STORMWATER DRAINAGE FLOOD PLAIN MANUAL

Document Sample
STORMWATER DRAINAGE FLOOD PLAIN MANUAL Powered By Docstoc
					S T O R M W AT E R D R A I N A G E
     AND      F LO O D P L A I N
              MANUAL

             SHORT TIT LE:
        “ D R A I N A G E M A N U A L”


            J U LY 1 1 , 2 0 0 7


            CIT Y OF IRV ING
 D E PA R T M E N T O F P U B L I C W O R K S
        8 2 5 W I R V I N G B LV D
  IRV ING TX (972) 721-2611
                         INDEX TO DRAINAGE MANUAL

1.   Design Storm Frequency
2.   Determination of Design Discharge
     a.   The Rational Method
     b.   Unit Hydrograph Method
3.   Open Channels
     a.   Hydraulic Design
     b.   Water Surface Profiles
4.   Closed Conduits
     a.   Design
     b.   Velocity
     c.   Manhole Location
     d.   Minor Head Losses At Structures
5.   Bridges and Culverts
     a.   Hydraulic Design of Bridges
     b.   Hydraulic Design of Culverts
     c.   Discharge Velocities in Culverts
6.   Permissible Spread of Water
7.   Inlets
     a.   Recessed inlets, where used, shall not interfere with the sidewalk.
     b.   Design of inlets shall take into consideration pedestrian and bicycle traffic.
8.   Off-site Drainage
9.   Floodplain Reclamation
     a.   Residential Construction
     b.   Nonresidential Construction
     c.   Mobile Homes
     d.   Floodways
          (1) Encroachments
          (2) Placement of mobile home
          (3) Encroachment within a floodway
     e.   Standards for areas of shallow flooding (AO and AH Zones)
          (1) All new construction and substantial improvements of residential
              structures shall have the lowest floor, including basement, elevated
            above the crown of the nearest street to or above the depth number or
            base flood evaluation specified on the FIRM; or
     (2)   All new construction and substantial improvements of non- residential
           structures shall:
           (a)    Have the lowest floor, including basement, elevated above the
                  crown of the nearest street or above the depth number specified
                  on the FIRM; or
           (b)    Together with attendant utility and sanitary facilities, be completely
                  flood-proof to or above that level so that any space below that level
                  is watertight, with walls substantially impermeable to the passage
                  of water and with structural components have the capability of
                  resisting hydrostatic and hydrodynamic loads and effects of
                  buoyancy.
           (c)    Certification
                  (i) Elevation, in relation to mean sea level, of the lowest floor
                      (including basement) of all proposed structures;
                  (ii) Elevation, in relation to mean sea level, to which any non-
                       residential structure shall be flood-proof;
                  (iii) A certificate from a registered professional engineer that the
                        non-residential flood-proof structure shall meet the flood-
                        proofing criteria of section 47-14(2) of the City Code;
                  (iv) Description of the extent to which any watercourse or natural
                       drainage will be altered or relocated as a result of proposed
                       development.
10. Federal Emergency Management Agency
11. 404 Permits
                 a.   Section 10 of the Rivers and Harbors Act of 1899 (33 U.S.C.
                      403)
                 b.   Section 404 of the Clean Water Act (33 U.S.C. 1344)
                                      City of Irving
                               Public Works/Engineering
                             Drainage Design Manual
1.    Design Storm Frequency

          The Storm Frequency period to be used in determining the size and capacity of
          drainage structures and elements is as follows:


                                           TABLE 1

                            DESIGN STORM FREQUENCY


       Type of             Description of        Maximum Time        Recommended
       Facility          Area to be Drained      of Concentration   Design Frequency
                                                     (Minutes)           (Years)


Storm Sewers*            Residential, Commercial
                         and Manufacturing             30                  25

Culverts, Bridges,       Any type of area less
Channels & Creeks*       than 100 acres                30                  25

Culverts, Bridges,       Any type of area greater
Channels & Creeks*       than 100 acres but less
                         than 640 acres                45                  50

Culverts, Bridges,       Any type of area greater
Channels & Creeks*       than 640 acres              No Limit              100


* When the maximum time of concentration of area to be drained is exceeded, the design shall
  be based on a 100 year frequency.

