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INSTALLATION AND HYDROLOGIC RESPONSE OF THE PENN STATE LOW
HEAD WEIRS
Larry Fennessey, James Hamlett, and Rob Cooper
University Park Campus
The Pennsylvania State UniversityUniversity Park, Pennsylvania
THE LOW HEAD WEIRS ARE LOCATED WITHIN THE UNIVERSITY’S FOX HOLLOW
DRAINAGE BASIN
POI
LOW HEAD WEIRS
1) SURFACE DRAINAGE AREA = 452.5 AC2) IMPERVIOUS AREA = 120.9 AC (26.7%)3) 100% CARBONATE GEOLOGY4) NO PERENNIAL FLOW IN BASIN5) SURFACE RUNOFF GENERALLY ONLY
GENERATED FROM IMPERVIOUS AREAS6) 110,000 LF OF STORM DRAINS7) BASIN MODELED WITH TR-20, HEC-HMS,
AND SWMM BEING DEVELOPED
FHDB FACTS TO POINT OF INTEREST
THE UNIVERSITY IS VERY CONCERNED WITH WATER QUALITY AND RECHARGE
BECAUSE OF ITS BIG HOLLOW WELL FIELD
LOW HEAD WEIRS
WELL HEADPROTECTIONAREA
THERE ARE SEVERAL SIGNIFICANT STORMWATER FACILITIES WITHIN THE BASIN
POI
RECHARGE AREAWATER QUALITY FACILITYPEAK CONTROL FACILITY
THE UNIVERSITY PREFERS THE USE OF NATURAL SYSTEMS FOR CONTROL
IN 2000, THE PASTURE THE LOW HEAD WEIRS ARE LOCATED WITHIN WAS IDENTIFIED AS A CRITICAL
LAND AREA FOR STORMWATER RECHARGE
IF CRITICAL AREAS ARE DEVELOPED, A DOUBLE IMPACT OCCURS
NOT ONLY DO YOU HAVE INCREASED RUNOFF FROM THE DEVELOPMENT, BUT YOU ALSO LOSE AN
EXISTING NATURAL RECHARGE AREA, THEREBY MAKING THE UPSLOPE RUNOFF EFFECTIVE
THE IMPORTANT QUESTION WAS: COULD THE INFILTRATION CAPACITY OF THE AREA BE
INCREASED WITHOUT ADVERSE CONSEQUENCES
THEREFORE, PRIOR TO THE DESIGN OF THE WEIRS, AN EXTENSIVE SITE ANALYSIS WAS CONDUCTED
SOILS AT THE WEIRS ARE SILT LOAM WITH 2’ DEEP TOPSOIL AND EXCELLENT VEGETATIVE COVER
Soils mapped as HcB (Hagerstown Silty Clay Loam) by Centre County Soil Survey
THE GEOLOGY AT THE WEIRS IS THE GATESBURG FORMATION
THE WEIRS WERE DESIGNED SIMILAR TO A DAM ABUTMENT WITH THE EXCEPTION THAT THESE “LOW-
HEAD DAMS” WERE INTENDED TO HAVE 100% LEAKAGE ABOVE AND BELOW THE ABUTMENTS
THE WEIRS HAVE A 6” DEEP AND 20’ WIDE LOW FLOW CHUTE THAT DISCHARGES TO A PREFORMED SCOUR HOLE TO PREVENT DOWNSTREAM EROSION
THE MAXIMUM DEPTH OF PONDING FROM THE UPSLOPE TOE OF THE WEIRS TO THE LOW FLOW
CHUTE CREST IS APPROXIMATELY 2.5’
Fox Hollow Drainage Basin Event of 8131/2005
1041.25
1042.25
1043.25
1044.25
1045.25
8/30/0522:00
8/31/051:45
8/31/055:30
8/31/059:15
8/31/0513:00
8/31/0516:45
8/31/0520:30
9/1/050:15
9/1/054:00
Date
Wei
r A W
SE (f
t)
1044.25
1045.25
1046.25
1047.25
1048.25
Wei
r B W
SE (f
t)
Weir A (downstream) Weir B (upstream)
Crest Overtopping Elevation
THE UPSTREAM WEIR HAS ITS 6” PVC BLEED OUT PIPE OPEN TO ALLOW SOME RUNOFF TO REACH THE
