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HomeMy WebLinkAboutSpecial ReportSTRATUM GROUP 1451 Grant Street, B¢llingham, WA 98225 Phone (360) 714-9409 June 1 O, 2003 ,. Gene Baur 47333 Highway 112 Port Angeles, WA 98363 Re~ Proposed Timber Harvest And Building Feasibility Tax Parcel 702243002 Quilcene, Washington Dear Mr. Baur: Attached is a Harvest Plan and Building Site Sketch for Tax Parcel 702243002. Our feasibility investigation indicates that timber harvest can take place on the property without a risk of slope failure or delivery of sediment into streams as long as our recommendations are followed. As noted on the sketch plan, the steep 45 degrees or greater slopes along the base of the slope below the road and above the flood pain of the Big Quilcene River should not be harvested or disturbed in any manner. Otherwise timber harvest can take place anywhere south of the indicated line on the map. Because areas of limited or no harvest have been delineated, these areas may need to be marked off on the ground prior to harvest. In addition to the harvest feasibility for the property, the property was inspected for potential building sites. There are numerous potential building sites on the property. The potential building area is also indicated on the site sketch map. If a building is constructed on the property, it should be located south of the existing road, and if located on the western portion of the property, it should be located at least 45 feet from the steep road cut. GENERAL GEOLOGY Northwestern Washington has been occupied by continemal glaciers at. least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washington (Yount, Minard, and Dembroff 1993) indicate the property is underlain by glacial till and undifferentiated pre-Fraser glaciation deposits. Observations on the slopes on the subject property are consistent with the mapping described above. Most of the property appears to be underlain by very compact silts that we interpret as pre glacial deposits. Glacial till was observed on the eastern portion of the property. A thin colluvium of weathered glacial till and perhaps minor recessional deposits was observed in hand dug test pits in a few areas. SPECIFIC SITE OBSERVATIONS A site location map is provided on Figure 1 and a general site plan sketch is provided on Figure 2. The property slopes up from the Big Quilcene River Valley floor on the north side of the property up the south side of the valley. The slopes closest to the flood plain are generally close to 45 degrees and appear to be the result of past undermining by the river at the base of the slope. The property is vegetated with a mix of Douglas fir, western red cedar, maPle and alder with an understory consisting of ferns where the overstory is thick and other brush in more open areas. An access road to properties west of the subject property crosses the property from the southeast comer of the property to the west side of the property. The westernmost length of this road consists of a cut and fill road. Ravelling of the oversteepened slope above the road is evident and some side cast failures are evident on this section of road as well. The Big Quilcene River is presently undermining the base of the slope at the west end of the property. Recent shallow soil failures are evident at this location and additional slope failures are likely to occur at this location. The slope below the road appears to be result of past erosion at the base of the slope during times when the river meanders against the base of the slope. Except for the area on the west end of the property the river is presently well back from the base of the slope. We did not observe any evidence indicating an incipient large-scale global-type or deep-seated failure on the subject property. Slope failures appear to be limited to fairly shallow failures of upper soil on the steep slopes and minor shallow soil creep on the other slopes on the property. CONCLUSIONS AND RECOMMENDATIONS It is our opinion that the trees on the slopes of the proposed timber harvest can be removed with minimum impacts to the stability of the slope and with no risk to the slope stability as long as our recommendations are followed. No harvest or slope disturbance should take place on the slopes 40 degrees located immediately above the flood plain area. The removal of the trees from the slope will not cause significant impacts on the slope as long as the trees are removed as follows: Trees should all be cut, not pulled over, All' slash should be removed from the steep slopes (40 degrees) and areas immediately above the steep slopes, Roots and stumps should be left in place, The harvest area should be replanted with Douglas fir or western red cedar as soon as practical after harvest. Based on our visual inspection of the subject property and vicinity, we conclude that a residence can be located on area above (south) of the current road that crosses the property. If the residence is located towards the west end of the property it should be set back at least 45 feet from the top of the road cut. We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slopes as long as our recommendations are followed. We recommend that roof drains and any other drainage be discharged to a