12.    Determination of Design Discharge

          The Rational Method for computing storm water runoff is to be used for hydraulic
          design of facilities serving a drainage area of less than 300 acres. For drainage
          areas of more than 300 acres and less than 640 acres, the runoff shall be calculated
          by both the Rational Method and the Unit Hydrograph Method with the larger of the
          two values being used for hydraulic design. For drainage areas larger than 640
          acres, the runoff shall be calculated by the Unit Hydrograph Method.


July 11, 2007                                                       Public Works / Engineering
Page 4 of 24                                                                  Drainage Manual
          The Rational Method

               The Rational Method is an analysis of the runoff quantity for each individual
               drainage area along rational lines and includes the analysis of the flow of storm
               water runoff from the surface of the drainage area on which it falls to the
               receiving inlet which leads to the storm sewer system and then through the
               storm sewer system, culvert or channel to the point of disposal. As is the case
               in most empirical formulae, the accuracy of the calculations is dependent upon
               the values selected for the factors in the equation, and the manner in which the
               equation is applied. The total drainage area must be accurately determined.
               The runoff coefficient which is selected must reflect careful consideration of
               presently existing conditions as well as conditions which will exist after the
               drainage area is fully developed in a manner indicated by present zoning as
               well as foreseeable future changes in land use. The most demanding
               coefficient must then be applied in the formula. The intensity of rainfall must be
               based upon a time of concentration which is determined from consideration of
               the distance of runoff and the runoff velocity attained on the degree of slope of
               the runoff path. The basic equation of the rational method of runoff
               determination is:

                                                Q=CIA

               In which:

               Q = The quantity of water arriving at a given point measured in cubic feet per
                   second (cfs). It is customary to use a subscript with the value Q to
                   designate the frequency period for which it is calculated. For example Q50
                   indicates the quantity of water for a 50-year frequency rainfall.

               C = The runoff coefficient, or percentage of rainfall in an area which is not
                   absorbed and which must be carried away, expressed as a decimal
                   fraction.

               I=    The intensity of rainfall for the selected period of time and frequency of
                     return or recurrence in years as shown on the attached curve which
                     complies with data published by the Weather Bureau, U.S. Department of
                     Commerce on Technical Paper No. 25 (see Figure 1).

                     Determination of the average runoff velocity for use in determining time of
                     concentration shall be as follows:




Public Works / Engineering                                                          July 11, 2007
Drainage Manual                                                                     Page 5 of 24
                Figure 1

July 11, 2007              Public Works / Engineering
Page 6 of 24                         Drainage Manual
                                             TABLE 2

                              AVERAGE RUNOFF VELOCITY



                                           Slope in Percent
Description of            ______________________(ft/sec)_______________________
Water Course                 0-3%         4-7%            8-11%        12-15%


Surface Drainage             5 ft/sec        10 ft/sec         14 ft/sec         16 ft/sec
Channels                     Determine Velocity by Manning Formula
Storm Sewer                  Determine Velocity by Manning Formula*


*   Manning Formula V = 1.486 R2/3 S1/2
                          n

                 The average velocities shown in this table shall be used unless the designer
                 submits calculations indicating that other velocities are appropriate for use in
                 the area under study. By using the average velocity of runoff the calculation for
                 time of concentration in minutes (T) shall be as follows:

                 T = Inlet Time + ___L___
                                   (V)(60)    where

                 L = Distance in feet from point of concentration to upper end of drainage
                     system.

                 V = Velocity in feet per second.

                 Inlet Time = As indicated herein for various descriptions of areas.

                 A = The area covered by the rainfall and which must be handled by the system
                     under design, expressed in acres. This determination must include
                     consideration of the present drainage divides as well as any future change
                     in drainage divide resulting from planned street and area grading. An
                     accurate map showing the drainage area shall be submitted as an integral
                     part of any drainage plan.

