STORM CENTERING APPROACH FOR FLOOD PREDICIONS FROM LARGE WATERSHEDS

Transcription

1 Guo, James C.Y. (2011). Storm Centerng pproach for Flood Predctons from Large Watersheds, second revew, SCE J of Hydrologc Engneerng, May STORM CENTERING PPROCH FOR FLOOD PREDICIONS FROM LRGE WTERSHEDS By James C.Y. Guo Professor, Cvl Engneerng, Unversty of Colorado Denver, CO E-mal: [email protected] BSTRCT: Most stormwater numercal models assume that the entre watershed area s under the desgn storm and shall be consdered as the trbutary area to the desgn pont. Care must be taken when smulatng storm runoff generaton from a large watershed because the ran storm may only cover a porton of the watershed. Snce the area-averaged ranfall depth decays wth respect to the storm-cover area, the experence of the larger the watershed, the hgher the flood flow s no longer true. In the study, a storm centerng technque s developed to dentfy the conservatve sze of storm cell so that the desgn runoff rate and volume can be maxmzed among varous locatons of storm center. Wthout any stormwater detenton n the watershed, the product of trbutary area and area-weghted ranfall depth serves as the ndcator of runoff accumulaton through the waterway. When takng detenton basns nto consderaton, the effect of flow attenuaton s converted nto an equvalent trbutary area that s used to maxmze the runoff volume at the desgn pont. Ths maxmzaton procedure has been tested by the Low Detenton Basn desgned and bult n the Cty of Las Vegas, Nevada. The method s smple, but senstve enough to dentfy the crtcal storm sze for conservatve desgns. Key Words: Large Watershed, DRF, Storm Center, INTRODUCTION dvances n computng methods have revolutonzed stormwater smulaton technques through the dranage network n a watershed. Many useful numercal models have been developed for flood flow predctons, ncludng HEC-1, SWMM etc (HEC , Rossman 2005). The physcal laws developed for surface runoff movement are ncorporated nto numercal algorthms by whch the ranfall and runoff processes can be smulated by a seres of numercal procedures wth the specfed ntal and boundary condtons. The dranage features n a watershed are descrbed by a set of parameters. Each parameter vares n a range to reflect the development condton of the watershed. Impacts of a proposed development can be quantfed by selectng these parameters accordngly. Numercal modelng technques are senstve to the sze of the watershed because the ranfall dstrbuton s vared spatally and temporally. When the watershed s much smaller than the storm cell, a pont precptaton depth can be appled to the entre trbutary area. Ths practce leads to the concluson that the larger the trbutary area s, the hgher the runoff rate s. In fact, many large watersheds n the southwest US can encompass several hundred square mles. The storm cell s too small to cover the entre watershed. s a result, t s a challenge as to how to select the crtcal porton of the watershed to be under the desgn storm. Secondly, the representatve precptaton depth for a large watershed must be ts area-averaged value. Recognzng that the area-averaged ranfall depth s nversely proportonal to ts trbutary area, the concept of "the larger the watershed, the more the runoff" s no longer true because the tradeoff does exst between trbutary area and area-averaged precptaton depth. To be conservatve, t s necessary to conduct a storm-centerng test when the watershed s greater than 100 square mles (NO tlas 2, 1973). mong many hydrologc parameters, 1

