During the Risk Identification and Assessment Phase of a
project, engineering modeling and analysis is refined to further enhance the
flood risk identification. Existing
modeling may be refined to use more detailed methodology for calculating the
amount of water (hydrology) expected during a storm event, include additional
detail and gage analysis. The hydraulic
models to include additional refinement to cross-sections and stream crossings
that may restrict flow in larger events and improve channel and structure
information in existing models based on field survey.
Engineering modeling applies the flow volume calculated for
a certain storm interval and places that water into the natural channel
described in the hydraulic software. As
tributaries and other drainage features add into the main stream, the flow
volume increases as the model travels downstream. The modeling calculates the peak water
surface elevation determined at each cross-section and these peak values are
graphically described in a profile.
These peak values are then mapped on ground elevation information to
produce a floodplain delineation that identifies the expected flood extent
during the analyzed storm event.
Once these hydrologic and hydraulic information is prepared,
the models are used to produce a range of flood risk datasets that describe the
variability of flooding within the delineated floodplain. These flood risk datasets include:
- Changes
Since Last FIRM
- Water
Surface Elevation Grid
- Flood
Depth
Grids
- Annual
Percent Chance Grid
3
- 30
Year Percent Chance Grid
This phase of the project benefits greatly from community
interaction and coordination with local technical and operations staff,
providing an opportunity for FEMA and its mapping partners to engage local
knowledge as the modeling is prepared.
FEMA would like to work closely with communities to identify areas where
the modeling and floodplain mapping may not agree with on the ground accounts
of flooding equivalent to the 1% annual chance storm event. FEMA would like to use this phase to review
community comments and include any available technical information prior to
proceeding to the update of the Regulatory products (FIRM, FIS and DFIRM
database).
Flood
Hazard Refinement tasks undertaken for the study streams included:
- Provide
more detailed input information for stream crossings that had not been
surveyed during the GBRA or City of San Marcos analysis
- Include
the 2015 storm events in the historical analysis record
-
Analyze
additional sub basins and include new gage locations into the hydrologic
analysis
-
Prepare floodway analysis for selected streams