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HomeMy WebLinkAboutSP202100007 Dam Inspection 2021-03-15FROEHLING & ROBERTSON, INC. O® 5' T ter. N 4 ij6Y 1 c. X F&R Project No.: 62Y-0172 Natel Energy 2401 Monarch St. Alameda, CA 94501 Attention: Mr. Joe Head FROEHLING & ROBERTSON, INC. Engineering Stability Since 1881 1734 Seibel Drive, NE Roanoke, Virginia 24012 T 540.344.7939 1 F 540.344.3657 Subject: Jefferson Mill Dam —Albemarle County, VA Dear Mr. Head: 14 August 2020 The purpose of this report is to summarize observations made during our inspection visit on 9 July 2020 to the referenced dam. The dam is located 0.16 miles upstream of Jefferson Mill Road (Rte. 618) near Scottsville, VA, as depicted on the Site Vicinity Map, Drawing No. 1. There were no records or drawings of the dam available for our review. 1. INTRODUCTION The dam was constructed approximately 200 years ago to operate a mill (Photo No. 1). The dam is constructed of large angular stone and mortar and is 9.0 feet tall. It has a crest width between 4 and 5 feet, and a crest length of approximately 110 feet, and another 35 feet extending to the mill structure for the headrace. The dam impounds the Hardware River to form approximately an 8.0 acre pool. It is a run -of -the -river dam so the normal pool elevation of the lake is the top of the dam. Based on the area and height, the storage capacity is calculated to be 30 acre-feet which is below the DCR minimum storage of 50 acre-feet and confirms that this is not a DCR regulated dam. The watershed consists of mostly undeveloped agricultural lands and is large enough to provide a constant water supply. Jefferson Mill Road (Rte.618) is a paved state maintained road that crosses the river 0.12 miles downstream of the dam and its bridge would not be impacted by a dam break. The next crossing is Albevanna Spring Road (Rte. 620), approximately 1.5 miles away. Since this dam is size exempt, there is no need to perform a Dam Break Inundation study. The lake and dam provide aesthetic, historic, and recreational value to the property. They can also still be used to drive mechanical equipment. 2. CONDITION OF THE STONE AND MORTAR At the time of our visit, the dam had approximately 2 inches of water flowing over it so direct observation of the downstream face was not possible (Photo No. 2). The typical construction profile for a dam of this type is a near vertical downstream face with a stair stepped upstream face built with a 1H:1V slope or flatter. This makes the base of the dam width at least as broad as the height which serves to improve its long term stability. As is also typical of dams of this type Jefferson Mill dam is Corporate HQ: 3015 Dumbarton Road Richmond, Virginia 23228 T 804.264.2701 F 804.264.1202 www.fandr.com VIRGINIA • NORTH CAROLINA • SOUTH CAROLINA • MARYLAND • DISTRICT OF COLUMBIA A Minority -Owned Business n constructed with a slight upstream arc so that the pressure of the impounded water compresses the individual rocks together and against a fixed abutment. The left side of the dam (looking downstream) is well anchored into the bedrock for its full height. At the right end of the dam by the headrace there is exposed bedrock at the surface indicating the base of the wall is well secured on the right end as well. The stone and mortar wall construction extends all the way to the mill building. At least 35 feet of this end of the wall is further supported with earth backfill for the full dam height. The headrace uses the mill building on the right side and a stone wall constructed perpendicular to the dam wall and offset approximately 10 feet from the mill exterior wall. It is approximately 20 feet long, 10 feet wide, and 9 feet deep (Photo No. 3). It is of similar stone and mortar construction to the dam except that it is a near vertical face on both sides of the wall. We recommend that care is taken operationally to ensure that this volume is not allowed to be filled with water to the top without additional support structures. The original equipment within the headrace has been removed and new replacement mechanicals to generate hydropower are planned. The headrace inlet is temporarily blocked and will be reopened when the new equipment is installed and ready for use. Based on the visible bedrock on both the left and right side, it is anticipated that the entire base of the dam is well anchored into bedrock. For this type of construction it is very