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GEI Report Upper Roberts Meadow Dam 10-27-2010

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GEI Report Upper Roberts Meadow Dam 10-27-2010 GEI Report Upper Roberts Meadow Dam 10-27-2010 Document Transcript

  • GEl • Consultants October 27,2010 Geotechnical Environmental Project 10426-0 Water Resources Ecological The Friends of the Upper Roberts Meadow Reservoir and Dam P.O. Box 561 Leeds,MJ 01053 Dear Friends: Re: Hazard Class Review Upper Roberts Meadow Reservoir Dam Northampton, Massachusetts This letter presents the results of our investigation of the hazard potential classification assigned to the Upper Roberts Meadow Reservoir Dam and evaluation of the feasibility of requesting a Hazard Classification Change from the Massachusetts Office of Dam Safety (ODS). Summary We reviewed the studies performed by GZJ GeoEnvironmental, Inc. (GZJ) to estimate downstream flooding from a breach of the Upper Roberts Meadow Reservoir Dam during both a fair weather failure and a failure during a flood. The hazard rating for the dam is based on the amount of damage expected downstream following a dam failure under either of these two conditions, whichever is worse. The current hazard rating of "High Hazard Potential" appears to be based on failure of the Lower Roberts Meadow Reservoir Dam and associated incremental flooding caused by the failure of the Upper Roberts Meadow Reservoir Dam during a flood event equal to the 500-year flood. In our opinion, it might be possible to reduce the hazard rating to "Significant Hazard Potential" if the following factors are taken into account: 1. The Upper reservoir has significantly less storage capacity due to siltation than assumed in the GZJ analyses (19 acre-ft vs. 35 acre-ft) based on a recent bathymetric survey. This means that in a dam failure scenario, less water would be released downstream. 2. The available mapping used by GZJ was based on 10-ft contour intervals (which may have been all that was available at the time). Mapping with 5-ft contour intervals is now available from the City of Northampton. Combined with the additional computation power of the computer program HEC-MS and the use of GIS, use of the more detailed mapping may show that more water can be stored between the Upper and Middle reservoirs, attenuating the flood wave as it travels downstream and lessening the downstream impacts. Given that the GZJ report indicated that the Lower Dam was only "slightly overtopped" by a dam breach during the 500-year flood, only a small increase in the upstream storage might be enough to show the overtopping is prevented. www.geiconsultants.com GEl Consultants, Inc. 400 Unicorn Park Drive, Woburn, MA 01801 781.721.4000 fax: 781.721.4073
  • The Friends of the Upper Roberts -2- October 27,2010 Meadow Reservoir and Dam It is our opinion that we would not be able to show that the hazard classification should be "Low," because breach flows are likely to flood and potentially damage the adjacent secondary roads (Chesterfield Road andlor Reservoir Road). The Upper Roberts Meadow Reservoir Dam is in Poor condition and several of the deficiencies will need to be addressed regardless of the hazard potential classification. If the dam is reclassified to a Significant hazard potential dam, the spillway will still need to be enlarged to safely pass the 1~O-year flood or the right embankment and left dike will need to be raised or hardened to allow flow over the top without washing out the embankment fill. However, the spillway enlargement or embankment modifications will be significantly less expensive for a 1DO-year flood rather than for a SOD-year flood. In addition, the stability analyses performed by GZA indicated that the spillway was not stable for several required loading cases unless three-dimensional buttressing effects (due to the dam's curved shape) were included. A more detailed three-dimensional evaluation might show that the dam can meet stability criteria. Background The Upper Roberts Meadow Reservoir Dam (Upper Dam) is located in Leeds, Massachusetts, and is a curved gravity masonry (granite block) dam built in 1883. The dam includes an earth embankment with a masonry core wall to the right of the spillway and a low earthen dike to the left of the spillway. There