KEY HAVEN ESTATES BASIN A Drainage Calculations
Project Area Pavious Area lmpervbus Area % Impervious
Rsinfan for 2Syrl24hr event (P) Rainfall for 25yrTJhr event (P)
Depth to Water Table Developed Available Storage -1 Stwage (S)
proiect Pervkus Arm Impsrviarsh % Imm
Rainfall for 25yrL24hr went (P) Rainfall for 25yrBhr event (P)
Devdopad AveitabkStwase soil Storage (S)
Vdume = QA
PmSsct Area Roof Area lmpervkus Area % Impervious (Excluding Roof Am)
A) One inch d Rlnon lrwn drainage basin
Water QuiwfW Vs. WeW Que$ll!
RiiJ. MM,PE -15 201 Front St., Suite 207 Key west. FL 33040 KEYHAVENESTATES BASIN A
STAGE-STORAGE TABLE - BASIN A Assumptions: Site storage is linear, starting at min. road elev 4.0 up through slab elev.5.0. From elev. 5.0 to 5.5 storage is vertical.
stage Site Grading ac-lt acin
4.0 0 0 4.5 ((.5/1) x 4.65 ac) x (.5 fV2) = 0.58 6.98 5.0 ((1/1) x 4.65 ac) x (1 fU2) = 2.33 27.90 5.5 (0.5*4.65)+((1/1) x 4.65 ac) x (1 ft12) = 4.65 55.80
Runoff Volume from 25yrMay 39.5 ac-in
The zero discharge stage for the 25yrI3day storm is interpolated from the stage storage table.
The minimum perimeter elevation is 5.19 R
Richard J. Milelli 201 Front St.. Suite 207 L-- Key West, FL 33040 QE~$Y(
KEYHAVENESTATES BASIN B Dminage Calculations Water QuanbW - Predevelo-
Pmjact Area Pervious Area Impenrkus Area % Impsrvious
R8blfan for 25yrR4hr went (P) Rainfall for 25y1/3hrwent (P)
Depth to Water Table Developed Available !Storage soil SEwaga (S)
Pmjact Ana Pervkush lmpaviow Area % Imparvbus
Rainfall for went (P) RainfaH fa2!5yl13hr wart (P)
Dapth to WdaTa#e Devebpad Avaitmbb Storage Storage (S)
Volume = QA
Projact Aroa Roof Area lmpenrious Area % lmpsrvious (Exdudlng Roof Area)
A) Thm quaftm lnch of Mlon from balnage basin
Water QuanlilYv Vs. Water Qu* -0.34 adn < 0.78 adn I
Water Quality 50% 3630
Rictrard J. MlM, PE #58315 201 Fmnt St, Suits m Key west, Fkrida 33040 KEYHAVENESTATES BASIN C Drainage CalcuIetiom
Project Arsr Pavious~ Irnpsrvias Area % Impervious
Rainfail for 25yrR4hr event (P) Rainfan for 25yrm svsnt (P)
Dsplh to Water Table Dwdaped A~bSlapge soil Staage (S)
Water Quentit~- Postdevdoamed
Project Area Pmhous Arm Imperv#us Area % Impervious
RPinhll for 25yrR4hr evsnt (P) Rainfall for 25yr13hr went (P)
Do@ to Water Table DevebpdAvailaMbSt#age soil stuage (S)
Water QueIi&
Project Am Roof Area lrnparviaus Area % lmperviars (Exdudi Roof Arm)
Wetar Quentitiv Vs. Weter Quelity
Swale Volume Rewired
Water Quality '50% '3630
Swele Vdume Provided
RimJ. MM 201 Front St, Suite 207 KeyWesfFL33040 pc~1 r - ,(< lol KNHAVENESTATES BASIN C
STAGE-STORAGE TABLE - BASIN C Assumptions: Sie storage is linear, starting at min. road elev 4.0 up through elev.8.0
Stage Site Grading acR acin
4.0 0 0 4.5 ((.5/1) x 14.10 ac) x (-5 fV2) = 1.76 21.17 5.0 ((111) x 14.10 ac) x (1 fV2) = 7.06 84.66 5.5 (0.5*14.10)+((1/1) x 14.10 ac) x (1 Pt/2) = 14.1 1 169.32
Runoff Volume from 25yrMday 125.91 ac-in
The zero discharge stage for the 25yr/3day stom is interpolated from the stage storage table.
