Flood Frequency Analysis for Tributaries of River Ise

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					NORTHAMPTONSHIRE COUNTY
COUNCIL


Isham Bypass


Flood Frequency Analysis for
Tributaries of River Ise



FINAL – April 2004
Isham Bypass
Flood Frequency Analysis for Tributaries of River Ise




Notice
This document has been produced by ATKINS for Northamptonshire County Council solely for the purpose of the
estimation of the Flood Frequency Analysis for the Isham Bypass study.

It may not be used by any person for any other purpose other than that specified without the express written
permission of ATKINS. Any liability arising out of use by a third party of this document for purposes not wholly
connected with the above shall be the responsibility of that party who shall indemnify ATKINS against all claims
costs damages and losses arising out of such use.

Document History

JOB NUMBER: 4324001                                        DOCUMENT REF: 4324001-DG-004-1-D1.doc

                    Purpose and
Revision                                     Originated     Checked       Reviewed      Authorised       Date
                     Description

  1-D1            Draft – for review             LC              MH          MH            MH          30-Mar-04

    1                   Final                    LC              MH          MH            MH          05-Apr-04




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Flood Frequency Analysis for Tributaries of River Ise


                                                                  CONTENTS

1   Introduction ............................................................................................................... 1
2   Method ....................................................................................................................... 2
3   Results....................................................................................................................... 3
    3.1     Pytchley Brook ..................................................................................................................................... 3
    3.2     Hardwick Brook .................................................................................................................................... 6
    3.3     Little Harrowden Brook ......................................................................................................................... 9
4   Conclusions ............................................................................................................ 12
5   References .............................................................................................................. 13




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Flood Frequency Analysis for Tributaries of River Ise

                                  GLOSSARY OF TERMS
          Term                          Meaning / Definition
          FEH                           Flood Estimation Handbook
          SPRHOST                       Standard Percentage Runoff HOST classification
          SAAR                          Standard-period Average Annual Rainfall
          QMED                          Median Annual Flood




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Flood Frequency Analysis for Tributaries of River Ise
Introduction



1          Introduction
           This report presents the results of a flood frequency analysis carried out by Atkins to determine the 1
           in 100 year and 1 in 200 year design flows at three ungauged locations on the tributaries of the River
           Ise, near Kettering in Northamptonshire (Grid reference 487852, 274735; 488250, 273250 and
           488250, 271750). Figure 1-1 shows the location of these three sites. Results of this analysis will be
           used for the design of the culverts under the proposed Isham Bypass.




           Figure 1-1 Location of the three tributaries on River Ise.

           The analysis was carried out in accordance with the approach outlined in Volume III of Flood
           Estimation Handbook (FEH) (Institute of Hydrology, 1999) [Ref 1].

           It should be noted that, although these flows are the most reliable available for the site, they have
           been derived using regional rather than local data, and are therefore subject to significant error.

           Relationships between key variables, such as the median annual flood, QMED, and the catchment
           descriptors provide a technique for deriving a flood estimate at an ungauged site. “Estimates
           produced in this way are (in nearly all cases) far less reliable than those obtained using flood peak
           data” (Institute of Hydrology, 1999, Vol 5 p1).

           Flow values for the three sites for a range of return periods are shown in Section 3.2.




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Flood Frequency Analysis for Tributaries of River Ise
Method



2          Method
           The FEH statistical method was adopted as these three locations are ungauged catchments and the
           method described in the FEH Volume 3 to derive a flood frequency curve provide a suitable
           methodology [Ref 1].

           This method uses Catchment Descriptors to estimate QMED (2 year return period) for the catchment.
           Further analysis was undertaken, incorporating data from analogue catchments in the region, in order
           to improve the estimation of QMED for the three locations on the Ise catchment.

           The flood frequency curve was derived for the catchment using the pooling group methodology, that
           is based on the use of the information from catchments with similar hydrological characteristics. This
           flood frequency curve was adjusted by the QMED of the site of interest, determined by the
           adjustment factor of the analogue catchments.




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Flood Frequency Analysis for Tributaries of River Ise
Results



3          Results

3.1        Pytchley Brook
                                                                                                     2
           Pytchley Brook is an ungauged and rural catchment that encompasses an area of 11. 5 Km . It joins
           the River Ise on the right hand side after the confluence of Slade Brook (NGR 487852, 274735). The
           catchment descriptors for the site of interest are shown in Table 3-1.

