Water Resource Publication Airport Dam 4A OPP-WRP-DM-4A:(1 ...

Introduction
Water Resource Publication
Airport Dam 4A
OPP-WRP-DM-4A:(1)-2007
The Airport Dam 4A was constructed in 1997 for future runway expansion projects and additional land development
projects, and is located on the east side of the airport in Benner Township (refer to Figures 1 and 2). The dam has a
contributing drainage area of approximately 306 acres with approximately 61.9 acres (20%) of impervious area. The
original design by Michael Baker Jr. accounted for 74.8 acres of future impervious area. The imperviousness is almost all
connected via storm drain pipes and grass/rock lined conveyance swales. The dam (PaDEP permit # 14-123) is a 27.5 feet
tall earthen structure classified as a C-2 High Hazard structure by PADEP. The dam was designed to meet the local
municipal stormwater management ordinance, and the dam was also designed to pass the ½ PMF runoff event. The dam
discharges approximately 3,000 feet upslope of Spring Creek along an unnamed tributary. During construction, numerous
sinkholes occurred, ultimately resulting in the dam being structurally lined. The dam is a dry pond and has no baseflow;
however, because of the liner, extended low trickle flows occur due to the dewatering of the soils above the liner.
Runoff is discharged from the dam via a low flow spillway at the interior toe of slope and a high flow spillway that both
tie into a 36” RCP outflow pipe. Additionally, the dam has a 110’ wide earthen emergency spillway. On January 2 nd
2007, the Office of Physical Plant (OPP) installed a pressure transducer on the dam’s low flow spillway and began
collecting data at 5-minute intervals.
N
Drainage
Area
Fox Hill Road
Airport
Dam 4A
Big Hollow
Spring Creek
Figure 1. Airport Dam 4A Drainage Basin
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Water Resource Publication
Soils and Geology
Figure 2. Photograph of Dam 4A with High Flow Spillway Shown in the Foreground
The majority of the Dam’s drainage basin soils are identified as the Hublersburg Silt Loam Series by the Soil Survey of
Centre County. The Hublersburg series consists of deep, well-drained soils that formed in cherty limestone residuum.
The most significant limitation identified in the soil survey for Hublersburg soils is possible sinkhole formation. The dam
was built near a small area of Hagerstown Silty Clay Loam. Hublersburg soils have a hydrologic soil group (HSG) B as
defined by the USDA NRCS.
The basin is underlain by bedrock of the Mines Member of the Gatesburg Formation and the Lower Member of the
Gatesburg Formation undivided. The geologic contact between these two formations runs parallel and south of the
runways splitting the basin.
Hydrologic Response
The low flow spillway has a series of four 3” diameter holes located 2 feet below the normal basin ground elevation to
dewater soils above the liner. These holes are partially blocked by large boulders. The pressure transducer is located at
the elevation of these four under drain holes; however, minor trickle flows through the holes cannot be determined
accurately. These holes have no significant effect on the larger event peak runoff rates or runoff volumes.
The During the year 2007, 35.9 inches of rainfall were measured at the Walker Building; however, 44.2” of rainfall were
recorded at the Airport Weather Station (KUNV) located within the drainage basin. The precipitation resulted in 32
runoff events being measured at the dam’s outflow pipe (refer to Figure 3). The total annual runoff volume discharged
from the dam was 117.45 ac-ft, or 4.6 watershed inches. This runoff volume equates to 12.8% of the annual precipitation
at the Walker Building or 10.4% of the annual precipitation at the Airport.
As would be expected, four precipitation events (3/2, 8/8, 8/21, and 12/24) resulted in generating a significant amount of
the runoff volume equivalent to 51.0 ac-ft or 2.0 watershed inches, which represented 43% of the total annual runoff
volume. These four precipitation events had a total of 6.93” of rainfall, or 15.6% of the total annual precipitation.
