Although FEMA discourages development within the Special Flood Hazard Area (SFHA), the floodplain during a 100-year flood event (Base Flood), it is not always possible to avoid encroachment within the SFHA, especially when constructing new or replacement bridges and often when optimizing the feasibility of residential and commercial development. Even when encroaching within the SFHA, replacement bridges are usually designed to preserve existing flood hydraulic conditions. When optimizing the feasibility of residential or commercial development, replacing culverts or hydraulically deficient bridges with a new bridge, modifying existing bridges to meet current standards, constructing a bridge at a new location, or sometimes when replacing an obsolete bridge with a new bridge with an unavoidable increase in floodplain encroachment, the project may increase or decrease water surface elevations or the extent of inundation during the FEMA Base Flood. If this is the case for a project within an area having flood risk mapped by FEMA using detailed study methods, FEMA requires that their published Flood Insurance Rate Maps (FIRM) be corrected and that any increased risk of damage during the Base Flood be mitigated. Prior to construction FEMA approval of a Conditional Letter of Map Revision (CLOMR) is often required. After construction FEMA approval of a Letter of Map Revision (LOMR) is required. Other less rigorous FEMA applications may be required for projects involving fill in the SHFA with no associated impact on water surface elevations during the Base Flood and for correcting published FEMA maps based on more precise topographic data.
PHI has conducted and reviewed numerous flood studies prepared to identify potential flood impacts of development and to identify mitigation requirements necessary to avoid increases in flood risk. PHI has prepared map revision applications for several bridge projects and private developments where preservation of existing flood hydraulic conditions was not justified and flood risk impacts were mitigated as appropriate.
PHI also conducts flood studies for other purposes including meeting local agency requirements, supporting environmental studies, related to claims, and dam break flood studies required for state required Emergency Action Plans (EAP).
Example Projects:
Olney Creek Flood Study, Redding
Olney Creek has a perched channel as it approaches the Sacramento River. As a condition of development a levee was constructed to separate flood flows in Olney Creek from the left bank floodplain which decreases in elevation in a direction perpendicular to the channel and enters the Sacramento River farther away from the levee. Having inadequate data available to document the sufficiency of the levee to meet the minimum standards of FEMA for protection of structures during the most probable 100-year flood, FEMA de-accredited the levee and published a draft Flood Insurance Rate Map (FIRM) reflecting the interior area inundated to the Base Flood water surface elevations of the stream channel. The draft map included approximately 650 structures many of which were clearly not at risk of damage due to the vast opportunity for overflow to find alternative paths to the Sacramento River. The City of Redding was provided the opportunity to develop their own flood map of the interior area that FEMA would review and rely upon if sufficient.
Pacific Hydrologic Incorporated was retained to prepare this flood map early in 2010, well before any public announcement by FEMA of new mapping procedures. Recognizing that the PHI approach to mapping the interior area was not consistent with FEMA standards at the time, given the unique set of conditions at this site PHI chose to estimate flow entering the interior area by a weir equation over a bridge deck (not all flow entering the bridge deck returned to the channel downstream) and a breach equation at the hydraulically highest point along the levee. Within the interior area PHI identified the overflow floodplain using a backwater model and identified the breach floodplain using slope area calculations within road prisms. The interior flooding analysis also considered flow accretion from other sources and recognized the great opportunity for flow to enter the Sacramento River. Breach computations and assumptions were conservative and well documented. The resultant flood map indicated approximately 300 lots potentially affected by the Base Flood event with levee failure, and the structures on many of these lots were outside the flood limits. This study was submitted to FEMA in May 2010 for review with full expectation of comments requiring revision. Instead, the City of Redding received a letter with comments and agreement to accept the interior SFHA identified by PHI as Zone A. This study preceded public announcement of proposed new mapping procedures where the interior mapping would be identified as Zone D but stands as a successful early application of methods considered under the proposed new mapping procedures.
Foothill Boulevard over Stenner Creek, San Luis Obispo
During an inspection of a pair of large culverts conveying Stenner Creek under Foothill Boulevard around the year 2000, conditions associated with potential imminent failure were identified. The City of San Luis Obispo closed two lanes of traffic over the high risk portions of the culverts and programmed funds for replacement of the culverts with a bridge. The culverts were known to be inadequate to convey the Base Flood in Stenner Creek and therefore responsible for overflow affecting numerous residential and commercial structures included assisted living facilities.
Although the City of San Luis Obispo initially looked at bridge options that preserved flood hydraulic conditions, investment in a new bridge that continued to cause overflow was not considered a socially acceptable approach. Therefore a bridge capable of conveying the Base Flood without causing overflow was selected as the preferred replacement bridge. However, flow that once left the channel as overflow during the Base Flood would be contained in within the channel until reaching the next, even more restrictive bridge. Hence, even though many structures would no longer be inundated by overflow from upstream of Foothill Boulevard, the peak flow of the Base Flood and therefore flood risk along Stenner Creek between Foothill Boulevard and the next bridge downstream would be increased as a result of constructing the preferred replacement bridge.
Prior to constructing the preferred bridge PHI prepared a CLOMR application for the City of San Luis Obispo to submit to FEMA (FEMA map revision applications must be submitted by the participating local agency). Construction of the preferred replacement bridge commenced shortly after timely FEMA approval of the CLOMR. After the bridge was completed PHI prepared a LOMR application reflecting minor differences between the project as designed and as constructed. The LOMR was approved in a timely manner.
