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HomeMy WebLinkAboutKNIK HEIGHTS Block K Lots 3 & 4tl~ %/o MUNICIPALITY OF ANCHORAGE MEMORANDUM DATE: September 15, 1986 TO: See Distribution FROM: Susan K. Metcalf, Manager~.Public Services, Engineering Division, DPW~4~--~, ~'~T~ SUBJECT: Knik Heights Subdivision--~Letter of Determination On September 4, 1986, the owners of Lots 3 & 4, Block K~ Knik Heights Subdivision, met with Senator Jan Faiks and with repre- sentatives of the Municipal Departments of Health and Human Services and Public Works to determine once and for all: whether water which is conveyed along their common property line jeopardizes either the legal compliance or practical safety of their private septic systems; a design for improved conveyance of the water which would be acceptable to all concerned; and c the various responsibilities for making those improvements. During the meeting it was unanimously agreed that design of the drainage ditch or swale involved an uncomplicated continuation of an existing ditch, and that actual construction should pose no great expense. The concern which loomed largest in the property owners' minds was the possibility that their septic systems might not be safe, or that new personnel, definitions, or conditions in some government agency, would threaten their status of technical compliance, whether or not conditions changed significantly. The attached draft letter is based on months of investigation and consideration, and seeks to not only allay anxieties over ,the changing bureaucratic breezes, but also to save future investiga- tors' time and troub].e. Please review, revise, and return the draft letter to me no later than Tuesday, September~2~D1986. I can be reached at 786-8173 if further information is necessary. SKM/bmco 1/skm/601 Distribution: J. David Norton, P.E., Municipal Engineer Bill Mans/Steve Morris, Dept. of Health and Human Services Bruce Erickson, Alaska Dept. of Environmental Conservation ," ~ ~ ANCHORAGE, ALASKA 99519-6650 ::'~'~,~d , (907) 786-8160 DEPAllTMENT OF PUBLIC WORKS (3500 Easi Tudor Road) September 9, 1986 Mr. & Mrs. Craig Richards P.O. Box 110-346 Anchorage, Alaska 99511 Dear Mr o & Mrs. Richards: RE: PRIVATE SEPTIC SYSTEM LOT 3, BLOCK K, KNIK HEIGHTS SUBDIVISION Based upon the attached report of a hydrologist retained for examination of the area of Knik Heights Subdivision, the agencies listed below have concluded that the surface water which is conveyed along your south and west property lines does not constitute a threat to the safe and legal operation of your private septic system. As mentioned in the report, run-off evident during spring breakup flows over the ground because the ground is frozen; therefore water cannot infiltrate the system, nor products of the system be leached away. On-site inspection during heavy rains of August of this year found no surface drainage along either of the subject property lines. If improvements to the ditch or swale along the south property line are considered, bear in mind that the water should be conveyed along a route which maximizes the distance between the water and both your septic system and the one located on Lot 4, Block K, Knik Heights Subdivision. Mr. & Mrs. Craig Richards September 9, 1986 Page 2 It should also be noted that the review of zonings and soils logs from areas uphill from your lot reveals that future development in those areas should not increase surface water flows to levels hazardous to your septic system. However, should conditions change sufficiently to allow sewage products to flow from or surface water to flow into your septic system, part or ali. of the ditch or swale mayrequ..re~ an impervious lining. Sincerely, Jo David Norton, P.E. Municipal Engineer Engineering Division Concur: NAME TITLE Alaska Department of Environmental Conservation Concur: NAME TITLE MOA Department of Health and Human Services JDN/SKM/bmco 2/jdn/4 Attachment LETTER REPORT Furrow Creek Channel Drainage Evaluation This report consists of three documents prepared on April 9, 1986, April 23, 1985 and June 3, 1986. The latter two documents were written to supplement the original report. All three documents should be read and considered as one. I986 LETTER REPORT Furrow Creek Channel Drainage Evaluation This report consists of three documents prepared on April 9, 1986, April 23, 1986 and June 3, 1986. The latter two documents were written to supplement the original report. All three documents should be read and considered as one. LETTER REPORT Lots 3 and 4, Block K Knik Heights Subdivision Drainage Evaluation Apt'il 9, 1986 D/6 INTRODUCTION As di~'ected in the Notice to Pt-oceed issued on March 27, 1986, I have p~'epared this letter report on the d~'ainage for Lots 3 and 4, Block K, Knik Heights Subdivision. This lette~- report is organized into five sections. Recommendations ai'e included in the last section. Based on information in the p~'oject file and fi'om Public Wot-ks staff, I understand the existing drainage situation to be as follows: A drainage problem was reported on Lots 3 and 4, Block K, Knik Heights Subdivision. This problem was alle9edly caused by the development of McCabe Subdivision East, prope~-ty located to the east and at a higher' elevation than Lots 3 and 4. Accor'dingly, I have investigated the local area dt'ainage, and my findings a~'e contained in the following sections. D/6 -] - SECTION '1 - GENERAL FEATURES Lots 3 and 4, Block K, Knik Heights Subdivision lie within Section 27, T12N, R3W, Municipal Grid 2936. This general area is known as the "hillside" portion of the Municipality of Anchorage. The area reviewed in this letter report is shown on the vicinity map. The main surface features in this area are shown on the attached map. A glacial moraine step traverses