INSTALLATION AND HYDROLOGIC RESPONSE OF THE PENN STATE LOW HEAD WEIRS

Transcription

1 INSTALLATION AND HYDROLOGIC RESPONSE OF THE PENN STATE LOW HEAD WEIRS Larry Fennessey, James Hamlett, and Rob Cooper University Park Campus The Pennsylvania State University University Park, Pennsylvania

2 THE LOW HEAD WEIRS ARE LOCATED WITHIN THE UNIVERSITY S FOX HOLLOW DRAINAGE BASIN LOW HEAD WEIRS POI

3 FHDB FACTS TO POINT OF INTEREST 1) SURFACE DRAINAGE AREA = AC 2) IMPERVIOUS AREA = AC (26.7%) 3) 100% CARBONATE GEOLOGY 4) NO PERENNIAL FLOW IN BASIN 5) SURFACE RUNOFF GENERALLY ONLY GENERATED FROM IMPERVIOUS AREAS 6) 110,000 LF OF STORM DRAINS 7) BASIN MODELED WITH TR-20, HEC-HMS, AND SWMM BEING DEVELOPED

4 THE UNIVERSITY IS VERY CONCERNED WITH WATER QUALITY AND RECHARGE BECAUSE OF ITS BIG HOLLOW WELL FIELD WELL HEAD PROTECTION AREA LOW HEAD WEIRS

5 THERE ARE SEVERAL SIGNIFICANT STORMWATER FACILITIES WITHIN THE BASIN RECHARGE AREA WATER QUALITY FACILITY PEAK CONTROL FACILITY POI

6 THE UNIVERSITY PREFERS THE USE OF NATURAL SYSTEMS FOR CONTROL

7 IN 2000, THE PASTURE THE LOW HEAD WEIRS ARE LOCATED WITHIN WAS IDENTIFIED AS A CRITICAL LAND AREA FOR STORMWATER RECHARGE

8 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

9 THEREFORE, PRIOR TO THE DESIGN OF THE WEIRS, AN EXTENSIVE SITE ANALYSIS WAS CONDUCTED

10 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

11 THE GEOLOGY AT THE WEIRS IS THE GATESBURG FORMATION

12 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

13 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

14 THE UPSTREAM WEIR HAS ITS 6 PVC BLEED OUT PIPE OPEN TO ALLOW SOME RUNOFF TO REACH THE LOWER WEIR WITHOUT OVERTOPPING THE CREST Fox Hollow Drainage Basin Event of 8131/ Volume below the crest of Weir A infiltrates Weir A WSE (ft) Crest Overtopping Elevation Weir B WSE (ft) /30/05 22:00 8/31/05 1:45 8/31/05 5:30 8/31/05 9:15 8/31/05 13:00 8/31/05 16:45 8/31/05 20:30 9/1/05 0:15 9/1/05 4:00 Date Weir A (downstream) Weir B (upstream)

15 THE WEIRS ARE COVERED WITH INTERLOCKING CONCRETE PAVERS THAT WERE BACKFILLED WITH SOIL AND GRAVEL IN THE LOW FLOW CHUTES

16 THE WEIRS WERE INUNDATED NUMEROUS TIMES DURING CONSTRUCTION

17 WHICH PRESENTED ADDITIONAL CHALLENGES TO THEIR CONSTRUCTION

18 INCLUDING AN EARLY SNOWFALL

19 NONETHELESS, TODAY THE WEIRS ARE ALMOST INDISTINGUISHABLE FROM THE SURROUNDING PASTURE, WHICH IS NOW LEFT UNMOWED

20 BETWEEN 2002 AND 2005 THE WEIRS WERE INSTRUMENTED SUCH THAT A MASS BALANCE COULD BE CONDUCTED Watershed Outlet V-notch Weir Flow Meters (typ) Filtration/ Backwater Area Mini Wells Deep Wells Storm Drain (typ)

21 RUNOFF AT V-NOTCH WEIR DURING ONE OF THREE RUNOFF EVENTS IN 2005 TO HAVE GENERATED SURFACE RUNOFF AT THE OUTLET OF THE FHDB

