard N.

English

Lehigh University: Lehigh Preserve

Lehigh University
Lehigh Preserve
Fritz Laboratory Reports Civil and Environmental Engineering
1986
Stormwater study for the yellis tract watershed for
the city of bethlehem, July 1986, 10p.
Richard N. Weisman
George A. Krallis
Gerard P. Lennon
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Recommended Citation
Weisman, Richard N.; Krallis, George A.; and Lennon, Gerard P., "Stormwater study for the yellis tract watershed for the city of
bethlehem, July 1986, 10p." (1986). Fritz Laboratory Reports. Paper 2329.
http://preserve.lehigh.edu/engr-civil-environmental-fritz-lab-reports/2329
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STORMWATER STUDY FOR THE 
YELLIS TRACT WATERSHED 
by 
Richard N. Weisman 
George A. Krallis 
Gerard P. Lennon 
Department of Civil Engineering 
and 
Center for Marine and Environmental Studies 
Lehigh University 
Bethlehem, PA 18015 
July, 1986 
for 
City of Bethlehem 
IMBT Hydraulics Lab Division Report 
IHL-107-86 
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1. INTRODUCTION 
The City of Bethlehem asked the writers to assess possible 
changes in stormwater runoff patterns from the Yellis Tract 
Watershed as a result of developing existing meadows and modifying 
the detention pond system. The watershed is in north Bethlehem 
bordered roughly by Holy Savior Cemetery, Nazareth Pike, and the 
Monocacy Creek (see Figure 1). The Soil Conservation Service 
(SCS) TR-20 Computer Program for Project Formulation Hydrology was 
used to determine the response of the watershed to possible 
modifications. 
Basically, three major cases were studied: 
1) 
2) 
3) 
The pre-existing, minimally developed basin, circa 1955. 
The existing watershed conditions with current land use 
and development wit~ limited detention available along 
the main swale. The swale becomes a well defined 
channel north of the Pine Top Trail. 
Developing the remainder of the basin and increasing 
detention along the main swale. 
The central questions answered by this study are: 
1) Has development increased peak flows from the pre-
existing situation to the present? 
2) What will be the effect of future development on runoff? 
3) Can stormwater runoff be managed by providing detention, 
especially in the main swale? Specifically, can flows 
in downstream channels be contained within the existing 
banks? 
This report contains a brief description of the model and its 
inputs, and then highlights the results of this study. Another 
report submitted to the City Engineer entitled "Background Report: 
Computations and Data for the Yellis Tract Stormwater Study" 
(herein referred to as the Background Report) contains a detailed 
listing of the input data for the model as well as detailed model 
outputs. 
2. THE MODEL 
The TR-20 Computer Program for Project Formulation Hydrology 
uses the procedures described in the SCS National Engineering 
Handbook, Section 4, Hydrology (NEH-4) except for the reach flood 
routing procedure which is described in Appendix G & H of the 
TR-20 manual. The TR-20 model is useful in generating hydrographs 
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and routing runoff through stream reaches and detention storage. 
It can be used to readily assess effects of land use and detention 
changes. The TR-20 input data characterizes the drainage areas, 
reaches, and detention structures. The drainage areas require an 
area, a time-of-concentration, and a curve number, (which is an 
index of runoff potential determined by the soil type, slope, and 
land use). The reaches are modeled using information such as 
reach geometry, slope, and roughness characteristics. Detention 
structures require elevation, discharge, and storage volume data. 
The model is formulated as a series of contributing subareas, 
reaches, and structures that route the stormwater through the 
watershed. A flow chart schematic of the overall flow through the 
watershed is shown in Figure 2. Corresponding areas and 
structures are shown in Figure 1. 
3. THE BASIN 
The watershed is composed of 0.6 square miles of suburban 
residential land as shown in Figure 1. Its soils are Washington, 
Conestoga, and Clarksburg Silt Loams with an SCS hydrologic soil 
group designation B. Slopes generally range between 3 and 8% (see 
NEH-4 and Northampton County Soils Report). Based upon SCS 
procedures, curve numbers (CN's) were chosen for these 
combinations of land use, soil type and for average antecedent 
moisture conditions. Times of concentration (tc) for each subarea 
were chosen using the Upland Method (see NEH-4). The watershed 
was sub-divided into twelve subbasins according to their drainage 
characteristics as shown in Figure 1 (see Background Report for 
specific values of CN's and tc). 
Swales, gutters, and sewers carry stormwater from the upper 
subareas of the watershed to the structures on the Yellis tract. 
A channel north of Pine Top Trail then carries runoff to the 
Monocacy Creek. This channel has a capacity of carrying 
approximately 120 cubic feet per second within its banks over a 
majority of its length. 
The structures within the watershed consist of: 
a) depression resulting from a sinkhole south of Macada 