the study area from southeast to northwest. The main drainage feature lies in a east-west direction and can easily be distinguished on the topographic map. A second drainageway south of DeArmoun Road can also be found on the topographic map. The glacial moraine step was likely produced by a retreating glacier in Cook Inlet. The soils in this step can be expected to contain glacial outwash material that contains fewer fines (silts and clays) than the sur- rounding steeper slopes. Test holes in this area indicate that the soils are predominantly gravels (GP,GM AND GW groups) with silt and sand lenses. The main drainage feature is a channel of the Furrow Creek drainage sys- tem, but it is apparently only active seasonally. Based upon analysis of available surficial soils information, surface water is likely to be found in this channel only during spring breakup when the soils are frozen, or during exceptionally heavy precipitation events. During warmer months, the soil porosity causes most surface runoff to infiltrate into the soil. A second drainageway was likely connected to the main dr-ainage feature in the past. Construction of DeArmoun Road pr'obably interrupted the connection. Surface water from this drainageway collects in a Iow area and seeps into the soil. I examined this area on foot and verified the general features found on the topographic maps. Due to the presence of ice and snow, detailed drainage observations could not be made. D/6 -2- Ab 'R~ J · O'MaHev Rd SECTION 2 - PROBLEM DESCRIPTION In order to understand the drainage problem on Lots 3 and 4, the surface drainage of the local area must be examined. The main drainageway in the local area, as described in Section 1, is a westerly sloping channel of Furrow Creek. This drainageway is the main surface drainage collector for Knik Heights Subdivision and McCabe Subdi- vision East. Flow in the channel is seasonal, and according to local resi- dents, water is found in the channel only during a snow melt event or after a large precipitation event. The Fum-ow Creek channel enters Knil< Heights Subdivision through a culvert under Bainbridge Road at Lot 3, Block K. The main contributors to surface drainage at this location are: the Fum-ow Creek channel, the Bainbridge Road ditch and the DeArmoun Road ditch. The Furrow Creek channel in McCabe Subdivision East, immediately east of Knik Heights, is contained within a 25-foot drainage easement and is the main surface drainage collector for the subdivision. The Bainbt'idge Road ditch intercepts westerly sheet flow and ditch drain- a9e and routes the surface water to the channel. Photogr'aphs of the ditch taken last fall indicate that the existing water' flow is causing erosion in the ditch. Surface runoff fi'om the Bainbridge Road ditch contributes to the Furrow Creek channel and affects the drainage on Lots 3 and 4. The DeArmoun Road ditch is at least a half mile long and may be the largest contributor of surface water. Field observations indicate that lar9e quantities of water from this ditch reaches the Furrow Creek channel, but local residents commented that the surface water' contribution fi-om the ditch is not consistent. The drainage collector in Knik Heights Subdivision is the Furrow Creek channel. However, unlike in McCabe Subdivision East, the channel tra- verses subdivision lots without dr'ainage easements. Since less than fifty percent of the lots in the subdivision are developed, dr'ainage within the subdivision has not been a problem except on those lots where the drain- age channel lies. The historic Furrow Creek channel lies within Lot 3, Block K, and the development of that lot has interrupted the drainage in the channel. The developer of McCabe Subdivision East has constructed mitigation impt-ovements in Knik Heights Subdivision as shown on the attached map. A ditch was constructed along Ridgewood Circle from Lot 3 to Leyden Road, then along Leyden Road to Ridgewood Road, then north on Ridge- wood Road to a culvert under Ridgewood Road that outfalls to the Furrow Creek channel. A ditch along the common property line of Lots 3 and 4, Block K was proposed, but was not completed due to objections from the Municipal Department of Health and Fluman Services and the State Depart- ment of Environmental Conservation. These two agencies were concerned that the ditch might affect the septic drain fields on Lots 3 and 4. The D/6 -3- JlB~)NI~'8 state agency pointed out that the ditch might be classified as a creek and would then require lO0-foot setbacks for septic tanks and dt-ain fields. Lots 3 and 4 have been developed and each contains a single family resi- dence, well, septic tank and drain field. The improvements have been constructed within the past eight years. Residents of Lots 3 and 4 have sustained property damage as a result o¢ drainage through their property. D/6 -4h SECTION 3 - REVIEW OF PREVIOUS WORK The previous work found in the file consisted of two memorandums to the file from Tom Bacon, one dated December 20, 1985, and tile other dated February 4, 1986. Each of these are discussed separately as follows: December 20, 1985 Memorandum: This memorandum is a summary of a meeting between Tom Bacon and Mike Lewis of ADEC at which the drainage problem on Lots 3 and 4, Block K, Knik Heights Subdivision was dis- cussed. I offer the {ollowing comments. Comment 1: Para9raph 3 - Mr. Bacon and Mt'. Lewis a9ree "that the only expected surface runoff will be from snowmelt in the sprin9." Mt-. Lewis's concern "is that this runoff will saturate and cause the overflow of the existing septic draina9e fields." During the warmer months, surface runoff is absorbed into the porous soil and becomes subsurface flow. Apparently, this subsurface flow is insufficient to raise tile groundwater to the level that would