Atwell Regional Stormwater Management Master Planning

Banning, CA

PACE designed the re-envisioned Smith Creek, extending over 2 miles through the TriPointe Homes’ Atwell community in Banning, CA, to creatively balance stormwater management and flood protection with open space, native vegetation, and wildlife habitat. Stormwater management and flood control comprise an integrated system of manmade and naturalized stream channels and basins located throughout the 1,500-acre community. 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.

A major goal of the project was to create a stable, naturalized creek channel that supports native vegetation and hydrology, safely conveying flood flows through the site, and providing a natural amenity for residents and wildlife. The community resides within an alluvial fan that is too steep to support a soft-bottom channel without erosion protection. Therefore, PACE integrated more than 40 drop structures separated by reaches of soft-bottom channel to allow the creek to descend more than 320 feet over the 2.1-mile length of the main channel of Smith Creek. An innovative drop structure design composed mostly of rock reduced construction costs, while blending in with the natural desert landscape of the area.

Smith Creek and portions of the new Atwell Community are within a Federal Emergency Management Agency (FEMA) floodplain and floodway that existed before the community development was initiated. The lack of an effective FEMA model required PACE to remap the effective floodplains before initiating floodplain revisions that reflect the site’s flood control measures. In addition to the difficult natural site conditions, the community was developed in phases, necessitating phased revisions to the floodplain. PACE provided hydrologic and hydraulic analyses of 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 2.5 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 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 to provide flood protection for both Atwell and much of Banning, which has historically 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 functional as stormwater infrastructure, and also offer a beautiful aesthetic, providing residents with a natural amenity and trails for 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