Road (designated as structure 10 (510) on Figure 1), 
b) detention pond at the corner of Johnston Drive & Powder 
Mill Road (57), 
c) detention pond south of Redfern Lane (51), 
d) detention pond north of Redfern Lane (56), 
e) detention pond south of Pine Top Drive (52), 
f) culvert at Pine Top Trail (54) and 
g) culvert at Biery's Bridge Road (55). 
According to city procedures, the design storm for this basin 
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I Macada Road 
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I FIGURE 1 
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is the 24-hour, 25-year storm using the SCS Type II cumulative 
rainfall distribution. Simulations were also conducted for 
24-hour SCS Type II storms with recurrence intervals of 2 and 10 
years. 
A complete listing of the data for the basin is found in the 
Background Report. 
4. TEST CASES 
Many simulations were performed and the input/output of each 
is contained in the Background Report. Presented here is a 
selection of seven simulations that allow answers to the relevant 
questions that need resolution. These selected simulations are 
summarized in Table 1. 
Table 1·: TEST CASE DESCRIPTIONS 
CASE DESCRIPTION 
I Pre-existing Conditions 
Curve Numbers Equal 60; No 56 or 57; 51, 52, 54, 55 
remain but with smaller storage volumes to represent 
roads that would impede flow; subarea 1 is 10% of 
existing size. 
II a Existing Conditions 
Curve Numbers equal 70 in all areas except subareas·6 
and 3, which are 60; structures 1 , 2 , 4, 5, 6, 7, and 10 
included (see Figure 1) . 
IIb Develo}2ed Subarea 6 
Existing Condition with subarea 6 Curve Number changed 
from 60 to 70. 
IIc Removed 57 
Existing Condition (Case IIa) without detention at 57. 
III a Increased Storage at 52. 
Expanded the available storage at 52 (only change from 
Case IIa). 
IIIb Increased Storage at 52 and 53. 
Expanded storage at 52 (Case III a) and created storage 
at 53. 
IIIc More Storage at 52 and 53. 
Doubled the storage at 52 from case IIIa with storage at 
53 from Case IIIb. 
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5. RESULTS 
Following are the results of the test cases. Peak outflows 
from selected structures and reaches are listed as well as the 
time-to-peak. The Background Report has a complete listing of the 
hydrographs at all of the structures and reaches. The results 
presented here are most pertinent to the questions asked in the 
Introduction. Flows are presented in cubic feet per second, and 
time of peaks are in hours since the beginning of rainfall of the 
design storm. The results are for the 25-year storm. 
CASE I LOCATION 
PRE-EXISTING S10 S1 S2 S3/4 R10 
Peak Flow 78 cfs 99 116 206 299 
Time of Peak (10.10 hrs) (10.22) (10.03) (10.06) (10.13) 
CASE II A6 S7 S1 S2 S3/4 R10 
a. EXISTING 
Peak Flow 47 7 226 299 392 467 
Time of (10.08) (10.15) (10.11) (10.08) (10.06) (10.08) 
Peak 
b. DEVELOP A6 
Peak Flow 71 7 240 326 423 495 
Time of (10.06) (10.15) (10.05) (10.05) (10.04) (10.07) 
Peak 
c.REMOVE S7 
Peak Flow 47 14 226 3"06 402 476 
Time of (10.08) (9.97) (10.11) (10.07) (10.05) (10.08) 
Peak 
CASE III INFLOW OUTFLOW 
S2 S2 S3/4 R10 
a . INCREASED 
STORAGE S2 
Peak Flow 330 279 361 440 
Time of Peak (10.03) (10.16) (10.07) (10.09) 
b. INCREASED 
STORAGE S2 & S3 
Peak Flow 330 279 363 441 
Time of Peak (10.03) (10.16) (10.07) (10.09) 
c.DOUBLED STORAGE 
AT S2 
Peak Flow 330 252 328 404 
Time of Peak (10.03) (10.22) (10.09) (10.10) 
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6. DISCUSSION 
A. Comparison of Pre-Existing and Existing Simulations 
(Case I vs IIa) 
The effect of development from the pre-existing to the 
existing simulation is shown by comparing Cases I and IIa. 
For the 25-year storm, flow in the reach north of Pine Top 
Trail (RlO) shows a 56% increase with development and the 
inclusion of some structures. At S3/4, the increase in peak 
flow is 91%, and at Sl, 127%. The results indicate that 
development of the watershed from 1955 to present (1986) had 
a significant effect on the peak runoff. Note that the 
capacity of the existing channel, RlO, is exceeded by the 
design storm for the pre-existing watershed condition. 
B. Comparison of Development in Subarea 6 to the Existing 
Case (Case IIb vs IIa); and Comparison of Removal 
of S7 to the Existing Case (Case IIc vs IIa). 
For future development in Subarea 6, Case IIb vs IIa 
shows a 51% increase in discharge from that area, but further 
downstream the effects are attenuated to 7% at Sl, 8% at 
S3/4, and 6% in RlO. The effect of this development is 
significant just downstream, but it is less noticeable 
in its impact at Sl, S3/4, and RlO. 
Case IIc vs case IIa illustrates the effect of removing 
S7. Flow increased 88% just below that structure. At Sl, 
there is no difference, 2% at S3/4, and 2% at RlO which are 
considered negligible. It is emphasized that S7 has a 
significant impact on flows just downstream (on Johnston 
Driv~), but its effect is minimal f~rther downstream. The 
lack of flow increase at Sl is due to the timing of peak 
flows arriving from S7 compared to other contributing areas. 
c. Comparison of Increasing Storage in the Main Swale to 
the Existing Case (Cases IIIa through IIIc vs IIa) 
