The Atwell project is situated on approximately 1,500 acres of alluvial fan rangeland 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 from the mountains and the development itself, so control of flooding is an important concern for the residential development project. On the other hand, the nearly three miles of creek channel that cross the project site represent an important natural habitat and a source of groundwater recharge, so it is important to preserve or enhance the water resources of the site. Balancing flood control with environmental enhancement is a major goal of the project and the reason PACE was hired to design an integrated water resources system for Atwell. PACE collaborated with the project owner site designers, geotechnical engineers, biologists, and landscape engineers to design an integrated system of manmade, naturalized stream channels and stormwater management basins located within the project’s open spaces. The streams and basins were designed to resemble natural features in function and appearance. The open space includes trails and active use areas that fit seamlessly with the water resources features. The open spaces provide wildlife habitat, recreation, flood control, and stormwater management.
PACE provided the hydrologic and hydraulic design and analysis of the project’s numerous large water resources features, including the development of hydrologic and hydraulic models to analyze and optimize proposed designs and ensure that the engineering methods used follow City, County and Federal standards. The HEC-1 and HEC-HMS hydrology models were used to simulate runoff from several design storms both on the proposed project site and on the mountainous watersheds uphill of the site, following County standard methods. The XPSWMM hydraulic model was used to route runoff hydrographs through the proposed system’s basins and channels and demonstrate a significant reduction of peak runoff rates compared to standard designs. The Flow2D and HEC-RAS 2D hydraulic models were used to evaluate existing and proposed conditions flooding on the project site and to demonstrate that the project will not adversely impact any adjacent or downstream areas. HEC-RAS and WSPG were used to model channel hydraulics. In addition to modeling, PACE provided FEMA floodplain revisions, sediment basin designs, coordination with several revue agencies, and construction drawings for key hydraulic features of the project.
The Atwell project includes the reconstruction of more than three miles of steep, unstable, impacted alluvial fan creek with a FEMA floodplain and floodway that existed before the initiation of the project. In addition to the difficult natural site conditions, the large Atwell project is being implemented in phases, necessitating phased floodplain revisions. PACE provided hydrologic and hydraulic analyses of existing and proposed conditions and prepared and processed FEMA submittals for the phased construction of the project, including CLOMR and LOMR submittals. Unique challenges with this project include the lack of a FEMA effective model at the onset of the project, requiring PACE to remap effective floodplains, and a steep proposed creek with alternating subcritical and supercritical flow regimes, which challenge the limitations of the standard HEC-RAS modeling software.