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 As written, the draft guidance document may be interpreted as saying that the low-level drain is always used to solve any threat at a dam, which is not true.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 Even when designed in accordance with the references included in the draft guidance document, many low-level drain systems would have inconsequential capacity when compared to the reservoir inflow and spillway capacity during a design storm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 As written, the draft guidance appears to state that the low-level drain systems are always used during extreme floods and that the low-level drain should have the capacity to drawdown the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