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Home My WebLink AboutMOSSFLOWER HILLS S12596Onsite File Hills AD EN AL 1111W�Iff In _9� I ��J' • Engineer Soils Report Prepared By: Grant Mathews, P.E. �F. A�,q°Sl *:491H - ...... H•••• GRANT.W. MATHEWS p �'� •, CE 135444 ,•cv�� pis'. .• ��'® ��PROfESS10NP�'.,. February 2021 Introduction The following report details the feasibility of the Mossflower Hills Subdivision to support on-site water and wastewater systems for residential single-family homes. The preliminary plat is attached with this report. The site is located in the Lower Hillside area of Anchorage Alaska, directly south of O'Malley Road and east of Elmore road. See below for a location map. Location Map The parcel is approximately 4.5 acres in size, zoned R6. Based on the zoning and current code, the parcel could be subdivided into a maximum of four lots, but the existing house on site will remain in place, limiting the lot count to three total lots. 1 The site is bordered by Wild Iris Subdivision to the south, Amazing Grace Lutheran to the west, O'Malley Road to the north, and BLM lots to the east. The surrounding lots are fully developed with residential single-family homes and the church, all of which are equipped with on-site water and wastewater systems. Water Availability Data from the MOA On-site department indicates the production of existing wells in the area range from 8 to 30 GPM with an average rate of 15 GPM. The depth of wells ranges from 227 to 282 feet with an average static water level of roughly 200 feet below ground surface. See below for a table of the well depths and production rates for the wells surrounding the subject parcel. *Data taken from COSA documents filed with the MOA On -Site Department Future wells for Mossflower Hills Subdivision are anticipated to have similar performance with an average anticipated depth of 250 feet and a production ranging from 5-15 GPM. Individual wells are anticipated to be adequate for the subject property if it is developed under the current zoning designation. The development of this subdivision is not anticipated to have any negative affect on the water availability of the surrounding subdivision or properties. Soils and Septic System Information Test holes were excavated on two new proposed lots. Native sands and gravels with varying silt content were found throughout the site. Soils logs are attached with this report. Percolation testing was performed for each lot. See below for a table showing the percolation results. Note, Lot 1 was not tested as the existing house and on-site systems are to remain in place as -is. Lot Perc Rate Testing Depth (BGS) 2 6.7 min/inch 4-5 ft 3 6.4 min/inch 4-5 ft Both proposed lots are suitable for conventional on-site wastewater disposal systems. Development of the individual lots will require an engineered plan for well and septic locations that will be reviewed and approved by MOA On-site to ensure that the septictanks and absorption fields are placed outside of the existing and proposed well radii. A grading plan showing conceptual locations is attached for reference. 2 Static Water Production Subdivision Lot Well Depth (ft) Level (BGS) (GPM) Wild Iris 2 227 175 30 Wild Iris 3 239 193 9 T12N R3W SEC 22 PTN N2 14 Unknown 135 4 T12N R3W SEC 22 PTN S2 & W30' 14 282 227 15 T12N R3W SEC 22 15 260 206 8 (Subject Property) T12N R3W SEC 22 16 Unknown Unknown Unknown (Amazing Grace Lutheran) *Data taken from COSA documents filed with the MOA On -Site Department Future wells for Mossflower Hills Subdivision are anticipated to have similar performance with an average anticipated depth of 250 feet and a production ranging from 5-15 GPM. Individual wells are anticipated to be adequate for the subject property if it is developed under the current zoning designation. The development of this subdivision is not anticipated to have any negative affect on the water availability of the