IS THERE ENOUGH SEDIMENT? Presentation at the State of the Estuary Conference San Francisco, CA, October 9-10, By Philip B.

Transcription

1 IS THERE ENOUGH SEDIMENT? Presentation at the State of the Estuary Conference San Francisco, CA, October 9-10, 2001 By Philip B. Williams 1. INTRO I AM TEMPTED TO MAKE THIS THE SHORTEST TALK OF THE CONFERENCE: [2] NO BUT SUCH AN ABBREVIATED PRESENTATION DOES NOT CAPTURE THE IMPORTANCE OF THIS QUESTION FOR ALL OF US WHO ARE INVOLVED IN PROTECTING AND RESTORING THE VALUES OF THE SF BAY ESTUARY. I WILL EXPLAIN WHY THOSE CONCERNED WITH THE LONG TERM MANAGEMENT OF THE ESTUARY S RESOURCES ARE CHANGING THEIR ATTITUDES ABOUT THE SEDIMENT THAT MOVES AROUND THE BAY. ONLY A FEW YEARS AGO WE CONSIDERED BAY MUD TO BE AN EXPENSIVE NUISANCE THAT SILTED UP HARBORS, LIKE HERE AT THE ALVISO MARINA [3] 15/8/2002IsThereEnoughSedimentImages.doc 1

2 TEN YEARS AGO THE PROBLEM SEEMED TO BE; WHERE DO WE PUT ALL THE MUD THAT WE ARE DREDGING? NOW WE ARE STARTING TO VALUE SEDIMENT AS AN ESSENTIAL RESOURCE THAT FORMS THE HABITATS THAT SUSTAIN THE ESTUARINE ECOSYSTEM. TO START I WILL EXPLAIN HOW SEDIMENTARY PROCESSES HAVE CREATED THE HISTORIC HABITATS OF THE BAY, HOW WE HAVE AND WILL BE CHANGING THOSE PROCESSES, AND THEN DISCUSS THE IMPLICATIONS FOR MANAGING THE ESTUARY IN THE FUTURE. HOW SF BAY FORMED AND FUNCTIONS SF BAY LIKE ALL ESTUARIES IS A DYNAMIC EVOLVING SYSTEM, WHO S SHAPE, OR MORPHOLOGY, IS AN EXPRESSION OF A CHANGING EQUILIBRIUM BETWEEN COMPETING PHYSICAL PROCESSES: A DYNAMIC BALANCE BETWEEN SEDIMENTATION, AND SEA LEVEL RISE, RIVER FLOWS, TIDAL FLOWS AND WIND WAVE ACTION. WE CAN UNDERSTAND THIS WHEN WE SEE HOW THE ESTUARY HAS EVOLVED SINCE THE END OF THE ICE AGE AS SHOWN IN THIS SEQUENCE OF MAPS 25/8/2002IsThereEnoughSedimentImages.doc 2

3 [4] SF BAY IS A GEOMORPHICALLY YOUNG ESTUARY FORMED ONLY ABOUT 10,000 YEARS AGO WHEN RAPIDLY RISING SEA LEVEL FROM MELTING ICE CAPS INUNDATED THE MOUTH OF THE SACRAMENTO RIVER THROUGH THE GOLDEN GATE. ABOUT 7000 YEARS AGO THE RATE OF RISE OF SEA LEVEL SLOWED. MARSH PLANTS WERE ABLE TO KEEP PACE WITH THE 1 TO 2 MM/YR SEA LEVEL RISE BY CAPTURING SEDIMENT AND BUILDING PEAT, CREATING EXTENSIVE MARSH PLAINS THAT GRADUALLY ROSE AND ENGULFED THE SURROUNDING TOPOGRAPHY AS WE SEE HERE AT THE CHINA CAMP MARSH 35/8/2002IsThereEnoughSedimentImages.doc 3

