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1577215- -2 IE- 15- -2 L ft, .� ST. FRANCIS OF ASSISI PRELIMINARY HYDROLOGY FOR PROPOSED SITE DEVELOPMENT PLAN Prepared by: Watson Engineering 50200 Monroe Street Indio, CA 92201 (760) 342 -7766 December 9, 2009 Q�OFESS /O O � Q� y� W. ►�,q� q� 0 0. 26662 z W MARCH 31;,12010 R' s� qTF C/ IL OF CA0I 0 i to o� �Nr �... �qti+.. aM� i� . � � �� a� �;�1 �. � ` �� r ,� .. �2 � '.:tt � ; _ �, .T �.. � - � � � ,� i' ,, i\ � � _ , 1 ��,�° _ �„ ,.� :,. w HYDROLOGY STUDY ST. FRANCIS OF ASSISI PARKING LOT EXPANSION PURPOSE The purpose of this study is to demonstrate that the proposed parking lot project can be developed based upon the site development plan. The 100 -year storm runoff volumes have been calculated in conformance with the requirements. of the city to demonstrate that the proposed on -site retention basin is adequate for the expansion project. EXISTING SITE DESCRIPTION The church property consists of approximately 49 acres. A portion of this 49 acres consists of the existing church structure and accompanying facilities which were constructed in the mid- 1980's. There is approximately 47 acres to the north of the church property that is tributary to the existing low point in Washington Street. The city contracted NAI consulting to provide a drainage study to determine what responsibility this ' project has for the drainage generated from the existing development within the tributary area which was determined to be 1.54 acre feet. DRAINAGE DESIGN CRITERIA This study determines the retention volume required by the proposed new development. The ' existing church development drainage patterns are not being altered with this development and will remain as originally approved and constructed. The proposed project area is used to calculate the volume of retention due to development of a portion of the site. Added to this volume is the 1.54 ' acre feet determined to be attributed to the property for the existing condition. The remaining portion of the property not developed by this project will remain vacant and will not be disturbed. When this portion of the property is developed an additional retention basin will be provided as required. IHISTORICAL DRAINAGE PATTERNS ' The St.. Francis Church property is severely impacted by off site drainage from the approximately 44 acres of mountain and developed property adjacent to the north boundary of the property. We researched existing records in order to find topography that was done before the development of the property that is now tributary to the church property. We were able to obtain topography from the Coachella Valley Water District that was done by the Department of Interior Bureau of Reclamation. ' We show the pre - development flow patterns which indicate that the drainage from the area impacting the church property flowed easterly north of Avenue 47 and southerly along Washington Street to a low point in Washington Street and then easterly. Over the years with the development along the ' eastside of Washington Street, the historical drainage patterns have been compromised. CONCLUSION ' The proposed development requires retention capacity of 1.42 acre feet. The existing conditions responsibility as determined the NAI study for retention is 1.54 acre feet for a total project ' responsibility of 2.96 acre feet. Percolation was not considered in the volume determination for the basin. As requested by the City, the basin was deepened to provide additional capacity to be used for off -site drainage. The basin capacity at elevation 56.0 is 5.19 acre feet which provides for a ' minimum of 2.23 acre feet additional storage for off -site drainage. 1 t R R IARIMMOF os�s o> O , i �• o• �• 4 � 4 °e 1 a N q 1 1 � _ 1 q g. DEPARTMENT OF THE INTERIOR BUREAU OF RECGAMAT /ON 80VLD£R CANYON PRO✓ECT ALL-AM£R /CAN CANAL SYSTEM- GAL /F. SEC'829 &30-- T�^r5S -R7 7E --SBB, &M TOPO'-'r' If Y 0 TOPOG. : - -R Q� ----------'SUBM/TTEO:---------------------- TRACm:_ 0IM- CKED_gLt APPROVED: C 2�T 308 YUMA, ARIZONA - FEB. ZB, /Sd3g 212— 30 5 -3M onn -7r vvv I v I— 0 V a Ila 00 PO `ogi 09 Pri92 dt7J ssb2lA b 7^9 L/ I wip X 4e�j 1 1 1 I I ee — i I M 1 1 t 4 1 ' 1 I C� C 1 � � C m O� 059Z �C J (` i�C WIN s o ph e gi p9 R R IARIMMOF os�s o> O , i �• o• �• 4 � 4 °e 1 a N q 1 1 � _ 1 q g. DEPARTMENT OF THE INTERIOR BUREAU OF RECGAMAT /ON 80VLD£R CANYON PRO✓ECT ALL-AM£R /CAN CANAL SYSTEM- GAL /F. SEC'829 &30-- T�^r5S -R7 7E --SBB, &M TOPO'-'r' If Y 0 TOPOG. : - -R Q� ----------'SUBM/TTEO:---------------------- TRACm:_ 0IM- CKED_gLt APPROVED: C 2�T 308 YUMA, ARIZONA - FEB. ZB, /Sd3g 212— 30 5 -3M onn -7r vvv I v I— 0 V a Ila 00 PO `ogi 09 Pri92 dt7J ssb2lA b 7^9 L/ I wip X 4e�j 1 1 1 I I ee — i I M 1 1 t 4 1 ' 1 I C� C 1 � � C m O� 059Z �C J (` i�C WIN LOCATION MAP FRED i WARING Hey ll1 MILES I AVENU HWY. 111 PROJECT SITE i • 4th 1 O�1D�AN WELLS L LSS 1 W 48th N W nn nn ��p� A W LA � ' N • •Z ° 50th AVENUE c� _z z 400e W 3 � W 0 52th AVENUE Ck: 3 0 N W AVENUE AVENUE m e m Dow DmdOpmmy Day wavo@a [29fl@99fll HG 9m@ ahL E4Gl4EER14G • LAND ?W4414G 81 -735 AY 111, SWE B * U 42201 Rt (760) 342 -7766 FA74 (760) 342 -7716 V 1. fo� 4A NO r. 21� ;.7 BASIN 1 BASIN CALCULATI ®NS i LOW LOSS RATE Al .10 X 1.63 = .163 A2 .90 X 2.85 = 2.565 A3 .80 X 2.33 = 1.864 A4 .95 X 0.46 = .437 5.029 C= 5.029_ 7.270 =0.692 .9- (.8X.692) = .346 =35% 1 HR = 2.20 IN 3 HR = 2.80 IN 6 HR = 3.40 IN 24 HR = 4.50 IN BASIN 1 13 1 i AVERAGE ADJPSTED LOSS RATE 113 123 133 141 151 163 173 183. 193 1103 0 SOIL GROUP COVER TYPE RI. NUMBER PERVIOUS AREA. LAND USE DECIMAL PERCENT ADJUSTED INFILTRATION AREA 88- tM�NE6 �[g� iH+ AVERAGE ADJUSTED `< l7 0 (PLATE C -1) (PLATE E-6.1) INFILTRATION OF AREA RATE -IN /HR /}�QEj INFILTRATION z M RATE -IN /HR (PLAT E E-6.2) IMPERVIOUS ( PLATE E -6.3) 143(1 - .9163) RATE -IN /HR cT 39 c9 3 C r- 0 CSI 5, 58 o !3 5I►V , /o ,b3 22 lob r < 0 0 A. ;DZ-V Z P�%rtnw c e4l 2 ASS' '394 o Sf c�a Z E1 TJeV 32 ,m Z. ,wry 80 Zo Z.33 ,32 ve -4 � rn � R&C-C _ ,040 ) o V0• n 0 Z C) a =� Cn 0 o U) c 0 � � D � 3 � M 0 v � �CL 0 ~ . fc63- �.z7 fclo3- ,ZL3 D VARIABLE LOSS RATE CURVE (24 -HOUR STORM ONLY) Fm= Minimum Loss Rate=— F/2 =i C10]/2 = 112- IN. /HR, g $ C =(F— Fm) /54 = (I CIO] — FM) /54= ,002 FT =C(24— (T /60)). +Fm= (24— (T /60))I.ss+ IIZ IN. /HR. Where: u T =Time in minutes. To get an average value for each unit time period, Use T= 2 the unit time for the first time period,T =12 unit time for the second period,etc. RCFC & WCD HYDROLOGY MANUAL SYNTHETIC UNIT HYDROGRAPH METHOD Unit Hydrograph and Effective Rain Calculation Form P�o�ec� • Sheet ` BY Date Checked Date [1] CONCENTRATION POINT /3,+!51 A) .