BOARD00185 ‎(2)‎

• Board Meeting Date: 5/23/2007
• Description: Intrastate Water Management and Development Section - Water Supply Reserve Account Applications for Grants
• Board Meetings - Doc Type: Memo

>>

\

storage capacity is also included with this supplement. Generally, the needs can be placed in one of the .'
following categories, in this order of priority:

5.

1.

Outlet Works - Deterioration of outlet components such as pipes and gates is one of the greatest
safety and serviceability concerns for all dams, particularly older ones which may have un-protected
steel pipes. Erosion and removal of embankment or foundation material along the outside or into and
through the pipe can result in sudden catastrophic failure even for dams that have had a long history
of stability. Additionally, the risk is increased at sdme of the project dams which have downstream
control valves. Downstream valves pressurize the ~utlet pipe. Therefore, the proposed work includes
focusing attention on the outlet systems for all of tHe dams.

Seepage - Uncontrolled seepage, particularly seepjge which exits on the embankment, at the
embankment/foundation contact, in the foundation klightly downstream from the toe of the
embankment or along the outside of the outlet pipe': is of a concern. If the seepage forces or pressures
are great enough, material can be eroded from the etnbankment or foundation, or the pressures can
de-stabilize the dam slopes. Methods to address s~epage range from channeling and measuring to
installation of filters and drains. The proposed work includes identifying significant seepage areas.
I
Slope Stability - As experienced with the Carl Smith failure, a long history of embankment stability
does not assure either perpetual security or adequat~ safety factors under all possible conditions.
Slope stability is affected mainly by the steepness or the slopes, the characteristics and strength of the
embankment and foundation soils and the seepage forces and pressures. In order to adequately
address slope stability issues a considerable amoun~ of work is needed including drilling test holes
through the embankment and foundation and installing and monitoring piezometers. That degree of
effort is considered well beyond the scope of work iIeeded to prepare the proposed inventory.
,
However, it is possible to perform a preliminary stability analysis using soil samples from shallow
hand-auger holes and based on observations of dOwPstream surface seepage. The proposed project
includes this preliminary level of stability evaluation.

Spillway/Flood Hydrology - Except for transient mLntenance-related issues such as debris and ice
accumulation, a long history of spillway adequacy 9an indicate some degree of spillway security.
Therefore, this is the lowest priority item and relatecl aspects of the proposed study will be limited to
field observations of spillway conditions, identification of spillway adequacy modeling previously
performed by the Division of Water Resources, and!research of typical means used to identify
spillways that are susceptible to ice damming and e~fective means used to control ice damming.
Future investigations may pursue this aspect of the facilities at a more detailed level. If, at some
future point, it appears appropriate to pursue spillwJy and flood hydrology issues in a more
com~rehensive m~nner, it wo~ld b~ best to evaluate I the. enti~e basin rather than individual dams.
This IS because spIllway conSIderatIOns can be subs~antIally Impacted by both upstream and
downstream dams. For example, a small dam may qot pose a great breach hazard if a larger
downstream dam could contain the entire capacity of the smaller dam within its freeboard. On the
I
other hand, a dam which has adequate spillway capacity when considered by itself may pose a much
greater hazard if the breach of an upstream dam cre~tes a cascading series of breaches in this dam
and, perhaps, other downstream dams. A comprehe~sive evaluation would likely consist of creating
a flood hydrology model of the entire basin that could be used to evaluate the impact of a wide range
of precipitation events. I
I
General Dam Condition - Overall characteristics of the dam and appurtenant features which are
I
relevant to safety and serviceability will be observedl and noted.

2.

3.

4.

2

.

.