2 watershed trbutary area, pont ranfall depth, and depth-area-reducton factor (DRF) are the key factors. The major task n a storm-centerng test s to dentfy the crtcal storm coverng area from whch the hghest predcton of storm runoff can be generated at the desgn pont. In practce, accordng to the drecton of storm movement and the orographc and topographc condtons n the watershed, the watershed s dvded nto several possble storm-cover areas. The one that produces the hghest runoff volume shall be chosen for conservatve desgns. Of course, n an urban area, predctons of flood flows are also senstve to the man-made flood mtgaton facltes such as detenton basns. Consequently, the selecton of the crtcal stormcover area depends on both the local DRF and detenton capactes wthn the trbutary area. Current practce on storm centerng technque s to conduct a seres of numercal tral-and-error tests untl the peak flows are maxmzed (CCRFCD Manual 1999). Often the floodng condton assocated wth the proposed development depends on the upstream storm-cover area plus many as-bult detenton basns. Therefore, a complete storm centerng test can become tme consumng repettons. Ths paper presents a volume-based approach to maxmze the product of equvalent trbutary area and DRF as an ndcator when selectng the storm-cover area n a large watershed. The extended release from a detenton basn s converted nto an equvalent trbutary area based on the rato of peak nflow to peak outflow. lthough ths method s smple, t can effectvely and accurately assst the engneer n selectng the crtcal storm-cover area. The storm-centerng technque developed n ths study has been tested and appled to the Master Dranage Plan update studes developed for the Las Vegas metropoltan area (CCRFCD Las Vegas Valley Master Plan Update 1997), and Moapa wld lfe valleys, Nevada (CCRFCD Muddy Rver Master Plan Update 1997). DEPTH REL REDUCTION FCTOR ran gage only reports the pont temporal ranfall dstrbuton coverng up to 1 to 5 square klometers. It takes a ran gage network and proper spatal nterpolaton schemes to derve the spatal ranfall dstrbuton over a large area (Guo and Harrgan 2009, Durrans et al. 2002). s recommended by the Natonal Ocean and tmospherc dmnstraton (NO), the depth-areareducton factor (DRF) represents the rato between the area-averaged ranfall depth to the maxmal depth at the storm center (NO tlas 2, 1973). Durng a storm event, the ranfall depth decays wth respect to the dstance from the storm center. The depth-area curve s an attempt to relate the area-averaged ranfall depths wthn the watershed to the maxmal pont ranfall depth under the same storm event. Generally, there are two types of depth-area relatons (Hershfeld 1963). The frst s the storm-centered relaton; that s, the maxmum precptaton occurrng when the storm s centered on the watershed area affected. The second type s the geographcally fxed-area relaton where the watershed area s fxed and the storm s ether centered over t or s dsplaced so only a porton of the storm affects the watershed area. In comparson, the stormcentered ranfall data represent profles of dscrete storms, whereas the fxed-area data are statstcal averages n whch the maxmum pont values frequently come from dfferent storms. Generally, the storm-centered relatons are used for preparng estmates of probable maxmum precptaton, whle the geographcally fxed relatons are used for studes of precptatonfrequency values for watersheds. computer model, TITN, was developed by the Natonal Center for tmospherc Research to dentfy ndvdual storm cells from radar ranfall data for the purpose of determnng ranfall temporal and spatal characterstcs (Dxon 1993). s recommended by NO tlas 14 (Bonnn et el. 2003), the area-averaged ranfall depth s related to the pont depth as: P RP o (1) 2

3 n whch P o = pont ranfall depth, P = area-averaged ranfall depth for trbutary area,, and R = value of DRF. In ths study, a decay curve s proposed to depct the varaton of DRF wth respect to storm-cover area as: R R ) c k ( 1 Rc e (2) n whch = storm-cover area n square mles, k = decay coeffcent, R c = fnal constant when storm-cover area >300 square mles. In ths study, the best-ftted Eq 2 was derved for the deptharea curves recommended n NO TP 40 as (Hershfeld 1963): R e for 24-hour storm (3) R e for 6-hour storm (4) R e for 3-hour storm (5) Eq s (3) through (5) serve as a general gudelne when the local ranfall nformaton s not avalable. Otherwse, a localzed ranfall depth-area curve should be derved from the observed extreme storm events. Havng corrected the ranfall data due to wnd and vegetal canopy effects (Guo at al. 2001), the records from a ran gage network provde the spatal nformaton of the sohyetal ranfall depth dstrbuton wthn the storm-cover area. Under the storm cell n Fgure 1, the area-averaged ranfall depth for a crcled area can be calculated by the area-weghtng method. Usng the maxmal ranfall depth at the storm center as the pont value, the DRF for a crcled area s the rato of the area-averaged ranfall depth to the pont value. Repeatng the same procedure as the crcled area ncreases, a set of DRF can be derved on a decay curve. Fgure 1 Illustraton of Decay of Ranfall Depth from the Storm Center. Ths procedure can be processed wth GIS tools to analyze a large amount of pont ranfall data. In ths study, as lsted n Table 1, seven hstorc extreme events observed from 1955 to 2006, were selected from the McCarran rport ran gage network at the Cty of Las Vegas, Nv. The DRF were derved usng the area-weghted method for each selected storm event, and then averaged to represent ths regon. Between 200 to 400 square mles, the best ftted lne was used to determne the DRF values. The duraton for these events s approxmately 6 to 12 hours. ded by Eq (2), the best ftted equaton s derved as: R e for the Las Vegas area (6) 3