[2] AREA DESIGNATION [3] DRAINAGE AREA Z7 C [4] ULTIMATE DISCHARGE - CFS -HRS/IN (645• [3]) [5] UNIT TRV E- MINUTES /"I Pj [6] LAG TIME- MINUTES [7] UNIT TIME- PERCENT OF LAG (100'[5]/[6] [8] S -CURVE [9] STORM FREQUENCY & DURATION j6CYEAR HOUR [10] TOTAL ADJUSTED STORM RAIN -INCHES [I I] VARIABLE LOSS RATE (AVG)- INCHES/HOUR [12] MINIMUM LOSS RATE (FOR VAR. LOSS) -IN/HR [13] CONSTANT LOSS RATE- INCHES/HOUR , ?_V5 [14] LOW LOSS RATE- PERCENT UNIT HYDROGRAPH EFFECTIVE RAIN FLOOD HYDROGRAPH [15] UNIT TIME PERIOD M [161 TIME PERCENT OF LAG [7]•[15] [171 CUMULATIVE AVERAGE PERCENT OF ULTIMATE DISCHARGE (S- GRAPH) [18] DISTRIB GRAPH PERCENT [17]m- [17]m- 1191 UNIT HYDROGRAPH CFS -HRS/IN 4 • 18 100 [201 PATTERN PERCENT (PL E -5.9) [211 STORM 'RAIN IN/HR 60 10 20 100 [5] [221 LOSS RATE IN/HR [231 EFFECTIVE RAIN INIHR [21] - [22] [241 FLOW CFS MAX LOW 1 3,40 .95'0 ,ZZ 727 5.te?S Z _,L 1, io , BS ` 6.4 I 3 4.4 1,162- , 93 6I,9L7 4 ,L S .o ,320 G ,478 I•t5 9, L 7 6• /, &,90 1, 67 Io,Lt� $ 8.1 21139 1 -915 13,191 LL 13.1 3,458 3,'?,35 23.518 l0 34. S c1,10 g S, 865- 64,554 (..7 ,i6 1,541- r1,Z3 7So 5,61 1331723A RSA t-V.416 I,-i6 IZ x 7. a> = 1, 93 )iF SAMPLE CALCULATION NO.3 SHORTCUT SYNTHETIC HYDROGRAPH (Example of Plate E -2.2 used for Shortcut Synthetic Hydrograph) L�� RCFC & WCD HYDROLOGY MANUAL, SYNTHETIC UNIT HYDROGRAPH METHOD Unit Hydrograph and Effective Rain Calculation Form Project S+ Fra►, c I s. Sheet i t BY Date Checked Date []j CONCENTRATION POINT a ,46/A) 1 [2] AREA DESIGNATION [3] DRAINAGE AREA ; 27 AC. [4] ULTIMATE DISCHARGE - CFS -HRS/IN (645' [31) [5] UNIT TIME - MINUTES 0 MIA) [6] LAG TD.4E- MINUTES [7] UNIT TIME- PERCENT OF LAG (100•[5]/[6] [8] S -CURVE [9] STORM FREQUENCY & DURATION 166 YEAR 3 HOUR [10] TOTAL ADJUSTED STORM RAIN- LNCHES Z.S /Its [1l] VARIABLE LOSS RATE (AVG)- INCHES/HOUR [12] MINIMUM LOSS RATE (FOR VAR. LOSS) -IN/HR [13] CONSTANT LOSS RATE- INCHES/HOUR 22 3 [14] LOW LOSS RATE- PERCENT -35-9. UNIT HYDROGRAPH EFFECTIVE RAIN FLOOD. HYDROGRAPH .[15] UNIT TIME PERIOD M [16] TIME PERCENT OF LAG [71•[151 [17] CUMULATIVE AVERAGE PERCENT OF ULTIMATE DISCHARGE' (S- GRAPH) [18] DISTRIB GRAPH PERCENT [17]m- [17]m- [19] UNIT HYDROGRAPH CFS- HRS/IN f4l*[181 100 [20] PATTERN PERCENT (PL E -5.9) [21] STORM RAIN IN/HR 60 r101[201 100 [5) [22] LOSS RATE IN/HR [23] EFFECTIVE RAIN IN/HR [21] -[221 [24] FLOW CFS MAXI LOW 1 Z.f. .43 ,u3 ,ZI 1,5 L L ,6 437 ZI !.5 L 3 3.3 , SS4 - 31 1,46L 3.3 .SS4 ),-520 ZMcL • ss4 331 2140 L .571 q. 4 '7317 41Z .701- 1484 3 S1 513 SID 467 4,8 to '1 ,1357 .63 do 1l 6.4 1 1,0 6 b5 b,tc 1 J J•Z2L I,DD 7,Z9L / S' .�i ZS I,ZoS 8,76c Al 0.36 —2,144. /5,` 'D 1 6 14•I 12-50 Z,Is4` J�,Lv ,g ,638 A / 2.4 ,403 .ZZ3 , 16,2 h 3o 2.,110 !iX>,V =--L?-78 4/= SAMPLE CALCULATION NO.3 SHORTCUT' SYNTHETIC 'HYDROGRAPH (Example of Plate E -2.2 used for Shortcut Synthetic Hydrograph) RCFC & WCD HYDROLOGY MANUAL SYNTHETIC UNIT HYDROGRAPH METHOD Unit Hydrograph and Effective Rain Calculation Form Project $f 6e^ a c I S Shee�t/ 1 By Date Checked Date [1] CONCENTRATION POINT JSA -51 A) 1 [2] AREA DESIGNATION [3] DRAINAGE AREA 7 Z C [4] ULTIMATE DISCHARGE - CFS- HRS/IN (645• [3]) [5] UNIT TIME- MINUTES p / Ni [6] LAG TIME- MINUTES [7] UNIT TIME- PERCENT OF LAG (100•[5]/[6] [8] S -CURVE [9] STORM FREQUENCY & DURATION / OOYEAR HOUR [10] TOTAL ADJUSTED STORM RAIN-INCHES 3,414) [1 l] VARIABLE LOSS RATE (AVG)- INCHES/HOUR [12] MINIMUM LOSS RATE (FOR VAR. LOSS) -IN/HR [13] CONSTANT LOSS RATE- INCHES/HOUR 223 [14] LOW LOSS RATE - PERCENT UNIT HYDROGRAPH EFFECTIVE RAIN FLOOD HYDROGRAPH [15] UNIT TDa PERIOD m [16] TIME PERCENT OF LAG [71•[151 [17] CUMULATIVE AVERAGE PERCENT OF ULTTMATE DISCHARGE (S- GRAPH) [18] DISTRIB GRAPH PERCENT [17]m- [17]m- [19] UNIT HYDROGRAPH CFS -HRS/IN f4J*f18I 100 [20] PATTERN PERCENT (PL E-5.9) [21] STORM RAIN IN/HR 60 f101[201 100 [5] [22] LOSS RATE IN/HR [23] EFFECTIVE RAIN IN/HR [21] - [22] [241 FLOW CFS MAX LOW Za l too Ica 2 1,2 .295 , 6?.?. 1.3 . Z45- .o4a '305 1,4 .7st- 'C'63 I ore 5 ,4 %LSC 1063 PASS '083 1,66 1 ID3 i4 8 I•G .3zL ,1&3 74 LG , )D , >4 /I .t. 1371L 1 163 24 L .7 .3,S) , It •9c /3 0 1347 114. •90 / lS l.8 , 36 '144 /,047 14, ,S .3&7 114 1104 /7 ?.C> .4109 , /S,r /,345- /B 7,0 ,408 ,186' ),34!r i 2.1 •4Z>; 14.0 5' ) 9 D 20 ,t .449 ,224. ), 4.1 Z•S .Slo 136 2.084. 2z 2.$ I g7l , 348 z, saa 2 3,0 ,GZ 1 , 398 Z.g9 24 3,4. .4-53 ,Zz3 4_;Z, 3,IZ4e SAMPLE CALCULATION NO.3 SHORTCUT SYNTHETIC HYDROGRAPH (Example of Plate E -2.2 used for Shortcut Synthetic Hydrograph) I RCFC & WCD HYDROLOGY MANUAL SYNTHETIC UNIT HYDROGRAPH METHOD Unit Hydrograph and Effective Rain Calculation Form Project S� r�G�als Sheet/ 2 / Z BY Date Checked Date [1] CONCENTRATION POINT f}�j 1 yV [2] AREA DESIGNATION [3] DRAINAGE AREA 7. 2, [4] ULTIMATE DISCHARGE - CFS- HRS/IN (645• [3]) [5] UNIT TIME - MINUTES / p [6] LAG TIME - MINUTES [7] UNIT TIME- PERCENT OF LAG (100'[5]/[6] [8] S -CURVE [9] STORM FREQUENCY & DURATION /QpYEAR 6 HOUR [10] TOTAL ADJUSTED STORM RAIN-INCHES 34 1 N [11] VARIABLE LOSS RATE (AVG)- INCHES/HOUR [12] MINIMUM LOSS RATE (FOR VAR. LOSS) -IN/HR [13] CONSTANT LOSS RATE- INCHES/HOUR . 