Hydrodynamic Modeling Analysis of Tidal Wetland Restoration in Snohomish River, Washington. Zhaoqing Yang 1 and Taiping Wang

Transcription

1 Proceedngs of the 12 th Internatonal Conference on Estuarne and Coastal Modelng, ed. by M. L. Spauldng, Aercan Socety of Cvl Engneers, St. Augustne, FL Hydrodynac Modelng Analyss of Tdal Wetland Restoraton n Snohosh Rver, Washngton Zhaoqng Yang 1 and Tapng Wang Pacfc Northwest Natonal Laboratory, 1100 Dexter Avenue North, Sute 400, Seattle, WA 98109, USA To re-establsh the ntertdal wetlands wth full tdal nteracton and prove salond rearng habtat n the lower Snohosh Rver estuary, a dked wetland along Unon Slough of the Snohosh Rver was restored by breachng the exstng dke and constructng brdges across the breaches. However, postrestoraton ontorng ndcated that the restored project ste could not dran as effcently as desred. To prove the dranage condtons at the restoraton ste durng low tdes, a odelng study was conducted to evaluate addtonal restoraton scenaros and to provde recoendatons for fnsh-grade ground elevatons to acheve the desred dranage. To accurately sulate the dranage of the project ste, an unstructured-grd hydrodynac odel wth fne-grd resoluton down to a few eters was used n ths study. The odel was frst valdated wth observed water level data collected n the project ste and then appled to assess the feasblty of dfferent proposed restoraton scenaros. A spatal varyng botto roughness opton n the odel s also pleented to better represent the hgh roughness due to the presence of dense vegetaton n the project ste. The ethodology, error statstcs of odel valdaton and uncertanty of the odelng analyss are presented and dscussed. * Correspondng author: Tel: ; E-al zhaoqng.yang@pnnl.gov 1

2 Introducton The Snohosh Rver s located n the Whdbey Basn of Puget Sound, Washngton State, USA. It enters Puget Sound near the cty of Everett, WA through ultple dstrbutary branches that provde vtal nearshore habtat for arne fsh and wld lfe. To better understand the hydrodynac condtons n the lower Snohosh Rver, a detaled three-densonal (3D) hydrodynac odel for the Snohosh Rver was developed to study the estuarne crculaton and salt ntruson n the estuary (Yang and Khangaonkar, 2008) usng the unstructured-grd fnte-volue coastal ocean odel FVCOM (Chen et al., 2003). To provde gudance to nearshore restoraton efforts for provng tdal wetland habtat, the odel was further appled to sulate the hydrodynac response to varous proposed restoraton condtons a (Yang et al., 2010). The Unon Slough arne wetland restoraton project s located on the west sde of Unon Slough (Fgure 1), whch was restored n 2007 by breachng the exstng dke and constructng brdges across the breaches at three locatons. The objectve of the restoraton project s to re-establsh the ntertdal wetlands wth full tdal nteracton wth Unon Slough and prove salond rearng habtat. However, post-restoraton ontorng ndcated that the project ste could not dran effcently as antcpated durng low tdes, prarly due to the hgh elevatons of breaches wth respect to the ground elevatons nsde the project ste. To prove the dranage condtons, a odelng feasblty study was conducted to evaluate addtonal restoraton scenaros at the project ste and to provde recoendatons for fnsh-grade elevatons to acheve the desred dranage. Ths paper descrbes a odelng feasblty study for the Unon Slough arne wetland restoraton project usng an unstructured-grd odelng approach. Model valdaton wth new observaton data after ntal restoraton, uncertanty analyss and odel applcaton to the new restoraton scenaros are presented and dscussed. Model Setup A hydrodynac odel grd for the Unon Slough project ste was developed based on an exstng odel grd of the Snohosh Rver estuary (Yang et al. 2010). The odel bathyetry was orgnally developed usng Puget Sound Dgtal Elevaton Model (Fnlayson, 2005) and LDAR bathyetrc data fro Puget Sound LDAR Consortu. Bathyetry n the project ste was updated wth new survey data and the odel grd was refned to represent the sall channels as accurately as possble. The presence of the north, central, and south brdges/breaches n the restored Unon Slough project ste was ncorporated nto 2

