  Two searches for new phenomena in final states containing a same-flavour
opposite-lepton (electron or muon) pair, jets, and large missing transverse
momentum are presented. These searches make use of proton--proton collision
data, collected during 2015 and 2016 at a centre-of-mass energy $\sqrt{s}=13$
TeV by the ATLAS detector at the Large Hadron Collider, which correspond to an
integrated luminosity of 14.7 fb$^{-1}$. Both searches target the pair
production of supersymmetric particles, squarks or gluinos, which decay to
final states containing a same-flavour opposite-sign lepton pair via one of two
mechanisms: a leptonically decaying Z boson in the final state, leading to a
peak in the dilepton invariant-mass distribution around the Z boson mass; and
decays of neutralinos (e.g. $\tilde{\chi}_2^0 \rightarrow \ell^+\ell^-
\tilde{\chi}_1^0$), yielding a kinematic endpoint in the dilepton
invariant-mass spectrum. The data are found to be consistent with the Standard
Model expectation. Results are interpreted in simplified models of gluino-pair
(squark-pair) production, and provide sensitivity to gluinos (squarks) with
masses as large as 1.70 TeV (980 GeV).