                     Parameter       Value               Parameter       Value

                     AREA            11.51               SAAR            628
                     FARL            1                   SAAR4170        622
                     PROPWET         0.3                 SPRHOST         35.6
                     ALTBAR          103                 URBCONC         -999999
                     ASPBAR          93                  URBEXT199       0.003
                                                         0
                     ASPVAR          0.37                URBLOC          -999999
                     BFIHOST         0.524               C               -0.026
                     DPLBAR          4.32                D1              0.313
                     DPSBAR          39                  D2              0.25
                     LDP             7.53                D3              0.285
                     RMED-1H         12.7                E               0.308
                     RMED-1D         29                  F               2.559
                     RMED-2D         37.4
                   Table 3-1 FEH Catchment Descriptors for Pytchley Brook


3.1.1      Pooling Group
           Following the FEH methodology for ungauged catchments a pooling group of 37 catchments with
           hydrological similarity was abstracted for Pytchley Brook, see Table 3-2.        This group gives a
           combined flow record of 988 years, long enough to predict a 1 in 200 year return period flow.

           The initial pooling group derived was reviewed statistically and found to be strongly heterogeneous.
           After removing some gauging stations the pooling group continued to be strongly heterogeneous,
           however the statistical parameter H2 reduce from 4.58 to 3.805. Despite the heterogeneity in the
           pooling group there are not discordant sites and following the principle of “better to tolerate
           heterogeneity than use too few data”, the group was adopted to estimate the Flood Frequency Curve.
           The removed gauging stations are listed in Table 3-3 with reasons for their exclusion.

           The sites in the pooling group had the same flood seasonality, with floods most commonly occurring
           in the period January to April.

           Experience has shown that after the initial review of the pooling group where homogeneity of the
           catchments is assessed and quality of the gauge records in relation to drowning and bypassing is
           investigated, removing further catchments from the pooling group has little effect on the flood
           frequency curve produced.




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Flood Frequency Analysis for Tributaries of River Ise
Results



                      Gauge            Gauge Name and                  Gauge           Gauge Name and
           Rank        No.                Watercourse          Rank     No.              Watercourse
            1         33045       (Wittle @ Quidenham)          20     27058     (Riccal @ Crook House
                                                                                 Farm)
             2        20002        (West Peffer Burn @          21      54034    (Dowles Brook @ Dowles)
                                  Luffness)
             3        30012        (Stainfield Beck @           22      20007    (Gifford Water @ Lennoxlove)
                                  Stainfield)
             4        25019        (Leven @ Easby)              23      41807    (Bevern Stream @ East
                                                                                 Chiltington)
             5        54052       (Bailey Brook @ Ternhill)     24      52011    (Cary @ Somerton)
             6        68015       (Gowy @ Huxley)               25      37003    (Ter @ Crabbs Bridge)
             7        29009       (Ancholme @ Toft Newton)      26      52016    (Currypool Stream @
                                                                                 Currypool Farm)
             8        41016       (Cuckmere @ Cowbeech)         27      40017    (Dudwell @ Burwash)
             9        41028       (Chess Stream @ Chess         28      41022    (Lod @ Halfway Bridge)
                                  Bridge)
             10       29005       (Rase @ Bishopbridge)         29      54036    (Isbourne @ Hinton on the
                                                                                 Green)
             11       29004       (Ancholme @                   30      28055    (Ecclesbourne @ Duffield)
                                  Bishopbridge)
             12       36007       (Belchamp Brook @             31      30017    (Witham @ Colsterworth)
                                  Bardfield Bridge)
             13       36003       (Box @ Polstead)              32      28058    (Henmore Brook @
                                                                                 Ashbourne)
             14       31010    (Chater @ Fosters Bridge)        33      53013    (Marden @ Stanley)
             15       20006    (Biel Water @ Belton             34      33030    (Clipstone Brook @
                               House)                                            Clipstone)
             16      30004     (Partney Lymn @ Partney          35      32008    (Nene/Kislingbury @ Dodford)
                               Mill)
             17      33031     (Broughton Brook @               36      20005    (Birns Water @ Saltoun Hall)
                               Broughton)
             18      53017     (Boyd @ Bitton)                  37      40006    (Bourne @ Hadlow)
             19      38002     (Ash @ Mardock)
           Table 3-2 FEH Pooling Group for Pytchley BrooK