are several dams downstream of the Upper dam, including the Middle Roberts Meadow Reservoir Dam (Middle Dam) and the Lower Roberts Meadow Dam (Lower Dam). Previous studies by GZA have indicated that flows in the Roberts Meadow Brook will overtop the Upper Dam during the SOD-year flood and pass through the Middle and Lower Dam reservoirs without overtopping the dams. Flows from a dam breach at the Upper Dam during the SOD-year storm would pass through the Middle Roberts Meadow Reservoir and over the Middle Dam spillway and could slightly overtop (by 0.2 ft) the Lower Dam, potentially breaching that dam and threatening lives, homes, and property downstream. Therefore, the Upper Dam is currently listed as a High hazard potential dam by ODS. Scope of Work Our scope of work was as follows: 1. Perform a site visit. 2. Review relevant portions of Massachusetts dam safety regulations. 3. Review existing studies, reports, presentations, correspondence and other relevant information. 4. Based on items 1through 3, above, prepare a concise letter report which addresses the following: a. History of the hazard classification( s) of the Upper Roberts Meadow Reservoir Dam; b. The overall completeness and degree of detail of the body of work to date; c. The reasonableness of the existing inundation mapping prepared by GZA to assess the probable impacts of failure of the Upper Roberts Meadow Reservoir Dam; d. The appropriateness of the existing high hazard potential classification; e. The feasibility of filing a request with ODS to reclassify the hazard potential;
  • The Friends of the Upper Roberts -3- October 27, 2010 Meadow Reservoir and Dam f. The estimated impact of reclassification to a lower hazard potential on existing cost estimates to rehabilitate the dam to meet the design standards of a high hazard potential dam. g. The estimated scope and cost to prepare and submit a hazard reclassification request to the state. Site Visit GEl made a visit on October 21, 2010 to the Upper Roberts Meadow Reservoir Dam to view the dam and the surrounding area. In addition to viewing the dam, portions of the upstream watershed were viewed. The downstream channel, Middle Roberts Meadow Reservoir Dam, Lower Roberts Meadow Reservoir Dam, and the Village of Leeds were also visited. Massachusetts Dam Safety Regulations The state dam safety regulations are codified in 302 CMR 10.00: Dam Safety. Several portions of the regulations are relevant to the consideration of hazard classification. Jurisdictional Status First, it is important to determine whether the dam is regulated by the state, i.e., is it jurisdictional? To be non-jurisdictional, a dam must be less than 6 ft high or store less than 15 acre-ft of water. The Upper Roberts Meadow Reservoir Dam is about 30 ft high and stores about 19 acre-ft of water (based on recent bathymetry) and therefore the dam is jurisdictional and will be regulated by the state Office of Dam Safety. Size Classification The size classification of the dam drives several of the requirements of the regulations. The size classification is determined by the height of the dam and the volume of water stored in the reservoir, whichever gives the larger size classification. A dam 30 ft high with 19 acre-ft of water stored falls into the Intermediate size category. Hazard Classification Hazard classification is assigned based on the likelihood for loss of life and!or property damage should the dam fail (breach) either during a flood or on a sunny day. The Upper Roberts Meadow Reservoir Dam has been assigned a hazard potential of High. • A high hazard potential dam by definition is "located where failure will likely cause loss of life and serious damage to home(s), industrial or commercial facilities, important public utilities, main highway(s), or railroad(s)" [320 CMR 10.06 (3)]. The other hazard potential categories are Significant and Low. • A significant hazard potential dam is "located where failure may cause loss of life and damage home(s), industrial or commercial facilities, secondary highway(s) or railroad(s) or cause interruption of use or service of relatively important facilities" [320 CMR 10.06 (3)]. • A low hazard potential dam is "located where failure may cause minimal property damage to others. Loss of life is not expected" [320 CMR 10.06 (3)].
