This report describes the results of a low flow study of Northern Ireland. The work was commissioned by the Departme nt of the Environment(N.I.) and was carried out by the Institute of Hydrology. The main objective of the study was to improve techniques for low flow estimation at the ungauged site. The study was based on mean daily discharge for 21 flow records using data provided by the Water Data Unit, Water Services Department. The main conclusion of the study was that relationships between low flow statistics and drift geology can be used to estimate low flows at

sites without flow records. It is recommended that the estimation equations are revised when the current revision of the water archive for Northern Ireland is complete

Gustard, A.

Sutcliffe, F.

English

NERC Open Research Archive

Water resource i n v e s t i g a t i o n s  r equ i re  information on low r i v e r  flows f o r  designing r i v e r  a b s t r a c t i o n  schemes, a s ses s ing  t h e  d i l u t i o n  of domestic and i n d u s t r i a l  e f f l u e n t  and f o r  e s t ima t ing  the y i e ld  of d i r e c t  supply r e se rvo i r s .  Where r i v e r  flow records a r e  a v a i l a b l e  a t  t h e  s i t e  of i n t e r e s t ,  then these  can be analysed t o  provide es t imates  f o r  t h e  above des ign  problems. However, t h i s  i s  not  always poss ib l e  because flow data  may be absent  o r  the  records too s h o r t  f o r  accura te  es t imat ion  of extreme low flows. An e a r l i e r  study (Department of  t h e  Environment (NI) 1984) included the  development of r eg iona l  es t imat ion  techniques f o r  determining the  y i e l d  of a  r e s e r v o i r  of given s to rage  f o r  a  s p e c i f i c  r e t u r n  period of f a i l u r e .  The ob jec t ive  of the  cu r ren t  i n v e s t i g a t i o n  is t o  develop methods f o r  e s t ima t ing  low flows f o r  e f f l u e n t  d i l u t i o n  and r i v e r  a b s t r a c t i o n  des ign  problems a t  l oca t ions  without h i s t o r i c a l  flow records.  A number of r eg iona l  low flow s t u d i e s  ( I n s t i t u t e  of Hydrology 1980; P i r t  and Douglas, 1982; Hanna and Wilcock, i n  press)  have noted t h e  importance of catchment geology i n  c o n t r o l l i n g  low flow regimes. I n  Northern I r e l a n d ,  rocks of low permeabil i ty  a r e  dominant, no tably ,  t h e  S i l u r i a n  and Ordovician greywackes i n  the south e a s t  and c r y s t a l l i n e  basement rocks of Moinian and Dal radian  age i n  t h e  no r th  e a s t ,  no r th  west and west.  Product ive s o l i d  rock a q u i f e r s  a r e  l imi t ed  i n  ex ten t  t o  t h e  Permian and T r i a s s i c  sandstones of t h e  Lagan v a l l e y  and west of Lough Neagh. The outcrop of Cretaceous chalk i s  a l s o  of l imi t ed  ex ten t  and al though i t  i s  of lower permeabil i ty  than i t s  Engl i sh  coun te rpa r t ,  it can s u s t a i n  low r i v e r  flows from a  number of sp r ings  which i s s u e  where t h e  base  of t h e  chalk o v e r l i e s  impermeable s t r a t a .  With t h e  except ion  of t h e  upland a r e a s ,  t h e  s o l i d  geology of t h e  Province i s  covered by a  veneer of d r i f t  ma te r i a l  of varying l i t ho logy .  Extensive d e p o s i t s  of impermeable boulder c l ay  r e s t r i c t s  recharge t o  and d ischarge  from, underlying permeable s t r a t a .  The baseflow component of t h e  s tream hydrograph i s  thus  very smal l  d e s p i t e  t h e  presence of s a t u r a t e d  a q u i f e r s  below t h e  impermeable horizons.  