            Gauge No.         Watercourse         Gauge Name            Reason for exclusion from pooling group
              54058       Stoke Park Brook      Stoke Park           Highly discordant
              32029       Flore                 Experimental         Only 5 years data, so L-moments cannot be
                                                Catchment            calculated as record too short.
              20004       East Peffer Burn      Lochhouses           Highly discordant and only 8 years data so
                                                                     estimates derived from L-moments cannot be
                                                                     considered reliable
              68011       Arley Brook           Gore Farm            Highly discordant and only 8 years data so
                                                                     estimates derived from L-moments cannot be
                                                                     considered reliable
              45801       Back Brook            Hawkerland           Highly discordant and only 5 years data so L-
                                                                     moments cannot be calculated as record too short
              33057       Ouzel                 Leighton Buzzard     Very different value of AREA to subject site
              40809       Pippingford Brook     Paygate            Highly discordant
              41026       Cockhaise Brook       Holywell           Very different value of FARL to subject site
              12004       Girnock Burn          Littlemill         Very different value of PROWET to subject site
              20005       Birns Water           Saltoun Hall       Very different value of PROWET to subject site
              25007       Clow Beck             Croft              Very different value of PROWET to subject site
              40006       Bourne                Hadlow             Very different value of URBEXT to subject site and
                                                                   drowns at high flows
           Table 3-3 Sites removed from Pytchley Brook pooling group



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Flood Frequency Analysis for Tributaries of River Ise
Results

3.1.2      Index Flood - QMED
                                                                               3
           The QMED for Pytchley Brook is 1.511 m /s, obtained from the QMED derived from catchment
                              3
           descriptors (1.66 m /s) and from the adjustment factor (0.910).

           The adjustment factor was derived using the analogue catchments listed in Table 3-4. These four
           analogue catchments were chosen from the full pooling group list, ensuring similar baseflow,
           SPRHOST, FARL (Index of flood attenuation due to reservoirs and lakes) and SAAR. The area of
           these catchments is not very similar to the site, since Pytchley Brook has a small catchment area
                     2
           (11.51 km ), therefore this is a source of uncertainty in the estimation of the index flood and might
           introduce some error, but it is the only data available. The ratios were combined from these
           catchments using the Geometric Mean as recommended by the FEH.

                                                                                                                               QMED from
                                                                                           QMED from
            Station                                                                                                            catchment              Adjustment
                                      Watercourse              Gauge Name                   flow data
              No.                                                                                                              descriptors            Ratio (A/B)
                                                                                               (A)
                                                                                                                                   (B)
            33045                   Wittle                 Quidenham                       1.169                           3.287                      0.356
            20002                   West Peffer Burn       Luffness                        3.588                           4.322                      0.830
            30012                   Stainfield Beck        Stainfield                      5.935                           5.017                      1.183
            25019                   Leven                  Easby                           6.110                           3.104                      1.968

           Table 3-4 Analogue catchments for Pytchley Brook

           The number of analogue catchments included in the estimation of the index flood was obtained by
           adding catchments until the geometric mean became constant.


3.1.3      Flood frequency curve
           The growth curve was determined by use of the Generalised Logistic Distribution that is not only
           recommended by the FEH as the representative of UK catchments, but also in this case provided the
           best fit to the data (Z=0.5781).

           The growth curve was then adjusted by the index flood, QMED, of the site of interest, determined by
           the adjustment factor of the analogue catchments to obtain the flood frequency curve shown in Figure
           3-1.