4 [5] AS THE ESTUARY GREW LARGER THE TIDAL CHANNELS FEEDING THE MARSHES GREW BIGGER; AND WIND WAVE FETCHES GREW LONGER. WAVE ACTION ERODED THE BAYFRONT EDGE OF THE MARSH CREATING EXTENSIVE INTERTIDAL MUDFLATS. THE MARSHES, MUDFLATS, AND TIDAL CHANNELS HABITATS HAVE EVOLVED AND PERSISTED FOR THOUSANDS OF YEARS, SUSTAINED BE THE INFLOW OF SEDIMENT. MOST OF THE SEDIMENT ENTERING THE ESTUARY HAS BEEN ERODED FROM THE WATERSHEDS OF THE CENTRAL VALLEY RIVERS AND WAS CONVEYED DURING LARGE WINTER FLOODS ON THE SACRAMENTO AND SAN JOAQUIN RIVERS. COARSER SEDIMENTS WERE DEPOSITED UPSTREAM ON THE VAST FLOODPLAINS, LEAVING THE CLAYS AND SILTS TO SETTLE OUT IN THE SHALLOWS OF SUISUN AND SAN PABLO BAYS AND OCCASIONALLY, DURING LARGE FLOOD PULSES, INTOTHE SOUTH BAY. TYPICALLY LATER IN THE YEAR, WIND WAVES WOULD RESUSPEND THESE MUDS, AND TIDAL CURRENTS WOULD REDISTIBUTE THEM TO ALL PARTS OF THE ESTUARY. AS 45/8/2002IsThereEnoughSedimentImages.doc 4

5 WE CAN SEE, SEDIMENTS ARE IN MOTION IN ALL SEASONS IN SF BAY AND THE MUDFLATS AND SHALLOWS ACT AS HUGE SEDIMENT RESERVOIRS.. [6] THE AMOUNT OF THIS SEDIMENT RECIRCULATION IS VERY LARGE. PROBABLY MORE THAN 100 TIMES AS MUCH SEDIMENT IS MOVED AROUND WITHIN THE ESTUARY IN A GIVEN YEAR AS IS SUPPLIED BY RIVER FLOODS. THIS EXPLAINS WHY SAN FRANCISCO BAY WATERS ARE. SO MUDDY. 55/8/2002IsThereEnoughSedimentImages.doc 5

6 2. HUMAN INDUCED CHANGES OVER THE LAST TWO CENTURIES EUROPEAN COLONIZATION HAS TRANSFORMED NOT ONLY THE PHYSICAL LANDSCAPE OF THE ESTUARY, BY DIKING 90% OF THE TIDAL WETLANDS, BUT HAS CHANGED THE PROCESSES THAT SUSTAINED THAT LANDSCAPE. THE FORM AND EXTENT OF ESTUARINE HABITATS IS ULTIMATELY SHAPED BY THE SEDIMENT BUDGET OF THE ESTUARY, THE BALANCE BETWEEN SEDIMENT DELIVERED BY THE RIVERS, SEDIMENT STORED ON MARSH PLAINS OR IN DEEP WATER, AND SEDIMENT DISCHARGED TO THE OCEAN. SEDIMENT DELIVERY HAS CHANGED DRAMATICALLY IN THE LAST TWO CENTURIES AS ILLUSTRATED IN THIS GRAPH; [7] BEFORE COLONIZATION THE WATERSHED WAS LARGELY UNDISTURBED AND FLOODPLAINS INTACT. THIS MEANT THAT SEDIMENT DISCHARGE WAS QUITE LOW. BY 1880 DRASTIC CHANGES HAD OCCURRED. THE COMBINATION OF OVERGRAZING, DEFORESTATION, FLOODPLAIN RECLAMATION, AND MOST 65/8/2002IsThereEnoughSedimentImages.doc 6