cause surface flows in the drainage channel. During a snow melt event, however, the frozen 9round, ice and snow present an impermeable surface over which the snow melt waters flow. This impermeable surface prevents snow melt water from percolating into the 9round, and consequently, the snow melt water could not affect subsurface featur-es, such as septic drain fields. Comment 2: Number 1 on Page 2- The "A.C." pavement must have a proper subgrade to prevent differential heaving. On a 9ravel road, the A.C. paved ditch may be a maintenance problem. Comment 3: Number 2 on Page 2 - The proposed shallow bury 24" CMP would be difficult to maintain and would likely become blocked with ice. February 4, 1986 Memorandum: This memorandum is a summary of a meeting with the owners of Lots 3 and 4, Block K. Presented with the meeting summary were three proposed solutions to the drainage problem. I will confine my comments to these three alternates. Comment 1: Alternate 1, the pipe/ditch system is described in the Decem- ber 20, 1985 memorandum, and I have previously commented on the memo- randum. Comment 2: Alternate 2, routing the runoff to another drainage cell, is not viable. Comment 3: Alternate 3 is a significant underground system in tile DeArmoun Road right-of-way. Cost considerations aside, this option should not be undertaken without performing a drainage study of this area so that the pipe can be sized and located to accommodate future drainage needs of the lar9er draina9e area. D/6 -5-- SECTION 4 - ANALYSIS AN[) DRAINAGE ALTERNATES Furrow Creek Channel: Acco~'din9 to 'the information reviewed in prepara- tion of this document, the drainage feature identified as an existing Fur- row Creek channel should not be considered an active creek. This feature is a channel only and surface water is evident only for- short periods during the year'. Unless other evidence is found to the contrary, this feature should be classified as a seasonal drainageway. Without performing a more detailed draina9e analysis of tile subbasin, tile significance of this channel is unknown, but it appears to be of local importance for seasonal drainage, and the channel should be protected and maintained. Analysis: The ideal method for drainage in this area is to use the exist- ing Fum'ow Creek channel to route surface water. Since improvements have been constructed in this channel within Knik Heights Subdivision, and since the Knik Heights Subdivision design did not provide drainage easements for' the preservation of the channel, this method may be difficult to apply to this situation. Since the McCabe Subdivision East runoff quantities were not computed for this report, it is not possible to numerically assess the affect of the subdivision development on the drainage problem on Lots 3 and 4 in Knik Hei9hts Subdivision. It is likely that the development within McCabe Subdivision East has increased the surface runoff in the Furrow Creel< channel that lies within Knik Heights Subdivision. However, the large lots in McCabe Subdivision East and the rural nature of the development will produce only small inc,'eases in the surface runoff, according to similar drainage studies in the Municipality. The design and development of Knik Heights Subdivision has ignored tile existing natural drainage channel that lies within its boundary. This situation has created several problems for the subdivision. First, lots that contain the Furrow Creek channel, if developed, will have continual sea- sonal drainage problems. Second, development of property along the channel at a higher elevation than Knik Heights Subdivision will increase runoff concentrations in the channel and will affect pt'operty that contain the channel, such as Lot 3. Third, complete development of Knik Heights Subdivision will require a d~-ainage system, and if the existing Furrow Creel< channel is blocked and unusable, it will be a prohibitively expensive underground pipe system. Since tile design of Knik Heights Subdivision has ignored the channel, development of upslope property (such as McCabe Subdivision East) that use tile channel for' surface runoff will create drainage problems within Knik Heights Subdivision. Possible solutions to the dr'ainage problem on Lots 3 and 4 have been examined and are briefly discussed as follows. Solution 1: Route surface runoff to another drainage subbasin: Since the scope of this letter report is confined to the local area, this solution was not fully explored. Proper evaluation of this possible solution would D/6 require a detailed drainage study of a large surrounding area (500 to 1,000 acres). Solution 2: Surface runoff infiltration: Since most runoff in warmer months infiltrates into the soil and does not pose a problem, a possible solution is construction of drywelIs or infiltration galleries to route drain- age under'ground. Evaluation of this solution requires 1) a subbasin drainage study to provide estimated runoff and snow melt quantities, and 2) detailed subsurface investigations and testing to determine infiltration capacity at proposed facility locations. Infiltration facilities may affect other subsurface facilities such as septic drain fields. Solution 3: Route runo{{ with a pipe/ditch system: t\ pipe/ditch system to route the runoff within the cell and subbasin is likely the least expen- sive and most practical solution, Numerous combinations on pipes and ditches can be constructed that will satisfactorily solve the drainage prob- lem. My recommendations are presented in Section 5. D/6 -7- SECTION 5 - RECOMMENDATIONS Lots 3 and 4 Drainage The drainage system in Knik Heights Subdivision began by the developer of McCabe Subdivision East should be completed. Specifically, a shallow ditch with a 24-inch half culvert is recommended in the newly acquired 