FLOOD05157

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: Manual For Estimating Flood Characterstics of Natural Flow Streams in Colorado
• Date: 1/1/1976
• Prepared For: State of Colorado
• Prepared By: CWCB
• Floodplain - Doc Type: Educational/Technical/Reference Information

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TECHNICAL MANUAL

DELINEATION OF FLOOD-PRONE AREAS

The following procedures can be used for outl ining flood-prone areas.

Direct field measurement:

I. Compute flood-depth information for selected sites along streams.

2. Visit selected sites and locate points of zero flow in channel near
site for base reference points (see fig. 12).

3. Add computed flood depths to points on streambed profile and u.e
surveying techniques to locate points on each side of stream which represent
1 imits of flood plains for the selected recurrence-interval flood.

4. Repeat above steps at enough sites to enable interpolation of flood-
plain boundary for study reach.

5. Flood-plain boundary points may be transferred onto topographic map.

Office measurement:

I. Using the methods described in this manual, compute discharges and
depths at several locations along each stream. Computations always need to be
made just downstream from each major tributary.

2. Plot profile of streambed based upon measurements on topographic map
of distance between contour crossings.

3. Plot depths computed in I as vertical distances above streambed
profile.

4. Draw flood profile using depths from 3.

5. From map study, locate where flood profile intersects land-surface
contours and mark locations on map. Some subjective judgment often is
required.

6. Draw flood-plain boundary between marked locations, being guided
by contour shape. Again, subjective judgment is required in many instances.
Accuracy of flood-prone area maps prepared by this method is closely related
to the contour interval of the map used. Large errors can result when maps
with large contour intervals are used.