Storm Water Engineering

Coe & Van Loo’s Storm Water Engineering team can prepare comprehensive drainage feasibility studies in order to understand the technical design issues and the financial constraints for our client’s development project. We thoroughly research the watershed and regulatory challenges before costly investments have been made. We understand that drainage infrastructure construction costs and the lengthy regulatory permit process, associated with drainage issues, can be a major factor in a project’s chance for success.

Coe & Van Loo has extensive experience preparing the conceptual level planning studies that precede detailed design of structural improvements.  The planning studies provide regional regulatory, environmental, and hydrologic context for a proposed project in order to identify significant issues, both technical and managerial, requiring on going and future resolution in order to establish analysis and design protocols that are appropriate for the site-specific constraints.  The purpose of the master planning studies is to provide direction that is both general and specific regarding the scope and the design intent of the proposed structural improvements.

Coe & Van Loo has extensive experience in preparing of floodplain delineation studies for government agencies, as well as private development clients. These studies are prepared to identify land areas (floodplains) subject to inundation by a flood that has a one-percent probability (100-year flood) of being equaled or exceeded in any given year. These floodplain delineations are ultimately incorporated onto the Flood Insurance Rate Maps (FIRM). This occurs after extensive reviews from local agencies, the State of Arizona, and the Federal Emergency Management Agency (FEMA).

We have conducted approximate, detailed and alluvial fan delineation studies. We use methods consistent with the level of detail or complexity needed for performing the hydraulic analysis, including; HEC-RAS models, 2-dimensional models and geomorphic analysis. The studies are typically submitted to FEMA as a Conditional Map Revision (CLOMR) and/or a Letter of Map Revision (LOMR).

Coe & Van Loo has an extensive background in both hydrologic and hydraulic analysis. We have applied our expert knowledge of hydrology and hydraulics for a wide variety of projects. We apply the appropriate modeling techniques to meet the diverse demands of site constraints and regulatory requirements. Streets or channels and some desert washes require very simplistic hydraulic computations, such as Manning’s calculations or HEC-RAS modeling. However, terrain that is mostly flat, alluvial fans or braided flow regimes may require 2-dimensional modeling methods.   

We have designed a vast amount of flood conveyance facilities including overland channels of various sizes and materials, spillways, grade control structures, and culvert crossings. We have designed an extensive amount of storm drain pipe for municipal, infrastructure, and subdivision applications. The site constraints and design elements are considered to ensure a system that is efficient and effective. Value engineering is a key component in the design of these facilities. We investigate all available options to ensure construction costs are minimized while land use is optimized.   

Hydrologic simulation modeling of the regional watershed is typically performed at the master planning stage in order to develop hydraulic design constraints for proposed regional flood control facilities and general civil site design. Regional modeling uses hydrograph analysis as a more accurate way of evaluating the impact of the proposed development on flood runoff within a specific watershed having unique climate and hydro-meteorological characteristic. The simulation modeling is typically regional in scope but it is also used to integrate specific local drainage improvements associated with smaller areas that are designed with non-hydrograph methods in order to evaluate cumulative impact at selected locations of interest.

Coe & Van Loo has considerable experience applying the traditional hydrologic modeling techniques to the alluvial watersheds common in the southwest. The additional element of risk and uncertainty associated with flow distributions in the broad flat floodplains common in alluvial environments creates a level of complexity that is addressed by combining geomorphic analysis with traditional hydrologic modeling. Geomorphic analysis provides an estimate of the magnitude of ‘breakout’ flows – avulsed flows that abruptly change path – based on topography, surface deposits indicating geologic age, aerial photography and other empirical data available for the watershed. The results of the geomorphic analysis are additively superimposed with the results of the hydrologic modeling to provide design flows for the structural facilities and water surface elevations for civil grading design.

Development in arid and semi-arid alluvial environments create serious flooding problems that cause the technical community to develop guidelines and procedures for more accurately estimating flood hazards in areas subject to overbank flooding and active geomorphic response patterns, including both scour and deposition. The general procedure is the same in all communities – scour is estimated as the sum of multiple components that are known to induce scour. Local communities and specific developments differ in the way these scour components are defined.

Several of the components are defined through simulation modeling, which requires a specific design event as one input and the selection of a sediment transport methodology from one of six or seven that have been developed and refined over the last forty years. Many of the scour components are defined by soil gradations specifying the size and distribution of soil particles available for scour and transport. In addition to the local specificity that must be researched and identified, the issue of future sediment availability within a developing watershed must be addressed.

