Flood Control Study Using Ultimate Conditions Discharges for a Proposed Residential Development in Southern California
Exponent conducted an investigation of the flood potential at a proposed residential development located in Indio, California. The existing floodplain area adjacent to the project area is designated to receive flood flows originating from Thousand Palms Wash, The Indio Hills, and the Riverine Drainage Area along Interstate 10. These flood flows pass through the existing Sun City Palm Desert development via three flood control channels. A two-dimensional flood routing model (FLO-2D) was applied to study the progression of a flood flow in the interim floodplain area. The model computational domain for the interim floodplain area was discretized using uniform square grids. The elevation of each grid element was based on U.S. Geological Survey (USGS) Digital Elevation Model (DEM), a rough grading plan for the project area, and high-resolution Light Detection and Ranging (LiDAR) survey data. Topographic features identified from a field reconnaissance survey were also included in the model. Model simulations were carried out for a period of 20 hours. Initial conditions to the model included grid elevations, floodplain storage, change in floodplain depth, average hydraulic conductivity, average capillary suction head, initial and final saturation, initial abstraction, soil porosity, and Manning’s roughness coefficient. Model results were analyzed to depict contour plots of the maximum flow depths in the interim floodplain for the two discharge conditions simulated. Based on the model results, the peak discharge estimated to enter the project area was conveyed through a driving range located in the area. The U.S. Army Corps of Engineers’ (Corps) River Analysis System computer program (HEC-RAS) was used to compute water surface profiles for this peak discharge. Model results show that the computed water surface elevations in the channel are below the pad elevations specified in the rough grading plan of the project area. An examination of historical flood records indicated one major flood in the Thousand Palms alluvial fan. Aerial photographs taken one day after the storm indicate that floodwaters approached the site from two locations. Flow-through channels located along the project boundaries accept these floodwaters.

Investigation of Flood Hazard on Alluvial Floodplains
Exponent conducted an investigation of the 100-year flood hazard for a proposed development in Desert Hot Springs, California. To facilitate this investigation, detailed topographic mapping was obtained for the study area using LiDAR technology. The project site is located at the confluence of the Long Canyon alluvial fan on the east and an alluvial piedmont plain on the west. Tectonic processes and climatic factors continue to play a major role in defining the flood hazard in the region. To evaluate the most probable routes of future storm water flows in the study area, we reviewed historical aerial photo coverage of three floods that struck the area between 1974 and 1991. Data from these floods provide useful insight into the normal conveyance of storm water into the upper Coachella Valley from sources in the San Bernardino and Little San Bernardino Mountains. Flooding on the alluvial piedmont from this source was modeled between Pierson Boulevard and the site using FLO-2D. Two simulations were carried out to estimate the maximum discharge entering the project area from this source. Existing topographic conditions were used for the first simulation. The second simulation was carried out to investigate the effect of an overflow from Morongo Wash. Simulation results were used to establish the maximum discharge entering the project area. The HEC-RAS computer program was used to compute water surface profiles in the onsite flood control channels. Model results show that the computed water surface profiles in the channel are approximately at or below existing street elevations. The final pad elevations will accommodate the necessary freeboard in the channels. Proposed flood hazard mitigation measures protect the project from 100-year alluvial fan flooding hazards. A Letter of Map Amendment Revision (LOMR) request was sent to the Federal Emergency Management Agency (FEMA) to remove effective flood hazards from the map.

Flood Protection Plan for a Proposed Residential Development
Exponent conducted an investigation to develop a flood protection plan for a proposed residential development located in Indio, California. Flood control infrastructure in the upstream Sun City development, combined with the geomorphology and water distribution features on the intervening lands, dictate the location and timing of peak flows on the subject property. The objective of the study was to estimate maximum flood flows in the existing floodplain area for an ultimate conditions discharge at Sun City Palm Desert, to estimate the peak discharge into the project area, and to illustrate how this peak discharge could be conveyed out of the project.

FLO-2D was applied to study the progression of a flood flow in the existing floodplain for the ultimate conditions discharge. Elevations were based on USGS DEM, rough grading plans, and high-resolution LiDAR survey data. Topographic features identified from a field reconnaissance survey were also included in the model. Model results were analyzed for maximum flow depths in the existing floodplain and peak discharges into the project area. A channel was proposed to intercept the flows crossing the western boundary of the project and to convey the floodwaters southward along the western boundary and then eastward along the southern boundary of the project. To account for the progressive interception of flows along the western boundary, the flows in the proposed channel were progressively increased. Because of existing sheet flow in a southerly direction across the southern boundary, the flows in the proposed channel decrease with increasing distance in the easterly direction. This simulates the outflows across the southern boundary; lateral weir flow was assumed to occur.