223 [14] LOW LOSS RATE- PERCENT UNIT HYDROGRAPH EFFECTIVE RAIN FLOOD HYDROGRAPH [151 UNIT TIME PERIOD M [161 TIME PERCENT OF LAG [71'[15] [171 CUMULATIVE AVERAGE PERCENT OF ULTIMATE DISCHARGE (S- GRAPH) [181 DISTRB GRAPH PERCENT [17]m- [17]m- (19] UNIT HYDROGRAPH CFS- HRS/IN f4j*f181 100 [201 PATTERN PERCENT (PL E -5.9) [211 STORM RAIN IN/HR 60 f I 01f201 100 [51 [221 LOSS RATE INIHR [231 EFFECTIVE RAIN IN/HR (21) -[22] (24). FLOW CFS MAX LOW Z� 3,5r .7/4 ,u ,49I 3, a Z` 3.Rr .7-f4 , 52 1,)4.1- Z1 4.Z , 857 16 '{,6a ZS 4.!r .11 S 6°t S,oS 3o v�.l 1, o4a 5,9 0 3Z S. I /, G Z!r ,4ct Io.l9 33 2,101 /, 8 1 A- 5 34 2.8 571 , 346 Z, 534; 3.5- , 2a¢ 121.3 toot , vo 36 D•S , /DL ,o3L , 061i. •9B� FLocD DLVwIE o 2.09 +1 727 = 1, 2-4 S SAMPLE CALCULATION NO.3 SHORTCUT SYNTHETIC HYDROGRAPH (Example of Plate E -2.2 used for Shortcut Synthetic Hydrograph) RCF.0 & WCD HYDROLOGY MANUAL SYNTHETIC UNIT HYDROGRAPH METHOD Unit Hydrograph and Effective Rain Calculation Form Project Sheet Z BY Date Checked Date [I] CONCENTRATION POINT 5 �) [2] AREA DESIGNATION [3] DRAINAGE AREA 7 L ` [4] ULTIMATE DISCHARGE - CFS -HRS/IN (645' [3]) [5] UNIT TD.AE- MINUTES C> yy l y� [6] LAG TRAE- MINUTES [7] UNIT TRAE- PERCENT OF LAG (100•[5]/[6] [8] S -CURVE [9] STORM FREQUENCY & DURATION /00YEAR 04. HOUR [10] TOTAL ADJUSTED STORM RAIN- INCHES S� [11] VARIABLE LOSS RATE (AVG)- INCHES/HOUR [12] MINMUM LOSS RATE (FOR VAR. LOSS) -IN/HR [13] CONSTANT LOSS RATE- INCHES/HOUR , 2. Z-3 [14] LOW LOSS RATE- PERCENT 3S %n UNIT HYDROGRAPH EFFECTIVE RAIN FLOOD HYDROGRAPH [15] UNIT TIME PERIOD M [16] TIME PERCENT OF LAG [7]•[]5] [17] CUMULATIVE AVERAGE PERCENT OF ULTIMATE DISCHARGE (S- GRAPH) [18) DISTRIB GRAPH PERCENT [17> -[17]m [19] UNIT HYDROGRAPH CFS -HRSRN 4 • 18 100 [20) PATTERN PERCENT (PL E -5.9) [21] STORM RAIN IN/HR 60 10 20 100 [51 [22] LOSS RATE IN/HR 1231 EFFECTIVE RAIN IN/HR [21]-[22] [241 FLOW CFS . MAX I LOW / 16" , 04r �IJ7 ,ca_ 2) Z .7 , 06 'SS ,a22. '641. Z4 e . I. '054 ,3'78 ,c1 ,D3j , L 7 , 063 30 01,2 z>4 1. , 29B S . o)Pi, ,36D ,tat, ,047. '34a 6 ,v ,090 352 ,43Z 4)S$ , ,422 ? 0 "010 ,34 ,032 .05 , AZZ I , ,117 , 3Z%o , Olt I )6;P'4- , 553 640 14 ,3c '05,01 ,D ,619 r /IS '/6z 293 X057 rod , N-3 14 1/00 ,285 ,e43 o7 , 6-,</ 2, r , /g 'ns au 1. ,123 g14 '071 ,1 c 1,ab / . 3.0 , 2>0 .?,63 eo , /8 3.3 Z17 .19' i 2.- , 3c,r ! 3.5 1 1248 [ t . 1741 Zo 4.3 .387 mi , I G. . I 10t, 310 ;�o ,L3 c36 '26L ZL 4,0 .360 .21 '10. ,"7!e 318 , :34L .zzl ► 2.1. -ego 2/ o . )3 SAMPLE CALCULATION NO.3 SHORTCUT SYNTHETIC HYDROGRAPH (Example of Plate E -2.2 used for Shortcut Synthetic Hydrograph) RCFC & WCD HYDROLOGY MANUAL SYNTHETIC UNIT HYDROGRAPH METHOD rograph and E Unit Hydffective Rain Calculation Form Project 5 f ' �i� a.+t c r s Sheet / Z i Z BY Date Checked Date [1] CONCENTRATION POINT /� c� W [2] AREA DESIGNATION [3] DRAINAGE AREA Z [4] ULTIMATE DISCHARGE - CFS- HRS/IN (645* [3]) [5] UNIT TIME- MINUTES 3Q M W [6] LAG TRvIE- MINUTES [7] UNIT TDAE- PERCENT OF LAG (100 *[5] /[61 [8] S -CURVE (9) STORM FREQUENCY & DURATION OOYEAR 2,4 HOUR [10] TOTAL ADJUSTED STORM RAIN -INCHES -4, 5— !A-) [11] VARIABLE LOSS RATE (AVG)- INCHES/HOUR [12] MINIMUM LOSS RATE (FOR VAR. LOSS) -IN/HR [13] CONSTANT LOSS RATE- INCHES/HOUR , Z2,3 [14] LOW LOSS RATE- PERCENT 3 S 9s UNIT HYDROGRAPH EFFECTIVE RAIN FLOOD HYDROGRAPH [15] UNIT TIME PERIOD M 1 [16] TIME PERCENT OF LAG [7]•[15] [17] CUMULATIVE AVERAGE PERCENT OF ULTIMATE DISCHARGE (S- GRAPH) [18] DISTRM GRAPH ' PERCENT (17]m- (17]m- [19] UNIT HYDROGRAPH CFS -HRS/IN [41*[181 ]00 [20] PATTERN PERCENT (PL E-5.9) [21] STORM RAIN IN/HR 60 ]b 20 100 [5] [22] LOSS RATE IN/HR [23] EFFECTIVE RAIN IN/HR (211-(221 [24] FLOW CFS MAX I LOW 2,4 5.? , S13 , o 131 Z12�� Z-1 6.s :6t2 28 4.G '414 deci 2,2 2,9 tr' 3 -477 IS `i 2.13® " 30 6"1 0 ,178 4911 , Z,a9 31 4.7 42 , ] ,ArA ►,$L$ Z 3.8 , 34Z ,►6 I 72e, 33 , 8 .072 .16L AZC 642 342 34 16 ,oS titr7 ,0►9 ,0 5, ,L5 S /.o ,a o , ►S3 ,0& I of& A OR- 31� If og/ Idig ,02$ o6' & 438C0 7 .8 . 672- .144 024 , 04 - , 84L 3S' .S .045 1 ,016 )021, all 3 fi •? ,063 13L ,elZl o , 1216 ,1 L A31 of-2, ,45o 41 0 124+ o)91 o 3 ,?,54 gt. .S ,D4S ,/Z ,014 oL ,LI.I 143 S ,04.- d Z'L toll. 1 Az ' 1211 44 15' 1 ,o43 otq OIL o • 1211 45- 1.6 •00' i 1 ,014 104 11.1 ( 46 .o _3 4 11!' 14'1 , CO. 110 47 .4 ,o e_ 11 ,013 1X23- '47 8 o L ,) iZ ,o13 ,o 0 _ 4,6 4,623K,Sa 21341 FLOOD V640410 2,342 +ILX 7,s7: 1,419 AP SAMPLE CALCULATION NO.3 SHORTCUT SYNTHETIC HYDROGRAPH (Example of Plate E -2.2 used for Shortcut Synthetic Hydrograph) ELEV 44 45 46 47 48 49 50 51 52 53 54 55 56 AREA (S.F.) 9,674 10,987 12,338 13,764 15,276 16,896 18,560 20,218 21,949 23,719 25,514 27,344 29,432 BASIN 1 VOLUME VOLUME (C.F.) . ACCUMULATED VOLUME (C.F.) 10,331 10,331 11,663 21,994 13,051 35,045 14,520 49,565 16,086 65,651 17,728 83,379 19,389 102,768 21,084 123,852 22,834 146,686 24,617 171,303 26,429 .197,732 28,388 226,120 BASIN VOLUME = 5.191 AF BASIN DRAWDOWN BASIN FULL TO ELEV. 56.0 BASIN CAPACITY: 5.191 AF 226,120 CF @ ELE. 56.0 AVERAGE BASIN BOTTOM AREA (ELEV. 50): 18,560 SF PERC. RATE: I" / HR PERC. PER HOUR: .083' X 18,560 SF = 1,540 CF / HR 2 - MAXWELLS PERC RATE: 0.50 CFS PERC. RATE PER HOUR: 1,800 CF / HR TOTAL PERCOLATION PER HOUR: 3,340 CF / HR 226,120 CF - 3,340 CF / HR = 67.70 HRS 67.70 HRS _ 24 = 2.82 DAYS ST. FRANCIS VOLUME: EXISTING CONDITION: 1.54 AF SITE IMPROVEMENTS: 1.42 AF TOTAL: 2,96 AF / 128,938 CF AVERAGE BASIN BOTTOM AREA (ELEV. 48): 15,276 SF PERC. RATE: I" / HR PERC. PER HOUR:.083' X 15,276 SF = 1,268 CF / HR MAXWELL 900 CF / HR TOTAL PERCOLATION PER HOUR: 2,168 CF / HR 128,938 CF _ 2,168 CF / M = 59.47 HRS 59.47 - 24 = 2.48 DAYS JI sip sva w + 92 Aln rn C; otil, 1.17 Jw FW Pu cn )vp aw OVA § jrh t S rk jw+ V Ids- 11? 'IV rn lt,4 oom, 7EY tv All .rw dw X0Y9 o 0 0 Cl) vw _ i i G7 � . N I JJJI( I r�,.