3 the odel grd. The dranage channel data, whch were specfcally collected for ths project, were used to gude the specfcatons of nternal channel elevatons near the three brdges. Model grd of the study doan and bathyetry dstrbuton n the project ste are shown n Fgure 2. Fgure 1 Unon Slough project ste n Snohosh Rver estuary (crcles denote water level ontorng statons on the project ste) The refned Snohosh Rver odel grd for the new exstng condton conssts of 44,885 eleents and 24,213 nodes. Ten unfor vertcal layers were specfed n the water colun n a sga-stretched coordnate syste. The odel was set up n Unversal Transverse Mercator (UTM) North Aercan Datu (NAD) 83 (Zone 10) coordnates n the horzontal plane wth reference to North Aercan Vertcal Datu 88 (NAVD 88) n the vertcal drecton. Tdal elevatons were specfed at the odel open boundares usng XTde (Flater, 1996) predctons for the odel valdaton perod (5/27/2009 6/26/2009) (Fgure 3). Tdal elevatons along the three open boundares show very slar patterns wth sall phase lags. Rver-nflow data obtaned fro the real-te USGS gauge were specfed at the upstrea boundary near the Cty of Monroe. The 3

4 flow rate for the odel sulaton perod vared fro around 200 to /s (Fgure 4). The ean flow rate for the sulaton perod s /s, whch was a lttle hgher than the long-ter, ean rver flow of /s esated fro longter gauge records. Wnd speed and drectons were specfed unforly n the entre odel doan usng osberved wnd data fro the Pane Feld Arport near the Cty of Everett. The average wnd speed for the sulaton perod was 1.9 /s, and wnd drectons were towards the south ost of the te. Fgure 2 Model grd of Snohosh Rver and bathyetry n the project ste. 4

5 Fgure 3 Water surface elevaton at glendale open boundary staton Fgure 4 Snohosh Rver dscharge at USGS gauge Model Valdaton The updated hydrodynac odel was valdated usng contnuous water surface elevaton data collected at four locatons (HOBO-1 to 4) n the project ste as well 5

6 as n Unon Slough (HOBO-5) after the ntal restoraton acton n 2007 (Fgure 1). Data were sapled at 10-nute ntervals durng the perod fro May 27, 2009 to June 26, Model valdaton was conducted by coparng odeled water levels to observatons and adjustng botto roughness and refnng bathyetry n the project ste. Intal coparsons of odel results to observatons ndcated that odel results atched the observed data reasonably well at all observed statons. Water surface elevaton dstrbutons nsde the project ste were very slar and reached about the sae hgh water levels as that n Unon Slough durng hgh tdes. However, water levels n the project ste dd not drop to the sae low-tde levels as seen n Unon Slough because of the hgher ground elevatons wthn the project ste. Fgures 5 and 6 showed coparsons of water levels at staton HOBO-5 n Unon Slough and at staton HOBO-4 n the project ste. The odel re-produced the general tdal features observed n the data n Unon Slough, such as the sprng-neap cycle and durnal nequalty (Fgure 5). 4.5 HOBO 5 Water Level (eter) Data Model /29/2009 5/31/2009 6/2/2009 6/4/2009 6/6/2009 6/8/2009 Date (PST) Fgure 5 Coparson of sulated and observed water levels at HOBO-5. 6

