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HomeMy WebLinkAboutPettygroves 2nd Block 106 - Geology and Potential Bank Stability Impacts - 1993.04.00GEOLOGY AND POTENTIAL BANK STABILITY IMPACTS due to PROPOSED DEVELOPMENT OF MARZAI\ PROPERTY PORT TOWNSEND for City of Port Townsend Randall Brackett, City Engineer 5210 Kuhn Street Port Townsend, Washington 98368 by G. W. Thonen & Associates C,onsulting Geologists 1926 Lincoln Street Port Townsend, Washington 98368 April 1993 INTRODUCTION As I understood my assignment, it was to evaluate the potential impacts of the construction of six single family iesidences on the block between R and S Streets, east of Walnut, oD the stability of nearby shoreline banks. The following report is based on our March 3L recon of the upland and beach areas, a recon of the bank face on April 6, and previous examinations of banks in the area since L988. It is atso based on experience gained from geologic mapping and landslide investigations of Puget Sound shoreline bluffs since l-968. (Sorne of the observations from that work are summarized in the attached article). Obviously, site-specific date are somewhat incomplete, but available exposures of local geologic materials are felt to be sufficient to justify the conclusions reported herein. Residents along bluff areas such as this have reason to be concerned about potential changes in hydrology due to ne$t development. Almost a1t deep-seated landslides, as well as most shallow slope failures, are triggered by ground water. Whether or not destabilizing concentrations of ground water occur in a coastal bluff depends to a large extent on the local geology. Sites where changes in the geologic materials impede the downward seepage of ground water are commonly areas of instability. Bluff resl-dents, however, often contribute to stability problems in many ways. Some are evident on the bank near the terminus of 'Sf Street. Loading" a shoreline bluff with clearing debris, graded soil, and conitruction waste commonly creates slides by disturbing vegetation in areas where the roots of that vegetation have alieady had difficulty penetrating the glacially-compacted subsoils. Cutting trees for better views generally results in the loss of their Soil- anchoring roots. Excessive yard waterj-ng, concentrations of roof drainage, or leaking water and sanitary systems can contribute to surface erosion aS well as instability. Beach access trails, Or worse yet, bank traffic and digging, can undercut banks or trample and smother young vegetation. Probably the most fundamental people problem" of bank edge development is building so close that even natural erosion may eventually threaten a structure. L GEOLOGIC MATERIALS The geologic materials of the bank off the terminus of 'Sf Street ar-e largely made up of glacial sediments, almost certainly those of the last glaciation. The approximate thickness of the sedimentary units here, aS well aS a rough bank profile, are shown on the following "cross sectionTprofile". The foltowing descriptions focus on the physical characteristics of each of the seven geologic units ls-they may relate to slope stability. Formal geologic names are largely avoided because correlations are unknown and/or are irrelevant to stability considerations. The descriptions begin with the lowest (oldest) unit, and include those at beach level and on nearbY uPlands. Sub oravel: Description of Units This unit underlies modern beach gravels in the area fronting the end of 'Sr Street. I iron oxide-cemented pebbly gravel of unknown t consists of an thickness. The rusty orange color is generally obscured by slime and other vegetation. The relationship of this unit with the nearby s6a-level silt" is not clear, but they could both be part of the Whidbey Formation. That unit, of largely flood plain deposits, i" mainly made up of silt, silty sand, and silty pelt, but it also contains iron-oxide cemented gavel in ptaces. Both this, and the following described unit would tend to impede the vertical movement of ground water wherever they occurred. Sea-level silt: This gray silt unit can now be seen at the toe of the slop", a couple hundred yards north of the bank in question, under overhanging alder and brush. I have seen it exposed in the lower 2-or'3-feet of the bank, closer to the site in previous years. However, it is now largely obscured by dry tlows of sand from above which