20-foot drainage easement along the common property line between Lots 3 and 4. The culvert should be integrally connected to the culvert under Bainbridge Road, and an ener9y dissipator should be constructed at the culvert outfall in the ditch on Ridgewood Circle. This solution will im- prove the drainage on Lot 3. The half culvert will contain the surface runoff and protect the adjacent septic drain fields without the problems associated with a buried conduit. Heat tracing in a metal conduit attached to the culvert is recommended, but since this area is not in a roads and drainage service area, the monthly electricity will have to be paid by tile residents on Lots 3 and 4 or by a Knik Heights Subdivision Homeowners Association. Knik Heights Subdivision Drainage Unless the existing Furrow Creek channel is preserved as the subdivision drainage collector, Knik Heights Subdivision will experience increased drainage problems as the subdivision lots are developed. It is recommend- ed that 25-foot wide drainage easements be acquired along the Furrow Creek channel. Approximate easement locations are shown on the attached map. Prior to acquisition, tile channel thread should be surveyed so easements can be accurately located and described. Private Development Review of McCabe Subdivision East Development of McCabe Subdivision East was reviewed by the Municipality, and the engineerin9 plans were reviewed and approved by the Department of Public Works. The shortcomings of tile review process are evident in the McCabe Subdivision East project, since a significant drainage problem was caused as a result of tile developer's drainage improvements con- struction. Development should be consistent with an approved area drain- age study, and tile Municipal reviewers should use the area drainage studies as a reference during their reviews. Unfortunately, a drainage study for this area has not been performed. The best method to insure that similar drainage problems do not occur in the future in areas where area drainage studies have not been performed is to require the develop- er's engineer to prepare and submit a local drainage study for Municipal review. Development can then proceed in accordance with this local drain- age study. Developers must be responsible for the downstream drainage effects of their development improvements and should construct mitigation improvements when necessary. D/6 -8- /<2 · · '1 Ye.' .i ......... ~:~. '!' b,:~.k.f.~.,?:::::: ..... H.~I!F: :Co I.v..'e:,t.t:::::: Recon~ended Drainage Im~provements Lots 3 and 4, Block K, Knik Heights Subdivision Hillside Drainage Study I strongly ~'ecommend the Municipality commission a drainage study of the developable hillside area. The absence of such a study likely contributed to the situation that resulted in the drainage problems described in this report. As more property is subdivided and developed in the hillside area, drainage problems will increase. Without a comprehensive plan that an areawide d~'ainage study provides, the Municipality will be poorly equipped to evaluate and direct hillside improvements. Respectfully submitted Jim Maclnnis, P.E. D/6 I:I&M CONSULTANTS, INC. 11/15 1 LOTS 3 AND 4, BLOCK K KNIK HEIGHTS SUBDIVISION DRAINAGE EVALUATION SUPPLEMENT -FO APRIL 9, 1986 LETTER REPORT APRIL 23, 1986 11/15 2 A. Aptfil 17, 1986 Field Investi9ations On April 17, 1986, Susan Metcalf telephoned and requested that I visit Knik Heights and observe the snowmelt surface runoff. I arrived at the site at 3:00 p.m. on April 17, 1986, and spend three hours observing, photographing and talking. My field observations are presented below: DeArmoun Road East of Bainbr!~ge Road The north DeArmoun Road draina9e lies in a well defined ditch until 20-30 feet from Bainbridge Road right-of-way where it forms braided channels. Flows in the ditch were estimated as 0.17 cfs with a velocity of 2.5 fps. Most of the braided channels travel overland through Lot 5 or in the east Bainbridge Road ditch to the culvert under Bainbridge Road near Lots 3/4 (hereinafter called Lots 3/4 culvert). A small percentage of the water- crosses Bainbridge, onto Lot 4 and sheet/ditch flows into tile ditch along the property line common to Lots 3 and 4 (hereinafter called Lots 3/4 ditch). DeArmoun Road South of Bainbridge Road DeArmoun Road has ditches on both east and west sides. The ditches are generally wide (10' - 15') and moderately deep (2' - 3'). Most ditches contained either small quantities of standing water or small, unmeasurable amounts of flowing water'. One west ditch near Bainbridge Road was blocked by a driveway without a culvert, and it contained 1'-1.5' of standing water. Ditch side slopes are composed of a sandy material, that when saturated, fails easily. Tract A, McCabe Subdivision East Tract A was observed from the DeArmoun Road right-of-way, and 20% - 40% of the property had standing water. Bainbridge Road Between DeArmoun Road and Bainbridg_e Circle The Lots 3/4 culvert under' Bainbridge Road was completely blocked, probably with ice. Runoff was overtopping the road at the culvert cation. A well-defined ditch exists between this culvert and the culvert under Bainbridge Circle. The flow in the ditch was estimated as 0.42 cfs with a velocity of 2.9 fps. Bainbridge Circle A small amount of runoff with a high sediment content was observed in the north ditch on Bainbridge Circle. The estimated flow in this ditch was 0.03 cfs at a velocity df 2.0 fps. Erosion and slope failure was found on the ditch cut slope, and a large spruce tree had fallen into the ditch. Tile culvert under Bainbridge Circle at Bainbridge Road was open. -1- 11/15 3 Bainbridge Road North of Bainbridge Circle The east Bainbridge Road ditch contained a large quantity of swiftly flowing runoff. The estimated flow in