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HomeMy WebLinkAbout989600020 Drainage Report';'· ~ NORTHWESTERN TERRITORIES, INC. Engineers · Land Surveyors · Planners Construction Coordination · Materials Testing ii "-:/~ , ~.. ...... ........... :~:~. May 19, 1999 _ Sue Theiss ' ..... Jefferson Oounty Permit Oenter 624 Sheridan Street Po~ Townsend WA 98368 Subject: Prospect View Estates- Drainage Dear Ms. Theiss: The developers of Prospect View Estates have made roadway drainage modifications to various streets as discussed with Jefferson County Public Works Department personnel earlier this year. These activities, along with-similar work to be performed by another land owner to the west, will substantially redirect the street drainage south of Prospect Avenue and east of Elston Avenue. The net result is to reduce the water currently reaching the bluff area by an estimated 75%. This work is consistent with the recommendations contained in the May 18, 1999, Geologic Report prepared by G. W. Thorsen, Consulting Geologist, which is attached hereto as an appendix. Storm water had been flowing southeasterly to ElSton Avenue soUth of Cascade Avenue and onward to the bluff where it had-caused some erosion south of the Prospect View Estates project. The installation of Jefferson County standard ditches and culverts will redirect the runoff to a more natural southerly direction for sheet flow disbursal within an unopened county-road right-of-way along the more gentle slope towards Chimacum Creek. The attached drawing illustrates the flow routes. This will allow natural biologic water treatment such as a bio-swale and, ultimately, the water will be disposed of by absorption. It is anticipated that this drainage revision will not only reduce the potential for bluff erosion, but also improve the vicinity's water quality by providing biological filtering. However, as a "safety net" if erosion is experienced, there is sufficient area in the un' opened right of way of Main Street between Elston and OlYmpic Avenues to install a sufficiently sized infiltration gallery (dry-well) for the disposal of the roadway runoff. 717 SOUTH PEABODY, PORT ANGELES, WA 98362 (360) 452-8491 1-800:654-5545 FAX 452-8498 Sue Theiss Jefferson County Permit Center Prospect View Estates May 20, 1999 Page 2 of 2 Infiltration of storm water in ditches, ponds, dry wells or infiltration galleries is the preferred method of disposal identified in' the Department of Ecology'S Storm Water Management Manual as this method provides local recharge-of aquifer Systems. It is anticipated that each new home constructed will have its own on-site storm water disposal system (most likely infiltration) and that the roadway ditches will carry only the roadway runoff. It is, therefore, my recommendation that time be given to observe the operation of. the ditching over time and as homes are developed in the region to ensure that the system is operating as expected before installing an infiltration gallery that may not be needed. If you have any questions or comments on this issue, please don't hesita[e to call me. Sincerely, NORTHWESTERN TERRITORIES, INC. Robert A. Leach, P.E., MBA Senior Civil Engineer RAL:eas I EXPIRES C: Gene Seton Jeff Ingman G. W. Thorsen, Consulting Geologist 1926 Lincoln Street, Port Townsend, WA 98368 (360) 385-6002 Jeff lngman May 18, 1999 23611 Glen Allen PI. Mt. Vernon, WA 98274 Subject: Lots 17 - 21 Prospect View Estates just south of Prospect Avenue and Kala Point in Jefferson County. Also, a single building site composed of lots 20-24 of the old Irondale Block 161, adjacent to and north of the end of Prospect Avenue (see accompanying plat map). Purpose and scope: This report is to help provide guidance for the residential development of the lots described above. It is also in support of the 50-foot setback for construction on those lots. Lots 17-21 were given such a variance by the county commissioners on April 24, 1989. (County staff have asked for an update on the geologic report upon which that variance was based.) Geologic input for the report consisted of a review of the old report and available maps and literature (see Selected References). New field work consisted of examination of exploratory pits (not available in 1989), another recon of the bank (see Profile), and photographing the site from the uplands, beach, and the air. Site description: The subject lots include about 800 feet of uplands along an easterly facing bank just south of Kala Point (see area map). In general, the uplands are planar, with average slopes of about 3-4 degrees (5-7 percent) to the southeast. The bank averages 40-43 degrees, including a near-vertical segment as high as 15 feet fronting lot 2 I. This steeper segment is lower or missing in places to the north. Total bank height ranges from about 100 feet at the south lot (#21) to about 110 feet on the north. The bank is largely underlain by sand and pebbly sand of glacial outwash origin (Grimstad and Carson, 1981). This unit was compacted by the last ice sheet, here about 4,000 feet thick. (Thus, it is