                      The values for runoff coefficient "C" and for the period of time for runoff to
                      reach the first inlet of the drainage system (Inlet Time) based upon zoning
                      and/or existing land use are as follows:



Public Works / Engineering                                                             July 11, 2007
Drainage Manual                                                                        Page 7 of 24
                                             TABLE 3

                          RUNOFF COEFFICIENT AND INLET TIME



         Description of                  _______Range Coefficient "C"_____        Inlet Time
          Area                               Range       Recommended              (Minutes)
Commercial, Shopping Centers                0.70-0.90   0.80                         5
Parking Areas, Downtown
Business, Paving

Industrial, Manufacturing and                0.40-0.70      0.70                    5
Warehouses - New Heavy Areas

Industrial, Manufacturing and                0.40-0.70      0.70                    10
Warehouses - New Light Areas

Apartments                                   0.60-0.90      0.80                    10

Duplexes                                     0.60-0.90      0.70                    15

Residential
  R – 40                                     0.40-0.60      0.40                    15
  R – 15                                     0.40-0.60      0.45                    10
  R – 10                                     0.40-0.60      0.45                    10
  R - 7.5                                    0.40-0.60      0.50                    10
  R–6                                        0.40-0.60      0.50                    5
  R - 3.5, 2.5                               0.40-0.60      0.60                    5

Schools                                      0. 20-0.50     0.60                    15

Parks                                        0.20-0.50      0.40                    20



13.     Open Channels

           In all instances, save and except where the width and/or size of the drainage
           channel renders it uneconomical and/or impractical as determined by the City
           Council, it is the policy of the City to have improved all drainage channels by owner
           installation of reinforced concrete pipe (RCP) or concrete lining. The final decision as
           to the size and/or type of channel improvements shall reside with the City Council.




July 11, 2007                                                           Public Works / Engineering
Page 8 of 24                                                                      Drainage Manual
          c.   Hydraulic Design

               The design water surface in an open channel is to be located a minimum of one
               foot below the top of the channel section to provide a margin of safety for
               channel obstructions and for flows which exceed the design storm frequency.

               Special care must be taken at entrances to closed conduits and culverts to
               provide for the headwater requirements.

          d.   Water Surface Profiles

               Open channel flow in urban drainage systems is usually non-uniform because
               of bridge openings, curves and structures. This necessitates the use of
               backwater computations for all final channel design work.

               A surface water profile must be computed for all channels and shown on all
               final drawings. Computation of the water surface profile should utilize standard
               backwater methods or acceptable computer routines, taking into consideration
               all losses due to changes in velocity, drops, bridge openings and other
               obstructions.

       14. Closed Conduits

          All storm drains shall be designed by the application of the continuity equation and
          Manning’s Equation either through the appropriate charts and nomographs or by
          direct solutions of the equations as follows:

          Q = AV, and
          Q = 1.486 AR2/3 Sf1/2
                n
          Q = Pipe Flow (cfs)
          A = Cross sectional area of pipe (ft2)
          V = Velocity of flow (fps)
          n = Coefficient of roughness of pipe
          R = Hydraulic radius = A/W p (ft)
          Sf = Friction slope in pipe (ft/ft)
          W p = Wetted perimeter (ft)




Public Works / Engineering                                                        July 11, 2007
Drainage Manual                                                                   Page 9 of 24
          There are several general rules to be observed when designing storm sewer runs.
          When followed, they will tend to alleviate or eliminate the common mistakes made in
          storm sewer design. These rules are as follows:

          •     Select pipe size and slope so that the velocity of flow will increase progressively,
                or at least will not appreciably decrease, at inlets, bends or other changes in
                geometry or configuration.
          •     Do not discharge the contents of a larger pipe into a smaller one, even though
                the capacity of the smaller pipe may be greater due to steeper slope.
          •     At changes in pipe size, match the soffits of the two pipes at the same level
                rather than matching the flow lines.
          •     Conduits are to be checked at the time of their design with reference to critical
                slope. If the slope of the line is greater than critical slope, the unit will likely be
                operating under entrance control instead of the originally assumed normal flow.
          •     Conduit slope should be kept below critical slope if at all possible. This also
                removes the possibility of a hydraulic jump within the line.