water infiltration system that will distribute water into the ground at the site. The access road should be graded such that storm water mn off is directed to the forest floor instead of all being channeled to the same discharge point. Site grading soils or debris, landscape debris, or any other material should not be disposed of on the steep slopes on the property. The septic drain field should not cause problems for slope stability as long as it is not located with within 100 feet of the top of the steep slope above the river. If conditions appear differem than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify oUr recommendations as required. We appreciate the opportunity to be Of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. · Sincerely yours, Stratum Group ~,~, Dan McShane, M.S., P.E.G. Professional Engineering Geologist t ! I t ,,,' i Figure 1. Vimily M~p NAD27 I22~5U34" W' ' )1~3~ W'~flower ~ns NAD//122o5[r34" w' March 18, 2004 STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 RECE VED Eugene Bower 47333 Highway 112 Port Angeles, WA 98363 Re~ Proposed Timber Harvest FPA 2605529, Tax Parcel 702243002 Quilcene, Washington Dear Mr. Bower: My investigation of Tax Parcel 702243002 indicates that timber harvest can take place on the property without a risk of slope failure or delivery of sediment into streams as long as our recommendations are followed. As noted on the attached site plan sketch map the no harvest or soil disturbance should take place in the following areas: 1) The steep 45 degrees or greater slopes along the base of the slope below the road and above the flood pain of the Big Quilcene River should not be harvested or disturbed in any manner. The cut-no cut boundary had been flagged at the time of my last site visit in January 2004. 2) No harvest of disturbance of the slope above the road cut in the central portion of the property where the road cut exceeds 10 feet in height. 3) The stream drainage on the southeast portion of the property should not be disturbed. This drainage stream is primarily the result of imercepted shallow ground water in the ditch on the uphill side of Clay Banks Road being directed omo the subject property and concentrated on the forest floor above a seasonal stream channel east of the subject property. This concentrated flow has caused the stream channel down stream of the subject property to begin incising dOwnWard. I recommend that the area of the forest floor where this ditch water is directed be undisturbed. 4) No harvest or soil disturbance should take place in an area of convergent topography on the southwest portion of the property. Stratum Group 3.1.04 March 18, 2004 FPA 2605529, Bower Parcel Quilcene, WA Trees should all be cut, not pulled over, All slash should be removed from the steep slopes (40 degrees) and areas immediately above the steep slopes, ~ Roots and stumps should be left in place, The harvest area should be replanted with suitable trees as soon as practical after harvest. These findings are consistent with timber feasibility report I provided you in June 2003 with the exception of the numbers 3 and 4 described above. At the time of my June 2003 site visit the property had not been surveyed and hence areas 3 and 4 were not fully evaluated. The following portions of the report are very similar to the original feasibility report, but the format has been altered to specifically note the Forest Board Manual recommendations for geotechnical reports in a bulleted manner. Geographic, geologic and soil conditions of the area in and around the application site. The subject property is located on the south side of the Big Quilcene River south approximately one half-mile south of the town of Quilcene. The northem portion of the property is located within the flood plan meander zone of the river. The property slopes up towards the south from the river plain. The property is accessed via Clay Banks Road and is crossed by an existing road that also is used to access properties and home sites to the west of the subject property. The location of the property is indicated on the attached Figure 1 The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washington (You~t, Minard, and Dembroff 1993) indicate the property is underlain by glacial till and undifferentiated pre-Fraser glaciation deposits as well as recent alluvial deposits along the flood plain of the Big Quilcene River. Observations on the slopes on the subject property are consistent with the mapping described above. Glacial till consisting of compact silty clay with sand and gravel embedded within the fines was observed over most of the eastern portion of the property except for the lower slopes. A thin colluvium of weathered glacial till and perhaps minor recessional deposits consisting of sand and gravel were observed in hand dug test pits in these areas. The western half of the property and most of the lower slopes on the eastern half of the property appear to be underlain by very compact silt, clay and sand that we interpret as pre glacial deposits. Stratum Group 3.1.04 . March 18, 2004 FPA 2605529, Bower Parcel Quilcene, WA The relatively level ground along the northern portion of the property within the flood plain/meander zone of the Big Quilcene River is