this ditch was 0.37 cfs with a velocity of 5.0 fps. The west ditch runoff continued behind the bollards blocking Bainbridge Road and flowed into tile culvert end at Bainbridge Ci~'cle; this ditch was contained by a dike behind the bollards, and the dike had been breeched at one location, allowing water to I'un across the driveway and onto Lot ]. Flow in the east ditch could not be measured, but is estimated at 0.03-0.0,5 cfs. Ditch Along Lots 3 and ,I Property Line Water running over' the culvert across Bainbridge Road travels in a ditch between Lots 3 and 4 to a ditch in Ridgewood Circle. Flow measurements in this ditch neat' Bainbridge Road indicated approximately 0.67 cfs in the ditch at a velocity of 3.3 fps. Approximately 30' before Ridgewood Circle, the ditch flow split into two forks; one fork continued directly to the roadside ditch, and the other fork meandered north across the front of Lot 3 for' ovel' fifty feet, then into the east ditch on Ridgewood Circle. Rid~gewood Circle from Lot 3, Block K to Leyden Road The water' fi'om tile Lot 3/4 ditch is contained in the east ditch along Ridgewood Circle until it reaches the driveway to Lot 2, where a partially or fully blocked driveway culvert diverts 30°6 - 40% of the ditch water onto the road surface. A snow berm on the Lot 2 driveway prevents water from travelling over the driveway. The driveway culvert for Lot 1 is sufficiently open to allow the ditch water to flow through it to tile north ditch on Leyden Road. Leyden Road from Ridg_ewood Circle to P, idcjewood Road Where the ditch water' from ~he Lot 1 driveway culvert curves into the north Leyden Road ditch, the cut slope is saturated and is failing. It appears that this slope was not stabilized after ditch construction. Water in the north Leyden Road ditch continues until it reaches the Lot 10 driveway culvert; this culvert is blocked and water runs over the drive- way. Since the driveway and house on Lot 10 are lower than Leyden Road, some water runs down the driveway and onto Lot 10. The resi- dents, tile Baxtel"s, were using hand shovelled berms of slush and snow in an attempt to keep tile water from entering their property. Flow on the Ridgewood Circle road surface continued to the Leyden Road surface whel-e it continued downhill the road to Ridgewood Road. ~Ridgewood Road from Leyden Road to Culvert Crossin~ The road surface and ditch water together flow into the ditch on the east side of Ridgewood Road dowel to the culvert crossing under Ridgewood Road. Flow from the natural channel on Lot 10 may contribute a signifi- cant quantity of water to this ditch but it is blocked by a berm -2- 11/15 4 constructed with the roadside ditch. Consequently, several hundred square feet of this natural channel was flooded. Tile culvert under Ridgewood Road was open and water continued down the natural drainageway from this culvert. B. COMMENTS ON FIELD INVESTIGATIONS Comment 1: The major contributor to the runoff reaching Lots 3 and 4 originates on Bainbridge Road above Bainbridge Circle. The flows estimat- ed from field measurements are as follows: Flow Percent Bainbridge Road E E- W ditches (North of Bainbridge Circle) Bainbridge Circle DeArmoun Right-of-Way Sheet Flow (estimated) 0.40 cfs 60% 0.03 cfs 4% 0.17 cfs 25% 0.07 cfs 11% Total (at Lots 3 and 4) 0.67 cfs 100% The large flow from Bainbridge Road is surprising. Mrs. Baxter, the resident on Lot 10, stated that tile improvements to Bainbridge Road that provided access to Bainbridge Circle has caused the problem on her lot. She claims that cutting the road down, and installing culverts has in- creased the water reaching her property. Evidently, the Bainbridge Road ditch is intercepting surface sheet flow that would otherwise flow overland in small concentrations to the natural drainage features. Comment 2: The drainage improvements in Knik Heights subdivision operated well considering the system deficiencies. The problems encoun- tered during the field investigations could be satisfactorily solved with completion of the system and annual maintenance. The problems I ob- served, recommended solutions, and estimated costs are presented below: a) Problem: Slope failure along Bainbridge Road and Bainbridge Circle._S°lution: Re-evaluate material stability under saturated conditions such as exist during spring break-up. Cut slope back as required. Re-install erosion protection matting and hydro-seed slope early in the season to encourage a mature growth by fall. Estimated Cost: 1,000 s.f. x $5/s.f. : $5,000. b) Problem: Dike failure on west ditch Bainbridge Road. Solution: Reconstruct dike for approximately 100', deepen ditch, and hydro-seed ditch and dike to encourage a stabilizing vegetative mat. Estimated Cost: 100 I.f. x $30/I.f. = $3,000. c) Problem: Frozen culvert under Bainbridge Road. Solution: Thaw culvert using existing thaw-steanl pipe at begin- ring of break-up. Estimated Cost: Annual Maintenance $300. -3- 11/15 5 d) Problem: Water in Lot 3/4 ditch meandering out of channel. Solution: Install ditch with half culvert and heat trace as recommended in the previous letter-report. Estimated Cost: 190 I.f. x $60/I.f. = $11,400. Annual Electrical Useage = $300. e) Problem: Blocked driveway culverts diverting water out of ditch. Solution: Install largest culvert as practical - preferably a 24" diamete~t CMP with provisions for ttaawing (heat trace, thaw wire, or steam pipe). Thaw culverts, if froze, at begin- ning of break-up. Estimated Cost: 3 each x $1,000/ea = $3,000. Annual Maintenance $300. f) Problem: Roadside ditches blocked with snow and snow berms keep water from reaching ditches. Solution: Grade berms down and pull snow out of ditches with road grader at beginning of break-up. Estimated Cost: Annual Maintenance $150. COST SUMMARY Item Capital Cost