Flood Insurance Studies

Detailed Flood Insurance Study for a Property Located in Huntington Beach, California
Exponent conducted a detailed flood insurance study (FIS) in support of a request for a Conditional Letters of Map Revision (CLOMR) for a property located in the City of Huntington Beach, California. FEMA requires a detailed FIS because the revised flood insurance rate map (FIRM) issued by FEMA in June 2000, based on an approximate study, shows the flood hazard at the property as an unnumbered A-Zone. Where FEMA has not provided detailed hydraulic studies, FEMA regulations state that the floodplain administrator must require a project proponent to prepare the FIS. Exponent studied the appropriate 100-year (1% annual chance) flood depth for the underlying floodplain in order to achieve adequate flood protection. Both riverine, and combined riverine and storm surge events were modeled. The study used detailed contour mapping, a Corps levee breach analysis, hydrology consistent with FEMA and Corps published discharges, an unsteady flow model which accounts for flood storage and unsteady tidal control, and proposed improvements with the property development.

CLOMR Submittal - SunCity Palm Desert
Detailed geomorphic studies were performed to provide a more accurate delineation of the flood hazard on the Pushawalla Canyon alluvial fan. A terrain feature known as a fosse, which is the intersection between two fan surfaces, defines the southern boundary of flooding on the fan. A CLOMR was prepared and submitted to FEMA for a revised fan based on physical change and improved methodology.

CLOMR-F and LOMR-F Submittals for a Residential Development
Exponent prepared a request for a Conditional Letters of Map Revision - based on Fill (CLOMR-F) and a Letters of Map Revision - based on Fill (LOMR-F) for a residential development partially within a special flood hazard area. Base flood elevations (BFEs) from a detailed study were projected across the proposed development. Graded pads were set to be above the 100-year (1% annual chance) BFE. The CLOMR-F was approved by FEMA, and with the submittal of as-constructed grading, the LOMR-F was also obtained.

Investigation of Fire-Flood-Erosion Sequence – Lowden Ranch Fire
A multi-disciplinary team from Exponent performed an evaluation of the fire-flood-erosion sequence accompanying the 1999 fire that occurred in a rugged, forested area near Lewiston, California. It began as a prescribed fire set by the Bureau of Land Management (BLM) that jumped control lines, burning 1,904 forested acres and 23 residences in four days.

According to the BLM Burned Area Emergency Rehabilitation Plan (BAER report), 30 percent of the burn area experienced high forest mortality. Most of the burn occurred on steep north-facing slopes underlain by deeply weathered Mesozoic granite. The slopes are incised by flat-bottomed drainages containing thick accumulations of granitic sand. Lewiston receives 35 to 40 inches of rain per year.

Investigation of Flood and Debris Flow Hazard
An investigation was conducted to evaluate the existing flood and debris flow hazard at the mouth of Andreas Canyon, a major, gauged watershed that drains from the rugged eastern slope of the San Jacinto Mountains in California. Previous studies indicated that the fan was different from archetypical alluvial fans, which form as a result of streams of water spreading sediment and cutting new channels on the fan surface. Instead, flood and debris flow processes had constructed a very coarse-grained debris flow fan at the mouth of the canyon, similar to other fans bounding the northern and eastern margins of the San Jacinto Range.

A single incised channel, Andreas Creek, currently conveys clear water drainage down the length of the fan from Andreas Canyon. A secondary channel branches off from the main channel near the mouth of the fan. During a 100-year flood event, the main channel is currently expected to carry about 96% of the total flow, and the secondary channel about 4%. Because the fan is constructed of coarse, boulder-sized material, the anticipated clear water flows are not sufficient to rework the material, and the existing flow paths can be relied upon to convey the flood flows.

Debris flow processes affecting the fan are much less understood. Eleven debris flow deposits of varying ages were mapped on the fan surface, ten of which originated in Andreas Canyon. Typical debris flows from Andreas Canyon are composed of cobbles and boulders in a sand-silt matrix. Granitic megaliths measure up to 7.6 m in diameter and are estimated to weigh up to 180 metric tons. The debris flows are very large, with a typical volume of about 105 m3. This volume implies that each time a flow occurs, essentially all available loose material is scoured from Andreas Canyon and transported onto the fan. The canyon is currently choked with coarse, boulder-sized material, conditions that are conducive to production of another debris flow under the appropriate hydrologic triggering event.