� � � 10 ow m< m �� Aw ,vvt 77 w Ills + F" —%I FF ms 's 1 PERCOLATION REPORT I� Sladden Engineering 77 -723 Enfield bane, Suite 100, Palm Desert, CA 92211 (760) 772 -3893 Fn: (760) 772 -3893 6782 Stanton Ave., Suite A, Buena Park, CA 90621 (714) 523 -M52 Fax (714) 523 -1369 450 Egan Avenuc. Bzaumc*mt. CA 92223 (951) 845 -7743 Fax (951) 845 -$863 October 19, 2007 Prest- VuksiC Architect$ 44-530 San Pablo Avenue, Suite 200 Palm Desert, California 92260 Attention: Mr. John Vuksic Project St. Francis of Assisi Church Expansion 47 -225 Washington Street La Quinta, California Project No. 544 -3253 07- 10-691 Subject: Supplemental Infiltration/Percolation Testing for Storm Water Retention As requested, we have performed supplemental subsurface investigation and percolation/inMtration testing on the subject site to evaluate the infiltration potential of the'soil at the bottom of the proposed storm water retention /percolation system: The current plans indicate that it is proposed to collect storm water runoff within large multi -plate arch type subsurface percolation/storage chambers manufactured by Contech. The plans indicate a proposed bottom elevation of approximate 35 feet above mean sea level (AMSL). Tn order to evaluate the infiltration potential of the native soil underlying the proposed multi- plate arch system, 2 additional test holes were excavated to a depth of approximately 3 feet below the anticipated bottom elevation. Percolation testing was performed on September 21, 2007. In general, approximately 3 feet of water was maintained within the test holes and the flow rate was monitored with time. Testing was performed in general accordance with the U.S. Bureau of Reclamation (BOR) Test Method for Unsaturated Soil above Groundwater. The infiltration rates determined are.reported in approximate inches per hour. Tests results are summarized below: Rate Teat Hole (incheAour) B1 -A 7.8 B2 -A 8.6 ' October 19, 2007 -2- Project No. 5443253 07- 10-691 ' It should be noted that the infiltration rates determined are ultimate rates based upon field test results. An appropriate safety factor should be applied to account for subsoil inconsistencies and ' potential silting of the percolating soil. The safety factor should be determined with consideration to other factors in the storm water retention system design (particularly storm water volume estimates) and the safety factors associated with the design components. ' Our exploratory bore holes indicate that the native soil at an elevation of approximately 35 feet AMSL consists of fine - grained silty sand and cleaner fine - grained sand. Although some isolated thin silt or clay layers were observed below the test elevations, the silt /clay layers were generally ' less than one inch in thickness and were not continuous. The presence of isolated widely thin and discontinuous silt/clay layers should not significantly limit infiltration potential. The supplemental testing indicates that the maximum allowable infiltration rate of 2 inches per hour ' should be applicable for use in the design of the multi -plate arch Sturm water retention system. The approximate test/bore locations utilized for testing are indicated on the attached plan. The corresponding bore logs are also attached. ' We appreciate the opportunity to provide continued service to you on this project. if you have questions regarding this letter or the data included, please contact the undersigned. ' Respectfully submitted, SLADDEN E.NGINFAMNG Brett L. AndersdW ' Principal Engineer Peres /nd Nu. G 45369 F�xp. 5- 3U -2U� Copies: 2/Prest- vuksic Architects 2/Watson & Watson Sladden Engineering SITE PLAN WITH APPROXIMATE BORE HOLE & PERC TEST LOCATIONS ::SITE DEVELOPMENT PERMIT 2408- - .:�. + <,• ` '� 'I' . f?•. —eau t'F "•- �. +��.Q Caw �— •� - ,.li • - .. .. .. w� . ,sue• -�- i ,` � �. -'�- -� l jr ,;,.•.:' �$�t...•,- n'.� 8-1A '�p B-� - ®; � ; :t.�` -� gy=p. .�- -,� '• ' �_ ,. ... a . .� �7ii Lp S(�TpE�= �y LLOO_PNWT SCALE: As Shown MAP SOURCE: MDS Consulting LEGEND JOB NAME: SL Francis of Assisi Church APPROXIMATE BORE HOLE LOCATION JOB NO, 544 -3253 APPROXIMATE PERC TEST LOCATION REPORT NO: 07- 07-492 I I I I I I I I I I Proposed Retention Basins (Basin. # 4 Area) St. Francis of Assisi Church, 47-225 Washington Street, La Quints ate: 7/31/7007 Bore No. 1A Job Number* 544-3253 Description to 80, ernarks 0 ANLSL Elevation SS Feet alive Soil 10/12 Silty Sand: Fine Grained SM 0.9 17.9 Grey in color /Dry 5 - 6114 Silty Send: Fine Grained SM 1.2 19.4 rvy in color/Dry 516 Sand: Fine Grained Sp 1.9 7.1 rey in color/Moist 10 S/8 Sd: -Fine Grained SP 2.3 6&, 4 rey in 0ofofVM0;rt 3E 4/6 Sand: Fine Grained SP 22.5 7.3 Grey Brown in color 15 Moist 414 Silty Sand, Fine Grained and Sandy Silt Interbedded SM, 5,9 39A Grey & Brown in color f20 Moist 419 San4: Fine Grained and Sandy Silt (1/2 & 1/2) SM 7.6 48.6 Grey & Yellowish Brown AMSL Elevation 35 Feet Dr y - 619 Sand: Fine Grained SP 1.2 4.6 Grey in colorA)ty 416 Sad: Fine Grained Se I's 11.2 Grey in color/Dry 2 4/6 Silty Sand: fine Grained with Silt L=ses SM 3.2 22-7 Grey in color/Dry 517 Silty Sand: Fine Grained and Sandy Silt (1/2 8t 12) SM 15.9 56.8 rey & Olive in color/Moist 30 515 Silty Sand: Fine Grained with Claycy Silt IntcrboddW SM 14.6 51.0 live in oolor/Moist AL 7/12 Sand: Fine Grained SP 3.1 9.4 Gruy in color/Moist 35 6J13 Sand: Fine to Modium Grained SP 2.6 5.7 Grey in color/Moist 3E 8/12 Sand. Fine G d raine SP 2.8 6.8 Grey in color/Moist 40 5/6 Sand: Fine Grained SP 3.2 4.5 Note The stratification lines (Grey in color/Moist) up-ew t the approximate boundaries between the soil types, the transition may be - 4/4 Silty Sand: Fine Grained and Sandy Silt (1/2 & 1/2) musm 25.5 70,8 45 (Olive & Grey in oolor/Moist) ual. - 6/7 - Silty Sand; Fine Grained SM 5.1 3D.5 (Grey in color/Moist) L 7/9/12 Sand: Fine Grained with Silt Layer -4 Inches SM 13.6 43.6 Total Depth --51 Fed so (Grey in color/Moist) Bedro& not encountered Groundwater not cncountcrcd Sladden Engineering I Proposed Retention Basins St. Francis of Assisi Church 47 -225 Washington Street, Le Quint' Date: 7/31/2007 Bore No. 2A Job Number: 544 -3253 b u � -all rWl U Description ks 0 AM.SL Elevation 60 Feet ative Soil - 4/4 Sand: Fine Grained and Sandy Silt(1/2 & 1/2) SM 1.8 373 Grey & Tan in color/Dry 5' - 4/6 Sand: Fine Grained with Sandy Silt Layer --2 Inches SM 3.5 33.6 Grey & Tan in color/Dry - 4/5 Sand. Fine Grained with Thin Silt Leases . SM 1.8 19.8 Grey in color/Moist 10 - 5/9 Silty Sand: Fine Grained and Silt Interbedded SM 2.3 47.2 reyish Tan in onlor/Moha - 7/9 Clayey Sih ML 5.9 $0.2 an in color/Dry 15 - 7/10 Clayey Silt ML 2.4 80.2 Buff in color/Dry - 416 Silt ML 1.1 78.5 ufi•in oolor/Dry 20 - 517 Silty Sand: Fine Grained with Silt layer -2 Inches ML 1.0 60.3 re yish Tan in dolor /Dry - 416 Sandy Silt Ml, 0.6 58.8 uff in eolor/Dry 25 AMSL Elevatiun 35 Feet - 4!7 Silty Sand: FiriC Gcainal SM 0.3 22.1 in color/Dry - 9114 Silty Sand: Fine Grained SM 1.1 22.7 rey in color /Dry 30 - 6110 Sample Not Recovered - 7/9 Silty Sand: Fine Grained SM 0.5 20.0 Grey in color /Moist 35 - 8/13 Sand: Fine Grained SP 0.7 9.0 in AolodMoist - 9/12 Silty Sand: Fine Grained with Silt Interbedded Sm 0.6 21.4 rey In color /Moist 40 - 9/17 Silty Sand: Fine to Medium Grained SM 0.5 17.6 rcy in oolor/Moist - otc: The stratification lines ep*esvnt the upproximatc - 11/26 Sand: Fine Grained SP 0.7 9.3 45 `:.