          e. Design

                In the preliminary design, calculations made to select the pipe or channel size
                after the design flow is determined are based on calculating the approximate
                average velocity by use of Manning's Formula. This is a widely used formula that
                has been found to be reliable for designs of this nature. In order to apply this
                formula, however, it is necessary to take into consideration the roughness of the
                water carrying surfaces by the selection of an appropriate coefficient.
                Roughness coefficients used in the design are given as shown in Table 4.

          f. Velocity

                Storm sewers should operate within certain velocity limits to prevent excessive
                depositing of solids due to low velocities and to prevent invert erosion and
                undesirable outlet conditions due to excessively high velocity. A minimum
                velocity of 2.5 feet per second and a maximum velocity of 12 feet per second at
                the outfall shall be observed.

          g. Manhole Location

                Manholes shall be located at points where design indicates entrance into the
                conduit is desirable; however, in no case shall the distance between openings or
                entrances be greater than 1,200 feet.




July 11, 2007                                                              Public Works / Engineering
Page 10 of 24                                                                        Drainage Manual
                                                                TABLE 4

                                            ROUGHNESS COEFFICIENTS


                                                     Materials of Construction


              Concrete Pipe ...........................................................................0.013
              Monolithic Concrete Structure...................................................0.015
              Concrete Lining.........................................................................0.015
              Earth Channel...........................................................................0.030



          h. Minor Head Losses At Structures

              The following total energy head losses at structures shall be determined for
              inlets, manholes, wye branches or bends in the design of closed circuits. See
              Figures 2 and 3 for details of each case. Minimum head loss used at any
              structure shall be 0.10 foot, unless otherwise approved.

              The basic equation for most cases, where there is both upstream and
              downstream velocity, takes the form as set forth below with the various
              conditions of the coefficient Kj shown in Tables 5, 6 and 7.

                                                   h=       v22 - Kj v12
                                                              2g

              hj =     Junction or structure head loss in feet.
              vI =     Velocity in upstream pipe in fps.
              v2 =     Velocity in downstream pipe in fps.
              Ki =     Junction or structure coefficient of loss.




Public Works / Engineering                                                                                      July 11, 2007
Drainage Manual                                                                                                Page 11 of 24
                Figure 2



July 11, 2007              Public Works / Engineering
Page 12 of 24                        Drainage Manual
                             Figure 3



Public Works / Engineering               July 11, 2007
Drainage Manual                         Page 13 of 24
                                            TABLE 5

                 JUNCTION OR STRUCTURE COEFFICIENT OF LOSS


     Case        Reference                                                      Coefficient
      No.         Figure                Description of Condition                   Ki


        I             2           Inlet on Main Line*                               0.50
       II             2           Inlet on Main Line with Branch Lateral*           0.25
      III             2           Manhole on Main Line with 45º                    0. 50
                                  Branch Lateral
      IV              2           Manhole on Main Line with 90º                     0.25
                                  Branch Lateral
       V              3           45º Wye Connection or Cut-in                      0.75
      VI              3           Inlet or Manhole at Beginning of Line             1.25
     VII              3           Conduit on Curves for 90º**
                                     Curve Radius = Diameter                        0.50
                                     Curve Radius = (2 to 8) Diameter               0.40
                                     Curve Radius = (8 to 20) Diameter              0.25
     VIII             3           Bends Where Radius is Equal to Diameter
                                       90º Bend                                     0.50
                                       60º Bend                                     0.43
                                       45º Bend                                     0.35
                                     22½º Bend                                      0.20
                                  Manhole on Line with 60º Lateral                  0.35
                                  Manhole on Line with 22½º Lateral                 0.75


*      Must be approved by Director of Engineering.
**     Where bends other than 90º are used, the 90º bend coefficient can be used with the
       following percentage factor applied:
       60º Bend – 85%; 45º Bend - 70%; 22½º Bend – 40%.
                In the case where the initial velocity is negligible, the equation for head loss
                becomes:

                                           hj =     v22
                                                     2g




July 11, 2007                                                             Public Works / Engineering
Page 14 of 24                                                                       Drainage Manual
              Short radius bends may be used on 24" and larger pipes when flow must
              undergo a direction change at a junction or bend. Reductions in head loss at
              manholes may be realized in this way. A manhole shall always be located at the
              end of such short radius bends.