likely that there are some minor flow passageways that have developed between the rock construction joints and through fissures in the bedrock. These are typically considered non -erosive but over time can degrade the rock immediately adjacent to the flows. This degradation over time is unlikely to create problems that cannot be identified and corrected well in advance of a serious problem developing. The other potential changes that can develop is scour in the river channel at the toe of the dam due to the constant impact of the water. Again these take a very long time to develop and can be managed appropriately. There is no low level drain for the dam so we recommend that at some time in the future a bypass flow arrangement be utilized during seasonal low flow conditions so that the dam wall and downstream toe may be observed to document any minor flow passageways. We anticipate that the dam will still show to stable but if anything is identified in that investigation, corrective actions may be planned and then executed. However, from what could be seen, there were no notable concerns in the profile of the dam top or face. The stone wall appears to be well keyed into the rock on the bottom and sides (Photo 4). Where the stones were observable they appeared to be sound with no indications of degradation or movement. The exposed mortared joints between the stones are hosting vegetative growth which should be discouraged (Photo 5). Roots, even small ones, contribute to loss of mortar in the joints. It would be appropriate and would also enhance the aesthetics of the wall to have the mortared joints repointed. Overall we have no reason to think this dam is nearing the end of its serviceable life or experiencing distress. Jefferson Mill Dam 2020Inspection F&R Proposal No 62Y-0172 2 August 14, 2020 3. SPILLWAY CONDITIONS 3.1 Principal Spillway In normal mill operation the headrace gate could be considered to be the Principle Spillway. As noted the gate is currently blocked and a new outlet and valve will be installed in the near future: There are several types of gates that may be used but for most dams like this a type that is opened by lifting is common. Other potential valve arrangements include a butterfly type valve that is part of a piping installation. Either is suitable in that the flow may be regulated for the entire opening or reduced to a portion of the opening that suits the flow requirements at the time. We do recommend that the headrace inlet is protected from debris with the use of debris guards and even the installed inlet position and direction. During low and possibly normal flow periods directing water through the headrace may enable lowering the pool below the top of the dam to allow more in depth inspections described above. We do note that there is what appears to be an intentional low point on the top of dam nearest the mill building that flows approximately 1 foot deep. As noted above, the flow during the inspection did cover the entire dam length so this low spot by itself may not be enough to handle typical daily flows. 3.2 Auxiliary Spillway Since this is a run -of -the -river dam, the entire top of the dam could be considered the Auxiliary Spillway. It is entirely open to flow, however, rafting trees can snag under water at the lip unless the flow rate is severe enough to carry them over the top. Lower flow events do not have the power to carry trees over the dam and a debris dam may build up on the upstream side. There was no rafting of trees or other debris in the upstream pool area during this inspection. We note that large debris moving fast during high flows can damage the dam, particularly the uppermost course of stones. It is difficult to protect a dam from this type of impact but it is something for the owners to consider. 