€ (Grey in color/Moist) undaries between the soil - 13/19 Sand: Fin. Gnti.,j SP 0.8 8.0 the transition may be - (Grey in color/Moist) "al. - 7/9/14 Silty Sand: Fine Grained SM 1.1 24.0 otal Depth =-51 Feet 50 (Grey in color/Moist) not encountered - toundwater not encountered Sladden Engineering CITY OF LA QUINTA DRAINAGE. STUDY o .�� 68 -955 Adelina Road Cathedral City, CA 92234 consulter [760] 323 -5344 • Fax [760] 323 -5699 Saint Francis of Assisi Catholic Church Drainage Capacity Study March 2009 Prepared for: City of La Quinta 78495 Calle Tampico La Quinta, CA . . Prepared for City of La Quinta Table of Contents TheTask ............................................ ............................... Page 1 Exhibit A Summaryof Findings ......... ............................... .................Page 2 Background............ ............................... .............:.............Page 2 TopographicPreparation ....................... ..........................Page 3 Calculation Methodology..* ............................................... Page 4 Exhibit. B Exhibit C Exhibit D Prepared by NAI Consulting, Inc. Prepared for City of La Quinta The Task The City of La Quinta (City) retained NAI Consulting, Inc., (NAI) to calculate how much storm water naturally ponds on the .low -lying property owned by Saint Francis of Assisi Catholic Church (Church) during a storm event. The February 28, 1939 Bureau of Reclamation Topographic Map is the earliest known documentation of the sump area on the undeveloped Church property. It should be noted, however, that even though the lowest point in the.sump area is located on the Saint Francis property, the sump area extends into the Washington Street right of way. It has been known for years that in large storm events, the natural sump area on the Saint Francis property does not have sufficient capacity to contain all of the storm water runoff from larger storms and as a result, the rising storm water elevation in the sump ultimately extends into the public right of way. It is also known that the depth and configuration of the sump area has changed through the years since 1939; sometimes by natural forces, sometimes by the forces of man. Thus, a key question -is:.what was the storm water retention capacity of the sump when the Church acquired the undeveloped property in December, 1998 via Lot Line Adjustment 98 -295 that separated its undivided ownership interest in a larger property it co -owned with the La Quinta Arts Foundation. As it turns out, the Church submitted a detailed topographic map of the undeveloped property to the City as part of its application seeking approval of Lot Line Adjustment 98 -295. City staff requested NAI to calculate how much water from a • 100 -year storm event draining to the combined sump area lies west of the Washington Street centerline, and how much of the water in the combined sump area lies east of the Washington Street Centerline. The centerline delineation aspect is a result of La Quinta Municipal Code section 13.24.120(D) which requires landowners to be responsible for storm water out to the centerline of streets adjacent to their property. NAI was tasked with calculating the sump capacity based on the 1998 topography since that is when the Church acquired the property.. .Prepared by NAI ConsuRing, Inc. 1 'Prepared for City of La Quints EXHIBIT A � 77 a 1980 r i r'., vt ia December 1998 ;�" � "• t'� ,7,':: gip:.' � . t :ai•` !• if= .cserkr :'i- cOOO 1 ,gt >• Ar - t c ` 1 i— } �.. : W. Photo courtesy of Google.com Aerial View of Church Property Prepared by NAI Consulting, Inc. Prepared for City of La Quinta Summary of the Findings Lloyd Watson of Watson and Watson, Inc, who was retained by the Church to prepare site plans for the project proposed by the Church, submitted a hydrology study to the City indicating the 100 -year storm event produces 8.4 acre -feet of storm water runoff that drains to the Church owned property from the neighborhood north of the Church. Since Mr. Watson's hydrology ' study was based on the current City rainfall specification (NOAA Atlas 14), City staff directed NAI to use the 8.4 acre -feet figure in its calculations. NAI discovered that the overflow elevation for the sump area is 58.7 feet and is located in the north flowline of Lake La Quinta Drive at the easterly end of the curb return. NAI determined that 1.54 acre -feet of the 8.4 acre - feet (18.4 %) is retained in.the combined sump area on the west side of the Washington Street centerline, and 0.44 acre -feet (5.3 %) is retained in the combined sump area on the east side of the Washington Street centerline. The balance of the 8.4 acre -feet of storm water runoff (6.42 acre -feet; 76.3 %) produced by a 100 -year storm event flows out of the combined sump area at the overflow point and flows easterly in the north flowline of Lake La Quinta Drive and eventually drains into Lake La Quinta. Le, lce i►a. �� w,j �� e, &t Pr � cr 'to 11.�� i.se. C✓ � � �... /ves.� �.� . � "E„s�! 