What is Floodplain Analysis?  Floodplain Analysis is the determination of the water surface elevation, for the 100-year flood, at all locations along an existing stream, based on the upstream drainage area.

What does a Floodplain Analysis entail?

The water surface elevation is determined through various Hydrologic & Hydraulic (H&H) calculations. Hydrologic calculations determine the volume and flow of stormwater draining to the stream based on upstream soil types and land uses. Hydraulic calculations determine the size and shape of the structure (pipe, box culvert, etc.) that will collect and pass the stormwater obtained from the hydrologic calculations. We assemble the H&H calculations in report form and prepare a set of construction documents indicating the proposed drainage structure(s) (i.e. pipe, culvert, etc.). Once assembled this package is delivered to Federal Emergency Management Agency (FEMA) where they determine the technical adequacy of the H&H calculations and the proposed drainage structure.

Why should you contact D.L. Howell & Associates for Floodplain Analysis services?

D.L. Howell & Associates has vast experience working with FEMA to obtain Letters of Map Amendments (LOMA), Letters of Map Revision (LOMR) and Conditional Letters of Map Amendment (CLOMA) which are the necessary permits required when part of your development will be constructed within the 100-year floodplain.

Previous Floodplain Analysis Projects

Crystal Springs - A 130 Unit Townhouse Subdivision where the site entrance from Route 10, in Parkesburg Borough, crossed through a stream. Our analysis concluded the minimum possible concrete box culvert to safely convey the 100-year flow under the proposed roadway, without passing over the road.

Lafayette Square - A 125 Unit Apartment Building Development where the site entrance encroached within the 100-year flood zone of the upstream side of an existing culvert under Business Route 30 in Sadsbury Township, Chester County.  Our analysis concluded that the minor grading involved with the site entrance construction will satisfactorily separate the proposed roadway from the 100-year flood without causing an unsafe condition along the existing highway.

The Sands Property - The Sands Property was a private, residential property bisected by a stream and floodplain.  We provided analysis of the limiting nature of the floodplain and associated riparian buffers and designed a residential subdivision and road system around those limiting areas.