quite resistant to root penetration.) This lower 80 feet (or so) or bank height is largely obscured by a wedge of colluvium that tapers from near zero in places at the top to as much as 8-10 feet thick along the upper beach. Exploratory pits and, in places, the upper bank expose a glacial drif~ made up of gravelly, silty sand. It appears to be of late glacial origin as it is not as compact as a "tree" till (hardpan"). Nevertheless, its well-graded nature permits it to stand in vertical banks. This description and interpretation is in general agreement with soils maps showing "Hoypus gravelly sandy loam" in this area. McCreary (1975, p. 25) further describes this soil as: ... somewhat excessively drained. Permeability is rapid. Roots penetrate to a depth of more than 60 inches. The soils holds 2 to 4 inches of water available to plants. Runoff is slow to medium, and the hazard of water erosion is slight to moderate. Slope stability: The bank here was cut by wave action since sea level stabilized about 5,000 years ago. Longshore drift later formed the spit at Kala Point. The spit and the easterly aspect of the bank both serve to limit wave action at the site. Wave erosion from winter southeasterly storms is also limited by the rather short (2 mile) fetch in that direction. Natural erosion at the site now consists largely of creep and localized debris avalanching (aerial photos A and B) of the colluvial soils that overlie the glacial sands. The role of wave action in recent centuries is merely to remove creep and avalanche deposits from the upper beach where they would otherwise accumulate and buttress the toe of the bank. The natural triggering mechanism for debris avalanching is generally an unusually intense rain storm or rain-on-snow event. The age of the alder in one slide scar suggests that it probably occurred during the New Year's Day 1997 rain and snowmelt that triggered shallow avalanching throughout Puget Sound shoreline banks. ic'.:~{d~rr~e~ dsliding ~waS seen.,. ~ch erosional forms tend to leave telltale signs, such as step-like topography, a deeply scalloped bank edge, or a sloping bowl-shaped "amphitheatre" on the bank. A broad scallop with a chord of about 15 feet was seen at lot 18, but the lack of a scarp and the presence of fir to 12-inch diameter on the bank suggest that this is either a very old feature or developed gradually. Seismic im acts on such materials are not apt to change current modes of erosion · P.... ' (springs, horizons of seepage)~.~...~t (concentrations of water-loving vegetation) s, the potential for quake-induced ground failure"~uch:~t~"~$11 ~-¥'a~i0n,- ~-eb~s i~em;tel ~ofll :the 1946 Vancouver Island and the 1965 Seattle earthquakes were strongly felt in the Port Townsend area, but there were no reports of soil failures. This was in spite of the fact that there are miles of shoreline banks in the area that are more susceptible to such triggering than here. Discussion: We have been asked to address the addition of septic waste water on building sites. (It should be noted that this will be "imported" water, not on-site well water simply being transferred from one depth to another.) This water will be in part offset by the street/ditch storm drainage system. That system, upslope from the subject lots, should intercept and "export" some of the surface runoff' that might otherwise reach the lots during extreme rain and/or snowmelt events. In regard to long-term day-to-day septic infiltration, it is important to consider potential impacts in the context of local soils and precipitation pattems. Grimstad and Carson (1981, p. 31, attached) show an annual "water budget" for nearby Port Townsend. Note that here there is a theoretical water surplus (available for recharge or runoff) only 5 weeks of the year (in contrast to nearly 6 months at Quilcene). The rest of the year there is a soil water deficit and/or soil moisture is being "recharged". Research in the Spokane Valley (Crosby and others, 1971), where mean annual precipitation is slightly less (17.5 inches) than at Port Townsend (18.3 inches) produced some interesting results in regard to possible ground-water recharge by drainfields. (Glacial outwash subsoils there are similar to soils at the subject site.) The researchers, drilled through drainfields at a nursing home, a trailer park, schools, and a commercial dairy, all sites receiving effluent at much higher rates than a single-family residence. In each situation, drill holes encountered dry soils at depths of 40 feet or less below the drainfields. The interpretation of the researchers was that there is a "capacity of the outwash deposits to retain much more water than is characteristically present" (Crosby and others, 1971). They explain the lack of long-term downward progression of saturation on such retention and subsequent evaporation to the atmosphere. They also speculate regarding possible "lateral dispersion" of soil moisture. This potential would be enhanced by well-stratified subsoils (not present at this site). ~:eiia~ ~t ~fi~d.