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HomeMy WebLinkAboutSDP201000054 Legacy Document 2010-09-15 (3)August 16, 2010 Petitioners: Norman L. Ourada & M. Rose Byrne 1420 Wilton Farm Road, Apt. 101 Charlottesville, VA 22911 Case No. SDP201000054 RESPONSE To Engineering Review Memorandum Dated 4 August 2010 for Site Review Committee, SDP201000054 Petitioner - Applicants respectfully submit this Response to the above - referenced Engineering Review Memorandum by Mr. Phil Custer, Current Development engineering review. Mr. Custer's competence, objectivity, and professionalism are sincerely acknowledged. However, Applicants respectfully - submit that the weight given to his opinion should reflect certain limitations on the applicability thereof, as described below. Mr. Custer acknowledges the concern raised in Applicants' Exhibit 6 (professional engineer's opinion letter) regarding increased erosion from runoff concentration in the southeast corner of the site. He then asserts that "most of the erosion and sediment suspension occurs from 'raindrop' erosion" and cites a USDA document for certain erosion comparison data (B.Y. Liu, et al., Slope Gradient Effects on Soil Loss for Steep Slopes, Transactions of the ASAE 37(6) 1835 -1840 (1994)). He uses the cited data to infer the proposition that "we can estimate that 28% slope erodes 3x more than a 15% slope (1.124/0.336)." Applicants respectfully assert that the cited document does not support Mr. Custer's inferred proposition. The soil erosion 1 data set forth in Table 3 of the reference reflects soil loss measurements from three locations on a certain well- characterized plateau in China. (Ibid at 1836, col. 2.) But contrary to Mr. Custer's statement emphasizing "raindrop" erosion, the reference's authors specifically note the importance of rivulet erosion for the reported data set: "[r]ill erosion on all three of these locations. was obvious. Some scientists estimate that half or more of the soil loss from these plots is caused by rill erosion." (Ibid.) In other words, because the data of the cited document predominantly relate to rill erosion, they "do not provide significant support for inferences about "raindrop" erosion on Applicants' parcel. Applicants acknowledge that rill erosion may in fact occur on their parcel. However, even with respect to rill erosion, the cited document appears to provide only indirect information about erosion rate as a function of runoff volume. Applicants nevertheless direct attention to Table 3 of the reference for an interesting comparison: plot 32 of the Suide site showed (on a 15.1% slope) that an annual runoff of 20.8 mm produced normalized soil loss of 0.336. Plot 2 of the Ansai site showed (on a similar, 17.6% slope) that an annual runoff of 52.0 mm produced normalized soil loss of 0.641. In other words, on similar slopes, an annual runoff of 52.0 mm produced greater soil loss than an annual runoff of only 20.8 mm. This .comparison arguably supports the commonsense notion that greater runoff volume leads to greater soil erosion. Applicants respectfully suggest that the cited document might well provide experimental support for the expert assertion in Applicants' Exhibit 6 that "[t]he construction site located in a drainage swale would cause . more siltation than a site located on the critical slope due to a the [sic] increased runoff .volume through the site during precipitation events." Applicants further note that Mr. Custer's engineering review appropriately addressed the specific public health, safety, and welfare factors set forth in Section 4.2.5 of the zoning ordinance (Chapter 18, Albemarle County Code). Application of the zoning ordinance in its entirety, however, requires consideration of a broader range of issues. In particular, Chapter 18 "is designed ...[t]o provide for adequate light, air, convenience of access and safety from fire, flood and other dangers; [and t]o protect against ... loss of life, health, or property from fire, flood, panic or other dangers." (See Sections 1.4.1 and 1.4.6.) Section 4.2 explicitly confers authority on the Planning Commission to grant a waiver in view of such broader issues: "The commission may grant a modification or waiver if it finds that ... [s]trict application of the requirements of section 4.2 would not forward the purposes of this chapter or otherwise serve the public health, safety or welfare." (See Section 4.2.5(a)(3).) In the present case, the subject area of non - critical slope lies within a drainage channel. The expert opinion from Old Dominion Engineering (Exhibit 6) notes that an increased risk of flooding exists for construction in that area. Applicants respectfully assert that they, as well as the Planning Commission, should appropriately consider not only how to reduce soil erosion, but also how to ensure safety against flooding for the completed structure. Mr. Custer speculates that "construction of the building on the 28% slope as proposed will result in more erosion than if the building was constructed in the southeast corner with a properly sized and lined diversion channel around the house." However, safety against flooding would unavoidably require more interventions than would be needed merely to control soil erosion. For construction in the drainage channel, the measures necessary to provide adequate protection against loss of life, health, or property from flooding would in fact require substantial alteration of the natural topography of the construction site. In particular, "[t]he grading required to build in the drainage channel would require significantly. more alteration of the natural - -- --paragraph-2,-item-2.) It follows that compliance with the expressed purposes of Chapter 18 supports grant of the waiver requested in the present case. Accordingly, Applicants request that such waiver or modification be promptly GRANTED. Norman L. Ourada RESPECTFULLY SUBMITTED, 2 Date: August 16, 2010 Elizabeth Marotta From: Philip Custer Sent: Friday, August 20, 2010 10:36 AM -To: Elizabeth Marotta Subject: Updated Critical Slope Waiver Analysis for the Ourada Building Site (SDP- 2010 - 00054) Attachments: E2 csw PBC_Ouranda.doc Good Morning Elizabeth, I have updated the engineering critical slope waiver analysis after the revised information-from-the applicant has been submitted. I have removed the data-table from the USDA study because it was adding unnecessary confusion'to, the discussion. It has been replaced with a general overview of the soil loss equation referenced in that study. My opinion remains unchanged. Please forward this memo to the applicant so they have the opportunity to prepare a response-to possibly be included as an appendix in your staff report. Please contact me if you have any questions.. -thanks, Phil .296 -5832 0072 1 -AL A'lA �RCIN�P County of Albemarle Department of Community Development Memorandum To: Elizabeth Marotta, Current