Atwell Regional Stormwater Management Master Planning

Banning, CA


PACE designed an integrated system of manmade, naturalized stream channels and stormwater management basins that are placed throughout the 1,500-acre Atwell residential development project. The site is situated at the base of the San Bernardino Mountains and receives runoff from several steep, sparsely vegetated watersheds. The rare but intense desert rainstorms can produce flash floods, so balancing flood control with environmental enhancement is a major goal of the project.


The project included the reconstruction of more than 3 miles of steep, unstable, impacted alluvial fan creek with a Federal Emergency Management Agency (FEMA) floodplain and floodway that existed before the project was initiated. The lack of a FEMA effective model at the start of the project required PACE to remap the effective floodplains. In addition to the difficult natural site conditions, the project was implemented in phases, necessitating phased floodplain revisions. PACE provided hydrologic and hydraulic analyses of the existing and proposed conditions and prepared and processed FEMA Conditional Letter of Map Revision (CLOMR) and Letter of Map Revision (LOMR) submittals along with the sediment basin designs and construction drawings for key hydraulic features of the project.


The nearly 3 miles of manmade creek channel and 44 basins throughout the project site resemble natural features and provide a source for groundwater recharge. The 40 smaller on-site basins are situated along the creek to capture the runoff from smaller storms, meet National Pollutant Discharge Elimination System (NPDES) requirements, and cut the peak discharges from large events. The four large off-site basins are designed to capture debris and stormwater that enter the site from nearby foothill canyons and provide flood protection both for Atwell and for much of Banning, which has been threatened by uncontrolled flooding from these off-site watersheds. The largest basin is Smith Creek Basin, holding more than 600 acre-feet of water and requiring an excavation more than 100 feet deep covering 10 acres of land.


The creek and basins are not only functional, but also offer a beautiful aesthetic that provides residents with a natural amenity and trails for dog walking or recreation. The manmade creek and stormwater basins are all located within a continuous swath of open space that runs the length of the project, offering wildlife habitat, native habitats, walking trails, and outdoor activity spaces in addition to flood control and stormwater management.



Key Features:

  • Integrated water resources management
  • Naturalized stormwater management / treatment
  • Stormwater master planning
  • FLO-2D floodplain hydraulic analysis / mapping
  • HEC-1 hydrology modeling
  • XP-SWMM hydraulic modeling for routing runoff hydrographs through the basins and channels

Pardee Homes

Floodplain Mapping, GIS Water Resource Applications, Hydraulic Modeling / Research, Stormwater Management / Flood Control, Watershed Analysis / Planning