have "buttressed" against the upper beach. The silt is probably thicker than its exposed thickness, and may extend to underneath the proposed development site, below beach level. Its upper sur?ace appears to slope southerly, and thus, if it exists under ths site it might be below mean sea level. Where exposed along the beach it is perennialty wet, probably due to the regionaf water table. This silt may correlate with the silt-and peat-rich unit that is exposed in the lower bank west of Point Wil.son. 2 Lower-bank sand: This predominantly sand unit makes up the fower naff-or-so of the bank here. It probably was deposited by meltwater streams in advance of the last glaciatlon. In the long-term, this sand tends to form a vegetated angle-of- repose stope. Remnants of Such slopes are common in the area, and trees can be seen in places growing on it. such vegetation, as well as grass and brush, catches eroding materiar from above and forms a veneering soil'/vegetationrrmatr' 2-g feet thick. As the permeability of the underlying sand is as high as the overlaying mat, there appears to be little tendency for shallow debris avalanches to occur during rainstorms such as is common elsewhere along Puget Sound where the mat overlies silt. (see enclosed article, fig. ). Where the vegetation mat is removed by wave erosion andr/or foot traffic this lower-bank sand can stand in vertical or even overhanging slopes due to some silt content which acts as a "binderi'. There appear to be no significant silt-rich horizons within the sand which might act as "water perchers" however. Where the natural silt/sand bonds are broken by digging or foot traffic the sand moves in dry flows that accumulate as cones along the base of the slope. Such bare and loose debris cones are very susceptible to erosion by wave action. Thus, they provide "feed" for the beach at Fort Warden but do not give much wave protection to adjacent slopes. Mid-bank sitt: This silt and sandy sitt unit, where exposed uv erosion, tends to stand as a vertical bank. That is because the fine silt particles tend to be cohesive and act as a "binder" between each other and with any included fine sands. This cohesLon was, of course, enhanced by compaction from the weight of the ]ast continental ice sheet (here, about 4O0O feet thick). Unlike the flood-plain silts near Point Wilson, this unit contains no wood or peat. That, and the delicate layering visible in places, suggests a lake-bed origin. Such silts (commonly called "bfue c}ay" where dar[ened by moisture) occur throughout the Puget Lowlands and adjacent river valleys. They were deposited on the floor of va;t lakes when the advancing ice sheet spanned between the Cascades and Olympics, damming the northerly drainage of aJ-I rivers. This unit, although less than 1,O-feet thick, would tend to impede downward seepage of ground water. Upper-bank sand: This sand tends to underlie angle-of-repose sfopes where it is largely obscured by vegetation. (See photo panorama). Thus, its overall textural patterns are not visinte. However, the upper few feet, visible in places, tends to coarsen in grain size upwards. The unit grades into fine gravel near the contact with the overlying glacial tiII. This -uggests deposition by melt-water from the advancing ice sheet w-hich deposited the overlying till. This upper-bank 3 sand was apparently cut away by glacial erosion and essentially replaced by tilt north of the area of concern. Its role in ground-water movement in the area would be largely as a ready conduit for any seepage that could infiltrate overlying materials (tilI or the "upland sand"). Upper-bank tiII: This glacial till unit can be seen to tnicken abruptly as the bluffs become higher to the southeast. It may extend under the Marzan propertY, but is only visible in a graded area just east of the property. If present under the site, it is probably thinner than along nearby bluffs and may "taper out" completety. Where exposed in coastal bluffs' this till stands typicatly in vertical banks. The till, even though sandier and less compact than some of the till expoiures elsewhere in Port Townsend, iS still rather impermeable and limits downward seepage of ground water. The till exposed on the nearby bluff is quite dry and thus not as susceptible to erosion by freeze/thaw and wet/dry cycles as are many titJ. exposures elsewhere along Puget Sound. Uoland sand: This is a medium-to-coarse-grained sand exposed in shallow exploratory holes on the property. been deposited on top of the till by late-glacia or, it could be merely the top of the'"upper-bank would mean that the titt is missing under It may have I meltwater, sand'r . ( This the site. ) Alternatively, this sand coul-d have been derived from the higher titl slopes to the south and east by post-glacial erosion. If so, these sands probably thicken downslope (to the northwest). If of post-till origin, these sands would tend to move seepage from any infJ-ltration trenches away from the bank instead of vertical. 