                          7.0

                          6.5

                          6.0

                          5.5

                          5.0
                                                                                                                          GL
                          4.5
             Q (cumecs)




                          4.0

                          3.5

                          3.0

                          2.5

                          2.0

                          1.5

                          1.0

                          0.5                                                                                                               T
                                                   2




                                                               5




                                                                          10




                                                                                   25




                                                                                            50

                                                                                                   75
                                             1.5




                                                                                                        100



                                                                                                                    200




                                                                                                                                      500




                          0.0
                             -2.0     -1.0         0.0   1.0        2.0            3.0       4.0              5.0               6.0             7.0
                                                               Logistic reduced variable




           Figure 3-1 Flood Frequency Curve for Pytchley Brook




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Results

3.2        Hardwick Brook
                                                                                                              2
           Hardwick Brook is an ungauged and rural catchment that encompasses an area of 12.34 Km . It
           joins the River Ise on the right hand side after the confluence of Slade Brook (NGR 488250, 273250).
           The catchment descriptors for the site of interest are shown in Table 3-5

             Parameter             Value            Parameter            Value

            AREA             12.34                SAAR             623
            FARL             1                    SAAR4170         615
            PROPWET          0.3                  SPRHOST          38.5
            ALTBAR           98                   URBCONC          0.506
            ASPBAR           88                   URBEXT199        0.007
                                                  0
            ASPVAR           0.36                 URBLOC           0.727
            BFIHOST          0.465                C                -0.026
            DPLBAR           4.48                 D1               0.307
            DPSBAR           37.1                 D2               0.264
            LDP              8.89                 D3               0.286
            RMED-1H          12.6                 E                0.31
            RMED-1D          28.5                 F                2.554
            RMED-2D          37.4
           Table 3-5 FEH Catchment Descriptors for Hardwick Brook


3.2.1      Pooling Group
           Following the FEH methodology for ungauged catchments a pooling group of 38 catchments with
           hydrological similarity was abstracted for Hardwick Brook, see Table 3-6. This group gives a
           combined flow record of 992 years, long enough to predict a 1 in 200 year return period flow.

           The initial pooling group derived was reviewed statistically and found to be strongly heterogeneous.
           After removing some gauging stations the pooling group became heterogeneous, and the statistical
           parameters H1 and H2 reduce from 4.186 and 4.106 to 3.34 and 3.05 respectively . Despite the
           heterogeneity in the pooling group there are not discordant sites and following the principle of “better
           to tolerate heterogeneity than use too few data”, the group was adopted to estimate the Flood
           Frequency Curve. The removed gauging stations are listed in Table 3-7 with reasons for their
           exclusion.

           The sites in the pooling group had the same flood seasonality, with floods most commonly occurring
           in the period January to April.

           Experience has shown that after the initial review of the pooling group where homogeneity of the
           catchments is assessed and quality of the gauge records in relation to drowning and bypassing is
           investigated, removing further catchments from the pooling group has little effect on the flood
           frequency curve produced.




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Flood Frequency Analysis for Tributaries of River Ise
Results



                     Gauge                Gauge Name and                 Gauge         Gauge Name and
           Rank       No.                    Watercourse         Rank     No.            Watercourse
           1        20002            (West Peffer Burn @         20      29004    (Ancholme @ Bishopbridge)
                                    Luffness)
           2        33045            (Wittle @ Quidenham)        21      36003    (Box @ Polstead)
           3        30012            (Stainfield Beck @          22      25007    (Clow Beck @ Croft)
                                    Stainfield)
           4        25019            (Leven @ Easby)             23      31023    (West Glen @ Easton Wood)
           5        41016            (Cuckmere @ Cowbeech)       24      27058    (Riccal @ Crook House
                                                                                  Farm)
           6        33030           (Clipstone Brook @           25      28055    (Ecclesbourne @ Duffield)
                                    Clipstone)
           7        33031           (Broughton Brook @           26      54036    (Isbourne @ Hinton on the
                                    Broughton)                                    Green)
           8        41028           (Chess Stream @ Chess        27      30004    (Partney Lymn @ Partney
                                    Bridge)                                       Mill)
           9        68015           (Gowy @ Huxley)              28      41022    (Lod @ Halfway Bridge)
           10       25010           (Baydale Beck @ Mowden       29      20007    (Gifford Water @ Lennoxlove)
                                    Bridge)
           11       29005           (Rase @ Bishopbridge)        30      52011    (Cary @ Somerton)
           12       36007           (Belchamp Brook @            31      29009    (Ancholme @ Toft Newton)
                                    Bardfield Bridge)
           13       37003           (Ter @ Crabbs Bridge)        32      52020    (Gallica Stream @ Gallica
                                                                                  Bridge)
           14       54052           (Bailey Brook @ Ternhill)    33      41807    (Bevern Stream @ East
                                                                                  Chiltington)
           15       31010           (Chater @ Fosters Bridge)    34      27042    (Dove @ Kirkby Mills)
           16       53017           (Boyd @ Bitton)              35      20005    (Birns Water @ Saltoun Hall)
           17       20006           (Biel Water @ Belton         36      38004    (Rib @ Wadesmill)
                                    House)
           18       38002           (Ash @ Mardock)              37      37020    (Chelmer @ Felsted)
           19       40017           (Dudwell @ Burwash)          38      28058    (Henmore Brook @
                                                                                  Ashbourne)
           Table 3-6FEH Pooling Group for Hardwick BrooK