LOWER WEIR WITHOUT OVERTOPPING THE CREST
Volume below the crestof Weir A infiltrates
THE WEIRS ARE COVERED WITH INTERLOCKING CONCRETE PAVERS THAT WERE BACKFILLED WITH
SOIL AND GRAVEL IN THE LOW FLOW CHUTES
THE WEIRS WERE INUNDATED NUMEROUS TIMES DURING CONSTRUCTION
WHICH PRESENTED ADDITIONAL CHALLENGES TO THEIR CONSTRUCTION
INCLUDING AN EARLY SNOWFALL
NONETHELESS, TODAY THE WEIRS ARE ALMOST INDISTINGUISHABLE FROM THE SURROUNDING
PASTURE, WHICH IS NOW LEFT UNMOWED
BETWEEN 2002 AND 2005 THE WEIRS WERE INSTRUMENTED SUCH THAT A MASS BALANCE COULD
BE CONDUCTED
Storm Drain (typ)
Flow Meters (typ)
Watershed OutletV-notch Weir
Mini Wells
Deep Wells
Filtration/Backwater
Area
RUNOFF AT V-NOTCH WEIR DURING ONE OF THREE RUNOFF EVENTS IN 2005 TO HAVE GENERATED
SURFACE RUNOFF AT THE OUTLET OF THE FHDB
HYDROLOGIC BUDGET FOR SPRING CREEK AT THE AXEMANN GAGE
Spring Creek at Axemann Hydrologic Budget from 10/1/41 to 9/30/04
Surface Drainage Area = 87.20 sq miSurface Drainage Area Percent Impervious = 12.00 % approxAverage Annual P = 38.63 in 100%Average Annual ET = 23.50 in 61%Average Annual Q = 14.79 in 38%Percent of Q from Baseflow (from Taylor 1997) = 85.29 %Average Annual change in G or S = 0.00 in assumed 0%Average Annual Runoff value = 0.70 mgd/sqmi
On an annual basis: P – Q ± G – E – T = Delta Storage Where: P = precipitation
Q = surface runoffG = groundwater flowE = EvaporationT = TranspirationDeltaS = Change in Storage
61%
HYDROLOGIC BUDGET FOR FHDBFOR A 2-YEAR PERIOD
(2ND AND 3RD WETTEST YEARS IN 110 YEARS)
CANNOT SIMPLIFY HYDROLOGIC BUDGET TO:P – Q = ET
Fox Hollow Hydrologic Budget from 6/8/03 to 6/7/05
Surface Drainage Area = 0.71 sq miSurface Drainage Area Percent Impervious = 26.72 %Average Annual P over study period = 53.19 in 100%Average Annual ET over study period = 33.04 in assumed 62%Average Annual Q over study period = 2.30 in 4%Theoretical Average Annual Q over study period = 11.91 in 22%Percent of Q from Baseflow = 0 %Average Annual change in G or S over study Period = 17.86 in 34%Average Annual Runoff value = 0.11 mgd/sqmi
61%
35%
ANALYSIS SHOWS THAT EVENTS LESS THAN 0.75” RAINFALL PRODUCE NEGLIGIBLE RUNOFF
Total Total PercentTotal Q (ac-ft) Basin DifferenceBasin Passing Theoretical Actual to
Precipitation Miniwell Runoff TheoreticalP (ac-ft) at Weir A Q (ac-ft) Q
P (in) In Range In Range In Range In Range>= 5" 195 32.2 58.4 55%
5">P>=4" 0 0.0 0.0 na4">P>=3" 135 13.9 34.6 40%
3">P>=2.5" 0 0.0 0.0 na2.5">P>=2" 236 15.8 71.4 22%2">P>=1.75" 73 8.9 18.8 47%
