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HomeMy WebLinkAboutDrainage Report GATTISENGINEERING ENGINEERS * CONSULTANTS January 23, 2015 Erika Bridges, EIT Engineering Services City of College Station College Station, Texas 77845 Drainage Summary Letter RE: Southwest Place Retail Center 1712 Southwest Parkway, #100 College Station, Texas 77845 Dear Mrs. Bridges, This letter is to summarize the site drainage impact from the proposed parking lot expansion for the above referenced project. EXISTING CONDITIONS The Southwest Place Retail Center is located at 1712 Southwest parkway. The retail center consists of a 9,967 square foot retail building with surrounding parking on 0.9454 acres. Currently, all storm water run-off from the building and parking lot areas, including the adjoining Shipley's Donut Shop is captured in the parking lots areas and conveyed southwesterly to the southwest property corner of the Southwest Place Retail center. There is an existing 2' wide, 12" high raised curb opening that acts a weir that meter-releases the collected storm water runoff off-site to the adjoining apartment complex parking lot. The weir shows to be performing correctly under all storm events as storm water currently ponds in the parking lot during rain events. No hydraulic modeling of the curb-weir was performed with this report. As stated above, storm water is discharged through the curb-weir and conveyed to the adjoining apartment complex through their parking lots areas and eventually combines with Bee Creek around the city cemetery. The subject property is not located in the 100 year FEMA floodplain per FEMA FIRM Map#48041C0310F, dated April 2, 2014. See attached Site Plan. X \\P^ mIC �cu� qd� { a `o, <k \7f,j251e7tt f. PROPOSED SITE IMPROVEMENTS \l The Southwest Place Retail Center is acquiring 0.17 acres of land(through re-platting) at is adjacent to the property in the rear of the building. This tract will be Sed to provi an additional parking lot(7,444 sq. ft.) that contains 16 parking spots. The parking1t will be constructed of 6"reinforced concrete pavement with raised curbs. This additional parking lot area will provide not only additional parking spots, but also additional storm water detention volume for any increases in storm water runoff as a result of the improvements. The 2' wide curb weir will remain in place to meter release storm water as it is currently performing today. See Tables 1 and 2 below. SITE HYDROLOGY SUMMARY The Rational Method— Q =C * I * A—was used for the hydrologic evaluation of the proposed site improvements. The Rainfall Intensity (I) was calculated based on the Texas Highway Department-Bridge Division Hydraulic Manual based on a Time of Concentration of Tc=10 minutes. As discussed above, the Bacon Baptist Church onsists of impervious cover areas (retail building, sidewalks, parking of area) and a so pervious areas (landscaping and grassy areas). A run-off coefficient of C=0.9 was used for impervious areas and a run-off coefficient of C=0.35 for pervious areas. The adjusted run-off coefficient of C=0.84 was calculated for the complete site and used for hydrologic analysis. See Table 1. Table 1 Hydrologic Data Summary Existing Proposed +1- Area(acres) 0.9459 ac 1.1159 ac 0.17 ac Coefficient(C) 0.84 0.84 --- Tc 10 min. 10 min. --- Impervious 40,096 sf 46,639 sf +6,543 sf Pervious 1,106 sf 1,977 sf + 871 sf The 0.17 acre increase is the land acquired for the parking lot addition. Table 2 PEAK FLOWS (cfs) SUMMARY Add'l Vol. Storm Existing Proposed +/- Volume Increase Provided Event cfs cfs c.f. c.f. 2 Yr 5.02 5.93 + 0.91 813 3,722 5 Yr 6.11 7.21 + 1.10 989 3,722 10 Yr 6.86 8.09 + 1.23 1,109 3,722 25 Yr 7.83 9.24 + 1.41 1,267 3,722 50 Yr 8.85 10.45 + 1.60 1,432 3,722 100 Yr 9.24 10.91 +1.67 1,496 3,722 CONCLUSSION As shown above in Table 2,there is a slight increase of storm water run-off maxing out at 1.67 cfs due to the 7,444 square foot parking lot addition. However,the additional parking lot also provides additional detention volume of 3,722 c.f. (7,444 sq ft x 6"curbs). Therefore,maintaining the existing 2' curb-weir as the current stormwater metering device in the parking lot area,there will not be any adverse effects to the adjoining property owners. Respectfully, GATTIS N n,INEERING,LLC If i I , li ira tot 4 0 ...lei bliiku Joe I PE � 5.