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

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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

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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

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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.

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