Shooks Run Drainage Study Basic Terminology

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Shooks Run Drainage Study – Basic Terminology

PREPARED FOR: City of Colorado Springs

PREPARED BY: CH2M

DATE: April 9, 2015

Introduction

This document is intended to provide an introduction to Colorado Springs’ Drainage Basin Planning Studies (DBPS) and to introduce, define and explain key terms and information. A Drainage Basin Planning Study (DBPS) is intended to establish a comprehensive approach to stormwater management within each drainage basin. Drainage Basin Planning Studies (DBPSs) should be completed to identify historic and future basin conditions and major system improvements so that existing deficiencies can be corrected and impacts from future land development can be adequately addressed according to the principles and policies defined in the Colorado Spring Drainage Criteria Manual. By identifying existing deficiencies and the costs of correcting them, a DBPS can also be used to budget for and schedule system improvements. The DBPS should include a method for rating system deficiencies and assigning a priority to each proposed improvement. In general, these studies are intended to address:

 Regional and basin‐wide drainage system issues.

 Economical use of resources.

 Environmental preservation and enhancement.

 Social and recreational enhancement.

 Compatibility with comprehensive plans.

 Responsibility for funding and implementation.

 Health, safety and welfare of citizenry.

The study will include an estimate of the cost of needed drainage facilities segregated by the cost of upgrading deficient existing facilities.

A Floodplain will also be delineated as part of this Drainage Basin Planning Study (DBPS). Mapping used to define the flooding limits will be developed from 2 foot contour mapping provided by the City of Colorado Springs. This study will provide relatively accurate representations of the floodplain limits. At this time this study will not be used as the basis for updating the FIRMs and the City may elect to update the FIRM at a future time. However, these studies are not a substitute for approved FIRMs and cannot be used for flood insurance purposes unless approved by FEMA. Important considerations for use of DBPSs include:

1. This DBPS analyzes the future conditions for land use only as the drainage basin is fully developed and, although there is redevelopment expected in the local area, the percentage of land uses considered to be impervious is not expected to change, based on the projected land use and associated impervious percentages within the basin.

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National Flood Insurance Program

The National Flood Insurance Program (NFIP) is a federal program enabling property owners to purchase insurance protection against losses from flooding. Participation in the NFIP is based on an agreement between local communities and the federal government, which states that if a community will implement and enforce measures to reduce future flood risks to new construction in Special Flood Hazard Areas (SFHAs) or “designated floodplains,” the federal government will make flood insurance available within the community. The SFHA is the land area covered by the base flood. Within a SFHA the NFIP must have been forced and the purchase of flood insurance is mandatory. The SFHA includes Zones A, AO, AH, A1‐30, AE, A99, AR, AR/A1‐30, AR/AE, AR/AO, AR/AH, AR/A, VO, V1‐30, VE and V. In the past, the national response to flooding disasters was generally limited to constructing flood control projects and providing disaster relief to flood victims after a flood occurred. This did not reduce losses or discourage unwise development in flood‐

prone areas. Additionally, the public could not buy flood coverage from private insurance companies. Faced with mounting flood losses and escalating costs to the general taxpayers, Congress created the NFIP. The City of Colorado Springs entered the Regular Program of the NFIP in June, 1984 and agreed to adopt and enforce floodplain development regulations that meet or exceed the minimum outlined in 44 Code of Federal Regulations, Part 60. If a community does not enforce the regulations that have been adopted, the community can be put on probation or suspended from the program. If suspended, the communities become “non‐participating” and flood insurance policies cannot be written or renewed.

To recognize and encourage communities that exceed the minimum NFIP requirements FEMA administers the Community Rating System (CRS), which is a voluntary incentive program. Depending on a community’s rating on a scale of 1 to 10 with a 1 being the best, flood insurance premium rates are discounted to reflect the reduced flood risk. To improve its rating a community can take actions that meet three goals:

1. Reduce flood damage to insurable property,

2. Strengthen and support the insurance aspects of the NFIP and 3. Encourage a comprehensive approach to floodplain management.

Colorado Springs and El Paso County participate in the CRS.

The following (Table 1) introduces terms and acronyms that will be typically used in the study.

TABLE 1

Definitions and Acronyms related to Hydrologic and Hydraulic Studies

Hydrology A science dealing with the characteristics, distribution, and circulation of water on and below the surface of the land and in the atmosphere. Hydrology defines “How Much” water are we working with in the stream.

Hydrologic (Water) Cycle The series of conditions through which water naturally passes from water vapor in the air to being deposited (as by rain or snow) on the earth's surface and finally back into the air especially as a result of evaporation

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TABLE 1

Runoff Water from rain or snow that flows over the surface of the ground and finally into streams

500‐YR Flood The 500‐Year flood refers to a flood that has a 0.2% chance of occurring each year. Typically, critical infrastructure such as Hospitals, Government Facilities and Airports are designed to withstand, or be elevated above, the 500‐Year flood. This is a FEMA designated flood zone but structures that are in the area do not require flood insurance.

