BOARD01965

• Board Meeting Date: 3/22/2005
• Description: Flood Section - Presentation of the CWCB's Draft Rules and Regulations for Regulatory Floodplains in Colorado
• Board Meetings - Doc Type: Memo

Gi/idelines and SpeclficationsJor Flood Hazard Mopping Partners [April 2003]

A.4

Data Requirements

[February 2002]

Requirements for topographic mapping, cross sections, and hydraulic structure surveys will vary
from Flood Map Project to Flood Map Project, depending on the flood hazard being addressed
(e.g., riverine, coastal, alluvial fan) and the option that is selected for generating flood hazard data.
Detailed information on the options for generating flood hazard data is presented in Volume I,
Section 1.4 and in Appendix C of these Guidelines.

New flood hazard data may be generated through detailed or approximate analyses. Updated flood
hazard data may be developed through detailed or approximate analyses or redelineation of
floodplain boundaries from the effective FIRM using more up-to-date and/or more accurate
topographic data than were used to prepare the effective FIRM. The topographic mapping, cross
section, and hydraulic survey requirements for each analysis option are summarized below.

.

For a detailedflood hazard analysis, which will generally include a detailed hydraulic analysis,
digital topographic data, cross sections (to include underwater elevations), and surveys of
hydraulic structures are required. Ground surveys and either photogrammetric mapping or
LIDAR-generated mapping are normally required unless suitable topographic information is
already available from other sources.

.

For an approximate flood hazard analysis, cross sections may be interpolated from contours on
topographic maps, and underwater elevations may be interpolated from uJlstream/downstream
data, assuming the channel bottom information has not changed significantly. Hydraulic
structure surveys are not required.

.

For a redelineation of floodplain boundaries using more up-ta-date or more accurate
topographic data. the topographic data needed to update the floodplain boundaries are required,
but no new cross sections or hydraulic structure surveys are required.

The requirements summarized in this Appendix are based on the assumption that suitable data do
not exist from alternative sources and that new ground/aerial surveys will be required.

A.4.1

Map Scale of DFIRMs and Base Maps

[February 2002]

Detailed information on how DFIRM map scales are determined is presented in Appendix K of
these Guidelines.

A.4.2

Data Models and Surfaces

[February 2002]

Mapping Partners may produce digital elevation datasets in many forms. For purposes of the
National Flood Insurance Program (NFIP), the default surface is the bare-earth terrain, devoid of
vegetation and manmade structures. The FEMA Lead for the Flood Map Project (usually, the
Regional Project Officer (RPO) or Project Officer (PO) at FEMA Headquarters) may specify the
use of one or more of the following data models:

. Digital contours;

A-tO

Section A.4