6r Background The Church acquired additional unimproved property in 1998 that was not part of its original property on which the existing church structure and accompanying facilities were .constructed in the mid- 1980's. The unimproved property is still unimproved, except for a temporary turf parking lot, as of the date when the City retained NAl to prepare the capacity calculation of the combined sink area. The Church was authorized by the City to construct the temporary turf parking lot shortly after the Church acquired the unimproved property. In doing so, the existing topography of the land was modified (minor leveling) to some degree as an expedient means to accommodate significant excess parking onsite instead of blocks of on- street street parking along busy Washington Street and in the neighborhood north of the Church. NAI acquired digital topographic data in AutoCAD format from Fiero Engineering that was used by Fiero to process a lot line adjustment in 1998 on behalf of the Church and the La Quinta, Arts Foundation. Fiero commissioned acquisition of the aerially acquired topographic data before the temporary parking lot was constructed on the unimproved property. ' Prepared by NAI Consulting, Inc. 2 i Prepared for City of La Quinta Topographic Background Preparation The digital topographic data acquired from Fiero Engineering does not contain sufficient elevation information in the Washington Street right of way to perform the desired capacity calculations without augmenting the Fiero topographic data. Thus NAI obtained elevation data from two sets of street improvement plans that were used to construct Washington Street in the area adjacent to the Church owned property, and onsite grading plans for the commercial building pads on the eastside of Washington Street. These three plans sets are: Plan Set 90015 — Plans for the west side of Washington Street prepared by BSI Consultants, Inc fora city sponsored capital improvement project. Plan Set 90030 - Plans for the east side of Washington Street prepared by Mainiero Smith /Spiska Engineering, a joint venture, for offsite street improvements associated with the Lake La Quinta development. Plan Set 90114 — Grading plans for the Lake La Quinta development, also prepared by Mainiero Smith /Spiska. NAI initially considered importing cropped TIFF images from the aforementioned plan sets into the Fiero AutoCad topographic file. However, it was determined that even though the TIFF images would provide a quick visual image of the existing street improvements, the imported images would interfere with, and hide the existing topographic data in the Fiero AutoCad file in the street right of way area, and moreover, the plan view of those TIFF images does not contain very many elevations that are needed to draw contour lines for the ponded water, because most of the elevation information on those plan sheets is contained in the profile portion of the plan sheet. Therefore, it was necessary for NAI to augment Fiero's topographic AutoCad file by re- drawing selected aspects of the grading on the east side of Washington Street, most notably, the long narrow retention basin (Exhibit C) sandwiched between Washington Street and the commercial building pads on the east side of Washington Street, plus implement a stationing system on the Fiero plan that matches the street improvement plans and then enter scores of new elevations on the Fiero plan that were obtained from the archived plans noted earlier in this report. Prepared by NAI Consulting, Inc. 3 Prepared for City of La Quinta Calculation Methodology In essence, the sump area where storm water from the surrounding neighborhood collects is a single sump, but it is bifurcated by the Washington Street centerline. -Thus pursuant to the City's request to determine how much of the 1998 storage capacity is on the west side of the Washington Street centerline and how much is on the east side of the centerline, NAI started drawing contours at the lowest elevation on each side of the Washington Street centerline. The contours were drawn in 0.2 foot increments until reaching elevation 58.6, then one additional contour was drawn with a 0.1 foot increment at elevation 58.7. If a contour line was found to naturally cross the centerline it was instead terminated at the centerline with the centerline acting as a phantom bulkhead between the storage areas located on each side of the centerline. Exhibits D contains the topographic drawing with the 0.2 foot contours defining the sump area. The area enclosed by each contour was then obtained from the AutoCad program, and entered into a. spreadsheet (See Exhibit B) prepared for calculating the storage capacity on each side of the centerline. It was discovered that the storage area of the combined sump area holds only 23.7% of the storm water runoff from a 100 -year storm. The capacity of the combined sump area is 1.98 acre -feet, while the 100 -year storm drains 8.4 acre -foot of storm water runoff into the combined sump area. Thus, 6.42 acre -feet of the 100 -year storm flows out of the sump area at the overflow point. See Exhibit B for a complete breakdown of the capacity calculation. It should be noted, that the lower elevations in the. long narrow man -made retention basin on the east side of Washington Street sandwiched between Washington Street and the commercial building pads in the Lake La Quinta development were reserved for storm water emanating from the Lake La Quinta.development. The upper 1.0 foot of the retention basin (ie the freeboard) was included in this capacity study. Prepared by NAI Consulting, Inc. 4 '0 ia io CL 0 0 :3 th Onsite, & Offsite Retention. in Sump Area West Sump East Sump Westside Washington Street. Centerline Elevation Scl Footage Average SF Incremental Volume (CF) Cumulative Volume (AF) 57.2 _ 8,259 7,988 57.4 .......... 8,803 8,531 1,706 57.6 0 9,077 1,815 0.04 57.8 1,265 633 127: 0.00 58.0 26,893 14,079 2,616 0.07 58.2 71,064 48,979 9,796 0.29 58.4 103,619 87,342 17,468 0.69 58.6 130,328 116,974 23,395 1.23 56.7 142,840 136,584 13,658 1.54 Eastside of Washington Street Centerline Sq Footage Average SF Incremental Volume (CF) Cumulative Volume (A 7,718 _ 8,259 7,988 1,598 0.12 8,803 8,531 1,706 0:08 9,350 9,077 1,815 0.04 9,900 9,625 1,925 01.09 19,981 14j941 2,988 0A5 23,936 21,959 4,392 0.26 28,125 26,030 5,206 0.37 30,904 29,514 2,951 0.44 Total Cumulative Volume (AF) (Both Sides) 0.12 0.08 0.04 0.15 0.45 0.95 1.61 M X -0 fJ I. Prepared for City of La Quinta EXHIBIT C ' Prepared by NAI Consulting, Inc. „•, 68 -955 Adelina Road Cathedral City, CA 92234 [760] 323 -5344 • Fax [760] 323 -5699 Saint Francis of Assisi Catholic Church Drainage Capacity Study March 2009 Prepared for: City of La Quinta 78495 Calle Tampico La Quinta, CA • Prepared for City of La Quinta Table of Contents TheTask ............................................ ............................... Page 1 Exhibit A Summary of Findings ............................. ...........................Page 2 4 Background............ ............................... ...........................Page 2 TopographicPreparation ...................... ...........................Page 3 CalculationMethodology ...................... ...........................Page 4 0 Exhibit B 0 Exhibit C Exhibit ,D Prepared by NAI Consulting, Inc. i • • Prepared for City of La Quinta The Task The City of La Quinta (City) retained NAI Consulting, Inc., (NAI) to calculate how'much storm water naturally ponds on the low -lying property owned by Saint Francis of Assisi Catholic Church (Church) during a storm event. The February 28, 1939 Bureau of Reclamation Topographic Map is the earliest known documentation of,the sump area on the undeveloped Church property. It should be noted, however, that even though the lowest point in the sump area is located on the Saint Francis property, the sump area extends into the Washington Street right of way. It has been known for years that in large storm events, the natural sump area on the Saint Francis property does not have sufficient capacity to contain all of the storm water runoff from larger storms and as a result, the rising storm water elevation in the sump ultimately extends into the public right of way. It is also known that the depth and configuration of the sump area has changed through the years since 1939; sometimes by natural forces, sometimes by the forces of man. Thus, a key question is: what was the storm water retention capacity of the sump when the Church acquired the undeveloped property in December, 1998 via Lot Line Adjustment 98 -295 that separated its undivided ownership interest in a larger property it co -owned with the La Quinta Arts Foundation. As it turns out, the Church submitted a detailed topographic map of the undeveloped property to the City as part of its application seeking approval of Lot Line Adjustment 98 -295. City staff requested NAI to calculate how much water from a 100 -year storm event draining to the combined sump area lies west of the Washington Street centerline, and how much of the water in the combined sump area lies east of the Washington Street Centerline. The centerline delineation aspect is a result of La Quinta Municipal Code section 13.24.120(D) which requires landowners to be responsible for storm water out to the centerline of streets adjacent to their property. NAI was tasked with calculating the sump capacity based on the 1998 topography since that is when the Church acquired the property. Prepared by NAI Consulting, Inc: 1 Prepared for City of La Quinta EXHIBIT A • Original Church property r� a aired in July 1980 "'A 11,11F TANI Unde�reloped Church - property acquired in December 7998 low . ,< _ r s h l� F~ 4 i j Photo courtesy of Google.c, Aerial View of Church Property Prepared by NAI Consulting, Inc. Prepared for City of La Quinta Summary of the Findings Lloyd Watson of Watson and Watson, Inc, who was retained by the Church to prepare site plans for the project proposed by the Church, submitted a hydrology study to the City indicating the 100 -year storm event produces 8.4 acre -feet of storm water runoff that drains to the Church owned property from the neighborhood north of the Church. Since Mr. Watson's hydrology study was based on the current City rainfall specification (NOAA Atlas 14), City staff directed NAI to use the 8.4 acre -feet figure in its calculations. NAI discovered that the overflow elevation for the sump area is 58.7 feet and is located in the north flowline of Lake La Quinta Drive at the easterly end of the curb return. NAI determined that 1.54 acre -feet of the 8.4 acre- feet (18.4 %) is retained in the combined sump area on the west side of the Washington Street centerline, and 0.44 acre -feet (5.3 %) is retained in the combined sump area on the east side of the Washington Street centerline. The balance of the 8.4 acre -feet of storm water runoff (6.42 acre -feet; 76.3 %) produced by a 100 -year storm event flows out of the combined sump area at the overflow point and flows easterly in the north flowline of Lake La Quinta Drive, and eventually drains into Lake La Quinta. Background The Church acquired additional unimproved property in 1998 that was not part of its original property on which the existing church structure and accompanying facilities were constructed in the mid- 1980's. The unimproved property is still unimproved, except for a temporary turf parking lot, as of the date when the City retained NAI to prepare the capacity calculation of the combined sink area. The Church was authorized by the City to construct the temporary turf parking lot shortly after the Church acquired the unimproved property. In doing so, the existing topography of the land was modified (minor leveling) to some degree as an expedient means to accommodate significant excess parking onsite instead of blocks of on- street street parking along busy Washington Street and in the neighborhood north of the Church. NAI acquired digital topographic data in AutoCAD format from Fiero Engineering that was used by Fiero to process a lot line ' adjustment in 1998 on behalf of the Church and the La Quinta Arts Foundation. Fiero commissioned acquisition of the aerially acquired topographic data before the temporary parking lot was constructed on the unimproved property. Prepared by NAI Consulting, Inc. 2 Prepared for City of La Quinta Topographic Background Preparation The digital topographic data acquired from Fiero Engineering does not contain sufficient elevation information in the Washington Street right of way to perform the desired capacity calculations without augmenting the Fiero topographic data. Thus NAI obtained elevation data from two sets of street improvement