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(1. Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003.1 Advisory Committee on Water Data, 1982) has been included in published flood discharge determinations, the Mapping Partner shall contact the RPO for approval before proceeding. Prior to performing a hydrologic analysis, the Mapping Partner shall work with the RPO to identify which, if any, of the hydraulic structures are to be included in the analysis (such as a large impoundment) and to identify appropriate methodologies for analyzing their impacts on peak flows and volumes. If effective FIS flood discharge data are to be used, the Mapping Partner shall verify that the data are current before proceeding. C.1.1.1 Floodplain Storage Considerations [February 2002] Large storage areas in a floodplain will significantly attenuate flooding within a community. The Mapping Partner shall evaluate attenuation using a standard flood routing technique. Storage in the floodplain may be uncontrolled, such as in detention ponds, isolated small natural depressions, and in wide floodplains of large rivers, or controlled with reservoirs. The requirements for performing hydrologic analyses of uncontrolled flood storage and controlled flood storage are presented below. Uncontrolled Flood Storage Uncontrolled detention ponds and natural depressions both provide uncontrolled flood storage. Detention ponds typically are used in developed areas for onsite storage, and these ponds limit post-development peak flow rates from a design storm to those of the pre-development stage. The ponds also are used for regional detention based on a master plan for the watershed area of interest. Depending on climate characteristics and local design standards that vary across the nation, detention ponds may be able to attenuate peak flow rates for a 1-percent-annual-chance storm for arid areas; however, in more humid areas, most ponds are designed for 20- to 50- percent-annual-chance stomlS. Usually, an ungated spillway and a low-level, ungated conduit comprise the detention pond outflow structure. The effectiveness of a detention pond in attenuating peak flow rates in the downstream reach depends on the pond's location in the watershed and its storage and release characteristics. While an onsite detention pond may be effective for a single development site, it may not be as effective for a large urban watershed that has many onsite detention facilities that are not located and designed systematically (Maidment, 1993). The Mapping Partner performing the hydrologic analysis shall analyze floodplain storage in small isolated natural depressions, where outflow is only through overflow, as uncontrolled detention ponds with appropriate outflow characteristics. The Mapping Partner may use both hydrologic and hydraulic routing methods to route the flow through ponds. Hydrologic routing methods are to be used when the outflow from the pond is not dependent on tailwater. Most of the single-event hydrologic models (e.g., HEC-HMS, HEC- 1, TR-20) use hydrologic routing methods. The Mapping Partner shall use hydraulic routing methods when outflow from the pond is dependent on tailwater conditions. For example, tailwater condition is a control factor where a series of interconnected detention ponds are used for flood attenuation in a relatively flat watershed. The hydraulic routing for ponds is often performed with an unsteady-flow model. A list of models accepted by FEMA for this purpose C-2 Section C.]