The only exception t o  t h i s  a r e  where permeable f luvo  g l a c i a l  d e p o s i t s  of sands and g r a v e l  maintain t h e  low flows a t  r e l a t i v e l y  h igh  d ischarges .  These outcrop most ex tens ive ly  t o  t h e  west and n o r t h  of Lough Neagh and i n  t h e  Lagan Valley. The approach used t o  develop es t imat ion  techniques at t h e  ungauged s i t e  was t o  r e l a t e  low flow s t a t i s t i c s  derived from the  d a i l y  d ischarge  d a t a  t o  a  number of c l imate ,  physiographic, s o i l  and geologica l  c h a r a c t e r i s t i c s .  Sec t ion  2 of the repor t  desc r ibes  the  a n a l y s i s  of the discharge da ta ,  s e c t i o n  3 t h e  catchment c h a r a c t e r i s t i c s  and s e c t i o n  4 t h e  d e r i v a t i o n  of t h e  es t imat ion  equat ions.  2.  LOV FLOW ANALYSIS A d a t a  s e t  of 21 mean d a i l y  flow records from t h e  Water Data Unit  (NI) were used throughout t h e  study. These s t a t i o n s  were s e l e c t e d  on the  b a s i s  of good accuracy of low flow measurement and a r e l a t i v e l y  low degree of a r t i f i c i a l  con t ro l .  Table 1 l i s t s  t h e  period of record f o r  each of t h e  2 1  s t a t i o n s  and a number of flow measures. These flow s t a t i s t i c s  were der ived  us ing  methods descr ibed  i n  t h e  Low Flow S tud ies  Report ( I n s t i t u t e  of Hydrology 1980). They inc lude  the mean d ischarge  (ADF), t h e  base flow index (BFI), t h e  mean annual  minimum (MAM) and t h e  95 p e r c e n t i l e  d i scha rge  (Q95) from the  flow dura t ion  curve. Values of MAM(1) and MAM(10) t h e  one and t e n  day mean annual  minima were c a l c u l a t e d  and s i m i l a r l y  Q95(1) and Q95(10) from the one and t e n  day flow dura t ion  curve. These discharge va lues  were expressed a s  a  percentage of t h e  mean flow. This  s t anda rd iza t ion  procedure permits comparison between d i f f e r e n t  r i v e r s  by reducing the e f f e c t  on t h e  s lope  and l o c a t i o n  of t h e  flow dura t ion  curve of d i f f e rences  i n  catchment a rea  and h igher  or  lower than average flows during the  period of record.  I n  comparison with low flows i n  o t h e r  regions of t h e  United Kingdom, d ischarges  a r e  r e l a t i v e l y  low i n  Northern I re land .  For example t h e r e  i s  only one catchment with a  va lue  of Q95(10) i n  excess of 18% ADF - t h e  average f o r  456 U.K. flow records. With t h e  exception of t h r e e  catchments a l l  va lues  of t h e  10 day mean annual minima a r e  below 15% ADF r e f l e c t i n g  t h e  gene ra l ly  low base  flows and f l a s h y  na tu re  of catchments i n  Northern I r e l a n d  The low flow i n d i c e s  descr ibed  i n  s e c t i o n  2 were c o r r e l a t e d  wi th  catchment c h a r a c t e r i s t i c s  t o  develop p red ic t ion  formulae f o r  u s e  on ungauged s i t e s .  The s e l e c t i o n  of catchment c h a r a c t e r i s t i c s  was decided by t h e i r  a v a i l a b i l i t y  i n  map form f o r  t h e  e n t i r e  province and t h e  f a c t o r s  considered t o  c o n t r o l  low flows. The fol lowing paragraphs summarize t h e  d e f i n i t i o n  and c a l c u l a t i o n  of t h e  c h a r a c t e r i s t i c s  l i s t e d  i n  Table 2 .  