3.3 Bottom Drain As noted there is no bottom drain outlet and given the age of this structure it would be unexpected to find one. It can be beneficial to have a mechanism to drain a dam but it is not practical to install a conventional bottom drain in this structure. The best recommendation we can provide is to continue with the plans to direct water through the headrace. We note that based on the observed flows it is not likely to be able to direct the entire flow away from the dam wall, but perhaps enough may be diverted to decrease the amount over the top to a manageable amount. Also for the age of this structure it is very likely that there is several feet of mud and debris deposited on the upstream side. This could be a concern if water is intended to be drawn off for use at lower elevations within the impounded pool. 3.4 Plunge Pool and Downstream Channel The wall of dam is only slightly wider than the normal stream channel so the flow does not spread before it drops over the edge into the riverbed below. The bottom is all exposed rock and the sedimentary nature of the bedrock is evident. The strike and dip of the folded sedimentary layers that are exposed angle downward to the upstream and right side of the river. This direction of the rock Jefferson Mill Dam 2020Inspection F&R Proposal No 62V--0172 3 August 14, 2020 n layers is also helpful to the long term durability of the dam. Due to the flow rate we were unable to probe the toe of the dam for any open pathways that may have developed under the dam, but no boils that would warrant further investigation were seen either. At some future date when the stream flow can be directed through the headrace, we recommend the toe be inspected and documented. 3.5 Abutments As previously mentioned the wall of the dam appears to be well keyed in to the rock on the bottom and sides. Also noted is that the length of the wall extends wider than the normal channel width towards the headrace and mill building on the right side. There are some small seeps to (estimated 1-2 gallons per minute) that were found coming out of the stone joints on the right side of the stone wall that is outside of the river flow area. These are characterized by a strong orange color due to iron eating algae that grows at the seepage point. This seep is very likely due to water tracking sideways along the upstream side of stone step construction until it finds a crevice through the wall. Presence of the algae indicates the seepage is moving slowly. At present these locations do not represent a concern but they do indicate that the earth backfill is eroding internally so eventually there may be a need or desire to address the seepage. 4. LIMITATIONS This report has been prepared for the exclusive use of the Natel Energy and their client for the specific application to Jefferson Mill Dam in Scottsville, Virginia and in accordance with generally accepted engineering practices. No other warranty, expressed or implied is made. Our conclusions and recommendations are based on information furnished to us, the previously described site visit observations and generally accepted engineering practices. The conclusions and recommendations do not reflect variations in conditions which could not be discerned by visual observations and should such variations become apparent, it will be necessary to re-evaluate our conclusions and recommendations based upon an additional on -site visit and observations of the conditions. 5. ACKNOWLEDGEMENT We have enjoyed working with you on this project, and we are prepared to assist you with additional services as may be required for this dam. Please contact us if you have any questions regarding this report or if we may be of further service. Sincerely, FROEHLING & ROBERTSON, David A. Krisnitski, P.E. Water Resources Engineer �its avid A Rrisn� Z No. 36465 y/ot�� F:\Pmjects 62Y\62Y-0172 (Jefferson Mill Dam Impection)\Jefferson MITI Dam �q Hoy Senior Engineer Jefferson Mill Dam 2020Inspection F&R Proposal No 62Y-0172 4 August 14, 2020 APPENDIX A Site Vicinity Map Jefferson Mill Dam 2020Inspection F&R Proposal No 62Y-0172 5 August 14, 2020 Nate Rr1 - m 1 �M SITE Bv� ea G W I s Scottsville 6 ,WVS to ad AlbevaWa Spec A G W I s Scottsville 6 ,WVS to ad AlbevaWa Spec A 0 •Pa Ys 00 V , old Sprino F,. r rrr .rrr (90 FROEHLING & ROBERTSON Date: August, 2020 Scale: As Shown Engineering Stability Since 1881 Drawn: HJM 62Y-0172 Jefferson Mill Dam Inspection Site Vicinity Drawing No. Natel Energy Scottsville, Virginia Map 1 APPENDIX B Photos Jefferson Mill Dom 2020Inspection F&R Proposal No 62Y 0172 6 August 14, 2020 1 a ' J _ � d Mo i � vr, A Sy,. . Sy,. . Y�- IS ♦ 1 h � oral l P b I'r ALL • • • • • • • _ a kc -' l . Me F&R Photo No. 5 — Vegetation in the Mortared Joints Photo No. 6 — Seepage Dk- Jefferson Mill Dam 2020Inspection F&R Proposal No 62Y-0172 9 August 14, 2020 SINCE Corporate HQ: 3015 Dumbarton Road Richmond, Virginia 23228 T 804.264.2701 F 804.264.1202 www.fandr.com VIRGINIA • NORTH CAROLINA • SOUTH CAROLINA • MARYLAND • DISTRICT OF COLUMBIA A Minority -Owned Business