              The values of the coefficient "Kj" for determining the loss of head due to
              obstructions in pipes are shown in Table 6 and the coefficients are used in the
              following equation to calculate the head loss at the obstruction:

                                                      hj =   Kj v22
                                                               2g

                                                    TABLE 6

                     HEAD LOSS COEFFICIENTS DUE TO OBSTRUCTIONS

                         A*                  Kj              A*                   Kj
                         A                                   A
                          1.05               0.10             3.0                15.0
                          1.1                0.21             4.0                27.3
                          1.2                0.50             5.0                42.0
                          1.4                1.15             6.0                57.0
                          1.6                2.40             7.0                72.5
                          1.8                4.00             8.0                88.0
                          2.0                5.55             9.0               104.0
                          2.2                7.05            10.0               121.0
                         2.5                 9.70




              * A = Ratio of area of pipe to area of opening at obstruction.
                A

                     The values of the coefficient "Kj" for determining the loss of head due to
                     sudden enlargements and sudden contractions in pipes are shown in
                     Table 7 and the coefficients are used in the following equation to calculate
                     the head loss at the change in section:

                     Hj = Kj    v2   where v = velocity in smaller pipe
                               2g




Public Works / Engineering                                                          July 11, 2007
Drainage Manual                                                                    Page 15 of 24
                                                    TABLE 7

                                HEAD LOSS COEFFICIENTS DUE TO SUDDEN
                                  ENLARGEMENTS AND CONTRACTIONS

                              D2               Sudden Enlargements       Sudden Contractions
                              D1                          Kj                  Kj
                               1.2                        0.10                 0.08
                               1.4                        0.23                 0.18
                               1.6                        0.35                 0.25
                               1.8                        0.44                 0.33
                               2.0                        0.52                 0.36
                               2.5                        0.65                 0.40
                               3.0                        0.72                 0.42
                               4.0                        0.80                 0.44
                               5.0                        0.84                 0.45
                              10.0                        0.89                 0.46
                             >10.0                        0.91                 0.47



                   D2 = Ratio of larger to smaller diameter.
                   D1

      15. Bridges and Culverts

         d. Hydraulic Design of Bridges

                Wherever possible, the proposed bridge should be designed to span a channel
                section equal to the approaching channel section. If a reduction in channel
                section is desired, this should be accomplished upstream of the bridge and
                appropriate adjustments made in the hydraulic gradient.

                A distance of 2 feet between the maximum design water surface and the lowest
                point of the bridge stringers should be maintained, based on Irving's criteria.

                The quantity of flow which the structure must convey shall be calculated in
                accordance with the Procedure for Determination of Design Discharge.




July 11, 2007                                                           Public Works / Engineering
Page 16 of 24                                                                     Drainage Manual
          e. Hydraulic Design of Culverts

               The function of a culvert or bridge is to pass storm water from the upstream side
               of a roadway to the downstream side without submerging the roadway or causing
               excessive backwater which floods upstream property.

               The Engineer shall keep head losses and velocities within reasonable limits while
               selecting the most economical structure. In general, this means selecting a
               structure which creates a headwater condition and has a maximum velocity of
               flow safely below the allowed maximum.

               The vertical distance between the upstream design water surface and the
               roadway elevation should be maintained to provide a safety factor to protect
               against unusual clogging of the culvert and to provide a margin for future
               modifications in surrounding physical conditions. In general, a minimum of two
               feet shall be considered reasonable when the structure is designed to pass a
               design storm frequency of 100 years calculated by Irving's criteria. Unusual
               surrounding physical conditions may be cause for an increase in this
               requirement.

          f.   Discharge Velocities in Culverts

               Velocities in culverts should be limited to no more than 15 feet per second, but
               downstream conditions very likely will impose more stringent controls.
               Consideration must be given to the effect of high velocities and turbulence on the
               channel, adjoining property and embankment. Table 8 is a tabulation of
               maximum allowable velocities based on downstream channel conditions.