The minimum perimeter elevation is 5.20 R
Richard J. Milelli 201 Front St., Suite 207 Key West, FL 33040 fg{e.slr
ENCHANTED ISLAND BASIN D Drainage Calculations
Roiath Pelviomh Impervious Area % lmpenrious
Roiath PelviomArw Impervious Am I- % I-
Rainfall for 25yrR4hr ernnt (P) Rainfall for 25yrM (PI
Ro$dh Rodh Impenrkus Ama % lmpervkus (Excluding Rod Arm)
Richad J. Midi. PE -15 201 FrnSt., SuRe 207 KeyWest.Florida33040 Mr. E. David Femandez Utilities Director City of Key West ,: 5701 College Road Key West, Florida 33040
Subject Technical Memorandum No. 1, Estimation of Drainage Well Capacities for the City of Key West
Dear David:
Enclosed are five final copies of Technkal Memorandum No. 1 for your use. Feel free to call me should you have any questions.
As noted, I have forwarded copies of this technical ntemorandurn to Annalise and Janet.
Sincerely,
Kenneth F. WiIliams, F.E. c Annalise Mannix-Lacher/ Key West JanetMuccino/Key West Steve Hillberg/CHUn HILL Mitch Griffin/CHZM HILL Rick OIson/CH2M HILL Dave McNabb/CHZM HILL Stanley Fardm/CH2M HILL TECHNICAL MEMORANDUM 1 C-MHILL Estimation of Drainage Well Capacities for the City of Key West
-ED WR: City of Key West PREPlWD By: CH2M HILL 4 DATE: April 19,2002
Contents Background...... 1 Tidal Influence ...... I Estimated Well Capacity ...... 5 Pressurized Drainage Wells ...... 6 Gravity Drainage Wells ...... 6 Conclusions...... 7 Background The City is planning to add both gravity and pressurized drainage wens to its Stomwater Disposal System. The capacities of these wells must be estimated to design the stonn water pumping system and to estimate the contributions these facilities wilI make to the City's storm water drainage system. Initially, it was proposed to estimate the capacity of the propoeed pressurized drainage wens by performing capacity tests on existing drainage wens. However, during the initial data gathering phase for these projects, data from well completion reports on gravity was installed in the Peary Court area in 1993 were made available to CH2M HILL The permeability test results demonstrated at these web is considered to be consistent through- out the island. These wen completian reports contained drawdown versus flow information for each well. CH2M HILL concluded that the capacity testing that was initially proposed was not able to provide Mter information than that already documented in the existing well completion reports. Therefore, well capacity will be estimated using the data from the Peary Court wells. Tidal lnffuence Capacities of both pressurized and gravity drainage wells are influenced by the tides. Consequently, tidaI influence must be considefed in both pressurized and gravity drainage well design, although tidal iduence is much more significant in gravity drainage well design. Tide elevations are defined as follows: Mean Lower Low Water (MLLVThe average of the lowest low tide that generally occurs each day Mean Low Wakr QviLW)-The average of all the low tides that generally occur twice each day Mean High Water O-The average of all the high tides that gendyoccur twice each day Mean Higher High Water (MHHW)-The average of the highest high tide that generally occurs each day Nautical Chart 11442 gives tidal informatian for Key West (24' 33'N/81° 48'W) that is summarized in Exhibit 1. This tidal information and Year 2002 tide prediction tables sub- sequently referenced in this technical memorandum (I'M) are on the same datum (MLLW), which, however, is different from the datum for all ground surface elevations in Key West and everywhere. The ground surface datum is the National Geodetic Vertical Datum of 1929 (NGVD1929). men1 Tilnfamakn kr Key West (24'33'N 1 81°48W) Mwn llkrn Moan Higher LowWr Hlgh Water Hlgh Wlbr (MLW) WW) (MHHW) 0.2 1.5 1.8 (Navigation Chert Datum d MUW) Nate:
Florida Department of Environmental Protection PEP)data provide two tidal stations that are referend to NGVD 1929 with the same latitude and longitude as that provided in Nautical Chart 11442. Tidal information for these two stations is provided in Exhibit 2 As shown in Exhiit 2,2average adjustment to convert from the Navigation Chart/Tide Prediction Table datum to the NGVD 1929 datum is approximately -0.6 feet. um2 Tidd !Won Irkmakm Mwn Noan M#n Higher LavrW.br High Wabr HIghW-r WLW) (MHW) (MHHW) Key West ldand -0.39 0.92 121 Site # 87WBO LaWude 24" 332N L#lgihrde 81" 48.5W Key West Hawkchamel -0.47 0.97 1.27 Site # 8724557 Letitude 24" 327'N LongUude 81" 47.6W Average of Tide Stations -0.43 0.99 124 Average~Be(wanTieCheR/ -0.63 . 4.56 -056 Tide Prediction Tables and NOVD1929 Nate: ~besedon#CM)1929 Exhibit 3 summarizes Key West tidal information referenced to NGVD 1929 that is used in this TM.
Extam3 Tidal I- fa Key West Referenoed to NGVD 1929 [Tzllwn Mean Mom Mnn High Lowwater HlghWatw Hl~hWatw (RIILW) (YHW) (MHHW) Eklwtion -0.4 0.9 1.2 (NGVD 1929)
In thie TM,low tide is cansidered to be the Mean Low Water (MLW)elevation of 4.4 NGVD 1929, and high tide is considered to be the Mean Higher High Water 0elevation of 1.2 NGVD 1929. Because high tide is limiting for both pressurized and gravity drainage well capacity, further discmion of the MHHW is provided. Because! MHHW is a mean, tides greater than and less than this mean elevation of 1.2 NGVD 1929 occur throughout the year. Exhibit 4 shows the percent of time the Higher High Water (HHW) ispredicted to be above the MHHW elevation of 1.2 NGVD 1929 in Year 202.
-4 Frequency Msblbulion for Year 2002 far Key West HiiHigh Wder Predded Elevakn,(NG\ID 1929 PercentdtineorWumberdDaysinYear Rwrtion (NGVD 1928) 20az Higher High W (HHW) is -tobeEqurlboarG-thn 1 1.3 1.4 1.5 1.6 1.7 1.8 13 20 Elevation M
%dTLne 61.4 50.7 40.3 29.6 171 7.4 4.1 22 0.8 Nlm&~d Daye m IS iu ioe ss n is a 3 -Hieher High - Although every month can be expected to have some higher high tides above the mean, September and Wber typically have the highest higher tides. For Year 2002, every day in September and October is predicted to have higher high tides greater than the MHHW elevation 1.2 NGVI) 1929. These months also typically have higher monthly precipitation amounts, as shown m Exhibit 5. Exhibit 5 also shows that 7 of the 12 months in Year 2002 are expected to have average monthly HHW equal to or greater than the MHHW of 1.2 NGVD 1929. For September 2002, tides are predicted to be higher ihan the MHHW of 1.2 NGVD 1929 24 percent of the time, or 6 hours every day, an the average. For October 2002, the amount of time the tides are predicted to be greater than the MHHW is even greater, to 32 percent of the time, or almost 8 hours evey day, on the average. For Year 2002, the highest lugher tides are predicted to occur an November 5,6, and 7 at elevation 2.0 NGVD 1929. , . I I ! 0 w-: .--.-- ;...--...--.-..'....------,-.-.:--.--.--1--1 -..,.--...- 1-.--- --...! 4 FMA MJJASON D Estimated Well Capacity Key West is under& by a highly porous oolitic limestone that is honeycombed with solution hoband has an extremely high permeability. Evidence of the ex&mely high permeability is provided in the Peary Court well compIetion reports. Testing data from these wells indicate that they were pumped at a rate of 2,900 gallons per minute (gpm) with 0.5 feet of drawdown during well development. Based on these data, the calculated specific capacity for these wells is 5,800 gpm per foot of drawdown while pumping out of the wens. To estimate drainage well capacity to aamnmodate stonn water discharge (ie., fhw into the well),b facto~~that negatively impact disposal capacity of the drainage w& must be considered. The fiist factor is wen efficiency when pumping water into a well, campared to that of pumping water out of a well. The second factor is the difference in density of fresh water (storm wafer) compared to that of sea water (the salinity of the ground water at the depth where the storm water discharges can be assumed to be the same as sea water m Key West). In