7 IMPORTANTLY, HYDRAULIC MINING; CAUSED HUGE INCREASES IN THE AMOUNTS OF SEDIMENT DELIVERED TO THE ESTUARY. THE SIERRA MUD WAVE CAUSED MUDFLATS IN SAN PABLO BAY TO RISE 3 TO 4 FT IN ELEVATION, ALLOWING FRINGING MARSHES TO ADVANCE. IN THE 2OTH CENTURY SEDIMENT DELIVERY STARTED TO DECLINE AS HYDRAULIC MINING WAS SHUT DOWN AND DAMS WERE CONSTRUCTED THAT CAPTURED SEDIMENT AND REDUCED FLOOD FLOWS. RESEARCHERS AT USGS [BRUCE JAFFE] HAVE SHOWN THAT THE INTERTIDAL MUDFLATS OF SAN PABLO BAY HAD LOWERED RESULTING IN LOSSES OF ABOUT 90ACRES A YEAR BETWEEN 1951 AND IF THIS TREND HAS CONTINUED IN THE LAST TWO DECADES WE NOW PROBABLY HAVE ONLY ABOUT 5000 ACRES OF MUDFLAT ABOUT THE SAME AS IN IF IT CONTINUES INTO THE FUTURE, THE MUDFLATS WILL BE PRACTICALLY GONE IN THE NEXT 50 YEARS.. 3. FUTURE CHANGES; FROM SURPLUS TO DEFECIT THIS PROJECTION IS AN EXAMPLE THAT MAY ACTUALLY UNDERSTATE THE RATE OF CHANGE IN ESTUARINE HABITATS OVER THE NEXT CENTURY. THESE CHANGES COULD BE AS DRAMATIC AS THE CHANGES THAT OCCURRED IN THE LAST TWO CENTURIES AND WILL BE DUE TO DELIBERATE OR INADVERTANT ALTERATIONS IN PHYSICAL PROCESSES THAT SUSTAIN THE MORPHOLOGY OF THE ESTUARY. 1. THERE WILL LIKELY BE FURTHER DECLINES IN SEDIMENT DELIVERY TO THE DELTA. AVERAGE SUSPENDED SEDIMENT CONCENTRATIONS ARE DECLINING AS THE FULL EFFECT OF DAMS AND DIVERSIONS ARE FELT. AVERAGE ANNUAL SEDIMENT INFLOW TO THE DELTA IS NOW PROBABLY ONLY ABOUT 4.5MCY/YR ABOUT HALF WHAT IT WAS IN THE 1960 S AND BARELY DOUBLE THAT OF THE UNDISTURBED LANDSCAPE OF 200 YEARS AGO. 2. SEA LEVEL RISE WILL ACCELERATE DUE TO THE GREENHOUSE EFFECT. THE LATEST PREDICTIONS ARE THAT RATES WILL DOUBLE OVER THE 75/8/2002IsThereEnoughSedimentImages.doc 7

8 NEXT 50 YEARS AND DOUBLE AGAIN IN THE SUCCEEDING 50 YEARS. TO PUT THIS IN PERSPECTIVE; NOW EVERY YEAR THE BAY INCREASES IN VOLUME BY ABOUT 2.5MCY AND THIS WILL INCREASE TO 10 MCY/YR IN THE NEXT CENTURY. AS THE BAY INCREASES IN VOLUME MORE RECIRCULATING SEDIMENT WILL BE CAPTURED IN DEEPER WATER. 3. INCREASING AMOUNTS OF ESTUARINE SEDIMENT WILL BE CAPTURED IN NEW SEDIMENT SINKS WITHIN THE ESTUARY. THE DIRECT EFFECT OF SEA LEVEL RISE IN CAPTURING SEDIMENT IS RELATIVELY SMALL IN COMPARISON TO ITS POTENTIAL INDIRECT EFFECT OF CAUSING FAILURE OF LEVEES SURROUNDING DIKED SUBSIDED LAND. OVER THE LAST 150 YEARS WE HAVE DIKED, DRAINED AND PUMPED ALMOST ALL OF THE FORMER TIDAL MARSHES OF THE ESTUARY. THE RESULT HAS BEEN THAT THESE LANDS HAVE SUBSIDED BELOW TIDE LEVEL CREATING A HUGE ARTIFICIAL HOLE. TO GIVE YOU AN IDEA OF THE SIZE OF THIS HOLE, OR POTENTIAL SEDIMENT SINK, HERE IS A GRAPH OF THE ESTUARY MORPHOLOGY 85/8/2002IsThereEnoughSedimentImages.doc 8