            Gauge No.         Watercourse          Gauge Name         Reason for exclusion from pooling group
            32029           Flore                 Experimental    Only 5 years data, so L-moments cannot be
                                                  Catchment       calculated as record too short.
            20004           East Peffer Burn      Lochhouses      Highly discordant and only 8 years data so
                                                                  estimates derived from L-moments cannot be
                                                                  considered reliable
            54058           Stoke Park Brook      Stoke Park      Highly discordant and only 6 years data so
                                                                  estimates derived from L-moments cannot be
                                                                  considered reliable
            68011           Arley Brook           Gore Farm       Highly discordant and only 8 years data so
                                                                  estimates derived from L-moments cannot be
                                                                  considered reliable
            21002           Whiteadder Water      Hungry Snout    Highly discordant and only 9 years data so
                                                                  estimates derived from L-moments cannot be
                                                                  considered reliable
            12004           Girnock Burn          Littlemill      Very different value of PROWET to subject site
            24007           Browney               Lanchester  Very different value of PROWET to subject site
            32008           Nene/Kislingbury      Dodford     Very different value of URBEXT to subject site.
                                                              Also bypasses and drowns at high flows
            41026          Cockhaise Brook   Holywell         Very different value of FARL to subject site.
           Table 3-7 Sites removed from Hardwick Brook pooling group




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Flood Frequency Analysis for Tributaries of River Ise
Results

3.2.2      Index Flood - QMED
                                                                        3
           The QMED for Hardwick Brook is 2.214 m /s, obtained from the QMED derived from catchment
                               3
           descriptors (2.098 m /s) and from the adjustment factor (1.055).

           The adjustment factor was derived using the analogue catchments listed in Table 3-8. These six
           analogue catchments were chosen from the full pooling group list, ensuring similar baseflow,
           SPRHOST, FARL (Index of flood attenuation due to reservoirs and lakes) and SAAR. The area of
           these catchments is not very similar to the site, since Hardwick Brook has a small catchment area
                     2
           (12.34 km ), therefore this is a source of uncertainty in the estimation of the index flood and might
           introduce some error, but it is the only data available. The ratios from these analogue catchments
           were combined using the Geometric Mean as recommended by the FEH.


                                                                                                                                 QMED from
                                                                                             QMED from
            Station                                                                                                              catchment                Adjustment
                                       Watercourse               Gauge Name                   flow data
              No.                                                                                                                descriptors              Ratio (A/B)
                                                                                                 (A)
                                                                                                                                     (B)
            20002                    West Peffer Burn       Luffness)                    3.588                              4.322                         0.830
            33045                    Wittle                 Quidenham)                   1.169                              3.287                         0.356
            30012                    Stainfield Beck        Stainfield)                  5.935                              5.017                         1.183
            25019                    Leven                  Easby)                       6.110                              3.104                         1.968
            41016                    Cuckmere               Cowbeech)                    5.399                              4.059                         1.330
            33030                    Clipstone Brook        Clipstone)                   11.064                             7.315                         1.512

           Table 3-8 Analogue catchments for Hardwick Brook

           The number of analogue catchments included in the estimation of the index flood was obtained by
           adding catchments until the geometric mean became constant.


3.2.3      Flood frequency curve
           The growth curve was determined by use of the Generalised Logistic Distribution that is not only
           recommended by the FEH as the representative of UK catchments, but also in this case provided the
           best fit to the data (Z=0.1583).

           The growth curve was then adjusted by the index flood, QMED, of the site of interest, determined by
           the adjustment factor of the analogue catchments to obtain the flood frequency curve shown in Figure
           3-2.