4 Storm Observed Ranfall Event verage Cover rea sq mles 6/13/ /21/1957 7/3/1975 DRF 8/10/1981 Derved 8/10/ /17/2006 DRF Table 1 Ranfall Events Observed n McCarran rport, Nv (Mark Group 1988) Fgure 2 presents the DRF curve derved n ths study. It agrees well wth the recommended DRF for the Las Vegas area (CCRFCD Manual 1999). In comparson, ths local DRF curve decays much faster than NO s DRF 6-hr curve (Bonnn et el. 2003, NO Hydro-40, 1984). Smlar concluson was also observed for the St. Lous regon, MI (Hoblt et al. 2002). Fgure 2 DRF derved for Las Vegas rea, Southern Nevada The man purpose of ths study s to ncorporate a local DRF curve nto the storm-centerng technque. The tradeoff between trbutary area and DRF s decay can provde a general gudance on how to conservatvely predct the desgn flood flows. STORM-CENTERING TEST Before the storm centerng test, a large watershed s supposed to have been dvded nto several trbutary sub-basns. s llustrated n Fgure 3, varous storm centers can be developed accordng to the storm cell movement, waterway network, orogaphc and topographc condtons. The case of all valley storm coverage has the largest trbutary area wth the lowest DRF, whle the Case of lower valley storm coverage has the smallest trbutary area wth the hghest DRF. 4

5 Fgure 3 Illustratons of Possble Storm Centers For each case of storm centerng, the accumulated trbutary area toward the desgn pont s the sum as: N 1 (7) n whch = total trbutary area for the selected case of storm centerng, = area of subbasn, = -th subbasn, N = number of sub-basns contrbutng to desgn pont. In essence, Eq (7) represents the sze of storm-cover area. s a result, the correspondng DRF can be computed usng Eq (2) as: N k ( 1 R ) e (8) 1 R Rc c In an urban area, flood flows are ntercepted by ether a conveyance faclty lke flood channels or a storage faclty lke a detenton basn. In comparson, an urban channel has neglgble attenuaton effect compared to the detenton basn. In ths study, all urban channels are treated as a drect release, whle all detenton basns are converted nto an extended release. For a drect release, the unt peak flow per trbutary area s defned as Q q (9) n whch q = unt peak release n cfs/acre or cms/ha from sub-basn,, and Q = peak dscharge generated and then released from sub-basn,. For an extended release, the detenton basn temporarly stores the runoff volume, and then gradually releases t over tme. Based on the concept of unt peak release n Eq (7), an extended release, O, can be converted to ts equvalent trbutary area as: O E (10) Q 5

6 n whch E = equvalent trbutary area for -th sub-basn, and O = extended peak dscharge released from -th sub-basn through ts detenton basn. Noted that when Q = O, Eq 10 also represents a drect release. Substtutng Eq (10) nto Eq (7) yelds: E N 1 O Q (11) In whch E = total equvalent trbutary area for the selected storm centerng. In essence, Eq 11 represents the runoff-producng area. In practce, the desgn nformaton requred n Eq (11) s readly avalable from the regonal master dranage plans. In ths study, the maxmal runoff volume at the desgn pont s determned as: Max V Max( P ) E (12) In whch V= runoff volume n nch-square mle. ded wth Eq s (8) and (11), Eq (12) s expanded nto: Max V MaxPo k N N 1 1 ( R (1 R ) e (13) c c O Q Eq (13) shall be tested for all possble storm-cover areas to dentfy the one that produces the maxmal runoff volume. In practce, the operaton of summaton n Eq (13) can be easly processed through tabulatons. DESIGN EXMPLE In ths study, the aforementoned maxmzaton procedure s appled to the desgn of the Lower Detenton Basn (LDB) located on the Western Trbutary Wash that drans nto the Las Vegas Wash n the Cty of North Las Vegas, Nevada. s llustrated n Fgure 4, there are two exstng regonal detenton systems bult upstream of the proposed LDB. They are the Kyle Canyon Detenton Basn (KCDB) and the Rancho Detenton Basn (RDB). The total trbutary area to the locaton of the proposed LDB s 92.0 square mles that s dvded nto four subareas, ncludng a trbutary area of 58.0 square mles that drans nto KCDB, another trbutary area of 6.0 square mles that drans nto RDB, and two more separate areas of and 5.4 square mles that drectly dran nto the proposed LDB. 6

