Chehalis Basin Strategy Causes of Extreme Flooding. October 11, 2016 Policy Workshop

Transcription

1 Chehalis Basin Strategy Causes of Extreme Flooding October 11, 2016 Policy Workshop

2 Agenda Hydrology o Precipitation o Flow Climate Change Effects Land Use o Channel straightening and incision o Riparian zone clearing o Forest practices o Urban runoff (increased impervious surface) 2

3 Precipitation and Flow 3

4 Chehalis Basin Hydrometeorologic Zones 3.5% Snow-dominated headwaters (dark blue) 17.5% rain-snow transitional (light blue) 79% rain-dominated (light green) Source: Perry et al. 2016

5 Source: United States Average Annual Precipitation, , published in 2012 by PRISM Climate Group, Oregon State University

6 Average Annual Precipitation 80 inches average over the entire basin (approximately 1% in the form of snow) Range from 46 to 50 inches in low-lying valley areas near Centralia and Chehalis To more than 140 inches in the Willapa Hills and 200 inches at some locations in the Olympics 6

7 Atmospheric Rivers Atmospheric Rivers are almost always the cause of major flood events (Nieman et al. 2011) Between 1980 and 2009 Atmospheric Rivers accounted for o 46 of the 48 peak flow events studied in Western Washington o All 12 of the flood events studied in the Satsop River o All peak daily flows that exceeded a 5-year return Heavy precipitation results when moisture source (atmospheric river) interacts with topography 7

8 100-year Precipitation Map (24 hour totals) Source: WSDOT (data analysis by Oregon Climate Service) 8

9 Estimated Peak Flows at Grand Mound Gage Percent Chance Exceedence Return Interval Flow (cfs) , , , , , , , ,600 9

10 Recent Historical Flood Events February 1996 broadly distributed rainfall and widespread flooding. Peak Flow at Grand Mound = 74,700 cfs December 2007 focused in Upper Chehalis and Olympics, Skookumchuck flow reduced by dam. Peak Flow at Grand Mound = 79,100 cfs January 2009 focused in Cascade tributaries and lower Chehalis Basin, still had large contribution from upper Chehalis Basin. Peak flow at Grand Mound = 50,700 cfs 10

11 Recent Historical Flood Events Relative Contributions from Upper Chehalis and Cascade Tributaries for Top 10 Historical Floods 1 Upper Chehalis Contribution Average: 66% Range: 58% to 85% Cascade Tributaries Contribution Average: 34% Range: 15% to 42% 1 Based on USGS historical peaks and mean daily flows 11

12 Relative Contribution of Flooding Water Year Gage Name CHEHALIS RIVER NEAR GRAND MOUND, WA (100-year peak = 75,000 cfs) CHEHALIS RIVER NEAR DOTY, WA (100-year peak = 35,000 cfs) SOUTH FORK CHEHALIS RIVER AT BOISTFORT, WA (100-year peak = 15,200 cfs) NEWAUKUM RIVER NEAR CHEHALIS, WA (100-year peak = 14,400 cfs) SKOOKUMCHUCK RIVER NEAR BUCODA, WA (100-year peak = 12,900 cfs) Date Peak flow (cfs) Peak flow (cfs) Peak flow (cfs) Peak flow (cfs) Peak flow (cfs) Dec-07 79,100 52,600 20,713 12,900 3, Feb-96 74,800 28,900 9,542 13,300 11, Jan-90 68,700 27,500 10,400 8, Nov-86 51,600 17,900 10,700 3, Jan-09 50,700 20,100 11,664 13,000 10, Jan-72 49,200 22,800 6,540 9,770 8, Dec-37 48, Nov-90 48,000 20,600 10,300 8, Dec-33 45, Dec-75 44,800 17,400 6,593 8,020 6, Jan-71 40,800 9,612 3,526 8,390 6,630 12

13 Relative Contribution of Flooding Water Year Date Peak flow at Grand Mound Gage (cfs) From Upper Chehalis DA = 437 mi 2 From Cascade Tribs DA = 400 mi Dec-07 79,100 85% 15% Feb-96 74,800 67% 33% Jan-90 68,700 72% 28% Nov-86 51,600 69% 31% Jan-09 50,700 63% 37% Jan-72 49,200 68% 32% Dec-37 48, Nov-90 48,000 66% 34% Dec-33 45, Dec-75 44,800 69% 31% Jan-71 40,800 53% 47% 13

14 Climate Change and the Chehalis River Guillaume Mauger, Se-Yeun Lee, Christina Bandaragoda, Yolande Serra, and Jason Won

15 Summary of CIG Study Storm dynamics (e.g., wind speeds) are not projected to change, but ocean warming will bring heavier precipitation Flooding is projected to increase Low flows are projected to decrease Models suggest drier conditions in summer New methods have led to improved streamflow estimates, but there remains a large spread among models 15