surrounding subdivision or properties. Soils and Septic System Information Test holes were excavated on two new proposed lots. Native sands and gravels with varying silt content were found throughout the site. Soils logs are attached with this report. Percolation testing was performed for each lot. See below for a table showing the percolation results. Note, Lot 1 was not tested as the existing house and on-site systems are to remain in place as -is. Lot Perc Rate Testing Depth (BGS) 2 6.7 min/inch 4-5 ft 3 6.4 min/inch 4-5 ft Both proposed lots are suitable for conventional on-site wastewater disposal systems. Development of the individual lots will require an engineered plan for well and septic locations that will be reviewed and approved by MOA On-site to ensure that the septictanks and absorption fields are placed outside of the existing and proposed well radii. A grading plan showing conceptual locations is attached for reference. 2 Nitrate Analysis No existing wells within 250 feet of the proposed subdivision have known nitrate levels exceeding 10 mg/I. Subdivision Lot Nitrate Reading (mg/L) Wild Iris 2 1.2 Wild Iris 3 Unknown T12N R3W SEC 22 PTN N2 14 5.64 T12N R3W SEC 22 PTN S2 & W30 14 4.17 T12N R3W SEC 22 (Subject Property 15 1.85 T12N R3W SEC 22 (Amazing Grace Lutheran) 16 Unknown *Data taken from COSA documents filed with the MOA On -Site Department Drainage The soils within the area are well-suited to infiltrate runoff and the large lot -sizes stipulated by the R6 zoning helps to ensure that the ratio of impervious area to vegetated area remains low. An engineered grading and drainage plan is included with this report. Drainage generated by this subdivision is not anticipated to have any negative affects to neighboring properties Conclusion The subject property and the associated preliminary plat is well-suited for on-site water and wastewater systems. Well and septic information from the immediate vicinity of the parcel indicate that aquifer production is adequate for single family homes; the soils are well -draining and will support conventional septic systems. 3 March 2"d, 2021 512596 Mossflower Hills Subdivision On -Site Comment Response Letter 1. Plan view is to have the engineers stamp and signature. Plan 6s now stamped atid signed. 2. Is the well serving Amazing Grace a community well with a 200' protective radius? `fes, tyle well is classified as Non -Transient, Non-C.arnmunity water system per ADEC . PlAJSit 21-4544. A 200' protective radius is nmv shown. 3. There are numerous locations that are within the septic reserve area or within 50' that appear o have slopes over 25%. The reserve area must meet the slope setback requirements of the code. The proposed locations Can rne:et the req uirernetits of "Che steep slope pro ViSionS o' 1.`.65.2.10. Trenches will noL L)e ii`Vstalled in sfooes areate9' than. 46%. If installed vJEthin a slope gr ater than 255-1, the trenches ` d l comply with 15.65.210.a. There is no area downgi ether' of the reserve areas with a slope exceeding 25%, and no risk for wastewater effluent to "daylight" from the proposed reserve area locations. 4. Show the well and septic locations for the lots to the east. The wells are now shop=,.tn. 5. Per property appraisal, the existing house has 4 bedrooms. The septic system is sized for 3. Please address. The appraisal was in error and has leen corrected to show 3 bedrooms. 6. Provide a design for an alternative site for the existing house. Alternative site.: is nolN shoMil. 7. The north arrow is in the wrong direction. Corrected, 8. Provide a nitrate table. Nitrate table has Deein ac ded to the soils report. 