Gauging s t a t i o n  201002 FAIRY WATER AT DUDGEON BR 201005 CAMOWEN AT CAMOWEN TERRACE 201006 DRUMRAGH AT CAMPSIE BRIDGE 201007 BURNDENNET AT BURNDENNET BR 201008 DERG AT CASTLEDERG 203010 BLACKWATER AT MAYDOWN BR 203011 MAIN AT DROMONA 203012 BALLINDERRY AT BALLINDERRY 203013 MAIN AT ANDRAID 203018 SIX MILE WATER AT ANTRIM 203020 MOYOLA AT MOYOLA NEW BR 203021 KELLS WATER AT CURRYS BR 203025 CALLAN AT CALLAN NEW BR 203027 BRAIDE AT BALLEE 203028 AGIVEY AT WHITE HILL 203029 SIX MILE WATER AT BALLYCLARE 203033 UPPER BANN AT BANNFIELD 204001 BUSH AT SENEIRL 205004 LAGAN AT NEWFORGE 205005 RAVERNET AT RAVERNET 205008 LAGAN AT DRUMILLER Period of (1) record MAMl MAM Mean annual  minimum f o r  1 and 10 days, % ADF 995 95 p e r c e n t i l e  f o r  1 and 10 days, % ADF BFI Base Flow Index ADF Average flow cumecs BFI ADF TABLE 1 Flow va r i ab le s  I Gauging S t a t i o n  D r i f t  Geology Pe rcen t age  cover  SOIL 201002 FAIRY WATER AT DUDGEON BR 25 44 18 0 13  .461 201005 CAMOWEN AT CAMOWEN TERRACE 35 16 28 12 9 .420 201006 DRUMRAGH AT CAMPSIE BRIDGE 58 14 15 2 11 .422 I 201007 BURNDENNET AT BURNDENNET B 40 18 18 19 5 .390 201008 DERG AT CASTLEDERG 10 40 45 0 5 .406 203010 BLACKWATER AT MAYDOWN BR 80 3 4 7 6  .437 203011 MAIN AT DROMONA 5 3 18 17 7 5 .445 203012 BALLINDERRY AT BALLINDERRY B 57 6 6 2 3 8  .303 203013 MAIN AT ANDRAID 57 25 8 6 4 .422 203018 SIX MILE WATER AT ANTRIM 69 17 4 5 5  .412 203020 MOYOLA AT MOYOLA NEW BR 44 11 11 24 10 .339 203021 KELLS WATER AT CURRYS BR 48 32 12 3 5 .470 203025 CALLAN AT CALLAN NEW BR 8 1 7 4 2 6  .375 203027 BRAID AT BALLEE 45 34 11 5 5 .385 I 203028 AGIVEY AT WHITE HILL 59 10 24 4 3 .417 203029 SIX MILE WATER AT BALLYCLARE 67 27 2 2 2 .426 203033 UPPER BANN AT BANNFIELD 74 19 4 0 3 .301 204001 BUSH AT SENEIRL 50 7 27 13 3 .410 205004 LAGAN AT NEWFORGE 69 16 2 7 6  .382 205005 RAVERNET AT RAVERNET 55 4 1  1 0 3 .450 205008 LAGAN AT DRUMILLER 68 21  2 0 9 .447 TL = till  RK = rock PT = p e a t  GL = sand and g r a v e l  AL = a l l uv ium SOIL Flood S t u d i e s  S o i l  Index  SAAR Annual Average R a i n f a l l  mm AREA Catchment Area km2 STMFRQ Number of s t ream junc t ions  per  squa re  k i l o m e t r e  from 1:63 360 and 1:50 000 s h e e t s  AREA STMFRQ Geology: The in f luence  of s o l i d  geology on low flows 1 4  r e s t r i c t e d  owing t o  t h e  s u b s t a n t i a l  depos i t s  of d r i f t  ma te r i a l  throughout t h e  a r e a .  At ten t ion  was thus  focussed on the  d r i f t  d e p o s i t s  of each catchment and these  were est imated by overlaying catchment boundaries on the 1:250,000 d r i f t  geology map produced by t h e  Geological  Survey of Northern I re land .  The propor t ion  of till (boulder c l a y ) ,  pea t ,  sand and g rave l ,  alluvium and rock dominant a r e a s  were ca l cu la t ed  f o r  each catchment. The s o l i d  geology map was a l s o  examined to  determine the  n a t u r e  of the  s o l i d  rock i n  catchments with a  s i g n i f i c a n t  percentage of rock dominant a reas .  S o i l :  The Flood S tud ies  (NERC 1975) Winter R a i n f a l l  Acceptance P o t e n t i a l  map was used t o  c a l c u l a t e  the  proport ions of each of t h e  f i v e  s o i l  c l a s s e s  from which t h e  FSR S o i l  Index (SOIL) was derived.  P r e c i p i t a t i o n :  Values of annual  average catchment p r e c i p i t a t i o n  (SAAR) were provided by the  Water Data Unit (NI). Catchment Area: The topographic catchment a r e a  was used. However because t h e  low flow v a r i a b l e s  have been s tandardized  by mean runoff  i t  was u n l i k e l y  t h a t  high c o r r e l a t i o n s  between low flows and catchment s i z e  would be observed. Stream frequency: Previous s t u d i e s  of both f loods  and low flows have ind ica t ed  t h a t  the  number of stream junct ions per square ki lometre (STMFRQ) is  a u s e f u l  p r e d i c t i v e  v a r i a b l e  i n  r eg iona l  hydrology. It is known f o r  example t h a t  a  high stream frequency w i l l  i nc rease  the  r a t e  of quick  response runoff and t h i s  might be expected t o  reduce low flow discharges.  