16 Flooding: Uncertainty Is Larger than Differences Among Tributaries Projected Change in 100-year FLOOD for all Bias-Corrected Chehalis streamflow sites 2050s: Average for all sites and all MACA models: rawwrf, SRES A1B bcwrf, SRES A1B bcmaca, RCP 4.5 bcmaca, RCP 8.5 DHSVM VIC DHSVM VIC DHSVM VIC DHSVM VIC min max min max min max min max avg min max avg min max avg min max avg min max ChehalisR-atPorter year = +66% ChehalisR-nrDoty ChehalisR-nrGrandMound year = +35% NewaukumR-nrChehalis SatsopR-nrSatsop SkookumchuckR-blwBldyRunCr year = +16% SkookumchuckR-nrBucoda SkookumchuckR-nrVail WynoocheeR-abvBlackCr WynoocheeR-abvSaveCr WynoocheeR-nrGrisdale Projected change for 25 a high greenhouse gas -9 scenario Average: All sites Average: Key sites (*) (RCP 8.5), for 2040 to 2099 relative to 1951 to 2005 KEY: +50%: wetter 0: no change 50%: drier 2080s: rawwrf, SRES A1B bcwrf, SRES A1B bcmaca, RCP 4.5 bcmaca, RCP 8.5 DHSVM VIC DHSVM VIC DHSVM VIC DHSVM VIC min max min max min max min max avg min max avg min max avg min max avg min max ChehalisR-atPorter ChehalisR-nrDoty ChehalisR-nrGrandMound

17 Low Flows: Projected to Decrease; Smaller Range Projected Change in 10-year LOW FLOW for all Bias-Corrected Chehalis streamflow sites 2050s: Average for all sites and all MACA models: rawwrf, SRES A1B bcwrf, SRES A1B bcmaca, RCP 4.5 bcmaca, RCP 8.5 DHSVM VIC DHSVM VIC DHSVM VIC DHSVM VIC min max min max min max min max avg min max avg min max avg min max avg min max ChehalisR-atPorter* ChehalisR-nrDoty* year = -7-6% ChehalisR-nrGrandMound* NewaukumR-nrChehalis* year = -14% SatsopR-nrSatsop* SkookumchuckR-blwBldyRunCr SkookumchuckR-nrBucoda* SkookumchuckR-nrVail WynoocheeR-abvBlackCr* Projected change 1-56 for -24 a -18 high 4 greenhouse gas -32 scenario WynoocheeR-abvSaveCr WynoocheeR-nrGrisdale (RCP ), for to relative to to Average: All sites Average: Key sites (*) KEY: +50%: wetter 0: no change 50%: drier 2080s: rawwrf, SRES A1B bcwrf, SRES A1B bcmaca, RCP 4.5 bcmaca, RCP 8.5 DHSVM VIC DHSVM VIC DHSVM VIC DHSVM VIC min max min max min max min max avg min max avg min max avg min max avg min max ChehalisR-atPorter* ChehalisR-nrDoty*

18 Climate Change Effects on 100-year Maximum Daily Flow For worst-case conditions, 100-year flow on upper Chehalis River (near Doty) is estimated to increase by 84% compared to historical flows (from 22,400 cfs to 40,400 cfs) 100-year flow on Newaukum River is estimated to increase by 94% (from 12,400 cfs to 23,900 cfs) While percent increase is slightly higher on the Newaukum, the magnitude of flow increase is greater on the Chehalis River 18

19 Land Use 19

20 Changes in Land Use that Affect Hydrology and Hydraulics Channel straightening and incision Riparian zone clearing Forest practices Land use activities such as increased impervious surfaces, loss of vegetation, and development in the floodplain

21 Floodplain storage The Chehalis watershed has lost much of its natural flood storage

22 Otter Creek (during Hurricane Irene) Time Delay 22

23 Peak Flow Reductions Newaukum Time Delay Peak Reduction Example from North Fork Newaukum River, just downriver from the confluence of South Fork and North Fork Newaukum Rivers, or where North Fork Road meets Jackson Highway 23

24 Land Use: Forest Practices

25 PRISM annual average precipitation in the Chehalis Basin Heavily managed forest Increasing precipitation with increasing elevation Source: Perry et al

26 Percent tree canopy cover in the Chehalis Basin Heavily managed forests Clearcuts on order of 1 square kilometers (240 acres) common Source: Perry et al

27 Hydrologic Processes with Snow Source: Perry et al Greater accumulation in the open snow lasts longer in the open Longer lasting snow increased chance for rain-on-snow events Rain-on-snow events: forest cover reduces snowmelt contributions 27

28 Hydrologic Processes without Snow Source: Perry et al Less evapotranspiration and interception in the open greater soil moisture in the open Road effects: subsurface flow interception routes runoff more efficiently to stream Road Maintenance and Abandonment Plans created to mitigate this 28

29 Literature Review Key Findings Peak Flows o Channel forming flows increase (statistically significant) o High flows (5-10 year return period) and extreme event flows likely increase, but with unknown statistical significance o Increase lasts 20 or more years Low Flows o Low flows increase, and this lasts about 5 years Source: Perry et al

30 Land Use: Increased Impervious Surfaces and Development

31 Development/Impervious Surface Impervious surfaces are generally confined to developed lands and transportation corridors Total developed area in the Chehalis Basin is 7% (residential homes, shopping centers, industrial facilities) Total impervious area in the Chehalis Basin in 2011 was approximately 1.49% (USGS 2011) o 2001: 1.44% (USGS 2001) o 2006: 1.47% (USGS 2006) 31

32 Twin Cities and Newaukum River Area 32

33 Conclusions In the upper Chehalis, Newaukum, and Skookumchuck River basins: o Land cover is dominated by forestlands and contains lowdensity rural and agricultural development in river valleys o These areas have much less impervious surface than the Chehalis-Centralia area Extreme floods on the Chehalis River, such as those experienced in 2007 and 2009, are the result of atmospheric rivers that deliver high rates of rainfall in the upper Chehalis Basin above the Chehalis-Centralia area However, land uses and floodplain conditions also influence downstream flood timing and extents 33

34 Questions