  • The Friends of the Upper Roberts -4- October 27,2010 Meadow Reservoir and Dam The dam safety regulations specifically address dams in series, such as the three Roberts Meadow Dams. The regulations state: "If an upstream dam has the capability to create a failure in a downstream dam because of its failure wave, it shall have the same or higher hazard classification as the downstream dam" [302 CMR 10.06 (4) (a)]. The hazard classification drives the size of the storm that the dam's spillway should be able to safely pass (without water flowing over the dam embankments and also from a stability standpoint). For a high hazard potential dam, the "design" storm is one-half the Probable Maximum Flood (1/2 PMF). The Probable Maximum Flood is "the most severe flood that is considered reasonably possible at a site as a result of the most severe combination of critical meteorological and hydrologic conditions possible in the region" [320 CMR 10.03 (2)]. A smaller storm can be used for design or analysis if it can be shown that the increase in flood damage downstream of the dam following a dam breach during that storm is less than about 2 ft greater than the flooding damage from the storm without a breach or is less than 2 ft above the lowest ground elevation adjacent to the outside foundation wall of an inhabited structure. Condition There are five condition categories for jurisdictional dams in Massachusetts. An inspection schedule is prescribed based on the hazard classification (every 2 years for High, every 5 years for Significant, and every 10 years for Low). The condition of a dam is assigned following an inspection and can be one of Good, Satisfactory, Fair, Poor, or Unsafe. If a dam is in Poor or Unsafe condition, the regulations permit ODS to order the dam owner to rehabilitate the dam to bring it to Fair, Satisfactory, or Good condition. Document Review The documents reviewed by GEl are listed in Table 1. The Phase II report and the Emergency Action Plan both refer to additional analysis details submitted by GZA under separate cover. Unfortunately, those additional details were not available for review. History of Hazard Classification At one time in the past (prior to 1987) it appears that the Upper Dam may have had a "low hazard potential" rating. However, according the GZA Phase I report prepared in 2006, a 1987 inspection report prepared by the Massachusetts Department of Environmental Management estimated that a failure of the Upper Roberts Meadow Reservoir Dam could cause a domino failure of downstream dams. Therefore, GZA proposed that the hazard potential classification should be High. In 2007, GZA prepared an Emergency Action Plan (EAP) for the Upper Roberts Meadow Reservoir Dam. The EAP includes maps showing the areas downstream of the dam where flooding is estimated to occur after a fair weather failure of the dam and after a failure during the design storm (the 112 PMF). The results of GZA's analysis indicated that the Middle Dam and Lower Dam would be overtopped by the flows from a breach (failure) of the Upper Dam and likely breach also. The resulting flooding would impact several properties in Leeds. Therefore, GZA affirmed their previous hazard potential classification of "High" for the Upper Roberts Meadow Reservoir Dam.
  • The Friends of the Upper Roberts -5- October 27,2010 Meadow Reservoir and Dam Spillway Design Flood Estimate The following presents a summary of the work perfonned by GZA. In 2008, GZA prepared a Phase II report on the Upper Roberts Meadow Reservoir Dam. This report was prepared in response to an order from ODS to investigate the condition of the Upper Dam and propose a plan for remediation of the darn, which had been found to be in Poor condition in 2006. As part of the Phase II studies, GZA re-evaluated the potential for flooding downstream of the Upper Dam from a failure of the dam during a flood. They evaluated flooding during the Y2 PMF, the 500-year flood, and the IOO-year flood. They concluded that the Upper Dam would be overtopped during by all three floods and could potentially breach. However, for the IOO-year flood the depth of overtopping over the earth embankment was calculated to be only 0.6 ft. The duration of the overtopping was not given. GZA also concluded that the Middle and Lower Dams would be overtopped during the Y2 PMF whether or not the Upper Dam failed. For the 500-year flood, the Middle Dam would not be overtopped whether or not the Upper Dam failed and the Lower Dam would be overtopped slightly (by 0.2 ft) if the Upper Dam failed, but would not be overtopped if the Upper Dam did not fail. GZA then estimated the extra flooding that a failure of the Upper Dam would cause if it failed during the 12 PMF or the 500-year flood. They estimated that a failure of the Upper Dam during the 112 PMF would cause less than 2 ft of additional flooding throughout the downstream area to the Village of Leeds. However, for the 500-year flood, GZA computed a failure of the Upper Dam would cause 2 to 3 ft of additional flooding over that caused by the stonn without failure. This additional flooding would