22 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 = sq mi Surface Drainage Area Percent Impervious = % approx Average Annual P = in 100% Average Annual ET = in 61% Average Annual Q = in 38% Percent of Q from Baseflow (from Taylor 1997) = % 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 runoff G = groundwater flow E = Evaporation T = Transpiration DeltaS = Change in Storage

23 HYDROLOGIC BUDGET FOR FHDB FOR A 2-YEAR PERIOD (2 ND AND 3 RD WETTEST YEARS IN 110 YEARS) Fox Hollow Hydrologic Budget from 6/8/03 to 6/7/05 Surface Drainage Area = 0.71 sq mi Surface Drainage Area Percent Impervious = % Average Annual P over study period = in 100% Average Annual ET over study period = in assumed 61% 62% Average Annual Q over study period = 2.30 in 4% Theoretical Average Annual Q over study period = in 22% Percent of Q from Baseflow = 0 % Average Annual change in G or S over study Period = in 35% 34% Average Annual Runoff value = 0.11 mgd/sqmi CANNOT SIMPLIFY HYDROLOGIC BUDGET TO: P Q = ET

24 ANALYSIS SHOWS THAT EVENTS LESS THAN 0.75 RAINFALL PRODUCE NEGLIGIBLE RUNOFF Total Total Percent Total Q (ac-ft) Basin Difference Basin Passing Theoretical Actual to Precipitation Miniwell Runoff Theoretical P (ac-ft) at Weir A Q (ac-ft) Q P (in) In Range In Range In Range In Range >= 5" % 5">P>=4" na 4">P>=3" % 3">P>=2.5" na 2.5">P>=2" % 2">P>=1.75" % 1.75">P>=1.5" % 1.5">P>=1.25" % 1.25">P>=1" % 1">P>=.75" %.75">P>=.5" %.5">P>=.25" %.25">P>=.1" %.1">P>=.06" % SUM % Additional loss between Weir A and V-notch weir not considered

25 HYDROLOGIC RESPONSE DURING HURRICANE KATRINA (1.24 RAINFALL, 0.98 IN ONE HOUR) Fox Hollow Drainage Basin Event of 8/31/2005 (provisional data by LAF at SEA) Flow (cfs) Runoff Volume Summary (ac-ft) Theoretical Basin Impervious Runoff = 10.3 Inflow to Lower Pasture = 4.4 Discharged From Weir B = 3.9 Discharged From Weir A = 2.9 Outflow From Basin at V-notch = :00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 Time Inflow V-notch Weir A Weir B

26 DRAW DOWN OF PONDING USUALLY TAKES BETWEEN ONE AND TWO DAYS Fox Hollow Drainage Basin Event of 8/31/2005 Inflow (cfs) /30/05 22:00 8/31/05 1:45 8/31/05 5:30 Dewatering Summary Time of Inflow Qp = 2:10 AM, 8/31 Response at Weir Starts = 4:00 AM, 8/31 Overtopping of Weir Ends = 10:30 AM, 8/31 Weir Dry = 3:45 AM, 9/1 8/31/05 9:15 INFILTRATION RATE = 1.7 IN/HR 8/31/05 13:00 8/31/05 16:45 8/31/05 20:30 9/1/05 0: /1/05 4:00 Weir A WSE (ft) Date

27 CONCLUSIONS 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 HOWEVER, THIS TYPE OF SYSTEM IS RADICALLY DIFFERENT THAN MANY ENGINEERED INFILTRATION SYSTEMS, WHICH ARE USUALLY NOTHING MORE THAN INJECTION WELLS

28 CONCLUSIONS 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. HOWEVER, THE LAST REMAINING CRITICAL AREAS (RECHARGE AREAS, WETLANDS, FLOODPLAINS, ETC.) WITHIN WATERSHEDS NEED TO BE AGGRESSIVELY PROTECTED.

29 CONCLUSIONS 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. THEREFORE, PEAK RUNOFF RATE ANALYSES SHOULD ALWAYS BE CONDUCTED INDEPENDENTLY OF WATER QUALITY OR VOLUME CRITERIA.

30 END