San Luis Obispo Creek at Avila Beach
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The feasibility of providing 100- year flood protection for developed
properties along all of the major streams in the watershed is limited by the large amounts of
runoff generated during storms. The terrain and hydrology of the watershed are conducive
to frequent and widespread flash flooding along the major streams. The hydrologic analysis performed for the SLO Waterway Management Plan
revealed that urbanization of the watershed has had only minor effects on larger flooding
events; severe flooding was common even under pre-development conditions. To a certain
extent flooding is inevitable and the most cost effective/best strategy may to avoid building
on flood prone areas in the lower watershed, selectively purchase buildings subject to
recurrent flooding, and fix only the generally localized areas where flooding occurs to
reduce the frequency and depth of flooding and flood damage. Flood-proofing of buildings
in flood prone areas can also be an important strategy.
The SLO Waterway Management Plan included a number of recommendations for managing vegetation in channels to decrease flood risk. (Please note that most of these activities require permits from regulatory agencies.)
"Management of willows at hardened bars (and other areas with dense, in-channel
vegetation) is a critical maintenance need and management problem. Informed management
of the flood-modified channel and banks affords an opportunity to reduce the impact of
future floods. Because of system wide channel capacity constraints, perennial vegetation in
the active, most efficient part of the channel should be maintained at a low level. Willows
should be discouraged from re-colonizing point bars for more than two or three consecutive
years. If the growth persists, there will be increased flow friction losses and sediment
deposition. This will increase the depth of flooding, or cause lateral erosion, particularly at
channel bends. In contrast, performing selective vegetation removal and replanting with
desirable species (replacing undesirable exotic vegetation, plugging gaps in canopy cover,
or planting areas denuded by the 1995 flood) will help stabilize the bank, direct deposition
into toe protecting berms, and reduce the sediment supply of future floods. Detailed
procedures for vegetation management are included in the SLO Land Conservancy Watershed Enhancement Plan."
"Even with vegetation management, structural channel modifications will be needed to
reduce flood risk in a number of areas of the City. Flooding problems along SLO Creek and
proposed flood management solutions for specific areas are discussed in Section 6 of the SLO Land Conservancy Watershed Enhancement Plan."
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