theory, well ef&ency should be the same whether pumping out of or into a well. However, experience has shown that wells in the Plorida Keys have approximately 70 to 80 percent of the eificiency when pumping into a well, as armpared to pumping out of the same well. Therefore, disposal capacity is cmly.approximately70 to 80 percent of the pump out capacity of the wens. Using a 70 percent effickmy factor dtsin a theoretical well disposal specific capacity of 4,060 gpm (9.1 cubic feet per second [cfs]) per foot of head build-up. The second factor, the difkrence in den~itybetweensaline and fresh water, also results in a decrease in well disposal efficiency when compared b the specific capacity testing per- kedon the Peary Court wells. The density of fresh water is 1gram per cubic centimeter (g/cm3), while that of sea water is 1.025 g/an3. To determine the head (i-e.,potential energy) of a column of fresh water within a sea water environment that is required before fresh water will begin to move into the sea water, the following equation is applied: Head = 0.025 x h, where h is the height of the fresh water column The proposed drainage wens will be 90 b 100 feet itr depth, and cased to 60 feet. Hence, the storm water (fresh water) will begin to discharge into the oolitic limestone formation at 60 fea It is expected that all stom waber will be discharged into the formation within the first 5 b 20 feet of open hole (65 to 80 feet of fresh water column). Thedore, the driving head of fresh water must be at least 15 feet (0.025 x 60 feet of casing), and is eqected to vary from approximately 1.6 feet (65 of fresh water column) to 20 feet (80 feet of fresh water column). The smaller driving heads would be typical of lower discharge rates down the drainage wells (i.e, gravity drainage wefts),while the larger driving heads would be required as the discharge rate down the drainage wells increase (i.e., pressurized drainage wells). This driving head of 15 to 20 feet of head must be overcome before the weIl'e theoretical disposal capacity of 4,060 gpm (9.1 ds) per foot of head build-up win occur, regardless of whether the drainage wells are pressurized web or gravity wells. Finally, it is common practice to indude a safety factor of 2 when predicting discharge to . subsurface drainage facilities. Indeed, this is required for permitting purposes. .
mwxwuamm 5 Pressurized Drainage Wells For pressurized wells, 8 feet of head build-up is the maximmn that FDEP will dow (i.e., a permit condition). The 2 feet of head required to overcome the density differential must be deducted from the 8 feet of atlowable head build-up. Fridion losses that vary between 0.3 to 05 feet for the wells (l8.O cfs to 25.0 cfs through 24-inch diameter WCwith a C of 140 for a distance of 80 feet) must also be subtracted born the available head. For low tide, the resulting available head is 5.5 feet for fresh water disposal (8 feet less 2 feet for salinity less 0.5 feet for friction losses). Based on the calculated theoretical disposal capacity of 4,060 gpm (9.1 cfs) per foot of head build-up and the 5.5 feet of available head for disposal, the wells have a theoretical disposal capacity of approximately 22,000 gpm (49.1 ds) at a total head of 8 feet at Iow tide. Adding a safety factor of 2 to the estimate results in a conservative disposal capacity of at least 11,000 gpm (24.6 ds) at a discharge head of 8 feet at low tide for each presswrized drainage well. The difference between high and low tide is 1.6 feet Therefore, at high tide, the available head for fresh water disposal would only be 41 feet (8 feet less 2 feet for salinity less 1.6 feet tide difference Iess 0.3 feet for friction lessee). With 4.1 feet of available head at high tide, the &netid disposal capacity is approximate@16,600 gpm (37.1 cfs). Adding a safety factor of 2 dtsin a conservative disposal capacity of at least 8,300 gpm 08.5 ds) at high tide for each pressurized drainage well.