Values were i n i t i a l l y  derived f o r  t h e  1:63 360 and these  showed s i g n i f i c a n t  c o r r e l a t i o n s  wi th  low flows. However, when stream frequency was c a l c u l a t e d  from t h e  more r ecen t  1:50 000 s e r i e s  the re  was l i t t l e  p r e d i c t i v e  va lue  i n  t h e  catchment c h a r a c t e r i s t i c s .  4- RELILTIONSEIPS BETVEEN LOW PLOWS AND CATCHIIENT CEARACTERISTICS 4.1 Est imation Equations The des ign  method recommended i n  t h e  Low Flow Studies  Report i s  based on r eg iona l  r eg res s ion  equat ions  between t h e  Base Flow Index and low flow s t a t i s t i c s  and l o c a l  r e l a t i o n s h i p s  between BFI and catchment geology. The -. c o r r e l a t i o n  between Q95(10) and BE1  i n  Northern I r e l a n d  was 0.61 compared t o  0.74 f o r  t h e  456 catchments used i n  t h e  Low Flow Studies  p r o j e c t .  The a b i l i t y  t o  p r e d i c t  B E 1  from catchment c h a r a c t e r i s t i c s  was a l s o  l i m i t e d  wi th  the  b e s t  two v a r i a b l e  equat ions  us ing  GVL and TILL expla in ing  only 35% of t h e  va r i ance  of B E I .  The r e l a t i o n s h i p  between B E 1  and SOIL was a l s o  explored i n  o rde r  t o  provide a  method f o r  est imating B E 1  a t  t h e  ungauged s i t e .  The r e s u l t s  of t h i s  were d isappoin t ing  wi th  very poor c o r r e l a t i o n s  between B E 1  and both the SOIL index and the proport ions of d i f f e r e n t  s o i l  c l a s s e s .  It was thus  decided t o  omit t h e  Base Flow Index from t h e  procedure and r e l a t e  low flow s t a t i s t i c s  d i r e c t l y  t o  catchment c h a r a c t e r i s t i c s .  The flow v a r i a b l e s  Q95(10) and MAM(10) were r e l a t e d  to  the catchment c h a r a c t e r i s t i c s  shown i n  Table 2  us ing  da ta  f o r  t h e  21  catchments. Logarithmic and square roo t  t ransformations were used but  these  showed no improvements over  untransformed va r i ab le s .  The most u se fu l  c o r r e l a t i o n s  were wi th  the  propor t ions  of d r i f t  depos i t s  p a r t i c u l a r l y  sands and g rave l  (GL) where t h e  c o r r e l a t i o n  c o e f f i c i e n t  was 0.80 wi th  Q95(10) and 0.73 wi th  MAM(10). (These were s i g n i f i c a n t l y  higher  than the  c o r r e l a t i o n  of 0.49 bestween GL and BFI). The recommended equat ions  f o r  Northern I r e l a n d  a r e  thus:- These permit  e s t ima t ion  of low flow indices  from t h e  1:250 000 d r i f t  map of Northern I r e l and  with an e r r o r  of j u s t  over 3  percent  of t h e  average flow. It has  not  been poss ib l e  t o  de r ive  new dura t ion  and frequency r e l a t i o n s h i p s  w i t h i n  t h e  scope of t h i s  cu r ren t  p ro jec t .  However, t h e  performance of t h e  Low Flow Studies  dura t ion  r e l a t i o n s h i p  was t e s t e d  on t h e  Northern I r e l a n d  d a t a  s e t .  This  was done by comparing t h e  observed r a t i o s  of Q95(1) and Q95(10) with t h e  p red ic t ed  r a t i o s  and t h i s  confirmed t h a t  t h e  n a t i o n a l  du ra t ion  r e l a t i o n s h i p  could be used i n  N. I r e l and .  