7 3.5 HOBO 4 Data Model Water Level (eter) /29/2009 5/31/2009 6/2/2009 6/4/2009 6/6/2009 6/8/2009 Date (PST) Fgure 6 Coparson of sulated and observed water levels at HOBO-4. However, n the project ste, sulated water levels tended to drop a lttle faster (1 to 1.5 hours) and lower (~0.12 ) than the observed water levels durng low tdes. These dfferences between odel results and observatons durng low tdes were lkely caused by the coplex bathyetry and rregular bed fors n the project ste that could not be accurately represented by the grd resoluton and a spatally unfor botto roughness. A ste vst to the project ste showed the presence of renant pedestals of eroded tdal arsh and rregular dranage channels that are coonly found n the tdal arshes n other estuares n Puget Sound (Hood, 2007). The dense vegetaton and coplex bed fors (Fgure 7) have hgher bed roughness that would affect the flow feld n the project ste. Therefore, the resstance of bed fors to tdal flows nsde the project ste would be uch greater than that n the natural rver channels and estuary. To prove the predcton of water levels nsde the project ste, botto roughness n the project ste was ncreased fro 0.1 c to 2 c, and reaned the sae (0.1 c) n the rest areas of the odel doan. Model results wth new bed roughness at staton HOBO-4 are shown n Fgure 8. It can be seen that predcated water levels were greatly proved than before (Fgure 6), especally durng low tdes. 7

8 Fgure 7 Coplex bed fors and dranage channels nsde the project ste. HOBO Water Level (eter) Data Model /29/2009 5/31/2009 6/2/2009 6/4/2009 6/6/2009 6/8/2009 Date (PST) Fgure 8 Coparson of odeled and observed water levels at HOBO-4 wth ncreased bed roughness nsde the restored project ste. 8

9 For a qualtatve coparson, an nundaton ap was generated based on odel results durng low tde and copared to the feld survey nundaton ap. Fgure 9 shows that the odeled dstrbuton pattern of tdal nundaton n the project ste s very slar to the survey data, whch confrs the capablty of the odel to predct tdal propagaton n and out of the restored project ste. Fgure 9 Coparson of odeled (left panel - dark shaded area denotes nundaton area) and feld survey nundaton areas (rght panel grdded area denoted surveyed nundaton area) Because the an objectve of ths odelng study s to evaluate the dranage effcency of after restoraton n the project ste, the dranage area percentage wth respect to the entre project ste was calculated as a functon of te. In the odel, a node or eleent s consdered as dry when the water depth s less than a nu depth (0.05 ), whch s the crteron used for sulatng the wettngdryng process. The predcted dranage area percentage vared fro nearly zero when the project ste was fully nundated at hgh tdes to above 40% at low tdes (Fgure 10). Calculaton of cuulatve ground surface area as a functon of ground elevaton ndcated that the cuulatve ground surface area n the project ste ncreases draatcally fro around 2% to alost 80% as the ground elevaton 9

10 changes fro 0.9 to 1.5 NAVD88 (Fgure 11). The flat dstrbuton pattern of the ground surface area ndcated that both the bathyetrc rregularty and the nner-connectvty would play an portant role n dranage of the syste. The neffcent dranage n the exstng restored condton was lkely caused by the cobnaton of narrow chokng dranage channels near the brdges, rregular and coplex bathyetry, and solated pondng areas n the project ste. Fgure 10 Predcted dranage area percentage n the project ste under exstng condtons Ground Elevaton (eter, NAVD88) Cuulatve Area vs. Ground Elevaton 0.0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Cuulatve Area Percentage (%) Fgure 11 Cuulatve ground surface area as a functon of ground elevaton n the project ste 10

11 Model Skll and Uncertanty A odel skll assessent, whch generally conssts of a atrx of error statstcs, was conducted. In ths study, the followng error statstcs were calculated based on odel results and observed data: Mean Error (ME), whch s defned by: N 1 o ME N 1 where N s the total data ponts of feld observatons; water level, and s the observed water level. 1 o Mean Absolute Error (MAE), whch s defned by: N 1 o MAE N 1 Root Mean Square Error (RMSE), whch s defned by: N 1 o 2 RMSE N Relatve Error (RE), whch s defned by: RE 100% o 1 where N 1 N N 1 N 1 o 2 o o o o 2 2 s the observed ean water level. s the odeled Error statstcs were calculated for all observed statons (HOBO-1 to HOBO-5 n Fgure 2). The staton-average error statstcs are provded n Table 1. Overall, odel valdaton errors for water level are sall. All error-statstc values are wthn 10% of the tdal range n the project ste. The negatve ean error ndcates the odel under-predcted the water level. It s portant to be aware that the errors n odel valdaton also contaned the errors that propagated fro the open boundary condtons that were specfed usng predcted water levels by the XTde progra. 11