OPP-WRP-DM-4A:(1)-2007

Penn State University
Water Resource Publication
1131
1129
1127
1125
Stage (ft)
1123
1121
1119
1117
1115
1113
1/1/2007 4/1/2007 6/30/2007 9/28/2007 12/27/2007
Date
Significant Runoff Events of 2007
Figure 3. Discharge from Dam 4A from 1/2/2007 to 12/31/2007
Five precipitation events over 1 inch in depth occurred during 2007, the largest being 2.76” of rainfall on August 21st.
These five precipitation events and a small rainfall event on snow (March 2 nd ) are shown in the table below along with
the event peak runoff rates and runoff volumes.
Walker Airport Max
Event Event WSE Outlet Out Out
Date P (in) P (in) (ft) Qp (cfs) Q (ac-ft) Q (in)
3/2/2007 0.53 0.88 1124.2 5.9 20.8 0.8
6/4/2007 1.01 1.13 1116.7 3.9 1.6 0.1
7/12/2007 1.09 0.47 1114.5 2.3 0.3 0.0
8/8/2007 1.21 1.10 1121 5.1 7.0 0.3
8/21/2007 2.18 2.76 1117.9 4.3 7.1 0.3
12/24/2007 1.02 2.19 1124.4 6.0 16.1 0.6
The minor rainfall event of March 2 nd occurred with an average of 3” of snow on the ground and resulted in the highest
annual runoff volume. The highest peak runoff rate was generated from the December 24 th precipitation event, which was
a high intensity event with approximately 1.4” of rainfall occurring over a 4-hour period. While these two events
represent completely different hydrologic conditions, the resulting extended hydrologic responses in the dam were similar
(compared Figures 4 and 5). This is most likely due to the drainage basin’s time of concentration and the low and fairly
constant discharge from the dam’s low flow spillway. The dam’s discharge at elevation 1131 (representing 18 feet of
head and 41.4 ac-ft of storage) is only approximately 14.5 cfs.
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Water Resource Publication
1131.0
1129.0
1127.0
1125.0
Stage (ft)
1123.0
1121.0
1119.0
1117.0
1115.0
1113.0
1111.0
3/1/07 0:00 3/2/07 0:00 3/3/07 0:00 3/4/07 0:00 3/5/07 0:00
Date
Figure 4. Runoff Event of March 2, 2007
1131.0
1129.0
1127.0
1125.0
Stage (ft)
1123.0
1121.0
1119.0
1117.0
1115.0
1113.0
1111.0
12/22/07 0:00 12/23/07 0:00 12/24/07 0:00 12/25/07 0:00 12/26/07 0:00
Date
Figure 5. Runoff Event of December 24, 2007
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Water Resource Publication
Water Quality, Thermal and Recharge Issues
No water quality sampling of surface water was conducted during the record period. Because there is no baseflow within
the Basin, and it is unknown at this time how much of the dam’s discharges are infiltrated downslope prior to reaching
Spring Creek, the thermal impact is not know. However, since the runoff volumes are so small compared to stream flow
within Spring Creek, the thermal impacts are assumed to be negligible. This topic may be explored more in the future as
additional data are collected.
Miscellaneous Information
Dam Permit Number: 14-123
Design storm: ½ PMF
Interior Embankment Bottom Elevation = 1113.00
Emergency Spillway Elevation = 1131.60
Top of Dam Elevation = 1135.00
Surface Drainage Basins: Spring Creek/Bald Eagle Creek/West Branch Susquehanna River
PA Title 25, Chapter 93 Water Quality Classification: High Quality-Cold Water Fishery (HQ-CWF)
USGS Quadrangle: State College
Hydrologic Unit Code: 02050204
Latitude/Longitude: 40° 51’ 15” / 77° 50’ 00”
Municipalities: Benner Township
Publisher: Penn State University, Office of Physical Plant, Energy and Engineering Division, Engineering Services
Editor: Larry Fennessey, OPP Stormwater Engineer, Ph: (814) 863-8743, email: laf8@psu.edu
OPP-WRP-DM-4A:(1)-2007