9 [8] IF ALL LEVEES SURROUNDING THE BAY WERE TO FAIL THE ESTUARY WOULD MORE THAN DOUBLE IN SIZE AND INCREASE ITS CURRENT VOLUME OF 15 BILLION CY BY ABOUT 80%. THIS POTENTIAL SEDIMENT SINK WOULD TAKE MORE THAN 1500 YEARS TO FILL AT PRESENT SEDIMENT DELIVERY RATES AND HAS BEEN GROWING AT AN AVERAGE RATE AT LEAST TEN TIMES THE PRESENT RATE OF SEDIMENT DELIVERY AS LAND AS SUBSIDED OVER THE LAST 150 YEARS. WHILE NO-ONE IS CONTEMPLATING ANY MANAGEMENT SCENARIO THAT CONSIDERS COMPLETE LEVEE FAILURE OR REMOVAL, ACCIDENTAL LEVEE FAILURES WILL OCCUR, PARTICULARLY IN THE DELTA WHERE SOME AREAS HAVE SUBSIDED MORE THAN 20FT BELOW SEA LEVEL. WHEN A BREACHED SUBSIDED SITE IS ABANDONED WE HAVE INADVERTANTLY CREATED A LARGE AND EFFICIENT SEDIMENT TRAP. IN THE LAST 20 YEARS ABOUT 3500 ACRES OF DIKED SUBSIDED LAND HAVE 95/8/2002IsThereEnoughSedimentImages.doc 9

10 BEEN RETURNED TO TIDAL ACTION THROUGH LEVEE FAILURE IN THE DELTA. FOR EXAMPLE AS HAPPENED HERE IN MILDRED ISLAND IN [9] MILDRED ISLAND IS APPROXIMATELY 1000 ACRES AND HAD SUBSIDED ABOUT 15FT. THE SIZE OF THE SEDIMENT SINK IS ABOUT 24 MCY. IF THIS ISLAND WERE TO GRADUALLY SILT IN AND RESTORE ITSELF BACK TO THE FRESHWATER TIDAL MARSH IT ONCE WAS, IT WOULD REQUIRE THE EQUIVALENT OF ABOUT 5 YEARS OF AVERAGE SEDIMENT DELIVERY. THIS ILLUSTRATES A MAJOR RESTORATION DILEMMA. FRESHWATER TIDAL MARSH IN THE DELTA IS A HABITAT THAT IS NOW NEARLY EXTINCT, SO THERE ARE NOW PROPOSALS TO RESTORE 20,000ACRES ODF DELTA ISLANDS AS TIDAL MARSH. THIS WOULD REQUIRE VERY ROUGHLY 320MCY OF SEDIMENTATION ASSUMING AN AVERAGE OF 10FT OF SUBSIDENCE. THIS IS EQUIVALENT TO ABOUT 70 YEARS OF SEDIMENT INFLOW. IT IS REASONABLE TO EXPECT THAT IN THE FUTURE A LARGER PORTION OF THE SEDIMENT DISCHARGED INTO THE ESTUARY WILL BE CAPTURED IN THE DELTA BEFORE IT REACHES SAN FRANCISCO BAY 105/8/2002IsThereEnoughSedimentImages.doc 10

11 IN SAN FRANCISCO BAY OUR PREFERENCE IS TO RESTORE SALT MARSHES BY BREACHING SUBSIDED SITES AND MAKING USE OF NATURAL SEDIMENTATION TO RECREATE WETLAND HABITAT. AN EARLY IF RATHER EXTREME EXAMPLE OF SUCH A RESTORATION PROJECTIS SHOWN HERE AT WARM SPRINGS. [10] THIS 200 ACRE PROJECT WAS EXCAVATED MORE THAN 15FT BELOW SEA LEVEL AS A BORROW PIT FOR BUILDING PADS FOR ADJACENT DEVELOPMENT. SINCE BREACHING IN 1986 IT HAS ALMOST COMPLETELY FILLED AND HAS ACCUMULATED ABOUT 5MCY OF SEDIMENT, OR ABOUT ONE YEARS SEDIMENT INFLOW, BROUGHT IN BY TIDES FROM THE SOUTH BAY. WE ARE NOW ENGAGED IN LARGE SCALE INITIATIVES TO RESTORE TIDAL HABITATS AROUND THE BAY. THE SF JOINT VENTURE S GOAL IS ABOUT ACRES OVER THE NEXT 20 YEARS. WE CAN MAKE A ROUGH ESTIMATE OF HOW MUCH SEDIMENT THIS WILL ULTIMATELY BE 115/8/2002IsThereEnoughSedimentImages.doc 11