4 GEOLOGY AND GROUND T{ATER General The geotogy of coastal btuffs are a major element in their staUlfity. - Obviously, wave action initially cut the bluffs and continues to remove material that fa1ls on the beach. However, since locaI Sea level approached its present elevation about 5 thousand years d9o, Iocal geology began to play a greater and greater role. This is because the geology of -a bluff commonty has a strong influence on not only the rate (e.g. how often, how fast) but also the mode of failure e.g. slump, slide, fall or flow). The primary source of this influence is due to the control geologic materials have on ground water. Not only are the physical characteristics of Ihese sediments important, but their sequence of deposition. FOr example, Sand abOve SiIt Can Congentrate Or "pergh" ground water, but silt above sand does not. For a ground-water concentration to occur on (rather than at the baie of) a shoreline bluff, several factors must exist. obviously, there must be a supply of recharge (ample rainfall). Second, there must be a reason for it to concentrate, and third, at least some of that concentration must move seaward. For the water to concentrate (i.e. saturate the sediments) it must, in its downward migration, either (1) encounter a less permeable layer (as just mentioned) or (2) it must reach the regional water table ( a surface below which all sediments are saturated). Near- horizontal commonly discontinuous "lines" of seeps or water- loving vegetation in mid-bluff areas are almost always e*a*ples of perched" ground water (See figures 5, _6, and 7, attaahed articte). berched ground water commonly may be Seasonal and disappear in summer. In the winter such horizons may be marked by rows of icicles. In contrast, a regional water table is perennial and low on the bluff. Existing Groundwater Conditions The only ground water currently exposed near the bank of concern-here is at the "Sea-level silt" north of the site" The abrupt contrast in permeability between this silt and the overlying sand may locally be a contributing factor to this moisture occurrence. However, the seepage is so low in the bank that it undoubtedly is the regional water table (rather than local, perched, and seasonal ground water). Ass such it would have a regional source of recharge, possibly extending beyond the City limits. Also, this silt is low enough to be 5 wetted by sea water, even if a fresh water source was not present. Thus, it would appear that even maior local bevelopments would have littte impact on this aquifer and that the currently proposed development is insignificant in area even if some of its recharge travelled L50 ft vertically to this depth. The annual "water budget" for Port Townsend (Figure 2) indicates that the local "water Surp1us" occurs only for about a month-and-a-half per year. Water surplus is defined as water available for recharge and surface runoff". Personal experience indicates that (except for developed surfaces) those areas of Port Townsend underlain by tiIl have such a great moisture deficit that there is probably tittle recharge ind no runoff even in the "Rainy season'r. Even mature trees in this setting (averaging less than L9 inches of annual rainfall) commonly exhibit stress signs due to lack of soil moisture. Thus, the concept of a water surplus in this area should probabty be confined to runoff from developed surfaces. Similariy, deep recharge is probably confined to closed depressions, anb nigUy permeable (sand and/or gravel) areas. Potential for ImPacts To get a better idea on the potential for bank destabilizing rec[arge from the proposed development, Iet's examine available data and make Some "worst Case'l aSsumptions. In examining the hydrologic setting of eastern Jefferson County, Grimstad and Caison (t981-) produced