12 Table 1 Staton-average, error statstcs of odel valdaton for water-level Error 12 Value () Mean Error (ME) Mean Absolute Error (MAE) 0.12 Root Mean Square Error (RMSE) 0.14 Relatve Error (RE) (%) 2.46% Because the dranage calculaton s based on the odel results of predcted water levels, t s expected that there would be errors n the dranage calculaton. However, t s dffcult to quantfy the errors n dranage calculaton because there are no quanttatve dranage/nundaton data avalable for odel valdaton. A ethodology was ntroduced n ths study to assess the uncertanty of dranage calculaton, whch s defned through followng steps. w 1. Defne wetted surface area S for eleent n the entre project ste at any gven te, t, usng wettng and dryng crteron: w S ( t) D H S ( t) 0 ( t) D H where D s the ground elevaton. H (=0.05 ) s the crteron for an eleent or node becong dry. 2. Calculate total dry (dranage) area A d n the project ste: M T d w A ( t) A S ( t) where A T s the total area of the project ste, and M s the total nuber of wetted eleents n the project ste at any gven te. 3. Defne the upper-bound and lower-bound uncertantes for predcted water levels usng RMSE: up ( t) ( t) RMSE and lo ( t) ( t) RMSE RMSE for the water-level predcton s 0.14 (Table 1). 4. Apply (t) and (t) to Step 1 and 2 to obtan the upper bound and up lo lower bound of the predcted total dranage area, A d up (t) and A d (t) lo. The te seres of the dranage area percentage under exstng baselne condton and ts upper and lower bounds (.e., ultplyng A d (t), A d up (t) and A d lo (t) by 100/A T ) are shown n Fgure 12. It can be seen that the uncertanty for the dranage area predcton s generally wthn the range of 5% ost of the te.

13 The average uncertanty range (upper bound lower bound) over the entre sulaton perod s 3.7% and the axu uncertanty value s 28.3%. Dranage Percentage (%) 50% 45% Base Scenaro 40% Lower Bound 35% Upper Bound 30% 25% 20% 15% 10% 5% 0% 5/29/09 12:00 AM 5/29/09 6:00 AM 5/29/09 12:00 PM 5/29/09 6:00 PM 5/30/09 12:00 AM Date (PST) Fgure 12 Uncertanty range of predcted dranage area over a durnal cycle Model Applcaton to New Restoraton Scenaros Once the odel was valdated wth observed data, t was appled to sulate a nuber of new restoraton scenaros and evaluate the dranage effcency n the project ste. A total of fve restoraton scenaros were sulated wth varous cobnatons of brdge reovals and creaton of low-tde dranage channels n the project ste. In ths paper, only the scenaro wth ost proveent of dranage effcency s presented. The confguraton of the restoraton scenaro ncludes reovng all three brdges and creatng low tde dranage channels. The confguraton of brdge reoval assued a clear openng along the full length of the brdge spans n the odel for the axu tdal dranage sulaton. A total of four low-tde channels were consdered n the project ste, wth one northern channel connectng the northern and central regons and three central channels connectng the low pondng areas to the central brdge openng. No channel was consdered n the southern regon near the south brdge. The channel wdth was around 2.5 and the lengths vared fro 90 to 150. The odel grd and bathyetry were odfed to represent the new restoraton condton n the project ste. All the boundary condtons and forcng functons for the new restoraton condton reaned the sae as the exstng condton. 13