12 REQUIRED TO BUILD UP SUBSIDED SITES EITHER THROUGH PLACING DREDGED MATERIALS OR BY ENCOURAGING NATURAL SEDIMENTATION TO ELEVATIONS THAT PICKLEWEED MARSHES FORM AT MHHW. ASSUMING TYPICAL SUBSIDENCE OF ABOUT 4 FT, WE WOULD NEED ABOUT 250MCY IN TOTAL TO EVENTUALLY RECREATE A MATURE VEGETATED MARSHPLAIN. 4. IMPLICATIONS FOR MORPHOLOGY AND HABITAT WITH REDUCTION OF SEDIMENT INFLOW TO THE BAY AND CREATION OF NEW SEDIMENT SINKS THROUGHOUT THE ESTUARY, WE CAN EXPECT MAJOR CHANGES IN SEDIMENT DYNAMICS WITHIN THE BAY. WE WILL HAVE CREATED A SEDIMENT BUDGET DEFECIT AND THE RESULT WILL BE THE DEPLETION OF THE SEDIMENT RESERVOIR IN THE SHALLOWS OF THE ESTUARY. THIS ILLUSTRATES HOW THIS WORKS [11] MUDFLATS AND SHALLOWS ARE FORMED BY THE BALANCE BETWEEN DEPOSITION AND WIND WAVE EROSION. WITH REDUCTION OF SEDIMENT DEPOSITION THE MUDFLATS WILL ERODE TO COMPENSATE. THE AMOUNT OF THIS EROSION IS LIMITED BY THE SIZE OF THE LARGEST WAVES SCOURING 125/8/2002IsThereEnoughSedimentImages.doc 12

13 THE SHALLOWS AT LOW TIDE. FOR SAN PABLO BAY THIS MIGHT BE TYPICALLY A 2 TO 3FT WAVE BREAKING IN 3 FT OF WATER. SO FOR PURPOSES OF ILLUSTRATION LET US ASSUME THAT THE ACTIVE SEDIMENT RESERVOIR IN THE ESTUARY IS THE ERODIBLE WEDGE BETWEEN MSL AND 6 FT BELOW SEA LEVEL. HOW BIG IS THIS SEDIMENT RESERVOIR? [12] IT IS NOT INFINITE AND GREATLY DEPENDS ON WHETHER WE LET THE NATURAL SHORELINE RETREAT LANDWARD RELEASING MORE SEDIMENT TO THE ESTUARY THROUGH EROSION OF THE MARSH EDGE. LET US ASSUME WE 135/8/2002IsThereEnoughSedimentImages.doc 13

14 DO NOT DO THIS AND INSTEAD REINFORCE OUR LEVEES. THERE ARE NOW [AS OF 1986] APPROXIMATELY 80,000 ACRES OF SHALLOWS BETWEEN MSL AND 6FT NGVD. IF THIS ENTIRE AREA WERE TO ERODE DOWN TO 6FT IT COULD PROVIDE ABOUT 400MCY OF SEDIMENT TO THE ESTUARY. THERE ARE MANY IMPORTANT CONSEQUENCES TO THIS FUTURE CHANGE IN SEDIMENT DYNAMICS, AND NOT ALL ARE NEGATIVE. IN THE LIMITED TIME I HAVE, I WANT TO DISCUSS AN IMPORTANT CONSEQUENCE FOR HABITAT RESTORATION PLANNING. SAN FRANCISCO BAY WATERS WILL BECOME LESS MUDDY. STATED MORE SCIENTIFICALLY, AVERAGE SUSPENDED SEDIMENT CONCENTRATIONS IN THE WATER COLUMN WILL DECREASE AND THIS DECREASE WILL DIRECTLY AFFECT HOW QUICKLY RESTORATION PROJECTS THAT RELY ON NATURAL SEDIMENTATION, WILL EVOLVE. I BELIEVE WE ALREADY HAVE BECOME SOMEWHAT OVERCONFIDENT OF HOW QUICKLY OR WHETHER WE CAN RESTORE VEGETATED MARSHES ON DEEPLY SUBSIDED SITES. MOST RESTORATION PROJECTS COMPLETED SO FAR HAVE BEEN IN FAVORABLE LOCATIONS WHERE THE SSC IS HIGH AND WIND WAVE ACTION LOW. THIS ALLOWS RAPID NATURAL SEDIMENTATION AS CAN BE SEEN AT CARLS MARSH AFTER 6 YEARS OF EVOLUTION. 145/8/2002IsThereEnoughSedimentImages.doc 14