Annual Maintenance Cost a) $ 5,000 b) 3,000 c) $ 300 d) 11,400 300 e) 3,000 300 f) 150 TOTAI_S $ 22,400 $ 1,050 Annual maintenance costs may be higher or lower depending upon break-up conditions. To pay fo~' these annual costs, I recommend that the residents form a Local Road Service Area or a Homeowners Association. Comment 3: The effects on downstream properties of channelizing and improving the drainage in McCabe Subdivision East and Knik Heights have not been evaluated. One disadvantage in closely examining a portion of a drainage system, such as Knik Heights, is that downstream drainage is largely ignored. For the same reasons entire sub-basins are examined during a drainage study, the downstream effects of the concentrated surface runoff must also be examined. C. DeArmoun Road Drainage Route Alternate At Susan Metcalf's request, I have examined and p~-epared a concept plan for routing t'he sub-face drainage east of Bainbridge to the DeAr-moun Road right-of-way. The enclosed plan and pro{lie was drawn using the 1"=200' scale, 4' contour interval maps and must be considered a concept plan only. A detailed route survey is required prior to preliminary design. The plan routes surface runoff from tile Furrow Creek Basin to the Rabbit -4- 11/15 6 Creek Basin shown on the drawing is 660' of 24" CMP with two manholes, 1,760' of ditch improvements and 900' of erosion resistant ditch lining. The 660' of underground pipe poses no technical problem. Crossing DeArmoun Road will require coordination with tile State DOT/PF. The DeArmoun right-of-way is large enough to adequately contain the ditch improvements. The east DeArmoun ditch rather than the west was chosen because of the outfall destination is Rabbit Creek. The existing DeArmoun ditch was observed during the April 17, 1986, field investigations to have very Iow flows. Although the ditches are sized to accommodate the addi- tional 0.5- 0.7 cfs, the ditch side slopes are composed of a silty-sand that is highly erodable. In addition, driveway and side road culverts must be installed or increased in size with thaw pipes. Approximately 900' of ditch has a slope greater than 706 and erosion protection must be in- stalled at those locations. The outfall will be at Rabbit Creel< through W-A Subdivision. A drainage easement must be acquired t:o the creek, and a water quality control device installed neat- the outfall to protect Rabbit Creek. A cost estimate for this work is presented below: 660 L.F. 24" CMP @ $120/L. F. 1,760 L.F. ditch reconstruction @ $20/L.F. 900 L.F. erosion protection @ $25/L.F. Misc. culverts and thaw pipes Drainage Easement (3,000 s.f.) Water Quality Facility and Outfall $ 79,200 35,200 22,500 7,500 6,000 30,000 Subtotal $ 180,400 10% Contingency 18,040 Total Construction Estimate 30% Engineering S Administration $ 198,440 59,532 TOTAL PROJECT ESTIIvlATE $ 257,972 D. April 23, 1986 Field Investigations On April 23, 1986, at 10:45 a.m., I again visited Knik Heights to observe drainage. The air temperature was 25°F- 30°F, the sky was overcast, and approximately 2" of new, wet snow lay on tile ground. Ditches and drainageways had frozen over-. Breaking through the ice cover, I ob- served ~ - ~ standing water' and a barely discernable flow (less than 0.01 cfs) in all ditches. Tile observations during this field visit support the "seasonal drainageway" concept for the Furro~ Creek channel. When the air temperature rises, additional runoff will be generated and similar conditions as observed on April 17, 1986, are anticipated. -5- 13/9 1 FURROW CREEl( DRAINAGE SYSTEM ANALYSIS A Second Supplement to The April 9, 1986 Letter Report Titled Lots 3 and 4, Block K Knik Hei9hts Subdivision Drainage Evaluation June 3, 1986 13/9 2 Introduction The information in this second supplement was requested by the Public Works Department as a result of discussions and meetings on the original letter report and first supplement.- The topics discussed in this supple- merit are: o Furrow Creek Basin Hydrology o West Extension of Knik Heights Drainage o Onsite Waste Disposal Systems near Furrow Creek Channel The information in this supplement complements the work previously sub- mitred and is intended to be read with the original letter report and first supplement. Furrow Creek Basin Hydrology The previous drainage study performed by URS Engineers was completed in February, 1983. The study boundary ended east of Bragaw Street. Using Municipal 200-scale 4-foot contour topographic maps, I determined the Furrow Creek basin boundary as shown on the Furrow Creek Drainage Topographic Map. The area neat- Hufflnan Road and Birch Road shown outside the basin boundary may be included in the basin, but a more detailed study must be performed to make that determination. This bound- ary extends approximately one mile east of the URS boundary. Twen- ty-three cells were defined within this basin boundary consisting of proximately 420 acres. The included 500' scale map is the same scale as the URS study map and can be considered an extension of the previous work. Much of the cell drainage east of Bainbridge Road is to local depressions. Runoff collects in these depressions and water both infiltrates into the soil and evaporates. A groundwater connection between these depressions may exist. Subsurface contributions from these depressions to lower elevation channels is also likely. Estimating the effects of runoff in this portion of the basin would be very difficult without subsurface investigations to determine groundwater parameters. The area west of Bainbridge Road consists of natural channels interrupted by roads and man-made ditches. One of these channels has caused drain- age problems on Ridgewood Circle, Leyden Drive and Shelburne Place. These problems have been discussed in the Letter Report and Supplement 1 and are summarized in the next section. Runoff in channels and ditches is seasonal and lasts only a few weeks each year, 9enerally during the spring snowmelt event. The soil is generally a silty, sandy gravel that allows most surface runoff to infiltrate into the soil. Lar9e flows during spring snoGmelt events are likely caused by frozen soil conditions that prevent infiltration. -1- Furrow Creek Cell Areas dON1 5/19/86 Planirneter Computed Computed ..c...e:.!.!....L.....A....r..?...