14 Dranage Area Percentage Coparson 60% Dranage Area (%) 50% 40% 30% 20% 10% Exstng Condton New Restored Scenaro Dfference 0% 5/27/2009 5/27/2009 5/28/2009 5/28/2009 5/29/2009 Date (PST) Fgure 13 Coparson of dranage area percentage between exstng and new restoraton condtons Copared to exstng condton, the dranage area percentage n the project ste under the new restoraton scenaro was proved consderably (Fgure 13). The project ste was nundated and draned faster under the new restoraton condton than the exstng condtons, ndcatng the tdal fluctuaton n the project ste was ore n phase wth tdes outsde the project ste n Unon Slough. The axu percentage of dranage area durng low tdes ncreased fro 40% under exstng condton to nearly 60% under the new restoraton condton. To further exane the proveent of dranage under new restoraton condtons, the dranage areas between exstng and restoraton condtons at low tdes are copared. Model results showed that the dranage n the northern and central regons of the project ste was proved consderably (Fgure 14). However, lttle proveent was seen n the south part of the project ste, lkely due to relatvely hgh ground elevaton near the locaton of south brdge. 14

15 Fgure 14 Coparson of dranage areas between exstng (left panel) and restoraton (rght panel) condtons. Suary In ths study, a 3D hydrodynac odel was developed wth a fne-grd resoluton for the Unon Slough project ste n the Snohosh Rver Estuary, Washngton State, USA. The odel was valdated wth observed data collected n the project ste and Unon Slough for the perod of May 27 to June 26, 2009, and appled to evaluate a nuber of proposed restoraton scenaros to prove the dranage functon n the project ste. 15

16 The aount of water contaned wthn the ste throughout the sprng-neap tdal cycle and error statstcs assocated wth the valdaton effort were calculated. Error statstcs for odel valdaton are wthn an acceptable range (10%), whch s consstent wth the standards n the coastal odelng county. The odel skll assessent showed that the odel has a good capablty for predctng water levels and dranage n the project ste. In ths study, a ethodology was developed for estatng the uncertanty for the predcted dranage area percentage. Whle the uncertanty of the predcted dranage area percentage vared over te durng dfferent tdal phases, the te-average uncertanty for dranage area predcton was sall (3.7%). To sulate hydrodynacs n tdal wetland envronents, the presence of dense vegetaton and rregular bed fors plays an portant role n bed roughness. Coparsons of odel results to observed water levels deonstrated that ncorporatng varyng botto roughness nto the odel based on vegetaton and geoorphologc nforaton can prove the predctons of water levels and dranage processes nsde the project ste. Bathyetry s a key factor n the odel confguraton. Wth hgher resoluton and ore accurate bathyetrc data, the nteror dtches and channels can be better defned n the odel. To ncrease the confdence of the odel, t s necessary to valdate the odel wth observed veloctes and water surface elevatons at varous locatons of the project ste. 16

17 References Chen, C., Lu, H., Beardsley, R.C., An unstructured, fnte-volue, threedensonal, prtve equaton ocean odel: applcaton to coastal ocean and estuares. J. At. & Oceanc Tech., 20, pp Fnlayson, D.P., Cobned Bathyetry and Topography of the Puget Lowland, Washngton State. Unversty of Washngton. Avalable fro: < >. Flater D A Bref Introducton to XTde. Lnux Journal, 32, Hood WG Scalng tdal channel geoetry wth arsh sland area: a tool for habtat restoraton, lnked to channel foraton process. Water Resources Research 43: W do: /2006WR JCF Jones & Stokes Alternatves Analyss for Adaptve Manageent of Ste Hydraulcs and Publc Access at the Sth Island/Unon Slough Restoraton Project. Study Report prepared for Everett Publc Works Departent, Everett, Washngton. Yang, Z. and T. Khangaonkar, Modelng of Salt Intruson, Intertdal Mxng, and Crculaton n a Braded Estuary. J. Coastal Research. 52, do: / sp1.171 Yang, Z., T. Khangaonkar, M. Calv, and K. Nelson, Sulaton of Cuulatve Effects of Nearshore Restoraton Projects on Estuarne Hydrodynacs. Ecol. Model. 221, do: /j.ecolodel