15 [13] ALTERNATIVELY OTHER SITES LIKE MUZZI 155/8/2002IsThereEnoughSedimentImages.doc 15

16 [14] HAVE USED DREDGED MATERIALS TO COMPENSATE FOR SUBSIDENCE ALSO ALLOWING RAPID REVEGETATION. WE HAVE LITTLE EXPERIENCE WITH LARGE SITES WITH LOW SUSPENDED SEDIMENT CONCENTRATIONS. HERE IS A PLOT OF HOW QUICKLY MUDFLATS BUILD UP IN A TYPICAL SUBSIDED SITE AT DIFFERENT AVERAGE SSCS 165/8/2002IsThereEnoughSedimentImages.doc 16

17 [15] AS CAN BE SEEN HERE IF SSC IS DREDUCED FROM 200 TO 100PPM IT CAN MAKE THE DIFFERENCE FOR A MUDFLAT IN A BREACHED SUBSIDED SITE REACHING THE COLONIZATION ELEVATION FOR SPARTINA IN 40 YRS INSTEAD OF 10 YRS. LOW SUSPENDED SEDIMENT CONCENTRATION COULD EXPLAIN WHY THE SLAUGHTERHOUSE POINT SITE SHOWN HERE 175/8/2002IsThereEnoughSedimentImages.doc 17

18 [16] STILL IS MAINLY MUDFLAT 20 YRS AFTER IT WAS ACCIDENTLY BREACHED. 5. IMPLICATIONS FOR PLANNING 185/8/2002IsThereEnoughSedimentImages.doc 18

19 [17] FIRST, WE NEED TO UNDERSTAND THE ESTUARY AS A DYNAMIC EVOLVING PHYSICAL SYSTEM WHOSE HABITATS WILL BE CHANGING WITH OR WITHOUT HUMAN INTERVENTION. THIS ALSO MEANS WE NEED TO FOCUS RESEARCH ON SEDIMENT DYNAMICS. SECOND, WE NEED TO BE PLANNING FOR THE FUTURE PHYSICAL EVOLUTION OF THE ESTUARY ON AN ESTUARY WIDE SCALE. THIS MEANS RECOGNIZING THAT ACTIONS THAT IMPACT SEDIMENT DYNAMICS LIKE DREDGING, HARDENING LEVEES, TIDAL RESTORATION, OR AIRPORT FILLS CAN HAVE SIGNIFICANT IMPACTS LONG TERM ON ESTUARINE HABITATS THIRD, WE NEED TO VALUE OUR SEDIMENT. SPECIFICALLY THIS MEANS MAXIMIZING THE REUSE OF DREDGED MATERIALS WITHIN THE TIDAL ZONE OF ESTUARY SYSTEM PARTICULARLY FOR WETLAND RESTORATION. 195/8/2002IsThereEnoughSedimentImages.doc 19

20 FOURTH, WE NEED TO BE REALISTIC IN ADDRESSING THE LONG TERM PHYSICAL CONSTRAINTS ON TIDAL WETLAND RESTORATION SPECIFICALLY UNDERSTANDING HOW QUICKLY OR WHETHER SITES WILL EVOLVE TO VEGETATED MARSH FOR LARGE DEEPLY SUBSIDED SITES WITH LIMITED SEDIMENT SUPPLY. FIFTH, WE NEED TO REVISIT OUR RESTORATION GOALS AND PRIORITIES TO MATCH THE HABITATS WE WANT TO RESTORE WITH THOSE THAT PHYSICAL PROCESSES WILL ACTUALLY SUSTAIN IN THE ESTUARY OF THE FUTURE. THIS MAY MEAN WE REFOCUS ON RESTORING OUR DIMINISHING INTERTIDAL MUDFLATS, OR GIVE HIGHER PRIORITY TO ALLOWING MARSHES TO EXPAND INLAND. LAST BUT NOT LEAST WE NEED TO RECOGNIZE THAT MANAGING MUD IS AS IMPORTANT TO SF BAY AS FRESHWATER. 205/8/2002IsThereEnoughSedimentImages.doc 20