occur downstream of the Lower Dam as a result of it being overtopped by 0.2 ft and subsequently failing. On the basis of these findings, GZA suggested that the 500-year flood be designated the Inflow Design Flood, instead of the 12 PMF. [F or comparison, in the Regulations, the Inflow Design Flood for a Significant Hazard potential dam is the IOO-year flood and for a Low Hazard potential dam is the 50-year flood. The current spillway at the Upper Dam cannot safely pass the IOO-year flood, but the costs would likely be much less to improve the dam to pass the IOO-year flood rather than the 500-year flood.] Evaluation of Completeness of Prior Studies In general, the analysis methods used by GZA were appropriate and the dam breach parameters assumed in the analysis were reasonable. There are one or two areas where the analyses could be refined: 1. A May 2009 presentation by the Northampton Department of Public Works (DPW) and GZA included a bathymetric survey of the Upper Roberts Meadow Reservoir that showed the impoundment is silted in (the exact date of the survey is unknown). Prior to that survey, the nonnal capacity of the reservoir was estimated to be 35 acre-ft (approximately 11.5 million gallons). After the survey, the nonnal capacity of the reservoir was estimated to be 19 acre-ft (approximately 6 million gallons). In the 2007 EAP, GZA states that they assumed a nonnal capacity of 35 acre-ft. Although the 2008 Phase II report doesn't explicitly state the storage capacity assumed in the dam failure analyses, we assume the same capacity of 35 acre-ft was used. Therefore, if the dam break analysis was to be redone with the smaller reservoir storage capacity used, it is possible that the incremental flooding from a dam break during a 500-year flood would be less than 2 ft. It would then be reasonable to estimate the
  • The Friends of the Upper Roberts -6- October 27,2010 Meadow Reservoir and Dam incremental flooding from a failure during the 100-year flood to estimate whether the 100- year flood might be a suitable design flood. 2. GZA used the computer program "DAMBRK" to perform the dam failure analysis and estimate the downstream flooding. An alternative computer program that could be used today would be HEC-RAS. One advantage of using HEC-RAS is that more detailed topographic information can be developed to more accurately define the geometry of the stream channel downstream of the dam using a geographic information system (GIS). In fact, the City of Northampton has mapping with 5-ft contour intervals available. GZA used USGS topographic maps with 10-ft contour intervals (which may have been the only information available at the time). Evaluation of the HEC-RAS results is enhanced by exporting the peak water surface elevations into GIS over a digital elevation model (DEM). The GIS software automatically delineates the inundated areas and allows us to efficiently compare and evaluate potentially impacted structures, properties, roads, etc. The incremental impacts are assessed by comparing the dam failure versus the non-darn-failure conditions. Although we do not know how many or how detailed the cross sections were that GZA used in their DAMBRK model, it is possible that the use of additional cross sections may allow better definition of the fairly flat area upstream of the Middle Dam. This flat area could store flood water and attenuate the flood wave as it travels downstream. An attenuated flood could result in less incremental damage or fewer impacts, which provide justification for reclassifying the hazard potential of the dam. Using the 5-ft contour map available on the City of Northampton website, we computed an area of about 72 acres between the Upper and Middle Dams at El. 405, the elevation of the emergency spillway at the Middle Dam. The spillway crest at the Middle Dam is about El. 399, implying that there is approximately 430 acre-ft of storage available above the Middle Dam spillway crest before the emergency spillway at the Middle Dam would be activated. Appropriateness of High Hazard Potential Classification Based on the work completed to date, it is our opinion that the High hazard potential classification was appropriate. However, it appears that the prior work was based on a storage capacity of 35 acre-ft rather than the storage of 19 acre-ft that is currently in the reservoir, accounting for siltation. If the reservoir is to remain a recreational asset and is not dredged, it would be appropriate to compute the downstream flooding effects using the lower storage volume. This reanalysis might justify a lower hazard potential rating such as Significant. However, it is our opinion that we would not be able to show that the hazard classification should be "Low," because breach flows are likely to flood and potentially damage the adjacent secondary roads (Chesterfield Road and/or Reservoir Road). Feasibility of Filing a Hazard Reclassification with ODS The Office of Dam Safety has a procedure for requesting a hazard classification change. The requesting party will need to provide data and calculations that justify that change. In this case the basis for pursuing the hazard classification change would include: the more detailed topographic data; the reduced reservoir storage as estimated from the bathymetric survey performed since the 2008 Phase II report was completed; and possibly the opportunity to use the HEC-RAS model. The bathymetric data were presented in the 2009 and 2010 presentations by the Northampton DPW
