Hydrologic Design for Wetlands

Roger Kilgore, P.E.
Kilgore Consulting and Management
2963 Ash Street
Denver, Colorado 80207
RKilgore@KCMwater.com

And

Joseph Krolak, P.E.
Eastern Resource Center
Federal Highway Administration
10 S. Howard Street, Suite 4000
Baltimore, MD 21201

The AASHTO Model Drainage Manual and Highway Drainage Guidelines recently added information pertaining to wetland hydrology and water budgets for wetland designs. The author is currently preparing a lesson in wetland hydrology to include in the Practical Highway Hydrology NHI course based on these AASHTO documents and other literature. The process of developing "practical" design guidance revealed many questions unanswered in the literature including:

1. Why should the designer select the driest and wettest years for analysis rather than years representing exceedence probabilities appropriate for the site, e.g. the 10% or 90% years?

2. Is there a better way of selecting a "normal" year than median annual precipitation? (Distribution of precipitation throughout the year may vary significantly, thereby affecting wetland hydroperiods.)

3. What differences in interpretation are likely when using monthly versus daily calculation intervals?

4. Should a rolling average of rainfall be used to identify runoff producing events rather than treating each 24-hour day as an independent event? (What role roes the size of watershed have in this issue?)

5. How can we better summarize hydroperiod variations beyond the common maximum depth versus month graph? Some suggestions include duration, depth, frequency curves, inundation versus month graphs; and average depth versus month graphs.

This paper will explore potential answers to some of these questions and suggest areas of development to support the hydrologic engineer in wetland mitigation design.