FLOOD10167

• Designation Number: 299
• County: Costilla
• Community: Unincorporated Costilla County
• Basin: Rio Grande
• Title: Floodplain Information Report - for the Rio Grande, Trinchera Creek, Culebra Creek, Colstilla Creek, and Selected Tributaries in Costilla County
• Date: 3/1/1989
• Designation Date: 11/1/1989
• Floodplain - Doc Type: Floodplain Report/Masterplan

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4.3 Review in the Field

The draft maps were reviewed in the field with 10cal staff
and residents. At the same time the approximate cross-sections
were reviewed. and the preliminary hydraulic calculations were
revised. On the basis of al1 of these steps the depths of
flooding listed in Table 8 were selected for the various stream
reaches in Costilla County.

In the field the basic question was asked. "Does the
combination of depth and width of flooding. in conjunction with
the estimated velocity of flow. make sense for this particular
stream reach?" Revisions were made both to the depth of
flooding shown and to the delineated floodplain boundaries. In
addition. in more urbanized areas, topographic details that
were lacking on the base maps were taken into account in
finalizing the delineations.

For the area immediately around the Town of San Luis. such
a field review was not necessary. The December 1988 detailed
study had already been field checked before it was finalized.
That study was coordinated with the CWCB. so no further review
was conducted.

On the Rio Grande a cursory field review of selected
portions of the CSU floodprone maps was conducted. By and
large, however. those maps were assumed to have been field
checked during their own preparation process. Because they
were based on gage analyses and rating curves developed for
field-surveyed cross-sections before they were ever field
checked. further field review was not deemed to be necessary.

4.4 Flooded Areas

Once all of the field review was complete. it was time to
finalize the maps. In most cases that meant drafting flood
boundaries that were slightly narrower than those shown on the
first draft maps. Where the field review indicated that the
floodplain was too wide on one side of the stream. an attempt
was made to assure that flood elevations were essentia11y equal
on both sides. within the limits of accuracy of the topographic
mapping. In several instances. roads and other features were
found to be higher in the field than the base maps led one to
believe. The maps were corrected, where appropriate. to show
such areas as being outside the floodplain or as islands within
the floodplain.

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