plans that were used to construct Washington Street in the area adjacent to the Church owned property, and onsite grading plans for the commercial building pads on the eastside of Washington Street. These three plans sets are: Plan Set 90015 — Plans for the west side of Washington Street prepared by BSI Consultants, Inc for a city sponsored capital improvement project. Plan Set 90030 - Plans for the east side of Washington Street prepared by Mainiero Smith /Spiska Engineering, a joint. venture, for offsite street improvements associated with the Lake La Quinta development. Plan Set 90114 — Grading plans for the Lake La Quinta development, also prepared by Mainiero Smith /Spiska. NAI initially considered importing cropped TIFF images from the aforementioned plan sets into the Fiero AutoCad topographic file. However, it was determined that even though the TIFF • images would provide a quick visual image of the existing street improvements, the imported images would interfere with, and hide the existing topographic data in the Fiero AutoCad file in the street right of way area, and moreover, the plan view of those TIFF images does not contain very many elevations that are needed to draw contour lines for the ponded water, because most of the elevation information on those plan sheets is contained in the profile portion of the • plan sheet. Therefore, it was necessary for NAI to' augment Fiero's topographic AutoCad file by re- drawing selected aspects of the grading on the east side of Washington Street, most notably, the long narrow retention basin (Exhibit C) sandwiched between Washington Street and the commercial building pads on the east side of Washington Street, plus implement a stationing system on the Fiero plan that matches the street improvement plans and then enter scores of new elevations on the Fiero plan that were obtained from the archived plans noted earlier in this report. Prepared by NAI Consulting, Inc. 3 • Prepared for City of La Quinta Calculation Methodology In essence, the sump area where storm water from the surrounding neighborhood collects is a single. sump, but it is bifurcated by the Washington Street centerline. Thus pursuant to the City's request to determine how much of the 1998 storage capacity is on the west side of the Washington Street centerline and how much is on the east side of the centerline, NAI started drawing contours at the lowest elevation on each side of the Washington Street centerline. The contours were drawn in 0.2 foot increments until reaching elevation 58.6, then one additional contour was drawn with a 0.1 foot increment at elevation 58.7. If a contour line was found to naturally cross the centerline it was instead terminated at the centerline with the centerline acting as a phantom bulkhead between the storage areas located on each side of the centerline. Exhibits D contains the topographic drawing with the 0.2 foot contours defining the sump area. The area enclosed by each contour was then obtained from the AutoCad program, and entered into a spreadsheet (See Exhibit B) prepared for calculating the storage capacity on each side of the centerline. It was discovered that the storage area of the combined sump area holds only 23.7% of the storm water runoff from a 100 -year storm. The capacity of the combined sump area is 1.98 acre -feet, while the 100 -year storm' drains 8.4 acre -foot of storm water runoff into the combined sump area. Thus, 6.42 acre -feet of the 100 -year storm flows out of the sump area at the overflow point. See Exhibit B for a complete breakdown of the capacity calculation. It should be noted, that the lower elevations in the long narrow man -made retention basin on the east side of Washington Street sandwiched between Washington Street and the commercial building pads in the Lake La Quinta development were reserved for storm water emanating from the Lake La Quinta development. The upper 1.0 foot of the retention basin (ie the freeboard) was included in this capacity study. Prepared by NAI Consulting, Inc. 4 v M d io CL Q Z D_ n 0 H C 7 l0 7 A Onsite & Offs ite Retention in Sump Area West Sump Westside Washington. Street Centerline Elevation Sq Footage Average SF Incremental Volume CF Cumulative Volume AF 57.2 ............. ................ 8,259 7,988 57.4 0.12 .................. 8,531 1,706 57.6 0 9,077 1,815 0.04 57.8 1,265 1 633 1.27...... 0.00 58.0 26,893 14,079 2,816 0.07 58.2 71,064 48,979 9,796 0.29 58.4 103,619 87,342 17,468 1 0.69 58.6 130,328 116,974 23,395 1.23 583 142,840 136,584 13,658 1.54 East Sump Eastside of Washington Street Centerline Sq Footage Average SF Incremental Volume CF Cumulative Volume AF 7,718 .......... ............. ................ 8,259 7,988 1,598 0.12 8.803 8,531 1,706 0.08 9,350 9,077 1,815 0.04 9,900 9,625 11925 0.09 19,981 14,941 2,988 0.15 23,936 21,959 4,392 0.26 28,125 26,030 5,206 0.37 30,904 1 29,514 1 2,951 1 0.44 G Total Cumulative Volume (AF) (Both Sides) . .........0.12.......... 0.08 0.04 0.15 0.45 0.95 1.61 1.98 M 2 IOU =i UD v m v d o_ 0 n 0 r G7 C 7 N �a�Ac yh !A ' wear poo�� rl Z Z 0 p 0 o t 0 p 0 F N �* v ` 0 D V: o o a - 0 t4 a 0 a u 4 W 0 u ro 9 u 0 G A` a 0w ON ZA n Y 4 as nn n 0 Y � D 0 ZN oa A? 5 - /',ic�c. _ _ NC40q m�luo N.B9•39'//"E. /9L6. 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LJ1 CUMULATIVE VOLUME 0.44 ACFT. APPROVED BY. CITY OF LA QUINTA APP'V'D I DATE TIMOTHY R. JONASSON, P.E. DATE PUBLIC WORKS DIRECTOR /CITY ENGINEER or%c - ki- Ar-DA7 rlvn nA-rc". 4 r)/-z Civil Engineering 68-955 Adelina Road �pFESS6��, Traffic Engineering Cathedral City, CA 92234 Project Management —5344 Contract Administration 760) 323 Consulti 760) 323-5699 (fox) ng No 3 PREP D UNDER THE SUPERVISION OF. C V OF Q STEVEN D. SPEER IN TH7= CITY OF LA QUINTA DESIGN: SS DATE- FEB 2009 ST. FRANCIS OF ASSISI DRAFT: PRIVATE REFSRENCE- MAINAGE CAPACITY STUDY ITC XXXXXXXXXXXX AS OF 1998 PURCHASE DATE CHECK: Ss SHEET EXHIBIT D SCALE- OF LOCATED IN A PORTION OF SECTION 30, TOWNSHIP 5 SOUTH, RANGE 7 EASt SBM 1"=40' 1 SHEETS I FOR: TH7 CrrY OF LA OUDUA' CITY REFERENCE- Olt: EXHIBIT C 0 IM CD z 0 w