100‐YR Flood The 100‐Year flood refers to a flood that has a 1% chance of happening each year. Also known as the “base flood”. For Colorado Springs, this flooding scenario is defined as 2.86 inches of rain falling within 2‐hours over the entire watershed. This is a FEMA designated flood zone and structures located with this zone are required to purchase flood insurance.

50‐YR Flood The 50‐Year flood refers to a flood that has a 2% chance of happening each year 25‐YR Flood The 25‐Year flood refers to a flood that has a 4% chance of happening each year 10‐YR Flood The 10‐Year flood refers to a flood that has a 10% chance of happening each year 5‐YR Flood The 5‐Year flood refers to a flood that has a 20% chance of happening each year 2‐YR Flood The 2‐Year flood refers to a flood that has a 50% chance of happening each year.

Peak Flow The point of the hydrograph that has the highest flow.

Discharge The volume of water that passes through a given cross section per unit time. Discharge is commonly measured in cubic feet per second (cfs). It is also referred to as Peak flow.

Hydrograph A description of flow versus time or a description of stage versus time.

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[cfs] Cubic Feet per Second, a measure of flow. 1 [cfs] is equal to 7.48 gallons per second

[ft/s] Feet per Second, a measure of Velocity

[sf] Square Feet, a measure of Area

[ac] Acre, a measure of Area. 1 [ac] is equal to 43,560 [sf]

[ac‐ft] Acre Feet, a measure of Volume. 1 [ac‐ft] is equal to 325,851 gallons

EURV Excess Urban Runoff Volume, a measure of runoff volume caused by development.

WQCV Water Quality Capture Volume, a measure of the runoff volume that gets treated by BMP and LID infrastructure

EDB (Detention Basin) Extended Detention Basin, A localized depression in the topography that is manmade. Water flows into the basin, is typically detained between 12‐72 hours and released by means of an outlet control structure.

Extended detention basins are designed to remain dry except during a flooding event. Water rights are generally not needed to construct a EDB.

Retention Basin Similar to a detention basin but water in storage is permanently obstructed from flowing downstream and generally will have a permanent pool of water. Water Rights would be required to install a retention basin.

Floodplain An area near a river which floods when the water level reaches flood stage

Floodway A certain channel width that is reserved to convey the 100‐Year flow rate without raising the water surface more than 1.0 [ft]

WSEL Water Surface Elevation

Flood Fringe The portion outside of the floodway and contained within the floodplain where development is allowed.

These areas do not experience a strong current when flooded

BFE Base Flood Elevation, typically reported out at key design cross sections

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TABLE 1

FEMA Federal Emergency Management Agency

NFIP National Flood Insurance Program, requires local governments to adopt and implement flood management programs that prevent developments in excess of national standards

NFF National Flood Frequency Program

FIS Flood Insurance Study Reports, provides general program and community background information and, where tabulated, information about the floodplain can be found

FIRM Flood Insurance Rate Map, provides delineations of the 100‐ and 500‐Year floodplains

FBHM Flood Boundary Hazard Maps, provide delineations of the 100‐ and 500‐Year floodplains and floodways as well as locations of surveyed floodplains and channel cross sections used in hydraulic analysis

USGS United States Geological Survey

USACE United States Army Corp of Engineers

HEC The Hydrologic Engineering Center, designated Center of Expertise for the United States Army Corp of Engineers

HEC‐RAS The Hydrologic Engineering Center River Analysis System, a program designed and maintained by HEC and is used in analyzing floodplains, and structures used to convey flow.

EPA The Environmental Protection Agency

EPA‐SWMM The Environmental Protection Agency Storm Water Management Model. Typically called “EAP SWIM” and is used in the analysis and design related to stormwater, sanitary sewers and other drainage systems in urban areas.

Watershed (Basin) An area of land where all of the water that drains off of it goes to the same place. Watersheds cross government boundaries

EDB Extended Detention Basin, A localized depression in the topography that is manmade. Water flows into the basin, is typically detained between 12‐72 hours and released by means of an outlet control structure.

BMP Best Management Practice, tools used by governments and developers to help reduce the impacts of urbanization

LID Low Impact Development, furthers usage of BMPs by integrating BMPs into every day amenities for the community

Drainageway The bed of a stream, river, or other waterway. A route or course along which water moves or may move to drain a region. Also Known as the Channel.

Storm Sewer (Storm Drain) A storm drain, storm sewer (US), is designed to drain excess rain and ground water from paved streets, parking lots, sidewalks, and roofs via a system of inlets and underground conduits.

Grade Control An earthen, wooden, concrete, or other structure used to prevent gully development and bed erosion

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Culvert A culvert is a structure that allows water to flow under a road, railroad, trail, or similar obstruction. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe, reinforced concrete or other material. A structure that carries water above land is known as an aqueduct.