• `.s ti 6 ii Lic' sed Professional Engineer `*'k:• i� ;*yk d 0 JOE I.GATTIS x Attachments 1''�90�;90964�a•�tty ,c ; ENS•• kielO AL iNAC" � /r 1 qiur � i � /-- 25- 15 FIRM#7698 • 2010 Moses Creek Ct. • College Station,Texas 77845 • Tel.979.575.5022 • Fax 979.690.6888 ATTACHMENTS flt w mcc I- W a:2 u- CI et �ca�o IW 20 L.�.. aEo WC) W� as E,00E o a o s m r, CO O o oLL Co a '- 2c�i wN . 8�IL N RO 7 2 �E5 Z wN v -0 11) O m CI a LI� ;g- Q � QJ DIII o m- W Z Zb el a W O J Z o a :e >, �E-$'08 O LL u W a o T c W a LL : o as DoE u` 1110 m V Q• . c w tLL-5g° a.Q o ll.4,E 4 0,c0.8 ‘=Ka OAR )9 §x� � in-41,0L $ E gm w o 0 § 0 5 a � - �O(WryW� Oc� �l C � ' G°` ` z `LctZ_j O� / `. • ��`� G�RC ice_ waa> CO esi = a`Z U ' 11‘. .e c=8VZ dA U J © BROTHERS , ..NitEcii_0 N-t iite & r` o v tzt lu 41 111 / 4‘ 8 0\-\0C" -I die % �j d 0110 _4,1/4 Q 41 w W �� d� ��b �� q z > •) '). °° N 2,\ 4 ,tiro I. d0 0, 7� °gid Ys co h 44 •o°w ° \'4, it. v I ww a 262 0 ''--(,, Zi,. liali 0a '1) )di *4. bpi-'d3 `� ,c.(4") aco -m 262 Q O d + W <{'4. Z W '4 Jz � Q `') -i lo o OQ v d ow N N N N N �p °`I OS a m __ E'9� ti_m_„, 3 = N cN, \ yOJ Allihh:_, C\I C° Z cn _0 SO£01NHd SNIOC I- N N o N q N q w CO L.L. •p Ma wZ piiau We zi w et> U : hj Z ,, ., aQ W cod V Or CL Hydraflow Table of Contents SWCenter.gpw Hydraflow Hydrographs by Intelisolve v9.22 Friday,Jan 23,2015 Watershed Model Schematic 1 Hydrograph Return Period Recap 2 2 - Year Hydrograph Reports 3 Hydrograph No. 3, Combine, SW Center Combined 3 5 - Year Hydrograph Reports 4 Hydrograph No. 3, Combine, SW Center Combined 4 10 - Year Hydrograph Reports 5 Hydrograph No. 3, Combine, SW Center Combined 5 25 - Year Hydrograph Reports 6 Hydrograph No. 3, Combine, SW Center Combined 6 50 - Year Hydrograph Reports 7 Hydrograph No. 3, Combine, SW Center Combined 7 100 - Year Hydrograph Reports 8 Hydrograph No. 3, Combine, SW Center Combined 8 IDF Report 9 1 Watershed Model Schematic Hydraflow Hydrographs by Intelisolve v9.22 1 -Existing SW Center 2-P-Lot Addition SW Center Combined Legend Hyd. Origin Description 1 Rational Existing SW Center 2 Rational P-Lot Addition 3 Combine SW Center Combined 2 Hydrograph Return Period Recap Hydraflow Hydrographs by Intelisolve v9.22 Hyd. Hydrograph Inflow Peak Outflow(cfs) Hydrograph No. type Hyd(s) description (origin) 1-Yr 2-Yr 3-Yr 5-Yr 10-Yr 25-Yr 50-Yr 100-Yr 1 Rational ---w ----- 5.027 -- 6.112 6.861 7.836 8.857 9.248 Existing SW Center 2 Rational ---- ---- 0.903 1.099 1.233 1.408 1.592 1.662 P-Lot Addition 3 Combine 1,2 -- 5.930 — 7.211 8.094 9.244 10.45 10.91 SW Center Combined 1 3 Hydrograph Report Hydraflow Hydrographs by Intelisolve v9.22 Friday,Jan 23,2015 Hyd. No. 3 SW Center Combined Hydrograph type = Combine Peak discharge = 5.930 cfs Storm frequency = 2 yrs Time to peak = 10 min Time interval = 1 min Hyd. volume = 5,337 cuft Inflow hyds. = 1, 2 Contrib. drain. area = 1.116 ac SW Center Combined Q (cfs) Hyd. No. 3--2 Year Q (cfs) 6.00 6.00 5.00 5.00 4.00 4.00 3.00 3.00 2.00 2.00 1.00 / 1.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (min) Hyd No. 3 Hyd No. 1 Hyd No. 2 4 Hydrograph Report Hydraflow Hydrographs by Intelisolve v9.22 Friday,Jan 23,2015 Hyd. No. 3 SW Center Combined Hydrograph type = Combine Peak discharge = 7.211 cfs Storm frequency = 5 yrs Time to peak = 10 min Time interval = 1 min Hyd. volume = 6,490 cult Inflow hyds. = 1, 2 Contrib. drain. area = 1.116 ac SW Center Combined Q (cfs) Hyd. No. 3-- 5 Year Q (cfs) 8.00 8.00 6.00 • 6.00 4.00 4.00 2.00 - 2.