  • The Friends of the Upper Roberts -7- October 27,2010 Meadow Reservoir and Dam and GZA. If the Friends cannot obtain the bathymetric data, a new bathymetric survey will need to be completed to update the DAMBRK model used in the previous study. If the DAMBRK model is not available, the dam failure analysis and hazard classification evaluation should be evaluated using recent bathymetric data, HEC-RAS, and GIS. Impact of Reclassification to a Lower Hazard Potential on Existing Cost Estimates to Rehabilitate the Dam The Upper Roberts Meadow Reservoir Dam is in Poor condition and several of the deficiencies will need to be addressed regardless of the hazard potential classification. These deficiencies include: • Trees and other vegetation on the embankments and dike. • Seepage through the spillway masonry blocks. • Seepage through the right embankment, particularly along the spillway retaining wall. • Sinkholes on the right embankment crest. • The loose fill upstream and downstream of the masonry core wall at the right embankment. • The lack of a functioning low-level outlet. If the dam is reclassified to a Significant hazard potential dam, the spillway will need to be enlarged to safely pass the 100-year flood or the right embankment and left dike will need to be raised to prevent overtopping or hardened to allow flow over the top without washing out the embankment fill. However, the spillway enlargement or embankment modifications will be significantly less expensive for a 100-year flood rather than a SOO-year flood. In addition, the stability analyses performed by GZA indicated that the masonry spillway was not stable for several required loading cases unless three-dimensional buttressing effects (due to the dam's curved shape) were included. We did not review the GZA stability analyses in detail, but changing the design flood from SOO-year flood to 100-year flood mayor may not resolve the stability issue. GZA used a simplified method to account for the 3-dimensional effects of the curved shape. A more detailed 3-dimensional evaluation might show that the dam can meet stability criteria. Although we cannot predict the full extent of the dam modifications needed without performing the additional spillway design flood/dam breach and stability analyses, it is our opinion that the costs to rehabilitate the dam if it is reclassified to Significant hazard potential would be less than half the costs to perform the full rehabilitation costs estimated by GZA for the High hazard potential classification. Given that the Lower Dam is only slightly overtopped (by 0.2 ft) by the breach of the Upper Dam during the SOO-year flood, it would be relatively inexpensive to raise the crest of the Lower Dam, if needed. .
  • The Friends of the Upper Roberts -8- October 27,2010 Meadow Reservoir and Dam Estimated Scope and Cost to Prepare and Submit a Hazard Reclassification Request to the State We have not prepared a detailed cost estimate to prepare and submit a hazard reclassification to the state. However, we can provide the following approximate budget numbers: Re-perform Inflow Design Flood/Dam break $25,000 studies to estimate the IDF Prepare and submit application to ODS $10,000 Bathymetric survey (if needed) $10,000 We have not provided a cost for additional ground survey work because we do not think it is necessary at this time. We appreciate the opportunity to be of service. Please let us know if we can provide any additional assistance. Sincerely, GEl CONSULTANTS, INC. Gillian M. Gregory, Ph.D., P.E. Senior Proj ect Manager GMG/rr M:PROJECf201O 10426Upper Roberts Meadow Dam Leeds MA Ltr Rpt Final.docx
  • Table 1 - Documents Reviewed by GEl Department of Public Works (2009). "Project Update, Upper Roberts Meadow Dam," City of Northampton, September 9. GZA GeoEnvironmental, Inc. (2006). "Phase I Inspection/Evaluation Report, Upper Roberts Meadow Reservoir Dam, Northampton, MA," prepared for Northampton Department of Public Works, March. GZA GeoEnvironmental, Inc. (2008). "Phase II Engineering Evaluation & Alternatives Analysis, Upper Roberts Meadow Reservoir Dam, Northampton, MA," prepared for Northampton Department of Public Works, March. GZA GeoEnvironmental, Inc. (2007). "Emergency Action Plan, Upper Roberts Meadow Reservoir Dam," prepared for City of Northampton, August. GZA GeoEnvironmental, Inc. (2009). Upper Roberts Meadow Dam Presentation 20090520, prepared for Northampton DPW, May. GZA GeoEnvironmental, Inc. (2010). Final Upper Roberts Meadow Dam Presentation 20100120, prepared for Northampton DPW, January. The Essex Partnership (2010). Letter to Mayor of Northampton, Northampton Board of Public Works, and Friends of the Upper Roberts Meadow Reservoir Dam, August 27.