               The velocity of discharge from culverts should be limited as shown in Table 8.
               Consideration must be given to the effect of high velocities, eddies or other
               turbulence on the natural channel, downstream property and roadway
               embankment.
                                                  TABLE 8

                         CULVERT DISCHARGE - VELOCITY LIMITATIONS


                                                                                      Maximum Allowable
                     Downstream Condition                                             Discharge Velocity


                     Earth ......................................................................... 6 fps
                     Sod Earth .................................................................. 8 fps
                     Shale....................................................................... 10 fps
                     Paved or Riprap Apron............................................ 15 fps
                     Rock........................................................................ 15 fps


Public Works / Engineering                                                                                    July 11, 2007
Drainage Manual                                                                                              Page 17 of 24
      16. Permissible Spread of Water

            The location of inlets and permissible flow of water in the street should be related to
            the extent and frequency of interference to traffic and the likelihood of flood damage
            to surrounding property. Interference to traffic is regulated by design limits on the
            spread of water into traffic lanes, especially in regard to arterials.

            The term spread of water as used in these design criteria refers to the amount of
            water that is allowed to collect in streets during the design storm with a 25-year
            frequency.

                The collection of storm water has been limited to the following:

                •   Expressway - Eight feet (8') from face of curb
                •   Major Thoroughfares (Divided) - One traffic lane on each side to remain clear
                •   Secondary Thoroughfare (Not Divided) - Five inch (5”) depth of flow at curb
                •   Secondary (Divided) - Five inch (5") depth of flow at curb
                •   Residential - 36' FF five inch (5") depth of flow at curb
                •   Residential - 30' FF five inch (5") depth of flow at curb

      17.       Inlets

                If, in the opinion of the Engineer, the flow in the gutter would be excessive under
                the conditions listed above, then consideration should be given to extending the
                storm sewer to a point where the gutter flow can be intercepted by more
                reasonable inlet locations.

                The primary purpose of storm drain inlets is to intercept excess surface runoff
                and deposit it in a drainage system, thereby reducing the possibility of surface
                flooding.

                The most common location for inlets (Figure 4) is in streets which collect and
                channelize surface flow, making it convenient to intercept. Because the primary
                purpose of a street is to carry vehicular traffic, inlets must be designed so as not
                to conflict with that purpose.




July 11, 2007                                                            Public Works / Engineering
Page 18 of 24                                                                      Drainage Manual
                             Figure 4



Public Works / Engineering               July 11, 2007
Drainage Manual                         Page 19 of 24
         The following guidelines shall be used in the design of inlets to be located in streets:

                c. When recessed inlets are used, they shall not interfere with the intended use
                   of the sidewalk.

                d. Design and location of inlets shall take into consideration pedestrian and
                   bicycle traffic.

      18.       Off-site Drainage

                In accordance with Irving City Code, under certain circumstances, the City will
                participate in the expense of storm sewer improvements designed for drainage
                originating off-site.

      19.       Floodplain Reclamation

                The City of Irving has adopted as part of its City Code, The Flood Insurance
                Study for the City of Irving, Dallas County, Texas, with accompanying Flood
                Insurance Rate Maps (FIRM) and Flood Boundary and Floodway Maps (FBFM)
                and any revisions thereof, as a basis for establishing the areas of special flood
                hazard. Within these areas of special flood hazard, the following provisions are
                required:

                f.    Residential Construction

                      New construction of any residential structure shall have the lowest floor,
                      including basement, elevated to at least two feet (2’) above the base flood
                      elevation. A registered professional engineer shall submit a certification to
                      the director of public works that the standard of this subsection is satisfied.
                      Further, once the above requirement has been met, the floor slab
                      elevation of all new residential structures shall be no lower than the top of
                      the street curb of the street on which the structure is located.

                g.    Nonresidential Construction

                      New construction of any commercial, industrial or other non-residential
                      structure shall have the lowest floor, including basement, elevated to at
                      least two feet (2’) above the level of the base flood elevation. A registered
                      engineer shall submit a certification to the Director of Public Works that
                      the standards of this subsection are satisfied.