1.75">P>=1.5" 367 17.3 101.7 17%1.5">P>=1.25" 313 18.9 70.4 27%1.25">P>=1" 258 19.1 56.0 34%1">P>=.75" 414 15.5 83.9 18%.75">P>=.5" 503 7.4 93.8 8%.5">P>=.25" 481 0.9 71.5 1%.25">P>=.1" 249 0.6 18.9 3%.1">P>=.06" 69 0.0 1.2 0%
SUM 3293 150.5 680.8 22%
8%
Additional loss between Weir A and V-notch weir not considered
Fox Hollow Drainage Basin Event of 8/31/2005(provisional data by LAF at SEA)
0
10
20
30
40
50
60
70
1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00
Time
Flow
(cfs
)
Inflow V-notch Weir A Weir B
Runoff Volume Summary (ac-ft)Theoretical Basin Impervious Runoff = 10.3Inflow to Lower Pasture = 4.4Discharged From Weir B = 3.9Discharged From Weir A = 2.9Outflow From Basin at V-notch = 2.5
HYDROLOGIC RESPONSE DURING HURRICANE KATRINA (1.24” RAINFALL, 0.98” IN ONE HOUR)
Fox Hollow Drainage Basin Event of 8/31/2005
0
10
20
30
40
50
60
70
8/30/0522:00
8/31/051:45
8/31/055:30
8/31/059:15
8/31/0513:00
8/31/0516:45
8/31/0520:30
9/1/050:15
9/1/054:00
Date
Inflo
w (c
fs)
1041.25
1042.25
1043.25
1044.25
1045.25
1046.25
Wei
r A W
SE (f
t)
Dewatering SummaryTime of Inflow Qp = 2:10 AM, 8/31Response at Weir Starts = 4:00 AM, 8/31Overtopping of Weir Ends = 10:30 AM, 8/31Weir Dry = 3:45 AM, 9/1
DRAW DOWN OF PONDING USUALLY TAKES BETWEEN ONE AND TWO DAYS
INFILTRATION RATE= 1.7 IN/HR
CONCLUSIONS
HOWEVER, THIS TYPE OF SYSTEM IS RADICALLY DIFFERENT THAN MANY “ENGINEERED
INFILTRATION” SYSTEMS, WHICH ARE USUALLY NOTHING MORE THAN INJECTION WELLS
THE PENN STATE LOW HEAD WEIRS HAVE SHOWN THAT IT IS POSSIBLE TO INCREASE AND MAINTAIN THE INFILTRATIVE CAPACITY OF AN AREA WHERE IT ALREADY NATURALLY OCCURS, WITH LIMITED
RISKS TO GROUNDWATER QUALITY; IF THE IN-SITU SOILS ARE ADEQUATELY MAINTAINED, AND THE
LOCAL HYDROLOGIC PROCESSES ARE CONSIDERED
CONCLUSIONS
HOWEVER, THE LAST REMAINING CRITICAL AREAS (RECHARGE AREAS, WETLANDS, FLOODPLAINS,
ETC.) WITHIN WATERSHEDS NEED TO BE AGGRESSIVELY PROTECTED.
THIS STUDY SHOWS THAT SOME WATERSHEDS CAN BE HIGHLY ALTERED AND STILL FUNCTION
HYDROLOGICALLY SIGNIFICANTLY BETTER THAN EXPECTED; AND THEREFORE, ARBITRARY RULES
OF THUMB REGARDING IMPERVIOUSNESS SHOULD ONLY BE USED FROM A PLANNING PERSPECTIVE.
CONCLUSIONS
THEREFORE, PEAK RUNOFF RATE ANALYSES SHOULD ALWAYS BE CONDUCTED
INDEPENDENTLY OF WATER QUALITY OR VOLUME CRITERIA.
WHILE THE WEIRS ARE ONE OF THE MOST EFFECTIVE INFILTRATION OR RECHARGE
FACILITIES WITHIN THE STATE, THEY STILL CANNOT EFFECTIVELY CONTROL PEAK RUNOFF
RATES FOR LARGER DESIGN TYPE RUNOFF EVENTS BY THEMSELVES.
ENDEND
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