The maximum age of the soils on the fan has been estimated to be about 1,000 to 3,000 years. This age suggests a recurrence interval of major debris flows from the canyon of about 100 to 300 years. Archeological records, however, suggest that Andreas Canyon has not experienced a major debris flow in at least 350 years. If debris flows are possible under current climatic conditions, a major debris flow is overdue. Alternatively, the absence of debris flows during this period may indicate that their occurrence is tied to longer-term climatic fluctuations.

To aid in construction planning, a statistical analysis of boulder sizes was performed using a high-resolution aerial photograph for boulders ≥ 2 ft (0.6 m) that permitted calculation of approximate boulder size and weight distributions for the site. Geotechnical investigations included assessment of nearby active faults, anticipated levels of seismic groundshaking, differential seismic settlement, liquefaction, and hydrocollapse. Construction alternatives evaluated included over excavation of the native alluvium and replacement with an engineered fill pad; excavation of shallow footings into the native alluvium, construction of a deep foundation system (i.e., piles or caissons), or construction of a mat-type foundation system by linking the surface boulders with steel reinforcement and concrete slurry.

Congaree River Floodway Evaluation
Columbia Venture LLC, a land development organization based in Columbia, South Carolina purchased a large area on the eastern over bank of the Congaree River, proposing a $1.5 billion development project. On September 26, 2000, FEMA issued an Appeal Resolution Report covering the proposed development area. The report designated most of the proposed development area as a floodway, thereby substantially reducing development potential in the area.

Exponent provided services to the development organization by revising the complex two-dimensional analysis performed by FEMA in their investigation, using the most updated topographic information and current numerical analyses technologies. The results of this investigation demonstrated that sections of the proposed development area were not behaving as a floodway path. The study required highly specialized knowledge in one- and two-dimensional hydrodynamic modeling, hydrology, and use of Geographical Information System (GIS) tools, combined with an in-depth knowledge of FEMA floodplain policies.

Eden Again: Mesopotamian Marshlands Restoration
As part of an international technical advisory panel led by the Iraq Foundation (a non-profit organization), Exponent worked to restore the Mesopotamian marshlands of southern Iraq, which were drained by the previous Iraqi regime. Exponent gathered updated topographic information for the region, evaluated existing hydrological conditions, organized GIS data, and simulated hydrodynamic circulation in the marsh regions using complex one- and two-dimensional hydrodynamic models. The focus of the study was a preliminary evaluation of marsh hydrodynamics on a macro-scale level. Topographic maps and satellite images were used to model many of the hydraulic structures blocking the flow of water into the marshes.

Six different modeling scenarios were developed to display the potential water distribution resulting from hypothetical water releases over the Al Hammar and Central Marshes. These models required assumptions to be made on the operation of some of the diversion and drainage structures. Computer models used for this project were based on non-proprietary, widely accepted models. Modeling results showed that, even for very conservative assumptions of water availability, enough water was available to partially restore the marshlands.

Click here for more information on the Exponent’s role in the Eden Again project.

Huntington Beach Wetlands Modeling
To investigate the possibility of restoring the Mills property in Huntington Beach, California as a fully-functional tidal wetlands area, Exponent developed a two-dimensional, vertically-averaged, finite element hydrodynamic model (RMA-2 from Corps Waterways Experiment Station [WES]) to study flood patterns, tidal influences, and salinity changes. The model extends from Atlanta Avenue, through the Talbert wetland, and through Talbert outlet to the ocean.

An existing mathematical hydraulic model of the Talbert Valley Watershed including Huntington Beach Channel, Talbert Channel, and Fountain Valley Channel and all urban areas from the Talbert Outlet to the I-405 freeway was used to develop the boundary conditions for the RMA-2 model. This hydraulic model is a two-dimensional, unsteady, unconfined finite difference model that uses rainfall input to generate flooding depths and outflow hydrographs for the regional flood control channels. Model input can be any precipitation event, or a continuous series of precipitation events and/or periods of no precipitation, combined with dry weather urban runoff.  The hydraulic model was employed to generate upstream boundary conditions for a two-dimensional, unsteady, finite element model RMA-2 developed for this study. Model boundary conditions were the ocean tidal regime at the downstream end, and unsteady flow inputs at the upstream ends. Initial conditions to the model included the bathymetric depths and initial water surface elevations.

The hydrodynamic model was coupled to the water quality model (RMA-4 from Corps WES) to determine the interaction between freshwater inflows at the upstream boundaries and the saline water at the ocean boundary. Model output included circulation patterns, velocities, and water depths resulting from the tidal flooding and ebbing. In addition, temporal and spatial distribution of salinity provided an assessment of the salinity regime resulting from tidal action. The RMA2/RMA4 modeling framework can easily be extended to include additional proposed wetland restoration areas.