underlain by river alluvial deposits. Portions of the base of the slope on the subject property are currently being actively eroded by the river. The river has eroded the base of the slope along other areas of the property in the past. This erosion has created steep lower slopes near the base of the slope. Potentially Unstable Landforms on the site:' Three potentially unstable landforms are present on the site. These potentially unstable areas are indicated on the attached site sketch map. Area 1 consists of over steep slopes above the meander zone of the river. These slopes are 45 degrees or steeper and are readily defined by a sharp slope break. Recent slope failures have occurred where the river has recently been undercutting the slope and where side cast materials had been placed on the over steep slope below the road that crosses the subject property. Area 2 consists of an over steep slope created by the cutting of the slope above the road on the subject property. Minor slope raveling and shallow soil failures have taken place along portions of this road cut in the past few years. Area 3 consists of an area of convergent topography on the slope above the road on the western portion of the property. No evidence of recent slope failures or earth movement is apparent, but the presence of low permeability soils'and the :convergent slopes indicate this area may be susceptible to slope failures. Analysis of the possibility that the proposed forestpractice will cause or contribute to movement on the potentially unstable slopes. No harvest or soil disturbance is planned on any of the potentially unstable slopes described · above and indicated on the site sketch map In addition, no harvest will take place within 30 feet of the top of the road cut where the road cut exceeds 10 feet in height. Assess the likelihood of delivery of sediment and/or debris to any public resource, or to a location and in a way that would threaten public safety, should slope movement occur. No harvest or soil disturbance is planned on any.of the potentially unstable slopes described above and indicated on the site sketch map. In addition, no harvest will take place within 30 feet of the top of the road cut where the road cut exceeds 10 feet in height. Stratum Group 3.1.04 March 18, 2004 FPA 2605529, Bower Parcel Quilcene, WA No road construction is planned for this forest practice harvest as an existing road already crosses the subject property. No alterations are planned on this road which is also used as an access route to properties and homes west of the subject property. A ditch along the uphill side of Clay Banks Road intercepts shallow perched ground water and directs that water onto the subject property via a the ditch. This water then passes through a culvert under the road on the subject property and is discharged onto the forest floor on a gemle slope below the road. The water appears to infiltrate into the ground most of the time, as there is no evidence of surface flow down slope of the discharge area. Although no slope instability has been caused by this discharge location, the discharge water flows in the subsurface to a small seasonal stream located east of the subject property. It appears that increased flow to this stream is causing the stream to incise downward into the compact silt and clay underlying the stream channel. Because of the potential for increasing the stream discharge in this area, no harvest or disturbance of the ground will take place in the area between the culvert discharge pipe and the east boundary of the property. This action will maintain the storage capacity of the forest floor in the infiltration area. No harvest is proposed within the river meander zone and, as described above on the steep lower portions of the slope above the meander zone of the river. It should be expected that the river will continue to change course in the future. By leaving trees in place within the meander zone and on the steep slope immediately above the meander zone, sediment or debris from timber harvesting will not impact the river even in the event that the river changes course. Harvest methods and replanting on the stable slopes to be harvested will also prevent increase sediment yields from the site. Trees should' all be cut, not pulled over. All slash should be removed from the steep slopes (40 degrees) and areas immediately above the steep slopes. Roots and stumps should be left in place. The harvest area should be replanted with suitable trees as soon as practical after harvest. Mitigation measures to address the identified hazards and risks. Except for harvest methods and replanting as deScribed in the immediate above paragraph no other mitigation is proposed. The forest practice will avoid the identified hazards on the site. Stratum Group 3.1.04 March 18, 2004 FPA 2605529, Bower Parcel Quilcene, WA We appreciate the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. If conditions appear different than those described in this report, or ~other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. Sincerely yours, Stratum Group Dan McShane, M. Sc., L.E.G. Licensed Engineering Geologist Stratum Group 3.1.04 F~ I. Vicinity Map NAD2? 122 NAD2'/122°50~' W :. ... /2 ,., TOPOI ~)1 NAD27 122~(J'34' W