Erosion The process where water removes soil and rock from one location in a channel and then transports it to another location where it is deposited.

Ground Water Ground Water is the water located beneath the ground surface in soil pore spaces and in the fractures of rock formations.

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TABLE 1

Degradation (Channel) Degradation refers to the lowering of a stream bed or floodplain, through erosional processes. It is the opposite of aggradation. Degradation is characteristic of channel networks in which either bedrock erosion is taking place, or in systems that are sediment‐starved and are therefore eroding more material than is being deposited.

Riprap Riprap is rock or other material used to armor shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour and water or ice erosion.

Precipitation Any form of water, such as rain, snow, sleet, or hail, that falls to the earth's surface

MS4 Municipal Separate Storm Sewer System ‐ An MS4 is a conveyance or system of conveyances that is:

 Owned by a state, city, town, village, or other public entity that discharges to waters of the U.S.;

 Designed or used to collect or convey stormwater (including storm drains, pipes, ditches, etc.);

 Not a combined sewer; and

 Not part of a Publicly Owned Treatment Works (sewage treatment plant).

Polluted stormwater runoff is commonly transported through Municipal Separate Storm Sewer Systems (MS4s), from which it is often discharged untreated into local waterbodies. To prevent harmful pollutants from being washed or dumped into an MS4, operators must obtain a NPDES permit and develop a stormwater management program

Low Flow Channel The lowest part of a channel that conveys base flows or perennial stream flows

Perennial Flow (Base Flow) Also called drought flow, low flow, low‐water flow, low‐water discharge and sustained or fair‐weather runoff, this is the portion of streamflow that comes from deep subsurface flow and delayed shallow subsurface flow. This is the flow that is found in a river or stream bed even during periods of low precipitation.

NPDES Permit National Pollutant Discharge Elimination System (NPDES) permit program controls water pollution by regulating point sources that discharge pollutants into waters of the United States. Point sources are discrete conveyances such as pipes or man‐made ditches

FEMA Flood Insurance Rate Maps (FIRMs) and Flood Insurance Study (FIS)

The purpose of FIRMs is to identify flood‐prone areas, by approximate or more detailed methods, and to establish flood risk zones for insurance rate purposes within those flood‐prone areas. FIRMs are based on watershed conditions at the time the engineering analyses and accompanying survey were completed. In addition, detailed contour mapping may not have been available or used in the preparation of the original FIRMs. The information provided on the FIRMs and in the FIS is not based on consideration of changes that

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floodplain or to identify areas prone to flooding. Further investigation of the assumptions, methodologies, and mapping that was used to produce the flood information on the FIRMs should be performed by a Professional Engineer registered in the State of Colorado. In some cases, the FIRMs are the only source of information available and can be used as an aid, but additional investigation and analyses may be required to define the actual floodplain limits on a particular parcel of land. FIRMs, however, are the official

regulatory maps published by FEMA for flood insurance purposes and, therefore, must be used when determining limits of the Special Flood Hazard Area (SFHA), and for complying with the floodplain regulations, as discussed previously. Important characteristics of FIRMs include:

1. Detailed Studies. FIRMs contain SFHA designations that were developed through a detailed study or by approximate methods. For drainageways that have a detailed study, Base Flood Elevations (BFEs) are provided on the maps and information is available in the FIS regarding floodplain and floodway widths, drainage areas, and peak discharges at select locations. In most cases, the BFEs can be used in

conjunction with detailed topographic information to produce a reasonable estimate of the floodplain limits on a particular parcel of land, as long as it can be verified that the topographic information and the BFEs are referenced to the same vertical datum.

2. Approximate Zones. SFHA designations that were developed by approximate methods (Zone A) are generally less accurate and BFEs are not provided. Typically, there is no published information regarding peak flow rates used to calculate the approximate limits. As a result, making floodplain determinations and correctly delineating the floodplain on a specific property is more difficult. When a project is adjacent to a Zone A floodplain, floodplain limits must be developed using topographic mapping and an acceptable level of hydrologic and hydraulic analysis, or a registered Professional Engineer must certify that flooding is unlikely. Procedures for making floodplain estimations in Zone A areas are outlined in the FEMA publication Managing Floodplain Development in Approximate Zone A Areas; however, the applicant’s engineer should consult with the governing jurisdiction prior to selection of a methodology or level of detail to confirm that they are reasonable and appropriate.

3. Map Revisions. FIRMs are often updated due to development or construction projects, changes in hydrology, the use of better topographic information, or other factors that affect the accuracy of the current SFHA limits. In most cases, the updates occur through a process called a Letter of Map Revision (LOMR). A LOMR provides revised floodplain information for a particular area, which supersedes the previous information and becomes the effective SFHA designation. However, the LOMR is a separate document, and the FIRMs typically are not re‐published with the changes resulting from a revision.

Figure 1 below identifies some of the terminology used above and the relationship to a home or structure.

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Figure 1: Special Flood Hazard Area Cross Section