Development planning and zoning review From: Phil Custer, Current Development engineering review Date: 20 August .2010 Subject: . Ourada Building Site (SDP- 2010- 00054) critical slope waiver request - - The critical slope. waiver request has been reviewed. The engineering analysis of the request follows: Description of critical slope area and proposed disturbance: The proposed plan disturbs critical slopes to construct a 1,023sf single family home, carport, driveway, and septic field on the northern end of TMP 88 -14A. The average slope of the area disturbed is around .28%. A smaller area. of steeper critical slope disturbance ( -50% grade) will.be required off of the existing private road when the driveway is constructed into the hillside. Areas Acres Total site 2.0 acres Critical slopes 1.18 59% of site Critical slopes disturbed 0.21 17.8% of critical slopes The numbers in the above table are a compilation of data provided by the applicant's surveyor and architect. These numbers do not include any disturbance necessary to establish a well and running the waterline to the house, but I believe this disturbance to be negligible. Exemptions to critical slopes waivers for driveways, roads and utilities without reasonable alternative locations: If the flatter area in the southern portion of the site was determined not to be a satisfactory building site, the driveway (accessway) could be considered exempt per 4.2.6.c. =Compliance with.Zoning Ordinance 18 -4:2: "movement of soil and rock" The USDA soil classification for all areas of the parcel is Chester very stony loam, 15 to 25 percent slopes (15D). Because of the steepness of the grade, the USDA soil survey notes that the hazard for -- - - erosion -of this- soil -as- severe. In order for any building permit to be approved in the rural area, the property owner is required to sign an Agreement in Lieu of Plan as provided in Virginia ESC Law. This agreement commits the applicant to follow all state ESC standards during construction. With agreements, however, ESC bonds are not held by the county and site inspection is more infrequent due to the wide dispersion of rural area building permits. The applicant has included in the waiver request package a letter from a professional engineer, Mr. Michael Craun, which asserts that construction in the flatter (15% grade), southeast corner of the site would cause more erosion because the earth disturbance would occur in an area where water from the uphill lands concentrate. While this is a noteworthy concern, I believe this disadvantage can be .mitigated by constructing a channel on the uphill side of the house to bypass this flow. A channel or Albemarle County Community Development Engineering Review comments Page 2 of 3 low point on the uphill side of the house will be needed in both building locations, but the channel would need to be larger if the lower corner of the property was the area of house construction. In my opinion, comparing the potential soil loss of the two slopes (15% and 28 %) is more critical than the construction of the channel around the house when evaluating potential soil loss. The Revised Universal Soil Loss Equation (RUSLE) is a formula endorsed by the USDA that is primarily used to estimate soil loss on tilled farmland. Though, it can be applied to construction sites as well. The equation is represented below: A = R * K * L * S * C * P where, A = Net Detachment (i.e. Soil Loss) R = Erosivity Factor K = Soil Erodibility Factor L = Slope Length (ft) S = Slope Steepness (ft/ft) C Cover- Management Factor P = Supporting Practices When applying this equation to the two potential building sites being analyzed in this application, the values R, K, C, and P will essentially be the same (assuming an upstream channel was installed properly). So, the equation reduces to: Soil Loss = X * Slope * Length Where, X = a constant (RKCP) . The average grade at the current proposed location of the building (�28 %) is almost twice as steep as the alternative at the lower end of the site (15 %). Consequently, the length of slope disturbed at the current proposed location will be slightly longer. So, according to the RUSLE, the soil loss is estimated to be twice as much at the current proposed building site than if the area of the property with flatter slopes were to be the site of the house. I acknowledge that some additional soil loss would result from the construction of the bypass channel. But the amount is impossible to quantify or estimate. I suspect the loss of soil that would result from the ditch .construction and stabilization would be much less than the difference that can be attributed to the steepness of the two slopes. The applicant believes there is a greater chance for soil loss with the cleanwater channel "excessive stormwater runoff' As with nearly all single family home construction, stormwater -runoff from the site will be increased, though the addition will likely be negligible. The applicant has indicated that they are thinking of including a cistern on the property which will help bring the property closer to pre - development hydrology. "siltation" Please refer to "movement of soil and rock" section. "loss of aesthetic resource" Engineering review provides no comment on the aesthetic resources on site. Albemarle County Community Development .Engineering Review comments Page 3 of 3 "septic effluent" County engineering review will defer to the Virginia Health Department with regards to the construction of septic systems on critical slopes. After reading several emails from VDOH officials, it's evident that there is no objection to the placement of the septic system on critical slopes. In fact, VDOH is prohibiting the construction of a septic system in the only area of the site where critical slopes are not present. Engineering review cannot conclusively agree with the applicant that all five "public health, safety, and welfare factors" are better served with the approval of the waiver. I suspect that the construction of the building on the 28% slope as proposed will result in more soil loss than if the building was constructed in the southeast corner with a properly sized and lined diversion channel around the house. Admittedly, there is not a lot of evidence available to support either position. (For septic effluent, I will defer to VDOH who has weighed in already.) However, I think it is important to remember that total soil loss from a site is influenced not only by existing and proposed grades but also by total disturbed area and time of construction. Considering these factors, the current proposal is likely below the county averages and, therefore, may lose less soil than many other properties that do not require waivers for critical slope disturbance. This may be helpful to keep in mind when the agent and Planning Commission are evaluating whether this waiver should or should not be granted.