San Elijo Lagoon Restoration Project
San Elijo Lagoon in Encinitas, California is separated from the ocean by a narrow gap in a railroad embankment. Circulation in the bay is a key element in preserving the bay’s freshwater ecosystem. Exponent was retained to conduct a feasibility study for the relocation of the inlet of the San Elijo Lagoon. A number of possible scenarios were investigated to improve the hydrodynamic circulation inside the lagoon and to reduce sedimentation rates at the inlet mouth.

The study successfully integrated results from one and two-dimensional hydrodynamic models. This allowed the project team to evaluate possible solutions for different configurations of the lagoon inlet. In addition, a two dimensional finite difference water quality model was used to predict salinity intrusion into the lagoon for various inlet relocation alternatives. Our investigations were founded upon extensive reviews of existing studies on the hydrology and sedimentology of the streams that feed the lagoon.

Ventura Keys and Arundell Barranca Project
The Ventura Keys is a residential waterfront development consisting of three main channels, a stub channel, and a connecting channel to Ventura Harbor, California. Flow patterns in the keys and the harbor were severely affected by adverse sediment deposition resulting from reconfiguration of the Arundell Barranca Channel outlet structure. Homeowners claimed that this reconfiguration led to shoaling in the channels, making the waterways unnavigable and susceptible to increased levels of bacteria in the water, effectively prohibiting both human contact with water and the recreational use of two private beaches in the keys. Exponent was retained by the City of San Buenaventura to perform an investigation and feasibility study to improve circulation in the Ventura Keys and Ventura Harbor. Our investigations included a review of the physical model and of the underlying conditions that resulted in the reconfiguration of the Arundell Barranca Channel.

Our team applied two- and three-dimensional models that simulated the effects of a proposed channel reconfiguration, of tide-induced circulation in the Keys and Ventura Harbor, of a proposed debris basin on the amount of sediment inflow into Ventura Harbor, and of a proposed change in channel geometry to create a density current that would reduce dredging costs. We studied the sources and quantities of bacteria deposition in the Keys and analyzed an array of scenarios to mitigate the bacteria problem. Our team of scientists and engineers has been instrumental in enabling the City of San Buenaventura develop a strategy to resolve residents’ concerns and improve circulation in both the Keys and Ventura Harbor without incurring the unrealistic expense of diverting all the flow in the Arundell Barranca Channel into the Santa Clara River. The proposed channel extension and debris basin will also allow for much reduced dredging operations.

The City of Beverly Hills Master Plan of DrainageThe City of Beverly Hills, California, is characterized by premium commercial and residential developments in which flood-associated damages could easily total millions of dollars. The City needed a comprehensive analysis to determine the adequacy of the existing storm drain system. The City asked Exponent to determine the locations of drainage deficiencies, the dimensions of any facilities needed to relieve these deficiencies, the associated costs to build or renovate the proposed facilities, and a facility construction prioritization based on an annual capital improvement program (CIP) budget.

Exponent developed hydrologic parameters and hydraulic characteristics in a GIS environment and imported them into drainage software that was developed by team members. We also developed several other decision-making modules to evaluate the existing drainage system deficiencies and the costs of proposed drainage improvements, in order to create an effective construction prioritization ranking. As a result of these analyses, the master plan we presented to the City identified and outlined approximately $11.5 million in drainage deficiencies and prioritized the construction of proposed relief drainage facilities into 23 annual CIP projects with annual budgets of $500,000 each. Pinpointing the priority drainage projects will allow the City to relieve all but the very lowest-risk drainage deficiencies in the first 10–12 years of CIP projects.

City of Northglenn Water Supply System ModelGrowth and development in the City of Northglenn, Colorado, are causing increased demands for water. The City meets these demands by drawing from a number of sources using a complex system of conveyance, storage, and treatment facilities. To ensure an adequate, reliable water supply now and into the future, the City uses a computer model to help make water resources planning and management decisions, but their current model does not have the flexibility to keep pace with changing needs.

Exponent and City personnel are working closely together to develop a new model that will serve the municipality's current and future needs. The model will be used to evaluate the benefit in yield that could be gained from adding new sources of supply, increasing storage, and changing operations. The modeling approach adopted is a radical departure from the proprietary, black box models of the past. The new model is an Excel-based tool that combines the inherent power and capability of Excel with a Visual Basic program. The fundamental compatibility and interrelation of Visual Basic and Excel provide several advantages. City staff can change and modify the program to fit their changing needs, and the utility of the model is enhanced by a graphical user interface that capitalizes on the graphic orientation of Visual Basic.

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