U n t i l  f u r t h e r  Low Flow Studies  a r e  c a r r i e d  o u t  t h e  dura t ion  and frequency r e l a t i o n s h i p s  i n  t h e  Low Flow S tud ies  Report should be used. The f i n a l  s t a g e  i n  t h e  e s t ima t ion  procedure i s  t o  c a l c u l a t e  t h e  mean flow a t  t h e  s i t e  of i n t e r e s t  us ing  t h e  methods ou t l ined  i n  LFSR number 3. (The a p p l i c a b i l i t y  of t h i s  method f o r  catchments w i t h  r a i n f a l l  i n  excess of 1000 mm was conEirmed i n  t h e  Study of Water Demand and supply i n  Northern 1 I r e l and . )  1 4.2 Winter Ra in fa l l  Acceptance P o t e n t i a l  Map It i s  i n t e r e s t i n g  t o  compare the c o r r e l a t i o n  between BFI and t h e  propor t ions  of d r i f t  geology wi th  t h a t  between BFI and t h e  Flood S tud ies  SOIL index. The bes t  t h r e e  va r i ab le s  were GVL, TILL and ROCK and t h e s e  explained 40% of t h e  var iance  of BFI. In  c o n t r a s t  t h e  SOIL index was not  co r re l a t ed  with BFI explaining only 2% of the  variance.  Although BFI was developed f o r  low flow es t imat ion  purposes, (1-BFI) i s  an index of the  r ap id  response runoff and t h i s  has been found use fu l  i n  f lood es t imat ion  (Boorman, 1985), expla in ing  near ly  60% of t h e  var iance  o f  s tandard percentage runoff .  Furthermore, the c o r r e l a t i o n  between BFI and SOIL i n  mainland B r i t a i n  of t h e  order  of -0.70 have been observed from 426 catchments. There i s  a l s o  evidence (Gustard 1986) t h a t  t h e  value of BFI observed from WRAP c l a s s e s  i n  Northern I r e l a n d  a r e  s i g n i f i c a n t l y  lower than  t h e i r  coun te rpa r t s  elsewhere i n  the  United Kingdom. The poor p r e d i c t i v e  a b i l i t y  of t h e  Northern I r e l a n d  WRAF' map and t h e  inconsis tency of c l a s s i f i c a t i o n  sugges ts  t h a t  a  r e v i s i o n  of t h e  Flood Studies  s o i l  map is  required.  Methods have been developed f o r  es t imat ing  t h e  95 p e r c e n t i l e  10 day d ischarge  and 10 day mean annual  minimum discharge a t  ungauged sites from the  d r i f t  map of Northern I r e l and .  The Water Data Unit (NI) a r e  c u r r e n t l y  v a l i d a t i n g  t h e  mean d a i l y  flow archive.  When t h i s  work is complete i t  i s  proposed t h a t  t h i s  s tudy should be updated us ing  t h e  improved da ta  and t h e  longer  t ime s e r i e s  which w i l l  then  be  ava i l ab le .  U n t i l  t h i s  t a s k  is completed t h e  recommended equat ions a r e  provis ional .  This  s tudy has  a l s o  i n d i c a t e d  a  need f o r  a  r e v i s i o n  of t h e  Flood Studies  s o i l  map of Northern I r e l and .  Boorman, D.B. 1985. A review of the  Flood Studies  Report Ra in fa l l  Runoff Model Parameter Es t imat ion  Equations. I n s t i t u t e  of Hydrology Report No.94. Department of  t h e  Environment f o r  Northern  I r e l a n d .  1984.  A Study of Water - Demand and  Supply i n  N o r t h e r n  I r e l a n d .  Gus ta rd ,  A .  1986. The u s e  of Regional  S o i l  Survey I n f o r m a t i o n  f o r  Flood and  Low Flow Design. Nordic  Hydro log ic  Programme Seminar on Water i n  t h e  u n s a t u r a t e d  zone. Oslo.  Hanna, J.E. and Wilcock,  D.N. 1986. The d e r i v a t i o n  of  f low d u r a t i o n  c u r v e s  i n  Nor the rn  I r e l a n d  ( i n  p r e s s ) .  I n s t i t u t e  of Hydrology.  1980. Low Flow S t u d i e s  Repor t .  P i r t ,  J .  and Douglas ,  J .P .  A s t u d y  of low f lows  u s i n g  d a t a  from t h e  Severn  and T r e n t  ca tchments .  JrWE 1982 No.4 p299-308. 

Low flow study of Northern Ireland

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Low flow study of Northern Ireland

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