Hydrology and Floodplains Sample Clauses

Hydrology and Floodplains. Executive Order 11988 (Floodplain Management) defines a floodplain as any lowland and relatively flat area adjoining inland (or coastal) waters, including that area subject to a one percent chance or greater of flooding in any given year (42 FR 26951). Tuacahn Wash qualifies as a floodplain due to flooding frequency, although Padre Canyon has not been characterized as a moderate or special flood hazard area by FEMA. Runoff from Tuacahn Wash drains through Santa Xxxxx City and eventually to the Santa Xxxxx River via Lava Flow Wash. The channel is 10-50 feet wide with banks up to 10 feet tall. Downstream from the Reserve, the channel has been narrowed by encroaching developments and the area previously available for water detention has been reduced. There is no aquatic habitat and no wetlands or other water-supporting features in the canyon. Additional information on Tuacahn Wash hydrology can be found in Alpha Engineering (2001).

Hydrology and Floodplains. As a result of dam and basin construction, Tuacahn Wash would convey sediment and floodwaters at a reduced rate. In the event of an average 100-year storm, water would be discharged at a rate of 386 cfs below the dam, relative to 1,456 cfs unobstructed. Although the basin would empty within 24 hours, water would be discharged over a longer period than under natural conditions due to the reduced rate of flow, and natural low-water flows may be impeded. Sediment load in discharged water would also be less as it would settle within the basin during detention. The width of the floodplain itself would be narrowed as a result of dam and basin construction. Alpha estimates that the existing area of flow is 31.26 acres (between State Parks property north of the proposed dam and subdivision property to the south) and would be reduced to 7.39 acres with the installation of the detention dam (see City of Xxxxx Detention Basin Project 30 Environmental Assessment Diagram 4, Appendix A). Connectivity of the floodplain to the larger watershed would be reduced as a result of this narrowing as well as the reduced rate of flow. No natural hydrologic processes that provide for ecological, biological, or fishery functions, however, would be adversely affected. Long-term impacts to hydrology and floodplains would be negligible.

Hydrology and Floodplains. Tuacahn Wash would convey sediment and floodwaters at a similar but slightly reduced rate under Alternative C. Water may be discharged over a longer period than under natural conditions due to the slightly reduced rate of flow but would likely be similar even under low-flow conditions. The width of the floodplain itself would be similar to its current state, with an area of flow unlikely to be much less than 31.26 acres (see Diagram 4, Appendix A). Connectivity of the floodplain to the larger watershed would also remain largely unaffected. Long-term impacts to hydrology and floodplains would be negligible.