9. Use the most recent well flow and nitrate readings (COSA data). Report has been ul paled, Grant Mathews, P.E. Municipality of Anchorage Development Services Department On -Site Water and Wastewater Section a� 4700 Elmore St. P.O, Box 196650 Anchorage, AK 99519-6650 www.muni.org/onsite (907)343-7904 Performed For: Legal Description: 1_ 2- 4- 5- 6- 7- 8- O- 10- 11- 12- 13- 14- 15- 16- 17- 18- 19- 20 - G ' T V. MATMr..: 31•ii}'G� ih_ 4QFESSt01�,..."" Soils Log - Percolation Test I Mossflower Hills Subdivision - Lot 2 Date Performed: 10/24/20 T12N R3W SEC 22 LT 15 Township, Range, Section: Slope Sita Plan SCAUFtcft OUTAt4ICS siL" Swat ul j 91RAVEL S"LL cov sto l3") SM-(bAA Ctem4 SRNN G-srt, /fit, �?_Sw 31VN SAtdD S' IbbA NrAwt~ WAS GROUND WATER Date ENCOUNTERED? No Depth to Water S IF YES, AT WHAT DEPTH? L Depth to Water After a Monitoring? NGWE e Date: 10/31/20 Reading Date Gross Time Net Time Depth to Water Net Drop 10/24/20 --- --- 5-13/16" --- 1 30 min 30 min 1-1/16" 4-3/4" --- 30 min 5-15/16" 2 30 min 60 min --- 60 min 5-3/4" -_- 3 30 min 90 min 1-1/4" ....... -1 — 11— c tmmutasttncn) t'EK(i HOLE DIAMETER 6" TEST RUN BETWEEN 4 FT AND 5 FT COMMENTS No presoak required. Pero readings measured to 1/16 of an inch PERFORMED BY: Grant Mathews I CERTIFY THAT THIS TEST WAS PERFORMED IN ACCORDANCE WITH ALL STATE AND MUNICIPAL UI EL ECT ON ES IN EFFTHIS DATE. DATE: -to -51-_ Municipality of Anchorage Development Services Department On -Site Water and Wastewater Section 4700 Elmore St. i P.O. Box 196650 Anchorage, AK 99519-6650 vANAv. muni.org/onsite (907) 343-7904 Performed For: Legal Description Soils Log - Percolation Test L Mossflower Hills Subdivision - Lot 3 Date Performed: 10/24/20 T12N R3W SEC 22 LT 15 Township, Range, Section: Slope Site Plan gtwtctAl. oytttAMte-S 1- 2 StLiY SAID tvf ttt?At�L SMI+u, C08$L�5 3- 5M-l�n� 5- 6- 7- 8- 9- 10- 11- 12- 13- 14- 15- 16- 17- 18- 19- 20- CLFi1r1 SANOV G? _GVj cows Ltp�v b" Readina VJAS GROUND WATER Gross Time ENCOUNTERED? No Depth to Water s IF YES, AT WHAT DEPTH?L P S i Depth to Water After $M Monitoring? NGWE E 1 30 min Date: 10/31120 0-11/16" 4-3/16" Readina Date Gross Time Net Time Depth to Water Net Drop 10/24/20 --- --- 5-3/4" --- 1 30 min 30 min 0-11/16" 4-3/16" --- 30 min 5-15/16" --- 2 30 min 60 min 1-1/16" 4-3/4" --- 60 min 5-15/16" --- 3 30 min 90 min 1-1/8" 4-11/16" PERCOLATION RATE U•4 (minutestinch) PERC HOLE DlXvlk:1 EX b TEST RUN BETWEEN 4 FT AND 5 FT COMMENTS No presoak required Perc readings measured to 1/16 of an inch. PERFORMED BY: Grant Mathews I ERTIFY THAT THIS TEST WAS PERFORMED IN ACCORDANCE WITH ALL STATE AND MUNICIPAL�GUIFINEFF�ECTONTHIS DATE. DATE: 10 -St -7,0 ® EX BLDG �a ®� / TRUE NORTH �®, m SCALE 1"= 50' P • �'jy � i l APPROX WELL LOCATION ®,� �� W APPROX LOCA TION /`1 1 Q OF EX SEPTIC PROPOSED WELL - SYSTEM \ \ \ / / RADIUS (TYP)low ADW z i �1 96 rf 200' COMMUNITY __ _ WELL RADIUS r y . i r BUILDING - BUILDING \ r LDIN SETBACK EX WELL RADIUS �' 125r �� SETBACK I ` I �7to / o"50"rn o , \ / I _ f , O ` o v 0'T i - „ i 25 LOTS' 1 \-�..�,�.. w 0 3 i `. Q O a o 0 3 rr s t r 12,000 SF , , C SEPT r, W RESERVE t TH ,l /7N.rr SO EX HOUSE �cr(3 BDRM) , rr ,w - A ;.} .. Tj ! 1 i r _ w , fig' - i .t ;. ......� - _ \ Y ' -__ ----------- - -7777- , _EX GRAVEL DRIVE77 - ---- - - t- 0 v , \ �' 1 .. • .. r _ — Z I tt - - •_- ( i\ !®♦tel } i C r_ 4 O f \ATG ", I i PROPOSED FIRE`' _ 000 S SEPTIC TURNAROUND F 1 1I LL Lam. I 1 RESERVE - v 0 j / 1 i �` tai � ,�•jQ S _ �d t y i a1,000 EX GAL ,' � � o � M SE NK �� 3 1 ,p r. SEP TA l 50' ; �`'� w En �' 3 l r „ \ 125 EX 4x50 `� �., _ ... �r ABSORPTION FIELD �\!� �. , Q 1 o i ' BUILDING - SETBACK i �.� v 4x50 REPLACEMENT FIELD ; ' 1 "` � I� �r - . �', �� _ _._/-. � , o � a r v' ----- '--- - _ cn O i 3:jf Ld _\ hi 'F i EXISTING GRAVEL -------- ---------= ----=----=- - --------;-- "� O z o o > a' r DRIVEWAYS TOCD =i -- " m� - ` ;; i BE ABANDONED ------------- --;------------- ------ w _ v r- ------------ ---- ------------------- ------ NOTE -� _," WELL AND SEPTIC DATA FOR 0 u.. � o rn (� EX HOUSE , ' �' 100 WELL RADII = SURROUNDING PROPERTIES TAKEN FROM SHEET \: (3 BDRM) ---- --�--- ---- ---- - WA ON-SITE ASBUILIS AND HAS NOT \4 - 1 ¢ FIELD ACCURACY. \ BEEN FlE D FOR OF