July 11, 2007                                                            Public Works / Engineering
Page 20 of 24                                                                      Drainage Manual
              h.     Mobile Homes

                     Other special and more detailed provisions apply to mobile homes and
                     mobile home parks. These provisions are described in detail in the Irving
                     City Code.

              i.     Floodways

                     Located within Special Flood Hazard Areas (SFHA) are zones designated
                     as floodways. Since the floodway is an extremely hazardous area due to
                     the velocity of floodwaters which carry debris and potential projectiles and
                     has a higher degree of erosion potential, the following provisions apply:

                     (4)     Encroachments, including fill, new construction, substantial
                             improvements and other developments are prohibited if such
                             encroachments will cumulatively increase the surface water
                             elevation by more than one foot within the channel of a river or
                             other watercourse and the adjacent land areas that must be re-
                             served in order to discharge the base flood.

                     (5)     All new construction and substantial improvements permitted by
                             this subsection shall comply with all applicable flood hazard
                             reduction provisions of Section 47-14 of the Civil Code.

                     (6)     The placement of any mobile home, except in an existing mobile
                             home park or subdivision, is prohibited.

                     (7)     A person proposing an encroachment within an area designated as
                             a floodway shall provide to the Director of Public Works sufficient
                             data, certified by a registered professional engineer, to establish
                             that the proposed encroachment, together with a comparable
                             encroachment, does not exist but is plausible, is not prohibited by
                             paragraph (a) above and will not increase flooding or divert waters
                             in such a way that any person's life or property will be endangered
                             or subject to significant increased flooding.

              j.     Standards for areas of shallow flooding (AO and AH Zones)

                     Within the areas of shallow flooding, the following provisions apply:

                     (3)     All new construction and substantial improvements of residential
                             structures shall have the lowest floor, including basement, elevated
                             above the crown of the nearest street to or above the depth number
                             or base flood evaluation specified on the FIRM; or

                     (4)     All new construction and substantial improvements of non-
                             residential structures shall:

Public Works / Engineering                                                           July 11, 2007
Drainage Manual                                                                     Page 21 of 24
                            (d)      Have the lowest floor, including basement, elevated above
                                     the crown of the nearest street or above the depth number
                                     specified on the FIRM; or

                            (e)      Together with attendant utility and sanitary facilities, be
                                     completely flood-proof to or above that level so that any
                                     space below that level is watertight, with walls substantially
                                     impermeable to the passage of water and with structural
                                     components have the capability of resisting hydrostatic and
                                     hydrodynamic loads and effects of buoyancy.

                      (5)   A registered professional engineer shall submit a certification to the
                            Director of Public Works that the standards of this section are
                            satisfied. A plat is required to assure conformance with the
                            provisions of the procedures for filling in a flood plain or floodway.
                            This plat is issued by the Director of Public Works and must be
                            obtained prior to the issuance of any building permit on property
                            located in a special flood hazard area.

                            Application for a plat shall be presented to the Director of Public
                            Works on forms furnished by him, which forms may include, but not
                            be limited to, plans in duplicate, drawn to scale, showing the
                            location, dimensions and elevation of existing and proposed
                            structures, and the location of the foregoing in relation to areas of
                            special flood hazard. Additionally, the following information is
                            required:

                                  2. Elevation, in relation to mean sea level, of the lowest floor
                                     (including basement) of all proposed structures;

                                  3. Elevation, in relation to mean sea level, to which any non-
                                     residential structure shall be flood-proof;

                                  4. A certificate from a registered professional engineer that the
                                     non-residential flood-proof structure shall meet the flood-
                                     proofing criteria of section 47-14(2) of the City Code;

                                  5. Description of the extent to which any watercourse or natural
                                     drainage will be altered or relocated as a result of proposed
                                     development.