GmByDrainage Weh As noted previously, the densitg difhmtial must also be e before gravity drainage web wiU begin to hcticm (1.6 feet of head). Because flow down vity webwill be substwiially less than down pressurized wells, friction loss wiIl subs tan^ h,and can be amsidered negligiile fw flows kss than 5 cfs. 3 If the ground water table is aseumed to be equal tide elevation of 1.2 feet NGVD 1929, then the water sarface must be at elevatbn drainage wells wotlld begin to fundim without flooding at high tide At of 6 inches (gained by either ponding on the surface by 6 inches or by the ground surface being higher by 6 inches, i.e., ground elevation 3.3), a disposaI capacity of at kast 1,WO gpm (22 cfs) could be expected for each gravity drainage well, at a safety factor of 2. There win be a higher effective head during low tide (an additional 1.6 feet) and disposal capacities would increase. With the pndsurface at elevation 28, ae above, a gravity drainage welt could be expected to have a disposal capacity at low tide of at least 3,200 gpm (7.1 ds) without ponding, at a wfety factor of 2 However, as shown above, thia same gravity drainage well would have no disposal capacity at high tide, unless the street flooded.
These estimates were made at two schemes of a amtinuously changing ground water condition. The achral flow rates in a gravity well will vary with time. For low ground' elevations (less than approximate elevation 4), there will be times that the wells will hwup to 9 cfs (or more), and other times where storm water would pond with negligible flow down a well, Hence, gravity wells are more suited for landscapes with higher elevations. For better and more reliable service, gravity weh should be placed at lo&tions higher than elevation 5.0. At this elevation, flows down a well could build up to elevation 5.0 before ponding occurs and a minimum flow rate down the well would be estimated at 3,600 gpm (8 ds,factor of safety 2) during high tide. During low tide, the flow rates into the wells would be approximately 6,000 gpm (13.4 ds), with a factor of safety of 2 Condusim Based onavailable well test data, a theoretical well dispasal specific capacity of 4,060 gpm (9.1 cfs) pkr foot of head build-up was used to predict the performance of drainage wells. This value was 70 percent of the measured capacity when pumping out of the well. Estimates of well capacity were made assuming that the groundwater table is essentially the same as the tide levels, and with a factor of safety of 2 It is recommended that the high tide (MeanHigher High Water) elevation of 1.2 NGVD 1929 be used aa the basis of the absolute minimum design high water table for determining the capacity of drainage wells. This is the recommended absolute minimum because this tide elevation is equaled or exceeded 61 percent of the time, or approximately 224 days in a year. For pressurized drainage wells, a conservative disposal capacity of8,300 gpm (18.5 ds) at high tide was estimated. This value win be used as the basis for determining the number of wells to achieve a desired drainage capacity. Under higher head conditions ww tide), flow rat- only increased up to approximately 25 cfs for pressure wells. For pitydrainage weIls, 1.6 feet of head is required to wemethe density differential before a gravity drainage well will begin to function. A well disposal capaaty of at least 3,600 gpm (8 cfs) without ponding can be expedd at high tide if the landscape is higher than elevation 5.0 and if ihe goal is to have Iittie or no etreet flooding. The flow into gravity web vary more between low and high tide conditions (almost double the flow rate). Gravity wells placed at elevatiom lower than 5.0 will have capacities lower than 8.0 ds during periods of the day, and at approximate elevation 4.0, gravity wells could have negligible dieposal capacity during periods of the day. Also, additional flow into a gravity well could be achieved where water ponds higher than he ground level. Pressurized wells have a more steady capacity rating (and higher per well) and will not vary with ponding.