graph comparing the annual water budget of Port Townsend and Quilcene (Figure 2). They point o,rf the water surplus areas ( shaded ) as bei-ngofnparticutarinterestbecausetheserepresentth9water available for aquifer recharge and surface runoff" (p. 30). In discussing soit moisture in Port Townsend they conclude that "the amount available for runoff and aquifer recharge is insignificant (0.6 inch - 6 inch soil water-holding capacity) ass 6o^pared with 29.7 inches in the Quilcene area'r (p. 32). Letts assume that the proposed development will increase the water surplus to 1- inch on-site (almost double the 0.6 calculateA). Further, Iet's assume that there is no runoff and that there is no till to impede downward seepage anywhere under the property. In seeping downward through about 5O feet of "upper-banX sand" (see Figure J.), let's assume that none of that'Lxtra" 0.4 inch of water surplus was trapped on sand particles or wicked back to the surface and evaporated during tne following sj-x months of soil water deficit. Further' Iet's assume that all of that extra water is stopped from further downward migration by the mid-bluff silt. How much potentially destabilizing saturation would reach the bluff iace, approximately L90 feet from the property boundary, probably weeks later than the storm that produced it? 6 WATER IIOLDING CAPACITYOFSOIL ?"6 PRECIPITA TION r8.3 25.2 t4.4 t7.7 WATER SUR PLUS 3.9 o.6 6 4 2 v,lr, I( Jz PORT TOWNSEND JAN FEB MAR APR QUILCENE 2SW WATER HOI-DING CAFAqTYOFSOIL 2'6' PRECIPITATIOF-50.0 25. I R t7.4 20.3 32.6 29_7 JAN FEB From: Geology atrd Groundwater Resources of Eastern Jefferson County. Depaflmenr of Ecology Water Suppiy Bulletin no. 54, l9g1 o MAY ,rUN JUL AUG SEP 0cT AUG SEP ocr NOV DEC NOV 0Ec to 8 6 4 2 o rtt! Jz MAR APR MAY ,rUN JUL Figure 2. Comparison of .,,annual water budgets,, forportTownsendandeuiicene. 'iiotentiar evaptlranspirati_on,, istheamountofwaterthatcoura-ue';;;"il'f;i_"rr.noration fromthesoilandtranspiration -i.o-* prants it enough water were|I"i*:i::'"J"nl""J,l"f ;*:id;X*,illl"rt"r.,,.,iJ-b;i"iperioJ TATION WATER DEFICIT POTENTIAL EVAPOTRANSPIRA TION LACTUA EVAPOTRA PRECIPI i,h6 WATER SURPLUS s \\_\qr STU L MOISTURE UTILIZATION PRECIPITATION PIRAT1ONACTUAL EVAPOTRANSPIRA TI L MO'STURE UTILIZATI ON It must be recognized that in the real world the extra water surplus wiII 5e seeping laterally as well as verticaL downward. This spreaAing tendency is further enhanced in iayered sediments because the layering lets water travel easier "with the grain" (laterally) than across the grain vertically). ThuJ, out hypothetical extra 0.4 inch of water surplus wouta, dt worst, amount to less than 0.1- inch over a much larger area by the time it reached the level of mid-bluff silt. Any saturation that might develop there would further spread laierally in all directions (not just towards the bluff). Some mbisture would also slowJ-y seep further down into the silt. The foregoing suggests that the minimal added soil water from this retitivefy smalt development would be undetectable at the level of the mid-bank silt, and have no impact on the stability of nearby banks. One might then logica1ly ask "What about cumulative impacts, aS more and more open space in this area is developed?i' Granted, this could eventually have an impact on local bank stability if major.development occurred in the area and most of that was required to have on-site infiltration systems. Such systems make good sense in such a geologic and ctimatic setting. However, it seems safe to ssume that there will eventually be a storm drainage system of some form along Wa1nut Street. Storm flows now gulley ditches in places here and/or deposit gravel and debris on the street. With engineered storm drainage in place, ground-water recharge would tnen become even less than it is today. 7 CONCLUSIONS L. The infiltration systems for the new residences proposed will introduce more water into the soils than in a "natural" setting. 2. The amount of that additional water will be small, and in many years largely be retained by the upper soils. 3. Any water "escaping" downward would be greatly slowed and dispelsed before it coutd reach a vulnerable bank surface. 4. It is highly unlikely that any additional recharge from the devetopment would be detectable at the bank and even less likely-that it might cause additLonal stability problems. e-//(U GERALD W.CPG I