AGENDA DATE: 8/1/2018
TITLE:
Title
Update on Dam Safety Program
BODY
SUBJECT/PROPOSAL/REQUEST: Information is provided to familiarize the Board with County activities associated with dam safety.
ITEM TYPE: Consent Information Item
STAFF CONTACT(S): Richardson, Henry, Harper
PRESENTER (S): Greg Harper
LEGAL REVIEW: Not Required
REVIEWED BY: Jeffrey B. Richardson
BACKGROUND: Since the 1980s, the County has been responsible for properly operating and maintaining six dams regulated by the Virginia Department of Conservation and Recreation (DCR). A summary of these dams is included as Attachment A. These dams are earthen dams (as opposed to concrete) and generally located on County-owned properties: Mint Springs Park (2 dams), Chris Greene Lake Park, Walnut Creek Park, and on land adjacent to Monticello High School (Hillcrest Dam). One dam (Lake Hollymead) was accepted by the County to facilitate the acceptance of Timberwood Parkway by the Virginia Department of Transportation.
New DCR rules issued in 2008 require that owners of regulated dams perform an analysis of a dam break, or failure. The analysis involves mapping flood inundations zones to determine what structures (homes, businesses, and roadways) would be affected by a dam break. The purpose of the analysis is to:
• reassess the hazard potential classification of the dam - the hazard potential classification is an indication of the consequences (to life, structures, and property) of a failure and does not characterize the possibility of failure or the integrity of the dam
• determine if the dam has adequate spillway capacity to safely pass the spillway design flood (SDF) - the SDF is the most severe storm a dam must withstand
• update Emergency Action Plans (EAPs) - EAPs include a listing of the downstream structures that would be flooded due to a dam break
• monitor for new development within inundation zones
STRATEGIC PLAN: Infrastructure Investment - Prioritize, plan, and invest in critical infrastructure that responds to past and future changes and improves the capacity to serve community needs.
DISCUSSION: The County contracted with Schnabel Engineering to perform dam break analyses and inundation zone mapping. This work began in 2013 and was completed just recently in 2018. The total cost for the analysis of the six dams was approximately $144,000. The County received a $33,600 grant from DCR to support the work.
In 2014, the Virginia General Assembly passed legislation authorizing an evaluation of the Probable Maximum Precipitation (PMP) - a theoretical extreme storm event used in dam break analyses. The evaluation resulted in minor changes to the PMP values. For example, the 24 hour PMP event for Lake Hollymead changed from 36.5 inches of rainfall to 32.2 inches. DCR issued guidance on the use of the new values in March 2016. Schnabel used the updated PMP values for all subsequent work. The new values were not consequential to work already completed.
As a result of the dam break analyses, the hazard potential classification of two dams - Lake Hollymead and Chris Greene Lake Park - was changed from a lower classification to high. The higher hazard potential classification requires that these dams withstand a more intense SDF.
Despite the higher classification, the dam at Chris Greene Lake Park was determined to have adequate capacity. During the SDF, the emergency spillway - discernable by a 150-foot wide dip in the roadway just before the dam - would prevent water from overtopping the dam itself. No modifications to this dam are required.
The dam at Lake Hollymead was determined to not have adequate capacity. The emergency spillway is only a 48-inch diameter pipe and would not prevent overtopping of the dam. The analysis predicts that during the SDF, the dam would be overtopped by water flowing at a depth of approximately 4.9 feet at its peak. The energy of flowing water on the downstream slope of the dam would cause severe erosion and could possibly lead to complete failure of the embankment. To prevent failure, the County began a capital project in FY 15 to “harden” the downstream slope using articulated concrete blocks (ACBs) (see depiction in Attachment B). The ACB will be covered with soil and permanent turf. Construction of this project is expected to begin this fall and be substantially complete within eight months. The total cost for this project - including alternatives analysis, design, and construction - is estimated to be approximately $2.8M. The Board appropriated funding to support the project in FY 15, FY 16, and FY 17. The County received a $85,200 grant from DCR to support the design work.
County staff are currently updating EAPs for all six dams based on the new inundation maps. Following DCR approval of the EAPs, staff intends to conduct an emergency preparedness table-top exercise in conjunction with local emergency management officials, DCR staff, and other applicable personnel.
In addition to being responsible for the operation and maintenance of dams, the County is obligated to maintain dam break inundation maps submitted for any dam located within the County. This includes dams operated by the Rivanna Water and Sewer Authority (RWSA) and privately-owned dams. If new development is proposed within an inundation zone, staff contacts DCR and notifies developers that - if the new development causes a change in the hazard potential classification of an upstream dam and the owner must make improvements to the dam - the developer would be responsible for half the cost of the improvements.
BUDGET IMPACT: There is no budget impact.
RECOMMENDATION:
Recommendation
Staff recommends that the Board receive the attached report as information only.
ATTACHMENTS:
Attachment A: Summary Table of Six State-Regulated Dams Attachment B: Image Depicting Installation of Articulated Concrete Block (ACB)