~em~-co~td r~h ,~e~ b~: ~d: have :a ? Conclusions: The setback variance of 50 feet granted by the Jefferson County Board of Commissioners (April 24, 1989) should be ample to protect homes well beyond their useful lifetime. The same setback distance seems appropriate for the nearby lot north of Prospect Avenue (see plat map). Recommendations: · Maintain and/or restore a 10-foot buffer strip of native vegetation such as salal along the bank edge. This can be trimmed low (2 feet) where necessary to enhance the view. · Minimize the disturbance of bank vegetation. Selectively limb (rather than top) trees where necessary for view enhancement. Control the blackberry fronting lot 21 as it will compete with desirable species. · Minimize the total area of impervious surfaces (e.g., decks, patios, and paths). All such areas should be designed to "leak". · Design drives and parking areas to disperse rather than collect runoff by employing techniques such as crowning, outsloping, grooving, or water bars. · Disperse roof runoff as far from the bank as possible. · Emphasize low-maintenance, drought-resistant species for landscaping to minimize the need for watering. · Monitor the maintenance of street ditches to ensure that runoff is not imported onto bank lots from upslope areas. Gerald W. Thorsen, C. P. G. 1525 · . · eeese eee ........ · ~ Selected References I~'- ~......_X~......:_~._,,.,/ Crosby, J. W., III; JoMstone, D. L.; Fenton, ~o~I~l.~-~on ofpollu~ts ~ a glacial outw~h enviro~ent, [P~] 3' Waer ResoumesKes~rcn,~, [~. 713-720. G~ms~d, P~er; C~son, R. J., 1981, G~logy ~d ground-waer resour~s of e~tem Jefferson County, W~n~on: W~h~on Depa~ent of Ecology Water Supply Bulletin 54, 125 p., 3 plates. McCrea~, F. R., 1975, Soil su~ey of Jefferson County area, W~n~on: U.S. Depa~ent of Ag~culture, Soil Conse~ation Se~ice, 100 p., 70 plates [sheas]. W~hin~on Depaament of Ecology, 1978, Co~l Zone Atl~ of W~n~on, Volume 11, Jefferson County: W~hin~on Depa~ent of Ecology, 10-p. text, 16 plaes, expl~ao~ maefials. ~~~ i ,~, 3 ' 5320 Had Vicinity map from the U.S, GeolOgical Sumey ~ ~ ' Po~ Townsend South quadr~gle (1 in. = 2,000 R; ~ ~ ( ~ contour ~te~a120 feet). Yellow da~ poims to ~e ~ ~ end of Prospect Avenue (see plat map). Adapted from .u,at ,,~-,av ~ vf ^ Prospect View Estates ; ', .... ~--; ~-:. ..........Subject lots enclosed by green line. ' .... , % i '. i ~ Approx. top of bank ____ CascadeAve.'J I . ;'~2, S~ope angles are averages. · PORT TOWNSEND " CAPACITyWATER'HOLDINGoFsOIL 2"I 6" From'. Grimsted and Carson PRECIPITATION 18.3 IPOTENTIAL EVAR3- 25.2 !TRANSPIRATION WATER SURPLUS 3.9 I 0.6 m '+'l ACTUAL EVAPOTRANSPIRATION\ ,,i I o / .~ PRECIPITATION '"'- ...........1 - .................... ~ .... SOIL MOISTURE UTILIZATION -I--~N I FEB I MAR I ApR' I MAY I aUN I JUL I AUG I SEP I OCT I NOV I DEC~ O QUILCENE 2 SW , = WATER HOLDING 2"I6" CAPAOTY OF sO! L PRECIPITATION 50.0 POTENTIAL EVAP(> 25. I TRANSPIRATION ACTUAL EVAPO- 17.4 12o.$ TRANSPIRATION WATER SURPLUS 32.6! 29.7 z ~ J~~ ~A~TRAN SPIRATION~ ~ ~ ~ ~ ~ ~ ~[~::~[~::~[~[::~ ~ ~~:::::::~ ~' ~ ~ ~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~ , ·, ~ ~ ~, v, ~ :::::::::::::::::::::::::::::::::::::::~:~:~:~:~:~:~: v ' i N~ II .:: · ,> 4- ':' · · ':<' - + ':' · ':':':<':':':':':': SOIL MOISTURE UTILIZATION -- ~--'~':~':'~:~ O-I ~AN I FEB I ~AR i APR I MAY I JUN I JUL I AU~ I S[P I OCT ! NOV I DEC Figure 19. MEAN ANNUAL WATER BUDGETS. 31 Photo A. The bank fronting lots 21 (left) to 18 (right). The shallow debris avalanche in the center was reportedly triggered accidentally during land-clearing operations. Older avalanche scars to the left apparently were natural (triggered by rain and/or snowmelt). Yellow darts mark common reference point in panorama. Photo B. The bank fronting the intersection of Prospect and Olympic Avenues (left to right). Note that the southern end of Kaia Point spit in the lower right is marked by drift logs. Lots 17 and 18 are on thc ie~. The ciearcd area right of the end of Prospect Avenue is the unnumbered lot included ~ner~';" (see plat n-~ ~ap~.~ ' Photo C. View to the southeast from the northwest comer of lot 17. Olympic Avenue is on the right. Maximum upland slopes (lots 18-21) average about 3 degrees (5 percent). The forest in upper right is the south boundaD, of Prospect View Estates. Photo D. View to thc northeast from the intersection of Prospect and Cascade Avenues, Slope is about 5 percent. The woodland on the lea is thc no~h bounda~: of this unnumbered lot north of Prospect Avenue (see plat map). · , ' Photo E. Note the asymmetry of the trunk and root system f Had this fir matured on a flat upland surface its trunk would be straight and there would now be roots jutting horizontally into the air. The tree's shape and root form indicate that it ~ seeded and grew near the bank edge, suggesting rather slow erosion. Photo F. Note the large ~flying buttress" root in this bank-edge 15- inch fir (more than 5 0 years old). This root developed in col!uvial soil (now largely eroded) along the face of the bank. Its exposure now suggests a long-term erosion rate on the order of 1 inch per year.