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (min) Hyd No. 3 Hyd No. 1 Hyd No. 2 5 Hydrograph Report Hydraflow Hydrographs by Intelisolve v9.22 Friday,Jan 23,2015 Hyd. No. 3 SW Center Combined Hydrograph type = Combine Peak discharge = 8.094 cfs Storm frequency = 10 yrs Time to peak = 10 min Time interval = 1 min Hyd. volume = 7,284 cult Inflow hyds. = 1, 2 Contrib. drain. area = 1.116 ac SW Center Combined Q (cfs) Hyd. No. 3-- 10 Year Q (cfs) 10.00 10.00 8.00 8.00 6.00 6.00 4.00 4.00 2.00 2.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (min) Hyd No. 3 Hyd No. 1 Hyd No. 2 6 Hydrograph Report Hydraflow Hydrographs by Intelisolve v9.22 Friday,Jan 23,2015 Hyd. No. 3 SW Center Combined Hydrograph type = Combine Peak discharge = 9.244 cfs Storm frequency = 25 yrs Time to peak = 10 min Time interval = 1 min Hyd. volume = 8,319 cult Inflow hyds. = 1, 2 Contrib. drain. area = 1.116 ac SW Center Combined Q (cfs) Hyd. No. 3--25 Year Q (cfs) 10.00 10.00 8.00 8.00 6.00 6.00 4.00 4.00 2.00 2.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (min) Hyd No. 3 Hyd No. 1 . Hyd No. 2 7 Hydrograph Report Hydraflow Hydrographs by Intelisolve v9.22 Friday,Jan 23,2015 Hyd. No. 3 SW Center Combined Hydrograph type = Combine Peak discharge = 10.45 cfs Storm frequency = 50 yrs Time to peak = 10 min Time interval = 1 min Hyd. volume = 9,404 cuft Inflow hyds. = 1, 2 Contrib. drain. area = 1.116 ac SW Center Combined Q (cfs) Hyd. No. 3--50 Year Q (cfs) 12.00 12.00 10.00 10.00 8.00 8.00 6.00 6.00 4.00 4.00 2.00 2.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (min) Hyd No. 3 Hyd No. 1 - Hyd No. 2 8 Hydrograph Report Hydraflow Hydrographs by Intelisolve v9.22 Friday,Jan 23,2015 Hyd. No. 3 SW Center Combined Hydrograph type = Combine Peak discharge = 10.91 cfs Storm frequency = 100 yrs Time to peak = 10 min Time interval = 1 min Hyd. volume = 9,819 cuft Inflow hyds. = 1, 2 Contrib. drain. area = 1.116 ac SW Center Combined Q (cfs) Hyd. No. 3 -- 100 Year Q (cfs) 12.00 12.00 10.00 10.00 8.00 8.00 6.00 6.00 4.00 4.00 2.00 — 2.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Time (min) Hyd No. 3 Hyd No. 1Hyd No. 2 9 Hydraflow Rainfall Report Hydraflow Hydrographs by Intelisolve v9.22 Friday,Jan 23,2015 Return Intensity-Duration-Frequency Equation Coefficients(FHA) Period (Yrs) B D E (N/A) 1 0.0000 0.0000 0.0000 2 65.0000 8.0000 0.8060 3 0.0000 0.0000 0.0000 5 76.0000 8.5000 0.7850 10 80.0000 8.5000 0.7630 25 89.0000 8.5000 0.7540 --- 50 98.0000 8.5000 0.7450 100 96.0000 8.0000 0.7300 ---- i File name: Brazos Rainfall-TxDOT.IDF Intensity= B/(Tc + D)^E Return Intensity Values(in/hr) Period (Yrs) 5 min 10 15 20 25 30 35 40 45 50 56 60 1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2 8.22 6.33 5.19 4.43 3.88 3.46 3.14 2.87 2.65 2.46 2.30 2.17 3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5 9.85 7.69 6.38 5.48 4.83 4.33 3.93 3.61 3.34 3.12 2.92 2.75 10 10.98 8.63 7.19 6.21 5.49 4.94 4.50 4.14 3.84 3.59 3.37 3.18 25 12.51 9.86 8.23 7.12 6.30 5.67 5.18 4.77 4.43 4.14 3.89 3.68 50 14.10 11.15 9.33 8.08 7.16 6.46 5.90 5.44 5.05 4.73 4.45 4.20 100 14.76 11.64 9.73 8.43 7.48 6.75 6.16 5.69 5.29 4.95 4.66 4.41 Tc=time in minutes.Values may exceed 60. Precip.file name: Brazos County Rainfall-24hr.pcp Rainfall Precipitation Table(in) Storm Distribution 1-yr 2-yr 3-yr 5-yr 10-yr 25-yr 50-yr 100-yr SCS 24-hour 0.00 4.50 0.00 6.20 7.40 8.40 9.80 11.00 SCS 6-Hr 0.00 3.33 0.00 0.00 0.00 0.00 0.00 7.90 Huff-1st 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-2nd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-3rd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-4th 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-Indy 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00