~...!.~..q..!~..! .......... 6!:.~.~...!.~..~..r..!..! .......... ..A.!:?:..~....(..o.~!:..e...}... FC-2 11.971200 478,848.00 10 99 ...[.gu~ ................ 5~.~.~.~.~ ................. !..g!..~.~.~.~.~.~ .............................. ~.~.~.! ..... FC-4 17.938702 717,548 Og 18,47 FC-5 9,258885 370,347,40 8 50 .... ~.g:.~ ............. t.~.~..~.~.! ................. ~.2~.~.~..!..~.g~ ............................ !.5,..~.~... FC-? 13.950027 558,001,08 IZ,81 .... ~.~.~ ............. ~.~.%~.!.~ ........... !..~.7.!.~.~.~.,..~.~ ........................... ~.,..~.? .... ..... E2U? ............. ~.Ld..!.§.~..!.~ .......... b..~.~.5,..!.!!.~.!! ........................... ~.~... ..~.~z!..~ .............. ~.~.~.~!~.~ ................. ~.!.~.,..~g~.~.!~ .............................. ~.,..~!... .E2u.!..~ ............ &!..~.~.~.~.~.~ ................ ~.,.~.~..!..,..7~ ........................... ~2~..~.~... ...E2~..[.~ .............. ~.,..~.~.~.t.~ ................. ~.~2~..~.~2,..~.~ .............................. ~.,..~.!... FC:-14 8.408879 258~287.17 5 88 FC-15 27.01138? i 1~080~455.48 24.80 ~c:j.~ .......... ~.,..Z~.~.~ ................ ~.!..!..,..~..!..,..~ ............................ ~.2,..~ .... ..[.~.:J..~ ........... ~.,..~!~.~ .......... !..,...!..~%.~.~.~ ............................ ~.~.,.d.~ .... FC-18 37,370573 . 1~494,822.92 34.32 .f. gu!.~ ........... !..5,..~.~5.~ ................ ~?.&,.~.g!..,...!..~ ............................ !.~.~.!..~ ..... FC-ZO 9.739188 38q~587,44 8 94 FC-21. 6.45319~ .......... ~}.~.,..[.~.~.~ ........................... ~.......f... FC-22 50.878300 i 2,035.052,00 48 72 ..[gu~ ............ ~.~.,..~.~ .......... !..,.~.,..~.~.,.d~ ............................ ~.,..~.~ .... 456,842779 i18~273~711,16 419.51 ~) LD BRAGAW Si ATHERTON ROAD I ) F\) (N CO (~ i// / / , i ';4-'- ".." - V:-p' DRt\ 13/9 3 This brief hydrologic analysis and accompanying maps were prepared with limited time and resources and should not be considered a complete drain- age study. Further work and detailed analysis is necessary in this basin. West Extension of Knik Heights Drainage At the Mun'c'pality s request, I extended my detailed examination of the drainage on Lots 3 and 4, Block K, Knik Heights wests of Ridgewood Drive to Br'agaw Street. The surface drainage collects in ditches along Ridgewood Drive and in the old drainage channel on Lot 10, Block H, Knik Heights and 'Flows under Ridgewood Drive in a culvert to the southeast corner of Lot 8, Block G, Knik Heights. Water flows diagonally across Lot 8 in braided channels and becomes better defined as it passes neat- the lot line common to Lots 4 and 5, Block G. RunoFf then was found flowing across the road in shallow swales in Shelburne Place. Seasonal drainage problems have been reported by residents living on Shelburne Place. The water' is collected in the east ditch along Shelburne Drive. During high flows, water' crosses Shelburne Drive to the west road side ditch, and caused the owner of Lot 19 to sand bag his driveway entrance to prevent flooding. Ditch water is routed south several hundred feet west through a culvert to a depression in the Leyden Drive right-of-way where the water is retained. This depression acts as a natural detention basin. When water in this depression collects to a sufficient level (several feet), the water flows northwest through Lot 19 in the surface vegetative mat. A well defined channel is not present. Following this water on May 1986 northwest through Lot 19, the channel and water blended with the existing vegetation and could not be found after- travelling approximately 100-200 feet. Continuing northwest across Atherton Road (several Iow areas on this road, but no cross culverts) toward Bragaw Street in the appa~-ent channel, water was found ponded on Lot 2, Block 2, Knik Heights West Subdivision. On Lot 2, a large home has been built in the lowest point on the property, and on May I a pond was observed in the back yard. In the pond was a pump, and a 1½" hose cam-led water from the pump to a ditch along the driveway in the front yard. The pumps was on and a large volume of water was obser-ved exiting tile hose in the ditch with no apparent decrease in the level of the pond. Water in the ditch travelled approximately 10 feet before infiltrating into the soil. It is likely that much of the flow northwest from Lot 19 is subsurface. I suspect that excavation and foundation construction on Lot 2 interrupted tt~is subsurface flow, causing the water' to pond on the surface. lVly recommendations for drainage improvements are shown on tile Furrow Creek Channel System Improvements table. Each improvement is described below: -2- Furrow Channel Sy,$tern Im.p. rovements: ,.8.~ j.!)...b..!:.!.~,g.~, ..~.9,g.~...tg.. ~.b.~.[~.~,C.~3.~...Bg.~ .................... ~ ................................................... i ....................................... ~,la~ ~1, 198~ JDM Cost Ea~Umate ........................................................................................................ ~....~. .............. J. ........... , ....................................... ~ ....................................... Improvement ~64uant~ty 6Dst/Unit ~ Total Cost U ................ [ .................... .... bet,../een Lots 3/4 Knik Hqts ....................................................................................................... ........................................................................................................ ~ .................... ~ ........... , ....................................... ~ ....................................... nstall Z4" Orive,,/a~f Culverts ,#ith~ 3 ~EA $1,000.00 ~ $3~000.00 I h~'.~, Protection Ditch Stabilization- Ridqewood ~ 800 iLF ~15.00 (~12, 000. 00 .m,~den , .~ .... } ...................................... ~ ....................................... ........................................................................................................................ ~.~.~.~.~.t~s~.~.t..5~ b~.[~:~.r:.~.~..?.~..~.~.~...?d.t~ ......... ~.~.~....~.[ .................... :~.~.9..~..~.~....}.~..[.~.,..~.~.~.,..~.~ .... ~che~/culverta/tho,~/ ~rotect~on ........................................................................................................ ~ .................... ~ ........... ~ ....................................... ~ ....................................... DitchConstractionalonqeast ~ 260 {LF ~ ~20.00 } $5~200.00 ';'~'~'~"'~'~';'f~'~iT;{~'"~'~;'fi'h ........................................... ~ .................... ~ ........... ~ ....................................... ; ..................................... : ........................................................................................................ , .................... ; ........... , ....................................... ~ ....................................... ~.~.o.~.~.~:.s~.~.L~.~.~.~.t~.~'.~.~.~.~.n.t~.0 .............. :. .............. L..}~[.,~, ..... [.5..,.~.~.~..,..9.~....?~.~..~.,..9~..,..0.~ .... Pond '~dth Dry?' Well Total Estimated Construction Cost~ ~ ~ ~$81,750.00 ............................................................................. ~ ......................... ~ .................... ~ ........... ~ ....................................... ~ ....................................... ~.~.~.Y~.~.~.~.~.~.~.~. .................................... E'~qineerina. Overhead. Etc. ~ ~ ~ 35 0C'/~$23,773 7~ ................................ ,.~ ................................ ~ ................................ ~ .................... ~ ........... ~ .................. ~......~ ........... ~: ........................... %...~ ..... ........................................................................................................ ~ .................... ~ ........... ~ ....................................... ~ ....................................... Total Estimated Proiect Costs i ~ ~ ~$91.698.75 ........................................................................................................ 4- 0 13/9 4 Install half culvert and ditch between Lots 3/4, Knik Heights - this improvement has been discussed in the Letter Report. Install 24" driveway culverts with thaw protection - this improvement has been discussed in the Letter' Report. Ditch stabilization along Ridgewood and Leyden has been discussed in the Letter Report. this improvement Reconstruct Shelburne Place with ditches, culverts and thaw pro- tection Shelburne Place is a narrow, poorly constructed road. Drainage improvements must be made in conjunction with road im- provements for' a permanent solution to the drainage problems. A ditch along the south side of Shelburne Place to Shelburne Road is recommended. Ditch construction along east side of Shelburne Road - the existing ditch must be deepened and graded, tile existing culverts cleaned, and thaw protection installed in the culverts. Construct detention/retention pond with dry well in the Leyden Road right-of-way construct a 3,000-4,000 square foot detention/ retention pond with a dry well. The pond will keep water off adja- cent private property. The dry well will assist with the natural soil infiltration. The total cost for these improvements is approximately $92,000. Onsite Waste Disposal Systems near Furrow Creel< Channel As requested by the Municipality, I have examined tile properties along the Furrow Creek drainage channel from Bainbridge Road to Shelburne Road as shown on the attached map. This information was compiled from Health and Human Services Department records. The affected zone is 100 feet each side of the channel. The lots that have onsite septic systems within the affected zones are: Lot 1, Block K, Knik Heights Subdivision Lot 2, Block K, Knik Heights Subdivision Lot 3, Block K, Knik Heights Subdivision Lot 4, Block K, Knik Heights Subdivision Lot 10, Block H, Knik Heights Subdivision Lot 5, Block I, Knik Heights Subdivision Lot 7, Block I, Knik Heights Subdivision Lot 8, McCabe Subdivision The lots that have onsite septic systems very near or partially within the affected zone are: -3- im !m ::i ; I E] 13/9 5 Lot 1, Block J, Knik Heights Subdivision Lot 4, Block J, Knik Heights Subdivision Lot 5, Block J, Knik Heights Subdivision Lot 4, Block I, Knik Heights Subdivision Lot 6, Block I, Knik Heights Subdivision Lot 8, Block I, Knik Heights Subdivision Lot 20, Block F, Knil< Heights Subdivision I strongly recommend that the Public Works Department work to obtain a change in the active creek classification of this d~'ainageway. The portion of the Furrow Creek channel described in this document contains water only a few weeks every year and can not be reasonably classified as an active creek or stream. This channel is no more active than hundreds of ditches and channels throughout the Anchorage area. Acceptance of this classification with the accompanying 200 foot wide affected zone may create a pI'ecedent that could affect hundreds or thousands of Anchoi'age property owners who live within 100 feet of a seasonally active ditch or channel. I::i&~Vl CONSULTANTS, INC,