Evidence suggests that development reduces the sediment supply within a watershed through structural designs that trap and strip the sediment from conveyance courses. Evidence also suggests that the sediment supply can increase during construction if activities are not properly monitored. Feedback from the current designs that are starting to come on–line will undoubtedly be used to further refine the analytic and design protocols used for structural mitigation in alluvial watersheds.

Coe & Van Loo has prepared a vast amount of Conditional Letters of Map Revisions (CLOMRs), Letters of Map Revisions (LOMRs), and elevation certificates for both government and private clients. A CLOMR allows for approval of anticipated map revisions that reflect proposed changes or conditions that are expected to exist in the future. LOMRs are an official revision of a current Flood Insurance Rate Map (FIRM) accepted by FEMA, which are based on changes in mapped areas for flood zones, floodplain area, floodways and flood elevations.

Coe & Van Loo has significant experience with drainage infrastructure design to support private land development projects from small subdivisions to Master Planned Communities. Coe & Van Loo provides the full suite of infrastructure design services, including grading, roads, potable water and sewer. A lengthy performance history is a strategic asset in large metropolitan areas with multiple municipalities and counties implementing detailed and often overlapping regulatory permitting, analysis and design criteria. Coe & Van Loo remains current with the regulatory environment as one means of adding value to its client services.

Coe & Van Loo provides expertise in storm drain system design providing cost effective solutions while meeting regulatory criteria. Storm drain systems include inlet catchbasins and structures, underground conduits, manholes, headwalls and outlet structures. The design of storm drain systems must consider a variety of factors including inlet clogging, headlosses, utility conflicts, hydraulic grade line, velocity, hydraulic jumps and outlet conditions.

We have considerable experience in the design storm drains ranging from simple to complex systems. Value engineering identifies design components and constraints that ultimately results in both a cost and hydraulically efficient storm drain system.

Because of improvements to land, due to development, there's generally an increase in the amount of impervious surfaces. This usually causes an increase in runoff volume and/or peak discharge. Therefore the local communities require storage of storm water runoff to decrease downstream peak discharges and associated impacts to drainage infrastructure.

Our professinals have extensive experience designing these storage facilities within each of the local communities and are very familiar with each agency’s requirements. We are very experienced in designing multi-use facilities. We work closely with other related disciplines at the very beginning of a project to ensure the development incorporates multi-use features. The project is designed with varying side slopes and land features meant to be aesthetically pleasing, while keeping public health and safety in mind.

Section 404 of the Clean Water Act is to protect the physical, biological, and chemical quality of our nation's water from irresponsible and unregulated discharges of dredged or fill material that could permanently alter or destroy these valuable resources. Any individual, firm, or agency (including local, state and federal government agencies) preparing to perform work or place fill material in Waters of the United States, must first obtain a permit from the United States Army Corps of Engineers (USACE).

The regulatory area is designated “Waters of the United States” or “jurisdictional waters." Waters of the United States includes essentially all navigable waters and their tributaries, all interstate waters and their tributaries, all wetlands adjacent to these waters, and all impoundment's of these waters. The regulations governing waters of the U.S. (including wetlands) apply to both public and private property

Determination of the presence and extent (if present) of jurisdictional waters should be undertaken during the early stages of project planning. A jurisdictional delineation establishes the USACE regulatory area. Ephemeral streams (washes) may be jurisdictional if they exhibit certain characteristics. The width of the wash, presence of hydraulic sorting, and the presence of riparian habitat, are factors, among other things, considered by the USACE.

A nationwide permit (NWP) is a form of general permit that authorizes a category of activities throughout the nation. These permits are valid only if the conditions applicable to the permits are met. If the conditions cannot be met, a regional or individual permit will be required.  

Individual permits are issued following a full public interest review of an individual application for a USACE permit. A public notice is distributed to all known interested persons. After evaluating all comments and information received, final decision on the application is made. An individual permit requires a 401 Water Quality Certification from the Arizona Department of Environmental Quality.

Coe & Van Loo can assist with the 404 delineation and permitting procedures related to development activities.

Coe & Van Loo has a very strong background in preparing Storm Water Pollution Prevention Plans (SWPPPs) and Notice of Intents (NOIs), along with the accompanying Best Management Practices (BMPs) for construction activities related to new and existing development projects.

Storm water runoff can transport pollutants to either municipal storm sewer systems or to Waters of the United States; therefore, they are subject to the requirements and permitting process of the National Pollutant Discharge Elimination System (NPDES), which is a U.S. Environmental Protection Agency (EPA) program and is the administrative mechanism chosen for storm water permitting.