Dear Ms. Watlington:

Fairfax Water, owner of the Upper and Lower Occoquan Dams, appreciates the opportunity to provide the following four comments on the Draft Dam Safety Guidance on Low-Level Outlets:

1. Page 4, Section IV, 2^nd Paragraph: “A low-level drain system is usually engaged to address problems posing an immediate threat to the safety of the dam...” 

Comment: Please clarify that the low-level drain system is intended to lower the reservoir when practicable and effective for addressing problems posing an immediate threat to the safety of the dam. As written, the draft guidance document may be interpreted as saying that the low-level drain is always used to solve any threat at a dam, which is not true. Consider adding a specific example, such as reducing the volume of water exerting force on a dam experiencing bulging. 

2.  Page 4, Section IV, 3^rd and 4^th Paragraph and list: “Below are specific examples of when a low-level drain system would be used; this list does not include all possible scenarios where reservoir lowering may be required. There is an immediate threat to the dam: Extreme floods threatening the safety of the dam and spillway system;” 

Comment:

A. Please change the first sentence of the 3^rd paragraph to, “Below are specific example of when a low-level drain system may be used…”  

B. Please clarify the statement that the low-level drain system is used for “Extreme floods threatening the safety of the dam and spillway systems.” Is this applicable only to dams where use of the low-level drain system is considered as part of the total hydraulic capacity of the dam, or when the EAP includes lowering the reservoir in advance of a major precipitation event? Even when designed in accordance with the references included in the draft guidance document, many low-level drain systems would have inconsequential capacity when compared to the reservoir inflow and spillway capacity during a design storm event. As written, the draft guidance appears to state that the low-level drain systems are always used during extreme floods and that the low-level drain should have the capacity to drawdown the reservoir even during an extreme flood event if the dam is experiencing a structural issue, which may not be practical or feasible in some cases.

3. Page 5, Section IV, 8^th Paragraph and list: “Dam owners are responsible for ensuring the following regarding the design of the low-level drain systems: Ensuring access to the low-level drain system is available at all times;”

Comment: Please remove this statement. It is not feasible to guarantee access at all times and under all conditions. 

4. Appendix 1 and references to it

Comment: Please clarify in the guidance document when the Code of Virginia Section 10.2-605 and Appendix 1 are referenced that Section 10.2-605 Part B applies to existing high hazard dams that do not meet the spillway design flood requirements in Table 1 of 4VAC50-20-50 and whose owners choose to meet the requirements set out in 4VAC50-20-53.  

Should you have questions or require clarification on these comments, please contact me or A.J. Wangner (awangner@fairfaxwater.org).

Regards,

Chad Coneway

Manager, Griffith WTP Production

Ms. Christine Watlington

Policy and Regulatory Coordinator

Virginia Department of Conservation and Recreation

Dear Ms. Watlington,

Please consider the following comments from the City of Lynchburg.

Installing or utilizing low level outlets on existing dams should be carefully considered on a case by case basis.  The guidance should be written to reflect that each circumstance and dam has unique challenges and that requiring the installation and operation of a low level outlet should take those circumstances into consideration and not automatically require this on existing dams. Retrofitting an existing dam to add a low-level drain system in most cases would be very costly and challenging and in some cases not possible. This would not be a one-time capital expense for installation; the dam owner would have to budget annually to ensure proper operation and maintenance of the drain. The Department should not impose such a significant financial obligation on all dam owners across the State.  

Also requiring retrofits, no matter the cost, makes little sense for low-hazard dams. By definition, there is no risk to people or property from a failure of these dams (“Low Hazard Potential is defined where an impounding structure failure would result in no expected loss of life and would cause no more than minimal economic damage.”) 4VAC50-20-40. Some low-hazard dams are even exempt from mapping, emergency preparedness planning, and other regulatory requirements because failure would “cause no expected loss of human life and no economic damage to any property” except to the property owned by the dam’s owner. 4VAC50-20-51. Asking a low-hazard dam owner to spend money to allow for drawdown is inconsistent with the State’s treatment under current safety regulations and is wasteful.

We have significant concerns related to utilizing the low level outlet to manage reservoir levels to respond to “Extreme floods threatening the safety of the dam and spillway system” as noted by DCR in the Draft Guidance.

There are many issues that a dam owner with a drain system must consider when deciding whether to drawdown reservoir levels in advance of an impending storm. The owner must try to predict the storm’s track and precipitation amounts. This is challenging, even for experienced meteorologists, and there are no established criteria for a dam owner to review that explain when drawdown should occur in advance of an impending storm. This assumes that it is even possible to access the dam during certain storms (some dams cannot physically be reached during flood stage).

The owner also must consider whether drawdown would be effective given the specific situation. In most cases, drawdowns would have minimal, if any, beneficial impact during significant storm events. Every new and existing dam should have or be in the process of upgrading the spillway based on the Spillway Design Flood as specified in Table 1 of 4VAC50-20-40 which would then safely pass any storm that was deemed a threat to public safety.  

In most cases, the additional storage capacity gained by lower reservoir levels and/or additional flow through capacity through the use of the low level outlet would be minimal at best.  In fact, the use of the low level outlet for either of these purposes could actually create a more significant public safety threat. The owner must also consider: (i) how fast water levels can safely be lowered without compromising the structural integrity of the dam which could mean lowering water levels would need to begin days in advance of a potential storm; (ii) whether lowering the water levels could impact a drinking water supply; (iii) whether the drawdown could result in negative environmental impacts such as large sediment discharges, low dissolved oxygen levels, fish kills, etc.; (iv) whether pressurization of the low level outlet pipe (especially in older structures) could potentially result in internal damage to the dam as a result of leaks and seeps; and (v) whether drawdown would be inconsistent with the dams existing Emergency Action Plan (and, if so, whether the plan should be revised).  

I am also submitting comments on behalf of the Virginia Municipal Stormwater Association as President that include recommended changes to the guidance that Lynchburg also supports.

Respectfully,

Timothy A. Mitchell, P.E.

Director

Lynchburg Water Resources