      20.       Federal Emergency Management Agency

                The National Flood Insurance Program makes flood insurance available to
                property owners in communities that adopt and enforce flood plain management
                measures to reduce future flood losses. The Program provides flood hazard
July 11, 2007                                                           Public Works / Engineering
Page 22 of 24                                                                     Drainage Manual
              maps (called “Flood Insurance Rate Maps, or FIRMs) and certain risk information
              upon which the communities base their local flood plain management program.

              One aspect of a sound flood plain management program is the maintenance of a
              floodway or channel, so as to assure that the elevations of future floods will not
              be increased significantly. The creation of a defined floodway preserves the
              conveyance cross-sectional area that is necessary for passage of floodwaters.
              By restricting certain actions, like filling the floodway, which would result in an
              increase in flood elevation, the impact of development is lessened.

              After a floodway is adopted, a community may encounter a compelling need to
              change the configuration of their floodway and therefore request a revision of the
              floodway map prepared by the Federal Emergency Management Agency
              (FEMA).

              There is no procedure whereby a floodway designation may be appealed by
              individuals at FEMA, since it is the community that selects and adopts the
              regulatory floodway. Thus, an individual should contact the community (the City
              of Irving) to appeal the floodway designation or seek amendments of the
              floodway maps. The community, in turn, may support the amendment request
              and forward the application to FEMA for revision of the FIRM. FEMA will review
              the request to ascertain that certain conditions are met before expending effort
              on the revision.

              It is recognized that while the floodway is adopted by a community, users of the
              Flood Insurance Study and FIRMs other than the community may request copies
              of the flood delineations. Therefore, in support of the community and other
              users, FEMA maintains a system for distributing floodway information and
              revision delineations as the need arises. When the revision is considered to be
              significant, FEMA will revise floodway maps and distribute them to recognized
              users. Significant floodway revisions are identified based on the size of area, as
              well as the number of interests affected by the revision.




Public Works / Engineering                                                          July 11, 2007
Drainage Manual                                                                    Page 23 of 24
      21.       404 Permits

                The U.S. Army Corps of Engineers (Corps) has been regulating activities in the
                nation's waters since 1890. Until the 1960's, the primary purpose of the
                regulatory program was to protect navigation. Since then, as a result of laws and
                court decisions, the program has been broadened so that it now considers the
                full public interest for both the protection and utilization of water resources.

                The regulatory authorities and responsibilities of the Corps are based on the
                following laws:

                c.    Section 10 of the Rivers and Harbors Act of 1899 (33 U.S.C. 403)
                      prohibits the obstruction or alteration of navigable waters of the United
                      States without a permit from the Corps.

                d.    Section 404 of the Clean Water Act (33 U.S.C. 1344) prohibits the
                      discharge of dredged or fill material into waters of the United States
                      without a permit from the Corps.

                Laws that govern the Corps regulatory activities have been passed such that
                wetlands are now defined as waters of the United States.

                Wetlands are areas that are periodically or permanently inundated by surface or
                ground water and support vegetation adapted for life in saturated soil. Wetlands
                include swamps, marshes, bogs and similar areas. A significant natural
                resource, wetlands serve important functions relating to fish and wildlife, food
                chain production, habitat, nesting, spawning, rearing and resting sites for aquatic
                and land species, protection of other areas from wave action and erosion,
                storage areas for storm and flood waters, natural recharge areas where ground
                and surface water are interconnected, and natural water filtration and purification
                functions.

                Although individual alterations of wetlands may constitute a minor change, the
                cumulative effect of numerous changes often results in major damage to wetland
                resources. The review of applications for alteration of wetlands will include
                consideration of whether the proposed activity is dependent upon being located
                in an aquatic environment.

                If property that is being considered for development contains areas to be altered
                in the course of development that may be classified as wetlands, the Corps
                should be contacted to determine the extent of their jurisdiction and the possible
                need for obtaining a Section 404 Permit.




July 11, 2007                                                           Public Works / Engineering
Page 24 of 24                                                                     Drainage Manual