                          10.0
                           9.5
                           9.0
                           8.5
                           8.0
                           7.5
                           7.0
                           6.5
                           6.0
             Q (cumecs)




                                                                                                                            GL
                           5.5
                           5.0
                           4.5
                           4.0
                           3.5
                           3.0
                           2.5
                           2.0
                           1.5
                           1.0
                                                                                                                                               Tr
                                               1.5



                                                     2




                                                                 5




                                                                            10




                                                                                 25




                                                                                              50

                                                                                                     75
                                                                                                          100



                                                                                                                      200




                                                                                                                                         500




                           0.5
                           0.0
                              -2.0      -1.0         0.0   1.0        2.0         3.0          4.0              5.0                6.0              7.0
                                                                 Logistic reduced variable




           Figure 3-2 Flood Frequency Curve for Hardwick Brook



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Flood Frequency Analysis for Tributaries of River Ise
Results

3.3        Little Harrowden Brook
                                                                                                                  2
           Little Harrowden Brook is an ungauged and rural catchment that encompasses an area of 1.8 Km .
                                                                                             2
           Note that this catchment area is only just larger than the minimum limit of 1.5 Km recommended for
           application of the FEH methodologies. It joins the River Ise on the right hand side after the
           confluence of Slade Brook (NGR 487852, 274735). The catchment descriptors for this site are
           shown in Table 3-9.

            Parameter        Value               Parameter        Value

            AREA             1.8                 SAAR             609
            FARL             1                   SAAR4170         604
            PROPWET          0.3                 SPRHOST          21.4
            ALTBAR           91                  URBCONC          0.3
            ASPBAR           59                  URBEXT199        0.009
                                                 0
            ASPVAR           0.27                URBLOC           1.024
            BFIHOST          0.75                C                -0.026
            DPLBAR           1.39                D1               0.3
            DPSBAR           47                  D2               0.273
            LDP              3.21                D3               0.286
            RMED-1H          12.4                E                0.311
            RMED-1D          27.9                F                2.543
            RMED-2D          36.8
           Table 3-9 FEH Catchment Descriptors for Little Harrowden Brook


3.3.1      Pooling Group
           Following the FEH methodology for ungauged catchments a pooling group of 38 catchments with
           hydrological similarity was abstracted for Little Harrowden Brook, see Table 3-10. This group gives
           a combined flow record of 986 years, long enough to predict a 1 in 200 year return period flow.

           The initial pooling group derived was reviewed statistically and found to be strongly heterogeneous.
           After removing some gauging stations the pooling group continuous being strongly heterogeneous,
           however the statistical parameters H1 and H2 reduce from 7.23 and 5.00 to 6.1 and 4.04
           respectively . Despite the heterogeneity in the pooling group there are not discordant sites and
           following the principle of “better to tolerate heterogeneity than use too few data”, the group was
           adopted to estimate the Flood Frequency Curve. The removed gauging stations are listed in Table
           3-11 with reasons for their exclusion.

           The sites in the pooling group had the same flood seasonality, with floods most commonly occurring
           in the period January to April.

           Experience has shown that after the initial review of the pooling group where homogeneity of the
           catchments is assessed and quality of the gauge records in relation to drowning and bypassing is
           investigated, removing further catchments from the pooling group has little effect on the flood
           frequency curve produced.




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Flood Frequency Analysis for Tributaries of River Ise
Results