Cases IIIa through IIIc show the effect of increased 
storage at S2 and S3. A simulation was conducted using a 
reasonably obtainable increase in storage at S2 (Case IIIa). 
Compared to the existing condition, this increased storage at 
S2 reduces the peak outflow by 7% at S2, 8% at S3/4, and 6% 
in Reach RlO (Case IIIa vs IIa). Coupling the increased 
storage at S2 with increased storage at S3 gives a 6% 
decrease in peak flow in RlO (Case IIIb vs IIa). 
For illustration purposes another simulation (Case 
IIIc) was conducted with a much greater storage at S2 than 
the reasonable storage used in Cases IIIa and IIIb. This 
simulation with a doubled storage at S2 and the Case IIIb 
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7. 
storage at S3 yield a 14% reduction in R10 compared to the 
existing Case IIa. These extensive modifications of Case 
IIIc may not be physically possible and do not yield 
significant peak flow reductions at the main swale/channel 
locations considered. 
CONCLUSIONS AND RECOMMENDATIONS 
Conclusions 
It is evident from this study that the natural ability of the 
existing swale/channel system to transmit water out of the basin 
has been overwhelmed by the combination of increased upstream 
development, possible inclusion of additional drainage area, and 
encroachment on the swale/channel system. It is also evident that 
stormwater runoff downstream of Pine Top Drive cannot be 
successfully managed by the simulated expansion of main swale 
detention facilities alone. The transition from the 
predevelopment condition to the present state has been accompanied 
by a 50 to 125 percent increase in the peak runoff for the design 
storm (24-hour, 25-year storm with the SCS Type II distribution). 
The types of mitigation options considered here fall into 
three general categories: 
1. 
2 . 
3. 
Upstream detention 
Main swale detention 
Increased flow capacity of downstream channels 
The conclusions based on the simulations are presented under these 
three headings. 
1. Upstream Detention 
Simulations conducted as part of this study indicate that 
small detention ponds scattered throughout the basin have a 
significant effect just downstream of that area and a lesser 
effect further downstream. Thus, the proposed development in one 
area (Subarea 6) would significantly increase flow immediately 
downstream (Johnston Drive) but would have a small impact further 
downstream. If developed, a detention facility immediately 
downstream of this area would be beneficial. 
The existing structure S7 at the corner of Johnston Drive and 
Powder Mill Road is effective in controlling stormwater flows 
and should not be removed from service. Although small, S7 
significantly reduces the 25-year design storm peak flow along 
Johnston Drive. 
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2 . Main Swale Detention 
Although reducing the design storm peak flows by 10-20% 
proposed improvements to the swale detention basin 52 will not 
solve the problem of flooding downstream. Less than a 15% 
reduction in design storm peak flow would result from a 
significant expansion of 52, which is extensive and may not be 
economically feasible (see Case IIIc). 
3. Increased Flow Capacity of Downstream Channels 
The reach downstream of Pine Top Trail has a bankfull 
capacity of approximately 120 cubic feet per second over a 
majority of its length. Simulations indicate that the capacity of 
the channel will be almost achieved by the two-year storm for the 
existing watershed conditions and would have been greatly exceeded 
by the 25-year storm for the pre-exisitng watershed conditions. 
As a result, even extensive detention facilities will not prevent 
overtopping of the channel banks from storms with return periods 
of 25 years. Additional mitigation would be achieved by 
increasing the capacity of the channel. 
Recommendations 
All stormwater mitigation techniques must be analyzed in 
terms of their economic benefits and costs. The recommendations 
made below, while all are technically feasible, must be 
investigated in terms of their relative costs and benefits. It is 
the judgement of the writers that the channel improvements will 
provide the most benefit for the least cost whereas swale 
detention facilities, although useful, may not achieve the desired 
mitigation effect for a reasonable cost. The following mitigation 
methods are presented in order of importance and 'should be 
investigated further from a cost stand point: 
1. Improvements to the culvert under Pine Top Trail and to the 
channel between Pine Top Trail and Biery's Bridge Road. 
2. Detention facilities for all newly developed areas off the 
main swale. 
3. Improved detention facilities between Redfern Lane and Pine 
Top Drive (52). 
Thus, it is recommended that main swale detention only be 
considered in conjunction with channel improvements and upstream 
detention. 
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Stormwater study for the yellis tract watershed for the city of bethlehem, July 1986, 10p.

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Stormwater study for the yellis tract watershed for the city of bethlehem, July 1986, 10p.

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