1987 Wetland Delineation Manual - The Wetlands Regulation Center

Wetland Delination Manual, 1987 36
(1) Routine approach. Determine the highest water level elevation reached during the growing
season for each of the most recent 10 years of gage data. Rank these elevations in descending
order and select the fifth highest elevation. Combine this elevation with the mean sea level
elevation of the gaging station to produce a mean sea level elevation for the highest water level
reached every other year. Stream gage data are often presented as flow rates in cubic feet per
second. In these cases, ask the CE District's Hydrology Branch to convert flow rates to
corresponding mean sea level elevations and adjust gage data to the site. Compare the
resulting elevations reached biennially with the project area elevations. If the water level
elevation exceeds the area elevation, the area is inundated during the growing season on
average at least biennially.
(2) Comprehensive approach. Complete the following:
(a) Decide whether hydrologic data reflect the apparent hydrology. Data available from the
gaging station may or may not accurately reflect the area hydrology. Answer the following
questions:
• Does the water level of the area appear to fluctuate in a manner that differs from that of the
water body on which the gaging station is located? (In ponded situations, the water level of the
area is usually higher than the water level at the gaging station.)
• Are less than 10 years of daily readings available for the gaging station?
• Do other water sources that would not be reflected by readings at the gaging station appear to
significantly affect the area? For example, do major tributaries enter the stream or tidal area
between the area and gaging station?
If the answer to any of the above questions is YES, the area hydrology cannot be determined
from the gaging station data. If the answer to all of the above questions is NO, PROCEED TO
(b).
(b) Analyze hydrologic data. Subject the hydrologic data to appropriate analytical procedures.
Either use duration curves or a computer program developed by WES (available from the
Environmental Laboratory upon request) for determining the mean sea level elevation
representing the upper limits of wetland hydrology. In the latter case, when the site elevation is
lower than the mean sea level elevation representing a 5-percent duration of inundation and
saturation during the growing season, the area has a hydrologic regime that may occur in
wetlands. NOTE: Duration curves do not reflect the period of soil saturation following
dewatering.
When all of the above have been considered, PROCEED TO STEP 9.
• STEP 9 - Determine Whether Hydrology Is Adequately Characterized. Examine the
summarized data and determine whether the hydrology of the project area is adequately
25 February 1997 Environmental Techncal Services Co. 834 Castle Ridge Rd Austin, Texas 78746