7 Fgure 4 Detenton Basns and Sub-areas n Case Study ccordng to the Master Dranage Plan publshed for the Las Vegas Valley (Master Plan Update 2000), the trbutary areas, desgn peak nflows and outflows to the proposed and exstng LDB, RDB, and KCDB are summarzed n Table 2. Based on the major waterways through the watershed, four possble storm centers are nvestgated for ths case as: (1) Storm Center 1: coverng Sub-basn 4 of 22.6 square mles to LDB, (2) Storm Center 2: coverng Sub-basns 4 and 2 or 28.0 square mles to LDB, (3) Storm Center 3: coverng Sub-basns 4, 2, and 3 or 34.0 square mles to LDB. (4) Storm Center 4: coverng the entre trbutary area of 92.0 square mles to LDB. Table 2 s the analyss of the equvalent trbutary area from these four subareas. For nstance, Sub-basns 4 and 2 are a drect release. s a result, ther trbutary areas are the equvalent areas. However, Sub-areas 3 and 1 are an extended release. Ther equvalent area shall be weghted by the rato of peak outflow to peak nflow. For nstance, Sub-basn 1 has a trbutary area of 58 square mles. ded by Eq (10), ts equvalent area s computed as: O1 360 E mle 2 Q The subscrpt 1 represents the parameters for Sub-Basn 1. 7

8 Sub rea Trbutary Peak Peak Equvalent rea Inflow Outflow Trbutary rea sq mle cfs Q cfs O sq mle E Sub basn 4 dranng nto LDB Sub basn 2 dranng nto LDB Sub basn 3 dranng nto RDB Sub basn 1 dranng nto KCDB Table 2 nalyss of Equvalent Trbutary rea to LDB Table 3 presents the computaton of runoff volumes for four cases. For example, Case 4 has a total trbutary area of 92 square mles under the desgn storm. Based on Eq (6), the value of DRF for Case 4 s computed as: R e e * for Case 4 Referrng to Table 3, the area-averaged ranfall depth s reduced from ts pont depth of 3.0 nches to 1.71 nch that shall be appled to the equvalent area of square mles that s accumulated as: N O E sq mles for Case 4 1 Q The runoff volume for Case 4 s the product of the area-averaged ranfall depth and ts equvalent area as: V nch-mle 2 for Case 4 Repeat the same process for Cases 1, 2, and 3 as shown n Table 3. The maxmal runoff volume s derved from Case 3 or the storm cell coverng Sub-areas 4, 3, and 2 shall produce the hghest peak flow at the proposed locaton for LDB. Case Storm Total ccumulated 6-hr Pont Las- real Runoff Vegas No Cover Trbutary Equvalent Ranfall DRF Ranfall Volume rea rea rea Depth Value Depth E P o R P V mle 2 mle 2 mle 2 nches Factor nches Inch-mle 2 1 Sub-basn Sub-basns Sub-basns ll sub-basns Table 3 Test of Storm Centerng for Proposed Lower Detenton Basn n Las Vegas, Nevada 8