                     Gauge                Gauge Name and                 Gauge           Gauge Name and
           Rank       No.                   Watercourse          Rank     No.              Watercourse
           1        39035           (Churn @ Cerney Wick)        20      33045     (Wittle @ Quidenham)
           2        41806           (North End Stream @          21      25019     (Leven @ Easby)
                                    Allington)
           3        99999           (Eastern Yar @ Budbridge)    22      33054    (Babingley @ Castle Rising)
           4        29009           (Ancholme @ Toft Newton)     23      44003    (Asker @ Bridport)
           5        39036           (Law Brook @ Albury)         24      66004    (Wheeler @ Bodfari)
           6        99999           (Lukely Brook @ Newport)     25      33805    (Beechamwell Brook @
                                                                                  Beechamwell)
           7        39033            (Winterbourne @ st          26      39015    (Whitewater @ Lodge Farm)
                                    Bagnor)
           8        33049            (Stanford Water @           27      39042     (Leach @ Priory Mill
                                    Buckenham Tofts)                              Lechlade)
           9        30017            (Witham @ Colsterworth)     28      30012     (Stainfield Beck @ Stainfield)
           10       54034            (Dowles Brook @ Dowles)     29      43014     (East Avon @ Upavon)
           11       44008            (Sth Winterbourne @         30      30004     (Partney Lymn @ Partney
                                    W'bourne Steepleton)                          Mill)
           12       29003            (Lud @ Louth)               31      39028     (Dun @ Hungerford)
           13       44006            (Sydling Water @ Sydling    32      36003     (Box @ Polstead)
                                    st Nicholas)
           14       52016            (Currypool Stream @         33      30005     (Witham @ Saltersford Total)
                                    Currypool Farm)
           15       41807            (Bevern Stream @ East       34      99999     (Churn @ Cirencester)
                                    Chiltington)
           16       40006            (Bourne @ Hadlow)           35      20002     (West Peffer Burn @
                                                                                  Luffness)
           17       41027           (Rother @ Princes Marsh)     36      41028     (Chess Stream @ Chess
                                                                                  Bridge)
            18      54052      (Bailey Brook @ Ternhill)      37         68015     (Gowy @ Huxley)
            19      30011      (Bain @ Goulceby Bridge)       38         29004     (Ancholme @ Bishopbridge)
           Table 3-10 FEH Pooling Group for Little Harrowden BrooK


            Gauge No.         Watercourse         Gauge Name          Reason for exclusion from pooling group
            54059           Allford Brook         Allford         Highly discordant and only 6 years data so
                                                                  estimates derived from L-moments cannot be
                                                                  considered reliable
            45801           Back Brook            Hawkerland      Only 5 years data, so L-moments cannot be
                                                                  calculated as record too short.
            54058           Stoke Park Brook      Stoke Park      Highly discordant and only 6 years data so
                                                                  estimates derived from L-moments cannot be
                                                                  considered reliable
            42011           Hamble                Frog Mill       Highly discordant and only 10 years data so
                                                                  estimates derived from L-moments cannot be
                                                                  considered reliable
            32029           Flore                 Experimental    Only 5 years data, so L-moments cannot be
                                                  Catchment       calculated as record too short.
            39035           Churn                 Cerney Wick     Very different value of FARL to subject site.
            39029           Tillingbourne         Shalford        Very different value of FARL to subject site and
                                                                  high flows often overestimated due to backing
                                                                  up from Wey causing drowning.
            33049           Stanford Water      Buckenham         Very different value of FARL to subject site.
                                                Tofts
            99999         North Wey             Alton          Very different value of BFI to subject site.
           Table 3-11 Sites removed from Little Harrowden Brook pooling group




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Isham Bypass
Flood Frequency Analysis for Tributaries of River Ise
Results

3.3.2      Index Flood - QMED
                                                                                  3
           The QMED for Hardwick Brook is 0.139 m /s, obtained from the QMED derived from catchment
                               3
           descriptors (0.131 m /s) and from the adjustment factor (1.067).

           The adjustment factor was derived using the analogue catchments listed in Table 3-12. These eight
           analogue catchments were chosen from the full pooling group list, ensuring similar baseflow,
           SPRHOST, FARL (Index of flood attenuation due to reservoirs and lakes) and SAAR. The area of
           these catchments is not very similar to the site, since Little Harrowden Brook has a small catchment
                       2
           area (1.8 km ), therefore this is a source of uncertainty in the estimation of the index flood and might
           introduce some error, but it is the only data available. The ratios from these analogue catchments
           were combined using the Geometric Mean as recommended by the FEH.


                                                                                                                                                   QMED from
                                                                                                           QMED from
            Station                                                                                                                                catchment     Adjustment
                                           Watercourse                  Gauge Name                          flow data
              No.                                                                                                                                  descriptors   Ratio (A/B)
                                                                                                               (A)
                                                                                                                                                       (B)
            39035                   Churn                        Cerney Wick                               3.510                                  0.206          17.007
            41806                   North End Stream             Allington                                 0.722                                  0.470          1.535
            99999                   Eastern Yar                  Budbridge                                 4.762                                  2.288          2.082
            29009                   Ancholme                     Toft Newton                               1.837                                  2.814          0.653
            39036                   Law Brook                    Albury                                    0.463                                  0.743          0.624
            99999                   Lukely Brook                 Newport                                   0.764                                  0.612          1.248
            39033                   Winterbourne                 St Bagnor                                 0.358                                  3.038          0.118
            33049                   Stanford Water               Buckenham Tofts                           0.719                                  1.397          0.515

           Table 3-12 Analogue catchments for Little Harrowden Brook

           The number of analogue catchments included in the estimation of the index flood was obtained by
           adding catchments until the geometric mean became constant.


3.3.3      Flood frequency curve
           The growth curve was determined by use of the Generalised Logistic Distribution that not only is
           recommended by the FEH as the representative of UK catchments, but also in this case provided the
           best fit to the data (Z=-1.79).

           The growth curve was then adjusted by the index flood, QMED, of the site of interest, determined by
           the adjustment factor of the analogue catchments to obtain the flood frequency curve shown in Figure
           3-3.
                          0.7




                          0.6




                          0.5
             Q (cumecs)




                          0.4

                                                                                                                       GL


                          0.3




                          0.2




                          0.1
                                           1.5



                                                 2




                                                             5




                                                                        10




                                                                             25




                                                                                         50

                                                                                                75
                                                                                                     100



                                                                                                                 200




                                                                                                                                  500




                                                                                                                                        T

                          0.0
                             -2.0   -1.0         0.0   1.0        2.0         3.0         4.0              5.0              6.0             7.0
                                                             Logistic reduced variable




           Figure 3-3 Flood Frequency Curve for Hardwick Brook




6ddb8e0d-cadc-498e-8d95-d65b55dba63f.doc                                          Page 11                                                                                 Final
Isham Bypass
Flood Frequency Analysis for Tributaries of River Ise
Conclusions



4          Conclusions
                     A regional method described in the FEH [Ref 1] has been used to determine return period
                      flows (2, 5, 10, 25, 50, 100 and 200 year) for three tributaries of the River Ise; Pytchley
                      Brook, Hardwick Brook and Little Harrowden Brook.

                     This is the best available method, but is still subject to error as:

                          o    No data is available for the catchment of interest

                          o    There is a large difference in catchment area between the catchment of interest
                               and the analogue catchments (the catchments for which data is available)

                     We suggest these results are treated with caution and that a safety factor is applied. The
                      size of the safety factor is best decided by the engineer responsible for the design of the
                      culverts because he/she is most conversant with the risks associated with potential failures
                      in the design. Our best estimate of flow associated with a range of return periods for each
                      site is provided in Table 4-1. The same table also shows these flows with a safety factor of
                      50%. The safety factor quoted here is only intended to be indicative.



                                Pytchley Brook                   Hardwick Brook              Little Harrowden Brook
                                487852, 274735                   488250, 273250                   488250, 271750
            Return
            Period                        Flood flow                       Flood flow                     Flood flow
                          Flood flow                        Flood flow                       Flood flow
            (years)                       with safety                      with safety                    with safety
                          from FEH                          from FEH                         from FEH
                             3 -1         factor 50%           3 -1        factor 50%           3 -1      factor 50%
                          (m s )             3 -1           (m s )            3 -1           (m s )          3 -1
                                          (m s )                           (m s )                         (m s )

                2          1.511           2.267        2.214           3.321                0.139        0.209
                5          2.267           3.400        3.330           4.995                0.206        0.309
               10          2.788           4.182        4.091           6.137                0.254        0.382
               25          3.513           5.270        5.139           7.708                0.324        0.487
               50          4.113           6.169        5.998           8.997                0.385        0.577
              100          4.773           7.160        6.932           10.398               0.453        0.679
              200          5.502           8.252        7.957           11.936               0.531        0.796
           Table 4-1 Flow estimates for the three tributaries of the River Ise




6ddb8e0d-cadc-498e-8d95-d65b55dba63f.doc                 Page 12                                                  Final
Isham Bypass
Flood Frequency Analysis for Tributaries of River Ise
References



5          References
           1. Institute of Hydrology (1999) Flood Estimation Handbook, Institute of Hydrology, Oxford




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