9 In ths study, detaled hydrologc models were derved for Cases 1 through 4 usng the HEC-1 model for the above four cases (HEC ). The entre 92-sqaure-mle watershed was dvded nto 35 small sub-areas. The 6-hour SCS ranfall dstrbuton and the SCS unt hydrograph method, supported by the Natonal Resources Conservaton Servce, were employed to predct the storm hydrographs from these 35 sub-basns. These storm hydrographs were then routed through the flood channels and detenton basns to predct the peak nflow dscharge at the LDB ste. The HEC-1 models confrm that Case 3 produces the hghest peak dscharge and storage volume for the proposed ste. In 1996, the Lower Detenton Basn was desgned for a trbutary area of 34.0 square mles. The constructon of the Lower Detenton Basn was completed n CONCLUSIONS The maxmzaton procedure derved n ths study has been tested for several large watersheds wth and wthout detenton basns n the southwest Nevada area (CCRFCD Las Vegas Valley Master Plan Update (1997), CCRFCD Muddy Rver Master Plan Update 1997). For a case of drect release, or no detenton at all, the soluton for maxmal runoff volume s senstve to how sharply the local DRF curve decays. flat DRF curve, or a slow decay, wll lead to the concluson that the more the trbutary area, the hgher the flow. Consequently, the conservatve desgn wth a flat DRF curve s the one that the desgn storm covers the entre watershed. On the contrary, a sharply decayed DRF curve wll lead to cases of small storm cells. In comparson, a large storm system tends to have a long duraton or more lke a general or wnter storm pattern, whle a small storm system tends to have a short duraton or more lke a thunder storm pattern. For the case that nvolves detenton basns or sgnfcant natural depresson areas, the extended releases serve as the key factor n determnaton of the crtcal storm center. In an urban area, a large regonal detenton system can act as a man-made topographc barrer that converts the upstream trbutary area nto a neglgble equvalent area. lthough the procedure presented n ths study s a lnear approach, t s senstve enough to dentfy the crtcal storm-cover area as long as the local DRF s readly avalable. REFERENCES Bonnn, G.M., Todd, D., Ln, B., Parzybok, T.,Yekta, M., and Rley, D. (2003). Precptaton- Frequency tlas of the Unted States, NO tlas 14, Volume 2, Verson 2. NO, Natonal Weather Servce, Slver Sprng, Maryland. CCRFCD Manual (1999). Hydrologc Crtera and Desgn Manual, publshed by Clark County Regonal Flood Control Dstrct, Las Vegas, Nevada. CCRFCD Las Vegas Valley Master Plan Update (1997). Clark County Regonal Flood Control Dstrct, Las Vegas, Nevada. CCRFCD Muddy Rver Master Plan Update (1997). Clark County Regonal Flood Control Dstrct, Las Vegas, Nevada. Dxon, M. (1993). TITN: Thunderstorm Identfcaton, Trackng, nalyss and Nowcastng - Radar-based Methodology. Journal of tmospherc and Oceanc Technology, 10(6): Durrans, S. R, Lesly T. J., and Yekta, M (2002). Estmaton of Depth-rea 9

10 Relatonshps usng Radar-Ranfall Data. Journal of Hydrologc Engneerng, 7(5): HEC-1 (2010). HEC-1 Flood hydrograph Package User s Manual, Corps of Engneers, the Hydrologc Engneerng Center, Davs, Calforna. Hershfeld, D. M. (1963).. Techncal Paper No. 40 Ranfall Frequency tlas of the Unted States for Duratons from 30 Mnutes to 24 Hours and Return Perods from 1 to 100 Years Unted States Department of Commerce, Weather Bureau. Washngton, D.C. Hoblt, B., Zelnka, S., Castello, C. and Curts, D. (2002) Spatal nalyss of Storms Usng GIS, Spepecal Report publshed by OneRan Inc Greenback Lane, Sute C-4, Orangevale, C Guo, James C.Y. and Harrgan, Kelly (2009). Conservatve Desgn Ranfall Dstrbuton, SCE J. of Hydrologc Engrg. Volume 14, Issue 5, pp Guo, James C.Y., Urbonas, Ben. and Stewart, Kevn. (2001). Ran Catch under Wnd and Vegetal Effects, SCE J. of Hydrologc Engneerng, Vol 6, No.1, n Jan. Mark Group (1988). Flood Control Master Plan for Maopa Valley, Nevada, Techncal Report, prepared by the Mark Group, Engneerng and Geologsts, Inc., Las Vegas, Nv, December. NO Hydro-40 (1984). Depth-rea Ratos n the Sem rd Southwest Unted States, NWS NO TM Hydro-40, US Department of Commerce, Slver Sprng, Maryland. NO tlas 2 (1973). Precptaton-Frequency tlas of the Western Unted States, Volume VII, Nevada, Slver Sprng, Maryland. Rossman, L.. (2005) Storm Water Management Model User s Manual. Verson 5, Water Supply and Water Resources Dvson, Natonal Rsk Management Research Laboratory, Cncnnat, OH. II. NOTTIONS = total trbutary area = area of -th sub-basn E = equvalent trbutary area for -th sub-basn E = total equvalent trbutary area = storm-cover area n square mles, = -th sub-basn k = decay coeffcent N = number of sub-basns. O = extended peak dscharge released from detenton basn P o = desgn pont ranfall depth, P = area-averaged ranfall depth for trbutary area R = value of DRF for selected storm-cover area R c = fnal constant when storm-cover area >300 square mles. V= runoff volume n nch-square mle 10